good point, also is it vibrating if you rev it out of gear? Put it in neutral and try to get the desired revs. If it vibrates then at least you can eliminate a couple of things

You say you have tried timing changes, have you removed vacuum can from distributor and regraphed it to suit the motor? Without the correct advance curve you could be doing serious damage to your motor at those rpm.

What about fuel flow and pressure, do you have a pressure gauge hooked up at top revs? You want about 2-3psi at idle never dropping below 1.5psi at high revs. If it does drop you need more flow ie. larger pump or 2 inline.

Hope this helps.

yeah it is right, sits at just under 4000rpm at 120 in 5th gear

Got video of the 1/4mile run somewhere, motor only had 300km's on it, did a few 15.1's and a 15.2, mus still take it to the track reving it to 6000rpm

The car is very quick, just needs some extra RPM now

Quote:

Assuming the 15.1 referred to is 1/4 Mile time that sounds pretty unlikely if revving to only 4500rpm. Are you sure your rev counter is correct/accurate? or Maybe its set for 6 cyls and 4500rpm is actually 6750rpm... but then 6200rpm would therefore be ... something horrendus!

Thanks Jmac

Just called the cam grinder, he didnt know what seat pressure was needed... these are guys that do cams for most of our racing industry he also told me now that it needs .4mm clearance not .3/.35 like I was told when they cut it...

Will check the seat pressure tonight, do any of you know if the shim is a fairly common size id/od? going to change the shim with the head on, got a tomei head gasket in there would rather just use the slight pressure method to hold valves in place.

Does .4mm rocker clearance sound right? sounds a big much?

Wish i had the money for a nice Tomei 304deg

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.

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

Intake ports are 34X41mm

Lobe center is aparently 104deg

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.

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?

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.