Merc 25HP 4Stroke Timing Problem???

Hey guys,
I'm tryin to sort out this little motor. My dad gave it to me after deciding he wasn't going to use it as a backup motor for his fishing boat. Its history is unknown, but it seems in good overall shape. Problem is when I start it, it seems like it has a fuel starvation issue. It starts great, and idles fine, but if I open the throttle too fast, it dies. I took the carb apart and cleaned it. Everything looked okay, though there was some salting and old fuel in there at first. It's all been cleaned out now. I checked the compression and got 160psi in both holes. Plugs look okay. When I went to check the timing (non-adjustable), it appeared to be way out of range. For this model, it should be 5BTDC at idle, and it's not even registering with the scale with a timing light. I'm not even sure if it's way advanced, or way retarted. The scale has numbers 30-0-10 reading left to right on the flywheel. The needle points to somewhere past the 10, like maybe where 20 would be if it were written in. I tried changing the rpm but the timing doesn't change. Can anyone comment on that? Is it normal?


I also have a question on how to read RPM on this sucker. It is setup for a remote control, but I don't have one. My timing light has a tach function, but at idle, it claims I'm doing 1700RPM. I thought these ran closer to 800. I couldn't adjust the idle stop screw to get me to 800, and past 1000 it stalled altogether. I have a voltmeter that has a HZ measuring function, but I don't know how to use it. Any help here would also be appreciated.

The timing is controlled by the ECM (computer) but it does rely on the Crank Position Sensor to (decide) when it should tell the plug to fire, so testing the CPS is probably in order - that is assuming your timing light is working properly. It seems to be reading "double" the rpm (reading 1700 when 850 would be more what you want to see at idle) when you have it set to read rpm.

I actually had 2 different timing lights tell me the same thing. Do you think timing being off could cause the hesitation I'm experiencing?

It could. If the engine is timed (wrong) then the spark is occurring too soon or too late during the pistons travel towards the top.

If it sparks "too soon" now the piston is trying to compress already expanding gas, so it working much harder to try and get a revolution.

If it sparks "too late" the piston is already past it's optimum compression so you are not get the "best bang" out of the expansion and it still may be expanding when the piston is travelling up again on the exhaust stroke - so crappy burn and has to continue to fight it's way towards exhaust.

Either situation would rob the engine of it's full power potential...

Well, here's what I found: I was reading the dang spark timing all wrong. Turns out, you have to read it not at the arrow stamped into the block, but rather through that window in the plastic flywheel cover. The timing is spot on 5deg BTDC. Also found out why the RPMs showed 2x as high. These are the same coils they used on the 2strokes. They fire every other revolution instead of once every 4 like a true 4stroke. So indeed, the sparks were doubled. I set it to about 1700 (850 real rpm). Now I still had that damn bog off idle. I took the carb back apart and realized that the accelerator pump plunger was stuck. I took a screw and screwed it into the plunger and managed to pull it free. I then cleaned the plunger and the cylinder it slides in. Now, it's all smooth. There was a bunch of old rusty looking crust in there. Now the only issue I have is the ripped plunger. I've called literally every dealer in my state and nobody stocks this item. Looks like I'll have to go through eBay to get it. There is a big hole around that plunger that goes right into the carb. It would be leaking air and gas if not sealed. Dumb design if you ask me...

That plunger is likely the equivalent of a choke butterfly and is fairly cheep to fix,. I tjink I paid about $25 for the kit on my 2 stroke. A lot of four stroke engines fire every cycle, Chevy cars still might

Well, I'm disappointed with this motor... I rebuilt the carb, cleaned every passage and orifice, and I still get a little bog off idle. In fact, it idles rougher now. It shakes and shimmies at 800-850rpm. I have it set to 900, but it's still bad. Did these come with an idle air screw? There is an orifice on the side that looks like where it would go, but there is no screw in it. I didn't take anything out. Putting my finger over it doesn't seem to do much, neither does spraying carb cleaner in there. My spark plugs seem okay, as the timing light doesn't seem to skip any sparks, and the tips don't look bad, just a little brown on one side. How do I measure the output of the fuel pump?

If it has an old gas line, that could be the problem. The new gas with ethanol eats the inside of the older gas lines and plugs the lines. The inside of the gas line will also separate from the outside and get sucked in and pinch off the gas.

I looked at one parts diagram for a 25HP 4 stroke and it showed an electrical solenoid on the enrichment valve? Does your engine have this as it might be stuck or defective

JeremyB said:

I looked at one parts diagram for a 25HP 4 stroke and it showed an electrical solenoid on the enrichment valve? Does your engine have this as it might be stuck or defective

Click to expand...

Hmm, indeed it does have an electric choke. I did the test in the book and it passes the ohm test, but when I hook up pos/neg to it, it doesn't do anything. The fact that both wires are yellow also is strange, as I would think one would be black... Not sure how that works honestly. I've seen electric chokes before and this doesn't look like such a system.

It might need replacing, I would bet if the electrical part doesn't work, then it is the same as having the choke on all the time

there is an idle adjustment screw- but it might be behind a brass plug. Indeed it could be running lean, you have to get the plug out, then adjust the screw.