Can't get timing right

I'm adjusting the ignition timing on my '72 250, but it just doesn't make sense. When I use a stroboscope lamp it reads about 20 deg ATDC at idle. 20 on the scale to the left of the zero mark, that is. According to the manual it should be about zero. At 1500 rpm it's about zero, when it should be 20something BTDC.

The thing is, when I turn the dizzy to adjust the timing, the engine runs really bad, it shudders and the idle speed drops. I can get it to 5-10 deg ATDC, but then the dizzy gets stuck. Apparently, either the vacuum pot or the condenser pushes against the spark plug cap on the first cylinder! Now that CAN'T be the way it's supposed to be.

So, what to do? It still runs best at 20 deg ATDC, which it really shouldn't.

[ctaylor738]

What has probably happened is that the rubber bushing between the crank hub and the harmonic balancer (which has the timing marks) has deteriorated and allowed the balancer to turn so that the marks are not showing the true position of the crank.

The long term fix is to replace the hub/balancer assembly.

In the short term, you could very carefully turn the engine to bring #1 to TDC with a small wooden dowel in the spark plug hole, observing its highest point. You could then "recalibrate" your timing marks with paint or a piece of chalk and time the eingine that way.

In the end, though, you want the timing where it runs best, so it may not matter much.

[joshhol]

Agreed,

My mechanic said most of the time they don't use timing lights on the old ones. There can be an margin of 5-10* over what the specs are. They're just worn out enough that spec isn't where it is supposed to be and timing it in spec often leaves it running poorly.

Time it by ear.

Thanks for the quick responses, I guess I'll just try and time it by ear.

Has anyone else had that problem with the distributor and spark plug cap pressing against each other? Perhaps an angled cap instead of straight would help.

Oh, when the rpm goes up as you turn the dizzy, is that good or bad? It always sounds more smooth with a bit of speed...

Olof Winterfjord

[Litton]

I sounds like you might have the plug wires off by one hole all the way around. If the dist. is like my 280....there is a little groove in the body where #1 wire goes.

[mzsmbs]

when you are doing the timing by ear, you are looking for the spot where you accelerate throught out the range and the engine runs very smooth. no coughs, stumbles, backfires etc..

this is all assuming that your carbs are in sync and vaccum system is tight etc...

[psfred]

I would strongly recommend finding TDC and remarking the balancer with paint over "timing by ear" -- inaccurate timing will cost you considerable milage at best, and detonation and a valve job at worst. In my experience, no one EVER gets timing close enough "by ear" to work well.

It's also possible someone replaced the pointer with the wrong one -- they are adjustable and several will fit (and sometimes the wrong one was installed at the factory).

Edit:

I just looked this up in the manual, and the pointer was changed to clear the level control pump. Some balancers therefore have two scales offset 40 degrees. If you have a wide (9mm) pointer, use the front scale, if you have a narrow (6mm) pointer, use the rear scale. If you have only one scale at the rear edge and a wide (later) pointer, you will be off 40 degrees (so that 20 ATDC would read 60).

I suspect you have either a pointer miss-match or a replacement balancer with the wrong scale -- some years have both, some have only one or the other, I think.

Peter

[cth350]

I was gonna say what Peter said. Likewise, the plug wires could be off a bit as per litton. I'd do this:

1. Find what the cam says is #1 TDC (take the valve cover off and line up the marks on the cam & tower).

2. Verify that the #1 piston is at the top of it's stroke. Use a long screw driver resting on the piston so you see the moving handle come to a point where it slows down and stops moving. Use a bit of "whiteout" or other bright paint to record the position on the crank relative to the pointer down there.

3. Eyeball the rotor position relative to the body of the distributor. It should be pointing to the mark cut on the top of the distributor.

4. Hand adjust the distribtor so that the points are just opening at this static position (use either a continuity light or a meter).

5. Check the plug wires. Your rotor should be pointing towards the #1 plug wire.

6. Test that theory by hand cranking, this time with the ignition on, the plugs out, and with the strobe light connected. You're looking for a 0 TDC strobe, give or take about 5 degrees.

7. Reassemble everything and...

7a. If you have an assistant around, or a trigger to run the starter w/o an extra person, set the timing at starter speed (about 200rpm) to 0 TDC. It should only be off a touch from your prior work. Then...

7b. Set timing at 4500 rpm as per the book.


note: At any point, if you hand crank backwards, do so an extra 20 degrees or so and go forward again to pick up the chain slack and get the relative markers back in sync.

-CTH

[autozen]

Peter beat me to it. The balancers of those years had two sets of timing marks depending on if you had load leveling or not. I always had to think it through when I had to set timing on those models.

Peter

[j9fd3s]

Originally posted by Winterfjord
Oh, when the rpm goes up as you turn the dizzy, is that good or bad? It always sounds more smooth with a bit of speed...

Olof Winterfjord

rpm going up is usually advancing the timing and dropping is usually retarding it. if you get it too far advanced it should slow down again/run wierd