Hesitation - 400 sbc 850cfm 4-Quadrajet

[tntbower]

Bogging down. Works fine with kick-down but hesitates/chatters with light pressing of the accelerator. Any thoughts? The carb has been rebuilt and new true dual exhaust

 

[RustyPile]

Did the engine do this before the carburetor rebuild?? Not sure what you mean by "chatters"???

Sounds like the accelerator pump isn't working properly.. Remove the air cleaner housing so you can look directly down the throat of the carburetor.. With the engine not running, check the accelerator pump operation.. You should see 2 definitive streams emitting from each nozzle (there are two).. Slowly move the throttle, the streams should start the instant you move the throttle..

Also check the vacuum and mechanical advance in the distributor.. Is the EGR hooked up and working.. Too much EGR will cause hesitation.. Also base timing not set "high" enough will cause hesitation...

 

[vkh]

X2 on accelerator pump. You can turn the linkage and look down the primaries to check for the squirt. (Running or not, long as there is fuel in the float bowl). It's really easy to put the little seal that goes over the spring on the accelerator pump on backwards (I have), that's the first thing I'd check if your accelerator pump isn't working. The the open lip of the seal should point down.

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If not that then I'd verify you got the check valve back in. It's the little BB the rests below a plug in the bottom of the carb, has a standard head.

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Oh, and to my knowledge factory quadrajets only came at 750 or 800. Only 850 I've heard of is the Edelbrock aftermarket one. But I've been wrong before and will be again.

 

[1987 GMC Jimmy]

Rochester never actually made a larger than 800 CFM Q-Jet, but the Edelbrock 1910 Q-Jet is 850 CFM. I guess that's some kind of modified reman that they used to do. Is that what's on it? Also, I third the acc. pump suggestions. Everything that has already been said is what I would say.

 

[RustyPile]

A word of caution!!! DO NOT check the accelerator pump while the engine is running.. If the engine should back fire while you're looking down the carburetor throats -- the flame will remove your eye brows, eye lashes, and quite possibly damage your eyes..

 

[crazy4offroad]

Sounds like it's running lean at part throttle. I would also check for vac leaks.

 

[78C10BigTen]

A word of caution!!! DO NOT check the accelerator pump while the engine is running.. If the engine should back fire while you're looking down the carburetor throats -- the flame will remove your eye brows, eye lashes, and quite possibly damage your eyes..

That happened to my dad with my old s10. No more eye brows.

 

[tntbower]

Thanks. Carb is fine. Wondering if it's the EGR valve or the solenoid?

 

[1987 GMC Jimmy]

EGR valve would fall under the vacuum leak category. They either fail by getting stuck, which the open position will manifest symptoms, or they'll lose the ability to hold vacuum, which is harder to diagnose. Some you should be able to just pump up with a Mityvac and see if they hold vacuum and some need exhaust pressure to work properly. You mentioned chattering. Are you running lockup?

 

[tntbower]

Not sure what lock up is?

 

[tntbower]

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[1987 GMC Jimmy]

Not sure what lock up is?

Torque converter lockup. 700R4s and TH350Cs came with it. When locking converters start to fail, they kind of chatter, which you mentioned that, and it made me think of it.

 

[tntbower]

Great to know

 

[RustyPile]

Sticking, worn out, or broken centrifugal advance mechanism in the distributor can cause hesitation also..

If the EGR valve is stuck open, most times the engine will idle rough, and might periodically stall out.. If it's stuck closed (not likely), the engine will detonate during light acceleration or pulling a load - sounds like marbles rattling around in a coffee can.. Disconnect the vacuum hose at the EGR valve and connect a hand vacuum pump to the valve.. Start the engine and let it idle. Apply vacuum to the valve.. If the valve diaphragm is not ruptured, the valve will open and cause the engine idle to become rough. The engine might even stall completely..

 

[vkh]

I highly doubt it's actually 850cfm. The only change they claim is a bigger needle and seat, and that wouldn't change cfm at all. Honestly I couldn't even find their supposed reman company "Topspeed". If it were me I'd verify that it's an 800 cfm casting and not a 750. Hopefully it's legit for you but I'm sceptical.