Blow By

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KSsteve

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Dec 26, 2006
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27
My 743 has a bad case of blow-by and its hard to start. (It likes the Ether) What would be one of the first things you would check? I was told the Valve seals would be a good place to check. Other than Blow-by it runs great. Thanks,
 

Tazza

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Blow-by = leakage between the piston rings and the cylinder walls, valve seats have nothing to do with this sorry. You could have a very small amount leaking past the valve guides but that won't cause hard starting.
How many hours are showing? these engines will usually give 5-6,000 hours of life before they need re-conditioning.
The only real way to check the engine is to pull it out, remove the sump and head, pull the pistons out and check the bores and pistons for wear. Sometimes you are able to get away with giving it a new set of rings and a flex hone of the bores but you need to make sure the bore is within tollerance. If you want to check this out, i have all the measurements at home.
The other thing you can do is a compression test. I had a machine that had low compression and was a little hard to start but when warm it was great. If you want it just rite be prepared to spend 1-2 grand. If you can live with it, live with it, if not they can be repaired. Don't over-look aftermarket parts!!! they are less than 1/2 the price of genuine but i think they still use genuine piston rings.
 

Tazza

Administrator
Staff member
Admin
Joined
Dec 7, 2004
Messages
16,697
Blow-by = leakage between the piston rings and the cylinder walls, valve seats have nothing to do with this sorry. You could have a very small amount leaking past the valve guides but that won't cause hard starting.
How many hours are showing? these engines will usually give 5-6,000 hours of life before they need re-conditioning.
The only real way to check the engine is to pull it out, remove the sump and head, pull the pistons out and check the bores and pistons for wear. Sometimes you are able to get away with giving it a new set of rings and a flex hone of the bores but you need to make sure the bore is within tollerance. If you want to check this out, i have all the measurements at home.
The other thing you can do is a compression test. I had a machine that had low compression and was a little hard to start but when warm it was great. If you want it just rite be prepared to spend 1-2 grand. If you can live with it, live with it, if not they can be repaired. Don't over-look aftermarket parts!!! they are less than 1/2 the price of genuine but i think they still use genuine piston rings.
One thing you can try is to check the valve clearance. It should be .007-.009 on both inlet and exhaust. I just use .008 and if a .009 will not fit after adjustment its correct.
 

Fishfiles

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Feb 8, 2007
Messages
1,698
There is a blow by tube coming off the top of the valve cover , if put your finger over it you can feel and see the blowby build up , if it's really bad it will probally blow the dip stick out the tube in goes in to , so don't stand right in front the tube when you do it as you will get sprayed with oil , it is probally the rings/pistons where the compression is being lost and needs a rebuild as stated , but before pulling it out and down you should do a a compression test to verify the problem and if it is one cylinder or all the same , it will help you to determine the cause of the wear down , it could be from not changing you air filter enough and dirt getting thru it to the engine , this is called dusting an engine , take the air filter out and run your clean fingers in the rubber hose going from the air cleaner to the engine and if they come out with dirt on them it's dusted , another way it could have lost compression is a wash out , in this senario the air filter is so dirty that the air flow is restricted and more fuel than air is introduced in the cylinders , the mix is wrong for burning and the excess fuel washes the oil off the walls of the cylinders and excessive wear takes place , this condition can also occur from tips blow off of the injectors , a compression test can be easily taken thru the glow plug holes , if you don't have the $90 Bobcat adaptor to do the glow plug compression test you can make one pretty easy , take a old glow plug and cut the top with a hack saw right under the electrical connection , take a punch and punch the heating element rod out , it will leave a hole right thru the fitting , braze or tig a 1/4 jic male fitting to the top of the fitting and you have an adaptor ---the specs for compression of a 743 with a 1702 Kubota are min = 325-350 max 425-475 , don't know about where you are at but around here the Kubota engine dealers prices on parts are half of what Bobcat wants and they are the same parts good luck fishfiles
 
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