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Left 2 cylinders running hot 14 Sep 2020 19:22 #835049

  • loudhvx
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Air has mass and it moves. Static compression ratio does not take that into account.

EDIT: I made a more thorough answer below.

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Left 2 cylinders running hot 14 Sep 2020 19:26 #835050

  • TexasKZ
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And blowby past the rings.
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough

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Left 2 cylinders running hot 14 Sep 2020 22:11 #835058

  • loudhvx
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A more accurate compression ratio would be the dynamic compression ratio. It is calculated from the point where the intake valve closes in the piston movement. The dynamic compression ratio is almost always less than the static compression ratio (and never greater). But that makes it an even worse predictor of compression pressure...so what gives?

I think there are at least three factors leading to the higher-than-expected pressure reading on the compression test.

First the air movement in the intake tract. The momentum, once the air is moving into the cylinder, carries in more air than the piston displacement would account for. This could lead to a slightly higher than atmospheric pressure at the start of compression.

Second would be the heat in the engine. The test is done "hot", so the air pressure in the cylinder would increase from this heat after the intake valve closes, even if the piston did not move after that.

Third would be heat due to compression. The temperature of the air rises greatly during compression. The simple static-comp-ratio times atmospheric pressure calculation totally ignores temperature. I assume everyone has heard of PV=nRT where T is temperature. R is constant and n is constant once the intake valve closes.

Volume V reduces by a fixed amount as the piston moves up. So if we hold T constant, pressure P must increase by a fixed amount to offset V. That would yield the pressure dictated by the static compression ratio assuming the valve closes at BDC. It will be less if we use the dynamic compression ratio with the valve being open beyond BDC.

But T increases greatly in practice during compression. So this causes an additional increase in P to account for that. This extra increase in P is what we see in a proper compression test using a properly designed gauge.

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Left 2 cylinders running hot 15 Sep 2020 00:46 #835060

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just out of interest when you say running hot compared to the others what do you really mean?
it sounds to me that the cold ones are not firing when under load as it according to what you are saying idles fine. my first although possibly unrealistic take would be tryk new plugs. i had a similar issue and fought for days trying everything and at some point decided to try the plugs and a new set sorted it.

low compression does not seem to be the issue as the temperature difference while running would not really be that noticeable without a temperature check (not sure if you have a laser thermometer to check)

suspect to me are three main things air leak in the intake, sticking diaphragm, or ignition issue i.e. as stated plugs not firing for whatever reason. as they are not on the same ignition coil the coils should be ok but the plugs are cheap to try out and eliminate.

hope it is a quick easy fix for you

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Left 2 cylinders running hot 15 Sep 2020 05:05 #835066

  • hardrockminer
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Yes, I understand the ideal gas equation. I'm a mechanical engineer.

That's a logical explanation of why compression should be higher than static, but it doesn't include the reasons why it will not reach theoretical maximum. Ring blow-by and valve leakage are two obvious ones. Also, when air moves, pressure actually drops, which is the physics behind carburetor function.

This is probably a subject for a separate thread. Apologies for bending this one.
I have several restored bikes along with a 2006 Goldwing with a sidecar. My wife has a 2019 Suzuki DR 650 for on and off road.

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Left 2 cylinders running hot 15 Sep 2020 06:31 #835071

  • dpivas7
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Regarding the compression... the manual says normal compression is 130-160 PSI. 180 sounded way high anyways.
Regarding the ideal gas law: T goes up because P goes up. They are proportional. P does not increase additionally because T goes up during compression due to P increasing - that would be circular logic wouldn't it? P goes up because V goes down which causes T to go up.

I can't speak for the ring blow-by and valve leakage, I don't yet have that level of understanding of it yet.

However, the manual states 130-160PSI is normal so I'll go with that.


Anywho... When I say it's running hot compared to the others, I mean that if I let it idle for a couple minutes, shut the engine off and take out the spark plugs, I can't hold the plugs from cylinder 1 and 2 in my hand because they are so hot. I can hold plugs from cylinders 3 and 4 though. Also, you can feel the heat coming from the left side of the engine from a few inches away. You can touch the right side of the engine - it just feels warm. The left side is much much hotter when you touch it after just idling for a bit.

I have spark in all the plugs, I've checked multiple time and I recently replaced the plug wire for one of the cylinders and confirmed it works multiple times. The engine doesn't misfire or anything. The bike rides totally fine except under mid speed (6000-7000rpm) WOT which I believe is due to the fact that there are pod filters without a dynojet kit. I can't recall if that issue was present a few years ago when I actually owned this bike. Once you get up to ~8000RPM at WOT, however, the bike all of a sudden takes off (accelerates) and it redlines real quick. If you roll the on the throttle thru the fuss point, you'll get acceleration but it noticeably peaks before redline. I would like to see the power curve for this engine. I don't see one in the manual.

There are no air leaks in the intake except at the pod filters. I notched the inside of the pod filter boots so that it doesn't constrict the little bean passage way (not sure what it's called). I used RTV on the outside of the air filters to make sure that no air can come in due to the loose fit between the boot and the filter. I used vinyl caps on the vacuum nipples on the carbs and on the reed case vacuum ports (however, I don't think the reed cases would affect intake. Seems too far downstream).

I've checked the diaphragms a bunch of times and I am pretty sue they aren't sticking. There are not holes in them as far as my eyes can tell. Maybe I should check again with a flashlight?
Current: '19 Harley Roadster, '72 XLCH, '84 GPz 550
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Left 2 cylinders running hot 15 Sep 2020 08:40 #835085

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In general I would expect the center cylinders to run a little hotter than the outside ones. As for left to right difference...I would think about the fuel/air mixture. A leaner mixture will run hotter.
I have several restored bikes along with a 2006 Goldwing with a sidecar. My wife has a 2019 Suzuki DR 650 for on and off road.

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Left 2 cylinders running hot 15 Sep 2020 09:03 #835086

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Have you checked your timing advancer? Look for proper fuel deliver to 1&2, Not sure on you bike but mine have 2 fuel lines the feed 4 carbs maybe 1&2 are not getting good fuel.
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Left 2 cylinders running hot 15 Sep 2020 09:16 #835088

  • loudhvx
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I've measured Zx550 motors that were exactly in the same range. .. 175 psi and up..

Which manual are you using? I'm using the Kz/Zx 550 factory base manual... both the white manual and the red manual have similar values with wide ranges going up to and over 200 psi.

From what I've found, Clymer just mentions 125 psi. The lowest numbers I could find anywhere in the factory manuals was 128 but the range was 128 to 198 psi, and that was for the 400cc version of the inline-four 550 family.

And I should reiterate, I don't believe there is anything wrong with your engine related to compression. I think it's the guage or some other external factor.









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Left 2 cylinders running hot 15 Sep 2020 10:32 #835097

  • F64
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You may try a cylinder balance test.
All of your plugs should be too hot to hold.
I'd almost rename the post, right 2 cylinders running cold.
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Left 2 cylinders running hot 15 Sep 2020 12:58 #835102

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F64 wrote: You may try a cylinder balance test.
All of your plugs should be too hot to hold.
I'd almost rename the post, right 2 cylinders running cold.


How do you do a cylinder balance test? Early this afternoon, I had the plugs out to check for spark. I had good spark in each plug. The engine would actually run when one plug was removed so I could see that it was sparking at idle.

Maybe I'm not getting fuel to cylinders 3 & 4.

The other day I tried spraying carb cleaner into the pod filter on the 4th cylinder and it didn't react at all. When I sprayed into cylinder 1, the rpms shot up.

This afternoon, none of the cylinders reacted to when I sprayed their filter. I could feel the suction of air around the filter if I put my hand there so I know it's drawing a vacuum.
Current: '19 Harley Roadster, '72 XLCH, '84 GPz 550
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Left 2 cylinders running hot 15 Sep 2020 14:32 #835111

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I confirmed I'm not getting fuel to cylinders 3 & 4. The float bowls are filling up, but for some reason it's not sending fuel to the cylinders. I had the bike idling and a turned the idle mixture screw on cylinders 3 & 4 and couldn't get the bike to react at all.

The bike reacted on cylinders 1 & 2. So now I have to figure out why the gas isn't getting from the bowl to the cylinder. I took the filters off and confirmed that the slide wasn't sticking - though I'm not sure that would contribute.
Current: '19 Harley Roadster, '72 XLCH, '84 GPz 550
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