Red hot exhaust valves

Matt Spencer said:

wouldnt stake my life on it ,

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Probably wise.

Matt Spencer said:

but if the recolections not to abysmal , around 18 months after the screw in crank timing rod was introduced ( or thereabouts . Was going to say 2 years . Approximate . If Im not misstaken !) The Position of the receptical for the rod was put in a less inaccessable posion . Thus differat degrees in relation to T D C .

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The timing plunger port was introduced in 1965 on the 650 models and although it was changed to a forward position around 1967 it soon reverted to the original position where it remained until the introduction of the 750 electric start models when it moved to a forward position once again.
It would be more than a "mismatch" as the crank timing slots would be somewhere around 180 degrees different, however, there's been no suggestion so far of the crank having been changed or that the timing plunger slots are not where they should be.

Tigernut said:

According to my compression tester gauge the compression is very high, at 210psi (within a few psi side to side).

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Yes, that does sound high.

Tigernut said:

The exhaust cam is a 70-9989, set using the stock timing marks so no idea what the actual timing is.

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What about the inlet cam timing?

By my own measurements on a 79 T140 and the words of several others who have checked factory 750 inlet cam timing,it's 92 or so degrees centerline ...This early cam timing will make the cylinder pressure very high at lower speeds making detonation more likely...Moving the cam timing to around 100 degrees like the later 650's makes for a better powerspread besides lowering the risk of detonation.. If it's actually 210 psi that's higher than normally mentioned...Ping ping...

L.A.B. said:

Yes, that does sound high.



What about the inlet cam timing?

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Inlet cam also set to factory marks, so probably timed as Hillbilly suggests. It did used to pink / ping a bit when I first started riding it, unless I was extremely careful with throttle & revs. This stopped once i'd got the mixture right for it. For the vast majority of its 13000 miles since new pistons etc, it hasn't AUDIBLY pinked. But maybe this kind of damage can be caused without audible pinking? Or, maybe it was caused right back in the first few hundred miles when it used to pink sometimes?

Wish I knew, so I could know how to make sure it doesn't do it again. I'm not sure whether the level of damage caused was relatively slight, or whether the engine came perilously close to a meltdown. The pistons look good, on their skirts - no signs of picking up / scuffing.

Do you know how to measure the compression ratio with a burette?

Triton Thrasher said:

Do you know how to measure the compression ratio with a burette?

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I do, but I haven't tried - maybe I will, out of curiosity. I can't see why it should be higher than about 9:1, as there hasn't been much shaved off the head. But the compression test readings suggest more like 10:1 or even higher, so it is a puzzle. I suppose a syringe would do the job if graduated finely enough...

Tigernut said:

I do, but I haven't tried - maybe I will, out of curiosity. I can't see why it should be higher than about 9:1, as there hasn't been much shaved off the head. But the compression test readings suggest more like 10:1 or even higher, so it is a puzzle. I suppose a syringe would do the job if graduated finely enough...

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You will need to know combustion chamber volume? I have CC'd the head volume of several late 60's 650 heads and a 79 750...They were all 69-70 cc's, none of heads were millled, minimal valve seat recession..

Tigernut said:

I do, but I haven't tried - maybe I will, out of curiosity. I can't see why it should be higher than about 9:1, as there hasn't been much shaved off the head. But the compression test readings suggest more like 10:1 or even higher, so it is a puzzle. I suppose a syringe would do the job if graduated finely enough...

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A syringe as commonly sold for fork oil works perfectly.