timing chain jumped a tooth?
 

I've just returned from my coast to coast drive in my 1960 190b. Car ran flawlessly - *until* I adjusted the points two weeks ago.

Next morning after the points work, car was tough to start. Made metallic klunk sound when the starter was turned. After several tries, engine started and ran smoothly all day.

Next morning, same thing.

I checked spark advance: spark is occurring 30 degrees advance, despite correct alignment of timing chain mark, cam bearing mark, and timing arrow at crankshaft. Prior to resetting the points, spark was occurring at 4 degrees advance (timing chain is slightly stretched).

When I reset the points, I now realize I was turning the engine backwards to move the distributor rotor. I had put the car in 2nd gear, then rolled it backwards to turn the rotor.

Could this have caused the (admittedly old and stretched) chain to slip a gear tooth, despite the chain lock screw being in place?

If so, how should this be corrected?

Slightly rotate and reset the oil pump/distributor gear?

or, lower chain tension and move chain one tooth?

or ... ?

Thanks.
Tom

I don't think you have a timing chain problem.

You can have precise ignition timing even with a slightly stretched chain. What you won't have is precise valve timing.

I am not familiar with that car but 30 degrees advance is way too far advanced. When you try to start, it kicks backwards and causes the mechanical noises.

How did you adjust the points? Did you use a dwell meter?

If it's all messed up and you're not sure what to do, set the timing static, then start the engine and set it more precisely with a dwell meter and strobe light.


Harry

1992 400SE
1986 325e
1974 2002
2001 CLK320

Harry - thanks for the reply.

I set the points with a feeler gauge 0.018", as the engine wouldn't start (set screw on points had loosened, so points had closed).

Cam and chain alignment marks are set correctly with crankshaft timing mark spot on. But spark occurs 30 degrees advanced. You can't rotate the distributor enough to correct that 30 degrees - it'll hit the bearing underneath (see picture).

http://www.britishcarforum.com/bcf/a...chmentid=29866

How would you recommend I move the timing back to TDC?

Thanks.
Tom

if it was me first I'd get a new chain order,then I align crank shaft to pointer and see how far off i was..remember on the compression stroke,then align cam,and the dist. should be at number one.

OK - crankshaft TDC mark is on the pointer. Cam and chain marks are aligned.

But distributor is 30 degrees off.

How do you fix this?

Distributor won't rotate enough to correct 30 degrees off. If I try, the vacuum advance would be pointing to the driver side fender, and the oil cup hits the adjuster dial beneath the dist., and the condenser would hit the valve cover.

Perplexed!
Tom

well on most engines you would pull dist. up and rotate to correct position and reinstall

Thanks. To change spark from 30 degrees advanced, to TDC, do I need to rotate the distributor a full 30 degrees?

Tom

yes with carnkshaft at zero and cam lined up.If cam not aligned move cam

If the only thing you touched was the points, and the advance increased from 4 degrees to 30 degrees, then the points gap is too small.

If you have the correct points gap, then distributor is installed in the incorrect position. To correct this condition, (assuming distributor rotation is clockwise), pull out the distributor, turn the rotor counter clockwise about the distance of one gear tooth, and re-insert. Now you won't have the problem of the distributor body touching something when you try to reduce the advance.

With the crank at TDC and cylinder 1 at compression, the rotor should point to a tiny mark that is on top of the distributor body.

Harry

1992 400SE
1986 325e
1974 2002
2001 CLK320

[QUOTE=HarryM;3232129]If the only thing you touched was the points, and the advance increased from 4 degrees to 30 degrees, then the points gap is too small. [QUOTE]

Actually, increasing the points gap advanced the firing point. For a given breaker cam lift, the opening point must come earlier for the gap to be larger, hence an earlier firing point. Large gap:advance. Small gap:retard.

To: Frank Reiner

You are correct. Apologies.:o

Thanks gentlemen. Only thing I did was to reset the points to 0.018" (4.5mm).

But to turn the rotor, I put the car in 2nd gear - and stupidly pushed the car backwards. Thus the engine was turned backwards. That's why I'm thinking the chain jumped a tooth on the distributor (idler) sprocket.

Crankshaft damper shows static timing is at TDC; cam mark and chain mark are aligned. Dist. rotor points to the mark on the dist base.

But crank the engine and strobe shows the spark firing at 30 advance.

I'm no mechanic - but if damper, cam, chain, and rotor are correct, then the only thing left to adjust is the chain.

(right?)

Edit: breaker plate isn't binding; vacuum advance works correctly. Rotor snaps back into position when twisted by hand.
Thanks.
Tom

NutmegCT:

When the crankshaft damper TDC mark is aligned with its pointer, is the camshaft hub (and/or thrust washer) notch mark aligned with the cast pointer on the camshaft front bearing stand? If yes, the crank and cam are in the correct phasing. If there is a mark on one side plate of the chain, and a corresponding mark on the cam sprocket, and they are also aligned, then that is simply icing on the cake. The corrective action that is required is to lift the distributor, and reinstall one tooth retarded.

Thanks Frank. When crankshaft damper TDC mark is aligned with pointer, the camshaft hub washer notch is aligned with the front bearing notch. At the same time, the dist. rotor is pointing to the notch in the base, showing it's aligned too.

I hear you on the "lift distributor and reinstall one tooth retarded". But how the heck did this happen? Did turning the engine backwards once while re-setting the point gap cause this? I didn't have the "klunk" and hard to start problem for 6000 miles - then I reset the point gap and the problem started immediately.

Thanks
Tom

NutmegCT:

If the timing chain was very slack when the engine was turned backwards, then there is the possibility that the chain skipped over the intermediate wheel (sprocket) by one tooth, which would lead to the situation of the distributor drive being advanced relative to the crank position. The Service Manuals do not, to the best of my knowledge, specify an initial timing procedure for the intermediate wheel during assembly, hence it may be installed at will, and the distributor will then also be installed to phase the rotor as needed.