Missfire

[andrew.chapman]

I have a VH that runs perfectly well through most of its operating range. It starts well, it ticks over well, throttle response is good and the plug is the right colour. However, if I open the throttle more that 3/4 open at higher engine speeds, it missfires. At first it just skips a beat or two but, if I hold the throttle open for an extended time, the misfire gets steadily worse. If I close the throttle a bit, the misfire goes away. That sounds to me like weak mixture.

I have had the misfire since I have had the bike, which is several years. Most of the time it doesn't bother me very much but it has a tendency to become a problem when you least need it, when you are overtaking something near the top end of its performance range.

I guessed that it was probably an issue with the main jet but experimenting with jet sizes has not fixed it. I am runnng with a Monoblock with a 200 main jet.

Has anybody experienced anything similar and, if so, did you succeed in fixing it?

[dave.owen]

It is often the case that the problem could be another sytem on the bike.Air leak it could be ,but is the ignition system set up correctly, the mag might just be overloaded at high revs if it isn't perfect. worth looking at

Dave

[chris.vredenbregt]

try to put the needle 1 position higher, when idling your engine runs on the stationary airmixture with a fixed jet.
when opening the trottle the needle starts working when opened 2/3 so by puting the needle higher you make the mixure richer...chris

[brenton.roy]

Hi Andrew, I'm working through a similar issue with my VH and will be interested in what you discover.
I have been reading that modern fuels are effectively a bit richer. Loading up (rich) can give symptoms like you describe.
From 3/4 you should be on the main jet. Do you remember which jets you tried?
I have had made a definite improvement by leaning the main jet (from 200 to 180), but have only been able to try short runs as the 70mph zone is a reasonable way from home.
I think the only way to really work out jetting properly is to try a "plug chop" and read off a colour chart.
Also, as Dave suggests - a simple thing, but do you have enough points gap? The Waller setting is 12 thou. The points might be floating a bit / the dwell might not be enough to make a decent spark at revs.
Brenton

[john.mitchell]

Andrew, I presume the little hole on top of the petrol cap is not wholly or partially blocked ...

[nevhunter]

Some of the oil additives can coat the plug insulator and short it out. Try a new plug. Also the plug may be a bit hot. Worth trying a colder plug
Air leaks show worse at low throttle opening when there is more vacuum. If it may be lean at the top of the range check main jet and fuel flow to carb through tap and lines and needle/seat in bowl. Nev

[chris.shearwood]

Hello Andy,
When my 51 VH exhibits those symptoms I replace the spark plug with a new one, gapped to 15 thou, and that usually solves the problem. I don't claim to be an expert on old bikes and their foibles but J.B.Nicholson sure was and this is what he has to say, in Modern Motorcycle Mechanics 7th Edition, about misfires (I strongly advise any owner of old bikes to buy this book, it is the best manual by far that I have come across):

In the trouble shooting section, under the column "Trouble": Motor cuts out and misfires on load, pronounced climbing hills in top gear when throttle is fairly well opened. "Probable Cause": dirty spark plugs. Carbon deposit on plug centre electrode insulator shorting high tension. Plug points spaced too wide. Weak spark, ignition trouble.
and again under "Trouble": Medium speed misfire - high speed misfire. "Probable Cause": Mixture too rich, magneto or coil defect. Under "What To Do": Throttle needle set too high, test out smaller main jets. Test with air cleaner disconnected. Check out mag armature or ignition coil.

If you still think your bike is suffering from too lean a mixture can't you close the air slide a bit when the problem occurs? From what Nicholson says, I would expect this would exacerbate the problem but one way or the other at least you would know in what direction you should be heading with your carb adjustments.
Regards, Chris

[david.anderson]

Andrew
There are a couple of things worth checking. What is the fuel supply like from the fuel tap after a couple of minutes. A dirty fuel tap filter screen or inadequate petrol cap breather as already mentioned can restrict flows out of the tap and the fuel flow will decrease with time. Next there is a fine filter under the fuel banjo fitting onto the carb. That filter can build up with gum from the fuel that will severely restrict flow. The gum deposit is virtually invisible and appears to be part of the fine gauze. Years ago I had a car that only missed up hill under wide open throttle. It took a bit of finding but was gum on the fuel filter inlet to the carb reducing fuel flow and causing starvation.
Finally 90% of carburettor problems are actually ignition.
Brenton
A 180 main in your 1 1/8 289 carb is probably very close. My 49 VH had been rejetted from 200 up to 210 main which was correct for our previous leaded fuels. I have jetted back to 190 for unleaded (Australian fuels). As you have a higher compression I would expect a slightly smaller main jet as you generally have to lean off with higher compression ratios. (E10 fuel seems to like original jet sizes)
David

[andrew.chapman]

Thanks a lot guys. There are a lot of good suggestions there. I had guessed that the problem was attributable to either the main jet or spark under high loads. I have already eliminated all of the things that relate to fuel starvation. However, my guess was that the mixture was too lean rather than too rich, so I have been experimenting with larger jets. I have now ordered a 180 & a 190 to play with. I will also try a smaller spark plug gap.

,Incidentally, I now use a Thorspark with a battery and coil. But the misfire was also present when I was still using the magneto. Therefore, I have assumed that it is not related to spark generation. Such assumptions can always be wrong, of course.

[john.nash]

However, my guess was that the mixture was too lean rather than too rich, so I have been experimenting with larger jets. I have now ordered a 180 & a 190 to play with. I will also try a smaller spark plug gap.

That one's easy.
Run it to where it usually plays up and then lower the choke (I assume you have the handlebar lever ??).
That will cut down the air getting through and richen the mixture.

If it starts to bog down, as soon as the you start lowering, then you are probably about right on jet/needle as the bogging down would inicate too rich.
If however you can merrily lower away with either an appreciable gain in speed or no effect at all, then it's bigger jets or a massive air leak somewhere....

Unfortunately, there isn't an easy way to check for running rich apart from reducing jet size.

When I went to a twin port head, I had to come down on the main jet by 15% as it was misfiring. Fine at idle, but as soon as you cracked it open at speed it all went tits up.