Hi there, questions for the Holley tuners among you.

I have an all forged 383 stroker with pro 1 heads and 10.3 to 1 yadda yadda yadda.
The cam is a Comp Cams NX288HR. (Hydro roller) 113 LSA 520/540 lift with 1.6 rollers. Initial timing is 20* and it is a msd 6a ignition with a blaster coil.



Sitting on top of it is 4150 850 holley double pumper. The jetting is 80 front/rear with a 3.5 power valve. Its fed by a holley black pump, 1/2" line, and 7 psi.

Runs great until full throttle. It breaks up pretty badly and starts to clear up toward the end of the track.

Any guesses? Float height? Ignition?

 

Do you have a secondary power valve?

 

Have you tried going much smaller on those jets?
Also, have you eliminated an ignition problem like bad wire or fouled or cracked plugs?

 

Plugs are new, wires are new, no secondary power valve. It had a secondary power valve but it was fouling plugs so I plugged it, and it actually studders more now, but the plugs are still turning black..... Plugs are gapped at .35

 

These are the new plugs that are fouled right?
New doesn't mean that they won't missfire.

Make sure the power valve isn't ruptured, and check for flooding/fuel level.

 

thats a big carb to start with,i think your jets are way too big ,i have a 350 bored 40 over.i have a 750 holley with 72s up front and 76s out back .mine runs great,the only way a plug turns black is too much gas ,it sounds like youre getting too much gas.this will make it break up bad.the reason it runs better at the end of the track is that it finally has enough rpms to burn most of the gas youre putting in it.also be sure the float level is correct.are you getting much black smoke from the exhaust ?hope this helps it can be really frustrating working on holleys sometimes good luck with it.bet its fast when you get it right.

 

What happens when you change jet sizes (the most basic tuning)?

 

Thanks for the replies, What jet sizes would you start with? 80/80 is the stock setting according to http://www.mortec.com/carbs.htm

I also bumped the primary power valve down to a 3.5 from a 5.5 due to the power valve coming on too early and making the engine blubber on fuel at quarter to half throttle. Now that problem is cleared up, but the studder seems worse, althought it never really was clean in the upper rpm's.

tom

 

You don't jet a Holley square when you're only running one power valve. Drop the primaries down to 72 or so to start, you may need to bump the power valve back up when you do that. The power valve should be based on cruise vacuum (mild cam) or idle vacuum (radical cam), not used as a mixture adjustment.

I wouldn't take mortec.com as gospel. I'd trust the Holley part number list long before.

 

Regardless of what the stock parts in the carb might be, the engine wants whatever it wants. That's what we call "tuning".

It would be nice if everything in the aftermarket performance world came out of the box perfectly calibrated to match whatever combo of stuff it gets put on; but unfortunately, it doesn't work like that. You gotta do a little work once in a while.

Put the secondary PV back in; and try adjusting the primary jets first until it runs right on primaries alone; then adjust the secondary jets until they also work right.

Sounds to me like right now, you've got it WAY rich on the primaries (black plugs), and WAY lean on the secondaries (high RPM fumble).

 

Jetting 78P 88S

power valve: 3.5P (maybe 4.5") blocked Sec
Jet extensions pri no sec yes

remove the carb, flip it over and adjust the throttle blades in relation to the idle fuel transfer slots. Make them even with about .030" of slot exposed.

use a PCV valve and a valve cover vent or two.
Clear all 4 air bleeds with compressed air.

Initial timing is 20 at idle check full high rpm timing 32-36deg max 51 with vacuum line connected.

If after this the idle is still too rich you can lean down the whole idle circuit by putting small pieces of V shaped wire in the idle feed restrictions to trim the idle fuel feed.

850's are calibrated with racing in mind. You can tame down the tune a little for the street by increasing the primary power valve channel restriction a small amount.
.002" at a time. This will allow running a smaller primary jet for clean lan cruising yet maintain the same WOT afr. Do every thing else first. Use a O2 sensor to find the best primary cruise jet (close to 14.7:1 at cruise) then enlargen the primary power valve restriction by the jet *area* change to restore WOT total fuel flow.

What intake manifold?

 

Have you looked on the underside of your air cleaner lid after a run and noticed any fuel on it? If there is, you need to read up on a condition called "reversion" It happens when the air and fuel flow into your intake is more than the engine is capable of useing and is common with a larger than nessasary induction system for your RPM range. This in turn creates pressure in the intake and literaly tries to spit fuel back up through the carb. which in turn creates a temp. rich condition. especially in mid range RPM under full throttle. Might want to try a smaller carb! unless your RPM range is greater than 7,000.

 

No fuel on the air cleaner, no fuel being spit back out. Valve float happens at about 7350, so 7k is about what I set my limiter at. Thanks fbird 88. Thats the kind of answer I was looking for.
The intake is a pro topline single plane similar to a victor jr. Ill change the jetting and adjust the throttle plates. The distributor is a MSD pro billet with the stock curve.