Cam-swap/engine won't start

[jambo]

Before taking any heads off I'd want to use a compression tester to get some numbers for front Vs rear compression.

Are you certain that there are no splits in the breather pipes that go from the air box to the rear of the crank case? They exit near the area you're describing.

A blown head gasket seems quite unlikely to be able to generate the sort of air movement you're describing. But obviously it's a little hard to tell without being in front of it.

Jambo

Rip the head off and have a look.

[yorkie_chris]

Head gasket would show loss of couple psi, bubbles in coolant etc. Look at a head gasket and how far air has to move through steel to get outside. It would jump to coolant or oil first.

And no for chrissake don't just tear into the thing before you've attacked it from every other angle! Cost you a new gasket to find a simple problem with timing or a valve clearance!

Originally Posted by yorkie_chris

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

It's been idling/starting erratically for a while. Figure that it was a small leak that's getting bigger

Originally Posted by yorkie_chris

Head gasket would show loss of couple psi, bubbles in coolant etc. Look at a head gasket and how far air has to move through steel to get outside. It would jump to coolant or oil first.

And no for chrissake don't just tear into the thing before you've attacked it from every other angle! Cost you a new gasket to find a simple problem with timing or a valve clearance!

It's definitely not a timing issue any more, as it's back on stock timing (reset from scratch, both cams out and back to stock exhaust cams), and clearances are good.

I just dropped a bit of oil, and it's clear, no emulsification. Not got time to check the coolant for bubbles, but that's my first job tomorrow. Having said that, given that the oil's perfectly clear (well, not perfect...but no more dirty than oil with less than 300 miles in it should be), I'm leaning towards bent valves now.

Originally Posted by jambo

Are you certain that there are no splits in the breather pipes that go from the air box to the rear of the crank case? They exit near the area you're describing.

Positive - I pulled it off the other day to check for this. No splits, cracks or leaks - plugging one end and blowing down the other just results in a red face. And a bad taste in the mouth

Have you pin pointed where this air is coming out yet?

[yorkie_chris]

Ok check valve clearances. Valve that is bent so far as not to seat will either have massive clearance or none.

Going to attempt a basic leakdown test tomorrow and check clearances.

Just a thought: If it's valves, would it be much easier to just transplant the entire head from the knackered engine? Saves messing about with swapping over the valves etc.

Ok, results are in and I'm confused.

Did a basic leakdown test on the rear cyl using a spare breather hose. Found TDC using a piece of thick wire, blew into it, air goes straight through into exhaust. Minor worry. Spin crank 360 degrees, repeat, absolutely nada. I go red in the face, air goes nowhere.

Right, so compression seems to be a non-issue afterall. Great.

Before pulling anything apart I decided to see what was happening with the rear cyl not sucking anything through the carbs, so I pulled the rear carb off and put my hand directly onto the carb rubber. Guess what? Suction. Carb back on, hand over carb-top, cover vacuum port, turn engine over, very little suction. Crank the engine a bit, engine fires to life. There's now suction on the rear carb. Not as much as on the front, but some. Rear header is now getting hot as well, so it's firing.

However, if I put my hand over the front carb the engine dies. If I do it over the rear it doesn't. Pull the front HT lead off, engine dies. Pull the rear one off, nada.