1975 BMW R90/6 - Rocinante

[ex119x]

Once again, Doc is right with what to do. However, that is not what I would do since I am cheap and I like to make things work.
To me, there are 3 potential problems;
1. the bore is glazed over and the rings are fine but not sealing well. Deglazing the cylinder and making sure the rings move freely in the groves might bring back the compression.
2. The rings are worn and fit too loose on the piston, don't have enough tension left to make a good seal and/or the ring gap is way to big. Honing the cylinder and replacing the rings might bring back the compression.
3. The cylinder is worn beyond the point that even new rings will seal so the cylinder needs to be bored and a new oversize piston and rings fitted. There are some that believe it is OK to only bore 1 cylinder of a twin but that has never sat well with me.

If it were me, I would go with option 2 but use the next oversize rings so I can set the ring gap to the tight side of specs to kinds cover any wear in the cylinder. It may or may not work and might be a temporary repair, but given how easy it is to pull one cylinder on an Airhead, that is what I'd do.

 

[doc_rot]

Option 1 and 2 are totally viable and reasonable repairs, but unless you know how worn the cylinder is its just a guessing game. You could go with option 2 and end up having to do option 3 anyway. Measure the bore first and know for sure. You could take it to a shop and they'll measure it for you or do it yourself. You don't need a super high end tool to do this because all the measurements will be relative to themselves so outright accuracy is somewhat irrelevant. You mostly just need to be able to make repeatable measurements.

that being said i would check the valve clearances and give it the ol' Italian tune-up before doing anything else

3-4" Precision Outside Micrometer 0.0001" Carbide Tipped,Standard US New | eBay

<li> Feature??</li> <li>3-4" x 0.0001" precision outside micrometer.</li> <li>carbide tipped measuring face</li> <li>ratched stop. lock nut.</li> <li>0.0001" graduation reading</li> <li>Woth 3" standard.</li> <li> </li> <li>Description??</li> <li>This product has undergone rigorous quality...

www.ebay.com

50-160MM Dial Bore Gage Gauge 0.01MM Engine Cylinder Indicator Measuring Hole | eBay

(Interchangeable probe depending on aperture size. 1 Set Dial Bore Gauge. Protective shell. 12-Piece changeable probe set. We can resolve the problem. excluded.). We will do out best to make you satisfied!

www.ebay.com

 

[ridesolo]

or are the valves out? the CX500 I did had like 80 and it was a tight exhaust valve. Fixed that and it went to the 150 it needed to be.

I'm pretty comfortable w/ the valve adjustment. I've gone through that a couple times on both sides and they're so easy to do, I'm pretty sure they are right. However, that's one of the things I'll check before actually pulling off the heads.

It could also be the valves have some crud on them, have you tried the “Italian tune up”?

You really need to measure the cylinder bore to see what’s going on. If there is taper, oval, or the piston/cyl clearance is too great no hone job is going to correct that and you will need to oversize.

If I do pull the heads and cylinders I'll check the bore. I've got inside mics for that... somewhere out there.

Once again, Doc is right with what to do. However, that is not what I would do since I am cheap and I like to make things work.
To me, there are 3 potential problems;
1. the bore is glazed over and the rings are fine but not sealing well. Deglazing the cylinder and making sure the rings move freely in the groves might bring back the compression.
2. The rings are worn and fit too loose on the piston, don't have enough tension left to make a good seal and/or the ring gap is way to big. Honing the cylinder and replacing the rings might bring back the compression.
3. The cylinder is worn beyond the point that even new rings will seal so the cylinder needs to be bored and a new oversize piston and rings fitted. There are some that believe it is OK to only bore 1 cylinder of a twin but that has never sat well with me.

If it were me, I would go with option 2 but use the next oversize rings so I can set the ring gap to the tight side of specs to kinds cover any wear in the cylinder. It may or may not work and might be a temporary repair, but given how easy it is to pull one cylinder on an Airhead, that is what I'd do.

1. Could be glazed. I know the rings moved freely, I had to reset them to 1/3 intervals before reassembly. One side, don't remember which, the gaps were all only a few degrees apart and the other side they were farther apart but not evenly spaced. I used assembly lube when I put things back together and I remember thinking at the time that the cylinders slid back on "not too easily but not too hard either" if that make sense.
2. Probably what I'll try: hone and new rings.
3. Hope not, but if that's it then so be it. I'd do both sides for sure... the bike would want to go around in circles, right?

Thank you all for the good comments.

 

[Hurco550]

If you pull the cylinders, I have a good set of telescoping gauges and micrometers here to measure them out well. Seems to me that you've likely found the culprit, and should be an easy enough fix. I would recommend that if you have them off and they need honed, lets just run them over to chris at Mototech in Mansfield. Hes an airhead guy to boot, and what he'll likely charge will be comparable or less than buying a hone....

 

[ridesolo]

If you pull the cylinders, I have a good set of telescoping gauges and micrometers here to measure them out well. Seems to me that you've likely found the culprit, and should be an easy enough fix. I would recommend that if you have them off and they need honed, lets just run them over to chris at Mototech in Mansfield. Hes an airhead guy to boot, and what he'll likely charge will be comparable or less than buying a hone....

Sounds good to me. I'll order rings and then we'll set up a time.

 

[ridesolo]

Anybody willing to donate a stick of dynamite?

Rings in + valves lapped = good compression. Still not running on the left side.

Completely stumped, but I'd say it's pretty much got to be the carb, guess I'll rebuild it again.

Open to more suggestions.

 

[Hurco550]

Did ya end up swapping the coils?

 

[ridesolo]

Did ya end up swapping the coils?

Yes I did. Got a, perhaps, fatter spark but otherwise it didn't make any difference.

 

[Maritime]

Yes I did. Got a, perhaps, fatter spark but otherwise it didn't make any difference.

can you swap the left and right carbs? or are they side specific?

 

[ridesolo]

can you swap the left and right carbs? or are they side specific?

Yea, they are side specific for sure. Probably could swap them to just do a quick try... don't hook up choke or throttle cables, etc. I've thought about it and might end up trying it if the rebuild that'll happen tomorrow doesn't change things. Even used ones are '$pencive so I'm hoping that a really super careful cleaning, inspection, and reassembly will cure things. There was so much stinky, slimy goo in the things, especially this one, that I'll admit that it's possible there's something hiding in there that isn't allowing proper air/fuel flow/mixture. I'm hoping there isn't a part or orifice in there that can't be replaced that's gotten corroded away. Almost everything in them can be replaced w/ available parts... almost.

 

[Maritime]

Yea, they are side specific for sure. Probably could swap them to just do a quick try... don't hook up choke or throttle cables, etc. I've thought about it and might end up trying it if the rebuild that'll happen tomorrow doesn't change things. Even used ones are '$pencive so I'm hoping that a really super careful cleaning, inspection, and reassembly will cure things. There was so much stinky, slimy goo in the things, especially this one, that I'll admit that it's possible there's something hiding in there that isn't allowing proper air/fuel flow/mixture. I'm hoping there isn't a part or orifice in there that can't be replaced that's gotten corroded away. Almost everything in them can be replaced w/ available parts... almost.

I feel your pain and hope another cleaning and rebuild do the trick. We've been messing with William's Hawk carbs trying to get it running right again since his crash and choke cable snapping. found one running crazy rich no matter what we did to the pilot and finally measured the float and it was way off so adjusted it and then when we were putting them back in the bike he cross threaded a spark plug and now we're waiting on a repair kit to see if we sorted the bad carb.

 

[adventurco]

Is that cylinder just flat out not running at all? Does the header warm up even a bit or is it stone cold after you've run it for a minute or two? I would think even with a little shit in the carb, it would still fire sporadically. If you pull the plug, is it wet?

 

[ridesolo]

I'm now certain that it's a carb issue. Everything back together and it'll run on both cylinders and, frankly, run pretty good, if the float is set so that it's puking gas out of the overflow. I adjust the float a tiny bit and it won't run on that side and even opening the throttle on just that side does nothing. So it's the carb for sure. I guess I'm just ham-handed or something. Honestly getting pretty tired of this.

 

[ex119x]

It may be time to get rid of the Bing CVs and put on some round slide Mikunis.

 

[Hurco550]

It may be time to get rid of the Bing CVs and put on some round slide Mikunis.

I have a 76 R90/6 north of 100k miles. About 25k of those I've put on myself since owning it. Recently the carbs decided to start acting a fool, and after multiple rebuilds and fiddling, I've decided it's time to do the same.

Unfortunately, the guy that ran Rocky Point Cycle that built the kits recently passed away I believe, and though SpeedMotoCo is now making the kit, after talking with them, I'm not convinced that it's a true plug n play as they just come with a generic UJM style throttle cable.

https://speedmotoco.com/r90-bmw-vm34-replacement-carburetor-kit-vm34-mikuni-kit-1973-1974-1975-1976/

 

[ridesolo]

Replacing the Bings IS tempting, that's for sure. I'm not ashamed to say that I'm getting pretty disheartened about this. I've put a huge amount of time and effort into the thing and have patiently been chipping away at trying to get it right. Mrs Ridesolo knows me well and will readily admit that I'm one of the most patient people she knows and at this point even she's amazed at how consistent and patient I've been with it. At this point I don't think I'd even be too upset or embarrassed if somebody came along and quickly pointed out some little stupid mistake... I just want to get to use the old girl!

 

[adventurco]

This was probably covered, but did you check the diaphragms for cracks and tears?

Probably worth replacing, if they are available. This has gotten me in the past on CV carbs.

 

[ridesolo]

Thanx, Nick... Yeah, I've replaced the diaphragm, I used the new one in the rebuild kit and I've got an extra set back as well and the one I replaced was good. Based on the "runs on both cylinders when it's puking gas out of the overflow" issue, I'm wondering if the seat for the float needle is bad. I did somewhat of a lapping on it and to my eye it looked cleaner and smoother. I haven't replaced the needle itself but i did do a clean-up of the end of the needle. It was a little ridged and i managed to smooth it out. The little ball and spring at the bottom are working fine. I don't know how long I should keep throwing $$ at things, but I could order up a new needle and I guess the seat can be replaced but I'm sure that's a job for a machinist.

If I take the float bowl off when is it isn't running right there's very little gas in it... needle not opening enough... When it runs good it's puking gas out the overflow... needle not seating. Seems to be no in between. So, I guess the question is: Bad needle or bad seat? Is my logic off?

 

[Maritime]

Thanx, Nick... Yeah, I've replaced the diaphragm, I used the new one in the rebuild kit and I've got an extra set back as well and the one I replaced was good. Based on the "runs on both cylinders when it's puking gas out of the overflow" issue, I'm wondering if the seat for the float needle is bad. I did somewhat of a lapping on it and to my eye it looked cleaner and smoother. I haven't replaced the needle itself but i did do a clean-up of the end of the needle. It was a little ridged and i managed to smooth it out. The little ball and spring at the bottom are working fine. I don't know how long I should keep throwing $$ at things, but I could order up a new needle and I guess the seat can be replaced but I'm sure that's a job for a machinist.

If I take the float bowl off when is it isn't running right there's very little gas in it... needle not opening enough... When it runs good it's puking gas out the overflow... needle not seating. Seems to be no in between. So, I guess the question is: Bad needle or bad seat? Is my logic off?

try swapping the needles between carbs and you'll get an answer on whether it is the needle or the seat. It's fun playing with this shit isn't it LOL. For some encouragement we fixed Magoo's hawk with the float adjustment and I stole it to ride to work this morning. Ran great so you'll get there on this one I am sure.

 

[Rider52]

I wonder if that carb is just worn out. Back when I actively restored antique motorcycles it was not uncommon.