Engine Question, dead til 3000RPM, ideas???

[chevy2racr]

ok, dad and have been workin with an 85 camaro all summer and we've fixed some quirks but theres one thing thats got us puzzeled. it a 355 with a Holley 750 Vac. 2ndaries. covette LT1 hydrolic lifter cam, headers, yada yada yada. has a MSD Digital 6 ignition. msd distrib..... and so on,well if you put a timing light on it(have a MSD light that reads as a tack as well) it never stays still it will be idoling and it willsay500RPM to 2000 RPMwithout any change in ga and it sounds like its idoling fairly steady, little rough, but nothing TERRIBLE. and according to that timgin light its timed somehere around 35 advanced, starts hard, but if we turn it so it should be timed closer to 20 on the road it pops back through the carb and runs like sh*t. so turn it back to where it was and the main problem still remains, if you lay into it it is slugish and jerky til it hits about 3000RPM and then it kicks ur a$$ back into the seat like none other. dad had te idea we needed a differnt dist. gear with the cam. ? anyone have anyideas??
anything may help.. thanks
Bryce

[chevy2racr]

heres the Camaro

[breeh]

Are you running your vacuum advance from a ported source on the carb? If so, try changing it to full manifold vacuum source. I did this on my Camaro and it made a big difference. The other thing that I did that made even more difference was to recurve the mechanical advance in my HEI with lighter springs. It was the difference between night and day. Here is an excerpt from an ignition guy off of the Camaro board that may help:

Now, for the rest of us: Vacuum advance.
Ported vacuum advance was, in fact, used by many makers of performance and stock engines in the past. Reason, the fuels were so good that the burn temperatures of the combustion chambers actually lowered as upper rpms were reached, and the added timing from ported was needed to heat the burn properly. In those days, the amount of degrees added in the cannister were fairly large, with some engines having 20+ degrees, but this number was usually not attained, as the total amount of vacuum added by the ported port was lower than that needed to get the vacuum advance to pull all the way in.
Today's pump vended fuels, and their additive packages, or lack thereof, have just the exact opposite of the earlier fuels, they always burn much too hot, even stuff like the Sunoco 280 have this problem. Add vacuum advance into the lace most succeptable to detonation, upper from mid to upper rpm ranges, and you can take detonation to the bank, you're gonna get it.
The only time today that ported timing is beneficial, to a point, is on Exhaust Gas Recirculation designed engines. These engines recirculate partially burned gasses from the exhaust system into the intake tract again to be re-burned and reduce emissions output, and the partially burned fuel needs the added combustion chamber heat to more completely burn the exhaust fuels recirculated.
In today's non-EGR performance engines, stock to performance, we do not require any upper rpm added timing for burn temperature control, in fact, it is detrimental to proper running.
So why use a vacuum cannister at all? Well, there is a simple silver lining to vacuum advance, engines as we are working with like timing at idle in the 18 to 24 degree range, but this isn't all that easy to do with some parts packages. How do we do this? Well, first, we need to think of the vacuum advance as not that, but an idle timing supplement to the initial timing.
As we now know, the idle timing can now be raised to a decent level at idle, to help with a stable idle, pull against a bigger than normal cam, pull against a converter and help cool the engine better from more complete idle fuel control. There is a catch, though, too many degrees of supplemental idle timing can be very counter-productive.
If we use the 24 degree figure, we can do things like set the initial timing at, say, 14 degrees, starts easily, then add the 10 extra with the cannister. OK, how do we get the 10 crankshaft degrees into the 20 degree cannister? We use either a Crane scroll plate to stop the advance pin that conects the diaphragm to the point/pickup plate to restrict plate movement down to the number of degrees we wish different from the static initial timing and the total idle timing.
OK, now that we have the number of degrees restricted, do we want to be able to change the vacuum level the canister comes in goes out with? Yes. We do this in two different ways, we look at the information of the stock cannister pull rate, and select the proper one for our vacuum levels, or we add the adjustable cannister from Crane, with the scroll plate that comes with it. Adjust away, you will find the correct amount of added timing for idle to make your engine run correctly.
Well, doesn't the idle added vacuum timing stay in the system when the gas pedal is hit? No, actually. It is immediately dropped as the off-idle vacuum goes away, and the centrifugal advance acts as usual . Only 180 on this is an engine that has no idle vacuum from a realy radical cam duration, and then, the vacuum actually rises after the engine is brought off idle. We just wouldn't use any kind of vacuum advance on an engine this radical anyway. Static idle timing would also be jumped up to 18 or so with the redical engine anyway, to try to stabilize the idle quality. Once again, adding initial timing helps the idle, just depends what you have available to help.
All HEI distributors manufactured for GM vehicles as production ignitions have way too much advance, mechanical degrees and vacuum degrees, and usually cause mucho problems in our performance and stock engines until they are set up correctly, and have full intake manifold vacuum sourced.
99 percent of all vacuum advances have way too many degrees of timing available and need to have the total number reduced to work with full manifold vacuum, as the manifold vacuum will pull all the degrees available into the cannister, not like the partial vacuum pull available with ported vacuum sources does at rpms.
Now, to dispell some myths about dyno tests and ported vacuum advance. Dynos always use more jetting to keep the chamber, engine and water temps cooled off, and ported timing just counteracts the rich mixtures of the rich jetting meeded for the dyno. In all the experiences I have had with crate and built performance engines, ported vacuum advance has been simply not productive, no matter what the engine did on the dyno.
If you are forced to use ported vacuum for your supplemental timing surce, then the curve and initial are not correct, and/or the total amount of vacuum supplemental timing is too large foruse as an idle supplemet from full intake manifold vacuum.

[Gee_Emm]

Does the car have a catalytic convertor, from what you descibed in the sudden burst of power and no power at low speeds it sounds like a clogged cat

[hanssolo]

breech- excellent read. Vacuum advance seems to work for an emissions engine, but IMO its unneccessary for a pre-emissions engine. I elongated my mech. advance on my HEI to 34 degrees and have great performance. Pure timing lead tied into RPM. High compression and no pinging.

[chevy2racr]

it doesn't have HEI or vaacume atvance, no catalitic in it either and what about the fluctuating tach on the MSN timing light? the timing light works, we had it on the drag car and it held steady. any other suggestions?

[chevy2racr]

ttt???

[breeh]

Sorry, it didn't click when you stated that you had MSD stuff. When you described the problem it just really sounded like ignition problems. I know nothing about that MSD system that you are using but I would still investigate the ignition system.

[RON WOODGEARD]

My guess is, If you recently changed CAMS, you got it off a tooth or so, or if you didn't just change cams it's slipped a tooth or so.

[chevy2racr]

you sayin you think my old man got the Cam degreeing off a tad? but would that explain the "jumping" tach on the timing light and the kick ur a$$ power at 3000RPM?

[slam33]

That was my first thought too. Sounds like your off a link. If thats not it, then check your MSD hookup.

[chevy2racr]

well I doubt its teh MSD cause we started off with a HEI and it did that to...

[RON WOODGEARD]

Whe you get the cam off a tooth or so they do some weird stuff. So we did get one thing straight, you did a cam change. How about lifters? what kind? are they too tight?
This along with the cam off would explain why it's jumping RPM.

[chevy2racr]

Yes, we did a cam change, the motor was a GM Performance 300HP 350 that was in our 83 Z for a while, and set, tried starting it one time and somehow water got into the odd cyclinder bank of the block. when draining oil there was water in the pan. had a busted rod too. all that from a starter! so we tore it down bored it .030 and have flat top pistons in it and a new Covetter hydrolic roller cam in it. we've adjusted vavle lash already and that did help some.. but it still is differnt. I just wanna make sure everyone know that the motor doesn't seem to flutuate RPM while idoling as bad as the Timing light tach feature says. it seems to idol aroune 1000+ or - and the tach on the timing light say 500 to 2000 within a couple seconds when it is just idoling. everyone still thing cam?? cause in a 85 camaro it will have to have the motor pulled to fix that. thanks for all your insites so far too.
Bryce

[tom hand]

If the cam timing was retarded it would give greater top end power at the exspence of low end torque. I am not sure what kind of tach you are using I have never seen that kind. Is it digital? If it is, it takes a second to stablize, and if the rpm changes before it does it loses its mind. I bet if you put an alalog tach on there it is only changing a few hundred rpm.

[chevy2racr]

the tach is a digital feature in out digital MSN Timing light, but it flutuates constantly, we put it on the nova and it holds real steady...

[73stepside]

In your first post you mentioned that you had a lt1 roller cam in. I think you have to change your distributor gear over to bronze. At least for aftermarket cams anyway. I'm not sure about factory cams though.
Have you tried any other tachs on it to see if it still does it at idle?It could be that the cam could possible not mtched right for your set-up. FI cams work good with FI but unsure of carbed motors. I do have to agree that you may be a tooth or two off.

[chevy2racr]

we have the bronze gear on there. if the cam is off a tooth or two waht way would it be off, the cam rotated forward or back?

[RON WOODGEARD]

The cam and timing gear have marks on them to line them up with, When you are "DEGREEING" in a cam it takes a special GEAR that you have the ability to advance and retard using small bushings. if you do not have that gear don''t fret. just line the tabs,circles or slightly drilled holes up. The cam one and the crank one should be lined up straight up and down!! If you were to lay a straight edge between the center of the crank hub and cam hub the straight edge should intersect each one of the marks. THEY HAVE TO BE IN LINE!!!!!

Now how they act, if you've retarded it it will be very BOGGY on the BOTTOM end and turn on in the upper RPM, If you've advanced it it will do just the opposite. And depending on how far off you are, it will back fire through the carb, POP and just run like SH**.
On my 350 I use for pulling I set the cam 2 degrees advance on purpose to get stronger bottom end.

RON