Inconsistent Qjet AFRs when returning to idle

[jamyers]

A couple of years ago I bought a Q-jet from Mark at Qjet Power for our '69 Chevy C10 w/ stock 350 engine. It's been a great carb, but lately has us stumped. We've had a wideband O2 gauge installed for the last couple of years, which is very helpful in setting idle mixtures and seeing what's going on as various issues have cropped up (like a crazy vacuum leak, etc). (it's almost too much info some times)

When it's running like it should, the jetting built into it is perfect - 14.7-15.0 at idle, 16 at cruise, and 12.5-13 under load, transitioning into the power circuit right when you'd want it to.

Symptoms: The A/F mixture ratio when returning to idle is erratic - that is, while driving and you come to a stop sign, sometimes it will be right at 14.7-15, and the next time it may well go as lean as 19-20+, with rough running and engine dying. Pump the pedal and restart it, and the idle may be normal, lean, or slightly rich. Next stop sign, it may be normal or lean again. It rarely goes rich, and then it might be 12-13, most often it leans out.
Above idle / transition (cruising down a road or highway), it's fine - generally right around 16, with great transition into power and secondaries.

History: 1. A year ago we had problems with the old fuel tank sending dirt to the carb, so now the truck has a new, clean aluminum fuel tank, an electric pump regulated to 5psi, two fuel filters, and we've taken the carb apart a couple of times, spraying out all passages and double-checking float height, etc. Since the new fuel tank I haven't seen any dirt, dust, or anything inside the carb.
2. We had a freaky issue with the intake gasket allowing a large vacuum leak, and replaced the gasket with a thick Cometic gasket - forgetting to open up the holes for the exhaust crossover passages, which meant we had to reset the choke.
3.Ignition is a GM HEI unit with relatively new plugs and wires, maybe 15K miles on it.
4.When it's idling correctly, it pulls 19 inches steady of vacuum and is rock steady. I haven't gotten a vacuum gauge on it when it's acting up yet - it behaves fine when at home (of course).
5. Engine temps are always a solid 195, Voltage is steady 14.5V when running.
6. The only vacuum ports are: Brakes, PCV, Distributor.
7. Transmission is a 700R4.

I'd think something is intermittently plugging something in the idle circuits, but for the life of me I can't find anything in there.

Maybe Carb icing because of no exhaust crossover - in West Texas?

I'm grasping at straws now - Any ideas?

[toolboxchev]

Odd thats for sure. I am told the Q-jets with the best metering where the ones from the mid 80's, right before fuel injection came about. Check your carb number.

I will tell you this for sure, Gasoline quality today sucks, period. I have had an Edelbrock on my rig for over 10 years, no issues, yet I do not run a wide band 02 on it.

Thinking I had a carb issue a while back I took off the top of the unit only to find the bowls were full of microscopic debris making its way past the fuel filters. I run 2, one before the mechanical pump and one before the carb.

Been running this setup for quite some time. I would venture to say you may have got some dirty fuel. I quick rebuild may verify this

[rpmerf]

Some thoughts.

Has it always done this, or has this problem come about over time?

Does your linkage move smoothly? Secondaries return all the way?

Running lean is not enough fuel, or a vacuum leak. I'm wondering if this could be brake booster related. Doesn't really explain why it is intermittent though. Are all your vacuum caps good? Unfortunately, the best time to test is when the idle is rough.

Is there anything you can note about the time before it idles rough - city / highway, going easy / driving it hard, warm / cold engine, ambient temperature? Might try to keep a mental log and jot down any time you notice the rough idle.

[geezer#99]

Always blame the carb!
It’s not the only thing that effects your idling/running conditions.
Have you checked your hei?

[tdangle]

I'm wondering if you may have a leaking brake power booster. Especially if it only happens when stopping.

[jeffahart]

Put the points distributor back in, trade your O2 and blood pressure gauges for a dwell meter. No need to grasp at straws, get back to the basics of a tuneup. Stop picking on your carb with all those diagnostics!


j

[cadillac_al]

If it's going lean it has to be something not flowing fuel like it should so, something is dirty in there.

[Wrenchbender Ret]

Check the throttle shaft & bore. A loose throttle shaft can cause this. If the return spring is pulling against the throttle on the opposite side of the arm like most Chevys do, try changing the spring to pull with throttle. This is what causes excessive wear on the shaft bore & the throttle does not return to idle consistently.
This may sound confusing but its the best way I can explain it.
George

[jamyers]

Originally Posted by rpmerf

Has it always done this, or has this problem come about over time?

Does your linkage move smoothly? Secondaries return all the way?

Are all your vacuum caps good?

Might try to keep a mental log and jot down any time you notice the rough idle.

Problem apparently started after we replaced the intake gasket - may have been going on before, but the old gasket was leaking something awful all the time anyways.

Originally Posted by geezer#99

Have you checked your hei?

Yes. Timing is consistent and rock solid. Spark is nice and strong. Advance can holds vacuum every time I check it.

Originally Posted by tdangle

I'm wondering if you may have a leaking brake power booster.

I've thought about that as well, but can't replicate the problem in the driveway by pushing on the brakes repeatedly, so I'm thinking it's not leaking.

Originally Posted by jeffahart

Put the points distributor back in, trade your O2 and blood pressure gauges for a dwell meter. No need to grasp at straws, get back to the basics of a tuneup. Stop picking on your carb with all those diagnostics! j

Umm, NOPE. Not going back to a worn-out points dizzy. GM HEI is probably the most reliable and durable ignition ever made.

Originally Posted by cadillac_al

If it's going lean it has to be something not flowing fuel like it should so, something is dirty in there.

That's what I'd think, but the last 2 times I've opened up the carb it's been super clean inside. 3 fuel filters.

Originally Posted by Wrenchbender Ret

Check the throttle shaft & bore. A loose throttle shaft can cause this. If the return spring is pulling against the throttle on the opposite side of the arm...

I understand, and why on earth Chevy put their return springs pulling back so they wear throttle shaft bores is beyond me...But nope, mine is in front of the carb, pulling against the throttle linkage.

The carb was built by a reputable builder, I believe he re-bushes all his carbs (I;ll check my notes on this one). I can't get any fore-aft or up-down play out of the throttle shafts that I can notice, but I'll take a closer look at that.

[geezer#99]

Timing good!
What’s your initial?
And checked at what rpm?
And checked lately?
Have you checked weights, springs, pins and shaft free play.

Side to side movement on the primary throttle should be checked as well as plates being tight.

[Dead Parrot]

Might be worth pulling the intake and opening the crossover holes. Carb icing is a possibility, especially if the THERMAC system was removed or not working correctly. Higher humidity will make icing more likely.

Don't overlook the possibility of loose connections/bad grounds in the O2 sensor circuit or gauge itself.