Stumble off idle - - Page 4

decooney · #61 (**permalink**) 09-01-2012, 01:43 PM

Interesting demarcation. My living grandmother at 95 thinks anyone under 70 is young. Our local tuner is 71 and he sure showed me and a few others a thing or two about how to tune a Holley and Quick Fuel carbs. I guess that makes him old school, new school, and cool.

Grubby · #62 (**permalink**) 09-01-2012, 02:25 PM

Guys,

Money is not an issue. If I thought spending $700 on a new Quick Fuel was the best solution, I would do it.

The original engine builder, who is a top notch builder, thought the 750 was correct.

I specificlly asked Promax which way he would go - forget the money. He said he would slightly lean toward the 750 double pumper assembled from his parts. So it would have similar parts to a Quickfuel, but hand assembled by him for the application. Given that the 750 vac secondary was in my hands he thought for minimal effort it would run very close to the double pumper. He had strong opinions against 850s for a street car.

Promax is a Quickfuel dealer.

For a situation like this, it is a benefit to have the dealer local.

Brent,

Is it possible for you to "tune" a carb to run like the 351C you mentioned, without the engine? If you have confidence you could do that well over the phone and ship me a great running carb, I am interested. Let me know your thoughts and I can call for the sales pitch. Although it is probably not practical for me to come visit your shop.

What steps or logic would you follow to spec the carb and then how much tuning and what type would be left for me?

John

patrickt · #63 (**permalink**) 09-01-2012, 02:38 PM

Quote:

Originally Posted by Grubby

Let me know your thoughts and I can call for the sales pitch.

Brent doesn't even give sales pitches. I've been harping on him for years to start concentrating on just making money and forget about "helping people." But, he just won't listen to me....

Grubby · #64 (**permalink**) 09-07-2012, 02:16 PM

Brent of B2 and I had a nice talk a few days ago and he sold me a custom built Quick Fuel 850 double pumper.

I will give a report of how good it is out of the box and my impressions of throttle response and overall feel.

John

patrickt · #65 (**permalink**) 09-07-2012, 06:59 PM

Quote:

Originally Posted by Grubby

... a custom built Quick Fuel 850 double pumper.

What was "custom" about it?

Grubby · #66 (**permalink**) 09-10-2012, 03:26 PM

Choke horn milled off and proper jetting, power valve, etc for my engine. Brent's carb guy has built numerous carbs for similar builds.

I am expecting it to be really close out of the box.

Full report will follow.

John

patrickt · #67 (**permalink**) 09-10-2012, 03:37 PM

Quote:

Originally Posted by Grubby

Full report will follow.

... with bated breath.

decooney · #68 (**permalink**) 09-10-2012, 06:41 PM

One way you can buy a new carb from Quickfuel is built to spec through the custom shop versus buying a standard off the shelf carb. As a builder or outside customer you can provide the specs of the vehicle, engine, use and driving characteristics and they will build it for you. There is no machining or milling for their all-new carbs, they use their QFT main bodies, slick billet metering blocks, and specific jets, restrictors set up for your needs. The best part is you can actually talk to them on the phone and they make it simple with no mystery.

blykins · #69 (**permalink**) 09-11-2012, 01:55 AM

I have a little more personal contact than that. One of the former carb designers for Quick Fuel builds my carbs.

bingo2 · #70 (**permalink**) 09-11-2012, 04:56 AM

Brent's "guy" built a HR 650 with mechanical secondaries for my motor and I couldn't be happier with the results.

After making the usual adjustments; idle speed, choke, float height, etc. the motor ran beautifully, albiet a bit lazy. Dyno revealed a slight lean condition and going up two sizes on the main jets cured that; 368/347 hp/tq at the wheels.

Grubby · #71 (**permalink**) 09-19-2012, 05:11 PM

The carb is here. It is installed with intial adjustments made. Time for a short road test only.

My first impression is the car has much more power. I will get an extended drive in this week.

John

patrickt · #72 (**permalink**) 10-02-2012, 10:04 AM

Quote:

Originally Posted by Grubby

The carb is here. It is installed with intial adjustments made. Time for a short road test only.

My first impression is the car has much more power. I will get an extended drive in this week.

John

So how did this finally turn out? Is the stumble now completely gone throughout the rev band?

Grubby · #73 (**permalink**) 10-02-2012, 01:45 PM

The Quickfuel needs some tweaking. I have 82 and 84 primary jets on order. It has 80s in it. Then we will see.

I will post the complete report when the driveability issues are all corrected.

John

MaSnaka · #74 (**permalink**) 10-02-2012, 06:22 PM

I have been following this thread and am very interested in the outcome. Please keep us posted.

Thanks,
John

kbutler971 · #75 (**permalink**) 10-02-2012, 09:17 PM

Quote:

Originally Posted by MaSnaka

I have been following this thread and am very interested in the outcome. Please keep us posted.

Thanks,
John

X2. Great thread and input/knowledge contributions.. Have a similar situation with mine.

Grubby · #76 (**permalink**) 10-03-2012, 06:32 PM

With the carb out of the box, I felt a lean surge on primary metering.

Installed the 82 primary jets tonight.

Verified fuel pressure at WOT on the road with my shop gauge - never below 5.5psi. Not enough time to do a thorough road test.

I previously hooked up my shop vac gauge and monitored during several drive conditions. I did this to try and verify I have the correct Power Valve. It seems the carb was somewhat lean on the primary circuit during light to moderate load. Conditions where the power valve would typically not open.

That is what drove the decision to install 2 sizes richer primary jets as the first tuning move.

I will get a drive in this weekend, weather permitting, and determine if more tuning needs done.

John

decooney · #77 (**permalink**) 10-03-2012, 07:04 PM

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.

Grubby · #78 (**permalink**) 10-05-2012, 01:50 PM

Quote:

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 · #79 (**permalink**) 10-05-2012, 06:35 PM

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

decooney · #80 (**permalink**) 10-05-2012, 07:47 PM

Quote:

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.