Carb or valve adjustments needed with new Siebenrock cylinders for a 1979 R100

[dfalatko]

I have just installed the Siebenrock 1000 cc cylinders with pistons and rings on a 1979 R100T with dual plug heads. In took a while over the winter to do everything, as I also had the crank redone and replaced all bearings among other items. It is up and running now, but there is a hesitation in acceleration, and I am wondering if there is a carburetor or valve clearance issue with the new cylinders running at slightly higher compression. I have the valves at 0.1 mm intake and 0.2 mm exhaust, run premium gas, and have the idle screw at ~1.5 turns open. It may be that the carbs are a bit fouled from lack of use over the winter, but I did drain the fuel when removing them last fall. Does anyone have thoughts/experience on carb/valve adjustments needed when doing this cylinder replacement?

 

[6322]

Following your modifications, did you adjust your timing with regards to dual plugging? This could be where you’re finding the hesitation.

 

[20774]

If you're not sure where you are relative to the carb synch, probably a good thing to repeat that process. Then you can re-evaluate the engine performance.

 

[dfalatko]

The heads were already dual plugged before the cylinder work, the timing is set to standard specs, should it be different with the dual plugs?

I did balance - sync the carbs, they seem good right now.

 

[20774]

the timing is set to standard specs, should it be different with the dual plugs?

Yes. With dual plugs, the flame front burns much fast in the combustion chamber, so there's the chance that the explosion of the fuel-air occurs sooner than desired. What should happen is that the full advance point occurs sooner, but that's hard to do. What I've read is that the static timing point should be much nearer to TDC if not right at TDC. Then the full advance happens and is reached sooner as desired.

Then do another carb synch.

 

[PGlaves]

When we had a double plugged R80/7 the OEM static timing was specified as 6 degrees before TDC. The revised specification provided by Oak Okleshen was 3 degrees before TDC. It ran fine.

 

[dfalatko]

Thank you all for your input. I went and looked into it more and found an old article by Tom Cutter on dual plugs and his recommendation was 4 degrees after TDC for a 1979 R100; but this seems pretty far advanced. I had it at close to TDC, and set at 4 ATDC, it did not make a noticeable difference; I may revisit this.

It seemed like a carb issue, so I took off the carburetors and took them apart, and found a couple of bits of old black gas line hose in them. Now awaiting new line and a couple of filters, hope this does it.

 

[Lendog]

If you have CV Bings the vacuum port for the bellows could have an insect cocoon restricting the vacuum, a bellow could be compromised or need the proper amount of oil in the bellows cavity

 

[dfalatko]

Yes, I took them apart and also replaced the bellows, I did not oil them however, is that a recommended step, how much/type of oil?

 

[GTRider]

Yes, I took them apart and also replaced the bellows, I did not oil them however, is that a recommended step, how much/type of oil?

If by “bellows” you mean the diaphragms in the Bings, they are run dry and require no oiling. And while there may be some diaphragm-equipped carburetors that have an oil reservoir (Stromberg?) Bings used on type 247 airheads are not one of them.

Best,
DeVern