# Troubleshooting carburation



## VettenGoat (Jun 19, 2007)

I'm having issues dialling in my carbs. While moving in 1st at low RPM, the motor wants to "hiccup". Also I noticed when I remove the air cleaner on the center carb, the idle speed slows down noticeably; put it back on and the RPM increase. Lastly, the motor is pulling about 12 lbs vacuum. I don't know how to interpret these indicators. There does not appear to be any vacuum "pull" on the in/outboard carbs.

The basics are correct (timing, gap, and plugs). Distributor is freshly rebuilt. Motor is a 30 over 421 SD (428) with #62 heads (2.11 1.77 valves). Ram Air exhaust manifolds. Comp Cam with .488 intake and .499 exhaust and durations or .230 and .236.
Standard Rochester tri-power. Motor runs like a bear at speed, but is sucking wind in parking lots.

I set the center carb to baseline mixture (all in then backed out 1 turn) and have on two subsequent occassions leaned left 1/4 turn for a total of 1/2 turn left.

Ideas?

Mike


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## Rukee (Feb 8, 2007)

I think you have a vacuum leak, perhaps at the base of the carbs, were the throttle shaft is is where most of them leak. You can buy repo base plates now, had to get them for my tri-power set up.


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## VettenGoat (Jun 19, 2007)

Rukee,
That possiblity has crossed my mind. I'll spray a little starter fluid around the shaft end and see if it spikes the idle. When you did yours, was it a simple swap out or did you have to get the carb overhauled by a shop? 
Mike


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## geeteeohguy (Feb 2, 2008)

Vette, if your RPM changes when you remove the air filter, you have a SEVERELY restricted air cleaner. It's acting like a choke. Rukee's right, though, you DO have a vacuum leak, and that's why it's hiccuping/running better with a restricted intake (clogged air filter). You may have sucked in the center carb base gasket, etc. Try swapping an end-carb air cleaner to compare how plugged your center one is. The small air filters on tripower cars plug easily, especially the center one, which is used 95% of the time. I use K&N's on mine, and it saves me a ton of money (paper filters around here are good for about 2500 miles). Find the vacuum leak, fix it, install a clean air filter, and you'll be fine.


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## Rukee (Feb 8, 2007)

Yea, if your running paper filters you should change them very often. Specially if you drove/drive in the rain.
I'm running K&Ns too.


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## D1147 (Sep 3, 2009)

Hi Mike, I Am Interested In What Jets You Are Using In The Carbs. I Also Am Running A 428 With Tripower, Ram Air Manifolds And A Comp Cams Xe 268. # 48 Heads. Very Strong Set Up, But Also Having Trouble Idling. It Seems That I Have To Set The Idle Screw So High That It Is Overriding The Idle Circut.


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## GroundHog (Oct 16, 2009)

If the idle is fine, but taking off from a stop in first gear i still experience a "hiccup" before it gets galloping, does that sounds like a carb tuning issue? I know that's not a very detailed description, but my motor seems happy otherwise.


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## Gran Turismo (Jan 12, 2010)

You have a nice motor setup' agree with the others in regards to centre carb try setting mixture screws 2 1/2 turns out from base. Good source for info or parts is Mike Wasson @ pontiactripower.com


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## Rukee (Feb 8, 2007)

GroundHog said:


> If the idle is fine, but taking off from a stop in first gear i still experience a "hiccup" before it gets galloping, does that sounds like a carb tuning issue? I know that's not a very detailed description, but my motor seems happy otherwise.


It could be a bad or weak accelerator pump. That could cause a hiccup on slow accel.


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## GroundHog (Oct 16, 2009)

Thanks! Carbs confuse the $#^@% out of me.


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## jetstang (Nov 5, 2008)

D1147, you can drill a small hole in the butterfly to allow a little air in, that will let you close the butterfly enough to keep the car in the idle circuit. Check out Jegs and Summit, they sell pre-drilled butterflies due to this, but probably not for your carb. Your carb wasn't built for a wild build like you have, so it needs modifying to compensate for all that motor.


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