[moparcolt]

I have been messing with this set up for a while now and am gaining on it but would like some input,

Car is a 67 firebird, 465 in. motor with 11.5 compression.
Kauffman d port heads
2 600 vac secondary carbs on a offy dual carb intake.
Petronix billet distributor with multi spark and rev limiter.

cam 572 lift 254 duration at .050 110 degree lobe seperation

vacuum at idle is 5 in.
initial timing is 12 BTDC
power valves are 3.5
jets 66
orange pump cams 25 squirters

so initially it was surging at idle and I found that the power valves were 5.5 and I think they were opening and making the car surge from 1000 to about 1500 RPM instead of a steady idle

changed to the 3.5 power valves but it was still not right. Idle screws werent doing anything. so I drilled a hole in each primary throttle blade and was then able to turn the idle screws down and get it to idle better.

The problem I have now, or I guess still is that it is breaking up above 4500 RPM

all the ignition parts are new this year and almost no miles on them.

Could the car be leaning out and causing the breaking up? or do you think it is ignition related? sounds kind of like how a rev limiter does.

pulls great if below 4500 starts good idles good.

Ideas?

[Stuckinda60s]

Fuel pump or valve springs. My guess would be the pump.

[screamingchief]

Do you have the vacuum pods using a common vacuum source?

FYI when using Holley VS carbs on a dual carb intake like that,you're supposed to use vacuum secondary "balance tube" so that both secondary pods see the same amount of vacuum so the secondaries operate together properly.

http://holley.com/20-73.asp



Not doing so can result in uneven secondary operation for one or both carbs.

And I assume these are still 4160 carbs right?

Any mods done to the secondary side yet?

What secondary metering plates are in those carbs?

And what springs are in the secondary pods?

Probably should bump up the initial timing some too (and limit the total timing if necessary),that combo will like it.

Anyhow,without a doubt,whatever the problem is,it can be fixed,just gotta figure out what needs to be dealt with first.

The more info you can give us,the better the chance is that we can help you figure it out.

HTH

Bret P.

[moparcolt]

I have not messed with the secondary jetting or springs, didn't know about the balance tube conversion, but I see summit sells another kit that has a spring assortment with it as well.
http://www.summitracing.com/parts/CMB-03-0101/

the plates arent cheap and I will need two so where do I start if I need to change them? and how do I determine if they are an issue?

Thanks,Chris

[moparcolt]

I see the plates can be modified to accept jets as well, is this a good option?