Ripped off the cease fire and went to a holley 600 + HEI. Set timing with vac advanced disconnected to 10 degrees. As soon as you connect it, RPM's surge and advanced goes way high(~36* at idle). Under part (<50% throttle) it runs fine. When you go anything over, it falls on its face, starts sputtering etc. Any suggestions on springs? Was thinking about putting in larger weight springs + an adjustable vac advanced. Thoughts?

 

You may be better of going to a distributor from a pre fuel injected 350 chev, bypass the electronic system that should solve your problems.

 

"sputtering" generally is fuel -starvation, or overloading. the timing is supposed to go to maybe 50* with vac advance attached. should drop back down as soon as you open throttle, if you have it hooked to a non-ported vac source. too much advance will cause spark knock, but not what you describe. I assume you have no connection to the EST.

look down the throat of the carb while opening throttle. you should see the spray into the venturis, also the accellerator pump action, if carb is working.

 

 

It sounds like you DO have an HEI for a carb, right?? Like stated above your problem sounds more like a carb problem, what fuel pressure are you running, assume you have about 6 psi for the holley??

 

Sounds like you DO have an vac. advance pre-injection HEI, correct?? How much fuel pressure are you running, Holley needs 6 or less. New carb or used?? Pull a spark plug and see if they are fouled from too rich of a mixture. Are you using a fuel pressure regulator and if so what kind??? I think your problem is carb or fuel related too.

 

I'd eliminate the vacuum advance and use light springs and heavy weights on the mechanical advance. If you really want to keep the vac advance hook it to a port up high on the carb.

 

The vacuum advance signal should be ported. At idle, there should be very little vacuum and your advance and idle should not rise significantly. Look for the ported signal from the carb.

 

Yep.

The port you need is in the passenger side of the metering block located between the float bowl & main body.

You may also need an adjustable vacuum advance to limit the amount of advance while crusing. If pinging happens when adding throttle @ cruise you can dial the amount of vacuum advance down (less advance).

 

NO!


There's a reason vacuum advance exists. It helps idle quality, fuel economy, and helps keep the engine cooler when idling on a hot day.
And use manifold sourced vacuum. Ported vacuum is just a 70's emissions bandaid.

 

I'll agree with you on the advantages of vacuum advance. However, the use of ported vacuum for vacuum advance is important for the motor to idle correctly. If a motor runs with 16* inital advance at idle and has another 20* added by the mechanical advance within the distributor, that gives 36* at wot. The vacuum canister adds say 10-12 degrees at cruise for a total of 46-48 degrees. If the vacuum advance added this 10-12 degrees at idle, that would be a total of 26-28 degrees of timing at idle which would make the motor very hard to start and may not even run. Hence the need for ported vacuum.

 

THIS IS FUNNY:

 

OK - Fundamentally- we're talking about setting the timing on a Small Block Chevy. First off - for anything but a dedicated track car - you want vacuum advance! BUT, having said that, setting the timing is about two things - initial (or Base) timing and max advance. Most SBC's want something on the order of 36 - 38 degrees of total advance at 3,500+ RPM. That 36 - 38 degrees includes the base timing plus the centrifugal advance. So -start with 8 - 10 degrees of base timing and see where you end up at 3,600 RPM. If you're in the ballpark take it out and see how it drives. If you've got too much timing pull out a little base timing or limit the amount of advance (on the HEI - this involves limiting the amount the weights can move). If you have access to a chassis dyno - the general rule of thumb is to keep bumping the timing till the power no longer increases - then back off 2 degrees. Things like cylinder head design and material, CR and cam duration affect the engine's need for timing - so once you change those things - the factory rules no longer apply.
Once you have the base timing and amount of advance figured out - then start working with vacuum advance -Yes - ported vacuum. Knocking during gentle acceleartion typically means too much vac advance - but adjusting that requires a special adjustable vacuum advance "can" on the distributor.

 

Uh, no. I have never seen an example of a properly tuned engine at cranking speeds maxing out the vacuum advance and causing the spark timing to cause rockback.

 

What would your base timing be with full vacuum and how many degrees of vacuum advance do you run?

 

I'm running 38* at >3200 RPM WOT, 12* initial, and 12* vacuum advance (tied to manifold vacuum), for about 24* at idle. I need the additional advance from the canister to get a decent idle quality with the cam I have.

When I get caught up with other things I want to play around with a different part number vacuum canister to give me a few more degrees down low which ought to further help the idle quality and keeping the engine cool.

 

.......... It sounds to me like you're running a carb intended for a 5.0 liter engine ....... rejet it for the 5.7 and your world will be better .......... there is little to no vacuum during cranking so no worries about too much vacuum advance there .... the ported vacuum was for emission reduction ... go with a full time vacuum source for the vacuum advance unit ... again at WOT there should be little vacuum to the advance unit ... HEI distributors all had emission oriented advance curves from the factory ... find an old speed shop that has a Sun distributor machine and get it set up properly ......... in the meantime jet up that 600 Holley ........ and , if it wasn't new , buy a Holley Trik kit and replace the power valve they don't tolerate backfire pops or shelf storage after having had gas in 'em ...........

 

There is also a reason race distributors have NO vac advance. That stuff was fine for stock equipment. On my old Nova the timing is locked at 34*. Just because the factory used it that does not mean you will die and go to hell if you dont.

 

No, but it also doesn't mean your engine will have optimal timing or optimal thermal efficiency.
Combustion burn rates vary with load and RPM. That is why the factory went to the trouble and expense to put in parts that vary the spark timing depending on the RPM and load.

 

Said Nova. Street legal, drive it to the track, straight liner. Only went out with no hood one time.