Engine falls off at high rpm

[Vladik41]

OK here we go, 302 long block from Blue Print engines with the e7 heads, Typhoon intake, holly 4150 600cfm single feed with vacuum secondary, mechanical fuel pump putting out 7 psi constant, HEI distributor with vacuum advance, T5 xmsn, 8.8 rear end with 373's . Car starts and idles perfect, Drives down the road great with good power, timing set at 14 deg at 800 rpm and 34 deg total at about 3500 rpm without the vacume advance hooked up. The problem is when I REALLY GET ON IT at A start or going down the road at any speed the car revs good to about 5500 rpm and then falls off fast and starts to run rough and has no power for a few seconds then starts to gradually smooth out after throttle is lifted. Put fuel regulator on car to check pressure which is steady at 6-7 psi. vacuum off the carb for the advance is strong at 25-35 hg. Help. I think its a distributor problem but not 100% sure, any Ideas would be very welcome.

[Gordon Levy]

You are running out of cam and heads. You did not give profiles but if you are using stock cam or one of the Ford letter cams such as b,e or f they quit making power between 5000-5500 revs.
As for the high rpm missing for a few seconds, you may want to take a look at the intake and AF ratio, you could be puddling fuel in the intake.

[NAZ]

"...starts to run rough and has no power for a few seconds then starts to gradually smooth out after throttle is lifted." That sounds like lean mixture at WOT and low vacuum. Of course if you see black smoke out the exhaust when this happens you know it's way too rich. If you simply installed a carb out of the box with no tuning I'd bet dollars to donuts you have a A/F issue. Possible A/F issues are easier to troubleshoot with an A/F meter but there are a couple of things you can look for. First make sure the floats are adjusted properly, don't assume a new carb is set-up correctly. Too low will cause lean mixture at high speed. Now the road test. Is the secondary side actually opening during this condition? This may require removing the hood and using a welding rod or coat hanger attached to the secondary linkage so you can see the linkage operate. It would be helpful to get manifold vacuum readings during your road test. If the condition only presents when at WOT with the secondaries open (and no black exhaust smoke) you have a lean mixture when the secondary system is in operation -- not uncommon.

[Vladik41]

Found problem, The floats were way way to low. Adjusted floats and problem went away. Installed wide band o2 sensor and found I was running way to rich, adjusted fast idle screws and got fuel ratio to 14.00 and it also helped a lot. thanks for all the help.

[mikeinatlanta]

My first thought when I saw the tread title? I hate when that happens.:)


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[NAZ]

Glad to hear it's better. Be advised that 14.0:1 is still a little lean. The stoichiometric ratio for gasoline is 14.7:1 but best power is usually obtained between 13.2 & 12.5 to 1. My experience is you will feel a big loss in power quickly when the A/F ratio goes lean, more so than a bit too rich. If there are any pilots out there reading this they may be noding their heads in the affirmative. Yes too rich will also rob power but it's not as exponential as a lean mixture and many times you won't feel it in the seat of your pants. If you have an O2 sensor and A/F meter have some fun with it and start tuning on that carb to optimize power. The needle valves used to set idle only affect the idle circuit. Once the carb has transferred to the main circuit the idle screws don't contribute much to A/F ratio. So hook up that A/F meter and go drive that hot rod and start tuning on that Holley 4160 carb. But if you're new to carb tuning go out and buy the Holley Carburetor book. It's fun, easy, and a good skill to learn.