My r90 restoration.

[Kai Ju]

Didn't you say that those were new diaphragms ?
If so, how did they get damaged so quickly ?
Is it possible that the replacements you just installed suffered the same fate ?

If they are torn again, and on the same side, you may have an issue with your carb body, or installation method.
Something to look at.

The more I look at that photo of your diaphragms, the more I see a cut caused by pinching. The cuts are evenly spaced and on the same vertical plane.
And from your description of the test ride, it sounds like it happened again.
I've had good luck by placing a spacer, like a dowel, under the slide so it's at full throttle and installing the carb top with the slide in that position.
Make sure to remove the dowel before you run it

[blaine.hale]

I wish it happened again and the solution were that easy. However, I pulled them both again to check and the new ones were still in perfect shape and keeping the top side dry.
I went to swap the carb sides today but didn't have spare fuel line to make the distance and the throttle cables need to be re-routed for that to function.
Here's a video of what's going on. Keep in mind, it only will start doing this after warming up and running around for maybe 10 minutes. Cold start, it will function normal.

[Beater]

Your solution is quite easy.

Mikuni.

[blaine.hale]

Originally Posted by Beater

Your solution is quite easy.

Mikuni.

*gasp!
If only Mikunis grew on trees :)

I've got the right carb soaking in some good ol' Berryman carb dip for tonight. Running out of ideas short of finding a new, matching carb somewhere.

[Airhead Wrangler]

Originally Posted by blaine.hale

I've got the right carb soaking in some good ol' Berryman carb dip for tonight. Running out of ideas short of finding a new, matching carb somewhere.

I hope you took every last piece of rubber out of it or the berryman's will do that for you. Be sure to follow it up by blasting out every last little passage with compressed air. Make sure the choke goes back in right too. Good luck. May the bing force be with you.

[brocktoon]

Nobody in the Georgia area has a matching, known-functioning carb to help this guy diagnose the problem? I thought there was a big Airhead community down there, as well as a spirit of southern hospitality.

[Beater]

Originally Posted by brocktoon

Nobody in the Georgia area has a matching, known-functioning carb to help this guy diagnose the problem? I thought there was a big Airhead community down there, as well as a spirit of southern hospitality.

Brad offered his which are set up for an R80 ... I have a set of 40mm Bings that are not tuned quite right for an R100 engine ... and my other bike has Mikunis which are dead on for a higher compression R100 engine.

None of us have an R90. Sad I know. I don't know of anyone close with an R90 and 32mm Bings.

[blaine.hale]

I am one of an isolated species :)

Maybe the carbdip will clean out some unknown oddity that I couldn't get back when I ultra sonic cleaned it and did the ol' pinesol bath.

Just to keep sanity...still sounds like a carb issue, right?
I mean...it's pretty damn obvious it is when I tug on the right throttle and nothing happens, heh.

[hardwaregrrl]

I offered mine as well...32 bings just rebuilt this summer.

[blaine.hale]

Bring some bings by this saturday for the tech day, we'll have a bing party!!

[Wirespokes]

It sounds to me like it's running rich at idle - that's what the popping sounds like. Unburnt fuel going off in the hot exhaust.

If that's the case, the problem isn't a blocked passage.

And just swapping the carbs without attaching cables would certainly tell whether the popping migrated also. Hell, you don't even need to attach fuel lines for a short test. Just have them full when swapping. I can run a couple miles on the contents of the float bowls.

[blaine.hale]

Originally Posted by Wirespokes

It sounds to me like it's running rich at idle - that's what the popping sounds like. Unburnt fuel going off in the hot exhaust.

If that's the case, the problem isn't a blocked passage.

And just swapping the carbs without attaching cables would certainly tell whether the popping migrated also. Hell, you don't even need to attach fuel lines for a short test. Just have them full when swapping. I can run a couple miles on the contents of the float bowls.

That would make sense with it only happening when it's warmed up. Would "over fueling" drop a cylinder like that, though? I assume the plug would be rather wet, when pulled, as well?

[bmwblake]

2 thoughts:

1. since the issue only happens once things are nice and warm i'd suspect a coil or something electrical that would have an issue only when warm but work well at colder temps. i'm guessing that you've testing everything with a meter but have you done so at fully warmed up temps when the issue is happening as well as at cold temps to compare the results?

2. the video sounds like a possible air leak, but that wouldn't happen only at full temp. have you sprayed ether around your carb to head connections to make sure they're tight? if you spray and the engine speed increases you have an air leak.

[supercreep]

are you using an electronic ignition? one of my harleys had an intermittent fault in the ignition module that would cause problems once it came up to temp.

i would be looking at the ignition on this bike if i were you - best of luck!

[blaine.hale]

Originally Posted by blaine.hale

It's time I crowd source this issue.
Fred and I have some ideas but I'm saving them for last effort kind of thing.

Symptoms:
The bike will crank (some hesitation) then run really really rough until 4-5k rpm. By rough, I mean one cylinder just bogging and dropping (right cylinder.) Once I get up to speed and warm it up a little bit on interstate...if I can convince it to do that, coming back off the throttle to slower speeds will kill the right cylinder and cause some nasty back firing.

Here's what's been done so far:
Pulled both carbs, cleaned and inspected all jets and tunnels. Everything was super spotless and float height was great and still is great for the past year. I blew compressed air through every single channel/tunnel to insure all blockages were clear. On top of this, being that I now have Hoske pipes with more flow, I bumped the idle jet to 50 (no difference made, at the moment.)
Next was electrical. Timing was slightly retarded, so I fixed that (no effect) spark was great when I pulled the plugs and cranked them a bit. Resistance is all fine. 3 sets of coils off of all working/running bikes were tried. 3 sets of plug cables all tested as well. Even crossed the cables to see if it was one side failing (nope.) 2 ignitions tested. Boyer electronic and stock points.
Finally, reset the valve lash. Right side was dead on. Left side was a bit tight (weird.) No valves are sticking, I can watch them go through their motions
All of that done and still the exact same issue, no improvement ANYWHERE.

Way back when, the top of the cam sheered off where you put a nut on to retain the points. This was drilled and tapped out for a screw. I know for a fact that it is not deaaaad center (pretty close.) Could this be the issue?
Could the slight off center bolt up there caused the bearings to warp a bit and not open the valves at the right time and/or screwed the cam?

I know I can pull one top end, put the piston at half on the cylinder then push to see if there is any give in order to check if the cam is off.

All of this feels like mechanical falliar, not spark/air/fuel. My understanding is that electrics generally break down in high RPMs more so than the low side.

Thoughts/suggestions/things I missed?

Anyone want to buy an r90? :)

Just to re-post what's been done since it's been a while.

Coils would show themselves as an issue when I crossed the cables (the issue would migrate to the other cylinder but did not.)
Point ignition is currently mounted up. Went through the boyer electronic as well. 3 sets of known good coils and spark cables were tried as well.

I'm going to try bumping the idle mixture back down to the stock jets tonight then swap the carbs, if I'm able to get the throttle cables to fit that way.
Other than that...that's all I have until I can find a new, matching carb to try out :)