Hey guys,

My built A15 is now fully broken in, seen a few track days and ran a 15.1 shifting at 4500rpm, and generaly runs beautiful under 6200rpm in my B110 4door..

Now comes the problem, as soon as I try rev her over 6300 I get an odd sound, almost like a missfire/reverbaration/ valve bounce and she stutters slightly then clears up and then at 7000rpm will not rev any higher, almost like a rev limiter.

in the A15 I have 11.5-1 comp, 280deg cam (.3mm intake and .35mm exhaust clearance), tomei dual valve springs, Tomei M-spec port layout (copied from one of the A13 touring motors) normalish header, 52mm exhaust, pertronix dissy pick up, MSD blaster SS coil, dual DCOE40's and a nice 3kg cromoly flywheel...

I have tried timing changes, plug changes (everything from range 6 to range 8) as well as jetting changes from 10.5-13afr at WOT, nothing seems to change the sudden sound at 6300rpm then total drop of power at 7000rpm

Here is the million dollar question:
1. Exhaust back pressure is to high
2. some form of intake reverbaration
3. Faulty pertronix?

Any idea's guys?

Video link

Took a little video, you can hear it top of 2nd gear right before i let off...

Are the valve clearances correct. I had an A14 that did the same years ago, turns out we were using the wrong gaps. You said you;ve just run the motor in,now everythings settled down,maybe the clearances are down.

Perhaps valve clearences,
IN .30mm (11.8 thou) sounds too tight
EX .35mm (14thou) sounds right
I have run .35mm (14thou) on both IN +
EX on hot motors with success, but don't
ignore instructions on your cam card either.

The specific rev range makes it a bit more
mysterious, almost like the carbs are not
making one of their transitions smoothly

Intake Reverb is usually assosiated with big
overlap on the camshaft, what lobe centre
is it?

just a thought

Edit: Are 40mm carbs big enough for what
sounds like a decent A15? What size chokes
are you Running? Could the thing be running
out of air at the top of its rev range?

40's would be fine for a rev range quoted.
Even the hottest of A15's would only require chokes of ~ 36mm size. 32mm - 34mm would be perfect for an all rounder.

i had the same problem with mine, i had done a similiar build, and my valve clearances were rooted, once they settled in i had to reset the gap.

Datsfullysik, what clearances did you run eventualy?

I run 35mm chokes, had a bit of a hard time with mid range air fuel not wanting to play along nice on the 36 chokes so I went back to a 35mm.

I run a local cut cam, not the best, not the worst but they dont give you a cam card! ... I also thought the clearances sounded a little tight. So most of you guys with warmer A15's run .35mm in and out?

Did you measure the spring rate before installing the springs? They may be used/worn out or counterfeit...and not actually the correct spring rate. Or perhaps installed at the wrong spring height spacers.

they were brand new in box tomei double springs... direct from tomei USA.

Intake ports are 34X41mm

Lobe center is aparently 104deg

everyone else has beat me to it, but based on the slight detectable change in note near the peak rpms, I'd put good money on either valve springs being too weak, or ignition starting to fall down.

If it was exhaust backpressure, it'd slowly fall down as rpms increased, same as for the carbs, this is more abrupt, so either the spark is breaking down, or the valve springs aren't being kept under control.

Rough and ready check - try reducing spark plug gaps down to around 25thou (imperial) -which is technically 'too narrow' for most apps, but just try and see what it does. If it doesn't improve whatsoever, it's almost certainly valve springs.

It's possible that the springs are 'right' but that in order to get the right seat (and by extension nose) spring loads right, they need spacer/shims under the bottom of them. There's really no good rough n ready way of checking. About the only thing you might try is to try (when the valves are seated) grasping the retainer/top of the spring on each in turn, and see if you can twist them around. Given the cam and it's likely rpm range, they should almost certainly have enough seat tension that you can't rotate them by hand, even when on the seat.

If they can be, bite the bullet, get the head off, and get them checked and re-installed at whatever height they need to be for the correct pre-load (which your cam supplier should be able to specify based on the cam they sold to you!)

The engine just sounds too strong for it to suddenly run out of air or have a restrictive exhaust.