From the number on the engine is there any way to identify a F3 motor vs an F2 motor? I pick up a bike that has had many hands in it and I am starting to think it might have an F3 motor. Bike is impossible to tune, but runs strong. My tuning issue could be explained by running F2 carbs on a F3 motor. I have been through everything other than a valve clearance check. So I want to check to see if someone dropped in a fresh motor. Any clue how to read the motor serial number?

 

Don't know how to tell from the engine number, but there are a bunch of other differences. F2 motor has a vaccum port on the head by the #1 cylinder, starters were slightly different, stock cam chain tensioners are different, F3 motor has sensor for speedo, shift drum was reverse so shift linkage will be upside down. Maybe some of this helps.

 

This vaccum port you speak of, is it the one that controls the fuel petcock valve? Since this is a 94 Frame there is not much I could go by if someone dropped a F3 motor in it.

 

yes, that vacuum port is usually connected to the fuel petcock. All the other stuff I mentioned is specific to the engine, so the frame year won't affect any of it.

The engines themselves are for all practical purposes are identical. So having an F2 or F3 engine wouldn't be the cause of your problems, unless there is something just plain wrong with the engine, ie bad compression. It's the stuff that bolts to the engines that were different. Not sure what you mean by impossible to tune, but F2 carbs on an F3 engine would work just fine. F3 carbs without the ram air would be a whole different story.

 

Wel long story, but I’ll give you the quick version. I have a problem with a lean mid range and a rich bottom range. It has to be a problem with the motor. The bike is all stock with the exception of a slip-on. Stock needles and jets should run ok with this setup. ON the stock needle it will barely accel thru the mid range (5-7K) because it’s so lean. I thought I had an issue with the carbs, so I put on another set. Same exact issue.

I can make it better with the dynojet kit, but it will not tune out. The further I drop the needle the better the mid range gets, but it makes my 4,000 range too rich. Very lump and wet sounding. If I rise the needle back up that range gets better but I experience 5-7K getting very lean. Its almost like there is a vacuum problem with the motor not allowing the slides to come up fast enough or all the way. I am out of ideas, I have tried every needle and jet combo there is with not much luck tuning out all 3 ranges.

 

Also to add to that on stock needle it will not cruise at all if the revs are above 4K. It stumbles and surges very bad on stock needle. Dynojet needle requires it to be on the 3rd clip down but even then it very lean at cruise. A bit of choke and it all disappears, really think I might have some vacuum problem with the motor. Compression is good. Maybe tight valves?

 

Well, if you've swapped the carbs out and still have the same problem. Then the carbs aren't the problem. Meaning it's not a tuning problem. You won't be able to tune it away. Make sure they are F2 carbs and not F3 carbs. Your problem sounds just like F3 carbs without the ram air hooked up.

Also do a compression check to make sure the motor isn't fubar. If it's got an aftermarket filter, make sure it's not over oiled or try a stock filter. Also check and see if someone put in an ignition advancer. Make sure you don't have any vacuum leaks. Also check to make sure the cams are in time with the crank. If someone adjusted the valves and put the cam chain back on one tooth off, it could act like that.

 

Yep, defiantly F2 carbs. I have yet the pull the valve cover off and check further. Defiantly don’t have any vacuum leaks around the carbs areas. Where would be the timing advancer if it had one? Would it really cause that much in the way of problems? I could see if the timing was off. The only other thing for concern is there is a box on the back tail of the bike with like a 25pin connector. I believe it’s the CDI?? It has some major road rash. I can see a little tiny bit of the circuit board, but nothing is exposed. Maybe that could be causing an issue.

The reason I kept tuning is because I can make the mid run better, but it take a bunch of fuel. I figured if it was a timing problem I couldn’t make it run better no matter what, but fuel helps it.

 

Yeah, that's the CDI and it sounds like it's taken a beating. You shouldn't be able to see the circuit board. They are suppose to be sealed to keep water out and if it's ever had water in it, it may not be working right.

Yeah, if the timing is off for any reason, the bike can run horrible at certain rpms. It can act rich because the fuel isn't being burned fully. Ignition advancer is behind the small cover on the right side of the engine.

You've definitely got more than just a tuning problem. In the end you're just going to have to recheck all the basics, make sure nothings broken. Then swap out things and test until you find something that fixes the problem. I can guess all day long at whats wrong with it, but it's never the same as working on it first hand.

 

If you can give me the serial number from the motor I will check it in my Haynes manual to see if it is an F2 or F3.