I have searched and tried what I believed the results have told me but nothing has worked. I have a 750 mech. secondaries on a built 355. It runs good at idle and when you ease into the throttle but if you bring it off of idle and try to launch it, it will stumble until you let off. It is freshly rebuilt and it has 70s in the front and 72s in the back for jets. I know this is a little big but I am still tuning. It has 30cc acc. pumps in front and back, and I am not sure what size power valves. I have played with the float level, idle mixture, fuel pressure, and timing. Nothing helps. Any suggestions.

 

It sounds like a rich condition. What size are the accelerator pump nozzles?

 

Sounds too lean in the secondaries. Most likely you don't have a secondary power valve (do you?), so you need to go 6-10 jets up in the secondaries vs. primaries.

 

okay, does it soundrich, or lean. I amnot a carb man, but I have thrown this together really fast and I need help. Thanks

 

Its lean... kick up the secondaries about 8 sizes bigger than the primaries if it does not have a powervalve on the secondary side. Then make sure your pump shot is set up properly. The linkage should have no slack at all. You should see the lever on the accelerator pump move if you move the throttle lever even the slightest bit. Start there, and get back to us.

 

I had the same problem. My secondary pump arm was set up with no play as described above. Looked up on holley's web site, and found out your supposed to have 15 thousandths play at the arm to properly function. Once I changed to the proper clearance the problem was solved.

 

The .015" is when the throttle is wide-open. The pump should have that much travel left at WOT. At idle, there should be no clearance between the lever and screw.

 

 

Sounds like you are lean to me. Check your accelerator pump lever like the others said..it could be an issue. Also check to see how much vacuum you have at idle and take your carb apart and see what size power valve is in it...make sure you have the right power valve first. I'm guessing you will have around 8-10 in. of vacuum at idle? If I remember your combo correctly..so a 6.5 power valve would be fine for that.
I would start with around 74/84 on the jets thats a good start for a 750DP. Reset your idle mixture screws again tuning for max vacuum..and maybe another 1/4 turn after max vacuum is achieved. Double check your float level too..I like to set it to the bottom of the sight glass or slightly above. Try that and see what happens. If you still have a problem you might need to swap in larger squirters maybe 35's...depending on whats in there now go a step or two larger.

 

As posted above, if you do not use a secondary side power valve, secondary jetting should be 80-85. + a holley strip dominator intake has a lot of internal volume. You'll need larger than stock accelerator shooters. maybe as big as .041" to .047"

A big rough idle camshaft needs lots of initial advance. recurve your mechanical advance for about 22 to 26 deg initial advance at idle and 36deg max advance at high rpm... Shorten the distributor mechanical advance travel from typical Out of the box 20-22 deg to
a limited travel of 10 to 14deg. Use one light and one mid tension advance spring as the lightest spring tend to be unstable at idle creating eratic ignition timing.

 

Ok
get it ideling and hook up to a vacume gage and thenget the proper power valve for the car if street driven you need a power valve
also check this link
MorTec lists Holley carb specifications, list #'s, jetting and tuning tips.MorTec lists Holley carb specifications, list #'s, jetting and tuning tips.
later and

GB

rk

 

I have a combo similar to yours in my sport coupe....roller cam 355, 236/248 dur. and .520/.540 lift with a holley 750dp mech. sec, no secondary power valve and I run 68s in front and 78s in the rear, .031 shooters in front and .035 in rear....awesome throttle response and pulls hard anywhere. Every engine is different but i'm just offering a guideline.

 

just chiming in with a question -

I noticed that my 4150 (dp /650cfm / mech 2nds) doesnt have a secondary power valve either. What is the reasoning behind this? Just because its a performance carb so the figure you want the power to always be there?
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O and one more thing. When I checked my vacuum, I was up around 15 i believe. Would the 6.5 be the correct PV for that reading?

 

just chiming in with a question -

"I noticed that my 4150 (dp /650cfm / mech 2nds) doesnt have a secondary power valve either. What is the reasoning behind this?" not using a power valve is usualy used for all out racing-- The carb will perform better on the street if tunned witha power valve.

"Just because its a performance carb so the figure you want the power to always be there?"
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"O and one more thing. When I checked my vacuum, I was up around 15 i believe. Would the 6.5 be the correct PV for that reading?"
check with holley on that one I don't have my chart handy right now
or check out
MorTec lists Holley carb specifications,
There is a ink in a prior repley on this post I put there.
later and
gb
rk

 

You don't use power valves in the secondary because they become "uncovered" during hard accelleration. As far as jetting, start with the jets holley used from the factory, they are set for sealevel, unless you've got a hood scoop shoving lots of air into the thing those jets will be rich enough. To the original post, do your idle air screws work properly?