Engine misfiring when warm

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johnpclynch

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I think it's an electrical problem, I've checked the basics but the problem still exists... here goes

My VMax has been running sweet for months now without any major problems. About 10 days ago the engine started coughing while I was driving to work, it happened to be the morning of the coldest day I can remember so I put it down to carb icing or some cold related problem. Later that day the bike wouldn't start so I went off, charged the battery & cleaned the plugs (they were pretty dirty) The bike started no problem and I drove home but on the way it started coughing again... :damn angry:

2 days later - new battery (Vmax Battery replacement 101 http://www.vmaxforum.net/showthread.php?t=113), plugs, HT cables, carbs, fuel additive (and a look over every connector on the bike) and the problem is still there :ummm:

The bike starts without a problem, runs fine with the choke on. I go for a drive and it takes about 5 minutes before it starts misfiring. Under acceleration there is no problem, only when idling/cruising. I tried unplugging each plug individually to see if a coil is blown but the engine revs always drop.

Q: ANYBODY have any clue as to what it might be? Could it be mechanical, head gasket/valves/oil pump etc?

PS: I had the carbs & new battery waiting to fit as part of a spring revamp when the weather improved. Regulator/rectifier was beefed up in November & I have an old CDI lying around that I'm going to try during the week
 
Sounds like a possible cracked airbox boot,carb boot or vboost boot leaking air or loose not sealing up good.

I've sen the air box kinda pop up off of the carbs,Boots seem to get hard and not want to stay on.
 
Sounds like a possible cracked airbox boot,carb boot or vboost boot leaking air or loose not sealing up good.

I've sen the air box kinda pop up off of the carbs,Boots seem to get hard and not want to stay on.

Never thought of it, carbs were in & out pretty often while I was getting the jetting right. The rubber has gone quite hard over the years

A quick rub of silicone should patch it up until I replace them, if that cures it then I shall be your slave

:worthy:
 
we've run into a rash of bad CDI boxes over the last year. Could be that is the problem too.

Sean
 
If it runs well under throttle then it's probably not a fuel issue per say but - You did say that your plugs were dirty so it make me wonder if at times your running rich and at other times it's fine. Are you getting a good spark you think all the time ?

The bike will run flat if rich, and stumble and surge and backfire if on the lean side as you transition through to the main jet.

I would look at that mid range for a possible problem - when the bike is doing that funky thing - hit the kill and pull in the clutch at the same if you can and hold the clutch in until your stopped and read your plugs.

Then I would look for possible causes of it either way - rich or lean for one or more cylinders.

It's where I'd start....just my little 2cent suggestion - good luck
 
So I spent the weekend going over all the electrical connectors & cleaning the contacts, then I drained the floatbowls & did the peashooter on the carbs. Finally I flushed out my fuel filter (running petrol the wrong way and shaking it about the place) you'd be amazed at the amount of fine rust that came out of it.

Conclusion: the problem went away :eusa_dance:

Some of the connectors were pretty dirty but it was the filter that got me thinking, what happens when the engine can't suck enough petrol from the tank? :bang head:
 
It goes way lean and starves. Opposite would be dirty air filter and then entire rev range would go rich.

Glad you got it sorted out!

Mark
#1098
 
I was worried about my CDI, valve clearances, wiring loom, carbs etc. All it turned out to be was a $5 fuel filter

The simplest answer is usually the correct answer
 

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