Author Topic: Need Help: 1969 RS200 MkII, Melting Hole In Piston & Crank Seal Leak  (Read 222 times)

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Offline robinkwmack

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Alright, reaching out to the knowledgeable folks here on the forum to try and figure out a reoccurring issue with my 1969 RS200 MkII.

First off some info on how I am running it.
87 oct fuel
40:1 yamalube premix in tank
cleaned points but haven't checked timing. Seems to be either fine or close to what it should be.

I got the bike from the grandson of the original owner who parked it in 1971 at just under 3k miles. I tore the cylinders apart and was able to figure out exactly why he had parked it. (Broken engine stud that then caused a head gasket leak.) I fixed the stud, put in a new gasket and the bike fired right up. I road it for maybe 300-500 miles with a gas leak on the left side. So I brought it to the local vintage motorcycle mechanic just to have them go through the carbs since I couldn't find why the leak was happening, had them make sure everything was set to "stock" so I could then tune it myself. They found the left float had a hole in it causing the leak and replaced it fixing that problem. with some carb cleaner I was able to find out there was a slight base gasket leak as well on the right cylinder. But about 50-100 miles after getting it back from them I lost compression on the right cylinder and found out that I had melted a hole in the center of the piston. Thinking that it was the base gasket issue I put a new piston on, new gaskets and it fired right up and ran like a top, but like an idiot, I did not check the carb seeing as it just came back from the shop, but then 50-100 miles later, same problem. Hole in the piston on the exact same road as when it happened before. I had noticed that the right fuel line was not filling up all the way and almost seemed "empty" only allowing a trickle at throttle, so I think the issue with the right cylinder is the right carb needs to be gone through again and readjusted and cleaned. But while I was in there I noticed some transmission oil was getting onto the crank when I turned it by hand (brown oil), very slight, but enough for me to realize I should do the crank seals as well.

So here are my questions:

Could my ignition coil be causing the spark plug to fire "hot" resulting in the melted piston?

Could it be an ignition timing issue?

Could the crank shaft seal leak be causing it to run lean as well?

What part number is that seal anyways? and what other parts will I need to do, or should do, while I have it open? I have had a hard time tracking it down in the parts catalogue and don't want to waste time and money trying to track it down seeing as I am 26 and basically broke and have already basically thrown away a few hundred bucks in a few hundred miles.

This time I will be sure to go through both carbs, all the cylinder gaskets again as well as that new right piston. I just can't afford to keep replacing that expensive piece.

Any insight would be a huge help and much appreciated.

Thank you.
-Robin

Online OldSwartout

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Re: Need Help: 1969 RS200 MkII, Melting Hole In Piston & Crank Seal Leak
« Reply #1 on: September 22, 2019, 05:26:04 PM »
The right side crankshaft seal you need is 09090-109, 28 oil seal.  The seal could cause a lean condition that would melt the piston, but a leak big enough to cause  melted piston would let a lot of transmission oil past and cause a lot of smoke while riding.  My best guess is it is the ignition timing advanced  too much on the right side. I don't think the coil itself will cause an early spark.  Of course, some of the other items you've considered are still possibilities. The carburetor might still be suspect - if you're certain there isn't a blocked jet, check the main jet size. It should be a 90 or 95. Compare to what the left side has. Check is to see if fuel will flow through the float valve freely. With the carburetor still connected to the fuel line, just leave the bowl off, hold over a container and turn on the gas. It should flow a steady stream. If that's good, check the float height per the service manual. I had a Suzuki that had the float set too low in the bowl, it would starve at high rpm, yet everything looked good otherwise.

Maybe others on here have more suggestions or experiences that will help.
Karl Swartout
Mooresville, IN
BS175 Roadracer
BS350 GTR

Online OldSwartout

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Re: Need Help: 1969 RS200 MkII, Melting Hole In Piston & Crank Seal Leak
« Reply #2 on: September 22, 2019, 05:31:19 PM »
An outside chance is that the fuel cap isn't vented properly. Usually, an unvented cap will just cause the bike to slowly die of fuel starvation on both cylinders as fuel flow slows; however, I know from experience that the right cylinder on these bikes will start to miss first when running out of gas.
Karl Swartout
Mooresville, IN
BS175 Roadracer
BS350 GTR

Offline BRT-GTR

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Re: Need Help: 1969 RS200 MkII, Melting Hole In Piston & Crank Seal Leak
« Reply #3 on: September 23, 2019, 05:04:32 PM »
          If the fuel cap vent is ok (just blow through it), take the fuel tap off and clean it thoroughly, tubes, filters etc.
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Offline Old BS Guy

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Re: Need Help: 1969 RS200 MkII, Melting Hole In Piston & Crank Seal Leak
« Reply #4 on: September 26, 2019, 03:37:33 AM »
Yes, of course, start from the top down and validate the tank cap, but it should affect both sides equally, not selectively. Still, good cause for checking. The only thing not mentioned here, unless I missed it, is the bowl vent for the carb, and under the circumstances seems most likely given the other carb is doing fine. I do agree, however, that validating each part from the cap down is required. Slim chance it's ignition, but ignition must always be vetted before carburetion in any diagnostic to prevent mistakes with fuel.
'65 BS Homer 50, '69 Mach II RS 200, '67 350 GTR,(2), '66 Sport 90 o.i., '65 90M Surf Rider, '68 100 Trail, '71 100 TMX ,'79 Honda CBX, '82 Yam 650 Turbo, '74 RD-350, '70 Triumph 500 Trophy, '65 Honda CL72 250 Scrambler, 67 Honda CL77 305 Scrambler.

Offline BRT-GTR

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Re: Need Help: 1969 RS200 MkII, Melting Hole In Piston & Crank Seal Leak
« Reply #5 on: September 26, 2019, 03:48:51 PM »
    Going back to basics, melting pistons are due to a cylinder running way too hot. Primary causes are weak petrol/air mixture IE, lack of fuel, too much air due to leak  and /or ignition timing advanced too far. Plug firing too early.
« Last Edit: September 26, 2019, 03:50:44 PM by BRT-GTR »
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Offline rwgibbon

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Re: Need Help: 1969 RS200 MkII, Melting Hole In Piston & Crank Seal Leak
« Reply #6 on: September 26, 2019, 04:57:48 PM »
Regarding what would melt a piston, I agree with Brian.

Randy

Offline robinkwmack

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Re: Need Help: 1969 RS200 MkII, Melting Hole In Piston & Crank Seal Leak
« Reply #7 on: September 26, 2019, 09:32:09 PM »
Thank you all for all of the great responses.

There was never any white smoke, I only noticed brown oil on the crank when I had the cylinders off and when I turned the engine over by hand. But I know that that right seal likes to go, so would rather do it now then later.

Are there any other gaskets or seals I will be encountering and might replace at the same time as the crank seal? Obviously know about the base and head gaskets, I already have another set of those ready to roll.

Petcock is new and fuel flows freely out of both nipples when nothing is attached. Fuel cap used to not vent, so I drilled a new hole, definitely is venting.

Also any help with how to get to and replace that crank seal would be much appreciated. Haven't opened this motor up that much yet but I do have the repair book to go by.

I am thinking that side keeps melting due to the carb, I will double check the timing and set it appropriately and also make sure the carb is adjusted and cleaned as it should. before firing it back up. Definitely seems like it has something to do with the right carb. Fuel will fill when you first turn on the gas, but after running it the fuel turns to an incredibly slow trickle, barely able to see it coming through and flicking the line/filter to move air bubbles doesn't help either.

Thanks again for all of the help and any future help. Once I know exactly what I need to order I will start getting this thing back on the road.

Offline BRT-GTR

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Re: Need Help: 1969 RS200 MkII, Melting Hole In Piston & Crank Seal Leak
« Reply #8 on: September 27, 2019, 09:03:01 AM »
        Fuel will flow at good rate when tap first switched on (bowls empty due to evaporation) but will reduce when the bowls are full. However, the lines should fill with petrol,  air running back up to the tank. Have a look at that new tap, could be an issue there.
     Check right carb for correct jets, needle jet, the needle itself and position of needle clip (all shown in manual). Also check float height, that's a likely source of weak mixture. Correct spark plugs ?, you can't just use any type, heat range has to be correct.                        Brian.
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Offline robinkwmack

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Re: Need Help: 1969 RS200 MkII, Melting Hole In Piston & Crank Seal Leak
« Reply #9 on: September 28, 2019, 08:10:08 PM »
Everything on the bike is incredibly stock, except for the home made air filter, the turn signals that I added, tires and chain. Spark plug is in range, I am going to check ignition timing after I get it back together so I am thinking something in the carb got blocked some how, or the float was not adjusted correctly. Going to order up parts soon and tear into it again. Petcock flows as it should and gas cap is venting properly. (It was not venting so now I know it is)


Are there any guides on this forum for getting to and replacing that oil seal? I like to have an idea of the direction I am going before just taking it all apart completely.

Thanks again for the help folks.

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Re: Need Help: 1969 RS200 MkII, Melting Hole In Piston & Crank Seal Leak
« Reply #10 on: September 30, 2019, 09:14:06 AM »
The service manual is available under the Downloads section of this forum. There is a 50-175 Service Manual and a 175 Technical Manual, both have similar info.

The general steps are:  Drain transmission oil, remove carburetor cover, remove carburetor (let it hang by it's cables out of the way), remove kickstarter, remove right engine side cover, remove nut on end of crankshaft (left hand thread), remove clutch thrust plate, springs and retaining nut and remove clutch assembly, remove rotary valve cover, replace seal.  This can be done with the engine in the frame of course, but you will have to move the footpeg.  When reassembling, be sure to align the punch marks on the pinion, clutch gear and alternator gear.

Plan to buy or make a new side cover gasket.
Karl Swartout
Mooresville, IN
BS175 Roadracer
BS350 GTR

 


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