Bog,stumble, hesitation - Chevelle Tech
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post #1 of 15 (permalink) Old May 29th, 09, 4:40 PM Thread Starter
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Unhappy Bog,stumble, hesitation

The 290 hp 350 crate motor I put in my daughters Nova awhile back, I am having a tuning problem with, It has a bog, flatspot, stumble, hesitation during normal driving right off idle then the car runs great. If you mash the throttle it smokes the tires and takes off fine.
All the ignition specs are set right and it has done the same thing with an old Holley 600 double pumper that it does with a new 590cfm Holley Street Avenger carb. It dosen't appear to have any vacuum leaks around the carb. It runs worse when cold before it reaches operating temp, dosen't have a choke but I have 3 other cars with Holleys and no choke and I have no problem with them.
Am I missing something here? any other ideas?
Gene
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post #2 of 15 (permalink) Old May 29th, 09, 6:40 PM
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Re: Bog,stumble, hesitation

You may to check accel. pump setting or the idle circuit in the 590. It may be set lean for emissions. You may have too bring the carb a shop and have the idle circuit richen up. Hope this helps.

Louis
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post #3 of 15 (permalink) Old May 29th, 09, 6:52 PM Thread Starter
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Re: Bog,stumble, hesitation

Thanks , Louis
Accel pump looks like it's giving a good shot , i'll try fattening up the idle circuit some.
Thanks, Gene
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post #4 of 15 (permalink) Old May 30th, 09, 7:46 PM
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Smile Re: Bog,stumble, hesitation

Since 2 carbs did the same thing esp 1 being a dbl pumper that inharently run rich esp at idle/off idle being lean off idle may not be the issue in this case .

I believe the 290hp gm 350 crate motor comes with a very mild perf cam and in that case i bet your not running enough base timing .

Also are you running vac adv and if yes for street that needs it is ithooked to full intake vacuum all the time wich in many cases is beter the ported with no vac at idle?

If vac adv is on ported try full int vac all the time.

Also, even though gm may not rec it try more base timing like at least 14-16 deg to see if that helps out. It only takes 5 mins to try and if no change and or motor doesnt run better with more base timing simply put it back to where it was prior to adv the timing.

Set base timing with low idle rpm like 600-650rpm to ensure mech adv in dist isnt partially activated upsetting base timig adj. Also unhook vac adv and plug it for timing check/adj.

Then post timing adj reattach vac avd to full int vac all the time and also readj idle mix & idle speed.

Scott

SCOTT
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post #5 of 15 (permalink) Old May 30th, 09, 10:39 PM Thread Starter
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Re: Bog,stumble, hesitation

This afternoon I fattened up the idle circut , I have about 6 turns out on the mixture screws and getting 20 inches of vacuum. I also advanced the inatial timing from 6 degs. to 12 degs. I don't know what the total advance is, the vacuum advance is on ported now.
I have another Holley 600 cfm single feed on my pickup that works perfectly , I was thinking of trying that on the Nova and see how it works, that should tell me if it's carb or something else.
Thanks for your help guys, Gene
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post #6 of 15 (permalink) Old May 30th, 09, 10:46 PM Thread Starter
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Re: Bog,stumble, hesitation

Scott
I'll try that timing thing tomorrow, more base timing and full vacuum not ported, and check mechanical adv. and vac advance. I have a degreed after market balancer that should show total advance,(right?????)
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post #7 of 15 (permalink) Old May 31st, 09, 4:04 PM Thread Starter
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Re: Bog,stumble, hesitation

Ok , I have 12 degrees inital advance with the vacuum disconected and pluged @650 rpm.
With the vacuum advance still pluged I get another 22 degrees mechanical advance @2700 rpm for a total of 34 degrees, am I adding this right ?
When I hookup the vacuum advance nothing changes, either port vac or direct manifold vac
so I put a vacuum pump and guage on the vacuum diaphram on the dist. and pump up to 20 inches of vacuum while the engine is running and nothing changes, vacuum advance dosen't work ????
With 34 degrees mechanical at 2700 rpm without vacuum advance shouldn't cause this bogging?
The car dosen't sound too bad reving it in the driveway , but if I drive it and your just barely moving and and give it more throttle to speed up it bogs then goes normally.
Gene
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post #8 of 15 (permalink) Old May 31st, 09, 9:08 PM Thread Starter
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Re: Bog,stumble, hesitation

Well , I put the 600 Holley off my pickup on the Nova this afternoon and it ran fine so I guess this a carb problem with this 570 cfm Street Avenger carb.
Tomorrow is another day, Thanks guys for your help.
Gene
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post #9 of 15 (permalink) Old Jun 1st, 09, 2:11 PM
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Smile Re: Bog,stumble, hesitation

Great you found the issue may be the carb but keep in mind a fatter/richer running carb will cover up a retarded base timing issue. That's because the motor needs more fuel comming off idle with retarded base timing to cover up an off idle stumble that the other carb may be providing hiding the true issue which could still be retartded base timing .

So with that said you take 5 mins to check base timng as i suggested to do because you could be leaving considerable perf/power on the table if the base timing is retarded . That's even true with a mild perf cam that you possibly bandaided with the other carb thats running richer off idle to cover up the stumble with the underlying retarded base timing issue still present loosing power & fuel mileage too if thats the case which it may or may not be untill you chk the base timing.

Scott

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2002 MAXIMA (DAILY DRIVER/1 owner,GOT 3/2013 w-44k miles)
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post #10 of 15 (permalink) Old Jun 1st, 09, 2:22 PM
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Re: Bog,stumble, hesitation

Gene, after you finish checking your timing as Scott suggested, what color cam do you have for the accelerator pump arm? A different cam will alter the timing of the shot as you first give it gas - once your timing is increased, I'd check that.

And yes, sounds like your vacuum advance, which will make a difference, is not working - did you get a replacement hooked up yet?

Mark

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post #11 of 15 (permalink) Old Jun 3rd, 09, 5:02 PM Thread Starter
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Re: Bog,stumble, hesitation

This Nova seems to be running good now , but I do need to take the distrubtor out and replace the vacuum advance or find out why it dosen't work. Maybe if things go right i'll have time this weekend.
Thanks , Gene
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post #12 of 15 (permalink) Old Jun 3rd, 09, 11:21 PM
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Smile Re: Bog,stumble, hesitation

Gene,if its a gm dist i change the vac adv with the dist still in the motor.

BTW,did you ever bother to check the base timing and if yes what was it at ?

Also,If you changed the base timing what did you change it to and how did the motor respond to it?

Scott

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post #13 of 15 (permalink) Old Jun 4th, 09, 1:24 AM
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Re: Bog,stumble, hesitation

See what squirter size is on the 600 from the truck
See what squirter size is on the other carbs.

Also, if you have "about 6 turns out on the mixture screws " this tells me your whole idle circuit is out of whack. My experience; 1 1/2 turns out minimum to 3 turn out tops.
The fix:
Engine off but warmed-up. Wind your idle speed screw until the primaries are closed. Open the idle speed screw one turn. This sets the tip of the primary carb blade just about right on the idle transfer slot. Run the idle mix screws in all the way and back out 1 1/2 to 2 turns. Start the engine. If it does not idle, dies, advance the timing until it does idle. Then set the idle mix for highest vacuum. Probably best to wait to do this until your vacuum advance is fixed. I usually attach the vacuum line to full vacuum.

George Kettler
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post #14 of 15 (permalink) Old Jun 4th, 09, 2:21 PM Thread Starter
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Re: Bog,stumble, hesitation

Scott , see post #7 thats where the timing is at now, will work on it more when I do the vacuum advance.
George, that sounds good i'll try that this weekend if I can get the car from my daughter for awhile.
Thanks guys, Gene
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post #15 of 15 (permalink) Old Jun 4th, 09, 3:29 PM
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Re: Bog,stumble, hesitation

Get one of the adjustable vacuum cans from a performance store or your local CSK/O'Reilly Auto Parts 'speed section.'

re post #7
12 degs initial could still be 4-6 even 8 degs retarded from best performance.

I've found that if the idle mix screws are off in the least bit, or cruddy, sure-fire hesitation. I also had a bog, the cause was the accelerator pump was WAY out of adjustment. Then I played with the squirters.

FYI, my first car was a 68 Nova.

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