Hey guys, I have a 1994 ZXR250C and I conducted a compression test to see the condition of the engine. I had to make a special adapter to fit into the tiny plug hole in order to conduct the compression test. The readings I obtained are as follows: Dry cylinders: 1: 85psi 2: 80psi 3: 85psi 4: 90psi I then sprayed inox in the spark plug hole and tested again and obtained the following readings: 1: 120psi 2: 115psi 3: 125psi 4: 125psi What do you guys think about these readings? Is this normal or is the engine worn? At least there is compression on this engine so I am relieved somewhat because it wasn't running too well when I bought it. Cheers Dean
A lot of variables but here are some of my thoughts for whatever they are worth. Those numbers sound low, however: Your compression gauge is not likely to be a certified instrument. Unless you have a lot of experience using it then you do not know how accurate it may be. Or how to compare these numbers to others. Personally, I place very little value on the results from equipment like this unless it has a certified level of accuracy. The readings across all cylinders are quite consistent. This is a good thing. Each cylinder only holds 62cc plus the volume of the cylinder head. How much inox did you squirt into each cylinder? If you put in 1 teaspoon then that is about 5cc you have displaced. Is the hose flexible in any way? How long is the hose? As an analogy, if the volume of your cylinder was 220 litres then the volume that will be absorbed by your hose and fittings would be negligible. But when the swept volume is only 62cc your hose and fittings become very significant. Did you force the throttle slides into the fully open position when you did this test? These engines are designed to develop peak torque at very high rpm. This means they will be terribly inefficient due to pumping losses at low rpm. You will need to compare apples to apples. In other words the compression values for your your 250cc four against other 250cc fours. Anything else will be quite meaningless. A leakdown test would be much more useful. You have to expect a 20 year old engine is going to need a top end overhaul at some stage. With the rpm that these engines run at I would consider rings as almost a service item. cheers Blair
Hey Dean, Blair is on the money with his advice, my 2c are that generally a compression test should be undertaken with the engine warmed up and the carbies either removed or pinned at Wide Open Throttle (WOT), the manual states between 142-218 PSI is the serviceable range on a ZXR250. Here is the some info on compression tests and leakdown tests http://2fiftycc.com/index.php?threads/rings-or-valves-how-to-test-compression.2932/
Thanks heaps for your thoughts Blair....You are spot on, right on the money! These compression test are just a rough idea/guide to see the condition of the engine. I do agree with you the engine will soon needed a complete rebuilt. I just want to get it running better than when I got it. When I bought this ZXR250, I took the risk because it was running sick and not revving or accelerating freely. I also found that the spark plugs are faulty and carburators are very dirty/shitty. Now, I just had to finish assembling it and hopefully it should run better.
Hi Dean Just a quick reading of your figures would indicate a somewhat low reading, Blair has it nailed I think. Would perform leak down test and take it from there. One thing that is encouraging is the consistency of the numbers wether wet or dry. Nothing worse than getting 3 reasonable readings and one poor one, you then have to debate wether to fix the lot or just the low one, you start second guessing yourself. Looking at the figures that Kiffsta put up, yours appears very much on the low side. These engines work very hard and rev like crazy so it shouldn't surprise anyone that the internals need some TLC. Keep us posted on the outcome.
Thanks for all the info Kiffsta and Phil. I had a read on the link on compression testing and yes it's been very informative. I did do my testing without the carby installed but in a cold engine. Yes, I will soon get this engine rebuilt...no doubt about that, but I just want to get it running so to pin point more problems in it. Thanks again guys. I will keep you posted for sure. Cheers, Dean
