Vintage Dirt Bike Q & A

Hi there,

Well to cut to the chase, my 79 CR125R restoration is now done, but I am having an engine problem. It appears that tranny oil is getting into the cylinder and eventually running out the exhaust manifold and pipe and coming out pure black. I rebuilt the engine the first time a couple years back and got this problem right out of the gate.

Part 1...So I split it again, insured the two halves were sealed properly and replaced the primary drive gear seal as well. Oil is still coming out of the exhaust. All the seals were replace on the inside as well.

Part 2...Over the winter I purchased another engine in case maybe the one I have has a case crack or flaw (which I never saw upon the first or second splitting of the first engine case). I never split the second one I purchased. I painted both engines in case I had to swap them at some point. I took a look under the primary drive gear of the second engine and there was silicon all over the underside of the drive gear from the previous owner. I cleaned it off, swapped it with the first one just yesterday, fired it up and yup......same thing. Oil puking out the exhaust.

Now this got me thinking that I am not the only one that this has happend to. Sooooooooooo?????? Is this a design flaw or is there something I am missing. Is it possible that oil is travelling down the spline of the drive gear into the crankcase? I really don't want to silicon it all up if I don't have to. That seems like a cheap way of doing things, but if I have to I will. I filled the tranny with the recommended 10W40 as well.

One other thing, I mixed the fuel as per the manual 20:1. Now I know this might be a bit rich, but whatever is coming out of the exhaust is way more than the rich fuel mixture would account for in my opinion. It is litterally running down the exhaust pipe and dripping onto the ground when the engine is running at an idle.

I don't think I am a "back-yarder" type mechanic, and consider myself relatively intelligent, but this issue has me questioning myself quite a bit!! Has this happened to anyone else? Any help would be greatly appreciated. I need some sleep!!!! Thanks

Is your transmission oil level dropping?
For your pre-mix maybe use a good quality modern 2 stroke oil and mix it per the manufacturer's instructions, not 20:1. 20:1 is a lot of oil! My bike would foul plugs like crazy at that ratio.
With less oil in the premix, ride the bike at an easy pace, check the transmission oil level periodically and see if the drooling is reduced.

My 79 dripped oil like crazy also. Not so much anymore now that I have the jetting correct. It pretty much cleared the oil issue up. I also went through changing seals and the like with no change. Do you have a Mikuni or the original Keihin carb?

I also run 25:1 now instead of 20:1. I use Blendzall racing castor oil.

I am running the original Kehin, but I did have a Mikuni in there as well with the same problem. If you guys think that it is the fuel/oil mix ( i do realize that 20:1 is a lot, what do you suggest? 35:1 or 40:1?

I have not run the bike long enough to check the oil level yet. I would like to take it out to the country and run it a bit an see. I do remember when I took the engine apart a few years back, I removed the cylinder head and the crank lobes were covered with a thin film of the tranny oil, if I remember correctly.

It just seems odd that two engines are doing the exact same thing.

I use Maxima Castor 927 at 32:1 in my 1975 CR125.

I will try reducing the ratio on the fuel and get a high end castor oil as you all have suggested and see where it leads from there. I will try to take it out this weeekend and see hat happens. Thanks for the help. I will let you know.

I use the Honda pre-mix that was developed for the CR's. The mix is 32 to 1.

Oil will tend to accumulate in the crank case of 2-strokes when the engine is allowed to idle for extended periods. You need to ride the bike to clear it out. Of course it pukes black out of the pipe when it clears out.

Good looking resto you have there!

~JM~

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WWW.CADDY500.COM
The High Performance
Big Block Cadillac Forum

If your exhaust is not white - and you've replaced the primary drive oil seal - I'll wager you do not have transmission oil being sucked into the crankcase. If the exhaust is blue / black and you have a high level of spooge at either the silencer / exhaust manifold you have a "not burning enough of the pre-mix" problem. I would run a modern oil at their recommended ratio which is going to be 32:1 to 50:1 depending. Then check your jetting against some form of benchmark - i.e. anyone else who is not having the same issue. Then if you can get the bike to run, really warm it up, and then ride it real hard. It is my understanding that the exhaust pipe temp needs to get to 1200F to really burn the pre-mix completely and it is an ugly reality that us mortals do not rev hard enough to achieve this. Coupled with conservative oil ratios and/or rich jetting = spooge.

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Michael Stiles
1979 Honda CR250R Elsinore | 2006 Husqvarna SM510R | 2007 Service Honda CR500R-AF

Thanks, I will reduce the ratio and grab some Honda pre-mix when I get a chance. I will take it out and ride it and see if it clears out. I did run it at idle when all the oil showed up. I have not even taken it out yet so I will get a better idea afterwards. It has only run at idle as of rigth now.

Thanks for the info and the compliment.

Cheers

The bike is running great, just rich in my opinion. I will lessen the ration as I have stated and give that a try. I think the smoke is more of a blue than white, but I will confirm that later. I really don't want to rev the snot out of it after a fresh rebuild, but we'll see, lol. I if run out of ideas I'll rev the hell out of it!! Thanks for all the info. It is nice to have people help out in these type of situations where experience is the only answer most times.

CHeers

Just would like to say thanks to everyone for helping out. I leaned out the mixture, adjusted the carb a bit and took it for a good boot, and the leaking went away. It has lots of snap now so I am quite pleased!! I can close the book on this resto. Well almost. My original Kehin carb needs a rebuild, so I will take care of that and drop it in. The Mikuni will do just fine for now however.

Thanks again to all!!!