kr1s compression test

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strokerdave250350500
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kr1s compression test

Post by strokerdave250350500 » Sun Apr 05, 2020 11:01 am

I have just done a compression test on my kr1s after lowering my squish clearance from 1.25mm down to 0.80mm. I`m now getting 140 psi on he left cylinder & 160 psi on the right cylinder where as before with rather large squish at 1.25mm I was getting 120psi on both cylinders, its in std tune and has had bores re-plated and fitted with Wiseco pistons. so would these different psi reading mean that it could put more strain on the crankshaft. I can`t seem to find any info on what the compression should be etc. Any help would be much appreciated

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500bernie
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Re: kr1s compression test

Post by 500bernie » Sun Apr 05, 2020 6:08 pm

107 - 164 psi from the manual
Has the engine been started up?
If not, give her a few heat cycles and see what it looks like then.
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JanBros
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Re: kr1s compression test

Post by JanBros » Sun Apr 05, 2020 11:02 pm

did you check both squish-heights after the modifications or did you just presume both would be at 0.8mm ?
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Re: kr1s compression test

Post by DougB » Mon Apr 06, 2020 10:29 am

If it's not been run since the full rebuild I'd say as Bernie and do a couple of heat cycles first then try again, the rings need to settle to the bores to develop a good seal and they'll only do that by running in. You've got good PSI already testing from cold so at least you know it's got compression!

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Re: kr1s compression test

Post by strokerdave250350500 » Mon Apr 06, 2020 12:00 pm

JanBros wrote:
Sun Apr 05, 2020 11:02 pm
did you check both squish-heights after the modifications or did you just presume both would be at 0.8mm ?
Yes both sides reading 0.80mm squish clearance.

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Re: kr1s compression test

Post by strokerdave250350500 » Mon Apr 06, 2020 12:02 pm

DougB wrote:
Mon Apr 06, 2020 10:29 am
If it's not been run since the full rebuild I'd say as Bernie and do a couple of heat cycles first then try again, the rings need to settle to the bores to develop a good seal and they'll only do that by running in. You've got good PSI already testing from cold so at least you know it's got compression!
Its done just over 150 miles since the rebuild, with a squish of 1.25mm.

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Re: kr1s compression test

Post by strokerdave250350500 » Mon Apr 06, 2020 12:05 pm

Yes its done about 150 miles since the rebuild on the 1.25mm squish clearance.

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Re: kr1s compression test

Post by strokerdave250350500 » Mon Apr 06, 2020 12:20 pm

Its now has 0.80mm squish clearance on both sides and has covered 150 miles since the rebuild on 125mm squish clearance . Just seems odd the the left-hand side reads 140psi and the right 160psi. Would this be ok to run as is or does it need attention. The compression test was done with a cold engine, will warm up engine and try again when hot.

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Re: kr1s compression test

Post by James P » Fri Apr 10, 2020 9:11 am

Dave,

I'm sorry I can't offer any meaningful advice on your compression pressure difference issue, other than to ask whether you did leak tests after the initial build and again after the squish clearance reduction (i.e. could there be a leak somewhere on the left-hand side which didn't exist beforehand?).

Whatever the reason for your difference in compression pressures, I am particularly curious about your squish clearance reduction.

Did you achieve the squish clearance reduction by lowering the cylinders using thinner base gaskets...or by machining the cylinder head gasket face...or by some other means?

Whichever path you took, I presume your overall higher pressure readings have resulted from the reduction in squish clearance with no modifications to the combustion chambers.
On my own bike I also reduced the squish clearance to 0.8mm, but was a little worried that the increase in compression may cause problems (fuel these days is not what it was when these bikes were new). For that reason, I milled the combustion chambers to get back to the standard compression ratio (7.4:1 corrected, about 12.2:1 geometric). Leaving the combustion chambers standard would have resulted in a compression ratio of about 8.2:1 corrected (around 13.5:1 geometric)...which is what I presume you have now.

I would be interested to know whether you have noticed any difference in performance (due to the higher compression ratio) since reducing the squish clearance and whether you intend to take any particular precautions to guard against trouble from hotter combustion temperatures (e.g. colder spark plugs, retarded ignition timing, use of special fuel etc.).

I have made a few assumptions about your set-up, so please let me know if they may be wrong!

Thanks & regards,
James

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Re: kr1s compression test

Post by strokerdave250350500 » Mon Apr 13, 2020 10:37 am

James I reduced the squish by fitting thinner base gaskets and nothing else, standard head with no mods at all. I only use shell v power 99 ron fuel, also run with std br10es plugs. I have not been able to get out on it since lowering the squish because of the lockdown but seems a bit more responsive on the throttle when stationary. I've not retarded the ignition at all as I didn't think it was possible on these bikes i assumed that they were fixed and couldn't be adjusted. I did another compression test yesterday but this time I did by putting the bike in 2nd gear and pushed it along , this time it read 155 psi on both sides. How strange is that. So now it has 0.80mm squish and 155 psi on both sides. Do you think that I need to check the timing, if so how many mm btdc should it be etc.

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Re: kr1s compression test

Post by JanBros » Tue Apr 14, 2020 12:23 am

timing is fixed, you can only change it a bit by making the holes for the bolts in the pickup "oval"

only real test you can do is with a strobe.
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Re: kr1s compression test

Post by James P » Tue Apr 14, 2020 12:00 pm

strokerdave250350500 wrote:
Mon Apr 13, 2020 10:37 am
James I reduced the squish by fitting thinner base gaskets and nothing else, standard head with no mods at all. I only use shell v power 99 ron fuel, also run with std br10es plugs. I have not been able to get out on it since lowering the squish because of the lockdown but seems a bit more responsive on the throttle when stationary. I've not retarded the ignition at all as I didn't think it was possible on these bikes i assumed that they were fixed and couldn't be adjusted. I did another compression test yesterday but this time I did by putting the bike in 2nd gear and pushed it along , this time it read 155 psi on both sides. How strange is that. So now it has 0.80mm squish and 155 psi on both sides. Do you think that I need to check the timing, if so how many mm btdc should it be etc.
Thanks for the update Dave. Jan is right in that the standard ignition system cannot be adjusted without modifications - I thought perhaps you may have a programmable ignition system fitted.
As your reduction in squish clearance has caused a rise in compression ratio, it should follow that the bike will run hotter, but it remains to be seen whether the extra heat will be enough to cause any problems.
Obviously you can check the ignition timing with a strobe light as Jan suggests, but (in the present configuration) it should just appear as standard (25 degrees BTDC between about 2000rpm and 7500rpm). Any adjustment to the ignition timing which may be necessary would have to be ascertained through rigorous trial and error (e.g. on a dyno, or using CHT and EGT figures under a range of riding conditions). One option for retarding the ignition timing somewhat is fitting a KR-1 CDI unit - the 'curve' is practically the same as KR-1S, but starts retarding 1900rpm earlier. Of course, a programmable ignition system would be a better option but it may be best to first decide whether the extra heat generated is enough to justify corrective action. To that end, it would be useful to hear from someone who has raised their compression to a similar level with little or no other modification and observed the effects...if any.

It is interesting that you now have identical pressure readings for both cylinders. Do the readings vary depending on the method used; e.g. kick-starting, pushing along in first gear, second gear etc.?
I have never used compression testers of the type I infer you are using (connects to spark plug hole?), as I'm not convinced that they give much meaningful information and they don't allow comparisons between different engines unless they are of the same specification.
Of course, any large difference in pressures between cylinders should indicate a problem, but one would expect to notice a difference in running conditions between cylinders if that was the case. I suppose they may be useful if you are buying a used bike and want to get a basic idea of the compression, but I would otherwise avoid drawing too many conclusions from the results. I am however open to input from others who may have found such pressure tests useful.

I would also be interested to know your observations on performance, heating etc. once you have had a chance to take the bike for longer rides in its present configuration.

Thanks again & regards,
James

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Re: kr1s compression test

Post by JanBros » Tue Apr 14, 2020 12:43 pm

extra heat (if any will occur) is no problem.
I run 10cc heads with 0.7mm squish and original ignition.
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Re: kr1s compression test

Post by James P » Thu Apr 16, 2020 12:23 pm

JanBros wrote:
Tue Apr 14, 2020 12:43 pm
extra heat (if any will occur) is no problem.
I run 10cc heads with 0.7mm squish and original ignition.
Thanks for the info Jan. It thus appears that reducing the squish clearance as Dave has done has benefits, with little chance of any detrimental effects. If anyone disagrees, please let us know!

Regards,
James

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Re: kr1s compression test

Post by strokerdave250350500 » Sun Apr 19, 2020 7:19 am

Thanks guys for the info, I will check timing with a strobe also check with a dial gauge, would I be right in thinking 25 degrees is about 3.13mm before tdc.

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