1986 Monoshock R80RT Running problems

[danparkin]

Hi,

Has anybody got any idea why my left cylinder on my R80 won't run properly. I have very little experience of these bikes but do have reasonable mechanical knowledge.The left hand cylinder is running but won't rev properly. I've connected a vacuum gauge and the right cylinder at tick over is pulling about 160mm vacuum but the left cylinder only about 30-40mm vacuum. Adjusting the mixture screw has very little effect.
Things I have tried:
Tested fuel lines
New air filter
New plugs
New Leads & plug caps
Carbs rebuilt
Float levels set
New ignition module
Timing checked
Valve clearances adjusted
Tried swapping carbs side to side (the problem stays on the left cylinder)
Swapped plugs side to side (the problem stays on the left cylinder)
New rings on both pistons.

Has anybody got an idea's as this is really baffling me now. I'd really appreciate any thing I could try

Thanks very much

Dan

[Rob Frankham]

The disparity in the depression figures indicates that there's something pretty drastically out of whack with the left cylinder.

Forgive me for asking the obvious question... when you set the valve clearances, did you make sure that each cylinder was at TDC with the valves closed. The sequence is to set to TDC. see which valves are rocking (i.e. one opening, the other closing) then set the other side. Next turn the crank through 360 degrees and set the first side. If you don't do this, the valves on one side will be closed far to long and only partially open when they're open at all.

Failing that yiou ar going to have to do some serious surgery to track down the problem. I'm fearing that there is a broken valve head, a holed piston or something else nasty at the bottom of it.... A compression test or leakdown test might give you a clue but I fear the wrenches are going to have to come out...

Sorry

Rob

[danparkin]

Hi Rob,

Thanks for this. Yeah, valves set correctly.
I replaced the rings as a wishful thinking exercise. Piston looks fine, valves looks fine although I've not actually removed them from the head yet as I don't have a spring compressor. Looking through the exhaust/inlet ports and turning the rear wheel, the valves look to go in and out at the correct times.
I'm just at a loss. I've got an eBay special compression tester on its way so I can test it but I'm pretty sure it's ok.
Thanks again
Dan

[melville]

Welcome to the board! I hope you work out your running situation. I have nowt to add but some admiration for your build. I have an incomplete R75/6 that may get a similar minimalist treatment. Thanks for some inspiration!

[danparkin]

Thanks, it’s a really simple build. Just a new rear subframe, seat and a load of motogadget stuff.

[gspd]

While your waiting for a compression check....
You never mentioned checking the coil(s)
switch the high tension leads at the coil(s) to see if the problem goes to the other side.

[danparkin]

Thanks, I've tried it and the problem remains with the left hand cylinder. It's really confusing, might have to swallow my pride and take it to someone professional to have a look

[gspd]

The fact that your engine idles on both cylinders implies that the valves/rings/compression are probably all good enough.
If the compression is good enough to start and idle, it's usually good enough to allow revving up.
If you had a serious compression loss, the bad side would not start or idle.
Before doing anything, just have a close look to be sure you don't have a broken valve spring; it's a common problem in many old engines that won't rev up, but very rare on airheads. A broken valve spring usually permits starting and idling, but not revving up. It is often NOT detectable with a compression test or leak test as cylinder pressure pushes the valve into the seat at low or no cranking speeds.

Suggestion:
Do yo have an air compressor?
If you don't have a leak tester or compression tester, a simple (farmer?) way to check the engine is to put the cylinder at TDC (valves closed) and improvise a way to blow compressed air into the spark plug hole. Remove the dipstick. If you hear air escaping from the intake, it's an intake valve leak, ditto for exhaust. If it's freely blowing out the dipstick hole, it's pistons or rings.


Question 1:
If you start the bike from stone cold and run it for 10 or 15 seconds, are both header pipes hot?

Question 2:
I don't know how much Bing carb experience you have, so just asking...
When you rebuilt the carbs, did you line up all the diaphragm notches?
Are you sure you checked ALL the passages for the idle circuit.

You said: It's really confusing, might have to swallow my pride and take it to someone professional to have a look
I resemble that remark!
You have more knowledge available on this forum than any 'professional'.
It's not rocket science, just internal combustion 101.
Try to approach the problem as if you had 2 independent single cylinder engines.
Remove the left spark plug and ground it. Now forget about that left side for now.
Imagine you are now working on a big single (right side).
Start the 'right' engine. warm it up, and establish if it runs properly and if the mixture screw works.
The engine should stall when the mixture screw is turned all the way in and idle acceptably with the screw 2 turns out.
It should rev up when opening the throttle.
If all appears OK, try that same cylinder, but with your other carb.
It should react the same.
This will confirm that both your carbs are good.
Then do the same but after switching coils. ht wires and plugs, that will confirm both ignition systems are good.

Now do the same for the other 'engine' and report your findings.

[danparkin]

Thanks so much for your time.
Question 1: yes, both headers get hot and with it being cold outside you can see both cylinders are combusting (not sure if thats a word) as there are exhaust gases.
Question 2: diaphragms installed correctly and i have checked all passages. I blasted carb cleaner through the passageways and all seem free flowing. The only thing I have noticed is that the pilot air passage hole is very close to being covered by the brass butterfly when it is closed (i.e. no throttle), is this correct?

I'm just going out now to brave the cold and try what you have detailed. I'll let you know my finding, thanks again for your time

[gspd]

danparkin asked : "The only thing I have noticed is that the pilot air passage hole is very close to being covered by the brass butterfly when it is closed (i.e. no throttle), is this correct?"

Here's a clear illustration of the idle circuit, more than one of those small holes may be blocked.
I've seen more than one tip of the needle broken off and stuck in the hole, and it is not uncommon to have a simple grain of sand blocking the hole at the mouth of the carb.

https://electricmotorglider.com/wp-cont ... -web-1.pdf