Alright I changed the accelerator pump, the old one actually looks like it was fine, oh well it has a new one now. Problem it still has a hesitation when you punch the gas. It's only like a half second hesitation, but it's not cool. Basically lets say your at a stop then just floor it. It'll climb up to about 1000rpm then drop down to about 800 for a second, then start accelerating like it should. Obviously getting lots of air and not enough fuel. Sounds like a typical accelerator pump issue, but that's new. It looks like there's plenty of fuel being spit in there, it seems like it does the hesitation at idle, even. I'll check in the morning, but I believe it has the hesitation w/o a load. Obviously something somewhere in the carb.

Electric choke q-jet for anyone curious.

 

Check idle mixture carefully. Some hints:
W/ stock Q-J you can probably turn mixtures to highest idle w/o getting too rich. The idle passages can be drilled to non-emission, esp. w/ modified engines.
Accel. pump lever typically has 2 holes, try the other one.

See p.126-132 in Roe's book.

 

I suspect it's timing.

 

foot problem?
see a podiatrist

 

Have you checked the tension of the spring of the secondary air valve door? Since the Q-Jet does not have an accelerator pump on the secondary side, the correct tension on that spring is critical especially since the tQ-Jet is a mechanical seconday carb.

Sounds to me like the air valve may not have enough tension, thus it is just flopping open causing too much air. Easy 10 min check.

For more info, get Lars tuning paper at www.corvettefaq.com .

 

this might be yr last thing to check?, I recently pulled my gas tank and found that the fuel line was pinched under the gas tank strap.I was scraching my head for months because I was having the same problem as you.during aceleration I would get a stumble, I did the same as yrself. check timing,carb,idel mixture, look for vacum leaks Ect ect. but found this to be my problem.... and I only went this route because Im going to be installing a new engine and wanted to make sure the tank was clean... hope this helps ..

 

This would be a serious consideration, however, it didn't do this before I changed the carburetor and distributor. There was no hesitation prior to the swap.

Also the hesitation doesn't occur if I push the pedal extremely GENTLY. If I just slowly push it in and drive slower than a granny it won't do it. However if I rush it a fraction of an inch it'll do a slight hesitation then pick up and haul ***.

 

Mine turned out to be electrical; Accel coil, distributor cap, plugs and wires solved it for me!

 

All of those are new, though.

 

Usually caused by a lean condition in the idle circult play with the idle mixture.

 

So basically twist a screw in? Erm I mean out. Out = rich and in = lean

 

Make sure your vacuum advance is plugged into the Passenger side vac port on the carb

 

It is. I made sure of that right after I installed them.

 

Hmm...try hooking the accelerator pump rod running from the throttle linkage to the inner hole on the pump lever...the outer hole is for california cars and will indeed give you some hesitation. I think California law comes very close to saying that it's illegal to floor it from idle, which is why they require the outer pump connection because the hesitation isn't there unless you give it too much gas from idle.

Another thing that will give you some hesitation is the choke plate...make sure that it's not being sucked closed...make sure the throttle linkage is there to force the choke all the way open at WOT.

 

These are all external fixes, I'm guessing? I'm lazy and hate unscrewing things.

The inner hole... I'll go check that.

 

It's on the inner hole. After tinkering with it and looking at my q-jet book it looks like the accelerator pump isn't shooting gas long enough. It's shooting a pure stream of fuel but it's only just a squirt. It looks like it needs to squirt just a hair bit more. When you hit the throttle it'll rev up at first then you'll hear it sucking air, which is when the engine drops RPMs, then it'll start revving like normal.

This have to do with the accelerator pump spring tension?

 

Still having issues. I've taken it apart and put it back together twice now and can't find the reason for it to bog. According to the Haynes book it says to adjust the accelerator pump shooters with really small drill bits ranging from .0135 to .04, in increments of .002. This would be a permanent modification to the carb and doesn't sound like it should be necessary. Lars isn't available till mid-september so I can't just send it to him and I need it to work before mid-august

Anyone got any other ideas?

By the way I don't have any problems with it started warm or cold. It's ONLY this problem.

 

There is another pump adjustment your missing. The arm that pushes on the plunger can be adjusted too. If its partially pushing the plunger down at rest/idle the stroke of the pump is reduced. This adjustment should be in the book or rebuild instructions. Basicly the end of the arm is bent up or down for this adjustment, I've forgotten but like a drill bit diameter between the arm and plunger.

It could be fuel delivery which you can check with the line going into a can for so many seconds. measure the amount. This is in the book.
Either crank it or run it on the fuel in the bowl. Plugged filter, if thats it then disconnecting prior in the above test won't show it, a new filter can be plugged.


The hole selections in the arm change the fulcrum and allow a short or long stroke.

Idle adjustment should be in the book too. open too much till idle drops, close for same affect then turn in so much for right amount.

While trouble shooting keep in mind that it may not be something you did- the recent work. It could be something that was due to happen and is a new problem. Would have happened anyway.

 

It's seeming more and more like it's just running lean, obviously not rich because there isn't any soot. Also it got really warm on the freeway today, above 200 degrees according to the guage. The hesitation was much shorter and had a lot less of an effect at the higher temperatures. I'll make note to double check the filter. The line is brand new from Mid-America because I kinked the old one.

To give an idea as to why it feels like such an anomaly to me I'll give a little more info on the engine. It was "remanufactured" back in April, bored to a 355, I had this done by private shops since I don't have the equipment or space to preform such a task. The engine looks brand new with very little dirt even on it, the oil pan is a little dirty now of course. It has broken in nicely as it has about 3000 miles on it since the rebuild. Early last month I began to remove the computer controlled components from the engine. The carburetor and distributor were replaced with their non-computerized versions (M4ME instead of E4ME and vacuum advance HEI instead of computer advance). The cam is a moderate cam with 214/224 at .050. The ignition system is essentially all new. The bottom and top end are all new, with the exception being the intake manifold. So in essense we're talking about an engine that is almost all new. It still has the stock exhaust manifold and pipes, but with Dynomax Super Turbo mufflers (replaced those because I had no choice since the original ones literally fell off.)

Now here's why I only consider the carburetor and distributor to be the only places that could cause issue. There was no issue even close to this with the original system in place.

My question now is should I attempt to tune it to run slightly richer or adjust something else? Right now, restricting airflow is preventing the hesitation a bit more.

 

you said new plugs, not bosch platinums or the like i hope