M600 running issue

Hi is anyone familiar with early monster 600 engines? I'm having an issue with throttle hang. It's nothing to do with the carbs or an air leak I am 100% about it. Also compression is excellent at 175psi hot and 150 cold. Valve clearances are good. Carbs are just rebuilt and balanced. Although the front cylinder is lower when hot. The throttle hang will only happen when the engine is hot. At idle when hot if I sprayed carb spray at where the head meets the cylinder the rpm would shoot up on the horizontal cylinder. Vertical one does not do this. I put it down to separating when hot. When I pulled the head and cylinder off there were no signs of loosing compression between the head and cylinder shock I thought was strange. At the same time I pulled the rear setup too. The rear piston is in better shape but I am putting that down to the piston slap in the front cylinder which they all suffer from. Should I swap the pistons on rebuild or leave them be? I will lap the heads to the cylinders before rebuild. Anyone ever have this issue? Thanks

[vince53]

Must be someone here who's got an idea ? Lud ? I think he's probably sussed it but be good if someone knows * )

[slob]

I wouldn’t swap pistons without accurate comparative measurements of all the relevant parts.
Lapping the heads/barrels can’t hurt.
Might be worth trying a crack detector spray on the front head.

Does it make a difference where on the front head you spray carb cleaner? from the description it still sounds like a leak around the front manifold when hot is most likely to me.

[Luddite]

The symptoms you describe do sound like an air leak. You say it only happens when hot, which suggests something is opening up with the heat but could it also be that the symptoms are being masked by using the choke when the engine is cold?

You say that the carbs have been cleaned so they've obviously been off. Are you sure the intake manifold was seated properly on reassembly? Does the vacuum fuel tap run off the front cylinder? How's the pipework? (Although, if it is only leaking when hot, it's probably not this.)

You say you're 100% sure it's not a leak along the inlet tract so where on the head are you spraying the WD40 when the rpm rises and how far is that from the intake manifold?

If the leak is not between the carb and cylinder, then there could possibly be a hairline crack in the head itself, which wouldn't be evident from the compression test, and is only opening up when hot.

If you're removing the head, you could try a crack detection spray like this:

https://www.gas-uk.co.uk/crack-detection-spray-set.html

Let us know how you get on.

Edit: You beat me to it, Rob!

Hi thanks for the replys. I am using a fuel cell suspended above the bike. Vaccum ports are either connected to balancing guages or blocked off with caps the result is the same either way. It's only when I spray between the head and cylinder with a pipe attached to the nozzle of the carb cleaner to get down there. If I sprayed directly onto the manifolds or carbs it didn't affect it. Yes a cracked head is a possibility. I have removed both cylinder and pistons and the front piston is in bad shape only on the intake side of the piston although the bore and rings are just in tolerance. It's very strange. It defenitly needs replacing. The rear or vertical piston and cylinder are fine. I have sourced a new piston and I will replace the rings on the vertical piston to keep them equal. I will lap everything in and refit everything and see where it takes me. Good call on the crack detection spray thanks. I filled the manifold up with petrol while the head was off and it held just fine but that was cold obviously. I think I should get it pressure tested maybe.

The two mating surfaces between the head and the cylinder had no signs of leaking under compression so I am doubtful that it was drawing on intake stroke, surely if it was it would have to have been leaking compression also and that would have been evident on the surfaces. Another reason a cracked head could be likely. When the long nozzle on the carb spray was inbetween the head and cylinder and pressed within 2 seconds the engine rpm would raise by at least 600rpm for a good 30 seconds until it used up the spray.

Quick update I managed to source a new old stock vertical piston and cylinder although this is the one I don't really need it makes it easier to for out the 300 pounds for a new horizontal piston and cylinder so it's looking like that's the route I'll take. While I'm at it I'll get the horizontal head pressure tested and obviously replace all gaskets. Should be right after all that hopefully��