'74 R90/6 misfire

[ryanjahn]

howdy!

Within a week the temperature has dropped from 70 to 18-20 degree weather (austin, tx). It's been 20 for a week now. Since then my bike is constantly loosing power while riding. It's like a sporadic mis-fire - the carbs will occasionally make this 'FffWAP- ffwp, ffwop-- FFWomp' sound while riding (at least it sounds like it's coming from the carbs) - and every time it happens the bike looses power for one rev. of the pistons - but that sound happens pretty consistently during the revolutions like 5 out of 30 revs i'll hear "FWAP" sound and the bike will ungulate like it wants to throw me off. It happens primarily once i get going in 1st - a whole lot in 2nd - less in 3rd - and rarely in 4th gear. got any tips. I'm browsing the forums for some answers - just not sure where to start and what to look for, but every day it gets worse - so i've parked it till i can figure this out.

Had this bike for about 2 years now. Super new to wrenching and airheads in general.
I've always taken her to the shop, but I think it's time I start wrenching on my own bike - so I need some guidance on where to start for this issue.

Any help is much appreciated!
Ryan

[Airbear]

G'day Ryan, and welcome to the forum. That's a nice looking bike.

I applaud your desire to start 'wrenching' on your bike. There's a lot to learn, and it is through dealing with the little issues along the way that will get you there. Your current problem sounds like an ignition issue to me but it would be good to get a clearer picture of what is happening. It would be useful if you could give some more information:

Does the bike still have points ignition?
Has anything been 'done' to it recently? Carb overhaul? A regular maintenance service?
Does the problem happen in any particular rev range?

My old girl, when she was still running on points, would start misfiring at about 4000 RPM. This would steadily worsen until I stopped, popped off the front cover on the timing chest and cleaned the points with a points file. This was a quick fix that would get me to where I wanted to go. I've recently switched to an Omega electronic ignition.

If you want to investigate the points, make sure you disconnect the battery negative lead before you remove the front cover - you really don't want to short out the diode board.

If you've been researching on the net you will have discovered Duane Auscherman's and Snowbum's sites. There are others - google is your friend. I'd recommend getting hold of the Clymer Manual too. I have found it to be the most useful of the hard copy material. It's got lots of pictures and step-by-step instructions.

Anyway, give us whatever info you know about your bike and what has happened to it recently. Others will be chime in with advice.

[ryanjahn]

Thanks for the direction. I took a look at the points and nothing seemed out of the ordinary. sparkling clean and the gap is perfect. So that got me thinking, what else should look super clean? The plugs! Took em out, cleaned em off - runs perfect.

Thanks for resources on the web!
R

[bbelk]

I bet if you brought that bike to the rally right up the road from you this may, it would get lots of loving attention and maybe even get fixed. There will for sure be lots of theorys thrown out.

More on the rally: http://boxerworks.com/forum/viewtopic.php?f=30&t=1476