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Discussion Starter · #1 ·
I have 2 identical 2006 carbureted Yamaha F40's. Cranking compression according to the shop manual should be 170psi. I put new timing belts on both engines. One cranks at 175 psi and runs perfect. The other engine cranks at 235 psi and stumbles and runs erratically at part throttle. The carburetors have been gone through and looked fine. Timing advances normally. Full throttle response is ok on both revving on the muffs. I double checked the timing marks on the one with high compression and the marks line up correctly. I thought maybe cam timing advanced or retarded might cause the high compression reading. Oil in the cylinders would cause it, but it does not smoke when running. It's hard to believe there could be enough carbon build up to cause the high reading, but it may be possible because this engine did a lot of low speed running at idle. On a leak down test it has less than 5 percent leakage on all cylinders. Has anyone ever come across this?
 

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Discussion Starter · #3 ·
I use Ring Free in my 90 2 stroke and F40. The engine with high compression is a recent acquisition and I only know a little about it's past history. I sprayed combustion chamber cleaner in the cylinders, but compression is still high. The plugs look clean but oily, and the engine has less than 5 percent leak down. I may try combustion chamber cleaner sprayed in through the vacuum plugs on the intake runners. There is no way to get the intake housing off to spray through the carbs without taking the whole intake off.
 

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Discussion Starter · #5 ·
Cam timing was my thought too, but the marks line up the same on my other F40 and it runs perfect. The cam looked fine when I adjusted the valves. I ran Yamaha Combustion Chamber cleaner through the intake port plugs again today and compression has come down to 210, still high but better. I tried advancing or retarding the cam one tooth and the marks are way off. I may try it one tooth retarded and then recheck compression.
 
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I went through the carbs. They were very clean and really did not need to be cleaned.
They need to be synchronized! If the butterflies are open a little then the compression will show high. Not sure if you are doing you comp. test with carbs at idle or at wot???
 

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Discussion Starter · #9 ·
Compression test is being done at wot. My known good engine tests at 175 wot. I retarded the cam one tooth and compression came down to 180, but it ran pretty ragged at that setting. I advanced the cam and it went back up to 235. With the marks lined up according to the manual it cranks at 215 at wot. I checked the cam gear dowel and the crank gear key and both are OK. The story behind this engine is the original owner bought it when we bought ours and had it on an identical Jon boat. The boat stopped moving one day and he was told the lower unit was bad. He gave my son the engine and he bought a new one. I took the lower unit off and found a broken driveshaft. It looked like it may have been cracked and the salt corroded it and it broke. Lots of corrosion on the lower unit. Power head looks new. This engine must have been kept in the water. I replaced the driveshaft and upper bearing and resealed the lower unit. Gears and other bearings looked fine. I replaced the timing belt on our engine and this spare engine. The timing marks were lined up on both engines exactly the same and the belts replaced. My engine runs great and has 175 psi cranking at wot. This spare engine has very high compression but it was running fine until the shaft broke according to the original owner. I have not had it on a boat yet, so it's hard to say if high compression will cause a problem. I just know it is not right to be that high on a low compression 4 stroke outboard.
 

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Discussion Starter · #11 ·
After reading some forums and watching YouTube videos about the F40, I ordered some jets for a F25FL. The F40 uses a 38 pilot and a 95 main jet. The new jets are a 45 pilot and 108 main. The new jets are only .003" larger. They made a huge difference in the way it runs. No more lean miss at part throttle. I still have not figured out the high compression but it now runs great. This engine was run very hard and not maintained to my standards. Our other well cared for F40 still has the stock jets and runs fine. I read that early F40's had very lean jets and the later engines had larger jets. I wont be taking the carbs apart on the good running F40 we use all the time just to check jet sizes though. My shop manual says the pilot jets are 40's. That would only be a hair larger.
 
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