SOHC.co.uk Forums > CB750

Engine full of fuel and seized up

<< < (6/50) > >>

K2-K6:

--- Quote from: rosco on August 31, 2020, 05:02:07 PM ---Have you thought it could be the gearbox stuck???  My brother bought a cb550 with a "seized engine " Turned out something in the gearbox let loose [ can't remember what ] and jammed the gears and broke a few teeth Try pulling in the clutch  and using the kickstart  If the kickstart don't move its the gearbox If it does move more than likely engine  Good luck   R

--- End quote ---

It doesn't turn on the starter either I thought.

Kickstart essentially bypasses the gearbox on these by just going down through the primary chain to the crankshaft, hence you  can pull the clutch and start it whether it's in gear or not.

Bryanj:
The only thing holding the advancer in time with the crank is a solid small dowel pin, if you put a lot of torque on the big nut you will snap off the ears on the advance cam, if you put too much torque on the 6mm stud you will snap it off.

It is only permissible to use that big nut with plugs out on a free turnung engine and its not really advised even then

royhall:
As always Bryan you are correct, that was me thinking out loud. I have made the incorrect assumption that everybody works like I do. By that I mean when I do anything on a bike if it wont turn or assemble correctly I assume there's something assembled wrongly, broken, or I am doing something wrong. I therefore never ever use force on anything without first appraising the situation as that usually just makes things worse.

I should make it clear now that I used the words gently and very gently and only millimetre movements. It was not my intention to use the timing end it break the engine free with a scaffold pole, just to establish which part of the engine is locked.

It may be wise to establish the facts here. Firstly what level of mechanical competence does PJM47 have, I assume as he's talking about repair engineers he's not a professional. Are you a competent DIY mechanic or a complete novice or somewhere in between. Was the bike left on the side or main stands? On the main stand the leak would fill the airbox first as the intake side is slightly lower (on the F2 anyway). If it cannot escape from the airbox as the petrol level rose it would then fill all 4 cylinders via the airbox. The cylinder with two closed valves would hold fuel in the intake so when the engine turns that fuel gets drawn into the cylinder and before it can leak down past the rings the engine comes back to compression and bends a rod or tips a piston jamming it into the bore.

Also, just how much petrol are we talking about here, you appear to say it could be around 2 to 3 gallons sat in there over a good few weeks? With the limited space inside a CB750 engine that is enough to fill the thing from top to bottom. If that is the case, a complete strip and rebuild is the only way forwards. The fuel will be in all the bearings, oil pump, oil galleries, alternator, clutch, scavenge lines, oil filter etc etc. Even if you can achieve turning the engine again it would require careful work to safely run it again. If there's fuel in the oil galleries clean oil would need to be slowly pumped through the engine to re-lubricate everything with the sump plug out to drain the fuel pushed through. You would need to remove the scavenge line to the oil tank to prevent fuel contaminating the oil in the tank. And don't forget were also talking Ethanol here that loves to remove lubricating oil.

In my view this engine needs stripping to clean everything out and re-lubricate everything and locate the damage. The alternator needs cleaning and drying off and testing. I many years ago foolishly cleaned some clutch plates off a Suzuki in petrol and the corks grew to about twice the thickness they should be which cost me a clutch, so that will need checking. Then lets not forget the failure in the carbs that allowed this to happen that also needs sorting, plus the airbox will be full of fuel.

If it were me I would take out the clutch plates to disconnect the engine from the gearbox physically then put the bike in gear and gently turn the back wheel and see if the clutch inner turns. That verifies the gearbox is not locked and it's therefore in the engine/kickstart mechanism. At that point see if the engine turns (please do this very gently). But that only establishes the area the damage lies and the engine will need stripping anyway.

Sorry to be the bearer of bad news and I do sympathise whole heartedly with you. But if the bike was mine I would bite the bullet and get the engine out. It's a big price to pay for forgetting to turn off the fuel tap, and not one of us on here can say we have never done that, we just got away with it.

PJM47:
I’ve taken the clutch out and the rear wheel/clutch inner turns ok when in gear. Kickstart still won’t move. Like you say no point forcing anything, if it wasn’t broken it would turn easily with the plugs out.
I’m a fairly competent diy mechanic and will tackle most jobs but obviously this is pretty major and might be best left to a professional, ouch!

rosco:
If your looking for a pro to rebuild your engine  "Trigger " on this forum is your man and Julie makes nice Tea!!!!

Navigation

[0] Message Index

[#] Next page

[*] Previous page