Smooth running engine only gets to 3400RPMs and 14MPH

sshotrodss

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SOLVED: !!!! when in doubt - check the easy things !!!!
Wish I could delete the thread - a little embarrassed but perhaps it will help someone

Prior to posting I read a tone of threads for those who appeared to have the same issue however they all appeared to start at a higher RPM and thus just needed a prop fix or their engine had issues like sputtering, dying, missing, cutting off and thus often needed electrical fixes (some fuel).
Engine: 1995 70HP Force by Mercury Marine, 2-stroke.
Boat: 20 foot pontoon (4 adults)
History: Purchased the boat having only started the engine out of water (hose hooked up). The engine ran fine and seemed to rev up fine. Seller indicated that he put a new fuel pump on it and other new electrical components - he said all but the starter and carb were basically new. He said he did a compression check and all readings were good.
Before putting on the lake, cleaned carburetor (appeared clean to begin with), removed and cleaned fuel cell with acetone (aluminum tank with ventilation – appeared clean to begin with), replaced all fuel lines and the filter, and put in new spark plugs. Out of water and in gear the motor will head right to 7000RPMs with the throttle lever fully forward. Out of water it appeared to run fine through the full RPM range.
At the lake it started fine, idled fine, and seemed to run smooth, sound smooth however it only got up to 3100RPMs and 8MPH with the throttle lever fully forward. Based on this forum and talking with boat shops changed out the 10.5X13.5 prop with a 12.5X8 prop as a first course of action. At the lake this yielded 3400RMPs and 12MPH. Motor still ran fine – purred along at 12 mph. Drove it around for 45 minutes (no change); would re-start with the flick of the key. For a 2 stroke no real noticeable exhaust – nothing abnormal.
Checked the throttle cable- appears to be the right one and moves the carb shutter the range I'd expect - I could be wrong though.

At this point I will begin rechecking what I can in regards to the fuel pump and electrical elements. Most likely beyond a visual inspection I’m going to have to take it to a boat shop eventually. Any advice on what the culprit may be for this do-it-yourselfer…..
 
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Chris1956

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Mar 25, 2004
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Re: Smooth running engine only gets to 3400RPMs and 14MPH

Gee, running that motor at 7000 RPM with no load will eliminate any need to find out your issue. It will just throw a rod. Don't do that, it proves nothing. Any idea what speed/RPM you got with the 10P prop? Normally you would expect to see 3800RPM, and a speed increase.
 

sshotrodss

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Re: Smooth running engine only gets to 3400RPMs and 14MPH

...running that motor at 7000 RPM with no load will eliminate any need to find out your issue... Any idea what speed/RPM you got with the 10P prop? Normally you would expect to see 3800RPM, and a speed increase.

The 10.5 inch - 13.5 pitch yielded 3100RPMs at 8MPH. The 12.5 inch - 8 pitch yielded 3400RPMs and 12MPH. On the 7000 about a second on that one .. put the hose on, started it put it in gear, made sure the prop was turning, took in up slowly to 4500, then 'goosed' it fully forward and it took off. Now I feel kind of foolish - I'm a car guy and with V8s if one plug isn't firing then you notice immediately at all RPM levels, in gear or out of gear, parked or on the road. Took three other guys on the boat (both boat owners not that that matters) and they thought it ran smooth and consistent. Took the spark plugs out today and the top two cylinders showed signs the spark plugs were firing ... the bottom one looked brand new and had only a nice 'blue' gas on it. I'll now have to read up on why it may not be firing plugs.jpg.
 

sshotrodss

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SOLVED: Smooth running engine only gets to 3400RPMs and 14MPH

SOLVED: Smooth running engine only gets to 3400RPMs and 14MPH

It appears a 3 cylinder 2 stroke can run smooth on 2 cylinders and the result is what one might have thought lower RPMs and MPH (less power). One of the three Coil packs was broken and you couldn't tell as it was covered by a rubber boot. I trusted the seller a bit two much when he said he went through all the electronics and replaced most everything.
 
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