doorgunner

1970 350 cu.in. engine/Edelbrock 1405 carb/upgraded HEI mechanical tach drive distributor---eliminated the resistor wire/set advance at 12* initial timing and connected dist. to ported vacuum at carb.

The engine idles perfectly.....it revs perfectly...........

BUT.......

between 800 rpm and 1200rpm it spits/stumbles/backfires/stalls out.

IF I REMOVE THE VACUUM LINE TO THE CARB....THE PROBLEMS GOES AWAY COMPLETELY.

Please remind me what I did wrong so I can undo it (PTSD has me "brain-locked").

SH-60B

Test the diaphragm in the vac advance can, it could be ruptured giving you a vacuum leak as soon as the throttle is cracked open.

bashcraft

connect it to manifold vacuum and see what happens.

toobroketoretire

You may have set it to 12 degrees but its very possible its not 12 degrees because of a bad damper. People don't realize their 40 year old 3-piece dampers have outer rings that will slowly slip to a retarded position so when they time it to say 12 degrees the actual timing could be 20 degrees or more. When you unplugged your vacuum advance and it suddenly ran good that indicates your damper's outer ring has slipped to a retarded position. Try timing it to 0 degrees and see if it runs good.

Never use the so-called "ported" vacuum for your vacuum advance. Only use manifold vacuum. Ported vacuum was used all thru the 1970's to reduce NOX levels for emission reasons and once computer controlled systems came about in the early 1980's the initial timing returned to it's previous 15 to 16 degrees of advance at idle.

Tampa Jerry

Did you set your initial timing with the vacuum advance disconnected and plugged? I agree with using manifold vacuum. There are a few timing papers on the forum from John Z and Lars. Both are great sources. Jerry

doorgunner

AAAAHAAA! Thanks Members! It's connected it to ported vacuum

I'll test the diaphram with my mighty-vac/then set the timing/then connect it to manifold vacuum/then adjust idle air screws.

I'll use a dial indicator on the #1 cylinder intake valve to find T.D.C. so I can see if the damper has slipped.

I'll check my notebook to see if I made a copy of Lar's paper's last year.

I'll post the results.

doorgunner

I'm back. I checked the diaphragm......it holds 20 with the mighty-vac forever.

I checked the harmonic balance with the dial indicator on the #1 cylinder intake valve,,,,,it's showing about 2* advance on the timing tab/close considering the method I'm using.

I removed the vacuum hose from carb since it was connected to ported vacuum.

I set the engine idle at 500 rpm....smooth idle.

I revved the engine and got about 20* advance at 3,000 rpm/no vacuum connected.

I adjust the idle air screws using a vacuum gauge/15 lbs/3 full turns from closed/700 rpm (2 full turns caused a rough idle).

The engine idles smoothly at 700 rpm.

I set the timing to 12*.

I turned off the engine/restarted it.....started well.

THEN.....I connected the vacuum hose to the small MANIFOLD port on the base of the Edelbrock 1405.

Next.....I connected the other end of the vacuum hose to the distributor vacuum advance.

THE ENGINE QUIT AS THOUGH I TURNED THE IGNITION OFF!

WHAT DID I DO WRONG???

NEED ADVICE..........................

nw2571

Use a piston stop to find TDC. If you can't find a thread here on how to do it, try a google search.

The intake valve isn't going to tell you when the piston is at TDC.

Big2Bird

First off, HEI or points?
If adding the vacuum advance killed it, look at the dizzy and see if adding vacuum is causing the breaker plate to pinch a wire. I have seen bare wires chaffed under them.
Next, set the mech advance at 36* at 3000 RPM. Forget static. No one drives at idle.
You want a smooth curve from 16* or so at idle up to 36* all in by 2800-3000RPM.
You want the vacuum advance to add 16* . at cruise you would be at 52* which is fine under little load.
The car will start at 16*, then go to 32* with vacuum.Floor it, and the vacuum drops out to 16*, the car accelerates to speed, then vacuum adds 16* to increase millage when cruising.
With me so far?

Big2Bird

Yep. Sorry I missed that.

doorgunner

You are right....the crank can be rotated a few more degrees to reach TDC even when the indicator needle stops moving.....I was just using the indicator on the valve to see when it actually closed fully before TDC...I needed a reference point to make sure the outer ring on the balance hadn't spun/slipped 10-15* over the years

It's HEI. I had the distributor completely apart two weeks ago testing the coil/checking for broken/bare wires....module was bad--replaced it.....but I'll check it again in the morning...I'll put a mark on the balance at 36* for reference and follow your directions.

Rookie question: If for some reason I set the initial advance too high with the engine idling at 500 rpm ( occasionally I do stupid things)----would attaching the vacuum hose to the distributor cause the timing to advance so much that it caused the engine to stall?

Big2Bird

Did it turn off, or cough and stall?

doorgunner

Thanks for sticking with me!

The engine stopped just as though someone turned the ignition off/no spit/no sputter/no cough/no backfire/nothing........makes me think that your "dead short" advice could be causing it to die...but I needed to ask the "extreme advance question just to be sure".

nw2571

I don't mean to keep harping on this, but I strongly recommend you find TDC for sure before proceeding much further. Otherwise none of the advance values referenced here mean anything.

The intake valve should OPEN near TDC, not CLOSE. The piston chases the exhaust valve closed, and the intake valve chases the piston down the bore.

doorgunner

Got it (I'm a slow learner).....Tomorrow I'll check the dist. for shorted wires at full advance and set the #1 piston on TDC/double-check the timing mark for 0*.

Thanks Members for sticking with me (P.T.S.D./A.D.D. SUCKS!)

Tomorrow

doorgunner

I removed the driver-side valve cover and used my 2"-travel dial indicator in the spark plug hole to find T.D.C. on the piston at the top of the compression stroke.....I used a drinking straw to feel the piston rising....then inserted the dial indicator rod to watch the needle as the balance mark began to become visible.

The timing tab showed that the harmonic balance was within 1* of 0* on the timing tab.

I also removed the dist. cap and rotor to double-check the pick-up coil wires which were in good shape since the dist. is only 2 years old/the wires were clear of the vacuum advance also.

That was as far as I got....then had to stop and go out of town.....I won't be home until late tonight

Tomorrow I will go through the steps that Big2Bird posted, and will update my results.

Thanks for the help.

doorgunner

ANOTHER ROOKIE QUESTION:

Do the individual marks on the timing cover TAB represent 1 degree for each mark........

OR.......

2 degrees for each mark?



Thanks in "advance/LOL" for the answer.

EDIT: Internet search says "2 degrees per mark". Does that sound correct (the internet NEVER lies?)

SH-60B

2 degrees per mark

doorgunner

Thanks! It's official......I (stupidly) set the total timing at 60* (24* base/add 20* mechanical/add 16" vacuum!)......thinking each mark represented 1*.

Now I can follow the other members advice and eliminate the stumble between 800-1200 rpm/along with the stalling when connecting to manifold vacuum.

I'm going to the garage now and reset the timing/no wonder it was hard to start once the car was driven for an hour....it was practically "firing" on the #8 cylinder instead of #1 cylinder on start-up!

Rookies!

toobroketoretire

Yeah, the initial timing plays a HUGE role in how hard it is to crank because if ignition starts too early it tries to force the piston back down.

Here's something else to consider. It has long been known Chevrolets, Fords, and Mopars make the most power with around 36 degrees of advance but that's for drag and circle track racing where the use of high octane gasoline is common. What works good in racing doesn't always work good for street engines because street engines operate in a very wide rpm range. Have you ever noticed how easy it is to hear your engine's noises when you're driving alongside those concrete freeway barriers? If you have the opportunity to do so give your engine a HEAVY throttle in high gear and listen carefully for signs of a "rattling" sound (pinging). Quite often 36 degrees is too much for street engines so dropping it back to say 30 to 32 degrees is often better for the engine because it gives you more flexibility.