In order for the engine to rev that high; It has to be getting quite a lot of BOTH air AND fuel.
An open PCV, open booster port, or a leak at the intake to head gasket, will only pass air, so the engine usually stalls at or near idle. Some carbs pull air down thru the air-cleaner stud hole, so make sure the stud is in there.
With ALL FOUR, CLOSED or NEARLY closed throttles, and assuming the bowls are not leaking directly into the engine; there are only a limited number of places that fuel can get into the intake.
But if it all looks good on the bench; namely the secondaries closed up tight but not sticking, and the primaries opened up just far enough to expose the transfer slots to make them square to a little taller than square, then after it is mounted,make sure that NONE of the throttles hang open on any gasket.
BTW
A 2bbl throttle bracket will not allow the primaries to close, with cable anchored in the usual way, clamped on the metal sleeve. If you clamp it on the black part, you're just looking for trouble.
If your carb does not have a throttle-shaft clocking spring, then you should run two return springs; one inside the other.
BTW
My engine has run at 7200 rpm for as long as maybe 10 seconds atta time, hundreds of times;
and normally goes 7000 for 6 to 8 seconds atta time, for thousands of times;
since being screwed together in 1999.
It doesn't seem to have bothered it. 4000 is just kicking back. 5000,just a walk in the park.
The afterfires that exploded your mufflers, were caused by an explosive, ready-to burn mixture. With a closed throttle, IMO, this can only happen if the exhaust is sucking fresh air somewhere, and the most likely place is at the head flange, with the collector coming in second. I don't think it can suck enough air thru the base gasket or thru worn-out throttle-shafts. But an open PCV or booster port would do it.
In any case
Happy Hunting.
