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Luke Guy

2 stroke Expansion Chamber/not starting/lumpy running

18 posts in this topic

Hi all,

I have a question that i haven't found some clear cut info about. Id just like to hear it from the community....

my mate and i are honda CRF boys , and sadly when we were recently on an out ride in the South African landscape, his engine came to a halt & seizure! what a day it was getting the bloody thing back to the car!

Anyway we are young adults at uni who dont really have a lot of mony spare! we managed to find a way to rapidly replace the bike and to save money. We found a running 2004 yama yz250 2t and the guy traded us a seized 2005 crf250 for this- he specialized in chop shopping Honda and obviously saw value in this trade.

 

it obviously came with its quirks....

this yz250 has:

-1998 yz 250 motor

-CDI from an 80cc  yz

 

When we first took the bike to the track it would not start. I noticed the following:

-small holes in the expansion chamber (a few, none bigger than the tip on an Aux jack)- what is the impact of these small holes on the bike? ( i will use fire-gum to fix)

-muffler does not tightly fit to the end of expansion chamber.

FYI, fuel mix was 50:1- recommended by the oil we used- i have been told that old Yamaha are better with more oil? 25:1?

 

So leading up to my question:

when we eventually got the bike started (Bump start) it really ran lumpy, and bogged when the accelerator was pulled and eventually died.

when i get the chance to look at the bike, what is the impact of holes in an expansion chamber?- could this be a reason for  starting and power issue and bogging under acceleration??

Could this be because of a CDI from an 80cc?

Could this be Fuel Mix?

Spark Plug?

apart from those questions, i am going to change the spark plug anyway- i can assume that maybe this could be due to a faulty plug?- maybe??

anyway, this is what i know. perhaps you guys would be able to add to my theories or tell me something more that i could investigate?

thanks,

 

Luke

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Oh boy, where to begin? Toss the CDI and the expansion chamber. Get the correct ones for a YZ250, make sure he hasn't also done some sort of hack job special to your subframe and muffler mounting points. You should tear that motor apart and make sure everything is properly in spec. 

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Oh, and I always mixed my YZ250 40:1 with Yamalube. You're not going to be wringing its neck as hard as a 125 so it doesn't need more oil than that.

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1 minute ago, WALKINGWOUNDED said:

2 things to be suspect of, is compression, and the pedals on the reed cage.

The CDI, if it really is one for a YZ80, Isn't going to properly work on a 250 as the ignition map is wildly different, the holes in the expansion chamber are going to screw up the pressure waves as they travel back and forth from the exhaust port to tip. This bike is a basket case at best.

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11 minutes ago, busted bones said:

The CDI, if it really is one for a YZ80, Isn't going to properly work on a 250 as the ignition map is wildly different, the holes in the expansion chamber are going to screw up the pressure waves as they travel back and forth from the exhaust port to tip. This bike is a basket case at best.

Yes, unfortunately it probably has a long list of possibilities.

He'll have to pick a most obvious defect, and proceed from there.

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Holes in the exhaust can melt the piston if large enough/enough of them. It will scavenge the cold air and cause a lean condition through the exhaust port.

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50:1 is too little oil.  32:1 is the proper ratio.  That will eliminate any issues due to insufficient lubrication.

Engines can also seize due to improper ignition timing or excessively lean jetting.  All two stroke CDI ignition advance curves have similar characteristics but they will vary depending on bore diameter, compression ratio and various other engine characteristics.  Might as well get the right stuff.

Holes in the pipe will just lose power, make noise and spray oil residue out.  Won't harm the engine.

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finding the time to try all these things is a challenge, I have had a brief look into the bike. Compression is okay @turbo dan . and placed a new plug in. fixed the holes in expansion chamber with fire gum @MaybeMe. I will need to try a new fuel mix. Recommended says 32:1 think i will use this as a baseline @turbo dan

I managed to get the bike started, but it is still bogging down under acceleration. i have gathered multiple opinions saying that the CDI shouldn't make much of a difference @busted bones - but i still shouldn't rule it out just yet. want to try one more thing before i go ordering parts.

 

so with the motor more or less being okay...

it occurred to me...I live in south Africa inland, about 1700m above sea level (5500 ft) which made me wonder if i may need to adjust the carb air/fuel with inside screw?- or even posibly a blocked/ jammed jet? @busted bones @MaybeMe @turbo dan

anyone know how many turns of the screw inside the carb cater for this kind of atmosphere?

again, this is a carb from a 1998 yz250- old but gold right? haha

i agree that this bike is really a zombie of a bike i even think it has been built onto a 4 stroke frame(tonnes of space around the engine compartment & carb)....not gonna hassle too much if i cant fix this simply....ill throw it to the dogs

but i want to try at least!

thanks for all the great ideas guys!

 

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2 hours ago, Luke Guy said:

finding the time to try all these things is a challenge, I have had a brief look into the bike. Compression is okay @turbo dan . and placed a new plug in. fixed the holes in expansion chamber with fire gum @MaybeMe. I will need to try a new fuel mix. Recommended says 32:1 think i will use this as a baseline @turbo dan

I managed to get the bike started, but it is still bogging down under acceleration. i have gathered multiple opinions saying that the CDI shouldn't make much of a difference @busted bones - but i still shouldn't rule it out just yet. want to try one more thing before i go ordering parts.

 

so with the motor more or less being okay...

it occurred to me...I live in south Africa inland, about 1700m above sea level (5500 ft) which made me wonder if i may need to adjust the carb air/fuel with inside screw?- or even posibly a blocked/ jammed jet? @busted bones @MaybeMe @turbo dan

anyone know how many turns of the screw inside the carb cater for this kind of atmosphere?

again, this is a carb from a 1998 yz250- old but gold right? haha

i agree that this bike is really a zombie of a bike i even think it has been built onto a 4 stroke frame(tonnes of space around the engine compartment & carb)....not gonna hassle too much if i cant fix this simply....ill throw it to the dogs

but i want to try at least!

thanks for all the great ideas guys!

 

Not sure of the exact jetting, but if you bought it from somewhere at a different elevation where it “ran” correctly, jetting will need to be changed. 

Have you checked to make sure the powervalve opens?

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@MaybeMe the time now is 12 noon. off to a meeting. when i get back it will be my task for the day. i'm going to crack open the carb and see what i can discover.

will let you know....

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21 hours ago, turbo dan said:

50:1 is too little oil.  32:1 is the proper ratio.  That will eliminate any issues due to insufficient lubrication.

Engines can also seize due to improper ignition timing or excessively lean jetting.  All two stroke CDI ignition advance curves have similar characteristics but they will vary depending on bore diameter, compression ratio and various other engine characteristics.  Might as well get the right stuff.

Holes in the pipe will just lose power, make noise and spray oil residue out.  Won't harm the engine.

That's a lot of misinformation. Oil ratio should be determined by the RPMs the engine runs at. Higher RPMs and temperatures require mlre oil. 40:1 for a 250 and 500 is fine, 32:1 for 125s, obviously more for 80s and smaller since their neck is wrung out. Oil will lean out a AF ratio and cause its own problems if you go too far.

 

The holes in the expansion chamber will cause all sorts of fueling and AF ratio problems and if it leans out too far the engine isn't going to last very long running hot. Been there done that.

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1 hour ago, busted bones said:

That's a lot of misinformation. Oil ratio should be determined by the RPMs the engine runs at. Higher RPMs and temperatures require mlre oil. 40:1 for a 250 and 500 is fine, 32:1 for 125s, obviously more for 80s and smaller since their neck is wrung out. Oil will lean out a AF ratio and cause its own problems if you go too far.

 

The holes in the expansion chamber will cause all sorts of fueling and AF ratio problems and if it leans out too far the engine isn't going to last very long running hot. Been there done that.

I would say you are misinformed.  32:1 is appropriate for anything from a 125cc engine on up.  The difference as far as AFR is negligible between 32:1 and 40:1.  The difference between 32:1 and 40:1 is eight tenths of an ounce out of a gallon, .6% by volume. 

I would be interested in how you were able to empirically determine that holes in the exhaust caused your engine problems.  Sounds likely that said engine was not being very well taken care of and it could have been any number of things. 

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Posted (edited)

Get the correct cdi first off 👍 and carb. Then maybe it will run if bottom end isn't whooped out 

Edited by Motox367

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39 minutes ago, turbo dan said:

I would say you are misinformed.  32:1 is appropriate for anything from a 125cc engine on up.  The difference as far as AFR is negligible between 32:1 and 40:1.  The difference between 32:1 and 40:1 is eight tenths of an ounce out of a gallon, .6% by volume. 

I would be interested in how you were able to empirically determine that holes in the exhaust caused your engine problems.  Sounds likely that said engine was not being very well taken care of and it could have been any number of things. 

Never had holes in the exhaust, I had a bike that was excessively lean causing it to run hot and cause a predetonation issue. With out a properly working exhaust(holes in it for example) it will suck air into the expansion chamber when it goes into vacuum and when the pressure wave returns from the end of the pipe, will lean out his AF mixture as it moves the air and unburnt fuel back into the cylinder.

 

Funny thing was, when I had that issue with my YZ, I thought I had possibly heard predetonation, posted a video hear asking for a second opinions, was told that's just how 2 strokes sound. At the time, I didnt have the greatest ear for it and was pretty inexperienced on how to diagnose issues. Sure enough a couple weeks later the piston grenaded. That was 11 years ago and I learned a lot about how to tell what an engine is saying since then, but it was a good lesson then.

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4 minutes ago, busted bones said:

Never had holes in the exhaust, I had a bike that was excessively lean causing it to run hot and cause a predetonation issue. With out a properly working exhaust(holes in it for example) it will suck air into the expansion chamber when it goes into vacuum and when the pressure wave returns from the end of the pipe, will lean out his AF mixture as it moves the air and unburnt fuel back into the cylinder.

Seems plausible in theory but I don't think it going to actually have an effect.  I have seen plenty of holed pipes, cracked welds and shoddy repairs.  They will make a mess, they make noise, they cost power but they won't burn a motor down.  A hole large enough to let a significant amount of air in would bleed off enough pressure to kill the pipe altogether. 

Detonation will definitely destroy a perfectly good engine very quickly.  It is very important to recognize that particular sound but I always have a difficult time describing it. 

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hello guys!

UPDATE!

eventually took apart the carb, yes there was some muck in the low speed jet....but this was not the issue!

i looked further and guess what i found!....an extremely damaged reed valve. possible problem?... i think so...

in my opinion this definitely explains:

-starting.

-no power!

Going to go get new ones in the morning, hope that i can bring this beast to life! man i love it when there is clear problem!

what do you guys think?

will keep you guys updated!

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9 hours ago, turbo dan said:

Seems plausible in theory but I don't think it going to actually have an effect.  I have seen plenty of holed pipes, cracked welds and shoddy repairs.  They will make a mess, they make noise, they cost power but they won't burn a motor down.  A hole large enough to let a significant amount of air in would bleed off enough pressure to kill the pipe altogether. 

Detonation will definitely destroy a perfectly good engine very quickly.  It is very important to recognize that particular sound but I always have a difficult time describing it. 

Haven’t seen it on a bike, but on a snowmobile; exhaust had a silver dollar-size hole about 2” after one exhaust port. The piston in the cylinder had a hole burn through it.

Maybe it’s not as prevalent on snowmobiles since multiple cylinders would mean less power loss across the board, as compared to a single cylinder.

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