383backinblack

im running into an interesting problem........my f1R blown 383 runs hard and strong at a nice steady 10.8:1 under boost......until appx. 5200rpm......then it starts breaking up, and reads lean around 12:1

heres the kicker though........it will do the same thing at rest with no load........so it can't be a fuel delivery problem (i.e. the pump/regulator) because theres no boost at no load conditions, so its not a boost reference problem......obviously it does it worse under load, but it also does with with no load.

im thinking air bleeds, but it could just be the primary jets can't keep up anymore after that RPM........i would suspect the whole curve would be FUBARED if the air bleeds needed teaking, not just stepping from dead-nuts to lean at a specific point......i say the primaries because it does it with no load, which doesnt even touch the secondaries

its a holley 950HP blow-through carb from the carb shop........at that point were looking at about 16psi or so of boost.....

any ideas?

Elephantismo

I would still put a guage on your fuel line and see if you get a pressure drop @ 5200rpm. If it holds constant pressure throughout your operating rpm, i would start jetting it up. If you dont have a holley tuning manual, i would get one. Theres a lot to be said for them.
It still could be your front, rear, or both end's jets.

The thing that gets me is that it does it @ 5200rpm with and without load.... does make it sound like primaries maybe. But with load and bost it would run out at a lower rpm because your increased air velocity with boost means it uses more fuel at a lower rpm. The fact that it does the same thing with and without load and hence boost suggest that either the charger is still boosting with no load and it has a jetting problem OR the problem is elsewhere. You might be looking at an ignition system problem. If it retards(or quits advancing?) at 5200 it might lean you out.

1978LT 406

Could the valvesprings possibly be weak

383backinblack

i hope not they're brand new lol

Damon

I think you're going to find it's not a carb problem. Given the tests you did I can't imagine how a carb could do that at a specific RPM both under load (WOT) and not under load (probably 10% throttle). You're using totally different throttle positions for the 2 tests. And carbs know only airflow and throttle position- both of which are drasically different in your 2 tests. A carb doesn't know anything about the RPMs your motor it spinning.

Only 2 other things on an engine know EXACTLY what RPM you are turning, regardless of load, throttle position or anything else:

1. Valvetrain (primarily valvesprings)

2. Ignition system

Given that you are showing lean on the O2 up high (and like throwing a light switch) I think you're getting unused Oxygen into the exhaust above that RPM point. Not becuase the carb isn't supplying the right mix, but becuase of incomplete combustion or valvetrain issues. Misfires throw unused O2 out the exhaust. And valve float can let intake air (unused oxygen) pass directly into the exhaust, plus it can cause misfires of it's own. Either way, unused O2 is making it into the ehxuast somehow. Ignition and valvetrain are the usual suspects here.