Engine Stumbling Issue

[Terry Stapley]

418W - QFT 850 DP - MSD 6AL - 12 degrees initial - 36 total

This is a new problem that has not been present before.
When VERY gradually accelerating, the engine would stumble, not really a miss but a stumble, until rpm reached about 1800 then it would catch and run fine. When quickly accelerating there is no issue at all! I checked the timing to make sure the dizzy was advancing which it was so I then advanced the initial timing 4 degrees and the problem went away but now the engine does not seem to have as much power as before. (Seat of the pants feel)
That would now put the initial @ 16 degrees with the total advance at 40! I have the weakest dist. advance springs in the MSD which is all in by 2000 rpm.

Any ideas? I do not detect any detonation with that much advance but am nervous to have it that high!

[Bobcat]

Terry , not familiar with the QFT 850 , but check your accelerator pump/cam as it may be giving you a full shot on fast throttle opening , but not enough, or too little on slow throttle opening . Had this problem on my 427 S/O with Holleys.

[wtm442]

Replace the spark plugs.
Retime engine .... 40 degrees is too much.
If that does not help .....

Check the float levels. If that does not fix the stumble ...

Turn the idle mixture screws out about 1/4 to 1/2 turn on each side. See if that reduces the stumble. It might be too lean. If not, try turning the screws in to make it leaner if its too rich now. Just remember where the idle screws were initially so you can put them back to the same place.

[patrickt]

Originally Posted by wtm442

Replace the spark plugs.
Retime engine .... 40 degrees is too much.
If that does not help .....

Check the float levels. If that does not fix the stumble ...

Turn the idle mixture screws out about 1/4 to 1/2 turn on each side. See if that reduces the stumble. It might be too lean. If not, try turning the screws in to make it leaner if its too rich now. Just remember where the idle screws were initially so you can put them back to the same place.

I agree -- it's probably the plugs. You might also squirt some carb cleaner down as well, but I think a new set of plugs will fix you up.

[jhv48]

First thing.

Back out the idle mixture screws about 1/8 of a turn. Sounds like your idle circuit is just a hair on the lean side. That will cause a slight stumble as you slowly roll on the throttle.

It's in the carb, would be my bet!

[Tom Kirkham]

Check and see if your accelerator pump arm is loose. The arm should be in contact with the accelerator pump cam.

[turnpike boy]

Check the power valve. If it's in the 6 to 6.5 range, try a 7 or 7.5 - and CHECK them with a vacuum source to verify their accuracy.

[Rick Parker]

You folks have begun to experience cold weather in your area. Is oxygenated fuel used/sold exclusively during the Winter months? It is in California and it sure plays havic with the way the cars run. Also, the cold air (which is denser) will pull and hold more fuel and could be causing it to run a little rich an thus require a minor retuning, and or temporary rejetting of the carb? Timing shouldn't need any work though.

[WildBill3]

Mine is doing this off idle also on pump gas also im going to get some of those carb cams and try some different combo's the pump shot is very important on when it comes in.Happy tinkering

[RICK LAKE]

Terry Stabley Terry Check the basics first. It sounds like you found the place where the carb is changing from the primary curcuits to the mains in the carb. 40 degrees of timing sounds a like high BUT if the motor is not knocking, and we don't know what cam,heads, compession, intake, and exhaust is on the car, Checking the plugs would need to be done to make sure about pre ignition. Some FE motors are runing as high as 44 degrees. You could drop the timing back 2 degrees and keep doing this until you feel a power drop off, then go back 2 degrees. This will be a save timing spot. MSD tells you with the springs about which ones and how fast you want the advance to work. If the car is running OK except for this narrow RPM range, there could be some other factors involved like temp of the motor, WEATHER ( this is a big one some times) Did this problem just start or have you had it all the time? The next thing would be to go to a Dyno shop or get an LM-2 meter and install this in both exhaust pipes and see what the A/F ratio is. This may be a lean spot. Make sure you have the CORRECT fuel pressure for your carb. You didn't say or I missed it about is this all the time or only doing certain things like racing or cruising? What color are your side or tail pipes? White is NO GOOD, black is rich and better than lean. Rick L.

[Terry Stapley]

Rick,
Problem started when cruising in Oregon in August! Gradually got a little more pronounced as time went on! Where fuel was purchased did not seem to make a difference.
Sidepipes are not black
Sparkplugs look a little lean (white)
Fuel Pressure is at 5.5 lbs
The problem was when cruising and VERY slowly rolling into the throttle. If you push on the go pedal quickly - no issues!
What is different is it seems down on power after advancing the timing, but the stumble went away!!
Weather permitting I will back off the timing and as Warren suggested richen up the idle circuit a tad to see if that helps. I also will install new spark plugs and report back!!

[RICK LAKE]

Terry Stapley Terry if you have a compression tester check the compression in each cylinder they should all be within 10%. If they are, See if you can barrow a leakdown gauge and check this too. They should be no worst than 8-10%. Lean is not good. The accelerator pump on the carb is covering the lean out under what you are saying. Go up 2 sizes on the primary jets for starters and turn in the mixture screws, 1/4 turn and see what happens. The LM-1 is what is really needed to check out the motor. Rick L. PS you are not using any E-85 gas? I know all gas can have up to 10% ethonall in the fuel now. wonder if this hurted the carb?

[Randy Rosenberg]

Originally Posted by Terry Stapley

The problem was when cruising and VERY slowly rolling into the throttle. If you push on the go pedal quickly - no issues!
What is different is it seems down on power after advancing the timing, but the stumble went away!!
Weather permitting I will back off the timing and as Warren suggested richen up the idle circuit a tad to see if that helps. I also will install new spark plugs and report back!!

This tip-in stumble sounds to me like a lean condition. You seem to confirm this by advancing the ignition to make the problem go away. Lean fuel burns more efficiently with greater advance (pump gas is designed to burn this exact fashion, which is why modern vehicles get better MPG - the compooper leans out the fuel and advances the ignition).

Richen up the idle circuit and/or install a more agressive pump cam and/or install a slighter larger squirter, so that you get a richer squirt at tip-in.

Good luck,
Randy R...

[Terry Stapley]

Great ideas all and thanks!! I will let you know what I come up with as soon as I get the chance to try your ideas!!

[greg schroeder]

The metering blocks(deals with the jets in them) may be plugged up. The fuel can evaporate to leave residual or the cast type can get corosion as well to have the pieces fall off and jam stuff up. Pull them off and check for obvious signs, scrape off the flakes of stuff and such and spray carb cleaner through the passages. Put it back together and it will probably work.

If you were dialed in to begin with and you're going into winter with cooler temps you may need a jet size up to snap up things a bit too.


generic looking metering block for part reference

[BlakeC]

Hi terry:

I have the same problem......just began to happen so I threw a thread up yesterday and saw later that you had one too. Have you tried any of the recommendations with any success????

[BlakeC]

Not sure if this is related, but I suffer from the same prob as Terry......I recently changed out my entire set of spark plug wires, not because I needed to, but because I just wanted to, yellow vs. black.......could plug wires cause this stumbling/stuttering issue???

[jhv48]

Not likely. Still sounds like a carb issue. A little lean on the idle circuit.

[danc30]

Mine has the same issue. It's currently being tuned on a Dyno in Daytona.

I'll let you all know what he did in the next couple of days. He's thinking the idle feed restrictors on my Q-950 need to go from 36 to 40.

[davidlee]

I had the same problem. It turned out to be the transition circuit in the carb. We replaced the metering block and the problem went away.