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2-Stroke Loses Power when hot

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Hello all,

This isn't a "bike" question per se, but it is related to 2-stroke engine/carb tuning. What I have is an old 83 Honda Odyssey FL250 (air cooled 2 stroke 250) which I spent part of the winter restoring, including new piston rings.

Anyway, the machine generally runs great throughout the RPM range and throttle range for 30-40 mins or so, but after that, an intermittent power loss seems to occur when transitioning between 3/4 to WOT. It is intermittent to the point where it's difficult to reproduce.

Do these symptoms seem characteristic of a jetting problem?

Any advice welcome!

Thank you.

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Thanks for the info!

I'm new to the world of 2-strokes so this is really helpful.

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Hmm i'm alittle suspicious!I seen many back in the day fl 250 with holes in the pistons due to ignition timing & lean mixture or leaking head gasket.So it runs fine 30 to 40 min mark,i think it's ignition break down,wrong spark plug & or to much resistance in secondary or primary ign coil or spark plug cap.

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30 to 40 mins. This is in fairly cold temps too, like 0 C to 7 Celsius. I have new head gasket and correct spark plug, but have not checked resistances on spark plug wires or coils. I know for a fact the spark plug wire is not factory, but am not able to find any resistance specs to determine if I am out of spec either. My manual indicates there is a noise reducer in the spark plug boot for the stock setup and the the wire/boot I have now looks like an old spark plug wire from a Ford truck cut off.

Looks like I have some research to do.

Thanks!

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Hello all,

Anyway, the machine generally runs great throughout the RPM range and throttle range for 30-40 mins or so, but after that, an intermittent power loss seems to occur when transitioning between 3/4 to WOT. It is intermittent to the point where it's difficult to reproduce.

Do these symptoms seem characteristic of a jetting problem?

Any advice welcome!

Thank you.

Been 30 years since I rode one of these. I don't remember, but how is the air ducted to cool the carb and cylinder? If either is not getting enough cooling you may get problems. The gas in the carb can boil and cause a power loss. Second question: describe the power loss. Is it a sharp cut-off, as if you just turned off the ignition? If so, look towards ignition problems. If it is a more gradual loss of power, as if you were rolling off on the throttle, I'd look first for fuel or overheating (possible partial piston seizure). A change to a larger main jet size is a simple and cheap way to get some diagnostic info. If the problem gets worse, and the engine starts "4-stroking" (firing on every other cycle, running rough, making the spark plug insulator black with carbon), your original jet size is probably OK or a bit too big. If it runs better, you have been running too lean. In any case, check the size of the jet by number (it should be stamped on the jet itself) against what is listed by Honda as "standard".

Good luck, Bluepill

Edited by BluePill

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Been 30 years since I rode one of these. I don't remember, but how is the air ducted to cool the carb and cylinder? If either is not getting enough cooling you may get problems. The gas in the carb can boil and cause a power loss. Second question: describe the power loss. Is it a sharp cut-off, as if you just turned off the ignition? If so, look towards ignition problems. If it is a more gradual loss of power, as if you were rolling off on the throttle, I'd look first for fuel or overheating (possible partial piston seizure). A change to a larger main jet size is a simple and cheap way to get some diagnostic info. If the problem gets worse, and the engine starts "4-stroking" (firing on every other cycle, running rough, making the spark plug insulator black with carbon), your original jet size is probably OK or a bit too big. If it runs better, you have been running too lean. In any case, check the size of the jet by number (it should be stamped on the jet itself) against what is listed by Honda as "standard".

Good luck

I'm pretty sure it's not a fuel boiling issue. The warmest day I've been out is 7celsius and my Odyssey is garaged so I doubt the fuel ever makes it much more than a few degrees, but are right, the design of these machines really sucks from a cooling standpoint. The engine is directly behind the seat so no moving air really ever gets to it.

I'm running stock jetting with the clip raised one notch - however, I'm at 4500ft elevation. I thought for a while that my problem may be running too lean in the cold temps, but I haven't had any improvement in performance as the temperature has increased.

Characteristically, I start the machine from ice cold and let it idle a bit and warm up, hop on and head out. Throttle response seems good from low rpm to WOT. However, after 30-40 mins of riding it, (I try not to bag the machine too much and keep it WOT all the time), I will let off the throttle going fairly quick then give it more throttle, the machine feels like it's going to stall - total loss of power. At that point, I let off the throttle and slowly get add more throttle, sometimes it's fine, sometimes it dies a bit.

Usually but this point (since I usually take my son for a quick ride) I start heading home in fear that the machine will die and I'll have to walk home and take it easy on the throtttle till I get home.

So, when it misbehaves, it's a fairly instanteous thing, performance great, the suddenly feels like it's going to die, then fine. Run great for 99% of the time.

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I'm pretty sure it's not a fuel boiling issue. The warmest day I've been out is 7celsius and my Odyssey is garaged so I doubt the fuel ever makes it much more than a few degrees, but are right, the design of these machines really sucks from a cooling standpoint. The engine is directly behind the seat so no moving air really ever gets to it.

I'm running stock jetting with the clip raised one notch - however, I'm at 4500ft elevation. I thought for a while that my problem may be running too lean in the cold temps, but I haven't had any improvement in performance as the temperature has increased.

Characteristically, I start the machine from ice cold and let it idle a bit and warm up, hop on and head out. Throttle response seems good from low rpm to WOT. However, after 30-40 mins of riding it, (I try not to bag the machine too much and keep it WOT all the time), I will let off the throttle going fairly quick then give it more throttle, the machine feels like it's going to stall - total loss of power. At that point, I let off the throttle and slowly get add more throttle, sometimes it's fine, sometimes it dies a bit.

Usually but this point (since I usually take my son for a quick ride) I start heading home in fear that the machine will die and I'll have to walk home and take it easy on the throtttle till I get home.

So, when it misbehaves, it's a fairly instanteous thing, performance great, the suddenly feels like it's going to die, then fine. Run great for 99% of the time.

Parts catalog shows that it has a fuel pump. This is more complicated than a simple gravity feed system. Older powersports machines are more likely to have fuel system problems than electrical problems IN GENERAL. I would dis-assemble the fuel system as much as possible, inspecting for any signs of rubber cracking, restriction, or contamination (dirt, water, sediment, etc.) and cleaning as necessary. Also change the fuel filter - looks like it has an automobile style pleated media type.

Back to the possibility of partial seizing when hot (more likely because it's newly rebuilt and tight), you might want to try a richer oil mixture and/or premium oil for a short time and see if that has any effect.

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Hi again,

So I measured the resistances of the various coils and I think I have found the smoking gun. The Exciter coil is supposed to measure 245 ohms, but I'm only seeing 189 ohms.

I wish I would have tested this before I put the whole thing back together again....

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atleast you got to the bottom of it, (hopefully) now get out and ride that thing

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