31K Poor Running Mystery

[midlifecrisisrd]

Swap carbs left to right and right to left

You will have to fit a longer choke joining tube

See if it swaps sides. I have had carbs that no matter what won't work

Also swap plug caps side to side

Steve

[Yogi]

Long shot,,have you left the pilot jet out
I bought a bike a few years back that baffled me until I noticed it was missing

[midlifecrisisrd]

Oh, and did you replace the float needle valves, seats and o ring

Steve

[reedpete]

Just because it’s lost spark doesn’t mean both sides will fire ok if one side does.
Here’s two possibility why...
1. There are two magnets on the rotor, so you might be getting two good sparks when one side needs it and two so so sparks when the other cylinder needs it.
2. Applies  to LC but not sure about this on the valve , anyway, timing is from left had cylinder, if the crank has slipped the timing might be totally out in the right hand side.

[Gitram]

what colour is the spark, big and blue or small and yellow?

marti?

[muttsnuts]

are both carbs the same model, e.g 31K, if not, then if they need to be jetted to the carb model, not what the bike is, plenty of bikes out there with mixed up carbs, but they are all for a YPVS I hear you say, surely they are all the same? definately not, 31k - 240 main, 1UA - 185 main, you put a 240 main in a 1UA carb and it will flood like hell on a stock bike

just a thought as I've seen all sorts come through my doors !

[geoffers997]

Hello Everyone,
Thanks for all the advice and support, I was sat on the garage floor yesterday spitting blood and nails at the rotten bloody thing, but a bit of help and advice has lifted my spirits and I've got my mo-jo back.

So:

Float needle valves, seats and o ring - all replaced with tourmax parts

The carbs are marked 31W 00, I've made the assumption that they're standard carbs and the jets and needles are all as per standard but now I'm not so sure. I'll take a closer look when I do the swap over test

I've ordered up some hose so I'll swap the carbs over and try that when it arrives

One thing that Gitram mentioned: The spark is a bit pathetic and yellow so I checked the plugs and caps I put on it - BR8ES plugs and LB05F caps - both are resisted. I wonder if that could make a difference? I'll get some LZFH un-resisted caps and new BR8ES plus when I'm out tomorrow.

[zed1015]

Mar 29, 2021 9:19:20 GMT 1 geoffers997 said:

 I checked the plugs and caps I put on it - BR8ES plugs and LB05F caps - both are resisted. I wonder if that could make a difference?.

Yes! You must not have two sources of resistance, either the plug or the cap but not both.
Having both can cause the issues you described.

[Gitram]

good, the weak yellow spark can get blown out by the rise in compression, see what it's like with the new bits and if it's not.. then some more investigation is required..

marti

[geoffers997]

Hi All,
So, swapped the carbs and plug leads over. No good.

Swapped plugs over - cold pipe swapped sides. Brand new plug was duff. Annoying as it was an expensive one from Halfords.

New plugs and it’s running on both cylinders. Minor result.

Road test: once it was up to temp it ran great to about 5500rpm then starting making what I can only describe as lady farts and wouldn’t pull past 6000rpm.

I’m thinking high speed windings or CDI

I’ve read mixed reports about West Country Windings - is there anyone else out there who could test the stator and CDI?

[geoffers997]

I think I made some progress this afternoon. Herself was at work so I had some time to look at the bike, drink tea and think about the problem.

I ended up checking the needles (both are standard 5K1). I changed them both from 4 from top to 3 from top.

Test ride: bike pulls away much more cleanly and feels less boggy and crisper up to 5500rpm where it still bogs and won’t rev past about 6000-6500rpm.

Plugs are starting to look black and sooty rather than sopping wet.

If I get time tomorrow I’ll try moving the needles to 2 from top.

I’m thinking I’m going to buy new 240 mains and maybe a couple of smaller sizes too maybe 230 and 220 but I’m a novice at this so would welcome advice on sizes to buy - should I get smaller/bigger needles too?

I’ve seen KMT kits on fleabay - any good?

Thanks

[mintyfresh]

Apr 4, 2021 21:18:18 GMT 1 geoffers997 said:

I think I made some progress this afternoon. Herself was at work so I had some time to look at the bike, drink tea and think about the problem.

I ended up checking the needles (both are standard 5K1). I changed them both from 4 from top to 3 from top.

Test ride: bike pulls away much more cleanly and feels less boggy and crisper up to 5500rpm where it still bogs and won’t rev past about 6000-6500rpm.

Plugs are starting to look black and sooty rather than sopping wet.

If I get time tomorrow I’ll try moving the needles to 2 from top.

I’m thinking I’m going to buy new 240 mains and maybe a couple of smaller sizes too maybe 230 and 220 but I’m a novice at this so would welcome advice on sizes to buy - should I get smaller/bigger needles too?

I’ve seen KMT kits on fleabay - any good?

Thanks

[mintyfresh]

Thts electrics. I wudnt bother messing around wiv the carbs. Good luck.

[jessy03]

I’d replace the cdi, if it’s a 29k one chances are it’s buggered
Jess

[geoffers997]

Thanks - I’ve held the suspicion it’s the CDI that’s at fault - it’s a 29K-50

I’ve put a request on the Wanted page to see if anyone would be willing to lend me a known good CDI to test with.

[geoffers997]

Hi All,
An update:
I sent the stator for testing - it needed a full rewind, both high and low speed. got it back and reassembled it.

A very brief test ride had it revving past 6000rpm but I didn't have the time or road room to test more thoroughly.

While I was putting it back together and in order to start from a known point, I put the needle clips back to 4th from top; all the jets are as standard.

Its running rich so my question is this:

If I start moving the needle clip up to say 3rd or 2nd position will this have the effect of leaning off the mixture - this may be enough to sort things out

Failing that I’m thinking new 240 mains along with a couple of smaller sizes too maybe 230 and 220. I’m a novice at this so would welcome advice on alternate sizes to buy - should I also get smaller/bigger needles and pilots too?

I’ve seen KMT kits on fleabay - any good?

Hope it stays fine at the weekend so I can get out and test - she's working so I've carte blanche

[geoffers997]

Hi All,
Work and stuff has got in the way on the important stuff, but here's an update:

Rewound stator installed; swapped the coil over, got another CDI from a forum member and swapped that too. Marginal improvements were seen so I turned my attention to the carbs.

I thried fiddling with the needle position and airscrews but it was way way too rich. Sent them to Mutts and he did a service to get them into the proper baseline state; again a marginal improvement but not quite fat lady singing time.

I read the Mikuni document someone's posted, marked the throttle twistgrip so I could see its position easily and did some plug chops. The low to mid-range seemed the main problem area, there was just way too much fuel.

I put in 17.5 sized pilots (from the 22.5 standard sized ones) and....it was a revelation, it went like a scalded cat.

After a 60 odd mile jaunt, mixed riding, the plugs are showing dark brown to black shading.

Based on what others say I'm thinking keep it a bit rich rather than leaning it off too far and risk overheating / melted piston drama.

There is still a small amount of hesitancy in the mid throttle position, I'll maybe play with the needle height to see if that irons that out when I've some time and it stops bloody raining.

The carb bodies are 31W-00 (German restricted) rather than UK 31K-00 I'm given to understand the bodies are the same its just the jets that are different, but can't help wondering if there is some subtle difference between them that needed a smaller pilot

Put it through its MOT and it went through without any advisories.

[muttsnuts]

good news, but be very careful with such small pilot jets, they are used more than the main jet when cruising at speed, you could find its way too lean under load.

I'd be more inclined to drop the main to say 230 and put the pilots back to stock, but it sounds like it needs to go on a dyno or at least an A/F analyser to be sure you are fuelling correctly at the various rev ranges