After top end (valve guide) work, hot misfire only runs on choke

Suicidalbadger

Suicidalbadger

New member
Location
Yelverton
Read several similar problems...tried most ideas for a cure.
76 180 Jota engine
Moto Witt DMC/2
starts on choke and gets (too) hot quite quickly When the strobe was working it showed 32 deg advance on flywheel
Spark is quite weak

My brain says it's running lean or retarded on centre piston
I've stripped carbs no dirt or blocked jets unless I'm missing something (accelerator doesn't seem to squirt petrol as it used to (but this should effect over all tick over?
I've tested the DMC with screw driver on magnetic senders, and get a weak spark

My next move is run +12v to coils direct from battery
Get new plugs
Is there a hidden area in carbs that can get blocked?

It feels like it is mixture...where would you go next? The ignition advance looks good
 
Regarding the acceleration pumps not working: if the membrane is still good and in place, you could try to fill them with gas to make them work again. Make sure they are not clocked.

Float hights measured and set accurately to 18 +/- 0,5mm? this is important.

Valve gaps controlled and within specs?

No water in the gas tank? Check float bowles for water bubbles in the gas...

Check filters of the fuel tabs for dirt

If the above is all done, Ultrasonic cleaning and all new jets and seals is the next way to go for the carbs I would say...

Weak spark is normal on original coils. you could try to improve it with new coils and wires. Are resistors still in place? If you recently changed to DMC, resistors need to be taken out if existing. What spark plugs and caps do you have? Only one should have resistance... So if you have a spark cap with 3 or 5 kOhm, plugs without "R" should be used (So B8ES, not BR8ES f.e.)
 
Laverdalothar said:
Regarding the acceleration pumps not working: if the membrane is still good and in place, you could try to fill them with gas to make them work again. Make sure they are not clocked.

Float hights measured and set accurately to 18 +/- 0,5mm? this is important.

Valve gaps controlled and within specs?

No water in the gas tank? Check float bowles for water bubbles in the gas...

Check filters of the fuel tabs for dirt

If the above is all done, Ultrasonic cleaning and all new jets and seals is the next way to go for the carbs I would say...

Weak spark is normal on original coils. you could try to improve it with new coils and wires. Are resistors still in place? If you recently changed to DMC, resistors need to be taken out if existing. What spark plugs and caps do you have? Only one should have resistance... So if you have a spark cap with 3 or 5 kOhm, plugs without "R" should be used (So B8ES, not BR8ES f.e.)
Click to expand...
Thanks, it has DMC coils and iridium BR8EIX (I have some old NGK B8ES I could change to) It seems like it would be carbs as ignition from DMC unit shouldn't change the advance degrees by much if it's working. Static test with screw driver gives sparks.

Thanks again
 
Suicidalbadger said:
Read several similar problems...tried most ideas for a cure.
76 180 Jota engine
Moto Witt DMC/2
starts on choke and gets (too) hot quite quickly When the strobe was working it showed 32 deg advance on flywheel
Spark is quite weak

My brain says it's running lean or retarded on centre piston
I've stripped carbs no dirt or blocked jets unless I'm missing something (accelerator doesn't seem to squirt petrol as it used to (but this should effect over all tick over?
I've tested the DMC with screw driver on magnetic senders, and get a weak spark

My next move is run +12v to coils direct from battery
Get new plugs
Is there a hidden area in carbs that can get blocked?

It feels like it is mixture...where would you go next? The ignition advance looks good
Click to expand...
Have you strobed the centre pot???

If running the stock pick-ups, their wiring is often swapped around, switching the polarity of the pick-up and seriously retarding (advancing?) ignition. In any case, waaaaay off.

...and ditch the Iridium plugs. Regular style plugs work far more reliably in the old clunkers. Carburetted engines soot the iridiums for too much to benefit from their merits or justify the cost.

What are "DMC coils"?

piet
 
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