preventing power loss

jaguar

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Nov 25, 2010
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If you have a head gasket leak (due to uneven head or cylinder top surface) or a crankcase leak then you will have power loss. Using sandpaper on top of glass you can easily plane the head and cylinder top.
How to check for crankshaft seal leaks
Remove the spark plug and magneto. Put water and about 10-20% dishwashing liquid soap in a spray bottle. Spray the left crank seal and push the bike forward and rearward so that the descending piston creates a momentary crank pressure. If there is a seal leak then it will blow soap bubbles. If the leak is big then it wont blow a bubble and you will have to listen for escaping air.
Put the magneto back on and remove the stator coil. Remove the primary drive gear. Spray the right crankshaft seal and turn the engine over by turning the magneto. It is possible also to put an electric drill onto the left crankshaft to turn the engine over. Turning it by hand is hard to do fast enough.
Both seals can be replaced by prying them out with a jewelers screwdriver and pressing the new ones in by hand after putting engine oil on the inner parts of the seal and also the exterior of it where it will be contacting the crankcase. You should have someone move the crankshaft CW and CCW by moving the magneto while installing the right seal. That is because there is a lip on the shaft that the seal sometimes will not go over. Best seals are available from Rock Solid Engines in Australia.
AS an alternative to turning the engine over while looking for bubbles you can have more internal pressure in a continuous way by sealing the exhaust and intake (only if no reed valve is installed) while the piston is at or near dead bottom. The exhaust port plate should have a motorcycle inner tube stem in the middle of it so you can use your air pump to pressurize the crank. I did this for a better idea of how much the seal was leaking after seeing some small bubbles doing it the easy way. I made the exhaust plate out of wood and I sealed it with silicone sealant. Here is a picture of it;
testerPSI.jpg
 


Fabian

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Sep 8, 2009
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Pressure testing the crankshaft seals will forever now be nothing more than a 5 minute job
 

jaguar

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I am in the process of using sealed crankshaft bearings as a first line seal. Last night I put it together and doing a pressure test now I see soap bubbles escaping from around the bearings. So I will split the cases again and move the bearings inward a bit and put Goop around the area that they will be pressed back onto so there will be a good seal. I'm hoping for a 100% seal since the seals I have are shot and I'm waiting for some more to arrive. Once my low psi pressure gauge arrives I can test for crank pressure to know exactly what the ratio is. I don't want it to be more than 1.5:1
 

jaguar

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yeah, of course, metal is better but I thought I'd try the wood to see if it would work and it did.
Today is a marathon day for engine repair. It is still leaking after using the Goop. I will tear it down again and use something else this time.
[1 hour later]
With time to eat lunch and think about it (instead of tearing into it again), I don't think the Goop was that ineffective. The bearing seals are a hard type of rubber and not designed to seal perfectly. It's either that or I damaged them when prying them off in order to stuff the bearing with Badfish grease. So I consider this a failed experiment. Oh well, you don't know until you try. I will put my barely functioning seals back on and wait for the new ones from Rock Solid.
 
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jaguar

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I forgot to mention that I found there was excess resistance in the clutch shaft bearings which was discernable just by turning the shaft by hand (after I removed the chain and clutch) On the sprocket side bearing I had to use my exacto knife to pry the seal off with the sprocket in place since I couldn't remove it. So I couldn't get as much BADFISH GREASE in there as I'd like but it's extra lubricating properties hopefully will make up for that. With both bearings greased the shaft turned much easier. People don't realize that a little turning resistance anywhere from crankshaft to rear wheel robs a big percentage of power when the engine is of small displacement.
 

geebt48cc

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Feb 2, 2010
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683
Of the 760 miles I've got now on the HT 66cc, 450 of those has been without a head gasket. I've done all kinds of mods, which includes milling,porting,reed,etc-- without using head gasket. Bike runs very strong without any signs of leaks. After reading this, I just wonder though? I can tell a small bit, that after riding a few miles with getting engine to temp, that maybe the engine isn't quite as strong as it was first mile. It's so close to beingthe same, that maybe it's just me???

Once again though, I do
keep everything torqued down, and it shows zero leaks?
 

jaguar

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Nov 25, 2010
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Almost every time I take mine apart I resurface the head and the cylinder mating surface and yet I still get leaks as is evident by the oil on the cylinder fins. A good design allows compensation for the mating surfaces to distort due to unequal heating (due to most of the heat at the exhaust side). I may make a head gasket out of fibrous gasket material. I've also toyed with the idea of grooving the head and cylinder to accept an o-ring. A lot of motocross engines have o-rings.
 

Purple Haze

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Aug 21, 2011
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409
Almost every time I take mine apart I resurface the head and the cylinder mating surface and yet I still get leaks as is evident by the oil on the cylinder fins. A good design allows compensation for the mating surfaces to distort due to unequal heating (due to most of the heat at the exhaust side). I may make a head gasket out of fibrous gasket material. I've also toyed with the idea of grooving the head and cylinder to accept an o-ring. A lot of motocross engines have o-rings.
I have been considering o-ringing, also. One more thing to point out concerning leaks, the sparkplug mounting surface was not flat on my last engine, luckily I work at a machine shop with a flycutter, otherwise I wouldn't know how to true it up. Finally, I've had sparkplugs that had a bad sealing ring on them.
 
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