Erratic timing during vacuum advance

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DusterDaddy

DusterDaddy

sledgehammer mechanic
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First let me say I have not owned a timing light since 1978 but will pick up a basic one today.

Did initial timing by ear and set it so it idled nice and sounded good. lol

Motor is a '74 mild build 318 with stock bottom, KB pistons, Mopar purple cam (dont know specs, bought the motor pan to carb complete from a third party) stock 74 318 heads with over size 360 valve job, gasket match porting and blending, 340 exhaust manifold and 2.5" duals, performer intake and 600 cfm Holley street avenger carb. New fuel system from tank pickup to carb with fuel pressure gauge.

Ignition is Petronix flamethrower HEI with matching flamethrower coil. Distributor is a re-manufactured stock Mopar electronic ignition unit, MSD wires NGK plugs gapped at.045

When I had the car on a trailer for exhaust install, I stopped by a friends and we checked the timing with his digital light. as I recall initial timing is 18*. I was in the car throttling up to 2500 rpm and my friend said the timing was jumping all over the place when the advance kicked in. I could feel the motor falling off as I gave it more gas.

I'm no mechanic by any means, any ideas why the timing jumps around like that during advance?

Thanks!

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Could be a number of things, but 1st check to make sure the vacuum advance on the distributor is connected to the ported manifold source, not to manifold vacuum under the carb.

What do you mean by "the motor falling off as I gave it more gas. " ? Running rough or?

With KB pistons, the CR is likely gonna be quite a bit higher than stock CR. So this timing be ear and at 18* initial may be too aggressive for that CR and iron heads. Add in a possible 22* mechanical advance and you are pushing 40* total.

BTW. buy a vacuum gauge too.
 
What I meam by falling off is it sputters and feels like if more throttle is given it will stall. Have not verified that will happen, that's what it feels like
 
More likely a carb or fuel delivery issue then. Could also be a weak ignition.

Was the engine well warmed up? And does the carb have a choke? It could simply not be set right.

How far open are the idle mixture screws on the carb? Is the idle very responsive to changes in idle mixture screw adjustments?
 
nm9stheham said:
More likely a carb or fuel delivery issue then. Could also be a weak ignition.

Was the engine well warmed up? And does the carb have a choke? It could simply not be set right.

How far open are the idle mixture screws on the carb? Is the idle very responsive to changes in idle mixture screw adjustments?
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I would defer any carb adjustments to someone more familiar than I am. I know how to set float levels and that's about it.

Could the timing being all over the place during advance be because the engine is starving for fuel and thus reducing the amount of vacuum it produces thus making the advance fluctuate?
 
As said above, sounds like fuel.

Do you know the cam specs? Compression ratio?
 
dano said:
As said above, sounds like fuel.

Do you know the cam specs? Compression ratio?
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All I know is the cam is a Mopar purple cam that measures .480 lift and the Kb pistons are said to get you to a tad over 9/1 with stock 318 heads.
That is what I have been told anyway
 
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nm9stheham said:
Could be a number of things, but 1st check to make sure the vacuum advance on the distributor is connected to the ported manifold source, not to manifold vacuum under the carb.

What do you mean by "the motor falling off as I gave it more gas. " ? Running rough or?

With KB pistons, the CR is likely gonna be quite a bit higher than stock CR. So this timing be ear and at 18* initial may be too aggressive for that CR and iron heads. Add in a possible 22* mechanical advance and you are pushing 40* total.

BTW. buy a vacuum gauge too.
Click to expand...
I agree....could be too much timing. Leave the dizzy a bit loose, and while the engine is running rough, back up the timing a bit at a time and see if it clears up. If you run up the engine without vacuum advance, does it still run rough?
 
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Pretty easy to tell if an engine is going lean just by putting your hand over the carb a little while throttling up and if it seems to clear up then you know.
If it goes flatter when you do it then you know it's probably going rich, or the problem is somewhere else.
Of course exhaust smell and plugs can verify that.
Not everything requires a special tool.:D

Also you want to make sure you are not over advancing at full advance with 18 initial as well as being sure the advance weight springs are not so light they let the weights jump around at whatever RPM's you are seeing it at.
Vertical slop in the distributor shaft or a loose timing chain can also cause that.

Remember the phrase "Trust but verify" in unknown engines.
All the above diags can be done with ZERO tools save for a plug wrench and timing light.
 
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Word of caution... My flamethrower distributor had the mech advance sticking after throttling the gas and could be part of your problem. Those cheapo tabs they use to stop the advance are NOT good.
 
Lelo Dart said:
Word of caution... My flamethrower distributor had the mech advance sticking after throttling the gas and could be part of your problem. Those cheapo tabs they use to stop the advance are NOT good.
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It's a Mopar stock dizzy. Only flamethrower module and coil
 
Duster Daddy,

One of the things that I have learned recently myself about Vac Advance Cans is that there are many available. One key feature to consider when selecting the right Can is the amount of vacuum present at idle which is dependent on your camshaft. The proper way to select a Can is to be sure it is "all in" 2 inches of vacuum below your camshaft at idle. If they are one in the same or higher than the camshaft the Can will "hunt" and provide erratic timing and idle characteristics. Basically, the plunger will oscillate. My Comp XE274 idles with about 11-12 inches of vacuum so I selected a Can that is all in by 9-10 inches of vacuum and only puts out 14 degree of advance to keep pinging down and allow for more mechanical timing too. My timing tape is now rock solid under the light and the idle very stable. I got mine through AutoZone but Echlin (Napa) and a whole lot more sell these for 12 bucks or so. I have a GM HEI Distributor FYI. I have a link I can provide if you are interested at home that is very informative on vacuum, vacuum sources and vac advance.

FYI,
Marion
 
The only way I've found to correctly measure the vac advance is to make sure the vac source is constant and enough to properly pull the advance in all the way. I use a vacuum pump
 
Sounds like the vacuum signal is what is erratic. Put a gauge on it. If the vacuum signal is not steady at a steady RPM such as 2500 for instance, that would be indicative of weak or broken valve springs.
 
I have been busy with work and honey do stuff and we are going on vacation next week. I ordered a timing light and a vacuum gauge. I considered a more expensive distributor, but I'll keep the stock Mopar unit and get an adjustable vacuum can if need be. I have educated myself about vacuum advance, what it's for and how it works and when I get back from my trip I'll tackle it.
 
Just remembered an engine I worked on several yrs ago that had a very similar problem. The problem was in the distributor. When the vac advance pulled in the reluctor gap closed up to the point of touching on a couple points (reluctor points aren't always the same height even though they should be) so it'd run real rough. I filed down the 2 high points it had so all points were within .001-.002" of each other and reset the reluctor gap for a happy medium with the vac advance engaged and disengaged and all was good.
 
Home from vacation and had the car out of the garage this past Sunday.
I put my vacuum gauge on the small nipple on the brake booster check valve and read almost no vacuum. I pulled the check valve from the booster, put my thumb over the inside of the check valve and the vacuum shot up to over 18"
Ten brake booster is a remanufactured booster/master unit I just bought from rock auto the past march. Rock says buy a new booster/master, install the new booster and return the leaker to them for a refund. Going to keep trouble shooting with the vacuum line to the booster plugged off until the new unit is installed.

So the motor idle slowed and had some of that nice loping that you might expect with a mild performance cam.

It seamed to run much better, but it did feel like it lost power during one series of revs to about 3000 rpm.

There isn't a huge amount of fuel in the tank so I'll add 5 gallons before the next round of diagnostics.

I didn't put the timing light on it or notice fuel pressure gauge but I certainly will this weekend.

Where should I set the timing at idle with the vacuum advance disconnected as a starting point?
 
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I'm running 19 initial on a bone stock 85 318 (exc for 4 deg adv timing set). unplug the can & bump the timing up to restore what the can was providing (as close as you can guestimate), a dialback sure would be nice! & see if it straightens out (rotor phasing issue).
 
Didnt read after the 1st post.

Disconnect the vac advance when you check your initial and mech adv timing.
Next thing is.. Unless you have light advance springs ,2500 rpm isn't full advancing....try over 3000,like 3500 rpm.

Timing chains slack out, once the cam isn't as loaded, as in not steadily climbing rpm...the cam will dance within the chain slack allowance.
That's the cam timing bouncing, transmitted through the intermediate gear to your distributor.
 
Timing chains slack out, once the cam isn't as loaded, as in not steadily climbing rpm...the cam will dance within the chain slack allowance.
That's the cam timing bouncing, transmitted through the intermediate gear to your distributor.
Click to expand...
& the dist lower tang can have a loose fit in the intergear slot causing more of the same. Twist the rotor back & forth lightly (not enough to get into the springs) & see how much rotational play you have. the tang can be peened or welded up/then filed down to fit better. Some have claimed the added slipon collar Dayton #1L660 will do the same & I would drive out the roll pin in the plastic collar further above & shim the axial play to .005". speedway motors has an excellent flat shim kit for this for $10
 
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