Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Poor Cold Running - Sputters, Backfires, Hesitates, Dies

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kuhnzoo:
Sorry to write another "my Q-Jet won't run cold" thread on my first posting, but I am at wits end. I have read many of the threads with similar problems, but just can't seem to understand what to do next... I am trying not to give up on the Q-Jet, but so far I am batting ZERO with these carbs (this is not my first struggle with a Q-Jet - last one I caved in and switched to a squarebore carb/manifold). 2BBL Rochesters, on the hand, I have never had a problem with them.

Carb 17084220. '85 Chevy K10 (federal) with GM Goodwrench rebuilt 305 CID engine (~35K miles). Stock cast iron manifolds. Timed per engine label (4 degrees BTDC). Vacuum advance hose connected to lower vacuum port (no vacuum signal at idle) on passenger side of carb under fuel inlet (angled toward passenger side of truck). EGR valve removed/blocked off. All vacuum leaks resolved (all rubber hoses replaced, new vapor canister, components tested for leakdown with vacuum pump, etc.). New plugs, new HEI distributor (No ESC). Vacuum at idle about 19 inHg (no unusual gage movements).

Carb was rebuilt by a reputable rebuilder (had many happy customers on many different forums). Would like to rebuild myself, but just don't have enough hours in the day. Jets are two sizes over stock. Throttle plate shaft rebushed. Fuel inlet helicoiled, new choke pull off, nice new coloring. Electric choke works, choke plate snaps closed cold and pull off opens choke plate for cold start/idle. Fires up fine when cold. Runs fine with Choke engaged (stays on about 2-3 minutes). Once choke opens, sputters / backfires through carb and hesitates/dies when starting from a stop and after 1-2 upshift.

Once warm, it runs much better, but still does not respond smoothly after 1-2 upshift and when pressing pedal from cruise at 40-50mph (slight hesitation). 700R4 TV cable is adjusted properly - upshifts, downshifts, TC lockup engages/disengages properly. ATF completely flushed / refilled, new tranny filter.

Idles fine, WOT it goes great and shifts nicely through range. Just looking to get this stocker running good for the cold NH winter. Need it to run and work properly for getting me to work, plowing snow and towing a snowmobile trailer!

Thanks for any help. And I apologize again for posting about a topic that seems ever present.

LouC:
I'm wondering if your electric choke is opening too fast. Does the choke open all the way before the engine reaches normal operating temps? If you don't have a temp gauge get an IR heat gun and take a temp reading of the intake manifold right below the thermostat housing. On my marine 4.3 I have found that IR temp readings from this area are pretty close to the dash temp gauge on the boat. If its opening before the engine hits 195* then that could be your problem. Electric chokes are run on a timer, they do not respond to actual engine temp like a hot air choke or a divorced choke with the coil mounted on the intake exhaust crossover area.

kuhnzoo:
I'll give the choke a slight turn to increase choke engagement time. Thank you for your reply and suggestion.

I still have slight hesitation (and occasional pop back through carb) when fully warmed though. I've read that with EGR removed, vacuum advance should be limited to prevent knock and other poor performance at cruise.

Gonna try. See what happens. Never had these problems with earlier chevy V8's (had a 57 Bel-air w/283 and have a 68 Impala w/307).

kuhnzoo:
OK, I put a mark on the electric choke cover for a baseline and then turned the choke cover CCW two notches. Choke stays engaged about 4.5 minutes and seemed to help expedite warm up and improve driveability when cold. After choke is open and it is getting to steady state temp, it still hesitates some but not as much.

Two things that are probably not helping warm up faster: the valve that diverts exhaust through the intake crossover under the carb does not function properly (sticks closed - so disconnected - new one is about $250-300 - YIKES) and the TVS for the distributor vacuum advance delay is gone. Both would speed up warming time/help with cold driveability, but have no affect on performance once the engine comes up to temp. Anyone have a good, used exhaust diverter valve for sale??

I also put in the Crane vacuum advance limiter plate and set to reduce about 10 degrees of vacuum advance (HEI vacuum can provides about 20-22 degrees of advance). Set initial timing to about 10 degrees BTDC (up from under hood label 4 degrees BTDC). That has helped eliminate most of the hesitation when giving it some pedal at cruise (40-50mph) and from a stop. Still seems there may be a small hesitation, but maybe I am being oversensitve to things (guess I am conditioned now to expect it to happed at this point!)?

I think I am going to reduce the initial timing a couple of degrees since the idle is a bit high and I cannot reduce it anymore with the idle speed screw (can't figure that one out - can back the screw right out). The only way I can get the idle down to 600rpm in D is by leaning out the idle mixture screws (which seems like a bad idea). My concern with the higher than normal idle is that the engine wants to "diesel on" at shut down.

Any other ideas from anyone to help tune this in? Thanks to one and all for any suggestions.

Cliff Ruggles:

I still have slight hesitation (and occasional pop back through carb) when fully warmed though.

You have a lean condition.  I would raise the APT screw until the lean symptoms go away.  Hopefully the "builder" installed a removable set screw in the airhorn so you can access the APT screw.

Not sure why the jets were replaced on a stock set-up 2 sizes larger?

The carb should have been installed first, APT adjusted correctly, then tested in all areas.  If it acted lean at heavy/part throttle, a larger jet would have been required.

I'll bet you $5 if you remove the top of the carb that the power piston hanger arms are NOT exactly even.  99.99 percent of the carbs we get in here after being "rebuilt" have bent hanger arms, and APT out of adjustment......FWIW....Cliff

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