This trucks kinda pissing me off lately.

[Irishman999]

Im getting weird performance out of my 305. I rebuilt my quadrajet a while ago, the carb seems to be working fine except for a couple problems. If i let the engine warm up it idles and runs great. If i try to take off with the engine cold it just wont go, it has no power and backfires out of the carb!

When its warmer outside and i've been driving around it idles very rough.

Im thinking a good start would be changing my air filters that at least 15 years old. Not sure why i haven't. #2 problem is my valve seals, they are pretty bad and im wondering if it could be fouling my plugs.

Any thoughts on this problem?

 

[Manbearpig]

Backfiring out of the carb would be a timing issue

 

[Irishman999]

Thats what i thought, Backfire out of the carb=timing, Backfiring out of exhaust=valves. My Dad told me if it starts perfect right after bumping the key the timing is right, so i haven't put a timing light on it yet. I should probibly get a timing light.

 

[Manbearpig]

I should probibly get a timing light.

Wouldn't hurt

 

[crazy4offroad]

I would also check for vacuum leaks. I had a quadrajet that kept me baffled with vacuum leaks till I finally saw the air horn was cracked, on the under side.

 

[Irishman999]

I was so proud after my carburetor worked, but now im wondering if i should give up on my quadrajet. There is an early 90's suburban in the pick-n-pull that is all complete in the engine compartment.

Would the heads, intake and throttle body swap over to my 305? Donor motor is a 5.7. I could probibly get everything i need for cheap next time they do the half off day.

 

[HotRodPC]

I'm for checking the timing just for GP, and vac leaks for certain. Not likley valve seals is your problem. A sign valve seals are bad is, when you first start it up, you get blue smoke, then shortly after start up, say 2-5 minutes, the smoke quits, then its your valve seals. Even if vavle seals were the problem, usually not enough oil can get down to the cylider fast enough to foul the plugs, even if the valve seals were completley gone. So that is not likey your issue. For god sakes, yes change the air filter, but that is not your problem either. Even on a rebuilt, or more like what I call, putting a kit in a carb, like you did, its common to have a vac leak at the primary throttle plate linkage. On a true rebuild they would normally ream out the bore where the linkage goes into the carb and install a bushing cuz its a common problem on Q jets. Get some carb cleaner or start fluid (Carb clean preffered), start it up, and spray the base of the carb, and the intake runners looking for a raise or stumble in idle telling the spray got vacuumed in through a vac leak.

 

[Irishman999]

Mr. Squarebody you are correct. The valve seals are bad, the truck does smoke when started. The carb was rebuilt before i ended up with the truck and the carb had the bushings installed, i left the brass bushings in it. I tried spraying an accelerate to check for vacuum leaks but i never get a raise in idle.

Im getting confused, maybe i need to tune the idle or metering rods?!

 

[crazy4offroad]

A worn-out cam can cause backfiring, as well as stretched timing chain. My brother-in-law could listen to an engine idle and tell if a lobe has gone flat. You or I may have to pull the valve covers and check the lift with a dial indicator.

 

[Irishman999]

Im glad you mentioned that, when i first got the truck i had horrible problems with the valve train. the intake pushrods on #2,4 and 8 cylinders where bent after the initial startup after sitting for 15 years. After bending more push rods i realized the valves were stuck.
Think it possibly trashed the cam?

 

[Manbearpig]

Eeeeek could be

 

[crazy4offroad]

LOL you should change the title of this thread to "I've been pissin this truck off for a while". A vacuum test can not only help diagnose a vacuum leak, but other things that might be wrong in the engine...

You must be registered for see images

I would run a vacuum test, if it seems to be valve train related (which I'm 99.9999% sure it is now) pull the valve covers and see which valve(s) are having trouble. Sounds like you need alot of work done on that motor.

 

[Manbearpig]

I wish I had 20" of vacuum at idle

 

[Irishman999]

That was very informative, I guess i will get a vacuum gage right after im done putting a transmission in it haha.

I've been keeping an eye on craigslist for a good set of heads, maybe i will keep an eye out for a cam and lifter set as well.

 

[85k10383]

Park the old bird in the pick-n-pull and go buy that cherry "Spirit of 76" truck you posted about. It sounds to me like it is a miracle the engine runs at all! LOL JFWY