B31 Running rich

Started by DGBland, 01 October, 2023, 07:31:42

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DGBland

I know this topic will have been covered many times but my B31 is running rich and I have to change or clean the plug quite often. On a run last weekend of 120 miles I changed the plug twice during the run.
It appears to have the standard monobloc carb and starts second kick with no choke and i don't need to use the advance and retard lever. With a clean plug it runs lovely.
Could you advise me please on where to start to identify the problem?
I was going to start by checking the needle height?

Thank you
David

Jim S

Hi David

There is something wrong if you don't need to use the advance/retard. I would start with that.
Remove the points cover on the magneto and determine if the face cam is rotating when you move the advance/retard lever. If the cam is moving, check the timing per service sheet 604. The advanced setting is with the cam rotated fully counter clock wise.

Next go to service sheet 708 for the carburetor.

What fuel are you using? A high content of ethanol and low octane may cause problems also.

Jim

DGBland

Thanks I wrongly assumed it was OK if I didn't have to use the advance and retard as I have spoken to others who don't use it either. But it makes sense what your saying. To be honest it had crossed my mind but I am not used to the magneto set ups. I haven't had the bike long and was a bit wary of messing about with the ignition. I will have a look though at this first though. I have been using the new E10 petrol so I will change back to E5
Cheers
David

Jim S

I start my B33 with the advance lever about in the middle position and when riding I move it to fully advanced position (although with ethanol fuel it seem to run better with it slightly backed off from full advance). I would expect the B31 would not operate much differently.

Maybe in the past you have been riding with the lever too much retarded? Remember,  the advanced setting is with the face cam rotated fully counter clock wise. This could be a very easy fix to your spark plug carbon problem and you should see better engine performance.

Jim

DGBland

Thank you, you are probably right about running it too retarded. I have never had a bike with the advance and retard system and it seems i just need to get used to it. I took it out for a short run yesterday, started it on retard
(I think) then advanced it a little and it appeared to run better. Once I find the best position with the lever I will mark it at that position for running

Thanks
David

EDDIE SIMPSON

what i do david is start in the retarded position then put the lever on full advance until the motor warms up and your moving. afterwards the motor will tell you it needs some retard and you need then to find its sweet spot which will not be a lot on the lever.

Pete Gill

I have experienced similar running rich. I have found that the monobloc carb fuel height as governed by the float and float valve is altered by me mistakenly placing a fibre washer between the float valve holder and carb body. Placing a washer there will raise the float level considerably. Under number 9 on the diagram. Some place shims here to finely tune the fuel level.

Jim S

Good point Pete.

Apparently if there is a counter bore in the carb body, you put a fibre washer. If no counter bore, no washer. I was schooled on this in another forum...

Pete Gill

Quote from: Jim S on 03 October, 2023, 23:00:13
Good point Pete.

Apparently if there is a counter bore in the carb body, you put a fibre washer. If no counter bore, no washer. I was schooled on this in another forum...
Thanks Jim, i knew some carbs used a washer but I wasn't sure how to work out which ones.
Pete