Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: adriangsmith on May 02, 2016, 08:24:23 PM
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Hi all,
I've had to strip my engine down (750 F2) as it got a little warm and i thought had partially seized whilst i was syncing the carbs and tweaking the mixture settings.
So engine out of bike, cam cover off and the right hand side of the camshaft was bone dry.
removed and cam tower and end of cam shaft have worn away and torn each other up.
continued top end strip down and found no further signs of damage.
however there are some metal fragments in the sump pan so think a full strip down is needed now to find the cause.
so gutted as i was 90% complete on my build.
So my question is where and how does oil enter the top end of the engine? and why would only the right hand end be dry? the rest was well covered with oil and never had the oil pressure light flicker at all during running.
Also now need another camshaft and cam tower. my cam shaft is stamped up R2.
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Is one of your oil metering jets blocked
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yep, sounds like a blocked head jet. Cam towers can be line bored but, you have to metal spray and grind the cam to fit ;)
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Yep these bad boys snapped my cam. Pull them out and check them. A is out. B is in place, so easy to miss them.
(http://i128.photobucket.com/albums/p188/MuckyDuck57/Motorcycles/Chopper%20CB750/Restrictor%20Jets.jpg) (http://s128.photobucket.com/user/MuckyDuck57/media/Motorcycles/Chopper%20CB750/Restrictor%20Jets.jpg.html)
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Thanks all.
Will check these out this evening.
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yep it has to be oil jets, a very common problem, the engine has to be spotless when rebuilding, and always check the jets are in and clear, also worth turning the engine over by starter or hand before running to see that oil is making it up the top, any future engines that i build will be bench run to save any hassles which require engine out
pete
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As stated the smallest particle of gasket debris will block those jets as will any silicone gasket that gets there
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You wouldn't see this loss of flow with the oil pressure light in any normal event.
The jets as far as I know are there to maintain the oil pressure to the crank bearings which is measured by the oil pressure switch. Past those oil restrictions is essentially a low pressure area that leaks out slowly from the cam bearings and leaves a pool of oil for the cam lobes to dip into.
The blockage of one or both jets is likely to have come from one of the gasket faces during assembly as it would not usually pass through first the oil pickup screen then the oil pump rotors and then via oil filter to get to the jets. so you are really looking at maybe joint face of crankcase or base gasket of barrels or the head gasket as the primary suspect.
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I always crank the motor over on the starter to check if the oil is reaching the camshaft. I feel your pain though.
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Hi All,
Thanks for the replies and advice,
That jet was definitely blocked with some gasket debris. now kicking myself for not ensuring all was clean.
Took all top end apart and now decided on a full rebuild of the engine as detected some play in the main bearings.
More work and new territory for me as never done any bottom end work on an engine before!
Still looking for a replacement cam tower and camshaft ready for the rebuild if anyone has one lying around in a box of spares!
Heres hoping for better luck this time round.
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You may need to spread your search for a cam tower far and wide. This has been a common problem over the years so supply of these parts is not great. Took me a month to locate one for mine. Not just the jets that block either, check that the drillings through the cam tower are clear as they block at the cam journals. Good luck with the rebuild.
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I have one spare 750 cam tower but not giving it away for free on here, as I normally would, 'cos it cost me in the 1st place !
If you are interested PM me but I won't have access to measure/mic it up it up till Tuesday now
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You are lucky you don't have this type of CB750K jet, which doesn't even have a part number ;)
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Well, I found the cause of the jet blockage today.
one single grain of sand stuck in the top of the jet!
This was so hard i thought someone had snapped a drill in the jet at first.
it took some shifting but eventually came out.
Put it under a microscope at work and a grain of sand is what it looks like, which is possible as the outside of the head was caked in it when i got the bike.
Just goes to show how careful and clean you need to be when reassembling one of these engines!
Now has anyone got a cam for an F2 engine and is it different from an F1 or K series cam?
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Any cam will fit and run but the F2 had the most aggressive profile after the original CB750