Check for voltage drop throughout the igniton circuit. I had stumbling, misses, fouling of plugs due to fouled contacts in the off/run/off switch. Biggest voltage loss was across it, and from there it feeds the coils. Coils were not getting sufficient voltage. I first tested before bringing out the vom, by cranking while in off position, and while spinning over, I flipped switch to run. It would start much better when I did that, so I opened the switch to find it grungy, and a clean up of it solved the fouling of forward cylinder plug and the stumbling and misses.

Gotcha, there is quite a drop at the coil - 7.5 at both, so that must be something upstream. Will have a look at the switch.

I can confirm that it's running on the front cylinder only. The weird bit is that I get spark at both. Tried a new plug and get some intermittent back fires and an occasional stumble when I pull the known-running plug cap and try to run on just the rear. With the front plug cap pulled, wouldn't I have enough juice to run just the rear? Should probably avoid too much logic. Will do some more poking tomorrow.

It's a wonder that it runs at all with 7.5V. Also have a look at the wiring upgrade. You`ll find the info in the downloads thread on top of the P750 forum.

Try cleaning the idle jets first, and run the bike for a while after the cleaning. The idle jets, being so small, tend to gum up when the bike has been sitting for a while. If that doesn't seem to help then adjust the idle mixture on both cylinders. I know that my bike can be sensitive to idle adjustment, with similar symptoms to what you describe, if the idle mixture screw is off by even 1/2 turn on one cylinder.

For reference, the idle jets are held by the screw-in carriers circled in yellow in the attached photo and the idle mixture adjustment screws are the ones circled in blue (sorry for the crappy picture quality!).

While you have the idle jets out of the carb, spray some carb cleaner through the open passages in the carb.

Also make sure that the air bypass screws are closed (circled in red), if one is open too far it may also have an effect on idle operation.



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One bypass screw should be part open ..... best not to dick about with them if you don't have a set of vacuum gauges ............... and 7.5 volts is way low , get that sorted 1st , mine was showing 9v and it played up

Woo, thanks guys.
So I got the relays in today and went through the pdf - thanks for that paso750. Now I get 11.5v at the coils AND BOTH CYLINDERS ARE RUNNING. Very fantastic. I haven't ridden it that much with our move this year and I suspect that the little I have done has been on one cylinder the entire time. That would explain a lot.

I also pulled and cleaned the idle jets and passages. Here's a video of it running now, with lots of smoke. The smoke eventually dissipated, hopefully it will keep doing so.

The smoke did clear up quickly after I pulled the air filter, wondering if it's over-jetted now. Mixture screws are out 2.5 turns and bypass screws are both closed. I have a set of vacuum gauges somewhere, will try to dig those out. Anyone have a link to a post on tuning w/ gauges?

Are the mixture screws air or fuel regulators? ie, if it's rich, do I screw in or out to meter fuel?

The idle mixture screws are fuel regulators. They screw in to make the mixture leaner. They are very sensitive, so use small increments.

The simplest way to adjust the idle is to set the idle speed to around 1000 rpm using the throttle stop, then pull one spark plug wire, adjust the idle mixture for the highest idle speed on the running cylinder, then do the same for the other cylinder, then re-adjust the idle speed back to normal via the throttle stop screw.

The air bleed screws are to adjust the vacuum balance between the cylinders at idle. If you have a vacuum gauge, all you need to do is compare the two cylinder readings, then open the air bleed screw on the cylinder with the highest vacuum to reduce the vacuum to the same level as the other cylinder. The bleed screw on the cylinder with the lowest vacuum remains closed. If a large adjustment is required (like 1 turn or more), you may need to re-adjust the idle mixture.

Sorry, forgot to point out that the vacuum gauge lines should be connected to the ports on each side of the carburetor, just above the outlet manifold. In the picture from the post above, one of the ports (and the blocking screw) can be seen just below the accelerator pump cam. The other one is in the same relative position on the other side of the carb.

Imho the engine still doesn't sound completely right. (But I`m guessing it was cold and maybe running with choke) It sounds ok at idle but when you open throttle or better when you close throttle and open again it chokes a little. But that's hard to say from the sound of a video and some may disagree. It may change with fresh fuel and once you bring the engine up to temperature. Setting the carb right will do the rest.

When you get it sorted buy yourself a set of new tires!

WTH G you selling tires now ...? that's 2 in one day . I have several new tires available..... original gear ... none of this fake modern stuff

Sorry, I pulled the little shrouds around the mixture screws so I could reach them w/ the airbox on. Set idle to 1000rpm once it warmed up. Raised the rpm with a mixture screw til it leveled off, adjusted idle, did the other. Pulled the spark plug caps to make sure it would run on each cylinder. I suspect the rear cylinder has less intake pressure, so I cracked its bypass screw about 1/8 turn. I know there's no science in that, but my gauges are still buried somewhere.

As it's been the wettest winter/spring on record here in Seattle, I'm just happy that it's ready to ride around the neighborhood to bring it up to temperature and continue tuning if the sun ever decides to stay out for more than an hour at a time.

Thanks for the link, I was looking for just that kind of tuning explanation. Will report back.

I read what you did but still don`t get it
You guessed that the rear cylinder creates less intake vacuum and therefore opened its air bypass screw? You have to open the bypass screw of the cylinder that creates the higher underpressure. If you`d open the one with the lower pressure the carb would be more out of balance. But then you mentioned before that the rear cylinder air bypass screw was less resistant than the one of the front cylinder. That might have been because that screw was seated meaning that the rear cylinder actually could have been the one with the higher intake vacuum this making opening its air bleed screw the right thing to do.
However I'd take the rainy days as a chance to look for your carb balancer!!!