36MX4 Running problems-hope someone can help me

[geoff lemar]

Hi I need some help this one has got me stumped!! My MX4 360 runs real lean when it warms up there is no adjustment on the pilot air screw it is all the way in and when you ride it and you back off slightly it pings its head off!! So this is what I have done I have completely pulled the engine apart cleaned up the faces on the crank cases and glued them back together I replaced the right side seal and on the left side fitted a sealed bearing and I have also sealed up the points plate as well put new base gaskets on and re assembled the engine I ran the bike today and it is still the same the only thing I have not replaced is the tufnol spacer's between the carby and the inlet manifold on saying that I have cleaned them up and silicon them together and to the inlet manifold so I have come to the end of my idea's does the tufnol become porous with age ? can someone help me ?

[Phil Hyde]

Pinging!!!!

Geoff

The pinging you refer to sounds like detonation, or pinking. Try some octane booster, or better still 50:50 super-unleaded with Avgas if you can get hold of it. Typically avgas is around 110 octane is fully leaded and also assists because it runs slightly cooler.



Sorry, I'm not much use on mixture settings. Start rich and turn back!

Regards

Phil

[geoff lemar]

Running lean

Thanks Phil I don't thinks its the fuel I use octane boost it seems like its sucking air from somewhere and I have sealed about everything i can think of there should be adjustment on the air screw but there is int .

[John Wakefield]

Crankshaft Bearings

I dont think a sealed bearing on the crankshaft is a good idea, the gease will soon dry out & sieze, as it will not get any lubrication from petroil mix. You need to remove at least the inner seal. These sealed bearings are only
suitable for slow revs, ie wheel bearings. Have you stripped the carb, could be a blocked pilot jet.

[John-Boy]

Geoff
I am not sure how effective is a sealed bearing in retaining crankcase pressure compared with a proper oil seal running directly on the crankshaft.
Sealed bearings, in my experience, are good at keeping dirt out and oil in at normal atmospheric pressure, but not necessarily a fuel/air mixture under pressure.
Your symptoms sound like a leaky crankshaft oil seal or a worn throttle slide/carb.

The Greeves manual specifies a 25 pilot jet but I have found my engine runs better with a 30 (I have a 380 cylinder on my bike).

It is possible that the tiny pilot jet channels are blocked on your carb and these can be difficult to clean. Have you got another carb you could try?

You could put some duct tape around the tufnol spacer, temporarily to seal off any possible air leak.

I agree you need some decent fuel, I use Bayford **** leaded which is widely available in the UK, see their website for details.

[geoff lemar]

sealed bearing

Thanks John but the bearing I used is rated at 12000 RPM and has high temp grease in it but I planned to keep the bearing serviced every couple of rides.

[geoff lemar]

Left side crank seal

Thanks John boy but they do not have a crank seal on the left side that is why I used the sealed bearing .

[geoff lemar]

New Carby

PS John Boy I put a brand new 900 series 32mil carby on it straight from the Amal carburetor company standard settings. 30 pilot with 300 main.

[John Wakefield]

Thanks John boy but they do not have a crank seal on the left side that is why I used the sealed bearing .

I cant believe it does not have a seal on LH side, all two strokes MUST have seals to keep in the primary compression. A sealed bearing is designed to keep out dirt but not compression. Dont know anything about the MX4engine, but principal is same on any 2 stroke. Has anyone got a cut away drawing of the MX4 engine?

[riflegreen]

There should be an oil seal on the left side , a sealed bearing will not work .

If it is running so weak then there is a problem with a blowhole in in the casting of the crankcases/cylinder or head . It has been known for the castings to fail after forty years .

Chris

[John-Boy]

Geoff,
There is a boss on the drive side crankcase to accommodate an oil seal. Perhaps yours has been machined off or used for a sealed bearing instead?
This picture is the best I have, the oil seal housing is partly obscured by the paper tissue.

[geoff lemar]

left side seal

yes it does have a seal but its right on the out side and the back of the drive sprocket fits into it but behind that there is only room for the bearing. this how it was when i pulled it apart. I thought this was a bit strange to have the seal hug the sprocket not the crank shaft

[Kim275]

Hi Geoff - that is correct and by design .........the seal is housed in the casing and the "seal" is made between the drive sprocket and the seal when the drive sprocket is located and tightenend onthe shaft (the shaft is airtight on the sprocket so achieves the same as if the seal was direct onto the shaft.
. As Chris states - it needs to be an oil seal........The reason for this is that (assuming it is the right way round) the pressure from the inside makes the "seal" onto the sprocket the other way round then the seal will leak...attached is a picture of my griffon 380 - but design is the same on the challenger.
Cheers Kim

[stormer254]

Kim is correct the seal grips the sprocket, I suppose it s possible that the fit on the crankshaft is not perfect or the sprocket running surface might not be perfect. You certainly need to replace the sealed bearing before it seizes on the shaft and damages your crankcases!

[geoff lemar]

Left side crank seal

Thanks Kim I got a seal kit from Frank Conley So it should be the correct seal I did replace both seals I only put the sealed bearing in to see if it would make any difference but it hasn't So I will remove it. I will have to keep tying until either I go mad or solve the problem.