spitting when hot

Hi

I have a 1969 750 commando that is easy to start but as it warms up any increase in the throttle the engine runs rough - its ok at idle but any increase in revs it start to spit and pop out the exhausts. It is impossible to take the bike for a spin because i can't open the throttle without the bike spitting and losing power

the carbs have been overhauled, cleaned (ultra) and new jets - new plugs, electronic ignition with single coil arrangement

the bike is still new to me and was not running when i got it - so i am working in the dark apart from the fact that it starts great

any help welcome to get this very nice example on the road

weak mixture, pick the throttle needle up one notch and screw in the air screw untill 1 turn out. failing that Timing?

Timing IMO.. Get a strobe/timing light onto it... as a "general" safe timing point to aim for can be 28-29Deg BTC @ 3000-4rpm on the centre stand.... At that setting you will ride all day in an "old fart" manner, and not have a problem.. (unless you want to race or have obsquire parts in engine, then thats different)

No point tuning/fiddling with carbs till the timing is right.. Have you checked the timing???

If its "spitting" cracking/backfiring from carbs when you heavily accelerate/gason it way toooooo far advanced....... (eg, firing before valve is closed)

And we really like feed back here!!! Good Luck

I had a coil failure that acted the same way. Fine when cold but ran terrible as it warmed up. Replaced it after eliminating everything servicable else first.
Clean carbs
Adjust valves
Check and recheck all wire
Adjust points and timing etc

If your bike is tuned correctly, as you take the chokes off when it is not fully warm it should spit back through the carbs. If it is doing it when fully warmed up you have a problem which is likely to cause damage. It might be an air leak in the manifold between the carbs and the head, or the carb needles one notch too low. If you think it is ignition timing, you should operate on it and get it right - you cannot afford to have that wrong. Check with a timing light. If you have altered the carburettors, the air jet in the bellmouth of some carbs affects the depression on the needle jet and can make the mixture lean.

'If its "spitting" cracking/backfiring from carbs when you heavily accelerate/gason it way toooooo far advanced....... (eg, firing before valve is closed)'

It might have swallowed a stone which has put a ding in an inlet valve ? If the motor has good compression, you wouldn't be able to start it without a serious kick-back if it had enough advance to fire before the inlet valve is fully closed. However the valve timing might be wrong. I always use the degree disc and dial mic. when rebuilding the motor. If the motor is fitted with electronic ignition, it can easily go radically wrong. All you have to do sometimes is fire it up with a plug lead detached and you can stuff a control box. We had that happen with a British Lextrix ignition unit on a two stroke - it advanced enough to crack a cylinder head and bugger a barrel.

I had some ignition timing issues but if your bike starts it's easy enough to fiddle with the stator to see if it runs better advanced and you'll soon know if it is too far retarded or drawing air, but you've probably done all of the above anyway and checked the compression. I think it's hard to give accurate advice without all the facts or the bike in front of you so best of good fortune to you . I have a 69 fastback and wouldn't mind a photo of yours if at all possible .
J

It sounds like the ignition timing is retarded to me.

Is it possible your choke is really "on" when you think it's "off".
I did this with my first Commando years ago because I didn't understand the action of the choke lever and rode it with the choke "on" for a couple days and it behaved exactly like you describe. Normally on a cable acuated choke, the lever is pulled "on" but with goofy Concentrics its backwards.

Just a thought.