Cam-swap/engine won't start

[5hort5]

torque wrench - got one you can borrow if need >28
Socket set 1/2 inch - got some you can borrow if needed

I agree though I would take it to the shop but I wouldn't want to f it up as I know I would

i use the standard sv handle bars as a brake bar lol and a draper torque wrench thats from 30>200+ that i got for £20 or so

[yorkie_chris]

Did you check valve clearance when you put it back together?

[jambo]

OK, the idea that the head gasket's gone seems entirely supported by the idea you can't hear air being drawn in through the intake. I'd scratch this train of thought unless something much more substantial comes up supporting it.

If the intake stroke does not seem to be drawing air down through the rear carb, and you are 100% certain that the cam positioning is correct at this time, then I'm afraid my first thought is that the exhaust valves have been bent.

I really hope I'm wrong, but I have seen this before (we've already established that the initial timing used was incorrect but hoped that it was in one of the incorrect positions that did not involve contact). If the intake cam was left alone but the exhaust timing was off it may have bent the exhaust valves due to piston / valve interface. This means on intake stroke the cylinder fills from both the open intake valves, but crucially from the non-sealing exhaust valves too. This drastically reduces the airflow through the carb, and sets compression to 0psi as the air escapes straight past the exhaust valves on compression stroke.

Do a compression test. If nothing else put your hand over the spark plug hole for the front and rear cylinders and see if the compression forces feel anything like equal. If the compression is low on the rear cylinder it's mechanical. Something isn't sealing, or the timing's out.

I do hope I'm wrong.

Jambo

[yorkie_chris]

Originally Posted by Neeja

When the front cylinder goes through the intake/exhaust cycle, you can hear the air being sucked in through the carb. When the rear cylinder does...you can't. It sounds like air's being sucked in from somewhere near the engine. The carb rubber looks good, and covering the carb (and all the relevant hoses connected to it) generates no suction when turning it over by hand.

Have you got a spark plug in it?

You can do "leakdown" test yourself. Set it to TDC compression, put a pipe down spark plug hole and get someone to blow on it, see if you can hear air going out of pipe/up carb etc.

[jambo]

Originally Posted by yorkie_chris

Did you check valve clearance when you put it back together?

It is also possible that the valve clearances were so tight with the different cam in that the exhaust valves were not sealing, however, since you've put the original parts back in this should be acceptable assuming the valve seat & valve are OK.

Jambo

[yorkie_chris]

With bad luck maybe a shim jumped out and is sat in between valve retainer and bucket.

[jambo]

Originally Posted by yorkie_chris

With bad luck maybe a shim jumped out and is sat in between valve retainer and bucket.

I think it's unlikely to have all gone together smoothly in this case but it's worth a look of course.

Originally Posted by yorkie_chris

Have you got a spark plug in it?

You can do "leakdown" test yourself. Set it to TDC compression, put a pipe down spark plug hole and get someone to blow on it, see if you can hear air going out of pipe/up carb etc.

This assumes it's TDC compression stroke, not TDC end of exhaust / start of intake stroke where the valves will be open slightly. Easy check by looking at the cam lobes but worth a mention. You'd also want a very good seal on that pipe. I'd probably recommend trying to borrow a compression tester if at all possible.

Jambo

There is some compression in the cylinder, but it's low. As mentioned above, can put my thumb into the plug hole and it'll pop it out, but not with much force. The reason I'm suspecting head gasket rather than valves is that when the engine's turning over, if you put your hand down the back of the cylinder (near to the exhaust header stub), there's a quite substantial amount of air being pushed around. Given that the header was put onto the new engine with a fresh gasket and was definitely sealed, I'm hesitant to suggset it's leakage from the exhaust.

As to the timing being off originally, it was turned over using the rear wheel with no noise, so I'd say it was unlikely (although possible, you never know) that it was valve/piston celebrity deathmatch. Valve clearances with new cams were also good.

If it is valves...shouldn't be that much of an issue. The head's got to come off either way for inspection, and I've got a spare engine to scavenge for parts.

[yorkie_chris]

Why would the head gasket go tits up because of playing with cams at low rpm?