CMB 90 and BVM Needle Settings

[Joe Warren]

Hi Joe,

I just went out and took the motor out of the boat and removed the rear cover. As the disc fell out again, I was ready for it and caught it so I could put it back the same way it came out. Looks like it was backwards! When it fell out before, the way I checked it with my other motor was by looking into the carb barrel of the other motor and spinning the flywheel. This casual inspection would have not enabled me to see the rougly 20 degrees of intake timing difference that reversing the disc would have caused since you can't see the intake port edges looking into the carb barrel. I wrongly assumed that a reversed disc would be way out of whack rather than just 20 degrees. Based on degreeing it as you said, the way I had it was more like 80 degrees ATDC at full closure rather than the correct 62.

Now I'm going to think about what this kind of intake timing retarding would do. Nothing good, I'm sure!

Hopefully, this is the problem.

Fuel tank inlet and outlet are verified and fine.

Thanks,

Doug

it will not run properly if it is flipped over. Air bubbles in the fuel line is not normal. make sure the fuel tubing is good & he fuel inlets are hooked up correctly.

 

[Doug Babcock]

Ran it on the hose just now and I think it's going to be OK.

For the first time, I can run with the ground off low speed needle and it will run at 1/3 throttle. With the BVM set at about 2 1/4 turns out, when I blip full throttle it will hesitate for a second, then clears itself out and the revs skyrocket.

I think I'm getting somewhere now.

As an interesting note, I took the back cover off of the never-been-run engine, slapped the degree wheel on it and found that it's intake disc was also reversed. It may never have been run, but it's surely been apart.

Looked at Gordon Jennings' book on two stroke engines and found that delayed closure of a rotary intake valve will tend to increase the RPM where the engine makes its power and obviously then reduce bottom end torque. Jennings gave a general closure timing of 65 degrees ATDC and cautioned about extending this by any more than about 2 degrees at a time. Things obviously go south in a hurry here. At 18-20 degrees beyond where it should have been, it's no wonder it ran so terribly.

No bubbles in the tank line this time.

Doug

 

[Joe Warren]

Ran it on the hose just now and I think it's going to be OK.

For the first time, I can run with the ground off low speed needle and it will run at 1/3 throttle. With the BVM set at about 2 1/4 turns out, when I blip full throttle it will hesitate for a second, then clears itself out and the revs skyrocket.

I think I'm getting somewhere now.

As an interesting note, I took the back cover off of the never-been-run engine, slapped the degree wheel on it and found that it's intake disc was also reversed. It may never have been run, but it's surely been apart.

Looked at Gordon Jennings' book on two stroke engines and found that delayed closure of a rotary intake valve will tend to increase the RPM where the engine makes its power and obviously then reduce bottom end torque. Jennings gave a general closure timing of 65 degrees ATDC and cautioned about extending this by any more than about 2 degrees at a time. Things obviously go south in a hurry here. At 18-20 degrees beyond where it should have been, it's no wonder it ran so terribly.

No bubbles in the tank line this time.

Doug

Doug glad to her you now got it running. One last thing. If it has a 90 degree exhaust pipe pressure fitting in the pipe make sure it is not stopped up. Many times this is a problem.... We normally run a larger straight fitting to correct this. A restricted pipe pressure fitting will cause a lean condiiton on launch, other things that cause a no launch are.... Wrong pipe too small), or too short of a pipe, or prop that is too big in diameter or too much cup. hope this gets you running. jw

 

[Doug Babcock]

The pressure nipple is a straight one, not plugged and I can see the tank top bow out slightly as I jazz the throttle. I don't think there's any problem with pressure.

I have several props in the X460-465 range that I can try it out with. Mark Sholund made me an X465 that has a diameter of 63mm and it's been pitched up to 3.75" which I will try once I get things in the ballpark.

I'll keep you posted.

Thanks,

Doug

 

[Joe Warren]

Should be Good, Time to go to the lake...... B)