Stumble off idle -

[Gaz64]

My bet is the carb idle circuit is borderline lean for the engine.

It WILL idle, but the transition circuit won't supply enough idle emulsion above idle.

With the fuel we have today, the whole carb including the idle circuit needs to be richer.

[decooney]

Originally Posted by Gaz64

My bet is the carb idle circuit is borderline lean for the engine. It WILL idle, but the transition circuit won't supply enough idle emulsion above idle. With the fuel we have today, the whole carb including the idle circuit needs to be richer.

EXACTLY. Transition is lean. Also, one other consideration I thought about is the older and earlier ERA Cobras had slightly more restrictive sidepipes too. I believe the newer ones flow a little better. With the slightly more freeflow sidepipes combined with today's standard (non-HP) 750 Holley, and more blended fuel - I'm betting this is the case; too lean. Especially with 70 primary jets he is using today. Most guys here run 850s/950s on our 428+ FE engines here now days and we don't see these problems like we did before using a borderline 750. I think he could make the 750 work though with some tweaks. Or, throw a Quickfuel 850 on it and be done with it.

[patrickt]

Alright. Grubby, order an 850 of your choosing and put it on there, run it, and if the problem is still there quickly clean it up and send it back for a refund. I hadn't thought of the sidepipe thing... I'm not sure about it, but maybe.

[decooney]

Originally Posted by patrickt

Alright. Grubby, order an 850 of your choosing and put it on there, run it, and if the problem is still there quickly clean it up and send it back for a refund. I hadn't thought of the sidepipe thing... I'm not sure about it, but maybe.

If he can get it to someone with a meter who can check the AF ratio (before buying another carb), might be money well spent. Would be interesting to know if the problem is better or worse in the summer heat vs. cold winter. Just recently came across a brand new Holley HP 750 that had casting flash on the base plate partially blocking one of the circuits; i.e. if everything is not perfect, little issues like this can pop up. A good carb blueprinter can find and fix his 750 and make it work better than it does now but its all time any money.

Note: Phil at Quickfuel is a good resource. I've used Phil a few times to validate my own assumptions. If Grubby provides all the engine/car specs, Phil at QFT can get him pretty close. They have spec'd out QFT carbs from 780 to 850 for FE engines set up like Grubby's. I like my own Holley HP 830 but may move to a Quickfuel 850 next. Super adjustable and better quality.

[Grubby]

Duane, Patrick and Gary-

OK guys I am feeling the love.

I used the Holley tool to determine correct carb size. If I choose "mildly modified" and 6000 rpm redline I get 750cfm. If I choose "highly modified" I get 850cfm. Clearly this engine is on the edge and I suspect Holley would error on the small side to make the car the most drivable for your average guy that just bolts it on.

Holley Interactive Carburetor Selector

With the definitions below my engine could fit into highly modified except my compression is 10.45. That means Holley suggest a 850cfm.

"Mildly modified - performance camshaft, performance intake manifold, headers and ignition upgrade

Highly modified - aggressive camshaft, performance intake manifold, headers and ignition upgrade, aluminum/ported iron cylinder heads and 11:1 or higher compression"

There is a engine tuner locally that comes highly rec. I am going to drive over and see him this week and see what he thinks. I would gladly spend $100 to "fix" my 750 vs $600+ for a new 850.

Duane - I would love to call your QF guy, but I don't have cam specs to give him. Do you think he would be much help with the information I have given you guys? Engine build sheet shows ultra Dyne cam with 108 deg cam ICL. No idea on lift.

John