More Carb (& other) problems

I now have a functional, running '75 R90/6 -- but just barely. Here's my latest tale.

Item 1: I can't adjust my left carb. Any changes I make to the mixture needle are irrelevant: no slowing down or speeding up. (I adjust and wait a few seconds, adjust and wait a few seconds, etc.: nothing.) Also, when I try to balance the carbs by shorting out the plugs, I can short out the left plug and the right side will slow but keep running. When I short out the right plug, the left side will not keep the motor running and it will stall. It stalls even if I open the throttle significantly. (I do this by screwing in the idle stop screw.)

I have checked the carb thoroughly. I have disassembled it six times: once for the initial cleaning and rebuild (new O-rings, diaphragm, and gaskets) and five additional times for re-cleaning and to check installation of all parts. I checked the diaphragm and enricher particularly for correct installation and I checked all the jets, which are numbered correctly and in the case of the needle jet area, installed as per the Bing diagrams.

My only thought at this juncture is to replace the carb. This is not an inexpensive proposition, however, and I am hoping someone will have a known, functioning carb that he would let me install only to verify that it is the carb that is the problem. I propose installing the loner carb, taking a warm-up ride, and checking for mixture and idle adjustment capability. Noting the carb's original needle/stop positions, I'd reset the carb and return it. (Hey, we could even do this together. It only takes a few minutes and I can bring the bike to the donor carb!)

As a secondary option, I am looking for a known functional, second-hand carb, 64/32/11, to purchase. (I suspect I need to do this anyway.)

The primary symptoms I now notice are: poor low speed power (1200-3000 rpm) and less than normal acceleration. The bike will run fast and will sustain 5000 rpm on the road, but power seems down.

Item 2: I can't set timing statically with any accuracy because the bike has Brandson Boyer ignition. I wanted to test whether this is working properly by swapping it out for the original system, but I don't have the points carrier plate and advance mechanism. (PO did not provide it.) So I'm looking for these parts, too.

Item 3: I can't time the engine using a strobe light because I can't see the marks when the strobe is flashing. I can only assume here that the previous owner has installed the flywheel other than with the OT mark visible when the pistons are at TDC. Is it possible that this has been done? Is it possible to do this? I have not replaced a flywheel, but the shop manual would seem to suggest that it is possible to instal it in error.

As always, I'd be most grateful for any information, assistance, and parts.

Thanks,

Stephen, 1975 R90/6

Hi Steve. This is a long shot.......... double check that the carb bowls did not get swapped to the wrong sides like mine were once. I had a couple weeks of hell until I finally saw that.
On the timing marks, pull the spark plug out and put in a small wooden dowel over 12 inches long. Turn over by hand until TDC when dowel sticks out farthest. Do you see a TDC mark on the flywheel now ? If not then something sounds screwy. There should be advance timing marks on the flywheel just before TDC also.

Hi Steve,
I had terrible carburetor problems where I couldn't adjust the idle mixture on one side. I assume you're waiting for the bike to really warm up before adjusting. I did the rebuild thing to no avail except I felt really good afterwards. My bike ran fine at normal speeds unlike yours, which meant I could use it. I think, based on a dead donor carb which I dissected that I may have had a gummed-up idle circuit which I understand is really difficult to remedy. I even looked into ultra-sonic cleaning but it somehow corrected itself during my search. It is also possible that the idle mixture seat is damaged from someone over-tightening the screw. Is it possible to swap your carbs left to right and see if the problem moves with the carb? I know the cables might be a mess but?? I suspect there are carbs sitting unused on basement shelves out there but people seem reluctant to share! If you do locate such an individual, nurture that relationship!!! I would loan you mine if it would fit but they are 26mm slides. I do have some valuable literature on carburetors, including the Bing manual with an excellent description of how the carburetor actually works as well as Keith Patchett's most EXCELLENT articles on Ping, Boyer Ignition, and voltage regulators. call me if you like 857-9569
Chris

Hi Steve,
Unfortunately for you I have Dellortos but I also have gone to the Boyer Electronic ignition. So I have, or should have the points carrier plate and advance mechanism. I can lend these to you.
However I would experiement around Tree's suggestion of relating actual TDC to the markings on the flywheel. There is also an 'S' and 'F' marks I think the 'F' is for Fulladvance.
I'd have to study the 'S' topic before commenting on that indicator.
Best of Luck.And let me know if you need to borrow the points parts.
Good Luck. Amédée

Thanks, guys. I'm working my way through all the possibles -- so far without a remedy, but I'm not done yet!

Unfortunately, I seem to have other responsibilities so bike work is intermittent. This work/real life balance can be a drag, but not quite as much as a non-running motorcycle.

Stephen

So Steve, did someone reinstall your flywheel off the marks ?
Do you have TDC marks showing anywhere near top centre ?

So... Finally I have some time (vacation this week) to get back to my poor bike and complete this installment of my tale.

I was picking up some carb bits from Chuck S. and relating my story when he reminded me - clearly and eloquently - that a motor needs fuel, air, spark, and compression. Had I, in my solitary focus on fuel, checked THOROUGHLY those others?

So, I came home and checked and found compression to be less than 30 psi.

Life has interfered with motorcycle repair for the last while until this afternoon when I pulled the head and there it is: a hole in the exhaust valve.

Now I can't wait to get one, so if there is an airhead exhaust valve that's useable kicking around your pile of parts, I'd love to acquire it.

And about the missing timing mark: It lines up perfectly at top dead centre. (Pretty easy to tell with the head off.) I am presuming that the reason I could not see it with the strobe was due to erratic running one one cylinder. Valve replacement, adjustments and tuning will be the proof.

Stephen

1975 R90/6 (less one exhaust valve)

Sounds like you are making good headway Stephen.
If the timing mark is still hard to see put a bit of white chalk or white out on it .

You'll be rolling soon if the valve gets swapped out.

Gee Steve,
It kinda begs the question: What caused it to hole the valve? You seemed to run well last year.
Chris

That's a good question, Chris, and I have no real idea. The bike was running a bit rough toward the end of the fall. I was driving it to SMU almost every day until late October when the roughness started. I adjusted valves and tried to adjust the carbs. I couldn't get the left carb idle to adjust, so that's when I decided to park it for the winter and do the carbs in the spring. Well, the rest is history now.

I don't recall any event that would indicate a blown valve. I'd expect a bang at the least and an obvious loss of power. I did have a loss of power, but no catastrophic event.

Anyway, I'm glad to finally have something I can actually fix!

Picture of valve at:

http://gallery.mac.com/zwerling

Stephen