Setting Timing

Bike is a 1970 750 Commando, engine rebuilt less than 1,000 miles ago (not by me) fitted with Trispark, Mikuni, Motogadget MUnit.

I statically time the EI using a degree wheel to 29deg BTDC. Bike starts and runs ok.

A few starts/rides later, starting becomes difficult with backfires etc indicating wrong timing.

Using a timing light, timing is more than 10 deg off, at less than 20 deg BTDC. Way more than expected.

I re-time statically again. All seems ok for a few more rides then the same issue occurs.

Initially, my guess is that the magnet rotor has slipped position on the end of the crankshaft. Position is right, rotor is tight and in the correct position.

The next culprit is the timing chain which was replaced when the bike was new.

Before I open the timing side, has anyone experienced this with a timing chain slipping position? Any suggestions?

I'm fairly sure it's not the Trispark as I have swapped out the head unit for the spare I have, and the symptoms are the same.

thanks.

Joe

freefly103 said:

Bike is a 1970 750 Commando, engine rebuilt less than 1,000 miles ago (not by me) fitted with Trispark, Mikuni, Motogadget MUnit.

I statically time the EI using a degree wheel to 29deg BTDC. Bike starts and runs ok.
A few starts/rides later, starting becomes difficult with backfires etc indicating wrong timing.

Using a timing light, timing is more than 10 deg off, at less than 20 deg BTDC. Way more than expected.

Click to expand...

But did you strobe it immediately after you set the timing statically?

freefly103 said:

I re-time statically again. All seems ok for a few more rides then the same issue occurs.

Click to expand...

Why reset timing statically which is not guaranteed to be completely accurate when you apparently have (or have access to) a strobe?

freefly103 said:

Initially, my guess is that the magnet rotor has slipped position on the end of the crankshaft. Position is right, rotor is tight and in the correct position.

The next culprit is the timing chain which was replaced when the bike was new.

Click to expand...

"1970" so presumably a 20M3S camshaft points engine. If the timing chain had jumped then the Tri-Spark rotor would also have changed position.

Put witness marks on things, then you’ll know.
Chain? Nope.

concours said:

Chain? Nope.

Click to expand...

A chain that jumps/slips will always retard the cam in relation to the crankshaft. You would almost assuredly bend the exhaust valve and the engine would no longer run.

As Dave pointed out. If the chain jumped a tooth the motor would no longer run, but it is possible that the chain could have stretched enough to retard the cam by 5 degrees -10 degrees at the crank.

1000 miles seems too soon for this but I have seen the new chains stretch 5 cam degrees in 10,000 miles.

Check your chain tension to see if it stretched a bunch.

It definitely could have done this if the chain was too tight to start with, but then you wouldn't see it in the chain play.

use your strobe to set the timing, not statically

your motor is timed to fire at 28 degrees so have an assistant raise the rpm's to what your Trispark
instructions say is fully advanced, about 3000? briefly while you check the timing with strobe

I always advance the timing to where the bike just kicks back when starting and then retard it
until it stops, on my 850 with Trispark that is at 29 degrees

A strobe will also tell you if there is slack in the timing chain: the timing mark will be unstable.

dynodave said:

A chain that jumps/slips will always retard the cam in relation to the crankshaft. You would almost assuredly bend the exhaust valve and the engine would no longer run.

Click to expand...

These are interference engines? I had not thought so.

MexicoMike said:

These are interference engines? I had not thought so.

Click to expand...

I will double check, but I thought I had seen one or two engines I disassembled with valve rash on the tops of the pistons.
Specific cam lifts and piston cut outs would dictate results.

Well, I learned that coil failure can have similar symptoms to incorrect timing.

The coil in question is the "no brand" version of a Dyna coil.

It seems that it's been breaking down over time. It passes resistance tests (5ohm primary resistance, 17kohm secondary) and only plays up when the engine is hot.

I replaced the coil, bike seems to be running fine now.