Carb Problem?

[K10bolts]

It's a manual choke Q-jet on a 305, not computer controlled. It is really hard to start, and when it does, it will slowly idle for at most 1 minute, then die. Opening the throttle has no effect on the engine speed either. I've played with the idle screws but it hasn't helped. With it running it's pulling 20hg of vac., so I don't suspect a vac. leak. Do you think I need a new carb? Any suggestions?
Thanks, Matt

 

[pauly383]

Have you tried covering it with your hand while its running and see if its running any better ? Maybe you need a choke , some later carbs are easier to swap the choke stuff back onto them

 

[K10bolts]

The choke is hooked up. It stays running longer with it closed but If I try to open it, it will die. The carb is from a '78 and mine is an '83. Do you think that could be the problem? I just want to be sure the problem is with the carb. Do you think anything else could be causing this?
Thanks for the help.

 

[pauly383]

Try waiting longer before opening it , try tightening up the choke coil so it stays choked longer too . Carbs were never meant to just get in and go first thing in the morning . If you don't wait for it to warm up it will bog on you

 

[K10bolts]

I have no coil adjustment because it is a manual choke. Maybe I didn't explain my problem well enough. If I close my manual choke completely and crank it, it is very hard to start. If it does start, it will idle very slowly for less than a minute and then die. I'm not talking about the engine boggin' down, I just mean it won't stay Idling.

 

[AkMudr]

. If you don't wait for it to warm up it will bog on you

My Qjet is the same way, if I let it warm up til the temp gauge hits 110 and kick the choke off it will run sluggish all day, but if I let it warm up to about 130* or so it runs ike a champ.

 

[K10bolts]

I'm not kicking the choke off though, I can't keep my motor running for more than a minute with the choke full closed.

 

[pauly383]

I don't rememver a manula choke on a Q-jet , 1978 would probably be hot stove choke , where a tube from in the intake supplys heated air . You don't need to tottaly close the choke , just close it some . Maybe rig up a manual cable if you have some skills and vary it yourself

 

[K10bolts]

So you think the choke is the reason it only runs for a minute or so, then dies? It does this very consistently.

 

[pauly383]

If it absolutely runs fine after its warm , I would imagine so

Of course I would imagine you have already checked for loose hoses , vacuum leaks etc

 

[1-ton]

The fact that your truck runs only when the choke is on, but not when it is off leads me to suspect you have some kind of lean condition with your carburetor. This could be caused by several things such as a clogged fuel filter, clogged air filter, clogged carburetor passages, or a vacuum leak.

 

[K10bolts]

Yeah, that makes more sense than the choke being at fault. A minute ago, i started the truck and it idled for 30 seconds, then started to die. I kept it alive by pumping the accel. pump. When I stoped doing that it died. This leads me to beleive the carb isnt pulling fuel for some reason. The filter is fine and it is getting 7Lbs of pressure. I don't think it's a vac. leak because I am getting 20"of Hg. at idle. By the way, does 145 sound right for cylinder compression?

 

[1-ton]

Did you check the small fuel filter that is located inside the carburater fuel inlet?

 

[zeroz400]

The carb is off my truck, its actually a 77. It worked on my truck but i replaced it with a electric choke carb. I then lent this carb to a freind with an 84 while we rebuilt his carb, he said this carb ran on his truck, it wasnt stellar but he drove it around so i highly doubt the carb is the problem. I brought my tester over today and his starter was drawing about 350amps, so i gave him the extra starter i had laying round. It supposedly cranks like a champ now. I told him to take the timing light and put it on every spark plug wire and make sure there is current going through them. The wires are all new, i also gave him my extra distributor with no cap, i doubt it will help. Tomorrow i think were gonna do a compression check on Every cylinder. We got about 7psi of fuel pressure and 20hg of vac at "idle" so i think were checked out on there. I have a feeling this motor is singing its death song, we'll post up if we notice anything tomorrow. With volumetric efficiency, fuel and air and hopefully ignition i cant think of what else can be wrong. Another thing to note, he took off his smog pump, the manifolds where the air injection goes to pop becuase they're not completely sealed, i dont think this would be a problem but it could be. If we dont figure it out by thurdays im gonna have my uncle come over and see if he can help. Thanks guys


Later

 

[zeroz400]

Well, we threw in the other distributor and i brought my carb over and had no luck. It actually ran better with the other carb. We tested compression on 3more cylinders and they were all about 150ish. I would suspect the other cylinders are the same. With all that clear, i think the cam chain jumped a tooth. The way it runs now is like an even firing engine running at real low rpms is the only way i can describe it. Are these motors interfering motors? Any other things that come to mind?


Later

 

[diesel4me]

bent pushrods....??

If you jumped the timing chain,it wont be timed with the distributor ,it might still run bur poorly,moving the distributor will"make up" for the difference to some degree,but it will never run right that way--a vacuum gauge might help diagnose whether the chain jumped...usually the motor cranks over hard,or throws flames out the carb or exhaust when the timing chain has jumped---
Chevy small block motors dont usually bend valves when they jump time,they can,but I havent seen it very often-(this doesnt apply to drag race engines!,at high rpm they can bend the valves)-the pushrods usually get bent in them quite often when the chain breaks or jumps(sometimes they just bend from over-revving the motor,or a rocker arm stud problem too)...keep looking,you'll find the problem ..