engine slows when advance timing

F

fried okra

Senior Member
Start the engine, slowly take the choke off and let it warm a bit, slowly advance timing from full retard to moderate advance.....and the idle speed starts to slow down and almost die out.

Retard the timing lever and the idle smooths back out.

Any thoughts on why that might be?
 
Thanks, AL....distributor was just rebuilt and used the NuRex timing tool to set timing and all seemed to go smoothly.

Hmmm.
 
Skip the NUREX tool and do it the old fashioned way. Had trouble with that tool on a club members car because the points plate was an after market item.
 
I will second what Al said.

Set timing lever to full retard. Set timing pin by cranking by hand in the normal direction. Open up distributor and turn cam by hand counter clockwise. The points should slightly open. You can check visually or with an ohm meter or with a trouble light.

You can turn the cam back (clockwise) and try again if you miss the opening.
 
Never assume the #1 piston is at TDC when the timing pin finds the dimple on the timing gear. Pull the #1 cylinder spark plug and use a flashlight to look in and verify.
 
You can also put your thumb on the spark plug hole to feel compression as you turn the crank with one of those bent up box wrenches.

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Thx, Fullraceflathead.....it looks the same (or similar) as others I've seen on line, the tab is offset for example, wonder how one would tell if it's one of the bad ones?

Bought it from one of the major suppliers recently.

You got 3 deuces on the SD455 in the GTO?
 
fried okra said:
Thx, Fullraceflathead.....it looks the same (or similar) as others I've seen on line, the tab is offset for example, wonder how one would tell if it's one of the bad ones?

Bought it from one of the major suppliers recently.

You got 3 deuces on the SD455 in the GTO?
Click to expand...

Other possibility is the distributor upper plate. Some of the repro plates position the points in a location so that when the points just open (to cause the coil to produce the spark), the rotor is too far forward advanced in its rotation. That causes the motor to run fine with the timing lever up, but as you advance the timing, the rotor ends up so far ahead of the button in the distributor cap that the spark has to jump further and further in order to make it to the plug. That has happened on a few distributors that club member have brought to my shop.

The fix is to move the points mounting location and re-drill the repro upper plate. Or, better yet, replace the upper plate with a better quality repro or an original.
 
The Model A engine is a very simple beast. Proper operation depends on first principles. These are the proper timing of the spark, the proper timing of the valves, and the rotor pointing to the proper spark plug (as Paul talked about above).

The dimple in the camshaft gear is a short cut, put there to make it easier to time. But observing the valves and pistons to find top dead center for #1 cylinder is a first principle. And observing when the valve open and close is also a first principle. Fortunately we can look into the combustion chamber on a Model A to observe the piston and valve action. Cylinder #1 should be firing when the #1 piston is at the top and both valves are closed. If the crank is 360 degrees out the valves will be transitioning from exhaust to intake with the exhaust valve closing and the intake opening when the piston is at the top. This can be a check on the valve timing, another first principle.

Given these first principles, if you find an antique engine, perhaps torn apart, and want to get it running, you can, even if there are no aids to help you with the valve and ignition timing. A lot of the early hit and miss engines are this way, with no aids to help with assembly or timing.
 
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Thanks, Paul....distributor in there has an original upper plate that it always ran well with.....but suddenly shut down idling in my driveway.

The cam screw would not loosen so had a local pro drill it out and replaced with a new drilled shaft, new hard wired lower plate to get rid of the NuRex "flex plate", etc..but the upper plate is the same as always was.

Am gonna try my Russian GAZ distributor next but heading outta town for bout a month.

Stay tuned group :)
 
nkaminar.....if the crank is 360 degrees out, the rotor would be 180 degrees out I am guessing.

That was the first issue I had, but got that resolved and timing dimple was visibly and tactically in place using pin on the front cover and piston was visually at exactly TDC when used the NuRex timing tool.

Am I missing something?

Thx!
 
You are thinking camshaft, which would be 180 degrees out. You have to rotate the crankshaft one whole revolution to get the #4 cylinder to fire and the #1 to be where the exhaust valve is closing and the intake is opening.

The dimple is usually in the correct place. The only reasons it would not be is if the camshaft gear was not made correctly or not installed correctly or a B front timing gear cover was used. You did the right thing by checking to see that the piston was at the top when the pin slipped into the dimple.

Some people have reported issues with the NuRex timing tool. It may have to do with the type of top plate used or the type of points used. I like to see the point just open when turning the cam counter clockwise. I have the NuRex tool for setting the points gap and that works great.

Check the position of the rotor as Paul suggested in post #13.
 
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I have a Nurex timing tool but have better luck using a screwdriver and setting the time. Most members of our club swear by them but I wasn't as impressed.
It does sound like your timing is too fast. Surprised it doesn't kick back a little on start up.
 
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