Here is a fun one for you guy's, I will just start at the top for everything that has happened to try and help,

Started with the transfer case blowing out which while i was finding a new one i changed a few thing's on the engine,

New distributor cap and rotor and plug wire's all msd,
A recurve kit and i installed the lightest spring's in the kit and it came with the adjustable vacuum advance,
MSD Blaster ignition box.

Now the problem, For some reason it will not run with the vacuum advance hooked up,

It run's like a raped ape with the vacuum advance unplugged with no issue's but as soon as you plug it back in and hit the gas and try to hold it at say 2000 RPM it start's backfiring through the carb.

I have the initial timing set at 8 degree's BTDC and this has be completely stumped.

Any idea's are great

 

If I had to guess, I'd say that the light springs plus the vacum advance are giving you too much timing.

 

Too soon.....
Spring don't affect total advance, just how soon it starts and how fast it reaches the limit of the slots in relation to rpms.

Where do you have the vacuum advance set to?

 

Here's the link to the instructions;
http://static.summitracing.com/globa...s/crn-271e.pdf

On page 4 they say;
"Due to the ever lowering octane rating of gasoline in the
US, we recommend you choose a centrifugal spring on
the heavy side rather than light."

I know because I'm tearing my hair out right now with this same issue.
Good luck!

 

I will take the distributor out sometime today or tomorrow and change back to some heavier spring's and see how that help's, It's just about the only thing i haven't tried.

And i have tried fiddling with the vacuum advance alot, From going all the way counter-clockwise to all the way clockwise and just about everywhere in between.

I will let ya know

 

I don't know what engine you have and what your engine is like; compression, cam, intake ect..
But I've found my best compromise so far with the light stock spring and one yellow spring @ 12* static and the 16L slot.

Still messing with the vacuum limits.

I would also suggest you buy 1 tank of low octane gas and time for that.
You never know when you might be stuck in the middle of nowhere without any choice.

 

I will agree...too much advance and too soon. Usually if you run really light springs you disable the vacuum advance. You want no more than 36 total degrees of advance. Purchase a timing tape (a degreed band that goes around the balancer so you can read past 20 BTDC) and run the vehicle. You timing light will show you how much advance you have at any RPM (if you hook up a tach) mechanically. For the vacuum use a vacuum pump and pull on the advance to see how much and when (inches of mercury) you are getting advance. The two combined give you total advance at any RPM/vacuum readings.

I'm betting your creasting 45 degree with both hooked up.

It will run fine without the vacuum advance.

Kenny

 

My new Crane vacuum advance will only give me 16* fully turned in at 18" hg, as read by my Mightyvac.
The stock one was more than twice that....wow!

12*static + 32* centrifugal + 16* vacuum = something like 60*
I'm still scratching my head, but no knocking on 86 octane so I'm not complaining!

 

16 degrees is a ton for a vacuum advance. Remember as you are really running it hard the vacuum will drop so at full stomp you are running nearly centrifigul only.

Vacuum is highest at low RPMs. So off idle you might have 12+16+0 or 28. lets say your advance starts at 2000 and maxs 16 degrees, 1 degree for every 100 rpms. At the same time the vacuum drops 1 degree for 100 rpms but no lower than 5 degrees:

2000 = 12+15+1
2100 = 12+14+2
2200 = 12+13+3
..
3100 = 12+5+12
3200 = 12+5+13
etc.

So you see it is a combo of rpm and vacuum (load).
Kenny

 

I understand it,
But i'm talking of squeaking up the revs on the interstate. Maybe I don't have the throttle *closed* but I do have the revs up to where I am at full advance and the darn thing still doesn't knock if I accelerate from cruise.

Highest is on the over rev w/ a closed throttle, but there's no load then.

Thanks for your insight!
I'm much more familiar w/ IR engines.

 

None of what you just said is correct.

You put in initial timing (static) of about 10-12 degrees, as the motor revs you put in more with EITHER the springs or the vacuum for a TOTAL of no more than 36 degrees.

Real light springs are usually used by Racers that want FULL ADVANCE to come in real quick and they generally don't run a vacuum advance.

For a street driven vehicle, you WANT to control timing with vacuum not springs.

 

Well played with it some more and put the heaviest spring's back in and still have the problem,

Not as bad though, It still backfire's but not as often.

I tried everywhere from 12 degree's advanced to 0 and same thing everywhere.

I might just run it for awhile with no vacuum advance and see how bad mileage suffer's and how it run's.

I also don't know about total timing but in the crane book it say's going down the freeway in cruise with the vacuum working and mechanical they reccoment around 45 to 55 degree's advanced. But what do they know

 

Crane is correct. With no load on the engine, it will take advance up in the 50's. This gives better fuel mileage. This is over and above the magic "38" degrees everyone talks about, which is calculated by the initial + centrifugal.

If you are messing around with timing curves and such, you should leave the vacuum advance disconnected till you get the engine running right on just initial and centrifugal. Then after that, you bring in the vacuum advance as "icing on the cake" for fuel mileage only.

Racers do not use vacuum advance because it is of no use. They usually have the throttle wide open, so they have no vacuum anyway, and the vacuum advance has nothing to do with full power output. It only comes into affect during cruising, and they are never in that mode.

 

Idaho86,
How easy is it to start, hot?
Are you sure your damper marks are correct? Try finding TDC#1 and looking at the marks with the engine off. It could have slipped.
Do you have a new timing set? installed correctly? If it's really sloppy your cam timing's jumping around.

I would think Crane knows a lot.
They have been grinding cams since the 1950's and selling this kit for almost 40 years.


'stosh,
Are you telling me If I'm on the highway turning, say 5k, and close the throttle, I don't have ALL my centrifugal in, AND high vacuum?....
12 initial and the 16L slot is giving me 44* by 3500 rpm with the vacuum disconnected.
I know that, Ive just seen it.

I do understand that it's mostly for economy, and that racers use only centrifugal for a reason.
My 347 stroker SB is making 440+ on the dyno and I intend to put that in a streetable car. (No vacuum advance there, but I do have ignition retard below 800 rpm )
I don't need as progressive a curve to get a 2200 lb kit car moving. And it never sees as much load as a work truck.

Thanks Franklin,
second paragraph right out of the 'book'
As usual, the voice of reason and experience......

 

It is very easy to start hot or cold which is nice, But i have the vacuum line hooked to the timed port so at idle it isn't getting any vacuum, And then as soon as you crack open the throttle and the timed port open's up it start's to sputter and backfire.

I will check the balancer sometime in the next couple day's and see what that look's like.

Thank's again for the help