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Discussion Starter · #1 ·
My carbed 408W cobra has a fine air fuel ratio at WOT but the plugs seem to foul pretty bad within 100 miles or so. Any ideas what could cause this?
The floats are set correctly.
I have a BIGs 950CFM carb on it.
Stage 5
http://www.bigsperformance.com/productpage.htm


Roger Ver
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[ June 03, 2004, 02:56 AM: Message edited by: Roger Ver ]
 

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Roger,

Have you had the car dyno'd yet, as this will tell you where your going to need to work on things.......For the 408W stroker, I suggest no more than a 700DP(believe it or not, probably a properly dyno tuned 650DP would do the trick).......950cfm carb on an engine like yours (unless your running 12:1 compression) is way too much carb for the motor.............


Bill S.
 

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Discussion Starter · #3 ·
I have 10.75 CR
AFR 225 HEADS
Huge cam
1 7/8 primary 4 into 4 headers

I had it dynoed and it made 424 RWHP
and the AF ratio was about 13:1

The whole setup was chosed and the cam is a custom design by the same guy that designed the wining combo for the mustang true street class drag racing leauge.
 

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Roger,

I agree with Bill S., the carb is way to big for the engine unless you run it at WOT all the time. For good street manners you should size the carb smaller than you would for maximum performance. A 650 to 750 would seem better suited for the combination.
 

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I'll add my nickles worth and agree with Bill and Speed merchant. You're running a race prepped carb on a street car. This carb seems like it was designed to work best on the strip, not the street.

I use to have problems with my 750DP fouling plugs on my 351C until I leaned out the idle mixture by drilling a a small hole in the butterfly valves. I also re-jetted the primaries, going to a smaller jet size.

Jim
 

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Roger,

Drag racing vs street are two completely different setups. Sure, at WOT the engine runs great, as any drag racer will tell you that you only run at WOT for that 1/8 or 1/4 mile at a time.....However, for any type of street use, I'll stand behind my original suggestion of a 650DP at most...Anything more than this will actually hurt your engines performance for anything but drag racing............

Once again, I hope you find this helpful.

Bill S.


Originally posted by Roger Ver:

The whole setup was chosed and the cam is a custom design by the same guy that designed the winning combo for the mustang true street class drag racing leauge.
 

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Here's a pretty good article on various carbs. It pretty much backs up what you are hearing on this thread. Carb shootout

Another good article on carb sizing shows pretty much the same thing. Carb sizing This test was with a chevy 383 and the 750 holly HP was about the best. The 600 CFM only made a few less HP with great throttle response. your engine is only about 6% bigger so I think the 650 to 750 would work best. As a final note - from the Holley website

"When in doubt, select a smaller
carburetor size because it will
typically give better acceleration
times - even though power may
fall off slightly at top RPM. You
can believe that you'll be happier
with the smaller carburetor
nearly every time!"
 
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