Tuning

[serpent710]

Trying to fine tune the Camaro. Hoping I can get away with giving basic info.
350 .030
scat crank
pistons,push rods, rockers all aftermarket
rpm air gap
Aluminum Heads
holley 650 dp 0-766651BL

I have it tuned so there is no hesitation or stumble when you hit the gas. At the same time the car after running for a bit will start to load up and the RPM starts to drop and go back up at a light. If I tune it so it idles good I get a stumble when I hammer the gas from a stop. let me know if I need more info to help. Thanks

 

[pomartin]

...If I tune it so it idles good I get a stumble when I hammer the gas from a stop. ...

As far as I understand you should not tune 0->wot transition with idle mixture but with pump volume and pump nozzle shot.
Idle screws are for idle and transition (cruise) only.
Then again main jets and power valve for WOT.

I'd reccomend you to tune this way:
First tune for good cruise conditions by adjusting idle cirtuit with ifr or lsab.
Ignore 0->wot for now.
When you are satisfied with cruising adjust idle, so it will idle fine.
Next step is tuning the pump for 0->wot transition. If you have big plenum you'll likely need 50cc pump. Here do not forget that bigger pump nozzle means faster discharge and that engine will starve for fuel faster if nozzle is too big. Shot should be long enough for main jets to kick in.
After 0->wot is fine all you have to do is tune the PV and main jets. Start rich and lean down not to cause lean condition at wot.
Also, forget the 1/2 idle vacuum for PV... this is not the best an idea in most cases.

 

[G72Zed]

Any chance that's a Holley 0-76650BL, Ultra DP with shiny aluminum and blue billet??

Assuming carb is clean and fuel system up to par, with no vacuum leaks, I look at the timing/ignition first.

What's the timing spec's ?

Is the carb ever been apart or played with and/or re-jetted?

 

[Rustbucket350]

Trying to fine tune the Camaro. Hoping I can get away with giving basic info.
350 .030
scat crank
pistons,push rods, rockers all aftermarket
rpm air gap
Aluminum Heads
holley 650 dp 0-766651BL

I have it tuned so there is no hesitation or stumble when you hit the gas. At the same time the car after running for a bit will start to load up and the RPM starts to drop and go back up at a light. If I tune it so it idles good I get a stumble when I hammer the gas from a stop. let me know if I need more info to help. Thanks

Your ignition timing I probably off. Try plugging the vacuum advance (if you have it) and setting the total timing at 32-36 degrees.

 

[serpent710]

Any chance that's a Holley 0-76650BL, Ultra DP with shiny aluminum and blue billet??

Assuming carb is clean and fuel system up to par, with no vacuum leaks, I look at the timing/ignition first.

What's the timing spec's ?

Is the carb ever been apart or played with and/or re-jetted?

Yes, Its the blue one. I'll check the timing #'s

 

[serpent710]

Friend thinks the timing is around 36-40°

 

[serpent710]

Carb is less than a year old but has been doing this since the beginning

 

[G72Zed]

What is your idle rpm and timing without VA? and are you using VA, if so is it ported or manifold.

Sounds like you have a nicely done up engine, and assume you have a more aggressive cam as well, spec's ? what's the CR & cranking pressure. Just getting to the fact that most "carb" tuning issues are more about ignition and timing issues. And the fact that the more aggressive cam, the more initial timing it likes and needs.

I've worked and repaired those carbs, IFS are in the high set, and are/could be over-drilled, that combined with overly machined transfer slots adds to this and could be your issue. You do not need 50cc REO pumps with this carb and it comes with dual 330 cams, more that enough to cover the engine's needs more than likely, unless something is way off.

 

[serpent710]

What is your idle rpm and timing without VA? and are you using VA, if so is it ported or manifold.

Sounds like you have a nicely done up engine, and assume you have a more aggressive cam as well, spec's ? what's the CR & cranking pressure. Just getting to the fact that most "carb" tuning issues are more about ignition and timing issues. And the fact that the more aggressive cam, the more initial timing it likes and needs.

I've worked and repaired those carbs, IFS are in the high set, and are/could be over-drilled, that combined with overly machined transfer slots adds to this and could be your issue. You do not need 50cc REO pumps with this carb and it comes with dual 330 cams, more that enough to cover the engine's needs more than likely, unless something is way off.

No VA, i usually set idle around 900 in park and 650 in drive.

 

[G72Zed]

OK, what's the timing at 600/900 rpm and how fast does it come in?

Vacuum advance is your friend once setup properly.