Gents,

I have two identical brand new stock engines that have not been run or opened (S-254 Evo's). Is it unusual for each engine to have different felt compression when pulled through? Like I said one engine is super tight where as the other turns over rather easy (still tight) compared to the other. I know that having an engine that turns over easy doesn't mean it wont perform. Thanks for the replies.


Sean

How do they compare with the sparkplugs removed?

Either way, you will not be satisfied until you take them apart and find out
what the difference is. You were going to anyway, right? It always pays to make
sure they are very clean inside before running them. Now you have two good
reasons to look inside.

Thanks for responding. I will take them apart to make sure I dont find any machining chips or trash inside. This doesnt seem like its a common problem but hopefully I wont find anything wrong.

After disassembling the engine I didnt find anything out of the ordinary. I ordered a new piston and ring. Maybe this will fix my problem.

One might have had more assembly oil in the top end. Just a few drops to many in the combustion chamber, and they can act like a hydraulic lock. If the ring looks like it is starting to wear all the way around (or nearly) from pull cord action, your compression should be as good as new. It would have been interesting to run them both for a bit, and then check for ring seating. One might be out of round and not sealing 100%. Rare, but it happens.

I use the tools shown in the photos to determine how good the piston ring is sealing. The Sullivan Mariner S-650 starter & the 19.2 volt Lithium-Ion Die Hard battery will turn any 30 cc size engine at approximately 4,000 RPM. A standard Snap-On compression gauge with a cone-type spark plug hole adaptor will tell you if the piston ring is doing it's job. Typical readings between 140 & 170 psi can be expected after running the engine at WOT for a period of time. I usually do this at the lake after a test session. The typical reading will be possible even though the standard ring gap for stock rings is well beyond the recommended amount of .0045" per inch of bore diameter for normally aspirated engines.

Jim Allen

If the motors have not even run - what problem are you fixing?? Ring might seat in 20 secs of run time -- not a problem till you ACTUALLY run it and see after if it has no compression. One could have more oil in it like ES stated - or could just be bearing sat in one place or sticking seals -- tons of stuff

But not of it matters till u run it ...... then check compression

just my 2 cents

Thanks all for responding to the post. Lots of good info and suggestions. I'm going to run it this weekend to break it in and I'll check the compression after that session. Like Canadian Hot Rod said, might just need seating.

Sean