Poor Cold Running - Sputters, Backfires, Hesitates, Dies

[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

[kuhnzoo]

I appreciate the response. The rebuilder told me he was increasing the jet sizing because the replacement Goodwrench 305 motor had more HP than the original 305. And, no, there was no provision added to adjust the APT.

I believe part of my problem was also excessive vacuum advance timing. I have since added a limiter plate to the vacuum adv can to reduce total vacuum advance to 12 degrees (I read most HEI cans are about 22 degrees) - confirmed with timing light. That has helped with driveability when warm.

I have two other peculiarities/curiosities with this carb.

The first has to do with the carb vacuum ports. They do not match my underhood label, so I doubt this is the original carb (not really surprised given the age and carb #). There is no "H" ported vacuum port (front driver's side) on this carb. My "B" vacuum port (where the label shows my distributor should be connected) reads manifold vacuum. I thought the distributor was supposed to have a ported vacuum source from factory? And I have a "K" ported vacuum port in the front of the baseplate (just to the driver's side of the manifold vacuum port for the PCV) that I have not found any info about. The other existing ports match the underhood label.

The second is that the idle speed screw can be completely backed out and I cannot reduce the idle speed (in D) to 600 rpm even with the timing set at 4 BTDC per the label (I prefer to run initial timing at 8 BTDC). It did this with the standard size jets also. Idle mixture screws were set with a vacuum gage. Vacuum at idle is about 19 inHg. There are no more vacuum leaks as all hoses and defective/leaky vacuum devices have been replaced, removed or capped. Never had a carb with this problem. Only way I can reduce idle speed is too lean out mixture screws or retard timing, neither of which are proper solutions.

Am I trying to fit a square peg in a round hole (wrong carb for application)? Or perhaps there is more wrong with this carb than I realize? I am sorry to dump all this out there. I do not have time to become a QJet expert (and trust me, as a mechanical engineer, I am very curious about all this). I would have preferred to send this carb to your shop for remanufacture, Cliff, but your published turnaround for a remanufacture is 12 months and I did not have 12 months to wait.

Thanks again for any and all replies. I do appreciate everyone that has taken the time to read and/or reply to my post.

Rob

[kuhnzoo]

Just curious if anyone knows anything about what seems to be an odd arrangement of the vacuum ports on this carb? I can't find any info about this port labeled "K" in the baseplate.

Anyone have ideas about the high idle speed? My concern with this (besides the speed being too high, of course) is that sometimes the engine "diesels on" after shutting off the igntion. This is somewhat embarassing.

Thanks, Rob

[kuhnzoo]

OK, I took the top off the carb to check out the primary hanger arms. They were not even as judged by my eyes, but only slightly off. I adjusted the arms to where they looked even.

Also looked at primary rods and jets for sizes. Jets = 72. Rods = 51M. I have no idea if these are what I should be using for my engine?

With the airhorn off, I drove out the plug above the APT screw and tapped the hole for a threaded plug. Just need to get a threaded plug for the hole now.

Adjusted the APT screw 2.5 turns up from fully seated as a starting point.

Just need to install on the truck and start adjusting APT to hopefully eliminate the off-idle and off-cruise hesitation/poor throttle response. If I get this resolved, I still need to figure out why I can't get the idle speed down to the proper RPM.

Thanks again for any responses.

Rob

[429bbf]

i would check if your rebuilder got one of the throttle plates not square. if you still have the carb off .tip it over and see how much light you can see with the plates closed. you said they put bushings in so they had to have the plates off.also i would check to see if theres a little play in the rod going to secondaries . with the pri. closed .id also look at the fast idle for any rubbing or binding.it all seems so simple but once in a while we all miss the obvious.ps. make sure the secondaries all the way shut. (fwiw )

[kuhnzoo]

Thank you for the suggestions. I did give the throttle plates a "once over", but I will double check for good measure. And I will check out the rod going to the secondaries, look for any hang up on the fast idle cam side and make sure the secondaries are fully closed.

Anyone have any feedback on the primary rod/jet combo I mentioned? I can't remember the secondary hanger / rod combo, but I am not so concerned with that for now since it runs fine at WOT (which is almost never anyway for this stock engine).

Thanks again.

Rob

[kuhnzoo]

OK. More info. Secondary rods = DP. Secondary hanger = L.

Also, primary and secondary throttle plates appear to close properly. There is a bit of light at the plate edges, but I would imagine this is typical since the plates and bores aren't going to conform to one another perfectly. Yes, No??

Rod to secondary has some play when throttle plates are closed.

Nothing hangs up on the fast idle cam.

So I made my plug for the APT access hole and I'm going to install the carb on the truck. Guess I'll see what happens. Hopefully I. can adjust the off idle hesitation away. No idea what to do about the high idle speed.

Thanks for replies/suggestions.

Rob

[429bbf]

this might sound stupid but ive seen dummer stuff. is there any slack in the throttle cable when it is hooked up?you didnt say you were testing it with the cable unhooked .

[kuhnzoo]

Sorry I left that detail out. yes, with the throttle cable disconnected the idle is still too high

[Schurkey]

Sorry I left that detail out. yes, with the throttle cable disconnected the idle is still too high

Carb have idle air bypass?  If so, maybe the metering holes are too big.

[kuhnzoo]

Not sure about the idle air bypass. When I had the airhorn section off, I look at the tubes, holes and other orifices to see if any looked to be modified (or damaged). Nothing looked to be modified or damage.

I am sure I am overlooking something, just don't know what it might be...

Snow is falling here. I need to get this truck up and running pronto. It is my snowplow rig!

Hope everyone has a very Happy Thanksgiving!