Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: csd4t on June 13, 2015, 02:18:46 PM
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Got my bike up and running but it was burning a little oil from number 4 cylinder, no big deal I thought as it had not been run for many months and was thinking maybe a sticking ring or valve stem oil seal. This week is my first ride due to a knee replacement and after 20 odd miles I find the following.
No smoke on start up when cold.
No smoke on overrun
No smoke on normal acceleration
Smoke on acceleration with big throttle openings
Smoke at standstill with hot motor upon revving.
What do the collective guru's think? Should I carry on and hope it clears or carry on smoking away for the summer?
If the motor has to come out then it can wait till winter methinks.
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It's probably a valve guide but could just be a sticking ring, take the no4 plug out and pour in some neat redex or parafin , rock the bike in gear to move the piston a little, leave it to soak overnight, in the morning place a towel over the plug hole and turn the engine over on the starter to expel any excess liquid, have a look down the plug hole with a torch to ensure you haven't got a pool of liquid , it could "hydraulic" and damage the engine if you have more than a surface coating still in there, put the plug in and run the engine, it will make a lot of white smoke initially, it might just unstick a gummed up ring, if it still smokes just run it and fix it in the winter as you say.
PS change the engine oil after the above procedure.
PPS the above is assuming the the smoke is blueish, i.e. oil smoke, if it's black smoke check your carburation.
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Cheers I'll give that a go, definately oil smoke (white blue ish) carbs have been ultrasonically cleaned and balanced so very close to where they should be, all jets and settings are standard, airbox is standard and a new set of 341's.
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100 mile update, the oil/smoke problem is still there but probably not as bad however it's running much too rich (oily petrol mess from the exhausts).
The bike had a yoshi copy 4 into 1 and pod filters when I got it mains were 95? I have put a standard airbox with K+N back on and 105 mains, the screws are now out 2 turns but it's still very rich. Not sure whether to put the 95's back in and/or drop the needles a notch. Any thoughts?
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What position are the needles in, standard they should be 3rd groove, bang in the middle, chances are they were moved up to make it run with yoshi pipe and pod filters. Although I am confused, normally you need to go UP jet sizes for race exhausts and pods, maybe the P.O. Drilled a set of 95 jets out to save a few quid:$
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Me too confused, yes I put everything to standard, if I remember then the clips were 3rd groove anyway. Never seen any reference to 95 jets in any manual or bulletin so it could have been very rich to start with. The tickover is now all to cock with he screws 2 turns out. looks like I'll have to pull the carbs off and move the needles down just not sure whether to do the jets at the same time.
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Don't forget that as a very rough guide the main jet doesn't affect running much below 3/4 throttle the slide cutaway and needle profile / position affects running from about 1/4 up to 3/4 throttle and the idle jet below that.
As you have the idle screws 2 turns out ( ie a complete turn more than standard ) that could be why the idle is to pot. If you are running around half throttle when riding then I would be looking at the slides / needles / needle jet and needle position.
Did you replace the needles? ( are they standard? )
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No needles are standard, I agree and really need to pull the carbs off again, I was just confused with the main jet sizes, as far as I remember the pilots are 40's. I'll also check the float heights again although I was pretty meticulous with them.
This is only the first tank of fuel through it so i was hoping it may settle a bit
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Have you done a compression test? Without and with a squirt of oil down the bores
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No I don't have a tester but a finger test!!! seems to be ok. The bike starts well (without choke) and runs well enough. If it is low compression then it can wait till winter before the engine comes out.
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You don't need a really expensive tester if all you are looking for is a drop in compression on one pot. Personally, given the symptoms, i would eliminate this before delving elsewhere.
http://www.ebay.co.uk/sch/i.html?_nkw=Compression%20tester&ssPageName=GSTL
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Got me a tester and it seems I'm very low on all 4 cylinders (between 70-90 psi) with oil it goes up around 30psi. Does this mean rings? Oddly enough No4 cylinder, the one that is smoking had the higher reading???
Anyway It'll have to wait until winter for a rebuild unless something serious happens.
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Got me a tester and it seems I'm very low on all 4 cylinders (between 70-90 psi) with oil it goes up around 30psi. Does this mean rings? Oddly enough No4 cylinder, the one that is smoking had the higher reading???
Anyway It'll have to wait until winter for a rebuild unless something serious happens.
Could be ring's, bores or valves
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Got me a tester and it seems I'm very low on all 4 cylinders (between 70-90 psi) with oil it goes up around 30psi. Does this mean rings? Oddly enough No4 cylinder, the one that is smoking had the higher reading???
Anyway It'll have to wait until winter for a rebuild unless something serious happens.
Did you do the test with the throttle fully open?
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It did indeed have pod filters on when I got it and looks like it had done a couple of thousand miles like that. I think it's a combination of everything mentioned, bores/rings and valves/guides. Hopefully it will be ok till winter. Having said all that it starts runs and performs well.
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Nasty knocks and rattles would probably require prompt attention, burning a bit of oil won't hurt anything, and its only 4 months til winter!, as bikers we get a bit overly concerned about our engines, I mean if we buy a second hand car with 80k miles we wouldn't dream of rebuilding the engine "just in case" but a bike with 30k miles and were replacing shells and god knows what .
I dynoed my 67k mile fireblade and it was making 2bhp more than Honda said it would when new, a friend of mine rebuilt his blades engine at 35k miles, a complete waste of money and time! :).