HELP!!: 1979 Yamaha ET300C 2-stroke Snowmobile

[throttlesteer]

Here's my latest project.  Getting this sled ready for a Christmas present this year and it's giving me fits.  It hasn't run in about 8 years and I don't know the history on it so I can't give any specifics really.  All I know is what my dad (old small engines technician) and I have done to test it.

1979 Yamaha ET300C
two cylinder in-line
2-stroke
single carb

PROBLEM:  Has trouble starting, sometimes not at all.  Once you take the plugs out and dry them, it will give a good kick the first time and then nothing after that.  Doesn't seem to matter whether you choke it or hold the throttle open, or a combination of both.  While we had it running, we left the throttle at idle and held the choke open.  It would not run unless the choke was open, but would not rev up at all.  Cracking the throttle would cause it to die, as would letting the choke close.  When the fuel supply ran out, it did rev up and jump on the pipe without cracking the throttle, which immediately makes me think that it's just pig rich and needs to be leaned out.

FUEL TANK
We have bypassed the fuel tank and fuel pump.  I am running an upside down soda bottle with a hole in the cap that doesn't leak.  It's held above the fuel bowls to allow gravity to feed the bowls to eliminate any possible pump and tank issues.

CYLINDERS
Compression test yielded 85psi on the first pull, 95psi on the second pull.  I can't find anyone that will tell me if this is good or not but it is equal for both cylinders.  Plugs in, plugs out, it doesn't make a difference.  Same reading every time.  What we could see of the pistons looks ok.  THOUGH, the belt cover does have a Wiseco sticker on it, we are unsure if they are stock pistons or not.

CARBURETOR
The carb is a unique piece.  It does not have a slide at all.  In fact, it is just a butterfly and that's it.  There are no reed valves or anything.  First thing dad and I did was clean the carb and check the floats.  It appears to be getting more than adequate fuel.  The plugs are always wet after one or two pulls.  We had to replace the choke cable spring and needle tipped piece that regulates fuel flow.  The needle-tip end was completely missing, we're not sure how or why, but that's besides the point.  Yamaha lists anywhere from a 200-240 main jet that comes stock.  When we pulled the bowl off a 210 main jet was installed.  We have no other reason not to believe that this was the stock jet.

ELECTRICAL
The handlebars are a rats nest of wiring due to the old grip warmers, but it does get adequate spark.  When placed on a ground, the plugs spark well and bright.  When we had it idling it didn't seem to have any hiccups.

EXHAUST
Exhaust is original, but is free flowing and does not have any restrictions.  The pipe appears clear and in decent shape for the age.  We've ran it with and without the exhaust to prove that it wasn't the cause.

AIR BOX
The air box is a maze of tubes and passages, but appears to be in good shape, we've taken it apart and it does not seem to have any problems with air flow.  We've ran it with and without the air box to prove that wasn't the cause.


I suppose it comes down to timing if compression isn't the issue, but I'm not sure how these 2-strokes adjust timing.  Obviously it's getting good spark so I have no reason to believe that the other ignition parts are working too.  Plus we had it running, which makes me think that's not the case.  Thoughts?

[Kamakazi]

#1- if the needle under the float is missing the end, or the end is wore, YOU WILL FLOOD THE CARB
#2- get some carb cleaner that comes in a can and let all ur parts soak in it overnight to clean up the gum
#3- ur engine is flooding plain and simple, im sure the needle is letting fuel by and flooding the engine
#4 check to make sure ur float doesnt have a hole in it and is taking on fuel, although i kinda doubt it

ur carb is your issue, get it clean!!!! and replace your needle and seat

[throttlesteer]

It's definitely getting more fuel than it needs. The needle under the floats does not appear to be worn nor is it missing.  We double checked the floats and they are floating properly.  Here's the picture of the carb setup on this beast. The choke plunger  was missing all together, not the needle.  As far as I can tell, this thing doesn't have a conventional needle, the diagram from Yamaha lists number 11 as the needle valve assembly. Regardless, that doesn't solve my issue of difficulty to start either, but those problems could be seperate issues.

[Krandall]

#11 is the needle assembly. You'll need the entire piece.

And if the choke plunger is non existant. I'm guessing that it's just dumping fuel in so.. choke open or closed. It's dumping fuel in.. If your plugs are wet after 1-2 pulls. Its flooding the heck outta that thing. I think if you get that needle and choke fixed. You'll be sitting better.

[Kamakazi]

i am sending you a present, check your email.  I work at a yammy dealer 

[Kamakazi]

also on the needle and seat, the needle may not look worn but im willing to bet it has small rings on the tip, and this is where fuel leaks through, also im not sure about the choke assembly missing being what is flooding it, Some carbs there isnt a problem, some there are.  Regardless  you want a choke assembly and i would order one from yamaha, should be roughly 100$ including cable.

[throttlesteer]

OK, that ended up being the float needle, not the midrange needle.  This one does not have a midrange jet.  It's the idle and main and that's IT.  I got the choke plunger and my cable is OK to use right now.  Thanks for the email!

OK, thanks for all the responses. The CORRECT compression test revealed 110psi on my gauge after 3-4 pulls on each cylinder. Given variance from gauge to gauge and only a 10psi difference than what suggested was, I'd call it good.

Tonight we pulled the carb off all together and used a spray bottle of gasoline instead. While spraying gasoline intermittently, it would kick over and fire for a few revolutions and then die. I'm fully convinced now that it has to do with a carburetor issue. The only problem is, I'm not sure that my carburetor is complete. We've already had to replace a missing choke plunger, what else has been messed with? Now, is it possible that after 8 years of sitting in gas, that the floats have absorbed some of the gas and just aren't floating like they should, thus letting in more fuel than is needed? I know this is true of some vehicle carbs, I had to do this on my 72 Ranchero due to soaked floats.

Thanks! Any help with the carb (parts, completely different unit) would be much appreciated.

[Xylifrost]

thank you  I will pass this info along. 

A mobile mechanic MM weve used in the past stopped by last night to look at it because thankfully my b-i-l is not too stupid to attempt to work on this car without guidance.  The MM said it was definitely a blown head gasket when he pulled the plugs and saw water in the pistons.  Fortunately, we dont have the proper tools to repair it so it will be done correctly and with a warranty.

Thank you all for your info, its much appreciated

btw, b-i-l did post on his BMW site, but got bs instead of help.  I laughed when the MM told him if he were driving a Chevy, hed still be driving.

[Krandall]

thank you  I will pass this info along. 

A mobile mechanic MM weve used in the past stopped by last night to look at it because thankfully my b-i-l is not too stupid to attempt to work on this car without guidance.  The MM said it was definitely a blown head gasket when he pulled the plugs and saw water in the pistons.  Fortunately, we dont have the proper tools to repair it so it will be done correctly and with a warranty.

Thank you all for your info, its much appreciated

btw, b-i-l did post on his BMW site, but got bs instead of help.  I laughed when the MM told him if he were driving a Chevy, hed still be driving.

huh?