Trojan Boat Forum

My port engine was not running right at the end of last season so I had both engines tuned recently. Now they both run smoothly, but the port engine goes to 2,600 RPM and no further. I push the throttle to full open, but anything past about 3/4 position gains no speed. The starboard engine speeds up normally and I can hear the 4bbl opening. I never hear the 4bbl open on the port engine.

When traveling this weekend I synchronized both at 2,600 and the fuel usage seemed to be the same.

I have Merc 255s and am searching for advice on where to begin to look for the problem.

Can you switch tanks to the engine? If so does it still do it on the other tank fuel supply?
I have seen that as a symptom of lack of fuel flow.... partially blocked fuel pickup or anti syphon valve.

That was one of my first thoughts and on the way back yesterday I did that with no effect.

When you synched the engines at the same speed, were the throttles advanced to the same position or did you have to advance the port one further? I have had a similar issue in the past that turned out to be insufficient advance in the distributor due to corrosion. If the throttles are in different positions to get the same RPM, it could be an advance issue.

They were basically in the same position.

check for sticking linage on the carb , as you said you could not here the 4 bl kick in .

I checked that also on the way back and the linkage is free.

I read in the Merc manual last night that it has a dual diaphragm fuel pump that has a sight bowl mounted to the side. The manual said that if there is any gas in the sight bowl the pump is bad. It sounds like it is possible for the pump to be pumping on just one diaphragm which would allow the engine to continue running.

I am assuming that these dual diaphragm pumps are similar to the ones that the company I work for makes for the chemical industry. If one of the diaphragms is bad the pump keeps pumping but at a reduced flow rate. Could that be what I am experiencing? I didn't look at the fuel pump while at the marina and didn't read the manual till I got home. I have never had an engine with a dual diaphragm pump before that I am aware of. Usually if the fuel pump is bad the engine won't run at all.

the Merc pump you're talkin about will keep you goin and the engine will not come up to full rpm, it'll drive you nutz.... if you have the small screen filters in the carb gas inlet check it for small pcs. of black rubber....

Thanks Tony - sounds exactly like what I am experiencing. I called my mechanic and he is going to check the pump this afternoon to see if it is bad.

no biggy I hope it's that simple....

Our neighbor at the marina had the same problem you had. Both his fuel pumps were bad as he had fuel in the glass bowls. Fuel pumps are an easy changeout. They are about $185 each though. Don't try to rebuild them. It's not worth the aggravation. 30 year old boat needs new fuel pumps. The other issue definitely could be spark advance. Sometimes the weights and springs get rusty and hang up and won't advance the spark causing the spark to be late at high rpms resulting in low power. A little WD-40 and manually moving the springs and weights might free them up and that might last you for a a while but the distributor really needs to be taken out and cleaned and possibly replace springs with new.
Damn boats just suck money out of you but they sure are fun. That's why my wife calls ours the mistress of the sea.

Thanks for the info Tom.

My mechanic was going to go check the pump either late yesterday afternoon or first thing this morning. He hasn't called yet. I am hoping it is the fuel pump. I told him if it is bad, I am replacing it on both engines.

The former owner made the switch-over to electronic ignition and he owned the boat for only ten years before I bought it almost three years ago. When the mechanic did the tuneup, he said everything looked good from that perspective but he didn't check the fuel pump because it had been running fine.

The only other thing I can think of would be the carburetor may need to be rebuilt.

Well the fuel pump was bad and will be replaced this week. Hopefully that will solve the problem. Thanks for all your help.

Please let me know what you find, i'm having the exact issue. Your supposed to see gas in the bowl when its bad, but that i'm not seeing. Doesn't mean its not bad.

Let me know, very interested.

Mine had no gas in the bowl but a gasket had failed and it was sucking air.