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[iain]

 Invercargill,, inbred capital of the world   :o :o :o :o :o :o

Iain
NZ

[dingleberry]

Sometimes it would be handy to have four thumbs 'tho

[The Prophet of Doom]

Can't see that listing.  Must have gone already at that price, but I'll never buy another bike from Invercargill

I have no real opinion about the compression readings other than to say it's pleasing that they are within 5%, and disappointing they are so far away from spec - though I expected some difference being a cold test and all.
Internet opinions on the difference vary so wildly that they are of no use whatsoever.

Surely someone else here must have done a compression test or am I the only one here who does any work on his bike.  The silence is deafening sometimes.

[Rikugun]

+1 about being within 5% and low expectations of actual compared to spec. My skepticism may be for reasons other than just a cold test though. Also agreed is trying to find consensus on the interwebs.

My feeling is a compression gage is more valuable as a comparative tool rather than something for measuring absolutes. The more experience you amass with that particular gage - especially on different engine types - the more valuable it becomes.  Building a history over time on a particular engine using a particular gage is also of great value. Use it on engines known to be healthy as well as sickly ones to build confidence.

I have several gages and none agree 100%.  I believe I may have the identical HF unit you now have which seems to read lower than my other two. The other 2 are a MAC with a rubber tipped short steel extension and a Craftsman with rubber hose.  Both utilize a Schrader at the spark plug hole end.

I measured my XZ shortly after buying it with approx. 6k miles using the HF gage.  The figures were 130-135 front and 125-130 rear using the HF gage. This was done cold with spark plugs removed, full battery, and WFO throttle. Both cylinders built pressure at the same rate.

Did you follow up the cold (and I'm assuming dry) test with a small amount of oil in each cylinder? Good luck finding consensus on what % improvement spells bad rings BTW. Also, did you happen to use the long rubber hose attachment for your test?

[The Prophet of Doom]

Yes I used the long hose.  That would pressurise the same but take a little longer is that right?

I didn't do the oil thing, I thought of it, but it was late, and I was just happy it wasn't 30PSI
There's not much I can do to fix it if I do discover problems.  400 parts are even less available than 550 ones

[Rikugun]

Yes I used the long hose.  That would pressurise the same but take a little longer is that right?

One would think so but I recall reading a paper on this subject that suggested otherwise. I think it had to do with having a (non leaking) Schrader at the hose end that fits into the plug hole.  It mattered more on small engines as compared to a 5.7 litre small block for instance. The author had some calculations and it all seemed reasonable but I'd have to look for the article.

[The Prophet of Doom]

Of course.  That took a moment to sink in  !!! - the valve is up by the gauge, not at the bottom.

Even if the gauge is accurate, without a valve at the base of the hose, you are effectively increasing the volume of the cylinder with your hose, and so you will get a lower compression reading than would actually be experienced in the cylinder with the spark plug in. 

With the valve up on the gauge the un-valved part gets released every cycle, and so will never build up to its true value.  The longer the hose, the less pressure.  If the hose was long enough, you would get almost zero pressure no matter how long you cranked.
If the valve were at the bottom, you would eventually get an accurate reading regardless of the hose length - a longer hose would just take longer.

None of this matters if comparing readings on the same setup, but it's a big deal if comparing between mates, or against a spec sheet where their gauges may have differing unvalved volume,

Yes it would be more of an issue on small engines because the percentage increase of volume would be greater

[Rikugun]

Yes, all that.  :)

[dingleberry]

progress report please  :)

[The Prophet of Doom]

Not great progress.

I found the floats were not floating so well.  With the float height set correctly, the float level was 15mm below the gasket, not 20mm. They seem watertight, so I reset and checked levels gain.  Spot on now.

Everything back together, and pressed the GO button.  It cranks and cranks, but not a sign of ignition.  Grrrr.  Spark is looking a bit weak,  but it should still go bang occasionally.  There's definitely fuel I can see it coming out the accelerator pump.

Then I noticed a pool of oil under the bike coming from the rear valve cover I think.  That one had a brand new gasket too.  Not sure what is going on there but it's coming off again.

[The Prophet of Doom]

I found the problem, there was a tiny chunk of gasket cement I'd missed when I cleaned it all out.  these don't need or benefit from cement in there.

Replaced the rear head gasket, and a couple of the bolts that were feeling like they were made of clay.  I'll be wanting a set for my racer but these are not available from Yamaha and they don't look like generic parts.

New spark plugs also as the ones in there were correct spec resistor plugs but with US spec resistor caps.  The two don't go together so well.

Started first time, and other than some smoke coming from the rear pipe.  Either my fix isn't quite fixed, or it's just burning off the exhaust cement, and oil that had leaked on to it.

I need to run it up on the manometer, do a road test and top up the fluids but otherwise (with any luck) I'm just about done.

[fret not]

Time to celebrate? ;)

[The Prophet of Doom]

Not quite.  I thought I was done about 3 days in to it, but that was wishful thinking.

[The Prophet of Doom]

I think I have a handle on the intermittent starting issue.  Seems it starts OK with 50% choke, NO throttle action.  Any more choke or throttle and nothing will start it for the rest of the day.  Have synced the carbs, adjusted mixtures best I can so time to get the tank back on and take it for a road test.

[Rikugun]

I guess she's a bit temperamental with the choke. Definitely looking forward to your riding impression.

[The Prophet of Doom]

Great progress
Today I discovered a split in the petcock vacuum hose.  It was brand new!  That's what you get for buying vacuum hose from people who make gumboots.

I listed the bike for sale last week, and a guy came round today and bought it on the spot.  I'm a bit out of pocket over the whole exercise, but at least I can concentrate on my proper bike again now.

[dingleberry]

Congrats on the sale. Let the real fun begin again with the racer.

[Rikugun]

So the split hose explained hard starting?

What about twingled riding impressions?

[The Prophet of Doom]

Yes.  Starting much better now, but still not going well at higher revs so it will be sold before I get an impression either way.  Sorry about that.

[dingleberry]

Did you get a chance to have a decent ride on it before it parted company? What you really need is a European bike to keep that European car company.... something with German precision ;D