falling flat under acceleration

Rebuilt and still falling flat... UPDATE

Have been running a mid-70's 800cfm QJ w/electric choke on my 350 (T/A manifold) for about 4 years, been great up to this point. Recently started falling flat under acceleration, sometimes only a mild stumble, others just a complete inability to feed the motor any gas and just being able to coast her along until the issue clears.

Running a clear filter in the fuel line and plenty of fuel being delivered in. Also running HEI ignition. No apparent vacuum issues.

In park, no stumble or idle quality problems and timing seems unaffected. Can't say if t-chain has ever been replaced, car has about 130k at this point.

UPDATE - had the carb completely rebuilt and it did have seemingly relevant issues needing attention.

Now back on the car, I've adjusted the mix screws to about 3.5 turns out (for highest vacuum reading of just over 20lbs at approx. 600 rpm idle).

Initial startup is ok, except within a minutes or so after, the motor stumbles badly and stalls. Hard to restart for several minutes thereafter.

After warmup, the same stumble / stall problem returns as I start driving - hard to restart for several minutes thereafter. The HEI is only a few months old with minimal mileage on it. Cap contacts and plugs look good.

Any new thoughts on carb adjustments or am I probably needing to look elsewhere at this point :idea2:

 

I would check out simple things first, like the vacuum break is ok, all linkage in hooked up and ok, no vacuum leaks, then if needed take carb apart and clean out all passages and check for proper positioning of all components. Reassemble with new gaskets.

It could be almost anything, you just need to check it all out looking for something broken or disconnected.

 

Also:
Make sure your distributor hasn't turned. Check the timming.
Make sure you have a free flowing, unimpeeded exhaust system.

Both of these happened to me, causing the engine to fall flat.

 

Yeah, I did check for timing and vacuum issues, all appears fine on those. It may need to come apart for a cleaning - still think it may need new accelerator pump though by the way motor's acting o No:

 

What kind of fuel pump do you have ? How old is it ? Might not be getting enough fuel. John B.

 

A bad accelerator pump could cause a stumble but not specifically under full power. If the pump is bad it will stumble everytime you nail the throttle, even in park. It sounds like a fuel delivery problem but there are a lot of areas to look for that. Try to get a fuel pressure gauge on it and see what you have.

Bill

 

Actually, the accel pump may NOT cause a stumble on accel in park. This is because the load on the engine is not present in park. A slight leanness will only show up under the constant load on the engine. The severity of the leanness will determine its presence. That said, there are several things to check before tearing into the carb.
First, did you just buy gas? Is there any possibility that the fuel quality is an issue? Does this issue appear when cold, hot, or all the time? Heavy or light throttle or both? Qjets are notorious for sticking power pistons, but the issue is more likely a peripheral issue, like a problematic heated air cleaner. Let us know when and if the problem disappears, like when fully warm, etc.

 

The accelerator pump cup could be breaking down due to the alcohols in the modern fuels. Best bet is to replace it with a pump cup that's alcohol resistant. (Like GHP-1010)

 

Check the rubber sections of the fuel line along the frame rail, they may look fine, but take your finger and squeeze the hose to feel if its squishy. I had a similar problem years ago, at idle it was fine, revving in park it was fine, under load it would start chugging and bogging, it was the inner section of rubber on the factory hoses getting sucked closed, specifically the rubber section of hose just under the drivers door along the frame rail. Mark

 

Something to consider here is the HEI. It is possible for the pick-up coil or the connections to be intermittent. This could cause problems with the ignition. You might try disconnecting the vacuum advance, and then drive the vehicle. If the stumble, hesitation is gone, or changed, I would look carefully at the pick-up. You could also check the coil with an ohm meter. Moving the connector while monitoring the readings will tell a lot. If the readings jump around, check the connections for the meter. If they are intact, it is likely the pick-up. Ray

 

Appreciate the great feedback -

I changed out the fuel pump at same time carb went on, both about 4 years ago. Car has seen 15-20k miles since then. I recently installed a new (repro) fuel tank and sending unit, changed the rubber lines running from the tubes to tank at that time as well.

I didn't have any issue that I recall up to the point that parked the car while awaiting funds to buy the tank - I ran the motor at least once a week for 10-15 minutes during that period (about 2 months).

No stumbling issue while running in park, only while driving with moderate to heavy acceleration. Seems to return to normal idle once I'm stopped again.

The HEI is a newer install - about 3-4 months old and the problem didn't occur for the first 2 months I drove the car with it in place. Would be interesting to check that out as a possible problem source.

Fuel running to carb is clean and plentiful (I'm running a clear filter in the fuel line between motor and carb). I don't think the pump is an issue.

Will keep you updated as I attempt to narrow things down -

Thanks for all the help in the meantime :3gears:

Scott

 

Something to consider is the q-jet's reputation for sticking power pistons. It is easy to check, without running the engine. Looking down the bowl vent, the one on the choke housing, the power piston is the device that appears to run front to rear. With the engine off, no vacuum should make the power piston rise to the top. Gently pushing down with a small screw driver, the power piston should drop, than rise as the pressure is released, and should move smoothly in both directions.
One other thing to consider is the temperature of the incoming air, as I mentioned in a previous post. These carbs are sensitive to air density, which rises quickly as the temperature drops. The additional air density can lean the mixture.
One additional thing to consider is the choke. It is very possible that the choke is coming off a little too soon, causing a lean out during warm up. As a side note, I have seen these temperature related issues for as long as ten minutes from the point of thermostat opening. You might try closing the choke adjustment some two notches. This adjustment controls the choke loading at start up as well as the rate at which is comes off.
The choke pull-off, or vacuum break, opens the choke when the engine starts, allowing it to breathe cold. If this adjustment is a little too wide, the result is a possible hesitation right after start up. You are letting the engine fully warm up before legging it, right?
As aheads up, these carbs are very sensitive to adjustments, and they should be made carefully, and a little at a time. Does the engine fast idle when cold? Does it return to curb idle soon after start up?
One more thing to check is the manifold heat riser. This is a valve in the exhaust that closes during cold operation, and forces additional exhaust through the cross over passage, heating the intake. If both the heated air cleaner, and the heat riser are both inoperative, it is possible for the engine to hesitate even after completely warm due to the cooling of the incoming air. At temps below about 45 degrees, it is also possible for the carb to ice due to the cooling resulting from the venturi pressure drop. This pressure drop will result in the moisture in the air to condense, and it will condense in the venturi. Once the engine stalls, the under hood temps will melt the ice, and you will never see the cause of the stall. A lot of info, but this advice comes from 45+ years of experience., If I can answer questions, just ask. Ray

 

Ray - appreciate the great info -

The manifolds are stock but I'm no longer running original intake or air cleaner parts up top - Have been running a TA manifold with open air cleaner assembly since installing the carb 4 years ago.

The stumble I'm having first became noticeable a few weeks back during a 30 mile road trip - car was fully warmed up and outside air temp was typical Florida mid 70's.

Was able to keep the car going by maintaining constant speed under 60mph but moderate acceleration from any point produced stumble. Been that way since.

Startup idle is still as it should be, as is the warmup and kickdown to normal idle speed... Only issue is acceleration stumble.

Checked all the vacuum caps, all are fine. Will check some of the other suggested things by the weekend.

thanks all :beer

 

70sgeek , remove the vacume hose to the adv. canister & plug it . Take it out for a road test, if the dead spot goes away or moves up or down the rpm range my guess is you have a wire broken inside the plastic sleeving on your HEI pick up coil ! Give it a shot! ick

 

I would try the test I suggested for the power piston. This sounds distinctly like the complaints I heard from GM customers when the power piston stuck in the carb. It is also possible that fuel quality may be an issue. Have you tried a different brand of NAME BRAND fuel? Food for thought... Ray

 

HEI looks good all around, can't tell that anything's wrong with it. I'm pretty convinced at this point the problems inside the carb. Had been using name fuel - 87 octane - all along and just recently installed new gas tank with sender.

I'm gonna have it rebuilt with the ethanol resistant upgrade parts at this point - puts a "dent" in my body budget but no point in having it all dressed up with no place to go :laugh:

 

It is very possible to service the Q-jet's typical ailments with only the air horn removed. This technique has saved many a customer's dollars, and created a reputation for me as honest and knowledgable. Since most of the heavy throttle fuel comes from the jets, getting the power piston un-stuck, if that is the case, does not require complete o/h. I might try a different fuel source first; I have seen my share of problems in that vein. Just a thought...

 

Before attacking the carb, or even giving it the hairy eye-ball, are you sure of the condition of the ignition, including the plugs? I have seen more problems with the ignition blamed on the carb. Even one fouled plug can feel like a stumble...