Quadrajet Hesitation When Accelerating

[jbwolfe]

Originally Posted by 68gmsee

I think you're gonna have to try what these guys are telling you otherwise it ain't gonna get fixed. Most of us speak from many, many years of experience.

First, I would look for another vacuum port if that one didn't fit or get some sort of adapter from local auto parts.

Next, I would tie the choke open regardless of what the vacuum does. It's still trying to close when the vacuum is low as when accelerating.

Re. vacuum testing: Go to Sears or the local auto parts store and pick up a vacuum gauge. It's something you need to have to work on these old trucks. One of the cheaper tools to own.

Re. checking for leaks: Yes, you can use the starter fluid carefully to see if the engine revs up higher when there's a vacuum leak. I prefer a 2-3 foot section of garden hose and place one end on the areas I suspect are leaking to listen for hissing sounds. Either way works.

Thank for your input. I'm going to order the book that was mentioned earlier to learn more about the Q-jets.

I'll tie the choke open and try that, I hadn't realized the vacuum would be lower when the throttle is open, makes sense.

Next time I'm in town I'm going to get a vac tester. I'll test the fuel pressure and vacuum. I will also test for vacuum leaks around the carb.

Originally Posted by jd7

Put a new holley on it and be done. That intake is way to much for that carb set up. A decent builder would have at least done a better job of cleaning the body.It looks to be a few carbs cannibalized.

I might if I had the money. I guess I wasn't very clear in the original post. The carb currently on the truck IS NOT REBUILT. That was the previous Q-jet, which had even more problems.

[jbwolfe]

Originally Posted by Bedpanbrian

I've used a Q-Jet wit that manifold in the past. Set up correctly the Q-Jet is a fantastic carb.

I wish you weren't on the other side of the state from me or I'd come see if I could help you. I don't know if someone else said something about the throttle return spring, but you have one of them hooked up wrong. You can run them both to the mount behidn the carb, with the smaller spring sitting inside the larger one, in your photo the larger one is stretched to its limit.

It will take a while for that setup to warm up all the way, so choke issues could be part of the problem. Part of the idea behind the air gap is keeping the carb setup cooler by having that air gap between it and the top of the motor. If I'm right, you are just shy of the Canadian border and even your summers aren't really toasty.

Switch over to an electric choke. The choke creates more vacuum in the airway of the carb so it sucks in more fuel. Just a thought but maybe you are running a little on the lean side. Get the vacuum guage and make sure you are pulling enough to advance the distributor.

Is everything inside the distributor moving freely? The mechanical weights and the vacuum to make sure you are getting enough advance in the timing?

Just some more thoughts.

You must have posted this while I was replying to another post, so I just missed it.

I'll check out those springs, I never was sure how to set them up because they were originally on the 2G. I tried putting the electric choke from the other Qjet on here, but it wouldn't fit right.

Everything in the distributor is new, and a shop did the timing not long ago. How much is the right amount of vacuum? Is that something the Qjet book will tell me?

Yes I'm about 10 minutes from the Canadian border, and we MIGHT get a few days above 70 in the summer. However, I've driven this truck in 20 degree weather and 90 degree weather, no difference at all once it's warmed up.

[flyingtim01]

jb, You're right, I did miss the part in your first post when you said you swapped out your carb for a used one from a garage.

In that case, get the used one rebuilt. The symptoms you describe sound like your garden variety gummed up carb. If you have plenty of power in the higher rpm ranges and no stumbling/hesitation issues, then your secondaries are more than likely functioning correctly. If that carb sat on their shelf for more than 6 months without being used, the fuel in it has probably blocked your primary circuits to some extent.

First, as some have suggested, move your accelerator pump rod to the inner hole on the arm. If it's in the other hole, as yours is, you're going to get too big/too soon of a shot of fuel, and that might be your issue, if you're flooding your carb a little each time you step on it.

Once you do that, try at least tuning it. The Qjet has two mixture screws in the front, sticking out of the throttle plate. 'Factory default' settings for those screws should be 2-2.5 turns out, as a baseline reference. Get the truck to operating temp, make sure the choke is wide open, then start turning one of the mixture screws in very slowly, no more than a 1/4 turn at a time, until you hear the engine start to miss/sputter. Note the position of the screw, then start to slowly turn it back out until you again hear the engine speed change, and note that position. Turn the screw back in until you've reached the middle point between your two limits, and then do it again on the other screw. This isn't the most technical way of doing it, but without a vacuum gauge, it will get you pretty darn close.

If those two things yield no results, then I would say its time to pull that carb apart and rebuild it. Good news is, it doesn't sound like you'll have to fiddle with your secondary spring tension. Put a good kit and a brass float in it. While you're in there, it wouldn't be a bad idea to epoxy the bottom of the float bowl (they have a tendency to leak, emptying your float bowl and making it hard to start) and check the play in your throttle shafts. When they get loose they suck air and wreak havoc with your mixture.

[jbwolfe]

Originally Posted by flyingtim01

jb, You're right, I did miss the part in your first post when you said you swapped out your carb for a used one from a garage.

In that case, get the used one rebuilt. The symptoms you describe sound like your garden variety gummed up carb. If you have plenty of power in the higher rpm ranges and no stumbling/hesitation issues, then your secondaries are more than likely functioning correctly. If that carb sat on their shelf for more than 6 months without being used, the fuel in it has probably blocked your primary circuits to some extent.

First, as some have suggested, move your accelerator pump rod to the inner hole on the arm. If it's in the other hole, as yours is, you're going to get too big/too soon of a shot of fuel, and that might be your issue, if you're flooding your carb a little each time you step on it.

Once you do that, try at least tuning it. The Qjet has two mixture screws in the front, sticking out of the throttle plate. 'Factory default' settings for those screws should be 2-2.5 turns out, as a baseline reference. Get the truck to operating temp, make sure the choke is wide open, then start turning one of the mixture screws in very slowly, no more than a 1/4 turn at a time, until you hear the engine start to miss/sputter. Note the position of the screw, then start to slowly turn it back out until you again hear the engine speed change, and note that position. Turn the screw back in until you've reached the middle point between your two limits, and then do it again on the other screw. This isn't the most technical way of doing it, but without a vacuum gauge, it will get you pretty darn close.

If those two things yield no results, then I would say its time to pull that carb apart and rebuild it. Good news is, it doesn't sound like you'll have to fiddle with your secondary spring tension. Put a good kit and a brass float in it. While you're in there, it wouldn't be a bad idea to epoxy the bottom of the float bowl (they have a tendency to leak, emptying your float bowl and making it hard to start) and check the play in your throttle shafts. When they get loose they suck air and wreak havoc with your mixture.

Thanks for the instructions. Now how would I tune the carb using a vac tester? I think I'm going to get one.

The throttle shafts are nice and tight, one of the reasons I picked this carb. I have a lead on a recently rebuilt Qjet, with an electric choke, so I'm really hoping I can get that one, either way I'm going to try some more things with this carb, and get a good book.

[68gmsee]

Put the vacuum tester in a vacuum port. The bottom manifold vacuum that I mentioned would be a good source. Any other manifold vacuum will work except for the ported vacuum that you have the advance on.

Next, adjust the idle screws as mentioned above by flyingtim but look for the highest vacuum on each one.

[BHyatt]

It would seem reasonable to me that a simple solution would be a return to the original carb and intake which you said ran fine. Problem solved.

[flyingtim01]

Just what 68 said. The vacuum gauge will get your mixture right on without having to rely on your ear.

I'd grab the carb with the electric choke. It's what I have on mine, and in my estimation, its just a simpler setup. No linkage to bend/adjust, don't have to worry about the choke stove getting gummed up when your inner intake bolts start to seep oil, and they're easier to adjust when the weather changes. Just loosen the screws holding it on and rotate. Just need to run a key on 12V wire. And if you want to be really trick, there was a member who remarked that he wired his choke in through his oil pressure wire, so it only got 12V when the engine was running, not just when the key was on (so you don't have to remember to make sure your key is in the OFF position when you don't want the choke to heat up).

BHyatt, the original setup that worked well was a 2 barrel, which for obvious reasons isn't acceptable...

[jbwolfe]

Originally Posted by 68gmsee

Put the vacuum tester in a vacuum port. The bottom manifold vacuum that I mentioned would be a good source. Any other manifold vacuum will work except for the ported vacuum that you have the advance on.

Next, adjust the idle screws as mentioned above by flyingtim but look for the highest vacuum on each one.

I just picked up a vac tester today, and a flair nut wrench set so I don't strip the bolt when I take the fuel line off next time. How do you tell which is ported vacuum and which isn't? You're talking about adjusting the two mixture screws in front right?

Originally Posted by BHyatt

It would seem reasonable to me that a simple solution would be a return to the original carb and intake which you said ran fine. Problem solved.

Aside from really wanting a 4bbl setup, the 2bbl setup had other problems of it's own.

Originally Posted by flyingtim01

Just what 68 said. The vacuum gauge will get your mixture right on without having to rely on your ear.

I'd grab the carb with the electric choke. It's what I have on mine, and in my estimation, its just a simpler setup. No linkage to bend/adjust, don't have to worry about the choke stove getting gummed up when your inner intake bolts start to seep oil, and they're easier to adjust when the weather changes. Just loosen the screws holding it on and rotate. Just need to run a key on 12V wire. And if you want to be really trick, there was a member who remarked that he wired his choke in through his oil pressure wire, so it only got 12V when the engine was running, not just when the key was on (so you don't have to remember to make sure your key is in the OFF position when you don't want the choke to heat up).

BHyatt, the original setup that worked well was a 2 barrel, which for obvious reasons isn't acceptable...

Yes I'm planning on putting the rebuilt Qjet with the electric choke on. I've already got the wire set up for the choke. In the mean time before I get the new carb, I am going to do some more tests on this carb because I'm curious about what's causing this problem. That and it wouldn't hurt to test my fuel pressure anyway.

I agree about the 2bbl being unacceptable.

[c10monkey]

I personally would swap the intake out. Looks like you have a single plane intake which is great for racing and higher rpms. But a dual plane intake is better at lower rpms and better for street use. I have a Qjet on an Eldelbrook dual plane intake and have no issues.

My 2 cents :-)

[jbwolfe]

Originally Posted by c10monkey

I personally would swap the intake out. Looks like you have a single plane intake which is great for racing and higher rpms. But a dual plane intake is better at lower rpms and better for street use. I have a Qjet on an Eldelbrook dual plane intake and have no issues.

My 2 cents :-)

How do you tell if it is a dual plane intake? I'm not really sure what that means...

Just for an update: I haven't been able to get that other used carb I was hoping for yet, and in the mean time I had to take the second used carb back to the mechanic who sold it to me for my money back, so I didn't get to mess with it anymore.

For now, I'm going to put the first Q-Jet back on, just so I can drive the truck. Hopefully I will get a different carb to try soon. If not, I'm considering buying a used Q-Jet and rebuilding it after ordering a good book.

[c10monkey]

Originally Posted by jbwolfe

How do you tell if it is a dual plane intake? I'm not really sure what that means...

Just for an update: I haven't been able to get that other used carb I was hoping for yet, and in the mean time I had to take the second used carb back to the mechanic who sold it to me for my money back, so I didn't get to mess with it anymore.

For now, I'm going to put the first Q-Jet back on, just so I can drive the truck. Hopefully I will get a different carb to try soon. If not, I'm considering buying a used Q-Jet and rebuilding it after ordering a good book.

Easiest way to tell the difference is with th carb off. When you look inside, is there a single level or dual level?
Here is a good article.... http://www.popularhotrodding.com/eng...e/viewall.html
Side thought but just to make sure.... you need a gasket between the carb and intake...

Hope this helps
Posted via Mobile Device

[jbwolfe]

Originally Posted by c10monkey

Easiest way to tell the difference is with th carb off. When you look inside, is there a single level or dual level?
Here is a good article.... http://www.popularhotrodding.com/eng...e/viewall.html
Side thought but just to make sure.... you need a gasket between the carb and intake...

Hope this helps
Posted via Mobile Device

Sorry for the late reply. I checked and the intake manifold IS for sure a dual level, and yes, there is a gasket in between the carb and intake.

Haven't had any time at all to even worry about the carb issue, just been running the rig with the old carb on there.

I did however just pick up a used Q-Jet at a friends garage sale for $10. He said it ran 20 years ago, so should be a good NON-FRANKENSTIEN rebuild candidate.

I just ordered the Cliff Ruggles Q-Jet book. As soon as I finish getting my shop cleaned up, and a work bench cleaned off, I'll do some reading in that book and get started.

Thank you all for your input on this thread!

[slikside]

Originally Posted by jbwolfe

Sorry for the late reply. I checked and the intake manifold IS for sure a dual level, and yes, there is a gasket in between the carb and intake.

Haven't had any time at all to even worry about the carb issue, just been running the rig with the old carb on there.

I did however just pick up a used Q-Jet at a friends garage sale for $10. He said it ran 20 years ago, so should be a good NON-FRANKENSTIEN rebuild candidate.

I just ordered the Cliff Ruggles Q-Jet book. As soon as I finish getting my shop cleaned up, and a work bench cleaned off, I'll do some reading in that book and get started.

Thank you all for your input on this thread!

You won't be disappointed. Just take your time...don't rush. I knew next to nothing about these carbs, and rebuilt mine with his book.....carb works great. Good luck.

[jbwolfe]

Originally Posted by slikside

You won't be disappointed. Just take your time...don't rush. I knew next to nothing about these carbs, and rebuilt mine with his book.....carb works great. Good luck.

That makes me feel better. I really don't know much about carbs in general, except having taken apart small carbs off lawnmowers and stuff like that. I plan to take it really slow, and make sure I've got a nice clean place to work with lots of room so I don't lose any parts.

I work on computers as a side job, and started tinkering with them when I was about 8 years old. Especially from working on laptops, I've gotten pretty good at keeping track of really small parts and screws and remembering were everything goes, so hopefully that experience will help me with rebuilding this carb. We'll see...

What do you recommend for cleaning the parts? Carb cleaner? Parts washer?

[68gmsee]

Originally Posted by jbwolfe

.....
What do you recommend for cleaning the parts? Carb cleaner? Parts washer?

I use carb cleaner for cleaning and if you are able to locate someone that plays guitar, the thin E or B strings are good for cleaning out the small holes inside the carb. While they're not looking snip one off and then run!

Don't buy chem dip one gallon can for soaking unless you live way out in the country. It stinks! Wife asked me to please remove it from the garage and luckily I have some land out in the country.

Working on computers, especially laptops does help. You should have most of the tools that you will need.

[slikside]

Originally Posted by jbwolfe

That makes me feel better. I really don't know much about carbs in general, except having taken apart small carbs off lawnmowers and stuff like that. I plan to take it really slow, and make sure I've got a nice clean place to work with lots of room so I don't lose any parts.

I work on computers as a side job, and started tinkering with them when I was about 8 years old. Especially from working on laptops, I've gotten pretty good at keeping track of really small parts and screws and remembering were everything goes, so hopefully that experience will help me with rebuilding this carb. We'll see...

What do you recommend for cleaning the parts? Carb cleaner? Parts washer?

I'm sure it will help, there are a lot of parts, but as you personally pull it apart, it will all make much more sense. The other thing I did though, was I took a lot of up close pictures with a digital camara as it was coming apart. I had a nice gallary of photos to refer to when I put it all back together....in addition to what's in the book.

I will back up 68gmsee on the smell of the one gallon dip can....It does stink, but if you just leave stuff sit overnight, it all falls away pretty good. Some compressed air and passages are cleared asap. One big limitation of the gallon can though, the full body of the carb will not fit in it. I had to buy a heavy foil/aluminum high walled pan, and put the body in that.

Another thing I will say that you will find in the book is that not all rochester q-jets were created equal. The earlier ones were not all that great. Later is better. My truck ('72) had a julian date on the carb from '76. I had always thought mine was original (I'm 3rd owner), but it wasn't, and that's a good thing. So, when you get the book, check the carb you bought at your buddies garage sale, and compare. Every carb will be better when it's rebuilt, but the later ones will be better still.

Looking forward to hearing how it goes for ya.

edit: Oh yeah, the one thing I had Cliff do for me was the primary shaft bushing. I really did not want to screw that up and have to go get/find a new base plate. Everything else I did myself with his parts/kit.

Originally Posted by 68gmsee

I use carb cleaner for cleaning and if you are able to locate someone that plays guitar, the thin E or B strings are good for cleaning out the small holes inside the carb. While they're not looking snip one off and then run!

Don't buy chem dip one gallon can for soaking unless you live way out in the country. It stinks! Wife asked me to please remove it from the garage and luckily I have some land out in the country.

Working on computers, especially laptops does help. You should have most of the tools that you will need.

[jbwolfe]

Originally Posted by 68gmsee

I use carb cleaner for cleaning and if you are able to locate someone that plays guitar, the thin E or B strings are good for cleaning out the small holes inside the carb. While they're not looking snip one off and then run!

Don't buy chem dip one gallon can for soaking unless you live way out in the country. It stinks! Wife asked me to please remove it from the garage and luckily I have some land out in the country.

Working on computers, especially laptops does help. You should have most of the tools that you will need.

Thanks for the tips. I do have a friend who plays guitar, and he might even have some extra strings so I don't have to steal any from his guitar.

I do live out in the country, so a one gallon dip can might work just fine.

Originally Posted by slikside

I'm sure it will help, there are a lot of parts, but as you personally pull it apart, it will all make much more sense. The other thing I did though, was I took a lot of up close pictures with a digital camara as it was coming apart. I had a nice gallary of photos to refer to when I put it all back together....in addition to what's in the book.

I will back up 68gmsee on the smell of the one gallon dip can....It does stink, but if you just leave stuff sit overnight, it all falls away pretty good. Some compressed air and passages are cleared asap. One big limitation of the gallon can though, the full body of the carb will not fit in it. I had to buy a heavy foil/aluminum high walled pan, and put the body in that.

Another thing I will say that you will find in the book is that not all rochester q-jets were created equal. The earlier ones were not all that great. Later is better. My truck ('72) had a julian date on the carb from '76. I had always thought mine was original (I'm 3rd owner), but it wasn't, and that's a good thing. So, when you get the book, check the carb you bought at your buddies garage sale, and compare. Every carb will be better when it's rebuilt, but the later ones will be better still.

Looking forward to hearing how it goes for ya.

edit: Oh yeah, the one thing I had Cliff do for me was the primary shaft bushing. I really did not want to screw that up and have to go get/find a new base plate. Everything else I did myself with his parts/kit.

Good idea on the camera, I'll be sure and do that. I also have the current carb on the truck I could refer to.

I have a buddy that works at a garage, he might be able to take the carb body in to work with him and run it through the parts washer.

I will check the date on this carb when the book gets here. I think it is a later one, maybe mid 70's.

[72CampSpecial]

Recently rebuilt/tuned my '75 Cali emissions carb..

-Cliff Rugles Book.. a must
-Cliff Rugles forum for Questions.. a must
-No one has mentioned adjusting the APT (or i missed it).. mine was way off after the re-build (this one requires a very easy carb modification, but worth it)
-Other things to consider.. Timing, Adjusting the Idle mixture screws, adjusting your idle and the APT made all the difference for me..

My truck was sluggish and ran like crap until i started dialing all those in..after many, many adjustments its running great now.

[capev86]

I personally would swap the intake out. Looks like you have a single plane intake which is great for racing and higher rpms. But a dual plane intake is better at lower rpms and better for street use. I have a Qjet on an Eldelbrook dual plane intake and have no issues.

that is not the kind of intake i would want to see on a truck, unless it is a stripped down, drag race only vehicle with a big bad engine tuned specifically for high rpm operation.

i did a similar swap on my 72 Suburban C20. i didn't care for the faulty 2bbl on the original 307, so i pulled it in favor of a Q-jet swap. i found a good deal locally on an a still in the box Edelbrock performer manifold that is designed to fit up to both square bore and spread bore carbs. this manifold is a low profile, dual plane unit that augments power in the idle-5500rpm range. it perfectly compliments the low end throttle response and top end punch the Q-jet is famous for on the street.

at the same time i swapped the rear gears from 4.57 to 4.10. the truck is soooo much more responsive and fuel economy has gone from 10/11 to 14/15. 2 out of the last 3 tank fulls have averaged 16mpg...and that is without overdrive! oh...and during idle the vacuum is over 20....according to my in dash reproduction vac gauge.

[jbwolfe]

Originally Posted by 72CampSpecial

Recently rebuilt/tuned my '75 Cali emissions carb..

-Cliff Rugles Book.. a must
-Cliff Rugles forum for Questions.. a must
-No one has mentioned adjusting the APT (or i missed it).. mine was way off after the re-build (this one requires a very easy carb modification, but worth it)
-Other things to consider.. Timing, Adjusting the Idle mixture screws, adjusting your idle and the APT made all the difference for me..

My truck was sluggish and ran like crap until i started dialing all those in..after many, many adjustments its running great now.

I'll check out that forum. What is the APT? I'd image all that is covered in the book...

Originally Posted by capev86

that is not the kind of intake i would want to see on a truck, unless it is a stripped down, drag race only vehicle with a big bad engine tuned specifically for high rpm operation.

i did a similar swap on my 72 Suburban C20. i didn't care for the faulty 2bbl on the original 307, so i pulled it in favor of a Q-jet swap. i found a good deal locally on an a still in the box Edelbrock performer manifold that is designed to fit up to both square bore and spread bore carbs. this manifold is a low profile, dual plane unit that augments power in the idle-5500rpm range. it perfectly compliments the low end throttle response and top end punch the Q-jet is famous for on the street.

at the same time i swapped the rear gears from 4.57 to 4.10. the truck is soooo much more responsive and fuel economy has gone from 10/11 to 14/15. 2 out of the last 3 tank fulls have averaged 16mpg...and that is without overdrive! oh...and during idle the vacuum is over 20....according to my in dash reproduction vac gauge.

Not to worry, the intake I put on the 350 is a DUAL plane Edelbrock Performer RPM Air Gap, and it is also designed for the idle-5500RPM range.

Sounds like your swap turned out great! Hope I can get close to those results with mine. I'd just be thrilled out of my mind if it ran well.

[snipescastle2]

There is a very small sockethead screw on the underside
of the top section of the quadrajet on the upper secondary shaft, if you
Try adjusting the spring tension on these secondary flaps you might
find the problem there.loosen the screw and release some of the
Tension, the secondaries will open sooner, lessoning the "Bog"
these carbs are famous for. Hope this helps.
Ben Smith
Posted via Mobile Device

[geezer#99]

You got that backward snipes!
Looser tension on the secondary air valve causes the bog.
As per the apt here's a link to explain it. Yours is likely in the baseplate JB.
http://www.crankshaftcoalition.com/w...#Trouble_spots

[luvbowties]

Originally Posted by jbwolfe

I'll check out that forum. What is the APT? I'd image all that is covered in the book...



Not to worry, the intake I put on the 350 is a DUAL plane Edelbrock Performer RPM Air Gap, and it is also designed for the idle-5500RPM range.

Sounds like your swap turned out great! Hope I can get close to those results with mine. I'd just be thrilled out of my mind if it ran well.

JB, I don't think anybody mentioned this: try adjusting timing with vac advance attached. Also, ensure vac adv is hooked to a port that is pulling vacuum even with engine idling. (If worse, u can always make changes.), but in my experience, these 2 things have worked better for me when using hei and q'jet.
Sam

[luvbowties]

*change to above: should read "with vacuum advance Un-attached and plugged, of course"...

[soxplayer]

I somebody tells you that you have the wrong powervalve in a QJET, try to forget everything they said to you.

You really need to get Ruggles book if you are serious about keeping this carb. If you get it right, it is very good. The book will show how to go about debugging the carb step-by-step.

I always find that the choke is never quite right. I set mine slightly loose so that it stall after a first cold start. Otherwise the choke hangs on too long and makes the engine overly rich.