Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: yozzer74 on October 17, 2016, 02:04:15 PM
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Hi I've just done about 20 miles on bike noticed she smoking a lot .I thought it may have been because of just rebuilding engine .but I think it's getting worse .it's kicking a little oil out of the breather pipe on crank case too .video attached https://youtu.be/jGkBzsxAnXE
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Any ideas what's wrong
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looks like piston ring blow by to me. did you replace the piston rings with the rebuild?
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Yeah I changed rings new valve guides seals recur seats
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The rings might just need bedding in, don't tear it down again just yet.
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Do we think putting different oil in will help
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20 miles today or 20 miles since rebuilding it?
What is the colour of the plugs? Consistent across all cyls?
What oil are you using? Doubt it will make any difference. Just to say I am using the Europarts semi-synth diesel stuff discussed on here without any apparent problems.
Edit: Watched the rebuild video - must say, lovely work.
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what oil is in it?
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after an engine rebuild,you should use the most basic mineral oil,
it helps the motor bed in,also a bit of firm acceleration and then shutting the throttle helps the ring to seal,
the smoke looks either the pump is not scavenging ,or the rings are not bedded in,or the valve stem seals are leaking
check the plugs,see if they all show oily tips,
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The plugs are black .this is the oil [attachimg=1]
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Did you prep the bores at all by glaze busting them to allow running in of new rings?
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try a basic oil,
the better oil prevents the parts bedding in,
http://www.sparesworld.co.uk/10w40-Mineral-Engine-Oil-5ltr.html
you may find it cheaper,but don't use any synthetic till the motor is well run in
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looks like car oil, might not be the best for the wet clutch in your cb.
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I had bores crossed hatched I think they call it .yeah it's car oil
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Hey yozzer, my F2 smoked like a pig after the rebuild, then after about 100 miles it started to run clear until nothing so it could just be bedding in, btw these old bikes really do f=ck with your head, it can be so frustrating, but stick with it fella
pete
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Ha cheers for that Pete and I'm starting to no what you mean about messing with your head .
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Just been thinking those 4 large o rings that go on bottom of cylinders .if there not on properly would they let oil into barrels and make it smoke .
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Just been thinking those 4 large o rings that go on bottom of cylinders .if there not on properly would they let oil into barrels and make it smoke .
those are to stop oil inside ,stay inside,should have affect on pistons and bores
go with Pete,run it a bit longer
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Yeah that's what I'm going to-do really don't want to pull engine out .
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Try to avoid constant revs, as above accelerating and shutting off is good for running in, keep going up and down the box, obviously don't thrash it but at the same time don't treat it like a baby, and yes mineral oil for the first 500 miles at least, my new Ducati comes with mineral oil from the factory, it swaps to fully synthetic after a minimum of 600 miles, not sure what revs you should do as max for the honda when running in, Ducati is 6k for 600 miles, the book recommends you find "hilly areas" then 7k for 600, then do as you like, very different engine though , anyway I hope it's just the bedding in that's causing the issue, really annoying if not!
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find "hilly areas"
Does it give the same advice in the Dutch manuals I wonder.
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This is what the 400 Owner's Manual has to say about 'Breaking In', suspect the principle is the same for the 750.
[attachimg=1]
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find "hilly areas"
Does it give the same advice in the Dutch manuals I wonder.
maybe all bikes should be run in either in Wales,or the Highlands of Scotland ????
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As Pete has said, it may take a few miles to get it right as the rings need to wear to fit the bores.
Looks like you've done the right things by getting the bores prepped as well. When new rings go onto existing pistons and used bores they just don't fit well enough initially to hold the compression up and the oil down below the pistons. They are then reliant on the cross hatching to literally grind the high points off the rings until they are properly matched to the bores. This will take a few miles to happen (probably as Pete has experienced around 100) give or take a bit.
You've an additional consideration using bores and pistons with some miles already on them as there will also be a small amount of wear compared to original new status, so the tolerances there will be more generous. I know there's always discussion about oil type (at any stage of an engines life) but don't feel in this case it will make much difference to you.
Also you are not running in a new motor so as long as there's nothing wrong with the assembly then you don't really need to be careful (apart from making sure you don't overheat it) during initial use. The care needed with brand new engines is principally centered around piston and bore clearance to avoid any heat build up causing any localised seizure of the piston skirt along with subsequent pickup to cause damage.
For your motor it needs to be used over its normal operational range and get some mileage on it to get the rings bedded in before the crosshatching is worn smooth as it won't happen after those sharp edges loose their bite.
I'd get out and ride it, then change the oil and filter at 500miles.
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Thanks for that info k2_k6 makes me feel a little better about it .
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Have a look at this instructional video clip...
https://subtletv.com/baaaUfY/Ichiban_Moto_shows_you_how_to_bore_and_hone_motorcycle_cylinders
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Ha Ha I've seen a lot of his videos .
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In the comercial world the Merc V6/8 burns oil if synthetic used but not when mineral used
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Bryan j you mentioned before that you used Tesco diesel oil can you remember which one
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From memory there only was one mineral diesel oil
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Bryan j you mentioned before that you used Tesco diesel oil can you remember which one
Yozz'
My K6 had a complete rebuild much the same as yours, the guy who built the engine & the bike is "shit hot" with the 750 sohc engines, he is in Rotherham
When my bike was finished he put Motul 10w40 semi' synth', there was not a hint of any smoke, the only issues we had was the PD carbs getting them jetted & set-up right, during that time frame when we were playing with the jets needle settings etc I had a lot of running on 3 cylinders, then 2 cylinders, & all this was going on whilst running it in, but it has not had any adverse affects at all on how the bike runs & performs, I have done nearly 2k on the bike now & still as clean as a whistle . I have changed the oil twice I now run on Motul 15w50 semi'
if you cant get it sorted have a chat to Steve who built mine he is on Alma Road Rotherham, his work shop is at the side of Motographics, it would be a good idea for you to pop over & see him just for some advice & he will be happy to help...its only a few miles away from where you are
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Thanks greebo yeah it's not too far
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By the way Yozzer, I assume the smoke is blue, and smells oily, that's what you'll get if your burning oil, if it's black it's an overly rich mixture, if it's a bit like steam it's water evaporating from the exhaust, if it's white you've got diesel in there! 😄
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It's definitely oil you can see it on end of pipe
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Sorry I only just saw the video, yes definitely oil smoke, fingers crossed that Pete's explanation proves correct, I had a CB900 years ago that smoked like that, it was worn valve guides, you could see the oil running down the valve stems!.