Can some one tell me what the compression was on a 503 fan new?

110-130 and both sides should be pretty close to each other, and it will run good.

My 1980 503 has 90 on each side. Is this too low?

my straton needs a head gasket because one side has 140 and the other has 90 and al kinds of oil splater over her

When I first checked the motor it had 90 on the laft and 60 on the right. The head bolts were real loose so I just tightened them up and now I got 90/90.

It's a little low. Try new head gaskets and if that doesnt solve it, I think it needs a ring job.

I think you are prolly right. I've been pricing a re-piston job and its about 250-300 depending on piston brand.

MY stratos has 85/90 psi so I was just looking if it needed a rebuild job..... thanks for the info

I can get pistons pretty cheap through work. For my 84 ET 300 They were 35$ each Canadian. Withour rings though.

I have taken compression on a handful of 503's form the 80's, 130ish is on the low side, 140-150 is about right

Are you sure about 150ish that's pushing the limits of pump gas and they were designed to run on reg.

very sure

just a side note: the cheap compression gauges with the rubber hoses give a much lower reading than the actual compression. I have one

I have to agree there. My tester is a Mac Tools kit.

Any one have a problem with the engine wanting to Flood when first started wth compression in the 90 psi range until warmed up?

are you sure it's flooding? both my 503's need to keep taking a hit of primer to keep them running when cold. I think it's because there's no choke

I have a problem with 90 psi...you see, two strokes can run quite well with even lots of scoring in the cylinders, and even a compression test won't show it. But with 90, I'd take the top end apart. I think you'll be surprised as to what you uncover. Just doo it...you'll be much better off in the long run. TNT FA is right...that motor should be closer to 150 than 100 for compression, assuming that they're both within 10% of each other...!

10% is a maximum too, it's a rule of thumb and it works OK with 4, 6, and 8 cyl engines and especially 4 stroke cycle motors. In cases where I have 10% or greater compression variance I then do a leakdown test. This process is not easily done on 2 stroke cycle engines. I believe it can be done but you have to seal off the intake and exhaust and I believe you will need a different type of leakdown tester.

With 90 PSI you better get ready to tear down the engine and find out what needs fixed. Fully expect to reseal the crankcase. If you are going to spend a couple hundred bux on pistons, rings and gaskets there is no reason to not replace the seals and o rings on the crank unless ofcourse you like melting a piston because of a faulty seal and want to spend the money once again.

Personally on a 503, anything below 120PSI should make you go hmmmm.

Bob, what does your 583 produce?

like said, it would be a good idea to take off the heads to look around. and new seals are cheap insurance.



the reason I mentioned the cheap gauges is because mine is only good for checking to see if both cylinders are even. it has NEVER read over 115(was on a 503), and is usually between 85 and 110.(outboards and sleds) I just didn't want sombody to buy new pistons if they didn't need to because of a faulty gauge. one day I need to borrow a good gauge so I can make an equivalancy sheet for my gauge vs. real world.

Remind me if/when we meetup to bring my tester.

that would be cool.

my dad's old 1 banger citation's got 90psi on the gauge, yet it starts and runs like a champ every time. My everest's got 120-130 compression and it needs to be babied a bit with the primer until it warms up a bit. 90 psi normally would make me concerned, but if it's not giving trouble, don't fix it.

90 PSI on a one lunger is a huge double red flag. Because of their larger bore and stroke they should often have a higher reading 130-140 would be my guess.

The liquid cooled RV engines such as is in your Everest tend to take more fiddling with the primer until they warm up because it takes longer to warm the liquid engines VS an air cooled. Also the reason they use primers on RV style engines you have to wet the rotary valve to get it to work VS a piston port or reed valve set up.

Don, I'm not sure what my 583 runs for compression right now, although I could doo it pretty quick and let you know I never really had a reason to, but I guess now since it's now officially up on the block I'll just Git 'R Done....did you see my thread about the 97 I bought this past weekend??

Yes, yes I read that post *eg*

The more I think about it the more I am becoming interested in a Formula III

my uncle checked mine on my 503 before we bought it and it was around 145 on both sides

That is almost as good as it gets.