Weird loss of power and sound

[clive]

Nothing concerning ignition timing has been altered or slipped since last riding, so I was thinking the piston must be taller above the wrist pin and that the ATD and cam assembly needs to be pulled off the drive taper on the crank and refitted in a more anti clockwise position.

Does that make sense?

To put it bluntly No. TDC is when the piston has risen to the top of its travel. The distance between the wrist pin and the crown of the piston is immaterial as tdc is when the big end has reached as high as it will go. However it may make a difference depending on how you are measuring TDC. Does the piston rise above the barrel at tdc? If so how are you securing your measuring tool before you introduce the 7/16 spacer? You are turning the engine backwards beyond the firing point and then coming forward to 7/16 before tdc to eliminate backlash aren't you, making sure points are fully advanced. I don't suppose the points have lost their 15 thou gap have they?
I am running out of other ideas but as you did not change anything on the points and it cannot be anything to do with the piston ...........

[Expat]

Back in the room now as have been out all day on another bike. Whilst riding, I was thinking about this when I probably should have been concentrating on riding but started looking at it from a different perspective. Looking at my theory, suppose the bike still had its original piston fitted, barrel hadn’t been removed, it had been running fine and cb gap and timing was spot on, verified by setting up the dti, finding tdc and placing a 7/16” gauge block on top of piston to see the contacts just separating at that point. If I now took the 7/16” gauge away and replaced it with a 1/4” inch thick one, found TDC with the dti zeroed on it, then added the original 7/16” gauge to give that amount before TDC. In this scenario, the points cam surely wouldn’t have rotated far enough to open the points because the piston crown position relative to the points cam position, has changed. Trouble is, there’s not enough adjustment available in the breaker plate slots to move it round so that the points separate.

It’s late here now, so will take another look and answer your questions tomorrow

Now my brain does hurt!

[Groily]

If you verify your position BTDC of the crankshaft (rotating the right way), then if necessary reposition your points cam, full advance etc, (respecting its sense of rotation) the job will be done.

If your piston comes up proud of the bore at TDC, a degree disc on the crank may be an easier way to do this with the head off, if you know the full advance figure in °. Measuring piston position with the crown at 7/16 from the top of the hole could put you miles out with a lumpy piston.)

If the relationship between the two is still wrong when you have verified the position of the crank again, the timing must have been 'wrong' before if nothing has been touched or slipped.

[Joker_Bones]

I had to get out the pencil and paper to convince myself...
The rotation of crankshaft (angle B) is the same for the height below TDC (distance A) irrespective of the height of the piston above the wrist pin.

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Good job I'm not an engine designer... I think my piston is going to hit the flywheel.

[SPRIDDLER]

Head isn’t on and engine is on firing stroke confirmed by the timing/breaker setting as when last running.......

This thread which started out as a loss of compression, rebore, new piston, valve grinding seems to have become very complex and confusing with use of blocks, dial gauge etc.

The orientation of the firing lobe is dictated by the cam's position on the inlet cam axle. The height of the piston crown is irrelevant, since regardless of the overall height of the piston, TDC will always be at the TDC point of the crankshaft stroke.
If you haven't changed anything and nothing has slipped out of register then you must surely be making a basic error?
Are you absolutely certain that you have the engine on the compression stroke?

(Ah, I see JB was posting his reply at the same time as me).

[clive]

Expat I think you need to believe the volume of response saying "it's not to do with the piston crown", some of which is from much more competent engineers than me. I think the fault in your last argument is that you say after putting in your 1/4 inch block "found TDC with the dti zeroed on it" and then proceeded to think about the setting without zeroing it.
Do you have a degree disc? This may be the only way to be certain what is going on.

[Andy G]

Or fit the head and revert to rod-in-ole technique

[Expat]

Well, the last thing I wanted when asking for suggestions and opinions of how to get around an issue that confounded me initially was to initiate such an intense debate. I just found that with the new piston installed and deciding to check timing, the original, working timing setting was way out and the breaker plate couldn’t be turned far enough make it right, hence my request of what to do.I do appreciate the suggestions offered, Groily pretty much confirmed what needed to be done. Thanks.

I’ve worked on different car and bike engines and I totally understand the concept of TDC and BTDC on the firing stroke and confirmed this on my bike by watching the inlet cam follower rise up then lower as the piston came up the bore. I do see that my methods here are far more complex than needed or that others use but as a tool/instrument maker with over fifty years experience, it’s in my nature to sometimes work this way and when I last removed the head years ago, set the timing successfully using the same method as now, with the original, undisturbed piston in place shown below
also with the new piston showing the first method of establishing TDC.

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[Expat]

Expat I think you need to believe the volume of response saying "it's not to do with the piston crown", some of which is from much more competent engineers than me. I think the fault in your last argument is that you say after putting in your 1/4 inch block "found TDC with the dti zeroed on it" and then proceeded to think about the setting without zeroing it.
Do you have a degree disc? This may be the only way to be certain what is going on.

Clive, perhaps my wording wasn’t clear but I meant that introducing the 1/4 inch block was artificially increasing the piston to wrist pin distance and TDC would need to be re established and dti zeroed on the block.
Here’s the piston position 7/16 BTDC and after the inlet follower had gone down.

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[Expat]

Finally, much to your relief I’m sure, pics show ATD wedged open for full advance. Next, showing the line between the bob weights on full retard and the center of the camshaft has to be further clockwise because…..next pic shows there was no more adjustment available to set timing correctly and align those two black sharpie lines that denoted correct position originally. Ignore the scratched lines done by p o. Steve

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