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Discussion Starter #1
Last week I bought a '74 F11-A dual sport 250cc two stroke. It seems to be in decent condition. It barely ran when I saw it but the guy siad it was fine until he had the carb rebuilt then never ran the same. It was very lean and would onyl run right if you worked the choke and throttle at the same time.

So I went thorugh the carb, and adjusted the cable for the oil pump and it now will start easier and idle alright, with a little smoke (concerned me before when there didn't seem to be any smoke). It's just very lacking in power. You gotta rev it up to about 3000 and let the clutch out slow. Doesn't feel like it has the power a 250cc two stroke should.

I'm used to riding my '79 250cc Elsinore, and that bike boogies and pulls the frot wheel up on demand. I figured this bike would have similar power, but a bit more tame.

I've rode a Blaster 200cc quad and you didn't have to wind that way out to get going. The onyl other two stroke bike I've rode is an '82 IT465... With 465cc it's just gonna be fast no matter what. That bike was insane. All the other bikes I've had are four strokes.

So anyhow, what should I be looking at on this bike to get some more power? I was reading someone elses thread with the same bike, different year, and he was saying the bike wanted to pull the front tire off the ground... I don't even feel like this bike comes close to pulling the front wheel up.
 

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Discussion Starter #2
One of my thoughts about this bike was the lack of a real expansion chamber. Are there any exhaust options out there? Anyone make their own expansion chamber?

How about compression? The owners manual says 6.4... How does it run?! Any way to bump the compression up to a more reasonable number? Piston options?

Any places that have a good amount of parts for these bikes?
 

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The first thing you can do is find out what the carb was rebuilt with. If an aftermarket carb kit was used, that could be it. They pretty much suck.
For the exhaust, the stock muffler should work ok. Expansion chambers for more power was available back in the day. Both for low end torque and for top end power. They sometimes appear on ebay. Today you'll have to make your own chamber.

Whisperin' Smith Torque Chamber




Bill Wirges Chamber (Aaron bought this)

 

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those were rotary valves, maybe suckin air somewhere, I rode one yrs ago but im thinkin for 250 of that yr they werent bad.
 

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those were rotary valves, maybe suckin air somewhere, I rode one yrs ago but im thinkin for 250 of that yr they werent bad.

You're wrong. The F-11 was not rotary valve. The F8 was.
 

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Yes, you're right. The F5/F9 350 Bighorns were rotary valves. The F8 Bison as well.
 

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always wondered why those werent kaws were big on rotarys back then. I think that was the only early 70s single kaw that wasnt.
 

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Discussion Starter #10
Yes it is a piston port engine. The intake doesn't look like the other two strike bikes I've had though, not a swuare/rectangle to easily hold a reed cage. Odd.

Anyhow I took the exhaust off and checked for carbon buildup in the exhaust port and it was minimal. Sprayed it and scraped out what I could. The factory pipe looks rather restrictive... Huge divots in it to clear the frame and hug it tight. Then out that tiny silencer.

Just seems like the bike has nowhere near the power I expected. Like I siad the other guy on here said his wanted to pull the front wheel up... This bike is nowhere near that and I"m only 175-280 lbs.

I'm gonna check the ignition and see what the timing looks like or if the points are bad. Source for points?
 

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Discussion Starter #11
I advanced the timing a little from factory setting and the bike is much nicer to ride. It still isn't what I would expect from a 250cc two stroke though. At least now I can take off from a stop and not feel like I'm totally killing the clutch. Also don't have to wind out every gear now, it has enough torque at moderate RPM to not bog too bad.

Still isn't waht I think it should be. Were these bikes just dogs from the factory? I'm not holding out much hope with only 6.4:1 compression. Thoughts?
 

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I have a stock 74 F11 that works correctly. I can let the clutch out at idle. It pulls very well down low then just fades away on top. It was designed as an enduro and should pull down low for riding in the woods.

Sounds lean. Check for air leaks, ignition side of crank, rubber mount for carb, wrong jets ( sometimes aftermarket jets are bad ) float not set correctly, needle in correct position.
 

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always wondered why those werent kaws were big on rotarys back then. I think that was the only early 70s single kaw that wasnt.
There was a few other models with piston port engines, but the majority of them had rotary valves.
 
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