Merc 1500xs won't rev like it should

[johndoeinvader]

I have a Merc 1500xs J-Block motor purchased from previous owners who use this site.

Recently a CDI switchbox was added to the motor to replace the original. Motor was tested and fired. Once at the lake the motor then lacked spark. After cleaning the distributor contacts the motor then ran well reaching about 6000 rpm on a 15' GW Invader.

After a number of runs where the motor ran well it started to lose power. It was found that the tank vent was closed so thought that is all it was. However now the engine isn't reving high enough. Nothing was changed. The CDI box tests OK according to the provided troubleshooting instructions.

It seems like perhaps timing or may be fuel related. I don't have a lot of experience with this engine so any suggestions from anyone familiar with the engine would certainly be appreciated.

[MercNuts]

Pull the sparkplugs and look for signs of aluminum deposits. Run a compression test. It is essential that you run good gas and enough oil and not too much timing. You need to also let it rev to 6K at full throttle, which it appears you have right.

[johndoeinvader]

Thanks for the input. I'll check plugs tomorrow. I am on vacation and don't have a compression tester with me, but it checked acceptable previously. I am running 98 octane Sunoco race fuel with Merc oil.

I have never checked or changed the timing. Should I try changing it slightly without knowing exactly where it's at?.

It just seems odd that it ran good yesterday for one session, but I can't see anything obviously wrong.