250 1985 Yamaha YZ250

RichyRich23 · Apr 17, 2016

These past few weeks I've been rebuilding a '85 YZ250. When I got the bike to a point that I could start it I tried kick starting it for at least a half hour(With and without the choke) When I finally gave up trying to start it I took the carb off and took it apart and cleaned it to the point it looked new(inside and out), I put it all back together and made another attempt to start it, Still nothing! I was starting to worry that there was some other problem. I then tried push starting it After six or seven attempts It finally started but died after I let off the throttle. Pushed the bike back to my house and called my dad, He told me to Make a couple adjustments on the carb(something like the air/fuel mixture screw and the idle screw) Made those adjustments and tried kicking it twice wouldn't start, I push started it a couple more times and it finally started again. I started heading back to my house and the bike acted like the clutch was slightly disengaging. It would drive fine with the throttle barley cracked opened. anymore throttle than that the bike wouldn't go any faster the engine would rev a little bit. I parked the bike for about five minutes went out and tried kick starting it. It fired up on the second or third kick, It ran for about ten seconds and died. What can I do to keep it from dying?? (It's getting gas, Has a new spark plug/getting spark) The gas/oil mix is 32:1 right now.(Side note it's a 2 stroke) The bike has all the plastics aside from the side plastics where can I find the side plastics?? Are older 2 strokes normally this hard to start after a rebuild? I have a '82 YZ80 i rebuilt a while ago and it wouldn't start when i tried to kick start it.. I finally gave up and tried push starting it on the second attempt it started.
Any Help will be greatly appreciated!! Thank you in advance for the help

Mihylo33 · Apr 17, 2016

Have you done anything other than clean the carb?
1: remove stator cover and make sure everything is tight in the flywheel section.
2: remove spark plug and take a picture of it and post it here. Also check to make sure it has bright blue spark.
3: remove the reed valve assembly to make sure the reeds are in excellent condition.
4: compression test. It should be around 190psi.
Report back here immediately if any one or all of these findings are out of spec.

RichyRich23 · Apr 17, 2016

Okay so the I'll have to check everything in the flywheel area tomorrow. As for the reeds what would I be looking for if there's something wrong? I did a couple compression test the first time I got roughly 90-95, The second test was a little more concerning it was only at 60. I plan on boring the cylinder to 70 or 71 mm The spark plug is brand new so that doesn't concern me very much..
I ended up tearing the top end down today and discovered that the crank case had some gas chilling in it. also when i pulled the expansion chamber off i was surprised to find that there was gas chilling in it also. Could the float sticking open be the problem to my "flooding" issue?
I've looked at the reed valve assembly and the only thing that concerns me is some reeds aren't sitting completely flush.

2strokesteve89 · Apr 18, 2016

RichyRich23:
Okay so the I'll have to check everything in the flywheel area tomorrow. As for the reeds what would I be looking for if there's something wrong? I did a couple compression test the first time I got roughly 90-95, The second test was a little more concerning it was only at 60. I plan on boring the cylinder to 70 or 71 mm The spark plug is brand new so that doesn't concern me very much..
I ended up tearing the top end down today and discovered that the crank case had some gas chilling in it. also when i pulled the expansion chamber off i was surprised to find that there was gas chilling in it also. Could the float sticking open be the problem to my "flooding" issue?
I've looked at the reed valve assembly and the only thing that concerns me is some reeds aren't sitting completely flush.
View attachment 28708

If you did the compression test correctly 95 is far too low, about half what it should be.

Mihylo33 · Apr 18, 2016

Time for rebuild.
Piston, rings, replate cylinder if applicable, reeds, gaskets etc.
might as well do the 1.23 waterpump seal and bearing while its there.

RichyRich23 · Apr 18, 2016

Yeah that's going to be the best thing I could do. The previous owner told me that the bike had been sitting for 10+ years. Could you tell me where I'd be able to get the side plastics?? I have one that only covers the air filter

RichyRich23 · Apr 18, 2016

2strokesteve89:
If you did the compression test correctly 95 is far too low, about half what it should be.

What's the proper way to do a compression test??
This is what i have to work with, I'm not sure if it makes a difference or not

Lineman353 · Apr 18, 2016

The compression tester looks good. Does it have a stop needle that holds as you kick the bike over. Also how many times did you kick the bike over while doing your compression test. I think you are supposed to kick it over 5 or 6 times for an accurate reading.

2strokesteve89 · Apr 18, 2016

There should be a check valve in the bottom of the hose to hold the compression sample from escaping while testing.but as far as the test, hold the throttle wide open and kick a full stroke 4 to 6 times. If you don't trust your tester either see if a friend has one your can compare it too or replace it.

ossagp · Apr 18, 2016

RichyRich23:
Okay so the I'll have to check everything in the flywheel area tomorrow. As for the reeds what would I be looking for if there's something wrong? I did a couple compression test the first time I got roughly 90-95, The second test was a little more concerning it was only at 60. I plan on boring the cylinder to 70 or 71 mm The spark plug is brand new so that doesn't concern me very much..
I ended up tearing the top end down today and discovered that the crank case had some gas chilling in it. also when i pulled the expansion chamber off i was surprised to find that there was gas chilling in it also. Could the float sticking open be the problem to my "flooding" issue?
I've looked at the reed valve assembly and the only thing that concerns me is some reeds aren't sitting completely flush.
View attachment 28708

ossagp · Apr 18, 2016

Everyone is big on compression tests!!! If you have raw fuel in your cases and up into the pipe, that is where I would be directing my attention first. Since you have it all the way "top down" you might as well see if the tolerances are close enough for rings and piston or if it's time to bore that one. Fyi trying to save money on the rebore will probably be a waste. A new float needle, Seat, and setting it would probably have had you riding for awhile. The cardinal rule is that when you are trying to start or tune any bike and you need advice, give a really detailed description of the spark plug.

RichyRich23 · Apr 18, 2016

Lineman353:
The compression tester looks good. Does it have a stop needle that holds as you kick the bike over. Also how many times did you kick the bike over while doing your compression test. I think you are supposed to kick it over 5 or 6 times for an accurate reading.

Yes it has stop needle and I kicked it 6 or 7 times just so I felt confident that 60 was its max.
Should i be using that brass piece??

RichyRich23 · Apr 18, 2016

2strokesteve89:
There should be a check valve in the bottom of the hose to hold the compression sample from escaping while testing.but as far as the test, hold the throttle wide open and kick a full stroke 4 to 6 times. If you don't trust your tester either see if a friend has one your can compare it too or replace it.

Uh when you say hold the throttle open you're saying I need to have my carb on the bike while I do the test. Oops I need to redo the compression test

RichyRich23 · Apr 18, 2016

ossagp:
Everyone is big on compression tests!!! If you have raw fuel in your cases and up into the pipe, that is where I would be directing my attention first. Since you have it all the way "top down" you might as well see if the tolerances are close enough for rings and piston or if it's time to bore that one. Fyi trying to save money on the rebore will probably be a waste. A new float needle, Seat, and setting it would probably have had you riding for awhile. The cardinal rule is that when you are trying to start or tune any bike and you need advice, give a really detailed description of the spark plug.

I have no idea what you mean by tolerances. Please explain what you're talking about.
Is the std piston size 68mm for that year of bike??
I'm not really sure what to say about it except its a NGK R br8es, Its brand new and portion that sparks is clean, i mean it looks like its never been used before. I have a spark plug gap gauge and In inches its telling me the gap is .020, it was fairly tight getting the the plug on it. I checked to see if i was getting any spark and the spark looked good to me. other than that i have no idea what else i could describe about it.

RichyRich23 · Apr 18, 2016

2strokesteve89:
There should be a check valve in the bottom of the hose to hold the compression sample from escaping while testing.but as far as the test, hold the throttle wide open and kick a full stroke 4 to 6 times. If you don't trust your tester either see if a friend has one your can compare it too or replace it.

I put the carb back on the bike and held the throttle wide open, I kicked it 7 times before the needle stopped moving. The difference wasn't very much more but it did add to it