Stumble off idle - - Page 5

Grubby · #81 (**permalink**) 10-06-2012, 03:37 PM

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 · #82 (**permalink**) 10-06-2012, 04:22 PM

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 · #83 (**permalink**) 10-06-2012, 06:53 PM

Quote:

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 · #84 (**permalink**) 10-07-2012, 11:19 AM

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 · #85 (**permalink**) 10-07-2012, 02:22 PM

Brent,

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

John

patrickt · #86 (**permalink**) 10-07-2012, 02:51 PM

Quote:

Originally Posted by Grubby

Brent,

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

John

It only takes about 20 minutes to switch the Holley back on there for a quick compare. Apples to Apples, temp to temp, gas to gas.... That would be definitive.

blykins · #87 (**permalink**) 10-07-2012, 02:55 PM

Temps are a lot cooler, air is a lot denser, which means more air than usual.

Grubby · #88 (**permalink**) 10-08-2012, 07:05 PM

No need to change carbs. I have put almost 8,000 miles on this car - most with the Holley 750. I drive it down to about 40 degrees. The cold did not have this much impact on the other carb for the last 3 years.

Reconfirmed no vac leaks. I used the propane torch method along the intake gaskets, carb base and carb fuel bowl gaskets. A leak on the inside of the intake is possible but i doubt it.

Quickfuel tech support suggested going down a few sizes on both IAB and HSAB. After reading up on the air bleeds; my guess is the IAB is the most likely. Decreasing the size of the IAB will richen the mixture at the high RPM range of the idle circuit (up to 2500rpm) and raise the RPM the carb transitions to main metering. The worst lean surge is between 1600 and 2000rpms in 5th gear.

I am inclined to drop IAB next. How many sizes? How many different sizes should I order and in what ranges? I assume the IAB change should be made to the primary side of the carb only. At least initially. Quickfuel tech says it is likely that with this change I will need to go down a few sizes in main metering jets.

I have been reading everything I can find to get up to speed on Holley 4150 tuning. I am feeling more comfortable.

I missed Brent today. So I will catch up with him or his carb builder later.

John

decooney · #89 (**permalink**) 10-08-2012, 07:24 PM

Hey John,
Other than this one last fine tuning issue to work out..., what is your overall impression moving to the QFT and up 100 more cfm on your FE - any noticeable difference in seat of the pants feel, response, midrange, or top end power?

cobra 53 · #90 (**permalink**) 10-09-2012, 02:11 AM

Hi John,

Very interesting thread.
I am new regarding Holley Carbs and searching for a good Tuning-Book of the Holley 1450.
Not easy to choose one because of the big choice.
Maybe you have a good hint for me?

Rico

blykins · #91 (**permalink**) 10-09-2012, 03:53 AM

Quote:

Originally Posted by Grubby

No need to change carbs. I have put almost 8,000 miles on this car - most with the Holley 750. I drive it down to about 40 degrees. The cold did not have this much impact on the other carb for the last 3 years.

Reconfirmed no vac leaks. I used the propane torch method along the intake gaskets, carb base and carb fuel bowl gaskets. A leak on the inside of the intake is possible but i doubt it.

Quickfuel tech support suggested going down a few sizes on both IAB and HSAB. After reading up on the air bleeds; my guess is the IAB is the most likely. Decreasing the size of the IAB will richen the mixture at the high RPM range of the idle circuit (up to 2500rpm) and raise the RPM the carb transitions to main metering. The worst lean surge is between 1600 and 2000rpms in 5th gear.

I am inclined to drop IAB next. How many sizes? How many different sizes should I order and in what ranges? I assume the IAB change should be made to the primary side of the carb only. At least initially. Quickfuel tech says it is likely that with this change I will need to go down a few sizes in main metering jets.

I have been reading everything I can find to get up to speed on Holley 4150 tuning. I am feeling more comfortable.

I missed Brent today. So I will catch up with him or his carb builder later.

John

John, do you have different contact info that I can use? I tried returning your phone call just a few minutes later yesterday and couldn't get an answer or a voice-mail. I also sent two emails without a response.

I guess I mis-read, or couldn't remember, but I thought your problem was on tip-in at a higher cruising speed. 1600 is pretty low.

I would give Scott a call today. I told him that I would probably put you in touch with him. I imagine that he will tell you to do some air bleed swaps as the carb is probably a little lean everywhere with the temp changes outside. It's about 38° right now.....a little different than several weeks ago when I sent you the carb.

Let me know how I can help, but I'd say you're not too far away from getting it nailed.

Grubby · #92 (**permalink**) 10-09-2012, 07:14 PM

Quote:

Originally Posted by decooney

Hey John,
Other than this one last fine tuning issue to work out..., what is your overall impression moving to the QFT and up 100 more cfm on your FE - any noticeable difference in seat of the pants feel, response, midrange, or top end power?

Duane,

Carb looks good and Brent had the choke horn milled off.

The seat of the pants feel is much stronger. The power seems to come on at a lower rpm and it comes in smoothly. Tire spin in the higher gears is much worse than before.

Anytime the secondaries are open or near opening the power is unbelievable.

If the low speed driveability can be fixed without a bunch of drama, I will be very happy.

John

decooney · #93 (**permalink**) 10-09-2012, 07:25 PM

John,

I have a dumb question. Now I'm really confused. Not that it matters now, but you've mentioned a few times about the "choke horn being milled off". When I buy quick fuel Q750s or Q850s, the main body and carb looks like this. No milling or anything needed. Pretty much bad a** right out of the box from quick fuel with some fine tuning and done. Hit the throttle on a cold day, and wham off you go. Everything is matched on these end-to-end, no mixing of metering blocks or main bodies.

Did you get a 100% Quick Fuel carb, a hybrid, or something else?

QFT Q Series

Grubby · #94 (**permalink**) 10-09-2012, 07:34 PM

HR 850 with mechanical secondary. The HR series comes with a choke. The higher end carbs are available as you show.

Brent suggested the HR based on my planned use.

John

decooney · #95 (**permalink**) 10-09-2012, 07:38 PM

Oh, please disregard all of my feedback to this point. I thought you bought a QFT Q series carb. The Q series carbs are different. I was racking my brain all of this time trying to figure out how you were lean with a Q series carb. I get it now.

Grubby · #96 (**permalink**) 10-10-2012, 04:36 PM

Duane,

Expand on your Q series vs HR series thoughts. Why would you think one might have a lean spot and the other would not.

John

decooney · #97 (**permalink**) 10-10-2012, 11:10 PM

Hi John,

For what its worth you might want to chat with Phil at QFT. He will get into aspects of metering, adjustability, tunability, and comparisons far beyond my basic knowledge of the various carb systems and circuits. After going through this analysis, trial and error, more than I care to admit for my own Cobras and builds for friends, I have strayed away from the the Avenger and HR street "efficient" carbs based on my own findings and tests. I learned way back to not listen to the conservative "its too big" or "its too much" crowd when there is no actual test data to back it up in many cases. I relied on first hand tests with my engine builders and beyond that in my own cars. I continued with the basic Holley HP or Quickfuel Q series performance carbs for my Cobras and buddies cars and builds. In the end, its paid off in terms of performance and improved mileage, clean running, and clean shutoff too, believe it or not. Result: at-will tire spin, and 16.2 mpg with an old 428CJ and 4-speed transmission using my Holley HP 830. My buddy got 15.2 mpg on his 482cu. in. running a HP 950 Holley. Not too bad for big sloppy FE motors with larger cams, etc. Prior to this I ran a 780 vacuum secondary carb on my Cobra, and it did not have the same performance or mileage. I'm rambling... I recommend speaking with Phil or anyone in the custom build shop at QFT. DISCLAIMER: What works on my car may not work on others cars, but since our engines and cars are so close, I thought it might be comparable for consideration. Keep up the good work, it will pay off.

Grubby · #98 (**permalink**) 10-15-2012, 06:52 PM

UPDATE:

Outside temp was near 70 today.

My car has 70 IAB. I ordered 66, 62 and 58s to try. The 84 main jets are still in place. Started with 80s.

Changed to 66 IABs and the car would barely run down the driveway and the the surging at steady cruise was much worse. The surge extended up to about 2400 rpm.

This was a successful test. I was able to make the condition worse. So, I must have gone the wrong way on the IABs.

I plan to order 76 and 80 IABs and see how that goes. As much worse as the car felt with 66s, I will guess going to something like 74s might be enough, but I want to move enough each time see the change.

John

blykins · #99 (**permalink**) 10-15-2012, 06:58 PM

IAB's control air, IFR's control fuel.

If you were stumbling right off idle, which indicates a lean condition, you went the right direction with the air bleeds. Something else is amiss...

Grubby · #**100** (**permalink**) 10-15-2012, 08:24 PM

Brent,

My primary problem before this change was lean at steady cruise between about 1600 and 2200. That condition is now much worse and the rpm range where it happens got wider. This is the top of the idle circuit range.

IABs control lean/rich at the top of the idle circuit and also how high the top of the idle circuit goes.

The problem of trying to drive down the driveway at less the 1500rpm arose with the IAB change.

Correct me if I am wrong. I think the IFR controls the low range of the idle circuit. If correct idle can't be achieved wth the idle mixture screws then IFR needs changed.

If I put the IABs back to original 70s I would not have a problem in the lower ranges of the idle circuit.

The ranges my problems are in are not even close to the ranges controlled by the HSABs, so I see no need in messing with them.

About the only thing left to do is go the other way on the IABs.

John