Quadrajet Hesitation When Accelerating

[jbwolfe]

Hi all, I'm about ready to go out of my mind trying to pin down the solution to my problem. Here's the deal:

My 71 CST/20 has a 350 with a Quadrajet. When I bought the truck, it had a Rochester 2G. It ran fine with the 2G, no hesitations or anything at all.

I put a new intake manifold on, and bought a rebuilt Quadrajet. It was nearly un-drivable right away. When you would step on the gas, it would just completely choke out, now if you were careful, you could get it to go, and once you did, it had plenty of power at higher rpms.

I had a mechanic work on it, who replaced the accelerator pump and did some adjustments, but it still was terrible, better but still terrible. Me and a friend messed with it, and then I took it to a different mechanic. They worked on it, and adjusted the timing. This made it a little bit better. They said the Power Valve in the carburetor was the wrong one for that carb.

I've been driving it for awhile, but it is getting really irritating. So I was talking to the guys at the shop, and I ended up buying a used Q-jet from them for $50. They said if it don't work bring it back. So I put it on the truck tonight. Here's the results:

It fired right up, even without the choke hooked up, (another problem...). Once it warmed up to normal temp, I took it down the road. It's a lot better than the old carb, when you're cruising down the road there is no hesitation at higher rpms, and no back firing. Only when you are starting off or shifting. If you're really light on the gas it's not too bad, but if you put your foot into it even a little bit, it gasps and chokes, and after a few seconds catches and then you've got tons of power, more than with the old carb.

Here's whats been done to the truck engine wise in the last 5k miles:

-Intake 4bbl
-Q-jet
-Plugs
-Cap and rotor (HEI Distributor)
-New springs in the distributor
-Wires
-Vacuum advance
-In line filter (in addition to the one in the carb)

I would really appreciate some help. I just want the truck to run right! The entire time I have had it, it's never run right and I'm sick and tired of it.

Please help me!

[jbwolfe]

Here are some pics I forgot

[Pont406]

In line filter (in addition to the one in the carb)

Try it with one fuel filter.

[jbwolfe]

I wondered about that... A friend was helping me when I put the carb on the first time, and added that filter, I didn't realize until tonight that the carb had one already. I'll give it a try without the second one.

[Pont406]

What's a fuel pressure? Looks like it's flooding a little.

[jbwolfe]

Originally Posted by Pont406

What's a fuel pressure? Looks like it's flooding a little.

I really don't know. How can I check?

[Lee H]

Bad accelerator pump.
Posted via Mobile Device

[Wrenchbender Ret]

With the engine off look down the carb throat & work the throttle. You should get 2 squirts of gas into the throat each time you work it. That tells you if the accel pump is working good or not. That looks like a high performance intake manifold. It will require a richer carb mixture to run good. It is not designed for cool weather street use. It probably will never work good with a Q-jet. That type manifold usually is used with a Holley.

[68gmsee]

Just wondering... I'm not familiar with that qjet and intake. How does the choke open up after it warms up? I would check that it opens up completely after it warms up to operating temperature.

The fuel pump pressure you can measure with a fuel pressure tester you can get at any auto parts store but since you say it runs okay except when taking off I wouldn't think that's the problem.

One thing I would change is the vacuum advance location. Try it on a vacuum source below the butterflies. The one below where you have it should be one. These old trucks seem to run better (my experience) with manifold vacuum rather than ported vacuum.

[jbwolfe]

Originally Posted by 68gmsee

Just wondering... I'm not familiar with that qjet and intake. How does the choke open up after it warms up? I would check that it opens up completely after it warms up to operating temperature.

The fuel pump pressure you can measure with a fuel pressure tester you can get at any auto parts store but since you say it runs okay except when taking off I wouldn't think that's the problem.

One thing I would change is the vacuum advance location. Try it on a vacuum source below the butterflies. The one below where you have it should be one. These old trucks seem to run better (my experience) with manifold vacuum rather than ported vacuum.

As for the choke, it currently isn't hooked up. That's another thing I need to deal with. Because it's not the stock intake, the divorced chock that this carb is set up for won't work. The other carb has an electric choke, but I tried to change it over to the new carb and it won't fit right on this carb. So currently it has no choke.

I was wondering what that port below the vac advance one was. I'll add that to the list.

Thanks!

[jbwolfe]

Originally Posted by Lee H

Bad accelerator pump.
Posted via Mobile Device

Originally Posted by Wrenchbender Ret

With the engine off look down the carb throat & work the throttle. You should get 2 squirts of gas into the throat each time you work it. That tells you if the accel pump is working good or not. That looks like a high performance intake manifold. It will require a richer carb mixture to run good. It is not designed for cool weather street use. It probably will never work good with a Q-jet. That type manifold usually is used with a Holley.

Alright, I'm making a list of things for tomorrow:

-Remove extra fuel filter
-Replace filter in carb
-Check accelerator pump

Would there be any reason to think it could be a problem with the timing? Or with the distributor springs?

Thanks for all the help so far!

[PatrickGMC70]

Qjets look intimidating, but are not as scary as they look. It seems like you have a "Frankenstein-carb" full of all kinds of different parts. For example, you could have the wrong size jets or rods (flooding out), vacuum leaks, and a host of other issues.

Before you just start replacing things, see what you already have. there are some great (and inexpensive) Qjet books out there. I like Cliff Ruggles' book. Also, look on YouTube for videos about rebuilding Qjets and get a sense for how they go together and what each part does.

Once you verify that your components are good, (and the right parts for that carb) you can begin to accurately solve your issue.

By the way, you can get retrofit electric choke conversions for your divorced-choke carb. But I would first get to the bottom of your driveability issue. Good luck.
Posted via Mobile Device

[Pont406]

Just noticed that accelerator pump linkage is in a wrong hole.

[jbwolfe]

Originally Posted by PatrickGMC70

Qjets look intimidating, but are not as scary as they look. It seems like you have a "Frankenstein-carb" full of all kinds of different parts. For example, you could have the wrong size jets or rods (flooding out), vacuum leaks, and a host of other issues.

Before you just start replacing things, see what you already have. there are some great (and inexpensive) Qjet books out there. I like Cliff Ruggles' book. Also, look on YouTube for videos about rebuilding Qjets and get a sense for how they go together and what each part does.

Once you verify that your components are good, (and the right parts for that carb) you can begin to accurately solve your issue.

By the way, you can get retrofit electric choke conversions for your divorced-choke carb. But I would first get to the bottom of your driveability issue. Good luck.
Posted via Mobile Device

The first Q-jet I had on there is a Frankenstein carb, but the one in the picture is the one I just put on there. Is there something in the picture that makes you say that?

I'll try to find one of those books. I agree, I'm going to get it running right first before messing with the choke.

Originally Posted by Pont406

Just noticed that accelerator pump linkage is in a wrong hole.

How so?

[Littletony]

What intake are you useing? Is the intake set up to run a spread pattern of the Q-jet? If not that may be your problem.

[trucks72]

If you are running it with the choke closed all the time it will flood out and not run good. The pictures you showed it is closed in them. It has no way to open when it warms up. You need to tie it open and see if that helps. You may have already tried this.

[flyingtim01]

One of the things that jumped out at me was that this was a 'rebuilt' carb that came off of a shelf, and I agree with some of the other posters, in all likelihood this is a frankencarb, who's pieces most likely didn't leave the factory together.

If it starts okay, but hesitates upon acceleration, I would check the basics first: make sure your timing is correct, make sure you have adequate fuel flow (pressure AND volume) from the pump and make sure that the choke is operating correctly. You'll have to figure that one out first, even if you operate it with a cable until you figure out how you're going to work it long term. Any carb adjustments you make should be done at operating temperature, with the choke fully open. That choke won't open on it's own, and I'm not a proponent of jamming a screwdriver into the air horn to prop it open. Probably don't need two filters, but I don't think that is your issue. You can remove the filter in the carb body though, without having to make up a new fuel line without an inline filter, if it's something you want to try.

Once you square all that away, you'll have to take a look at the tension on the secondary metering rods. The QJet is a pretty interesting carb, and one of the books recommended would be great reading for you, in learning how they work, especially under acceleration. When you step on it, engine vacuum drops and without going into too much detail, the secondaries start to open, meaning the metering rods start to pull out of the jets while the secondary air valve (the big butterflies on the top of the air horn) open, admitting air in. The secondary opening timing is adjusted by tension on a spring. The spring tension is held by a set screw, which is located under the air horn, by where the secondary linkage to the choke pull off is. It's sort of a pain to get to, and you'll probably need a little allen wrench to get to the set screw, I don't remember what size. Anyway, the tension is set with a little flat blade screw driver (you'll need to move the secondary linkage out of the way in that area) and you hold tension on the end of the adjustment, loosen the set screw and then tighten the adjustment a little bit at a time. When I say a little bit at a time, I mean like an 1/8th of a turn. Those springs break VERY easily. It could be too tight, it could be too loose. But I'm betting that is the source of your hesitation. I'm also betting that I'm not explaining this very well, and that getting Cliff Ruggles book would help immensely in this department, explaining the theory of how engine vacuum pulls the secondaries open and so forth, but also showing you some good pictures and procedures one how to adjust the spring tension. Unfortunately, there is no good way to adjust it once and be done, you have to adjust it, try it, adjust it, try it, until the hesitation is gone. I'm sure this procedure has been documented on here in a hundred different places, so you may want to search for a better explanation. But I would add this to your to-do list, once you rule out the simple stuff.

In regards to your intake, it's not that it won't work with a Qjet, but you have a choke to think about. That intake has no provision for a choke stove, and therefore won't work with your particular carb. You have a couple options: find a different intake that has a choke stove (plenty of them do) or find a different carb that originally came with an electric choke.

Personally, I would stay away from off the shelf reman carbs, and your issues are a good example of why. If I were you, I'd cruise the swap meets and find one. Qjets are all over the place, guys don't understand them, slap a Holley out of a box on their motor, and call it done, so you end up with a million perfectly good carbs at $5 a piece. Then take your Qjet bible and your $40 carb kit (make sure to use a brass float) and make yourself a Qjet expert, all while saving $300 (plus shipping) in the process and STILL smoking those Holley boys when you're done...

[68gmsee]

Looks like you have a long list of things to check. As you mentioned above, list them down and take one at a time doing the easiest first before going to the next. I wouldn't even mess with removing the inline filter for now. Just replace the one inside the carb if it hasn't been replaced.

The swapping of vacuum source is probably the easiest thing to try. Just use one that's below the throttle shaft. On my 4MC there's one on the back of the carb that I'm using. Also be sure and set the idle air adjustment screws for hightest vacuum using a vacuum gauge. Do it on each screw and restet the idle rpm as you make the changes.

The acc pump link usually goes on the inside hole. At least everyone I've had is like that. I'm not sure that's going to make a whole lot of difference but it gives you a little more fuel when stepping on the throttle. Easy thing to try. Just tap the small holding pin in with a punch.

[CC69Rat]

I had a similar situation and it was a small tear in the gasket between the carb and the manifold. So in other words, a gaping vac leak under the carb. I replaced EVERYTHING in the engine bay, including a brand new 350/290hp crate motor to get rid of this hesitation. Used the intake and carb off the old engine (stock 307) and of course the problem moved with it! Replace the gasket under your carb.. I know it sounds stupid, but its a very cheap thing to try. Look at it really close and see if you can tell if there's evidence of a leak / blow by on any of the ports.

Keep in mind, it ran fine with the Q2. .. I'm thinking a vacuum issue. Leak / ported vac, etc.

[jbwolfe]

Originally Posted by Littletony

What intake are you useing? Is the intake set up to run a spread pattern of the Q-jet? If not that may be your problem.

It's an Edelbrock Performer Air Gap. How would I find out if it is set up for this carb?

Originally Posted by trucks72

If you are running it with the choke closed all the time it will flood out and not run good. The pictures you showed it is closed in them. It has no way to open when it warms up. You need to tie it open and see if that helps. You may have already tried this.

That makes sense. I will try that right away.

Originally Posted by 68gmsee

Looks like you have a long list of things to check. As you mentioned above, list them down and take one at a time doing the easiest first before going to the next. I wouldn't even mess with removing the inline filter for now. Just replace the one inside the carb if it hasn't been replaced.

The swapping of vacuum source is probably the easiest thing to try. Just use one that's below the throttle shaft. On my 4MC there's one on the back of the carb that I'm using. Also be sure and set the idle air adjustment screws for hightest vacuum using a vacuum gauge. Do it on each screw and restet the idle rpm as you make the changes.

The acc pump link usually goes on the inside hole. At least everyone I've had is like that. I'm not sure that's going to make a whole lot of difference but it gives you a little more fuel when stepping on the throttle. Easy thing to try. Just tap the small holding pin in with a punch.

Good ideas. I'm adding them to the list.

Originally Posted by flyingtim01

One of the things that jumped out at me was that this was a 'rebuilt' carb that came off of a shelf, and I agree with some of the other posters, in all likelihood this is a frankencarb, who's pieces most likely didn't leave the factory together.

If it starts okay, but hesitates upon acceleration, I would check the basics first: make sure your timing is correct, make sure you have adequate fuel flow (pressure AND volume) from the pump and make sure that the choke is operating correctly. You'll have to figure that one out first, even if you operate it with a cable until you figure out how you're going to work it long term. Any carb adjustments you make should be done at operating temperature, with the choke fully open. That choke won't open on it's own, and I'm not a proponent of jamming a screwdriver into the air horn to prop it open. Probably don't need two filters, but I don't think that is your issue. You can remove the filter in the carb body though, without having to make up a new fuel line without an inline filter, if it's something you want to try.

Once you square all that away, you'll have to take a look at the tension on the secondary metering rods. The QJet is a pretty interesting carb, and one of the books recommended would be great reading for you, in learning how they work, especially under acceleration. When you step on it, engine vacuum drops and without going into too much detail, the secondaries start to open, meaning the metering rods start to pull out of the jets while the secondary air valve (the big butterflies on the top of the air horn) open, admitting air in. The secondary opening timing is adjusted by tension on a spring. The spring tension is held by a set screw, which is located under the air horn, by where the secondary linkage to the choke pull off is. It's sort of a pain to get to, and you'll probably need a little allen wrench to get to the set screw, I don't remember what size. Anyway, the tension is set with a little flat blade screw driver (you'll need to move the secondary linkage out of the way in that area) and you hold tension on the end of the adjustment, loosen the set screw and then tighten the adjustment a little bit at a time. When I say a little bit at a time, I mean like an 1/8th of a turn. Those springs break VERY easily. It could be too tight, it could be too loose. But I'm betting that is the source of your hesitation. I'm also betting that I'm not explaining this very well, and that getting Cliff Ruggles book would help immensely in this department, explaining the theory of how engine vacuum pulls the secondaries open and so forth, but also showing you some good pictures and procedures one how to adjust the spring tension. Unfortunately, there is no good way to adjust it once and be done, you have to adjust it, try it, adjust it, try it, until the hesitation is gone. I'm sure this procedure has been documented on here in a hundred different places, so you may want to search for a better explanation. But I would add this to your to-do list, once you rule out the simple stuff.

In regards to your intake, it's not that it won't work with a Qjet, but you have a choke to think about. That intake has no provision for a choke stove, and therefore won't work with your particular carb. You have a couple options: find a different intake that has a choke stove (plenty of them do) or find a different carb that originally came with an electric choke.

Personally, I would stay away from off the shelf reman carbs, and your issues are a good example of why. If I were you, I'd cruise the swap meets and find one. Qjets are all over the place, guys don't understand them, slap a Holley out of a box on their motor, and call it done, so you end up with a million perfectly good carbs at $5 a piece. Then take your Qjet bible and your $40 carb kit (make sure to use a brass float) and make yourself a Qjet expert, all while saving $300 (plus shipping) in the process and STILL smoking those Holley boys when you're done...

I guess I wasn't very clear in my original post. The carb CURRENTLY on there is just a plain old used carb, never rebuilt to my knowledge. It was the PREVIOUS carb that was a rebuilt nightmare, that had even more problems.

I'm going to prop the choke open and try that, and I'm definitely going to need one of those books. I found a few on eBay, I should get one.

As for the timing, I had a shop do the timing for me recently.

Originally Posted by CC69Rat

I had a similar situation and it was a small tear in the gasket between the carb and the manifold. So in other words, a gaping vac leak under the carb. I replaced EVERYTHING in the engine bay, including a brand new 350/290hp crate motor to get rid of this hesitation. Used the intake and carb off the old engine (stock 307) and of course the problem moved with it! Replace the gasket under your carb.. I know it sounds stupid, but its a very cheap thing to try. Look at it really close and see if you can tell if there's evidence of a leak / blow by on any of the ports.

Keep in mind, it ran fine with the Q2. .. I'm thinking a vacuum issue. Leak / ported vac, etc.

I will check for a vacuum leak around the carb gasket. It is always amazing how such big problems can sometimes have such simple solutions.

[jbwolfe]

Here's what I tried today:

-Removed fuel filter in carb.
-Checked accel pump. It sprayed out two nice streams of fuel.
-Moved accel pump linkage.

I also tried to move the vac advance line, but it didn't fit on the other port.

Also, as for the choke, when I started the engine, the choke opened up. Not really sure how that works, something to do with vacuum?

All of this resulted in no change.

How do I check vacuum level?
I've heard I can spray some starter fluid around the carb while the engine is running to test for a vacuum leak. Is that true?

[68gmsee]

Originally Posted by jbwolfe

Here's what I tried today: ......
I also tried to move the vac advance line, but it didn't fit on the other port.

Also, as for the choke, when I started the engine, the choke opened up. Not really sure how that works, something to do with vacuum?

All of this resulted in no change.

How do I check vacuum level?
I've heard I can spray some starter fluid around the carb while the engine is running to test for a vacuum leak. Is that true?

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.

[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.

[jbwolfe]

I got this truck almost three years ago. A friend helped me put on the new intake and carb. I'm really wishing I had just found a stock 4bbl intake. That would have been so much simpler in so many ways.

I'm learning lots though, just wish I had known a bit more before I bought this intake and carb setup.

[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.