Power valve help

[pnora]

For starters my car runs good. Idles fine and makes great power. Its a 10.0 pump gas 528 hemi on 91 octane. Was dynoed at 642 HP AND 650 TQ. Its a 5 speed manual 3.71 gear rear. Cam is a Comps hydraulic flat tappet 535 Int. and 541 Ex. 112 lobe separation. Indy roller rockers 1.6 Int. and 1.55 Ex. Ready to run distributor not using the vacuum advance. About 11 to 12 inches of vacuum at 1,050 to 1,100 rpms. Timing all in at 35. Pro systems 4150 style 950HP carb. Like I said idles and runs great except at a low speed rpm wise. At light throttle or even de-cel anything lower than 2,000 rpms it has a light surge or ramping condition. I am trying to find a logical way to tune it out. Car has only 250 miles since I put it together. Doing distributor bushings, springs, and timing change the running but not the area I would like it to be. Right now the PV is 6.5. I might be to fussy and have had EFI to long and going to a carbed Hemi is a different rodeo. Thoughts.

 

[pnora]

Have you tried to contacting Pro Systems yet? They are super helpful and tech-support for tuning issues is a lifesaver. I would call them before I trusted any of the 1000 opinions you’re going to receive on this forum. I run a PS carb and love it.

No. They have done many carbs for me. However the last time I called them on a different carb Patrick was not very friendly or nice. Maybe he had a bad day. Other times were positive. This carb has been sitting in the box for many years as my project dragged on. I did pull the bowls and made a chart of all the bleeds, jets etc. I realize what answers I might get and I know how to read them. I just figured I would try a couple easy inexpensive items. Hence my last word thoughts.

 

[Outsider]

11 to 12 inches of vacuum at 1,050 to 1,100 rpms in Park/Neutral? What is vacuum in gear foot on the brakes, and at cruise, where you are concerned, going down the road? I duct tape a vac gauge on my cowl to check.

P.S. Duh, ya a 5 spd. Seems most have these problems with an AT. So cruise in 3rd gear it surges, and you mean it doesn't stay at a steady RPM with a constant throttle pressure?

Assuming you have like a modern dual plane 4brl man and not like a difficult to tune old Rat Roaster man? I once had a new carb that surged, but didn't leak fuel; it had an internal "leak" between the metering block and the carb body. Found a high spot, filed, installed new gasket and it fixed it.

 

[Aron Gleason]

The way holley or pro system says to check vacuum at idle is wrong. The Best way is to see how much vacuum you have at cruise and devide it in half and add 2 inches. Which you would run either a 9.5 pv or a 10.5. And that will make it open sooner. Then tune for your power valve jets for wot.

 

[RemCharger]

Thinking outside the box; one traditional reason for surge can be too high timing.
I'm not a Hemi guru, but 35 seems high?

 

[Geoff 2]

First thing to check is how much transfer slot is showing at idle. Everything in carb/ign tuning starts with that.
Also, what initial timing. Please report back.

 

[lewtot184]

i don't see anything wrong with the 6.5 power valve and manual trans. i've been fighting a light surge in a stock 440 i have and have tried all different kinds of "tricks" with no real solution. throwing jet at it doesn't help but messing with the ignition does. my engine simply wants a more conservative timing curve and now i've started taking voltage out of the coil. the more i head in this direction the smoother the engine runs. i know this isn't by any means an apples to apples comparison but you may be experiencing something similar. i think the more efficient the engine becomes the less it's dependent on aggressive ignitions.

on another note other than ignition you may look at the idle jet to idle air bleed relationship. low rpm depends a lot on these two working together. for the cubic inches you have i don't see the cam as a real culprit. i'll bet low rpm is more efficient than you might think.

 

[pnora]

Thinking outside the box; one traditional reason for surge can be too high timing.
I'm not a Hemi guru, but 35 seems high?

I have had it as low as 26 with the same results. Timing and advance curve all change the overall running but does not deal with this light surge or ramping condition.

 

[pnora]

11 to 12 inches of vacuum at 1,050 to 1,100 rpms in Park/Neutral? What is vacuum in gear foot on the brakes, and at cruise, where you are concerned, going down the road? I duct tape a vac gauge on my cowl to check.

Read my first post it is a 5 speed manual. I have not driven it with the gauge attached. That will be my next test.

 

[pnora]

First thing to check is how much transfer slot is showing at idle. Everything in carb/ign tuning starts with that.
Also, what initial timing. Please report back.

The slots are just about all above the plate. Base timing is around 12 and all in at 35 right now. the car starts easy.

 

[hunt2elk]

Surging below 2000 rpm, I would think is lean and you are still in the idle circuit. Have you tried bigger idle feed restrictors? I'm sure you know that the power valve won't be open under 2k just cruising along. You can use a higher number pv than what your idle vac is to tune when it opens, but it will never open at idle or when you are in the idle circuit.

 

[pnora]

Surging below 2000 rpm, I would think is lean and you are still in the idle circuit. Have you tried bigger idle feed restrictors? I'm sure you know that the power valve won't be open under 2k just cruising along. You can use a higher number pv than what your idle vac is to tune when it opens, but it will never open at idle or when you are in the idle circuit.

Other than playing with timing all across the board and basic carb set up that's it. I believe it is a lean condition but I have no way to confirm it. I have no problem buying restrictors or what ever, I just am trying to be logical on the approach. The engine runs great except in that off idle to 2 grand. I can drive around it but I feel I should be able to tune it out. It would be nice to just let the clutch out and have it smoothly run along in first gear without it ramping.

 

[GTX5877]

Surging below 2000 rpm, I would think is lean and you are still in the idle circuit. Have you tried bigger idle feed restrictors? I'm sure you know that the power valve won't be open under 2k just cruising along. You can use a higher number pv than what your idle vac is to tune when it opens, but it will never open at idle or when you are in the idle circuit.

Also would agree on "lean surge", often occurring when the RPMs are 'under' the camshaft rpm range. You have not listed the cam duration @ .050", but by other specs, I would guess it to be in the 240's, which with A/T would require about a 2500-3000 stall converter. Just using this as an example, I know yours is a 4 speed. I would get with Pro Systems on changing the emulsions for a slightly richer fuel curve at lower RPM or richer idle feed restrictions as stated previously. Hope this helps.

 

[hunt2elk]

I use the $200 AEM A/F meters in my cars for carb tuning. Are they perfect? No, but you can tell when the different circuits of the carb are in play and make it easy to experiment.

 

[64 Sportfury]

I fought a similar issue with my cross ram Hemi with the 770 cfm Holleys. I have a 5 speed with a single disc clutch. Surging below 2000 rpm and at idle. I pinned and tuned the carbs ad nausem. I added an AEM air/fuel gauge which along with spark plug reading helped my dial in my carbs. I still had the surge. I have been running the MP distributor and chrome box for years. I opened up the distributor to find the springs allowed for quick advance but also allowed the timing to "bounce" which was causing my surge. I cleaned, lubed and swapped in heavier springs which slowed the curve and completely eliminated the surge. I read you have tuned your Ready-To-Run but if its not a lean tip-in it might be worth another look. I run a pair of 10.5 PV's, a little bigger solid FT cam with 3:55 gears. As for the AFR gauge you can install it for tuning then remove it and plug the O2 sensor bung. I hope this helps.

 

[Geoff 2]

T slot position is good. The next thing is timing. It wants & needs a LOT more than 12* initial.

Probably 30-35*. If you turn your dist slowly, engine idling, to advance timing, 3 things WILL happen [ how do I know this? ]:
- speed increases
- idle gets smoother
- vac increases

You need to find the optimum idle timing [ highest rpm achieved in the above test ]. Once you have that, adjust the centri curve accordingly; best done using an adj vac adv unit connected to MANIFOLD vacuum. [ see 2nd post ].
The t/blades will be able to closed slightly after doing this; this leaves MORE of the t slot to handle light throttle cruise. This, coupled with the engine making more HP from the timing it needs, will help with the surging & may even get rid of it. If you still have surging after dialling in the ign, you will probably have to increase the IFR slightly. A quick test for leanness: engine idling, close choke slowly until engine note changes. Wire choke in this position. Test drive. If surging is gone/better, carb is lean.
Importance of correct init timing:

 

[Geoff 2]

Why vac adv connected to MANIFOLD vac [ MVA ] is so important & not the USELESS ported PVA that Chrys used.
If you do not have a dist that takes an adj VA unit, do yourself a favour & get one.....Will be the best $$$$ you could spend....

Scroll down to post #6:

www/hotrodders.com/forum/vacuum-advance-hooked-up-directly-manifold-bad-47495.html

And about MVA from somebody who has forgotten more than most will ever know:

 

[RemCharger]

A good question was brought up.
How tight or loose is the choke set on your 650 horse Hemi with the pro systems 950.

 

[64 Sportfury]

I should have added, my distributor has the vacuum advance can disconnected. I have tried it with and without and the cleanest idle and best performance is without. I stilll get 13mpg. Also, my engine builder recommended to disconnect it. Granted a cross ram has less of a vacuum signal along with fuel distribution challenges. I agree your initial timing should be higher than 12*. I have mine at 18 or 20 depending on the Florida heat. Every set up is different, however.

 

[pnora]

A good question was brought up.
How tight or loose is the choke set on your 650 horse Hemi with the pro systems 950.

No choke.