Dyno Run shows very rich at low RPM but...

Sledge · #1 (**permalink**) 08-05-2012, 09:49 AM

...a little lean at high RPM. The pull was shut down at 6000 RPM, didn't go to the usual 6500. It was a club-sponsored dyno pull so no time was available for tuning.

So your help and advice are requested as I am NOT a carb expert.

On the dyno WOT was applied at about 3000 RPM and the AFR was 13; by 4100 RPM the AFR was 10.5; back to 13 AFR by 6000 RPM.

The carb is a 4150 HP-based Pro Systems 750 w/mechanical secondaries. I e-mailed Patrick with the dyno run printout, his advice was:

"The best plate (sic) to jet is really the racetrack. Jet up or down evenly 2 jets at a time and jet for best mph."

I don't have a handy racetrack so I'm doing the best I can.

Over the last week I've been trying different smaller primary jet sizes, but I notice more severe surging at 1/2 to 3/4 throttle with the 72 and 70 primary jets than with the 74's that came with the carb. At WOT it goes great, right up to 6500. I haven't tried a 75 or 76 yet. But I'm wondereing, should I also be looking at the secondary jets and trying some changes there? I haven't done anything there yet with the 84's that came with the carb. Should I try going larger with the primary jets and smaller with the secondaries?

I get lousy mileage and am ok with that if the performance is there. These dyno runs showed 16 less hp than previous runs--on a different dyno--with a Street Avenger, no cold air box, and more restrictive mufflers--but about 40 more lb-ft torque. I was hoping for better power but I'm thrilled with the increased torque.

Should I keep fiddling with the jets or should I be looking elsewhere? Ultimately I know I will probably be heading back to the dyno and spending $$ but if there is something obvious to those of you who are knowledgeable, I'd appreciate it.

lovehamr · #2 (**permalink**) 08-05-2012, 10:37 AM

Since I don't know, are your Pro System's primary air bleeds changable? If so, it seems like tuning for part throttle would be easier there than main jets. Have you contacted Pro Systems about it?

Sledge · #3 (**permalink**) 08-05-2012, 02:16 PM

Thanks, yes they are changeable but now I'm edging out of my limited carb skillset. I mentioned e-mailing Patrick--he is Pro Systems. He did not say anything about air bleeds, but I can call him tomorrow. Spare drillable air bleeds came with the carb.

Thanks

brandx · #4 (**permalink**) 08-05-2012, 07:16 PM

The one thing to be looking at is the power valve.But 13 is not what I would consider lean. The power valve is what is opening and closing with the signal from the vacuum.

JustOne · #5 (**permalink**) 08-08-2012, 08:46 PM

I have a Pro-Systems carb and so far it is very responsive to changes. Patrick is helping me get mine dialed in with the use of an LM-1.
For a WOT dyno pull, your numbers don't seem to far off; as long as the 13 is not over 13.3. As mentioned in the previous post it can be several items causing the rich spike at 4100, but that may be OK because you want a lower air fuel ratio right around your peak torque number. Hopefully, closer to 11.8 to 12.2 at peak torque.
If you want to tune your carb more, I would either schedule some dyno time or find a wide band that you can use. Start with your idle mixture, then WOT, and finally drivability and fuel economy.
So far I like the Pro System carb much better than the other double pumpers and the Avenger that I had.

Good luck,

Justin

Gaz64 · #6 (**permalink**) 08-09-2012, 12:23 AM

While your A/F ratio is near correct at 3000 and 6000, no main jet or high speed air bleed change will cure your heavy richness at 4100.

If you have a think about leaning the main system by either a smaller main or larger air bleed to make 4100 rpm correct, the rest of the band will go lean.

Your metering block would also have replaceable emulsion bleeds (most probably the issue here) which Pro-systems should be able to address for you.

PurpleVenom · #7 (**permalink**) 08-09-2012, 07:42 AM

What size secondary jets are you using? I have a different carb, QuickFuel, and mine are 74 primary / 82 Secondary. AFR is pretty steady at 12:1.

Sledge · #8 (**permalink**) 08-11-2012, 11:43 AM

I don't know, I must have cheesed Patrick off somewhere along the line as his responses so far haven't been all that helpful. I tried asking him again and was told that maybe I should try smaller squirters. So I'll try that and see. When I first got the carb 2 years ago he was very helpful. Anyway, thank you for the suggestions, soon as I heal up from a little surgery I'll get back on it.

Fiddling with the primary jets didn't do anything to help and made driveability a little less pleasant. I've got 84's in the secondaries.

Thanks

Gaz64 · #9 (**permalink**) 08-11-2012, 01:39 PM

A fuel curve that goes rich in the middle while the throttle is wide open won't be affected by changing the accelerator pump squirters.

The carb is wide open at 3000 rpm with correct A/F ratio, pump volumes have already been used up.
A/F ratio is correct again at high rpm, which shows it is not pump "pullover".

Google "emulsion tuning".

DAVID GAGNARD · #10 (**permalink**) 08-11-2012, 02:45 PM

Quote:

Originally Posted by Gaz64

A fuel curve that goes rich in the middle while the throttle is wide open won't be affected by changing the accelerator pump squirters.

The carb is wide open at 3000 rpm with correct A/F ratio, pump volumes have already been used up.
A/F ratio is correct again at high rpm, which shows it is not pump "pullover".

Google "emulsion tuning".

The other thing I'd check is the power valve........correct, the pump squirter only squirts extra fuel during accelaration from idle to full throttle,once the throttle blades are open (your right foot on the floor), the squirters are not pumping any fuel into the venturi.....

I'm guessing there is something internal in the carb that is casuing the richness at a certain rpm level.....

David