Mercury 25hp wont rev

[laglass69]

I have a early 90s Merc 25hp, been sitting for a while. Starts, idles fine. Revs up wide open on the hose but wont go past around half way in the water. My first thought was carb gunked up but i pulled it and it was clean as a whistle. Just put new plugs in it also. What else could cause this?

[rev.ronnie]

Is it hitting on both cylinders? I have one that had the same problem and it was a bad switch box.

[Dave Strong]

Fuel pump, check valve in the primer ball, fuel tank pickup screen?

Dave

[laglass69]

Is it hitting on both cylinders? I have one that had the same problem and it was a bad switch box.

I was leaning towards switchbox or coil. I pulled the plugs and top one was oily and had carbon like it should have and the bottom one looked like i just took it out of the box.


Fuel pump, primer ball and tank are all new. Its getting plenty fuel. It seems to be ignition related. I checked the spark and both had good spark.

[Dave Strong]

How is the compression? just covering the baisics before ya start buying parts. Don't think I have a manual for that one for electical diag. someone should, cheaper to dig than throw parts at it.

Dave

[laglass69]

Compression is around 120lbs give or take on both cylinders according to my gauge. Its a older motor but has very little use.

[laglass69]

I know i can do a ohm test on the coils, But how do i test the switchbox?

[rev.ronnie]

There is a test procedure to do it in the manual. Involves testing across the leads with everything disconnected.

[laglass69]

I ran it again yesterday evening and noticed it revs up just fine in reverse but not in forward. Its a shift in the throttle model.

[Dave Strong]

Was just told early 90's 25hp has same ign components as a 15hp up to 96. I have that manual will post test procedure later today.
The rev in rev but not fwd thing???


Dave

[Dave Strong]

Try this. If it has the stop button in the end of the handle make sure the 2 wires that come out of the tiller handle and go to switch box are not rubbed through somewhere.

Dave

[laglass69]

Thanks Dave, will test it this evening

Chase