Puzzling compression test

[crazy steve]

I'll throw my .02 in here...

First, I think the manual data is a bit suspect, and most of you are misreading it. If you're looking at a Clymer, it's particularly not clear.

My '78 factory manual puts it like this: cranking pressure (at sea level) = 142 psi. Below that, it says: maximum difference between cylinders = 14 psi (italics added). These are the same numbers Clymers uses, but they state it differently: cranking pressure 142 psi, +/- 14 psi. That is not the same thing. Let's look at the cranking pressure number first; Yamaha gives 142 as a sea level number for my '78. One, they're not showing a 'range', just one number. I would expect to see a 'range', say 137 to 147 as a rough guess. Well, the chances that all XS motors would show that single number are slim and none, given even slight variations in manufacturing, not to mention break-in and use. Two, Yamaha also made changes in the camshafts and static compression ratios over the years, so that number can't possibly be right for all years. The '78 had the longest duration cams, which reduce cranking pressure, so seeing a higher number on a later motor with the shorter duration cams is to be expected. And I'll also point out that I've run into sound, stock motors that have had better-than-posted numbers more than once; sometimes you just get one that everything is 'perfect' on.

Moving on to the 'difference' number. The factory manual number if you look at it is the standard 'acceptable' 10% allowable variation that you'll find in any generic repair book. The Clymer is actually showing a 20% variation (+/- 14 psi, for a 28 psi range), and would produce a poor-running motor if followed.

So back to the original question; are these numbers too high? Maybe, maybe not. The variation between cylinders is well within spec, and I personally wouldn't worry too much. If you've been riding it with no issues, no problem. If all this is part of a 'pre-start' check, it might pay to take look down the spark plug holes with a rifle 'bore scope' to see if there a ton of carbon in there, and if so, run some 'cleaner' through it as soon as you get it running. I will ask one question; did you perform this test with the carbs/air box installed? If not, that can account for the big numbers. The restriction of the carbs with closed slides limits the amount of air that can enter, lowering your cranking pressure. I've got a spare motor that I tested, and with the carbs removed got 205/200/125/195 (yep, got one weak one..).

[b.walker5]

Steve, you may well be right about the stated figures being misleading. I've just checked a few other service manuals I have (mostly cars) and while they all give only one figure, for instance 307, 327 and 350 V8's as fitted to early Australian Holdens are 150psi, the notes state that this is the minimum pressure at operating temperature and no upper limit or percentage is given.

Interestingly the Clymer for XS11's states the compression figure as COLD at sea level.

Based on that then perhaps 11Riders fiqures arn't too high, however I still maintain that numbers significantly higher than spec should be investigated.

Very good point about the carbs, that will definately affect the outcome.

[11Rider]

I must ask first, why are you performing a compression test, does it not run well? Does it have a dead miss that you cannot attribute to lack of fuel or spark?

Compression tests are used to provide information regarding the ability of the engine to ingest and compress air, nothing more. It is a diagnostic, not a health test. It is used to determine if a mechanical failure condition exists (bent valve, broken ring(s) etc..) after you have eliminated fuel or spark as being the cause of whatever performance problem you are trying to diagnose. (there is no need to test compression on an engine that runs well)

Over and over have I seen 'gearheads' get a 'new' (used) engine and perform a compression test on it, get all bent over the results, and chase their tails for months over a percieved problem that just does not exist!

Also, the numbers mean essentially nothing as absolute values, they are only usefull when compared to one another on a cylinder by cylinder basis.
i.e., if you have a dead miss, fuel and spark seem to be good and a compression test shows that cylinder to be at, say 80 psi and the others at 150, 160, and 180, you HAVE found a problem. Addind oil to that cylider and rechecking can provide more info: if the reading goes up significantly, the issue is with the rings, if it does not, the valves are suspect.

The readings you posted mean nothing unless you are trying to solve a specific problem, too many variables in the testing equipment and testing methods. Also, the exact minutae of how the test is performed is of little relevance, so long as you reproduce the testing method faithfully for each cylinder, again it is a compairative test and not an absolute test.
Remember the KISS principle. These bikes are pretty simple, keep it that way.

Very interesting. To answer your question, no, I have no running issues with the bike aside from it taking a little long to warm up and a somewhat clunky trans. I got it recently so I thought it would be good to check everything out. I am finding a consensus though that says it's the cylinder to cylinder readings that are most important. Thanks for your input!

[11Rider]

All great input - you guys are a wellspring of info. Not to beat a dead horse, but is there any chance of doing harm by spraying the carbon remover in there? I keep envisioning a nice big chunk breaking loose and getting stuck in an exhaust valve or something

[natemoen]

Should ne no problem using the cleaner

[b.walker5]

Never used it, but couldnt see it being an issue.

[cywelchjr]

+1 on what Brian says. I can't see the cleaner hurting, but my experience with gasoline engines over the years has shown me that water in through the intake at medium RPM's seems to clean the carbon out really well, and I'm not aware of it having damaged one if you didn't try to turn the engine into a water pump. Do it slow enough to make sure you don't hydrostatically lock the engine, that would be likley to kill it. But a small amount really rather steam cleans the inside of the engine, so make sure it's up to temp AND run it for a bit afterwards to make sure that it relubes everything.