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pembell

yz 426 cold start no compression

5 posts in this topic

I've searched on the subject and there were a couple of answers, but I have a numpty question as a result of the answers I've read!

My 426 always starts no problem, but this morning - unusually cold in Saudi Arabia, but still about 6-10C - I got kick through with no compression, as if the decompression lever was stuck.

I immediately consulted TT and found a couple of potential answers, and figured from those it was the cold causing a valve clearance problem.

So, I parked her on the sunny side of the street, waited a bit, then kicked her over a few times and compression returned and she started. Went for a ride - all good - pulling liking a train again.

Now, I will check the valve clearances etc, but what I don't quite get yet is why the cold would make the valves open? I would have thought that the opposite would happen? Is it the head that shrinks? Is something else going on?

Edited by pembell

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I don't quite get yet is why the cold would make the valves open? I would have thought that the opposite would happen? Is it the head that shrinks?

Yes. The head is aluminum, and with the bucket tappet layout, it's not unusual for the head to contract enough with cold to lower the cams against the tappets enough to to hold the valves off the seats.

If the clearances seem normal, the culprit was probably carbon on a valve stem or face holding one open.

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Yes. The head is aluminum, and with the bucket tappet layout, it's not unusual for the head to contract enough with cold to lower the cams against the tappets enough to to hold the valves off the seats.

If the clearances seem normal, the culprit was probably carbon on a valve stem or face holding one open.

Thanks, Gray. So is there a 'correct ambient temperature' to set your clearances at? Or doesn't it matter?

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Thanks, Gray. So is there a 'correct ambient temperature' to set your clearances at? Or doesn't it matter?
I don't think it matters a lot where the engine is if the ambient temp is between about 40-100 ℉, but outside of that, it could have an effect. But the thing to remember is that in order for a temp of <30 ℉ to cause a loss of compression by holding the valves open, the valve clearance would need to be way too tight in any case.

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I don't think it matters a lot where the engine is if the ambient temp is between about 40-100 ℉, but outside of that, it could have an effect. But the thing to remember is that in order for a temp of <30 ℉ to cause a loss of compression by holding the valves open, the valve clearance would need to be way too tight in any case.

That's what I figured, so will get in there this week and have a look. Thanks again.

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