Howard Gem JAP problem!

[vhgmcbuddy]

Hi. I have a Howard Gem JAP 600 rotavator (actually I have three, and occasionally get them to work). I am trying to get this one to start. This is what I have done:
Cleaned and re-set the points
Checked the magneto and plug: I have a fat blue spark,
Removed, stripped and cleaned out the carb.
Removed the head, and ground in the valves.
Set the points with a meter so that they open with the head 1/4″ before TDC

I can’t think what else to do: it doesn’t even fire up. The only additional information (which I think is important) is that the suction appears quite poor, although the compression seems ok. Even when I flood the carb, I am unable to flood the engine itself – the plug remains dry. Although the bore is unscratched, I wonder whether it could be rings. Any ideas on what I can do?

[vhgmcbuddy]

Hi, you could try a drop of petrol down the plug hole. If it runs and then stops, you will know its a fuel problem.

[vhgmcbuddy]

Thanks, Owen. I’ve tried that – no luck with petrol. I did get it to fire up (nicely) with easy-start (ether based) in the plug hole, but after a few revs it stopped again. Easy start into the carb intake didn’t work, though. I’m going to have a look at the intake valve again tomorrow; I wonder if there might be wear in the guide.

[stuart]

Have you checked your valve clearences ??.

[arikispencer]

could be an issue with a poor seal on your carby meaning its suckin air where carby fits on rather then the fuel from the bowl? although even with that easy start usually will let it fire

[vhgmcbuddy]

Perhaps try a little engine oil in the plug hole, turn a couple of times, replace plug and try to start. Improves the suction. Wont do any harm.

[vhgmcbuddy]

Update:

Many thanks for your various suggestions, I have tried them all, and I believe I have now found the cause. I have removed the head and valves, and found a lot of play in the inlet valve, so I think that on the induction stroke it was sucking air past the valve stem. Although the piston and bore do seem OK, I have removed the cylinder and am having it rebored with an oversized piston fitted and new valve guides. The existing valves will be re-faced or replaced as necessary. I’ll report back later!

[vhgmcbuddy]

Resolved! It was certainly an engine wear problem, with a whole raft of things working together. I think the main issue was the inlet valve guide wear. So, as in the previous post, it was bored out to the maximum (+40) because the cylinder had become mis-shapen. Oversize piston and rings provided, and new valve guides fitted. The existing valves were re-faced and the guides reamed to fit, and the valve seats were also re-cut.

I also fitted a small end bush, using a hand reamer. It all went back together yesterday and started well.

Looking back on the many years of frustration with these beautiful old machines, I regret never having done this before!

[Author]

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