Stumble off idle -

[decooney]

John,
I think QFT could get you very close on the first or second try with a few adjustments. If you have a really good local tuner who knows Holley carbs well, it would be worth a discussion at least. Our local tuner here truly knows Holley; sort of blew me away. You could do a Holley HP or a QF carb, but from what I just went through with QF, their carbs are a level in design, material and build quality above the Holley HP in my opinion. In addition QF will build you a carb from scratch from the custom shop for no extra charge; or they did for my car - you can ask them. I just did this for my best friends Lister project and all new motor. Your motor is close to mine in setup only you have a tad more cubic inches. What we all have to remember here is most of these places base specs on heavier street cars. You are running a 2550lb or so Cobra with fairly open sidepipes and I'm assuming periodic vs. daily use. You could even possibly get by with a custom built Quickfuel 750 (they are a different beast), but I'm thinking Phil will tell you to go with an 850 for a Cobra performance application. He did for my car. They use a combination of different throttle size bodies mixed with specific venturis so you don't lose throttle response and you get more flow. We did have to do some fine tuning on my buddies QFT 750 but they helped us over the phone and email. Its interesting, the metering blocks and what they are doing on the QFT carbs is really nice. Just the billet fully adjustable metering blocks (don't warp) alone are worth it. You could call the Holley Tech line and ask the same questions as QFT to get a good feel for both, before you see your tuner. If you were not so far, I'd send you my own Holley 830 HP to try it. Let us know how it goes and what model QFT or Holley anyone recommends to you for your particular application.

[Grubby]

My cam is Ultradyne 305000 - all I can find with google is that it is a custom grind.

Duane - can you send me Phil's contact info? Also, do you have a mech secondary or vac?

I plan to shop and see what a few different experts say and then go with the guy that gives me the best feeling.

I will let you guys know what happens.

John

[patrickt]

That would be nice if you could try out someone else's larger carb. You know the size differential is up there in the higher RPMs (not down where you're having the problem) -- but it's only a few hundred bucks either way, and you can always recoup a good bit of that regardless (and ERA Chas drops that kind of dough just on dinner), so I say "roll the dice" and see if the bigger carb fixes you up.

[decooney]

John,

Sent you a PM for both Ultradyne and Quickfuel Technology. DC

[Gaz64]

John,

My mates 355 runs a QFT 750 DP (my choice, I built the engine), your engine could quite easily handle a QFT 850. You won't look back,

[blykins]

I would recommend an 850 as well.

If you're looking to upgrade, let me know. We custom build our Quick Fuel carbs, specific to the engine.

[Grubby]

Brent,

I will call you next week and discuss.

Patrick,

I was a driveability engineer in the 90s with GMC Truck. I understand difference between a 750 and 850. My theory is the idle circuit is slightly lean and it can't keep up during the transition to main metering. Further, it is likely that max HP is being limited by the smaller carb. The HP is really a secondary issue since I don't race it.

John

[blykins]

A 750 would also work for this size engine, although IMO it would be down on horsepower. If you were to use a carb with basically infinite tuning, you could tune the transition a lot easier. The higher end Holley and QFT carbs have adjustable IAB's, HSAB's, IFR's, PVCR's, etc.

However, on 427's and up, I generally use 850's.

[decooney]

John,

This diagram helped me when I ran into this situation on the Lister COOX motor replacement project. Formerly we had a Holley HP DP on the prior 377 motor with a moderate cam, it worked okay but had a mild stumble at about 1500rpms. With the new 400ci dart motor and slightly larger cam we knew the former Holley HP DP 750 was not going to cut it, and did not want to risk running the new motor too lean. The thought at that time was a QFT 750 of the same size was "much more adjustable" so we tried it. Ugh. The lean stumble was there again on the 2nd 400ci Dart motor even with a stock Quickfuel 750Q carb stumbling and bumbling at 2100-2500rpms. Originally, Quickfuel said we could make a 750 work (being conservative), but it would take some jetting and metering plate jet changes to go with larger the idle feed restricters. In the end, we got it to work. However, reflecting back, they oringally recommended an 850 and I could have gone with that. We eventually got the 750Q to work well. However, if I had it to do again, I should have taken their first recommendation which was the 850. This was with a 400ci 10.25 compression motor with .248 cam at .050 and free flowing sidepipes just like the Cobras. Originally I was afraid the QFT 850 would be too much but later learned the whole carb looks similar to Holley but it is a different design in terms of sizing, metering, and adjustability. The only reason I'm typing this is we spent a lot of time on this same situation with two motors. Having the right carb and setup day-1 would have really helped avoid all of this.


__________________________________________________ ___________
If it helps anyone who goes through this in the future, in this diagram you can see where the idle circuit and transition circuit are shared. Even on the new 750Q we had to go with larger and larger idle feed restrictuers to richen the "transition" part of the shared circuit below. On the QFT carbs these are simple replacement jets. On the Holley HP DP 750 we had, you have to drill them out so we did not go there. My next carb will be a fully adjustable QFT on my Cobra.

[Grubby]

I spoke to Ultradyne today. My engine builder buys two cam grinds. I assume the first is mine and the second is used in 482s. The builder didn't remember. My cam is one of the following two with number one the most likely;
1. 235 int / 239 ex duration with 112 deg separation
2. 239 int / 244 ex duration with 112 deg separation

Bigger cam than I expected.

I spoke to my local carb tuner. He said he could put my 750 on his "dyno" and he could fix it. I am skeptical becaus he was a bit vague when I pushed for details. His shop is pretty close to my office so I plan to stop by this week and see if his "dyno" is a flow bench.

Not sure how a flow bench could be useful in finding or fixing a stumble.

I made contact with Quick Fuel for a suggestion and should get something soon.

John

[Gaz64]

Ask your dyno man if he owns a "number drill set" and if he is willing to drill the idle feed restrictions in your primary metering block.

[blykins]

Originally Posted by Grubby

I spoke to Ultradyne today. My engine builder buys two cam grinds. I assume the first is mine and the second is used in 482s. The builder didn't remember. My cam is one of the following two with number one the most likely;
1. 235 int / 239 ex duration with 112 deg separation
2. 239 int / 244 ex duration with 112 deg separation

Bigger cam than I expected.

I spoke to my local carb tuner. He said he could put my 750 on his "dyno" and he could fix it. I am skeptical becaus he was a bit vague when I pushed for details. His shop is pretty close to my office so I plan to stop by this week and see if his "dyno" is a flow bench.

Not sure how a flow bench could be useful in finding or fixing a stumble.

I made contact with Quick Fuel for a suggestion and should get something soon.

John

How about I loan you one to try out? Carbs are not always perfect coming straight from Holley, Quick Fuel, or whoever it is.

[patrickt]

Originally Posted by blykins

How about I loan you one to try out? Carbs are not always perfect coming straight from Holley, Quick Fuel, or whoever it is.

I'd take Brent up on the offer... just in case it's not the carb. And he can send you one that he knows works reasonably well.

[Grubby]

Brent,

Thank you for the kind offer. At this point I will go a different direction.

**

I visited the local Holley expert (Promax) and had a long discussion and shop visit. He uses test mule engines on a dyno to get basic tune on carbs and then experience to final tune. He is confident my carb can be made to work well.

Starting from square 1 he would have suggested a 750 double pumper. His opinion is that with about $175 in tuning work my carb will be very close in performance to a 750 double pumper.

I am going to give him a shot.

He says the 850 would not be as responsive for a street car, and the 750 does give up some max HP.

John

[patrickt]

Originally Posted by Grubby

Starting from square 1 he would have suggested a 750 double pumper. His opinion is that with about $175 in tuning work my carb will be very close in performance to a 750 double pumper.

I am going to give him a shot.

He says the 850 would not be as responsive for a street car, and the 750 does give up some max HP.

John, I discussed this same issue, at length, with the builder when I was having my engine spec'd and assembled. In one sentence, his opinion (which I will just pass on without editorial comment): Dual fours would have the most horsepower (but a TP won't fit over them), a larger CFM single that is greater than the 750 would have the second highest horsepower, but the 750 would feel more responsive, quicker, and more fun on the street. As between vac secondaries and mechanical secondaries, properly tuned vac secondaries were superior in non-race conditions, and when tuned properly, a second accelerator pump was not necessary. Now, mind you, dollars were not an issue on the carb choice. We talked it over, and that's what he thought was best. My engine, with the shorter 4.125 stroke, light pistons, light aluminum flywheel, solid flat tappet, was all meant to be a fun "quick-revver," and that was his considered opinion on the carb. If he had said, "I really recommend the blah-blah carb, but that's an additional $500..." I wouldn't have batted an eye, and he knew that.

[blykins]

I wonder if he's ever driven a Cobra with a 445FE in it? Not the same as a Ford Galaxie with a 289....

[patrickt]

Originally Posted by blykins

I wonder if he's ever driven a Cobra with a 445FE in it? Not the same as a Ford Galaxie with a 289....

I have yet to meet two engine builders that agreed on anything. And I offerred it up simply as one engine builder's opinion, and you know he's built thousands of FE engines, and hundreds for Cobras alone. But, like every other builder, some like him, some don't....

[blykins]

I was talking about the ProMax guy...

But throttle response depends on a whole lot more than the size of the carb. I use 850's on 408's and 427's and with a manual trans car that weighs 2400 lbs, I've never had anyone say, "This engine has horrible throttle response!"

The last 351C I did had 4V heads, a dual plane intake, a smaller hydraulic roller cam with plenty of advanced cam timing, 10:1 compression, and a 750 Quick Fuel carb. It went into a '73 Mustang. The customer's response after he got it into the carb was, "I don't know how you set the cam up, but this engine takes off like a rocket in any gear at almost any rpm. It's hard to bog it down!"

[decooney]

The former magic tuned Holley 780cfm vacuum secondary carbs I originally ran on both of my 428 Cobras don't hold a candle in throttle response and snap-rev I see with my current Holley 830HP double pumper. I agree, taking the next step into the future the QFT carbs are the next level in design and tunability with even more improved throttle response. As I'm finding out, if you ask them in person many old school engine builders and tuners don't know much yet about the new QFT carbs or the story of how two ex-Holley guys identified and corrected the flaws and started QFT. As others have said once you see or try one on your performance car or Cobra, you'll never look back.

[patrickt]

Originally Posted by decooney

As I'm finding out, if you ask them in person many old school engine builders and tuners...

Well, maybe.... But he's not 70 yet, and that is now my official demarcation line between "old" and "young."