Engine runs on etc

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bmcecosse
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Re: Engine runs on etc

Postby bmcecosse » Sat Mar 28, 2015 11:23 am

Well - assuming you set the original test gaps (my 60 thou = 1.5mm) absolutely the same - that now indicates 0.7 mm difference = 28 thou, which suggests an error in the cam timing. The exhaust is more closed than the inlet is open - suggesting the cam is running one tooth behind where it should be. Turn it one turn and check again the 'start' settings (1.5mm) and then turn it again (NB always turn the engine in the normal direction - never turn it back..) and check again at TDC.
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Rust bucket
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Re: Engine runs on etc

Postby Rust bucket » Sat Mar 28, 2015 12:59 pm

Just done as you said, turned it once and checked the gaps on #1 cylinder valves both closer to 1.55mm then 1.50 but they are identical.
Turn over once again, almost the same result (must get some new imperial feelers) .85mm inlet and a tight .20mm exhaust,
Can't believe I made such a silly mistake on assembly, I was sure I lined the timing dot's up spot on using both eye and straight edge, then turned over to check alinement, :oops:

bmcecosse
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Re: Engine runs on etc

Postby bmcecosse » Sat Mar 28, 2015 1:20 pm

It surely looks like it then..... :cry:
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IslipMinor
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Re: Engine runs on etc

Postby IslipMinor » Sat Mar 28, 2015 2:33 pm

Is 0.6mm really 1 tooth? This is a difference, so the gaps are +/- 0.3mm or 0.012". It doesn't seem like 1 tooth?
Richard



bmcecosse
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Re: Engine runs on etc

Postby bmcecosse » Sat Mar 28, 2015 3:00 pm

There's no plus/minus - the difference is 0.65mm, checked and double checked. The reading is wrong - and although I too wonder about '1 tooth' it is an unacceptable difference. And it's behind - so moving the cam forward a tooth ON THE CAM SPROCKET would be my remedy! I would of course check it again with the feelers before boxing up the timing case...
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IslipMinor
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Re: Engine runs on etc

Postby IslipMinor » Sat Mar 28, 2015 5:11 pm

the difference is 0.65mm
Exactly! If the gaps were the same they would be ~0.5mm, so the inlet gap is 0.3mm too big (plus) and the exhaust is 0.3mm too small (minus).

The point I am trying to make is that, although there is a difference of 0.65mm, I very much doubt is that equates to 1 tooth and is it actually significant? Or just down to various tolerances?

Does anyone have an engine partly stripped with access to the timing chain and who could do the same measurements with the cam timing set correctly and then advanced by 1 tooth, and retarded by 1 tooth? It would be very interesting to see what the differences are; I would hazard a guess that, with 1 tooth out from 'correct', one valve will already be on the move, so no gap to measure?
Richard



bmcecosse
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Re: Engine runs on etc

Postby bmcecosse » Sat Mar 28, 2015 5:22 pm

I believe it IS significant, and I would turn the cam one tooth forward. At the overlap - the followers are only on the gentle slopes of the cams, and one tooth will make enough difference to give equal gaps. Whether it will affect the very high 'compression' reading is another matter !
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IslipMinor
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Re: Engine runs on etc

Postby IslipMinor » Sat Mar 28, 2015 5:32 pm

Simple way to check. The cam sprocket has 38T, so each moving 1 tooth is 9.5°. Rotate the crankshaft clockwise ~20° (the equivalent of moving the cam sprocket 1 tooth) and see if the gaps become more equal. Saves stripping the timing cover off! If it makes it worse then rotate the crank 20° anti-clockwise and see what the results are. Then at least there is good information about what to do next.
Richard



bmcecosse
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Re: Engine runs on etc

Postby bmcecosse » Sat Mar 28, 2015 6:12 pm

Good idea - if you can measure 20 degrees!
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IslipMinor
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Re: Engine runs on etc

Postby IslipMinor » Sat Mar 28, 2015 11:14 pm

Dead easy - with the standard 1275 damper pulley 20° is 20mm around the periphery of the pulley!
Richard



Rust bucket
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Re: Engine runs on etc

Postby Rust bucket » Sun Mar 29, 2015 2:17 pm

Getting very technical now :o I have got radiator drained and out now so i'll whip the cover and check anyway.

bmcecosse
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Re: Engine runs on etc

Postby bmcecosse » Sun Mar 29, 2015 5:02 pm

Top tip there Richard! If the engine has a crank damper....
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Rust bucket
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Re: Engine runs on etc

Postby Rust bucket » Mon Mar 30, 2015 11:20 am

More work involved locking up the engine to remove the pulley nut, since I have a damper pulley with late type Mini self adjusting cover fitted, I've measured 20mm round each way round the pulley, from the TDC mark the results are as follows...20mm before TDC .120 inlet, nothing on exhaust. 20mm after TDC nothing on inlet, .120 exhaust.

IslipMinor
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Re: Engine runs on etc

Postby IslipMinor » Mon Mar 30, 2015 11:37 am

Sounds like the timing is correct to the nearest tooth?
Richard



bmcecosse
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Re: Engine runs on etc

Postby bmcecosse » Mon Mar 30, 2015 12:48 pm

That's interesting! Open up the initial settings and check again - because when it gets to 'nothing' you have lost track of what's happening. But I do agree - it seems as though it's maybe as good as you are going to get - but definitely not right at TDC. Any I have tested have been correct within 3 or 4 thou. If it was a competition engine you could obviously fit vernier timing gear - or fiddle about with shims, to get it right. But I doubt it's worth it for a road engine.
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