I have a vac advance issue ...this is long

mike 1978 · Feb 22, 2005

I switched from TPI to carb and was having problems getting the tip in stumble out of the car because i was running an Elgin cam that was 224-234 @ .050, but had 71 deg of overlap (8" of vac in gear). After plenty of help i finally got it decent with carb tuning ( tube type # 37 squirter and blue cam in #2 position)and dist. tuning. For the dist i had to weld up the slot to only allow 15 deg of mechanical advance, and i used the crane vac advance canister with the screw all the way in and set to the lowest amount of travel or advance. That worked good I had 30 deg at idle which really smoothed out that cam and made it driveable. Then the motor spun 4 rod bearings..long story.

So in the rebuild i tamed it down a little with a nitrous HP cam 218-230 @ .050 113 LSA 108 ICL 48 deg of overlap(15" of vac at idle).( stock #31 squirter and blue cam #2 position) The car runs great, EXCEPT when i hook up the vac adv, to either manifold or ported, the timing goes back to 30 and the car runs rough, and starts missing as soon as the throttle is cracked open. It runs good without the crane vac can hooked up, but i thought it would run cooler and keep the plugs cleaner longer if i put the vac advance back on ? Do i just need to readjust the screw in the end of the vac can since i have more vac ? Or is this something to do with the MSD digital 6 ?

One more question, has anyone put a mechanical pump on an L-98 that was TPI previously ? I see there's a block off plate covering the hole, but is there a opening for the push rod ? Reason i ask is because I HATE that darn loud holley blue.

sorry so long and thanks for any help.

Mike

onovakind67 · Feb 22, 2005

Something about your description doesn't make sense. If you tighten the spring all the in you are raising the amount of vacuum necessary to begin changing the advance. With 8" of vacuum, I can't see how you would have any effect at all by using the vacuum advance can.

The vacuum advance can physically rotates the trigger mechanism inside the distributor. I have seen problems caused by the mechanism being eccentric and creating problems with the air gap in the reluctor as the vacuum advance rotates it.

mike 1978 · Feb 22, 2005

sorry , let try to explain better.

the old cam had 8" of vac in gear. So i bought the adjustable crane vac canister and put it in my summit HEI and MSD digital 6 plus. I turned the screw inside the can so that i could get full advance with little vac. and hoked it to manifold vac so i had 30 deg timing at idle with 15 deg of initial and 15 deg of vac, this really smoothed out that old cam and made it tolerable on the street.

The new cam has 15" of vac and runs fine without the vac can hooked up with 15 deg of initial and 30 deg of total at 2500, but as soon as i hook up the vac advance the motor starts to run rough with the 30 initial and the vac hooked up, now if i hook it to ported it's ok at idle, but as soon as i crack the throttle open it starts to vibrate, miss and run rough ?

It's kind of tough to explain, sorry.

thanks for the help

Mike

67 Plum · Feb 22, 2005

It is a hit and miss on the hole for the fuel pump pushrod.Some blocks are drilled and some arent.Just have to pull the plate and see.

mike 1978 · Feb 22, 2005

ok thanks 67 plum

JimM · Feb 22, 2005

Mike,

It seems that your new setup just doesn't like as much advance as the old, which I suppose is a good thing.

How bout this: hook up the vacuam, then retard the initial till it sounds happy. Then reset your mechanical for the total you need?

Also, have you tried limiting the AMOUNT of vacuam advance, pull it back to 10 degrees or so? I think you have to do that inside the distributor, the screw on the can only adjusts "how much vac" not "how much advance".

mike 1978 · Feb 22, 2005

JimM,

Thats what confuses me , the dyno showed the most HP at 35 total , all mechanical, because the vac was giving me issues. I wonder if there's a leak in that vac canister ? I have the total already limited as much as i can with the adjuster that comes in the kit which is 15 deg, is all it has and also as low as it will go.

Could a leaky diaphram in the canister cause this ?

onovakind67 · Feb 22, 2005

Have you checked the concentricity of the advance plate? Is it interfering with the operation of another part in the distributor? I doubt whether a small vacuum leak like that would cause that many problems. Use your mighty-vac and actuate the mechanism to see what moves.

mike 1978 · Feb 22, 2005

I was thinking the same thing because if i disconnect the vac hose from th ecarb and leave it open the motor increases slightly but stays the same.

No i haven't checked it, but this dist has always been noisy to me when the car is running. I don't have a mighty vac, but i will check it with a hose and some suction.

This is one of those things like Eric 68 last year....WHY and work to find the solution and fix, and i really would like to have it for daily driving.

thanks guys

JimM · Feb 22, 2005

Did you change the heads and cam together, or which first?

I really doubt the distributor has gone "democratic" just because you pulled it out and put it back.

If you changed heads and cam separately, when did the problem show up?

If the motor's happy without it, why aren't you? (serious question, not messin wit you... what don't you have without vacuam advance that you wanna get back?)

davidpozzi · Feb 23, 2005

With a very liberal mech advance you probably need less degrees of vac advance. the screw adjusts when the vac advance drops out under low vac, but not the total degrees of advance it contributes.

There is another version that has a little adjustable stop for the vac advance, it comes with a little metal plate that is cam shaped to allow adjustment of the degrees of vac advance. I think it is also made by Crane.

You probably only need 10 to 12 deg vac advance.

SLEEPER 86 · Feb 23, 2005

you do know that the little screw inside the crane vac canister turns "in" counter clockwise?
i had a similar problem that turned out to be sticking mech. advance weights.
best of luck,and keep us posted!
Eric

mike 1978 · Feb 24, 2005

It's fixed, thanks to you guys for pointing me in the right direction.

I'll try to explain this the bast i can, but don't know all the parts exact names. I took the cap and rotor off and looked at the position of the two triangular ( magnets ? ) that come closer together as the advance works. I noticed as i pulled vac on the crane advance the two points went past lining up. I then used a stock one and when i pulled it with vac the two points lined up perfectly. I then compared the two advance cans and noticed that the crane slot was longer allowing more travel and the limit plate or adjuster goes to the initial side ( taking away the beginning point) which still allows over travel. Dave Ray sent me a pic of the limiting plate put on the other side to limit travel at the proper end. I used the stock one and only used half the limiter to get 10 deg of of vac advance. So I set 15 deg machanical initial timing with 30 total ( for the 100 shot pump gas) and hooked the vac advance to manifold vac and now have 25 deg when idleing, so if i have 30 mechanical plus 10 deg. vac i'll be at 40 max when highway crusing. So far the car runs great. I have alum heads and 9.8-1 static with 8.18 DCR and i run 92 octane

Mike

onovakind67 · Feb 24, 2005

Looks like this?

mike 1978 · Feb 24, 2005

nope , but i sent the pic to you if you want to post it here.

thanks

Mike

I have a vac advance issue ...this is long

mike 1978

onovakind67

mike 1978

67 Plum

mike 1978

JimM

mike 1978

onovakind67

mike 1978

JimM

davidpozzi

SLEEPER 86

mike 1978

onovakind67

mike 1978

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