carb pros

[91 foxbody]

Engine specs are listed in my sig. I have a problem with running rich. Dark smoke at idle and fouled plugs. Anything after 3000 rpm is no problem. But at idle and when i first hit the gas it smokes. I did a compression test to make sure no problems with rings or valves. All cylinders were between 180-185 psi. The HP 750 comes stock with a 6.5 power valve. Holley said to take vacuum reading @ idle and divide that number by 2. My vacuum at idle is 5.5 psi so i put a 2.5 power valve in and still have same problem. Never had any problems with car idling. Rpm at idle is around 1000-1100. The car runs great and is very fast. But is burning so rich at idle it makes my eyes water. Even with the set-up it is not bad on street, have never at it cut off on me. Trying to fix what i can before i get a dyno tune. And i am not sure that will help my rich problem. The car was built to scream but cant always do that on the street.
Thanks

 

[93 LX]

To start vacuum is measured in inches not pounds. You only have 5.5 inches of vacuum? WOW thats low you must have a huge cam in that thing. What jets do tou have in this carb? Is it mech secondaries or vacuum? Where do you have the idle screws set? A good starting spot st start it bottom out the screws and then back them out 2.5 turns. Screw in or out no more then 1.5 turns. If you have to go more one way or the other you need to rejet the carb.

Allen

 

[91 foxbody]

My mistake. I meant inches. It is a custom grind cam. Specs @ .050. Lift is 588/622and i have 1.7 roller rockers, not sure how much that increases lift. and duration is 246/249. Not sure of the jet sizes. Thanks for the tips. Will try that this weekend

 

[91 foxbody]

Forgot to add. The HP 750 has mech. secondaries and no choke horn.

 

[85_SS_302_Coupe]

For starters that's way too much carb even for a .30 over 351W. Those HP (i'm assuming you mean a Holley HP series 4150?) are nasty carbs and often run way more cfm than what they're listed at. It's probably jetted too rich for your combo from the factory.

Contrary to popular belief, bigger is not better when it comes to carbs. You'd probably be better off with a 650HP or even a jetted up 650 double pumper. Now, maybe if you were running big ass 2.20 valves on a 400+ci stroker, you could probably use that much carb...or if you run lot of spray and would need to run richer.

 

[91 foxbody]

It is the HP holley 4150 with mech sec. I have a bottle mounted and a BG fuel system hook up with a purge. The fuel lines are ran the i have all the wiring running through relays to switches inside car. I am just waiting to find a nice used wilson plate and plan on running a 150 shot. So i will need the carb for the track. I dont drive the car very often. It is more of a cruiser with occasional trips to track, may i can use smaller jets if plan on running street and when i go to track step the jets back up. The car runs awesome, it does not overload. Just at idle and when i am cruising at anything below 3000 it is running very rich. May be i cant have the best of both worlds. With my engine, tranny and suspension it was built for the quarter mile. It runs rich at idle, surpisingy it never cuts out on me at stop lights. I have longtubes with 3in collectors, 3in x-pipe going to flowmasters and the pipes are cut out about 3 inches behing the mufflers. I probably need to run the pipes to the back, maybe that would help on fumes.

 

[85_SS_302_Coupe]

You should try to find someone local with a 650 and try it out and see if it runs any different. The thing you have to remember is that 750 is just a cfm rating...the problem is it came with jets too big for your setup. If you jet it down, you're gonna get really soft throttle response because it'll more or less be tuned lean...even though it'll be the right amount of fuel, it'll be too much air.


I bought my 650dp for a 351 that i never ended up building, and i'm sort of in the same boat as you, only it doesn't run rich enough to foul out plugs or smoke, but the throttle response isn't as crisp as it could be with a 600.

 

[93 LX]

I have a 700 DP on my 82 Capri and it runs way better then it did with the 650. Oh wel I guess its all int he setup and tune.

Allen