Need some HELP! Promax modded Holley 3x2bbls installed, "surging" "pulsing"

[biomedtechguy]

I started this as a new thread to bring attention to a problem. A new problem.
Changed out stock fuel pump to a Carter M6903. Changed upper and lower fuel lines to Promax braided stainless steel lines w/pressure gauge. Asked for a WIX 3/8ths fuel filter to match my V-code/Hemi factory 3/8ths fuel lines and was happy to find the fuel filter that was on the car when I bought it 4 years ago was the same one I just sourced and installed. No fuel leaks. Carb to intake gaskets provided by Promax are nice, seem to have a slick-ish surface so as not to stick, but they are thinner than the fibre composite ones that were on it. Eliminated vapor separator and plugged the return line.
Carb intake surface is clean and smooth. Fuel is from a nearby station I have always used, 93 octane no ethanol.

So that brings me to the new problem.
As I drive, part throttle, the engine is "pulsing" and "surging" more than it ever has, which was something I had to try to make it do in the past. It always has had a little "surge" or "uneven" climb through the RPM range at part throttle only in the upmost 2k to upper 3k range, but now driving it "normally" it is much more noticeable.
WOT seems fine, idle is great. I checked the 6 idle screw settings with a timing light tach and vacuum gauge, and it is around 14 inches which is as good or better than ever, and really didn't need any idle screw adjustments to speak of. No strong fuel (rich) smell, but you can tell the engine runs on gasoline.
I am about to drive hundreds of miles along the coast for the next week and I would really like to smooth this out.
Timing checks out fine too.
Engine specs in my signature.
Thanks for any advice!

 

[NOSTRT980]

distributor vacuum advance???? if you are running one

 

[kiwigtx]

Is the surging happening during a transition between primary and secondaries? Or even when the engine RPM are hitting the sweet spot on the cam? Because you have had the carbs effectively re-engineered, you may have highlighted an issue that was already present, except now, it is more noticeable.

 

[biomedtechguy]

distributor vacuum advance???? if you are running one

Yes. I have a FBO ignition system and just installed a fitting in place of the plug in the rear of my Edelbrock/Chrysler aluminum 6bbl intake, with a little pipe thread dope, tight and no leaks I'm sure.

 

[biomedtechguy]

Is the surging happening during a transition between primary and secondaries?

This is running on the center carb only.

 

[NOSTRT980]

weak spring in one of the outboard carbs. just trying to help

 

[biomedtechguy]

Carbs were run and tuned on similar 440+6bbl at Promax.
Going to check for leaks at the carb base to manifold gasket surfaces tomorrow morning.
I will follow up with any significant news.
Thanks guys.

 

[493 Mike]

Surging is usually a symptom of a lean condition. Idle restrictions too small or idle air bleeds too large will cause this. This assumes you are cruising in the RPM of the idle circuit. Rear gear ratio/OD?
Mike

 

[moobster]

You mentioned that you plugged the return line.
Those fuel pumps can put out 8psi, maybe check your fuel pressure, could be too high.

You said the vacuum advance is now plugged into the manifold.
If you changed this from a port on the carburator, reset the ignition timing.
Vacuum being better then before, leans toward timing.

Hope this helps

 

[biomedtechguy]

Surging is usually a symptom of a lean condition. Idle restrictions too small or idle air bleeds too large will cause this. This assumes you are cruising in the RPM of the idle circuit. Rear gear ratio/OD?
Mike

4.10
What RPM/throttle position would have me "cruising in the RPM of the idle circuit" as best as you can guess?

 

[biomedtechguy]

You said the vacuum advance is now plugged into the manifold.
If you changed this from a port on the carburator, reset the ignition timing.
Vacuum being better then before, leans toward timing.

Don at FBO ignition systems is who I got my ignition system from, and he insists on using manifold vacuum for the vacuum advance signal to the distributor. Prior to the Promax center carb metering block, which has no metered vacuum nipple, I used the carb choke diaphragm manifold vacuum nipple for the vacuum advance signal, and made do with the metered vacuum to operate the choke diaphragm.
So the source for vacuum to the distributor has always been manifold vacuum.

 

[moobster]

Don at FBO ignition systems is who I got my ignition system from, and he insists on using manifold vacuum for the vacuum advance signal to the distributor. Prior to the Promax center carb metering block, which has no metered vacuum nipple, I used the carb choke diaphragm manifold vacuum nipple for the vacuum advance signal, and made do with the metered vacuum to operate the choke diaphragm.
So the source for vacuum to the distributor has always been manifold vacuum.

I agree with manifold vacuum being the better choice.
When you set this up on the other engine, did that engine use manifold vacuum also?
What is your fuel pressure at the carbs?
Was the ignition system changed at the same time, as the carbs?
If so, can you go back to the original ignition system, to take that variable out of the equation, for now?
Maybe their metering block has an issue.

 

[PurpleBeeper]

Well, I'm going to guess that with 4.10 gears (even with overdrive) you go beyond the idle circuit at 20 mph. "Maybe" try to lock out the outer carburetors (remove/block off vacuum lines & run a hardware store spring somewhere to keep the outer carbs closed) & see what happens? I think your choices are carbs or ignition, so it might be a matter of elimination.

You might also look again for a vacuum leak...very carefully.

 

[biomedtechguy]

You might also look again for a vacuum leak...very carefully.

I haven't looked for one yet, and that is the first thing I am going to do as soon as I can. Suspect the thinner carb-to-intake manifold gaskets may be the culprit.

Well, I'm going to guess that with 4.10 gears (even with overdrive) you go beyond the idle circuit at 20 mph.

Yes. I am specifically referring to 2,000-3,000 RPM operation when on the center carb only and part throttle.

I agree with manifold vacuum being the better choice.
When you set this up on the other engine, did that engine use manifold vacuum also?
What is your fuel pressure at the carbs?
Was the ignition system changed at the same time, as the carbs?
If so, can you go back to the original ignition system, to take that variable out of the equation, for now?
Maybe their metering block has an issue.

Don at FBO insists on (everything-lol) and his distributor has always operated off of manifold vacuum, I am just using a different (new) location to draw the vacuum from.
Nothing else is new or changed other than the Promax mods.

 

[PurpleBeeper]

Don't assume the brand new Promax carbs are dialed in for YOUR engine. Of the top of my head
1. Vacuum leak
2. Outboards trying to come in/out (re-adjust outer carb return linkage)
3. Center carb running lean
4. Timing/advance or "something" in the distributor

 

[biomedtechguy]

Thanks PB

. Vacuum leak

As soon as I can, sometime today, I will check for leaks w/carb cleaner spritz thru the little straw attachment.

2. Outboards trying to come in/out (re-adjust outer carb return linkage)

Already done. Had to reinstall the rod, which is marked on the threads to get me close. Only one rod one turn off.

3. Center carb running lean

I will see if he changed the jets I had in the center carb to a lower #.
I checked all 3 carbs w/timing light's tach (immediate, accurate readings) and a vacuum gauge, and they all seemed to be right on with very little to no adjustment needed.

4. Timing/advance or "something" in the distributor

I checked that, but I can be more thorough!

 

[493 Mike]

4.10
What RPM/throttle position would have me "cruising in the RPM of the idle circuit" as best as you can guess?

Well, knowing this I will ask what size main jets are in the center carb?
Mike

 

[biomedtechguy]

Well, knowing this I will ask what size main jets are in the center carb?
Mike

I have to double check, but if Promax didn't change them out, #65

 

[biomedtechguy]

So I drove the RR about 45 minutes to Mississippi to get ready for Crusin the Coast.
Same stuttering/missing/ba baba ba ba crap....on the center carb.
I haven't been able to detect any vacuum leaks.
My outboard carbs used to transition in flawlessly, BEAUTIFULLY irregardless of the circumstances. If I floored it at 5 mph after rolling away from the starting line, no bog, no stumble. If I eased into the outboards at fairly high (2,800-3,500) RPMs after being on the center carb only much higher RPMs than "usual" they would pin me to the seat!
NOW, "post Promax", there is a stutter when they open at any RPMs, any speed.
THIS REALLY SUCKS!
Right now, if I could, I would use a time machine and go back to before I ever sent them off...

PLEASE HELP!
I wrote Promax a 2nd email tonight. No response to the first one.

 

[OH71RRunner]

Might need to change springs in the vacuum pods. Could be end carb jets are incorrect size.
Trying to set up modified carbs is an absolute headache. I was (am) a complete novice to carbeuraters even though I've read an ungodly amount the past couple years.
Modified carbs dyno'd on their engine is not the same as setting them up on your engine.

I do hope you get it figured out. And when you do, please keep us updated on what you find.