Missfire

[steveww]

My Elan has developed an occasional missfire which is causing me some confusion. It generally happens under acceleration at higher rpm. Lifting off the loud pedal clears it. My guess is ignition related and that the spark is not able to make the leap across the gap under these conditions.

I am going to double check the ignition timing, plugs and leads etc. Anyone else got any ideas?

[Hamish Coutts]

Steve,

Just thinking out of the box - it's not float levels too low?

Lift off & the carb fills up enough to feed the engine properly?

Hamish.

[steveww]

Ah found it

The static timing was 20 degrees BTDC The dizzy had moved, this is not the first time that this has happened. I should have reconised the symptoms - doh. It would have been the 2.5 hour drive to Castle Coombe, the trash round the track then the 2.5 hour drive back home.

I do not want to over tighten the dizzy clamp as this will dammage the dizzy body but the bloody thing keeps moving

[alaric]

Does the dizzy shaft turn freely? Maybe it needs oiling / greasing etc. The Aldon ignitor is an option, as it's a hall effect device there's less energy involved in turning it.

[iain.hamlton]

I found the same after the rain. It turned out to be damp getting into a split in the HT lead (dist - coil). Dont normally go out in the wet! Replaced the leads and the problem's gone.

best regards, iain

[types26/36]

Ah found it The static timing was 20 degrees BTDC The dizzy had moved, this is not the first time that this has happened.-

Thats odd, the distributor turns anti-clockwise so if the distributor body was loose I would expect the distributor to turn that way, this would retard the timing giving ATDC, if it were BDC then I would check that the cam is not sticking on the shaft.

[rgh0]

Steve

The whole clamping arrangment for the distributor is a bit dodgy and needs careful assembling in the cramped location under the carbs.

I would do the following.

1. Pull the dizzy and clamp plate and check the clamping plate is flat and undamaged and the clamp surfaces on the dizzy flat and square.

2. Refit the clamping plate leaving the locating setcrew into the block slightly loose.

3. Refit the dizzy ensuring it goes all the way home into its mounting hole and that the clamp is properlely located on the dizzy clamping surface. This really needs a small mirror to properly check under the dizzy.

4. Tighten the dizzy clamp and clamp to block screw once everyting is seated properly.

5. Loosen the dizzy clamp slightly until it can be turned by hand and set timing and retighten. Check again to ensure the clamp is properly seated with a mirror after this work and that you have not overtightened the clamp causing it to twist and distort.

If you do all the above you should have not problems with it loosening again. The normal problem with them loosening is due to the clamp being twisted due to over tightening at some stage and not properly seating on the distributor or the distributor not being pushed in fully so it does not properly seat on the clamp.

regards
Rohan

[stuartgb100]

Just to add to Rohan's method:

be very careful not to overtighten the nut and bolt which secure the clamp to the dizzy. I've seen dizzys where the clamped part has sheared off through overtightening of the clamp.

In fact, one of them was my own !! (expensive lesson).

Regards,
Stuart.

[steveww]

I think the dizzy clamp is distorted from previous over tightening. I will order a new on Monday.

I am happy that the dizzy is in good working order as it was only recently professionally rebuilt. I also oil it regularly and the mechanism is free. No idea why it advanced and not retarded. May be the vibration moves it?

[rgh0]

Steve

There is lots of vibration around the nose of a twin cam especially if you use the full rev range to 6500 rpm so enough to make the distributor move what ever way it wants if its loose.

When you start getting up around 8000 rpm the vibration actually starts breaking things like oil pumps, distributor casings, alternators, mounting brackets and locating bolts. I have worked through most of these over the years.

regards
Rohan

[steveww]

Now that I have fitted a rev limiter (set to 6500) I was certainly using the full 6500, the shift light comes on at 6200. The engine is still pulling well but I am not too sure how well built the bottom end is (steel crank, balenced etc.) and it could get expensive going above 6500