Stumble off idle -

[Grubby]

Originally Posted by decooney

John,
For what its worth... I'm running primary 82s on my Holley HP830, and have tried both 6.5 and 5.0 PVs. One thought is I suspect the QFT carbs may be a tad leaner on the idle feed / transition circuit as we found out on my buddies QFT carb. He's got more freeflowing open sidepipes too. In the last run someone there at QFT told me they went down 2 sizes in the metering blocks for the idle feed restricters. So, we went back up two, and another two more again - something like 32 > 34 > 36 and it cured the problem. Not sure if this is your situation or not. Curious to know what you have in there now. If going with 82 primary jets up front and a jump in PV from 5 to 6 does not help, you might check with Phil at QFT and see (and measure) what size itty bitty jets are in the metering blocks for the idle feed restrictors. This is what I like about the QFT better than my Holley, way more adjustable. You can get the little restricter jets from Summit or Jegs too.

Duane,

I have the jets in but the weather is not cooperating. I will update all on what it takes to get the drivablility spot on.

After driving with the vacuum gage it became obvious the 65 PV is likely correct. Lean conditions occur when the vacuum is higher than 10 inches. It is possible a 75 PV may even work better, but let's see what the jetting does first.

I am thinking about idle feed restrictors as well. I am working through one piece at a time.

John

[blykins]

I think you're above the IFR/IAB rpms....

[decooney]

Originally Posted by Grubby

Duane,
I have the jets in but the weather is not cooperating. I will update all on what it takes to get the drivablility spot on. After driving with the vacuum gage it became obvious the 65 PV is likely correct. Lean conditions occur when the vacuum is higher than 10 inches. It is possible a 75 PV may even work better, but let's see what the jetting does first. I am thinking about idle feed restrictors as well. I am working through one piece at a time. John

I re-read your post and realized that you said, "occurs during light to moderate load" while above 10psi. As a first step, I would be trying the same as you are, going up 2 jets and to a 6.5pv. Its a good first test if anything, even if you had to go up 2 more primary jets just as another quick test before doing anything else. Your comment about how it occurs at "light" load is a key word. My buddies new QFT had the exact same issue. On his, after we did the sniffer test with the tuner we stopped there at bumping primary jetting and PV and started playing with IFRs (up) and (down) on PVR restrictors again. QFT thought we had larger IFRs in the carb until we measured it; nope - down 2. We had to reconfirm it twice with them, and that was it. Hope yours is solved with these primary circuit jet changes alone. Happy testing. It will be worth it once its dialed in. Can't wait to hear the final test report. Good Luck.

[Grubby]

Tried 82, 84 and 70 primary jets.

The difference in feel between the 82 and original 80 jets is barely noticable.

The 84s will pull smoothly down to 2000 rpm then it starts surging or bucking. The 80s had to be above 2200 during at steady cruise. When you stand on the gas the power feels much greater with the 84s. Moderate accel and WOT both seem OK with these jets. This is still strange. The Holley 750 would pull smoothly down the highway in 5th gear at down to 1600 rpms.

I tried the 70s for kicks, because I had them. They did help calibrate the seat of my pants to feel lean. They were super lean. The engine would barely get to 3000 rpms without the secondaries.

No other jets to try without placing another order.

It seems that 86 or 88 jets would be required to smooth things the rest of the way. However, that seems really rich for this application.

Another noticable difference between the Holley 750 and this Quickfuel is during warm up. The Holley ran decent after just a few minutes from cold start. The Quickfuel runs very poorly until full temperature. This makes me think lean to.

I will call Brent on Monday and discuss.

John

[decooney]

John,
are you referring to stumbling when you are just barely on the throttle, at steady cruise speed, say at 25-30mph mph in 2nd gear?

[Grubby]

Originally Posted by decooney

John,
are you referring to stumbling when you are just barely on the throttle, at steady cruise speed, say at 25-30mph mph in 2nd gear?

Yes and no. It did have a stumble when you moderately hit the gas from lower rpms. In addition it has some lean surge at steady cruise at the lower rpms.

Both conditions seem to have gotten better by going up 4 jet sizes. Seat of the pants power seems to be better as well at WOT.

John

[blykins]

We may need to look at the high speed air bleeds.

One thing that is different from when you had the 750 Holley and this new carb is the weather. We're around the same location as far as weather/temps go and the past few weeks have really seen a drop in temps.

[Grubby]

Brent,

It is certainly cooler out and I assume the gas suppliers have switched to the Winter fuel blend.

John