Damn misfire....

[cliveyandrews]

What could be the cause of a misfire under load (literally just after pulling away) a real poppy, farty misfire. Lots of backfiring. Ticks over fine. I'd swapped std coils for stick coils - and had taken tank right off onto Ps bike. Have gone back to OEM coils. Still misfiring. Rechecked all hoses. Still misfiring. The quandary is - was the misfire there before I did the work. What could fail stood over the winter. Fuel tap diaphragm? But then that should only show when float bowls empty. I'm flummoxed.

[Ckennedy]

Do you have good voltage coming to the coils? Wouldn't have thought the diaphragm would cause your systems. Is it just on one cylinder or both?

[cliveyandrews]

Just the one cylinder. The front I think - that's where the farting seems to come from. Plug was quite sooty too. I tried a spare, only had an ordinary DPR9 though.

[cliveyandrews]

Obviously I have disturbed the connections when I changed to stick coils. How would I check that - is it not a pulsed voltage?

[Ckennedy]

If you could get the probs from a multi meter into the back of the plug on the coil then that would show a plusing voltage. This would obviously have to be done carefully as touching the live and earth together hurt. You could even use the battery as the earth to make life a little safer. I would have suspected electrical fault. Is everything clean and hunky dory at the plugs just under the tank?

[VTRDark]

You could also try swapping the, electronics, Ignition Control Unit and Converter Unit in the tail fairing if you have spares knocking about just to rule it out. Also check the connectors while your at it to make sure they are all good. Water has been known to get in that area before, but unless you have a leaky garage roof that shouldn't be a problem.

Personally I would start with the spark plugs. I know you say you have swapped them but unless you know the condition of the plugs, which is hard to tell, new ones is the way to go. I have had that issue with Marty's bike before. It's annoying when one goes checking everything else only to miss the basics.

(:-})

[lloydie]

If you don't have spare black boxes pop over on it as I have a set or two spare you could try and I could have a look over it for you too

[cliveyandrews]

I'll give the plugs a go. Riding is out of the equation - it just won't pull. On one Cyl soon as I try any load....

[AMCQ46]

Have you trapped a vent pipe from the tank? Or damaged the vacuum tap?

My bike went onto 1 cyl when the tank airlocked on a hot day in Yorkshire. It runs on one as one carbs lower than the other but leaves you convinced it's electrical!

It has to be something that you fiddled with when you had the tank off to put on Pauline's bike, so keep checking

[cliveyandrews]

Good thought. I had trouble with the crankcase breather, and the other line (braided pipe, venting to tank) has been swinging round since I bought it.

[cliveyandrews]

So, new plugs in and all the hoses checked. Still misfiring. I'm running out of ideas and patience. It can only be the carbs. I think I might have to pass it to the experts.

[tony.mon]

Are the carbs properly seated?
It's easy for one to pop out of the rubbers and allow air in.
But this sounds electrical to me- try swapping the stick coils front to back, and see if the problem also moves.
if so, there's the issue.

[cliveyandrews]

I put OEM coils back on, same problem as the stick coils. I think it's something on front cylinder. I wonder if it's a needle sticking? It ticks over ok, but pops and bangs as you pick revs up. By 3,000 it's failing, 4,000 revs and it's dropping to 1 Cyl. Put load on it, and it drops to one quite rapidly. Lots of backfires....

[VTRDark]

It ticks over ok, but pops and bangs as you pick revs up. By 3,000 it's failing, 4,000

Now that description does sound like a slide has stuck open. If your feeling is that it could be a carb issue then you have to go with your instincts as were not hearing or feeling it and only have you description to go by.

You can easily check the slide just by removing the airbox lid. You may even be able to un-jam it if stuck open with your finger. A little squirt of carb spray ideally along the slide runners will help, but you can get away with a light squirt of WD. I had this happen once on the way down to Mac's with Marty, diagnosed it straight away as I was OK at higher revs wide open throttle. Below that the bike was virtually un-rideable. Defo worth investigating as it cost nothing and don't involve loads of work.

(:-})

[cliveyandrews]

Slides are both free. Had tank off last night :-( My only observation was there was no fuel came out of offside fuel pipe, which Cyl does that feed? It might just be gravity as bike is on side stand. When I take the tank off again, I'll give the vac pipe a suck to see if it flows both sides. I'm sure it just tees off at the back....