f3 motor with f2 carbs rpm problem
 

So I found someone who has a cbr f3 motor in an f2 frame (without title) I was looking to pick up to put into my cbr f2. The guy says it starts up just fine but it then slowly begins to Increase rpms all the way to redline. In which he then turns the bike off not to blow the motor. The f3 motor he has on the f2 frame has f2 carbs on it as well. Which im thinking is the problem. He said the f3 carbs wouldn't fit on the f2 frame. Is that right? I thought they were the same frame. I heard of issues with the airbox fitting due to the ram air style. I guess my question would be is do you think it has to due with the f2 carbs on an f3 motor or something else. I have seen and heard people doing the swap before and juat didn't know if the problem was related to the carbs or something else

https://www.youtube.com/watch?v=_B1L...ature=youtu.be

https://www.youtube.com/watch?v=pHhb...ature=youtu.be

This is some video he just took. Maybe its just me but sounds like a slight knock? Does anyone else here it?

To me it sounds like it's not firing on ally cylinders.

Any ideas as too why it continues to increase in rpms all the way to redline? Im assuming its a air or fuel issue. Just not sure if this has happen to anyone before with other swaps with the f2-f3. Or this issue with any other motor. Thought maybe if he is using the f3 fuel pump with the f2 carbs maybe thats causing the issue.

Have you tried turning your idle knob to the left and see if that helps?

Its not mine yet. Going to check it out when im off. Just trying to get some Insight and ideas before I go look

If the idle knob is turned to far in, it will rev too high. It can rev all the way to redline if things are out of whack. If somebody doesn't know what they are doing with carbs they attempt to use it as a tuning tool to get to idle better. That's not its purpose.

100% agree.

It's completely indicative of an engine that isn't running on one (more likely 2) cylinders and the fluctuation happens when a cylinder starts to intermittently fire. When that happens the engine is not pushing against as much compression and since the carbs are trying to get it to churn faster, the revs climb. IF you have the idle adjusted down to where it's supposed to be, it'd probably stall.

And after watching the videos, I'm positive it's not firing on all cylinders. I would start to diagnose it via an IR gun pointed at the headers to determine which cylinders are not running. Then I would test the spark. After that checks out, I'd move to the carbs. I'm pretty sure that F2 carbs run fine on an F3 engine because of the fact that the engines aren't all too different.

So my next question would be if its not firing on all cylinders could that mean the motor is bad.....or just something to due with fuel or spark. If there is a possibility the motor is shot there is no point in buying it for me

It's a possibility. Low compression due to worn rings or a bent valve(s) - etc.

By the same token, it could be a bad spark plug (a $3 fix...).


Now, statistics would say its more likely something easier than a bent valve or worn rings (like spark and fuel). BUT, the fact that someone else (who more than likely bought it as a project, and thus, likely has some mechanical aptitude) is at a loss on it and is ready to sell it like that + it's age - would make me nervous to say its more likely to be something simple.

You won't know until you can dive into it. And that's where you need to be from a purchasing standpoint. I wouldn't even think about buying that bike for more than $400-600 depending on what the fairings look like and where you're located. You need to assume the motor is beyond what you're mechanically willing to deal with, and thus, buy it for what you're willing get out of it parts-wise, or buy it with the assumption that you're going to have to buy a second-hand engine for it.