New engine tuning trouble. Any ideas? - Team Camaro Tech
Troubleshooting Diagnosing problems done here.

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post #1 of 6 (permalink) Old Dec 30th, 01, 03:12 PM Thread Starter
Join Date: Dec 2000
Location: Clermont-Ferrand, France
Posts: 17

I recently installed my new 409
(0.040" over 400).

Here are the current specifics:

AFR 210cc intake cylinder heads

Stahl raised port 1 3/4" primary headers
2 1/4" Borla exhaust

Demon 750 vacuum secondary
#72 jets in Primary's
#82 jets in Secondary's
#31 squirter
50cc accel pump (was 30 cc)
Light Vacuum Spring
Electric Choke

Carter Street/Strip Mechanical Fuel Pump
(8.5 psi), Hard line to pump, -6AN line to carb with Earl's sintered bronze filter.

Comp Cams Hyd. Roller 230/236 @ 0.050"
1.6 ratio roller rockers
13 inches of Mercury at Idle

Victor Junior intake with no spacer

10.3:1 compression ratio

Mallory Unilite Ignition with Accel Supercoil
(Electromotive ignition ordered 5 weeks ago!)
12 degrees advanced at idle
35 degrees total

NGK R5672A-9 plugs

6-inch rods, standard 3.75" stroke crank

Tremec TKO 5-speed, 3:08 rear end

02 sensor guage installed.

Here is the trouble I have been having:

The car started off very rich (78 primary, 86 secondary). I backed the jet sizes down to where they are now. The 02 guage still reads rich at part throttle and idle. (Idle set using max vacuum method). When full throttle is applied, it goes lean on the guage and stumbles for approximately 2 seconds. Eventually it clears out and goes back to rich if I keep the throttle down. I tried a #41 and a #28 squirter and could find no difference. I also tried several vacuum springs with no influence. I installed a 50cc accel pump and that seemed to help with the lean spot. It runs very well when the bog is not there and I have seen no black smoke. Now I am having a heck of a time getting it to cold start. I pulled a plug and it is completely black. They are fouled for sure.

My next idea is to replace the plugs (try a higher heat range) and keep reducing the jet sizes. The only thing that bothers me about continuing in this direction is that I am getting pretty far from the baseline that Demon set at the factory. Am I going the right direction?

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post #2 of 6 (permalink) Old Dec 30th, 01, 04:20 PM
Join Date: Nov 2001
Posts: 138

Have you checked float level? With that much fuel pressure you may be pushing the needle off it seat.Was this a new Demon? Looks to be too big on jet size for a 750. Don't mean to put down this carb but have heard of many tuning problems with it. Built one just like yours for a streetrod with holley 750(smaller cam) that runs great.From the looks of your post you have done everything right. Hope you get it figured out.
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post #3 of 6 (permalink) Old Dec 30th, 01, 05:46 PM Thread Starter
Join Date: Dec 2000
Location: Clermont-Ferrand, France
Posts: 17


I have checked the float levels (even tried moving them up and down) and they seem to be plenty stable at idle. What jet sizes worked for you on your similar engine?
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post #4 of 6 (permalink) Old Dec 30th, 01, 07:56 PM
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Join Date: Jan 2000
Location: Wilton, Ca. USA
Posts: 10,118

I think about 7 psi of fuel pressure is all they recommend. You might need a regulator if it's really 8.5 psi. I've never had much luck with that much pressure on a carb, usually makes it pretty rich along with flooding conditions.

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post #5 of 6 (permalink) Old Dec 31st, 01, 06:49 AM
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Join Date: Aug 2000
Location: seattle, wa
Posts: 172

Have you gone over the intake and carb for vacume leaks?

Stock 67 327 RS
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post #6 of 6 (permalink) Old Dec 31st, 01, 02:44 PM Thread Starter
Join Date: Dec 2000
Location: Clermont-Ferrand, France
Posts: 17

I think the problem is solved now. I was using the NGK equivalent of the AFR recommended plugs for the 210 heads. From what I understand these plugs are intended for pure race engines and are very cold. I looked at the recommended plugs for the Street 195's and they are a street plug (ACDelco FR3LS). After confirming that they have the same reach and tip, I tried a set of them and the car is behaving normally now. I just need to lean the carb a bit more and it should be good. Thanks for the ideas...
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