compression before vs. after exhaust cleaning?

mikehailwood · Post by **mikehailwood** » Mon Apr 13, 2009 11:40 am

Has anyone recorded engine compression readings both before and after cleaning their exhaust system? If so, what were those readings?

chevyguyjay · Post by **chevyguyjay** » Mon Apr 13, 2009 12:02 pm

i think in my opinion, the compression would be the same. now, i can c if u changed the piston rings, or the cylinder/ head.....but not the exhaust.

mousewheels · Post by **mousewheels** » Mon Apr 13, 2009 3:09 pm

Interesting idea, but sensitivity of the test is likely very low.
Cranking speed is well below idle. Gas flow thus pressure difference would be at the minimum.
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Taken from another angle, the Spree Iowa model had an obstruction in the exhaust as part of the changes to keep it at 25mph. The service manual does not list a different compression reading for that model....
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But an experiment would cost nothing. You could put a temporary partial obstruction on the tip of your exhaust and run a compression test.

mikehailwood · Post by **mikehailwood** » Mon Apr 13, 2009 3:29 pm

Wait a minute. I have had compression readings in excess of 165 lbs and I understand that 114-120 is acceptable. Likely cause of my higher compression (and accompanying poor performance) is said to be carbon accumulation on the piston (mine is clean), inside of head (mine is clean), clogged exhaust port (mine was 2/3 blocked but is now clean) and a clogged exhaust (I have not unclogged it yet). Am I incorrect in assuming that eliminating excess carbon in all these locations will lower my compression readings and enhance performance?
Thanks

mousewheels · Post by **mousewheels** » Mon Apr 13, 2009 3:40 pm

Compression going up with deposits on the piston top and head is straight forwards. Less volume. I guess if the exhaust is really obstructed, pressures could go up, but seems like the scooter would run very poor if at all. What's yours run like?

BTW - Ok I'm curious, will run an obstruction test on an exhaust this week.

Kenny_McCormic · Post by **Kenny_McCormic** » Mon Apr 13, 2009 3:45 pm

Somebody probably milled the head or something, does it run? Does it knock?

Wheelman-111 · Post by **Wheelman-111** » Mon Apr 13, 2009 3:51 pm

Greetings:

Mildly Higher compression, as in light accumulation in the combustion chamber, will slightly enhance performance, until the point where pre-ignition begins, on many engines.

However caked carbon in the exhaust doesn't so much increase compression as it does impede flow. When the spent gases can't as easily quit the scene of the crime, less room will be available for the intake charge waiting in the transfer port wings. That will not be good for performance, whatever the compression numbers may show, because you're compressing a mixture of intake charge and spent exhaust gases. CO2 doesn't burn too well...

mikehailwood · Post by **mikehailwood** » Tue Apr 14, 2009 8:55 pm

Okay. Back up. First, this is a 1983 Express. Not as much help out there for Expresses as can be found here for the Spree. Similar powerplant - 49cc 2 stroke of same brand and similar model year as Sprees. In fact, uses the same head gasket and the carb is suprisingly similar. Scooter came w/o an air filter element. I got it running w/o the air filter. It ran great for about 2 blocks and then would intermittently bog and hesitate. I removed the exhaust pipe, started the bike from cold and went about 2 blocks whereupon it would bog down completely when the throttle was opened. By the way, it's pretty incredible how much noise these little things make without any exhaust system attached. Since the scooter always ran well for 2 blocks when started when completely cold, I decided that since the enriching ciruit shut off (after about 2 blocks), the enriched mixture contributed to it running well for that brief distance. Drawing on Spree info, I decided that maybe it was getting too much air when the engine warmed up. Then I took a chunk of foam and cut out a makeshift filter element. Now tha scooter does not run well for the first couple blocks but is doing better after it warms up. It will bog and hesitate some when doing slightly downhill, it does it but less on level ground and it runs best on an uphill incline. I have the correct filter coming tomorrow. My plan is to install that filter and see what happens. Next step will probably be to cook the exhaust system in the Weber grill. Any thoughts? Is there additional info I can provide. I cannot explain the high compression readings but from the education you guys have provided, that is probably not a concern since the combustion chamber checks out clean. Thanks

Kenny_McCormic · Post by **Kenny_McCormic** » Tue Apr 14, 2009 9:08 pm

Filter system will screw you up like that.

nysnowborder · Post by **nysnowborder** » Tue Apr 14, 2009 10:16 pm

umm i dont even no why im commmenting on this.

yes the air/ fuel mixtures and system w/e have to be pretty percise on these things or they run like * hehe

and im suprised that thing didnt seize .

but i ran my elite 50 w/o a filter and i ran pretty d*** good and never siezed in about 80 degree weather so idk.

but yes deffinatly the stock air filter element should sole or mostly fix these simptoms

mikehailwood · Post by **mikehailwood** » Wed Apr 15, 2009 10:35 pm

I installed a new OEM filter element today. This made a huge improvement. I have discovered that this scooter is supposed to have (but does not currently have) and exhaust pipe gasket (header to head). I have purchased one but have not yet installed it. What effect does the lack of a pipe gasket have on these scooters. Guess I'll find out tomorrow.

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