So I got my 87 lt250r back together from the boxes it was once in. Its bored 80over with wiseco , has trail porting done on cylinder, full FMF exhaust, Reeds, Kelhin Flat side carb, and possibly hot rods crank/rod. Just wondering if 93 oct will be enough. I have been breaking in the new topend so keeping it under 1/3 throttle. I finally opened it up in 2nd and its a beast then I shift into 3rd and it acts like its out of gas.
I would think so but Has the head been cut?head squish band been recut?Might wabt to do a compression test as well?
Is it cutting out or does it tall on its face when wide open?
Sure, you can run 93 octane, you can also run kerosene and lighter fluid for that matter LOL
On a 250, octane isn't as critical as a 500 so 93 should be ok in most cases.
I've seen better performance and more consistent jetting with 100LL (avgas) though.
If you've done some performance upgrades like porting, pipe, and haven't had the head cut for optimum geometry, you can still run the risk of detonation even with a 250.
I had a mostly stock 250 (FMF pipe and aftermarket reeds) and when I ran 87 it destroyed the piston after some hard runs in the dunes.
Wish someone had told me the same thing beforehand, that was a long walk.
So I guess the risk is yours to take, I won't tell you not to but it's difficult to deny the convenience of running pump gas.
As for your other problem, you need to find out whether it's an instantaneous issue or something that takes a few seconds to develop.
Go easy with it and see if you can get up into 6th gear with light throttle.
If no problems arise, slowly give it more throttle while you hold the brake a bit, maintaining speed.
If it starts to bog down with throttle position, and gets better when you let off the throttle, you have a jetting issue.
If it starts to bog down while simply giving the engine a decent load and doesn't get much better when you let off slightly, it's a fuel flow issue.
Use caution with both, as they could be running dangerously close to melting a piston from a lean condition.
For the jetting, you'll need to find out whether it's rich or lean and adjust accordingly.
Engines tend to need slightly different jetting requirements after a top overhaul, and it's also starting to cool off which changes jetting as well.
Your jetting may need some attention, and it's easier said than done if it's your first tuning.
For fuel flow issues, close the fuel petcock and remove the fuel line from the carb.
Hold a bottle to the line and open the petcock...it should flow freely out of the tank.
If it doesn't, clean the tank and/or screen inside, or see if there's a problem with the petcock of fuel line.
Really good info Tex. It does light throttle just fine threw all the gears. I don't know the ratio of 6th but it did seem to bog a bit in 6th. As far as the head I don't know if it was ever cut. We did use a machinist stone to make sure the face was even but that shouldn't have taken much off.
As far as tuning this would be my first go round. I thought about soaking the carb in seafoam because the throttle was sticking a little. Also I never checked to see what jets were in it so this would be a good time to check. Also there is a fuel filter in line. I can replace that or should I just lose it all together?
As for how it was acting it would lose all power almost like if I had pulled the plug wire. I would let it and it was just fine part throttle. Even up over half throttle opening up the powervalve its good just when I give it the full load.
I jumped all over it in 2nd and she screams like normal but when I grab the next gear she fails me.
Sounds like your main jet/circuit is the problem.
The main jet circuit comes into play at around 75-100% throttle, which overlaps somewhat with the needle circuit.
It's hard to tune in low gears because the engine just muscles right past the bad spots due to insufficient loads.
If the engine isn't being loaded up like a high gear would, it can still have more than adequate power to accelerate under lightly loaded conditions.
The rough spots in tuning hardly even feel like hesitations, and before you realize something isn't tuned right it's already time to shift to another gear.
That's the purpose of putting it into high gear, because it takes the engine much longer to increase RPM due to high load conditions and even slightly out of tune circuits will be magnified.
If your main jet is the culprit, it'll feel like you're hitting a rev limiter at a certain point on the thumb throttle's travel, like at 75% or 85% throttle travel, for example.
If it's a fuel flow issue, it might scream in 6th for a few seconds before it starts falling on its face.
It might feel great at full throttle for a few seconds but once you empty the carb's bowl you might not be able to get it past 50% throttle until you let off the throttle long enough for the fuel to catch up again.
A minor fuel flow issue may require several runs or a long time at full throttle to drain the bowl and cause issues, and may flow fast enough that you wouldn't have to let it sit at low throttle position for it to catch up.
Fuel flow issues have varying degrees of severity, but main jet issues always occur at certain throttle positions.
Try cleaning the carb first, then tune the main jet.
It's not hard to completely clean the carb, just lay it out like you would when disassembling a gun and take a few pics just in case.
Use a can of carb cleaner and squirt it all the way through every passage ensuring that it comes out the other end (also a great way to learn what all the passages and such do in the carb!)
Reassemble and see if your problem is fixed, if not, then start changing out main jets.
Start off by going richer, one jet at a time (main jets get richer as the numbers get bigger).
You're probably 2 jets off...that's how it normally works, if it's running rough enough that you think it's a problem then you're several sizes off.
Not as easy to tell the difference in just one jet size.
If going richer is making it worse, go a size smaller than your original jet size and see if it gets better.
Be sure to buy plenty of jets, both larger and smaller, so you don't have to worry about running out of richer or leaner jets to try.
While you're at it, go ahead and buy pilot jets and a few needles since you'll probably be putting in an order online for them.
You do need to re-tune for summer and winter temps, there is no "one jet fits all" for year round riding, and there's no specific jetting that works for all 250's (not even baseline jetting, really).
Use your existing jetting as a baseline and work from there, write everything you've done in a little notepad to keep track of what all jets you've tried, how many turns on the pilot air screw, needle clip position, outside air temperature when you were tuning, etc.
Ok, a few things you should know about jetting.
-Learn about plug chops, they're vital to understanding your engine's state of tune.
-Learn about air leaks and how they affect your tuning (you can easily melt a piston with nothing more than a small air leak) and how a leak down tester works.
-It's best to start with the idle circuit, then needle, then work up to the main...but since you're only having trouble with the main it's probably not necessary.
-Tune your engine after all other factors have been addressed that might affect tuning (air leaks, air filter, fuel flow, fuel filter, float level, bad fuel, worn out reeds, low compression from high time rings, leaking head gasket, incorrect spark plug, etc.)
Basically, anything that might affect tuning should be fixed first, because tuning in itself is demanding enough.
As far as your fuel filter goes, replace it or simply remove it.
So long as your fuel is clean and your tank is clean, you don't need it.
I've run a 250 for 200+hrs without a filter and never had a problem with trash getting into the carb.
ChrisG sold me a few tank condoms, which basically pre-filters the fuel as you're pouring it in...so if you had one of those and your tank was clean a fuel filter would be completely unnecessary.
Not sure if he's still selling them, but they were pretty reasonably priced and you could wash contaminants and trash off whenever they caught anything.
For now, just remove the fuel filter and run a line straight to the carb, drain the tank and replace with fresh fuel, clean the carb, and see what happens.
You might even try a few gallons of 100LL just to be sure you're not detonating too, find it at the local airport.
Damn thats a lot of info. Well turned the fuel on and it was pouring out so I went ahead and ordered some new fuel line. So I am gonna see if that helps and or cures the issue. If not then I will pull the carb down. Thank you again for all the good stuff. Wanna get this handled so I can start putting a KX back together that I picked up for next to nothing.
Great writeups in here.One thing I would like to add is make sure the slide is in good shape on your carb.Won,t be your problem but if its worn tuning it can be a
****...I chased my tail for some time before realizing my carb was badly worn.
