What Manifold Vacuum and Timing w/ these mods?

[FullSizeJeeper]

I've got my rebuild in and running, but still not purring like I want. It's a 74 AMC 360, .040 over, summit 8600 cam, crane roller rockers, summit digital ignition, MSD disributor and coil, edelbock preformer intake, timing chain, ceramic headers, 650 carb, port and polished intake, +.10 pushrods to correct value geometery.

I set the timing at 8 degrees BTDC, had low vacuum 5hg in drive at 650RPM, about 8hg at 1000 rpms, 15hg at 2000 rpms. Hard to start and running very hot.

I decided to alter timing. Set the timing in drive at about 650
RPMs, with the advanced vacuum plugged and a vacuum gage on the manifold vacuum. I moved the distributor to achieve maximum vacuum reading then backed off a little. I achieved about 12hg, but the timing was of the scale on the timing cover. I figure about 25 Deg BTDC maybe more. Running much stronger and smoother. A little detonation on hard accelaration. Starts much better.

This seems like a lot of advance, the stock setting is 7 deg BTDC. What should I expect the the above mods when timing and on the vacuum gage?

[82Waggy]

Keep in mind that when you hook the Vac advance back up your timing at low throttle and at idle (high vacuum conditions) will be higher - since you have noticable ping, you should back off the advance quite a bit.

I would guess you do not need more than 32-34 degrees total advance from about 2500-3000rpm up.

If your timing mark is correct, 25 degrees initial is likely way too high.

[1980_Cj7]

Sounds like the EXACT thing we were running into with our 401 with the timing. I'm hoping it was due to our discovery that the machine shop had installed the timing gear incorrectly on the cam and that once that is corrected, the problem will be gone.

[Dusty]

unhook vacum and plug it run engine up to 2500-3k where full mech advance is in. use an advance timing light and set it at 32-34 degrees total. tighten down and let it fall back rpm wise this way you know where your top is. i fought timing issues for awhile. i set my total at 33* at WOT i get just a hair of detonation full load at less than 2k rpm with a manual tranny with the auto i had no detonation. i set my idle at 800-850 in neutral regardless of what the manual says. then adjust the carburator ect to keep it right there, helps with AC on and eliminated a little stubble i had off the line.

i have 22 in vacume at 850 idle

[AMX69PHATTY]

A lean condition can increase engine temps.

[FullSizeJeeper]

I am sure the timing gear is correct. I've put several on and matched the marks up carefuly and rechecked it several times during the build before putting the cover on.

It could definitely be running a little lean. I have not fine tumed the carb, trying to get the timing set first. I realize they go hand in hand, but until I'm close on the timing, the carb will change correct?

So the total advance should be in the 30 to 35 BTDC range? I have an MSD Billet dizzy, do I need to change the springs to get the mechanical to mesh with the vacuum?

Does that vacuum sound a little low?

[AMX69PHATTY]

I'd say the idle vacuum is low. Should be at least about 12 inches at idle.

Timing does not significantly affect carb mixture setting.
Do you have the mixture screws out about 2 or 2-1/2 turns ?
Are the Float Levels set ?

The 35* or so total advance does NOT include Vacuum advance.

Yes, you will have to change springs and advance stop bushing
to re-curve and dial-in the MSD Distributor for ~35* total at ~3000 rpm.
The 35* does NOT include the Initial timing advance, about 10*
so the distributor should have about 24* mech advance by 2500-3000 rpm.

Are the valves adjusted correctly ? Max. of .045 lifter pre-load.
Too Tight would make for low vacuum.

Vacuum Advance goes to ported Vacuum, no vacuum at idle.
Vac Advance is only "engaged" at steady state constant speed cruising.
Any time the throttle is opened slightly further engine vacuum pretty much goes to zero,
until a balanced steady state is re-established at the new throttle opening position.

[FullSizeJeeper]

Yes the valve adjustment may be the problem with the vacuum. I adjusted the rocker arms during the build, but a had a couple of lifters that we not fully loaded. After I had it running, warmed it up, and rotated through the engine adjusting the rockers by paired intake and exhaust. When #1 intake fully open, tighten #6 intake, (1-6, 2-3, 5-8, 4-7). I adjusted the lock nut tight until I just felt resistance when spinning the pushrod. Then tightened a 1/4 turn more. I tried to do the loosen till the clack and then tighten while the engine was running, but I caused more problems than solutions, so went to the process mentioned above. When I did this during the build up, I was finding that more than a 1/4 past first resistance was leaving the valves open slightly, though the crane insructions say 1/2 to a full turn. If I went a full turn the valves would never close.

So I have someone coming over to help with Vavle adjustment tomorrow. With the Summit 8600, will that bring the vacuum down some?

[FullSizeJeeper]

I have the floats at the stock levels in the Edelbrock manual, I've read on here that they may need to go a little lower for better offroad preformance.

I am at 2 2/3 turns currently. That is were I set them during the rebuild and have tried adjusting them a couple of times but the valve issue was causing some issues with the vacuum reading.

I will post some vacuum readings at different RPMs after I reduce the timing some. Could I be off a tooth on the dizzy, which is making it want to time so far advanced? As I said the 25* is a guess, just have a basic timing light. So once it goes off the scale, its all guessing. It runs at 10* but lacks power and idles pretty rough.

[1980_Cj7]

I am sure the timing gear is correct. I've put several on and matched the marks up carefuly and rechecked it several times during the build before putting the cover on.

The way they screwed ours up is they put the key in the oil groove in the cam timing gear instead of the keyway. The timing marks still lined up, cuz they just turned the cam until they did. Don't get me wrong, I'm not saying you made this same mistake, but our symptoms were exactly like yours in every respect. Sounds so much like ours it's scary.

Ours would run at 10* initial advance, but had no power, and strained to turn over when trying to start it. If we turned the initial advance up to 25* (we have an adjustable timing light), it turned over normally, and made a lot more power, but pinged. We didn't keep it at 25*, just tried it there. Around town, temperature ran fine, but out on the highway cruising, it would run hot.

It's back in a different machine shop, due to detonation tearing up the engine. Look up a post in here called "Collapsed Piston" and check out the pics. On tearing it down, they discovered the timing gear being on wrong. Hope to have it back any day now. This time I'm personally putting a degree wheel on it before it goes back in the Jeep. If that would have been done the first time, we probably wouldn't be having it rebuilt the second time in only 300 miles.