Runability issue. Minor. Question.

[D.PASSMORE]

Later model Q-Jets you could adjust the rod depth. Earlier models you had to re-jet, spring and the best meter rods for the set-up. But, 90% of the time a base stock 750 cfm Q-jet would run about any thing. If you added aftermarket parts, made it breath better (more air) - the carb would have to be sweeten" it up. Edelrbrock still sells Q-jet main jets, rods and springs. Jets are not that expensive. Summit racing...

Shoot me the numbers off the side of the carb. Lets see what year/application it belongs too.

[geezer#99]

Dave. Earlier models are adjustable too. In the base plate between the mixture screws there's a plug that covers a screw that threads in. The end of the screw spreads a hair pin type clip that raises or lowers the apt.
Find an old quad with a divorced choke and pull the base plate off. It's hidden between the base and the bowl.

[D.PASSMORE]

I'll have to look at that...

lyrikz,

Try these guys for CARB assistance. They're in your area. One of the best companies around for Q-Jets - IMO...

http://www.carburetion.com/

[lyrikz]

Originally Posted by geezer#99

Is that 16-18 degrees with the vacuum advance line plugged off!!
You don't need to buy new jets. You can adjust the metering rods so they sit higher in the jets. It's called an APT. Adjustable part throttle. It's accessible under that round plug on the carb body just in front of the vent hole. Unfortuneately you need to pull the carb top off to get the plug out.

Im not sure if it was plugged off or not. I think it was plugged.

Ok, so you pull the top of hte carb off, remove that plug, then how do you adjust that APT. How far would i adjust it out on the first time?

Originally Posted by D.PASSMORE

Later model Q-Jets you could adjust the rod depth. Earlier models you had to re-jet, spring and the best meter rods for the set-up. But, 90% of the time a base stock 750 cfm Q-jet would run about any thing. If you added aftermarket parts, made it breath better (more air) - the carb would have to be sweeten" it up. Edelrbrock still sells Q-jet main jets, rods and springs. Jets are not that expensive. Summit racing...

Shoot me the numbers off the side of the carb. Lets see what year/application it belongs too.

Its off a 1981 c2500 gmc.

Originally Posted by D.PASSMORE

I'll have to look at that...

lyrikz,

Try these guys for CARB assistance. They're in your area. One of the best companies around for Q-Jets - IMO...

http://www.carburetion.com/

Checking them out now.

[geezer#99]

JMHO but check your timing first. Pull the vacuum hose off the carb and cap off the port on the carb. Then check your timing.
I'll search for a link for adjusting your apt.
In this link on page 3 near the bottom he points out with a small screwdriver the apt. You can see the top of it in the hole. Odd shaped head. You can use good needle nose pliers to turn it. Turn it all the way in to see how many turns from bottom. I usually set them at 3 turns and adjust from there.
http://www.vetteprojects.com/kstyer/quadrajet_3.htm

[lyrikz]

Originally Posted by geezer#99

JMHO but check your timing first. Pull the vacuum hose off the carb and cap off the port on the carb. Then check your timing.
I'll search for a link for adjusting your apt.
In this link on page 3 near the bottom he points out with a small screwdriver the apt. You can see the top of it in the hole. Odd shaped head. You can use good needle nose pliers to turn it. Turn it all the way in to see how many turns from bottom. I usually set them at 3 turns and adjust from there.
http://www.vetteprojects.com/kstyer/quadrajet_3.htm

Being its stock, how much should i turn that apt out??

I will adjust the timing again, im pretty sure that it was capped off, but im not 100%.

[geezer#99]

I set the apt at 3 turns from bottom. See where yours is then adjust from there.
I've seen bbc ones set at 6-8 turns from bottom.
Have you taken the top off the carb yet.

[lyrikz]

Originally Posted by geezer#99

I set the apt at 3 turns from bottom. See where yours is then adjust from there.
I've seen bbc ones set at 6-8 turns from bottom.
Have you taken the top off the carb yet.

I have not. When traffic dies down, im going home and getting the truck and bringing it here to work. Here i can adjust the timing, then i can adjust that apt with the gas analyzer on the truck.

So say the APT is set at 3 turns right now. Should the first adjustment just be 1 turn??? 1/2 a turn?

[geezer#99]

You gotta find bottom to see where you're at.
You need to pull the top off the carb to get the plug out to access the apt.
You need to make a tool to adjust it from the outside. You can't reach inside with anything to adjust it. Hole is too small.
If you pull the plug you need to make a thread in plug to seal it up.
Check the timing.
Set up your gas analyzer to see if you're lean.
Above all I applaud your efforts to make it run right.

[lyrikz]

Originally Posted by geezer#99

I set the apt at 3 turns from bottom. See where yours is then adjust from there.
I've seen bbc ones set at 6-8 turns from bottom.
Have you taken the top off the carb yet.

So, im not saying that im giving up yet, but if i cant get it to idle today, what carb would you recommend that i buy geez???

[lyrikz]

Originally Posted by geezer#99

You gotta find bottom to see where you're at.
You need to pull the top off the carb to get the plug out to access the apt.
You need to make a tool to adjust it from the outside. You can't reach inside with anything to adjust it. Hole is too small.
If you pull the plug you need to make a thread in plug to seal it up.
Check the timing.
Set up your gas analyzer to see if you're lean.
Above all I applaud your efforts to make it run right.

Can do. Im going to mark it, so i can return it back to stock if this doesnt do anything.

Im going in this order.
timing
reverifying that there is no vacuum leak
gas analyzer to verify lean
then apt goodie thing you mentioned above.
then Putting it on ebay if it doesnt work. lol.

[geezer#99]

Sounds good.
We'll get to the apt later.

[lyrikz]

Originally Posted by geezer#99

Sounds good.
We'll get to the apt later.

Geez, i found this while reading on tuning the quadrajet. Kind of long read, but if you could. Let me know your thoughts.

"Chevy Truck Magazine
This is an insert from an article on Q-jet Tuning Tips that appeared in the January 2000 issue.
The holes in the baseplate where the idle mixture screws are located are called the idle discharge ports. These ports, together with the mixture screws, control how much fuel enters the engine at idle and won't need alteration with a stock engine. However, once the engine is modified with a bigger cam, these ports often need to be enlarged to avoid a lean miss at idle. Stock ports are usually 0.050-0.081 inch, and Murphy (Sean Murphy from Jet Performance Fuel Systems) says they can be enlarged up to 0.093 inch, according to engine needs (anything beyond .0093 inch, and you start losing idle-adjustment capability). Although application needs vary, Murphy recommends 0.081-0.086 inch for cams with 204 to 210 degrees of duration at 0.050, 0.086-0.089 for cams between 210 and 216 degrees of duration, and a maximum of 0.093 for cams above 216 degrees of duration. Big-block engines have different recommended specs.



I read the above insert in the article. My CompCams XE268H cam has 224 degrees of intake duration, so I said why not. I checked the holes and found out they were .088 inch. So I gathered up the nerve and drilled them out to 0.093 inch. Worked like a charm. Now I have adjustment with the screws. Before drilling them bigger, I could turn the screws all the way in and not affect the idle. I had to turn them out about 4 to 5 turns to achieve a decent idle mixture. I highly recommend you giving this a try if you have no adjustment with the idle mixture screws. Just for the heck of it, I checked the holes in the old stock Q-jet that I had removed and found out these were, get ready for this, 0.055 inch. That's right, no wonder we can't get a decent idle after we change cams.

""

[geezer#99]

That's a brave move.
Test drive and see how it works.
Curious! Did you do that with the carb on the motor?

[lyrikz]

Originally Posted by geezer#99

That's a brave move.
Test drive and see how it works.
Curious! Did you do that with the carb on the motor?

No, that was just something i read and posted here for you to read to see what you think..

This carb on my truck is 100% stock. Just cleaned up internally and stuff. Everything was VERY clean on the carb when i pulled it off the motor. it didnt need much.

I will be leaving in a bit to get the truck and bring it in...Cant wait to see what this analyzer says if its lean or rich. IM really leaning towards rich.


What were your thoughts on me buying a new one, what would you recommend?

[lyrikz]

Originally Posted by geezer#99

That's a brave move.
Test drive and see how it works.
Curious! Did you do that with the carb on the motor?

Didnt know if you wanted this info, or this changes anything. This is my cam.

[geezer#99]

Actually a fairly mild cam for a bbc.
You said you could screw the mixture screws in all the way with no change in idle. That means your idle is set to high and the idle discharge ports are fully uncovered.
If you hook your vacuum advance up to a full time source your rpm will go up. Then adjust your idle on the carb back down. That will get the screws back into adjustment.
If you're going to try another carb I'd go Holley 750 vacuum secondary. Easy to buy parts for, easy to adjust.

[lyrikz]

Originally Posted by geezer#99

Actually a fairly mild cam for a bbc.
You said you could screw the mixture screws in all the way with no change in idle. That means your idle is set to high and the idle discharge ports are fully uncovered.
If you hook your vacuum advance up to a full time source your rpm will go up. Then adjust your idle on the carb back down. That will get the screws back into adjustment.
If you're going to try another carb I'd go Holley 750 vacuum secondary. Easy to buy parts for, easy to adjust.

No, when i screw them in, the idle lowers and it will die.

So how should i run my vacuum advance, on full vacuum?? I literally have had 3 people over the time i had the truck look at and change where that damn vacuum advance should go on the carb. Seriously. The carb guy changed it to. Where do i hook the vacuum advance up to? Can you look at the pic and tell me where?

Ok, so disconnect the vacuum advance, cap it off, adjust the timing.
Then, hook the VA to full vacuum, readjust idle. Im going to reset the fuel screws on the front again, and then readjust those to get the smoothest idle.
Then start all over again. lol.

[geezer#99]

Got confused!! Read it in the quote.
Leave everything as you have it.
Check timing first. Leave vac hooked up where it is.
Adjust carb.
Check with analyzer.
Every motor likes to run differently. My bbf likes lots of advance at idle. Yours might not.

[lyrikz]

Originally Posted by geezer#99

Got confused!! Read it in the quote.
Leave everything as you have it.
Check timing first. Leave vac hooked up where it is.
Adjust carb.
Check with analyzer.
Every motor likes to run differently. My bbf likes lots of advance at idle. Yours might not.

You wont believe this ****, pardon my language, or you might believe it. So i drive it in to the old timer. I pull in.. its stumbling wanting to die. He revs the engine a bit, then revs it while he is covering up the top of the dist. then uncovers and vroom.. The idle smooths right out and the idle is high.. Obv cause i had been adjusting hte **** out of it.. Basically i had something stuck in the idle jet or something that cleared out. He lowers the idle, adjusts the two front settings. Its idling like a friggin DREAM. He then sees my vaccuum leak. one tiny hose with a crack.. Fixes that.

Now shes idling great, he notices that when he hooks the vacuum advance to vacuum that the rpms arent changin.. He shuts it down, pulls the cap off and verifies the VA is functioning.. It is. He then puts a timing light on and realizes the timing is WAAAAAAAAAAAAAAAAAAAAAAAAAAyyy off. There is a groove in the pully and then a painted on arrow.. Well, everyone set the timing based on that arrow.. Well, that put my initial timing WAY off. So, he fixed that.. That also explains why the engine had great initial power, but damn fell off in the higher rpms.. I dont know if there is any damage to the engine. WHo knows..

So, it idles now, and im going to take it for a test drive here in about 30 mins to let you know if this 454 now feels like a 454. More to come.

[geezer#99]

See! Sometimes you think it's the carb when all along it's the dizzy.
Don't burn off too much rubber!!!
And you know. Oldtimers rule!!!

[D.PASSMORE]

Glad it worked out...

[lyrikz]

Originally Posted by geezer#99

See! Sometimes you think it's the carb when all along it's the dizzy.
Don't burn off too much rubber!!!
And you know. Oldtimers rule!!!

Well, it was both. There was something stuck in the carb. right after he did the choke off and floored it, it idled great. He then set the fuel and then just took a peak at the timing and he was the only one to question the little white arrow. He thought that wasnt right. EVERYONE else figured thats where you timed it at. So we set it to TDC and verified that the actual groove was 0. So with the vacuum advance off, we set it to 16 and then hooked up the VA and readjusted the idle. ITs a TINY bit low still but i can fix that.

So nice. It has way more top end. Way more even power band... He had to pull the dist. out and move it so we could adjust it. This stud on the intake mani. got in the way.

I feel friggin awesome though. So nice to have that done. Now i can finish the other things.

[geezer#99]

Excellenty!!
Next you'll be posting about having too much power!! LOL!!!

[lyrikz]

Originally Posted by geezer#99

Excellenty!!
Next you'll be posting about having too much power!! LOL!!!

It does feel good. Im going to change my fuel filters again. Truck runs good though.

I would still like to put it on a dyno to do some tuning. But dont want to screw with anything.. I still cant get over how far off the timing was and that NO ONE caught it. Running 28-30 degrees timing at idle, can do waht to a motor?