Misc: Ignition Comet timing

[Piston Pete]

Hi All

I am just wondering if anyone knows the recommended number of degrees BTDC for setting the ignition timing on a Comet with a BTH electronic mag ? I have set it at 34 degrees , but its still kicking back , and blew the Mikuni carb off the spigot ,when I started it earlier.

Any help is much appreciated as I plan to have a look at it tomorrow.

 

[vibrac]

34 BTDC unless you have 1950 5 star and that should equate to about 4 deg BTDC fully retarded if your ATD is good
of course if you have high comp pistons then even less than 34
Thay are the figures I use on my road Vincents
Oh and dont get oil on that rubber sleeve they love popping off (clip too tight?)

 

[greg brillus]

Running a new type BTH you should have a solid drive pinion not an atd device.......Does it have a single or twin plug head.......and does it have a stock Burman gearbox or a Norton box........Any of these things would affect the starting.........Mikuni's tend to run quite lean, so any opening of the throttle could easily cause the engine to kick back....... Cheers.

 

[Piston Pete]

Thanks Greg - i have a single plug lead , and a solid pinion . I am trying to retime it after a bit of an issue with poor running. I have done over 40,000 miles on the Mikuni , I put new jets in and its running well ….once I can start it ! Here in Ireland , we are using ( not by choice !) E10 fuel , but that hasn't been an issue heretofore.

I will retime it another few degrees retarded ,and see what happens.

 

[greg brillus]

Do you use the choke for the initial start up....?.....I know on a 600 single I built years ago, with a 36 mm Mikuni, It seemed to like the choke on, no throttle, cycle the engine over using the decomp lever 2 or 3 times........ease it past TDC then kick, and it would fire up every time no problem.......If the idle is weak this might contribute to the kicking back, perhaps wind the mixture screw in say a 1/4 turn to see if that helps.

 

[chankly bore]

I had trouble with the Mikuni blowing off the inlet stub because the stub had no machined groove to seat the internal raised projection on the rubber mount. Once I set up the stub in a four- jawed chuck and machined a groove no more problems. Are you sure there's not an air leak somewhere? Mikunis are usually "set and forget."

 

[Michael Vane-Hunt]

Check the little orifice at the bottom of the enricher in the float bowl to see if it is blocked.

 

[oexing]

Maybe you could try a bigger pilot jet and certainly adjust idle mixture for correct air and fuel setting. With a small pilot jet you can only start the engine at part pilot , part needle jet and needle - not great and a pain to start the beast.

Vic

 

[Piston Pete]

I had trouble with the Mikuni blowing off the inlet stub because the stub had no machined groove to seat the internal raised projection on the rubber mount. Once I set up the stub in a four- jawed chuck and machined a groove no more problems. Are you sure there's not an air leak somewhere? Mikunis are usually "set and forget."

Thanks for the reply - I have the groove machined in , and its been great up until I mucked the bike around. Old exhaust rotted out, got a new one ....bit noisy ,so I fitted a baffle which upset the running . I made the rookie mistake of not undoing the last thing done before the bad running started , and changed jets etc. Everything is set back now to when it was running well , but after a run today i started it up to see how it would start when hot , and it blew off the stub ( grrrr !)

 

[Piston Pete]

Maybe you could try a bigger pilot jet and certainly adjust idle mixture for correct air and fuel setting. With a small pilot jet you can only start the engine at part pilot , part needle jet and needle - not great and a pain to start the beast.

Vic

Thanks for the reply. I have installed new jets , as per the Mikuni guy's (Motocarb.....highly recommended) recommended . These are the jets /settings that I have done over 40,000miles on , with no kickback or carb blowing off the stub.