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Camaro Research Group Discussion => Maintenance => Topic started by: j-rho on January 06, 2013, 05:17:25 AM

Title: Advice for tuning a stock-ish 302
Post by: j-rho on January 06, 2013, 05:17:25 AM
Finally got my project car running -    ;D

It is basically a freshly rebuilt clone of a stock '67 Z28 302.  Stock block, crank, rods, pistons, cam, heads, etc.  Carb is a stock re-issue carb I asked about here:

It came with what I believe are the stock jets, 68 primary and 76 secondary, and likely stock power valves, accelerator pump and cam, vacuum secondary spring, etc.  I'm not allowed to change any of those parts in the class I'm building the car for.  Here's an example of the sort of racing it is:

The car has Doug Thorley tri-y 1-5/8" headers (mid-range torque is more important than top-end HP for what I do), a 2.5" Magnaflow exhaust with x-pipe, and a high-end Mallory distributor and coil.  Has 100 octane in it at present, I have no problem running that long term.

Looking for advice from the group here - suggestions on initial timing advance, total advance?  I have some flexibility in advance curves, mechanical and vacuum.  (manual here: - I'm using the curve that has 6 degrees advance at 2000, and 19 at 5200).  Also have initial timing at 12 degrees but really have only started it a couple times and haven't done a basic idle tune yet.  Vacuum gauge is on its way.  At present it has a bit of a stumble off idle, even though idle seems rather high (12-1500rpm).  My engine builder suggested this advance curve, but I forget where we had initial timing when it was on the engine dyno.  I suppose to get to the normal 36 degrees total, would need 17 degrees initial?  Need to have a timing approach that is safe considering the car will probably be lean most of the time with stock jets.

Looking for suggestions on adjustments I can make to the carb (idle, float levels, etc.) to help it perform under a transient-throttle racing situation?  What I do is a lot different than drag racing; we are part-throttle most of the time and need good transient response.  Looking at some kind of tube to run between the primary and secondary bowl vent extensions, to keep the carb from flooding under heavy braking, which would be allowed since it's not an internal modification.

Thanks so much again for the help!

Title: Re: Advice for tuning a stock-ish 302
Post by: Jon Mello on January 07, 2013, 01:06:59 AM

I saw this as one of the rules of STX...

"The engine management system parameters and operations may be modified via an ECU chip. You can do the Evotech, AMD one-click, the Shark, and maybe someday the GIAC chips."

If the guys with on-board computers can make a change like this, plus apparently the rules say you can add headers, a cold air intake and use any spark plug, why do you think you can't even optimize the jets? That seems pretty ridiculous. If that's true, you need to petition the rule makers for an exception for cars that use a carburetor.
Title: Re: Advice for tuning a stock-ish 302
Post by: j-rho on January 07, 2013, 04:59:09 AM
It's a long story Jon, but a jet allowance isn't likely to happen.  Some conversation on the topic in this thread ( from another forum.  Warning - that site tends to get a bit "salty" as compared to here ;)

The short version, is I'm running the car in a category and class built for modern cars, where the rules makers don't really want a car like mine to do well.  There are other categories to race old musclecars in, where you're allowed to change carbs and cams and pistons and stuff, but those classes also basically require you to cut up the car and run monster wheels/tires, in addition to going much further with race prep than I really care to. 

Have to make the best of the allowances available.  Of note, factory authorized tuning procedures are OK to perform - I've thought about running the float level extra high to increase the static fluid pressure on the jets a little.  In combination with the a jet extension fuel slosh tube, maybe that could work?
Title: Re: Advice for tuning a stock-ish 302
Post by: Everett#2390 on January 07, 2013, 03:17:01 PM
You could run a slosh tube from Holley, they do work very well.
Or,  a hose between the bowl vents with the very top of the arched hose bologna sliced open for venting of both bowls.
As for the off idle stumble, 12-14 BTDC timing, idle, even with the -140 cam, s/b around 900-1000 rpm, need alittle more bypass from secondary side.

If you have to stay with OE, why not drill out an OE size jet to the size you want?
If stumble, then try a bigger squirter, jump two sizes a step until gone.
Again, measure the new squirter size and drill out an OE size squirter?
Title: Re: Advice for tuning a stock-ish 302
Post by: ss jim on January 07, 2013, 07:35:06 PM
Check accelerator pump clearance, .015-.020 @ wot. I prefer .015. I have found this to be an off idle problem a number of times. Check you vacuum also. You may need to change the power valve. Hope this helps. Jim
Title: Re: Advice for tuning a stock-ish 302
Post by: firstgenaddict on February 28, 2013, 05:43:48 PM
They don't want you changing Jets, do you think they will use an indexed drill set to check the orifice size on the jets? take a stock 68 and drill it... it will still say 68.
I hate to advocate this however the rules committee forces one's hand with their lack of equity. They are allowing a computer to do what they will not allow you to do manually. NO DIFFERENCE

 I hope you beat these idiots at their own game using their rules!

Title: Re: Advice for tuning a stock-ish 302
Post by: firstgenaddict on March 04, 2013, 03:30:34 PM
The stock 68 jets were lean for emissions.
Stock computer chip fuel curves are lean for emissions.
If you ran in Denver the stock 68's would be closer to "correct" with the thin air, the chip would alter to compensate for altitudes etc.

Have you dyno'd the engine and looked at the Air fuel ratios?
Is it anywhere close to 11:1?