New motor....tuning the carb - Team Camaro Tech
Troubleshooting Diagnosing problems done here.

 
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post #1 of 7 (permalink) Old Jun 29th, 02, 06:14 AM Thread Starter
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Join Date: Apr 2000
Location: Colorado
Posts: 59
Alright...pretty much a stock 350 /w a 268H comp cams 268H and a Edelbrock Performer Aluminum manifold. I took the Q-jet off of a 400, it ran fine atop the 400. The 350 is new...about 40 minutes of running in the garage and around the block. This is my first motor so it's probably a vacuum leak somewhere that I can't find.

My trouble is that I know nothing about tuning a carb. I can't get the thing to idle in the idle circuit. 1100 is about the best I can do and I still have to double foot the brake/accelerator to keep it idling around 800 or so. I haven't even messed with the mixture screws yet cause I know nothing will happen until I get the idle circuit to work.
- It pulls right above 10Hg on a manifold vacuum source and is irratic at times.
- There's no in between....it will idle poorly down low and a SLIGHT turn on the idle screw will bump it up to 1200 or so.
- I can get it to idle at 800 if I put a rag over top the primarys to make them smaller. Vacuum leak?? With the rag I can actually put it in gear without it dying, although it still runs pretty rough. Removing the rag, after idle has been lowered to 800, will cause it to die.
- Sprayed carb cleaner in various places, hard to tell if I have a vacuum leak.
- I spray carb cleaner along all the intake edges...it bubbles when it hits the heat from the heads. I hope its not from vacuum leaks. How/where do I look for vacuum leaks?
- I'd look at the plugs...but the motor is so new I doubt they'd tell me anything.
- I've disconnect the vacuum advance and it advanced to about 14degrees at 1000rpm or so...is that too retarded?
- It drives wonderfully around the block off idle.

So...what do you guys think should be my next move? Hope I gave enough details.

Also...what are the torque specs for the 4 carb to manifold bolts?

[This message has been edited by Dan-69rallysport (edited 06-29-2002).]
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post #2 of 7 (permalink) Old Jun 29th, 02, 06:55 AM
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My Olds service manual says carb bolts go 15 ft. lbs. Should be the same on all GM products. Memory(?) says short bolts 18-22, long bolts 16-20, tighten long bolts first. Couldn't find carb torque specs anywhere else on short notice. Old Rochesters are notorious for leaking fuel well plugs, could be part of your problem. Also check throttle shaft bushings as they egg out and pass air sometimes. Good luck with it, let us know what happens.

------------------
Steve
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post #3 of 7 (permalink) Old Jun 30th, 02, 12:44 AM
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Mark
 
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Don't be afraid of the mixture screws. Lightly seat them and back them out one and a half turns. Then try to lower the idle.

-Mark.
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post #4 of 7 (permalink) Old Jul 1st, 02, 02:22 AM
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I have been having the exact same problem with my 650 so I put a new 750 on and it runs and idles beautiful with a slight bog or hesitation when I floor it just above idle. I started to look at the 650 that I took off and the screws that hold the baseplate on where all loose, I am not sure if this was my vacuum leak or not. I also had to crack the secondaries open to get the car to idle. My car would idle at 1200 and then 800 in gear with erratic readings on the vacuum gauge. The 750 works for me until I find out what is wrong with the 650. The 750s throttle response isn't as crisp as the 650's. Good luck, Erik

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68 Camaro RS, Built 355, 274 Comp Cam, Sportsman II Heads 200 c.c. 2.02. Ported to Performer RPM Manifold and 1.75" Headers, 1.5 Roller Rockers, 650 Holley, Steel Crank, 12 Bolt posi with 4.11 Gears, TH350 tranny w/ shift kit.
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post #5 of 7 (permalink) Old Jul 1st, 02, 04:50 AM
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Did you pull the Q-Jet from a stock 400?

I assume your choke is functioning correctly.

It sounds like the power system is engaged at idle...the new cam has the vacuum way down (the Q-Jet had approx 17hg on the 400...now it's down to 10hg). You might try a weaker spring on the power piston...perhaps a 5-6hg spring.

...although, you say that covering the carb smooths out the idle which would indicate a lean condidtion.

What do the exhaust pipes look like?

You're right...adjusting the idle mixture screws will be impossible until you get the idle down to specs, but you might try half turn increments...from a half to three turns out...see if she smooths out.

Also, you're smart to look for vacuum leaks...plug all your vacuum ports...see if she smooths out...if so, reconnect one accessory at a time...see if you can find the culpret.

Lastly, how's your ignition...timing and advance...if you're using weak springs with your mechanical advance the idle can get weird...the weights don't always return all the way back to 'closed' position.

Let us know what you find.
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post #6 of 7 (permalink) Old Jul 3rd, 02, 11:55 AM Thread Starter
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Well...thanks for the suggestions. This is the greatest web site out there.

It would seem that things are getting better.

I started by tightening the carb to manifold bolts, checked fuel filter. I sure I was being paranoid about putting torque to anything aluminum. They're at just over 15ft-lbs. I also went through all the vacuum accessories. After adjusting the choke a bit it started right up and I was finally able to bring the idle down to 800 or so. It's a world of difference...sounds and feels a lot better.

Few things that I still don't like that'll hopefully be ironed out this weekend. It idles at around 1000 and drops to around 500 when in gear. It still lugs at 500...but at least it runs. The mixtures screws are four full turns out...seemed to give the highest vacuum reading.

It still is lacking right off idle, maybe timing is too retarded? Its at 8degs /w vacuum hooked up at idle. The carb was from a stock 400 and was rebuilt about 3years ago, the distributor assembly (also from 400) was bought new at that time.

I sure a bit more playing will give me better results.

Took if for a drive today and it made a squeaking, almost chirping sound as the engine rotated. It would only happen at idle and would get fast with engine rpm, but go away completely at around 900-1000rpm. Hope my first motor isn't falling apart already...so I might be asking for more help really soon. Kill me. What's the best to break it in? 50-60mph? 30-40mph?

Thanks again guys...I've learn a fair amount about carbs the last few days.
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post #7 of 7 (permalink) Old Jul 4th, 02, 03:27 AM Thread Starter
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Well...the continuously chirping squeal(the best way to describe it ) is definantly a problem. I could be completely wrong but it sounds like it's coming from back of motor...the distributor gear maybe?? It seems to go away if the engine is under load and comes back and idle or low cruising speeds. I don't know what it could be. Any ideas would be helpful.

Happy Fourth Everyone.
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