Intermittent power loss at high speed

Hi everybody.
I need some input from the experienced 2 stroke engine people here....
I have recently put on a killer expansion chamber cut up from a Kawasaki dirt bike. The engine has the most power I've ever had, but a new problem is starting to rear it's ugly head! At high engine speeds (starting around 6000 RPM), the engine will all the sudden loose all power, then it will regain, then loose all power again, over and over. I'm running my new intake which could be a contributing factor, but I'm not sure! Just the other day, I was test driving the bike, and I managed to get lucky, and I didn't get that massive power loss, and the bike kept going and going, I think I must have reached 45 MPH!, and I think it could have gone further. I don't think the bike is too rich, in fact, my spark plug color is grey, and I'm running really rich jets (.042" or 1.07MM)!
I keep going richer and richer, but I'm starting to suspect that I'm simply compensating for another problem. Is it possible that I'm getting an air leak through the crankshaft seals? I will rebuild the engine today and replace those seals, but I have a sneaking suspicion that the problem will still be there! This engine is really turning into a monster since I put this insane expansion chamber on. The other thing I'm thinking is that perhaps the engine is short circuiting because the vacuum from the exhaust is so strong, it sucks the incoming charge right into the exhaust port. The power loss is 100%, like someone putting the brakes on.

Thanks for your consideration.

-Fred

Yeah, you guys might really be targeting the right area. I did a flow test, by removing the carburetor drain plug, and it seems to flow pretty good, but I will check again.
Tooljunkie, you're right, the size of these jets is crazy! Every time I made an improvement to the exhaust, I found I had to enrichen the mixture further.
Well I did take the engine apart, and here's what I found:
The seals looked fine; the spring was in place, the lips seemed to have a sharp edge, the rubber seemed soft enough, but after I had removed the old seals, I checked to see how they fitted to the crankshaft, and found thin magneto side seal was not really tight on the shaft. It wasn't loose, but I could imagine how it could leak. Another thing I noticed was the the crankshaft bearing on the magneto side was a little dry, while the other side was all nice and oily. That led me to believe that in fact the magneto side seal was in fact leaking. The exhaust must be pulling hard. I'm going to experiment with different intake and exhaust configurations. Perhaps they need to be matched, so one or the other does not create a restriction. After I put the bike back together, I had to put a leaner jet (.89 MM), and now, I'm getting a little brown on the plug! That confirms I was getting a leak, but the problem still persists! Another reason why I'm running such rich jets is that the intake system I'm running tames the pulsations, so they are not so violent when they get to the carburetor venturi. A strong intake pulsation will tend to draw more fuel than a soft pulsation, even if the total volume of air is the same. That is a big reason why I'm running such rich jets.



Thanks for the help guys!

-Fred

Hey Norman, thanks for responding.
Well, I checked the fuel flow again, and it seems as good as I could expect it to be. The ignition is a pretty new DC fired CDI controlled system powered by an external 12V lead acid battery and a coil from a scooter. You know the problem became very apparent after I put a new cylinder on which I enlarged the width of the exhaust port. I also basically enlarged the whole port to match the huge exhaust system I cobbed together from that Kawaski dirt bike. It seems that the exhaust now has the ability to influence the engine in a major way, for the better or worse... In this case, definitely the worse! I think I need to sit down and design a race exhaust for this engine that is tuned properly, for a very wide power band. So far, I've only been playing around, or just guessing with these exhaust systems.
So, I swapped out that radical exhaust for earlier exhaust system I made using a modified BGF expansion chamber. It flows quite a bit better than the stock version, but more restricted than the crazy pipe. The bike now drives pretty much OK now, but I know the engine has the ability to perform better.
I found some interesting software to help two stroke engine tuners build expansion chambers to perfectly match the kind of results they would like to get. It even prints out templates for you to cut out all the pieces to build the chamber. And best of all, they are only asking $18.00 for it!
I think I'm going to get it, and I'll see what it comes up with!



-Fred

Well, OK... Thanks, that sounds like something I will try. It seemed like the trickle coming out of the tank would be enough fuel, but it's hard to tell for sure. Best way is to try replacing the petcock. I will try it!



-Fred

Well this has been interesting few days! Today, while taking a trip to the post office, the CNS "racing" carburetor casting broke. I was forced to put the pathetic little NG "speed" carburetor as a back up. Boy, I'm glad the turn of events happened because now my bike runs perfectly! Faster than ever; I can easily pass 40 MPH on the straight! I've spent countless hours fussing with my CNS carburetors and never really being happy with how the engine runs. I put that simple NG "speed" carb on, and didn't even make a jet modification!
Also, I remembered this forums members comments and concerns about the fuel delivery, I examined the stock fuel petcocks and found some interesting things. When I dismantled the petcock/banjo fitting on the NG "speed" carb the pathway through the valve seemed very restricted, so I drilled right through with a 1/8" drill (more or less), essentially bypassing the valve, so the valve is always on. I then took a look at the petcock that came with the gas tank.... When I removed the plastic fuel filter, I noticed what seemed to be a intentional restriction! I enlarged it so it was the same size as the ID of the barbed end. That seemed to improve fuel flow slightly. I still like the idea Baird mentioned about replacing the stock one alltogether.



-Fred

biknut said:

Sounds a lot like a faulty gas cap vent.

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Oh yeah, that gas cap problem. I noticed that problem a little while back, and I was forced to drill a little hole in the top of the cap. Not the best solution, but I don't know what else to do. It seems like a tricky situation.



-Fred

headtrama said:

That is some good advice , But in my case I wanted to replace the Chinese carb with a real carb . The motor runs so much smoother from idle to wide open .

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Hey Head trama.
I just got in a new carb. from Dax. It has a flat slide. It's a pretty neat looking design carb. I'm looking forward to trying it. Very inexpensive, simple design. It has a plastic float bowl and a plastic top which may break in time. Someday, I would like to try a Mikuni carb. though, since I've heard a lot of positive things about them.



-Fred