history first.

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

 

What is the initial timing@RPM?My first thinking is that you have the mechanical advanced locked at 15 and the vac advance at 15.

This is assuming that the mixture has been set to ge thte highest vac reading

 

mechanical is 15 deg at 700 RPM and 30 deg total at 2500 . i have the vac can limited to 15 deg. that it can give and the vac screw in the can set to pull full at low vac.

 

Take off your dist cap and look at your rotor.

Sounds like the tip is broken off and pointing sideways.

Leave the carb and the static ignition timing alone and put them back where they belong, and hook up the dist vacuum to the ported source. They're not the problem.

 

I'll look at that tonight, but if the tip were broken off wouldn't the car run rough even when the vac advance is off ? Also wouldn't it be hard if not impossible to time the engine ?

thanks for your help

Mike

 

Along th elines of what RB is saying. Maybe the added advance makes up for the miss

 

The added advance changes when the spark occurs, relative to the location of the dist shaft; so when the timing isn't very far advanced, the stationary contact in the dist is close enough to the bent-sideways tip, that the spark can still make it. However when there's more advance, the tip is so far behind where it's supposed to be (hasn't made it around close enough to the contact yet), that the spark can't jump that far.

This doesn't affect whether it will make a timing light blink at the right time.

Don't argue, just go look.

 

I switched to an l98 that was tpi a few months ago. I sold the tpi and used the mechanical pump for the carb. works great.

 

rb, i'm not arguing with you, just trying to picture in my mind what you were saying. I understand now . i have had the cap off several times and maybe i did this putting it back on, it's tight in there. I'll check this tonight.

thanks alot guys.

As for the mechanical pump has anyone had a bad experience with a edelbrock, holley or carter mechanical pump?


thanks again

Mike

 

Further , you can download the Crane vacuum advance instructions here

The limiter plate may need adjustment. When you adjust the limiter plate (amount) you have to readust the initial timing and the screw(rate of advance).

Which end of the mechanical advance slot did you weld up?

If you welded up the end of the slot that the pin sits on at idle (advanced weights closed), instead of the other end that the pin hits at high rpm/full advance,(advanced weights open), then you would throw off the * rotor/cap phasing.*
This could give the same fault as RB discribed about the bent rotor tip. {rotor tip misaligned with the cap contact at fireing)

The vacuum advance actually moves this *rotor/cap alignment as it advances.

You can check rotor/cap tip alignment by cutting a viewing hole on a old HEI cap so you can see the rotor through the cap and watch the rotor tip position while the motor is running, using a timing light. The timing light will freeze the rotor allowing you to watch the alignment change as you rev the motor with and without the vacuum advance attached.

The mounting point of the vacuum advance and the magnetic pick up plate has the biggest effect on Rotor tip/cap alignment. If the Crane vacuum advance was manufactured with the mounting holes in the wrong spot, you'll have to drill them out and move the whole mechanisms' mounting point relative to the distributor.
Readjusting the limiting plates' position should solve it thou.

 

F-Bird,

I welded up the high RPM side. This whole set up worked with the other combo ? And if i recall correctly initially worked on this set up when first fired, then it started giving me troubles so i unhooked it and all was fine. The part about the bent rotor tip that throws me off is I have 15 deg initial and 30 deg total at 2500, so why will the car run flawless at 30 deg WOT and have issues at 30 deg with the vac canister hooked up ? I will definatly check the rotor though.

If i hook up the vac advance it's fine for 1-2 min, then starts to run rough, it's not like an on / off switch as soon as i hook the vac up to the advance canister ? I have hooked a hose to it and sucked on it, it seems to pull and when i let the suction go i can hear it go back and hit the adjuster ?

What would a leaky vac canister react like ?

thanks for the help

Mike

 

 

***Because the vacuum advance affects the rotor alignment. The mechanical advance does not.***

+++I did not know this, thanks alot++


What would a leaky vac canister react like ?

***Slow or unstable vacuum advance rise.***

++I don't think that would be it, because the timing mark goes from 15 to 30 right away ?? Am i right in thinking this ?

thanks

Mike

 

You can also check the magnetic pickup air gap spec and distributor shaft side play. (worn bushing)

 

Probabily
Further.... if the rotor alignment is too far off from the caps contact point at any time or advance point, the spark will take the easiest point to ground which may not be through the spark plug. Its common for a HEI rotor to short through into the advance weights and plate. Eventually it will burn a hole right through the rotor. You should replace the rotor too. Spark plug gap should be .035" ot .040"

It may not show up right away as you disconnect and reconnect the vacuum advance becuase the air in the cap has to ionize, then all hell breaks loose. {spark scatter}
I would avoid full throttle WOT operation or long trips at hwy speeds untill you get thsi sorted out or you may be replacing rod bearings again.
Spark scatter is real hard on rod bearings.

 

I'll check the end play too, although this dist is only a year old with 1500 miles on it, but we all know that doesn't always mean anything.


thanks guys i'll check this out tonight and get back to you tomorrow with some results.

Mike

 

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

BTW, F-Bird i could actually see where the spark had been arcing under the cap on the weights.

Thanks alot to all you guys who helped me with this