Main Jet Washer

[gspd]

As for compression last time I tested it it was fine since then its had a Siebenrock Big bore kit which has improved compression if any thing.

I would recheck the compression before proceeding. Don't assume that it's good because some new parts were installed. You could have broken rings, leaking valves or a multitude of other ailments. It costs nothing to do a compression check. Do it with the carbs removed or in the very least assure that the butterflies are wide open AND that the slides are fully raised.

I plan to balance the idle circuit with the diaphragms disabled when the vacuum gauges arrive, see what that says.

That makes ABSOLUTELY no sense to me. How do you "disable the diaphragms", and why the hell would you want to? Please enlighten me. I'm assuming you are aware that the diaphragms have a "bump" on their inside diameter that engages in a notch in the slide, and another "bump" on the outside diameter that engages in a notch in the carb body. (pic below)

Oneday sombody will read and say either what an idiot or corr thats whats happening to my bike.

I'm sure many of us (more experienced forum members) could diagnose your problem in a few minutes in person. It's a lot harder doing it remotely. I can't speak for all forum members, but I doubt anyone here will ever call you an idiot for not being able to fix it. We're all here to help you as well as we can.

GO BACK TO STEP ONE: Do a proper compression check and report back.
We'll get that sucker running in no time

[Tom H Ca]

Have you tried swapping the spark plugs from one side to the other? A defective spark plug can work fine at idle, but fail at higher RPM. A coil can also have the same issue.

You have great answers so far. If I was you.....

Check compression on both side and let us know what it is (humor us). Swap from one side to the other..... Swap plugs, swap coils, swap plug wires (then try just the caps as well), swap carbs. Do not swap all of the items on the list at one time. Swap one item and see what happens, then put back and try the next one on the list.

Something on the list should move the issue to the other side.

Let us know and good luck!
Tom

[mkj]

So I will look out the compression tester, diaphragms are easily disabled with a little hole in each or by merely miss fitting them , I have spare ones and they are very cheap. As for swapping bits I have tried it and renewed all the ignition parts except the hall effect unit. The one thing the made a difference was swapping carbs over. So I bought a second hand carb but it seems to have the same fault.
The weekend is nearly upon us I'll have another go them.

[gspd]

So I will look out the compression tester, diaphragms are easily disabled with a little hole in each or by merely miss fitting them , I have spare ones and they are very cheap. As for swapping bits I have tried it and renewed all the ignition parts except the hall effect unit. The one thing the made a difference was swapping carbs over. So I bought a second hand carb but it seems to have the same fault.
The weekend is nearly upon us I'll have another go them.

What I interpret from this is that you are uncertain if the diaphragms were installed correctly and that they are possibly perforated. Check them and let us know. The carb will never function properly if those notches are in the wrong place or if there is even a tiny perforation. The notches assure that the slide is EXACTLY where it must be.

PS: FYI - The hall effect sensor and ignition module equally affect (pun intended) both cylinders.
Both cylinders will work, or both cylinders won't work.

[barryh]

These are flat tops so the diaphragm is secured with a plastic ring. To Get the orientation right when fitting the diaphragm make sure the air hole in the bottom of the slide is positioned nearest to the butterfly. This hole communicates venturi vacuum to the top side of the diaphragm and is therefore responsible for piston lift. If the diaphragm is fitted 180 deg out then the hole will not see the correct vacuum signal which will affect lift.

[mkj]

Thanks for your concern the diaphragms are installed correctly but are a little stretched and a bit tricky to fit. Yes I know about loss spark ignition. As for the manometer I discovered my 20 year pvc tubing was cracked and so it was never going to work.

[gspd]

These are flat tops so the diaphragm is secured with a plastic ring.

oops!

[mkj]

As promised found the the compression tester but sadly not a good result, for the compression tester not the engine. I bought on the ebay and remember being disappointed. The problem being the only adapter which fits the very standard BMW plug hole joins the gauge on a quick connecter and anyone who has spray painted using one will know they don't really hold any pressure. I could only get 50psi, both sides the same though. If this were a true reading i don't thing it would even start.
However on the plus side I took the main jets out, not the needle jets which I had replaced with new ones. Both jets are 145 and under the micro scope looked in similar condition, so just because I could I swapped them over and the fault moved from one side to they other. Gosh who would have believed it a worn main jet can cause such a major problem, which has taken me over a year to locate, this also seems to explain why my second hand carb had the same problem.
So another question, as I have fitted a 1000cc big bore kit should I now order 145 or 150 jets.
Some people may still be baffled by my desire to disable the diaphragm, this would be so I could get some pleasure out of riding it, I have been very frustrated, however now I think I have found the problem, how many times have I said that, I may not take such a radical step, but on the other hand I have spares and 80 mph is more fun the just managing 50mph and being over taken by learner bikes.

[gspd]

So another question, as I have fitted a 1000cc big bore kit should I now order 145 or 150 jets.

The only way to answer your question with certitude is to put the bike on a dynamometer.
You will NOT notice a huge 'seat of the pants' difference between a 145 or 150 main jet in normal riding. The needle position is what determines the mixture, except at full throttle.
My experience tells me your bike will run fine with either. If your bike is a US model, it was probably way on the lean side when stock although many bikes have lived a long and happy life that way.

Thanks to unleaded gas, the days of relying solely on spark plug colour to fine tune jetting are a thing of the past. Obviously, white is too lean and black is too rich, but there a more than '50 shades of grey' (and tan and brown) in between. If you're getting black smoke on hard acceleration and really lousy gas mileage, it's too rich, drop the needle a notch. If you get a slight hint of that feeling you get just before you're about to hit reserve, or audible pinging under load, it's too lean, raise the needle a notch.

I run 160 main jets and 2.68 needle jets.
1000cc, 40 mm carbs, 9.5 compression ratio, dual concentric valve springs and modified intake and exhaust systems. It runs real strong and sounds glorious at WOT, and I still get 5L/100km when I choose to ride conservatively. I use the highest octane gas available. Plugs are always an acceptable, albeit slightly greyish, tan.

I miss the good old days of real high octane (cherry red Kool-Aid) leaded gas and that beautiful rich tan colour it created on spark plugs AND inside exhaust pipe tips. You just don't see that anymore. Check any bike or car exhaust in a local parking lot, you'll see what I mean.

[mkj]

Have taken some pic of my main jets, the top one runs but the bottom one doesn't cannot see why, will replace both with 145 but may also buy 150 just for something to play with. Any ideas