Twin 115 mercurys won't rev over 3800 RPM

[brogers]

I have recently purchased an 18 foot Hydrodyne boat with twin 1971 115 mercury engines. Both motors star fine, idle fine, and pull strong up to about 3800 rpm but will not go over 3800. Speed increases slightly if you back off the trottle a little so there not quite wide open. Main jets look clean but are .066. The service manual I purchased says they should be .072 below 2500 feet elevation. Could this be the problem? The boat came from the Los Angeles area but is now in South Dakota (alt around 1800). They have 21 pitch props, oil is mixed 40:1. I have installed an 18 gallon fuel tank in the bow and run two 3/8 inch fuel lines with electric fuel pumps. Tank has a 5/8" vent hose. Is high fuel consumption normal for these engines?

 

[Solittle]

Re: Twin 115 mercurys won't rev over 3800 RPM

Welcome - Not a Merc guy but I think your motors are the 6 cyl jobs so you got 12 cylinders thirsty for fuel so - - <br /><br />I'm not comfortable on commenting on the jets but a couple of questions. Have the carbs been overhauled? Crud will do a number on carbs and you have a bunch of carbs. Also how old is the fuel? Could be a factor.

 

[Hooty]

Re: Twin 115 mercurys won't rev over 3800 RPM

I'm wondering if they're not over-proped? Try putting a couple of lower-pitched props on and see if the revs come up. That's an easy fix. Let us know the results.<br /><br />g'luck & c/6<br /><br />Hooty

 

[jim dozier]

Re: Twin 115 mercurys won't rev over 3800 RPM

Since both engines are pulling the same low rpms it seems unlikely that a dirty carb or hinky ignition is the culprit. I think Hooty's right, it probably something common to both engines, like the prop or engine height/trim. Are these the original props?

 

[Gold Bear]

Re: Twin 115 mercurys won't rev over 3800 RPM

brogers <br /> <br />"Speed increases slightly if you back off the trottle a little so there not quite wide open."<br /><br />With both engines shut off pop the cowls and check the setting of the throttle stop screws. Move the throttle levers to WOT then check to see that the throttle stop screws are set so the carb butterflys are straight in-line with the bore of the carb - offering maximum air flow.<br /><br />I agree with the other posters that you are probably over propped. This is just something else that you should check.<br /><br />Good Luck,<br /><br />Gold Bear

 

[brogers]

Re: Twin 115 mercurys won't rev over 3800 RPM

changed both props from 21 pitch to 17 pitch. Wide open trottle RPM did not change. Still about 3800

 

[kenimpzoom]

Re: Twin 115 mercurys won't rev over 3800 RPM

Could one engine be bad and not be pulling hard and thus keeping the good one down?<br /><br />Are they too far down in the water?<br /><br />Did the previous owner have the same problem? Has it always been this way, or is this something recent.<br /><br />How much throttle do you have to give to get 3800 rpm? Does half throttle get you to 3800 rpm, and then the last half throttle do nothing?<br /><br />Ken

 

[Hooty]

Re: Twin 115 mercurys won't rev over 3800 RPM

Now that's strange. The only other thing that I can think of that's common to both engines is the fuel. <br /><br />I know you're not suppose to do it but see if they'll blip higher then that 3800 rpm out of gear, in the water, of course.<br /><br />c/6<br /><br />Hooty

 

[Boatin Bob]

Re: Twin 115 mercurys won't rev over 3800 RPM

Wow....I find it very hard to believe that 4 inches of pitch did not change the RPM's, do you have 2 tachs? I would have expected the RPMS's to jump up approx 200 per inch, did the diameter of the props stay the same?

 

[brogers]

Re: Twin 115 mercurys won't rev over 3800 RPM

UPDATE: Throttles are opening straight in line with the carb bore. I don't know the history on this boat as it was bought at an auction. Was owned by the Boy Scouts. Throttle must be wide open to get 3800 RPM. Will bury both tachs in when not in gear VERY QUICKLY! New props are a little larger in diameter than the old ones but I think 17 pitch should be over reving. Running at wide open throttle if you squirt gas in the carbs it makes no difference. Also under full load if you pull plug wires off all cylinders drop RPM. Can't believe how well theses engines start and run but can't get the RPM.

 

[Hooty]

Re: Twin 115 mercurys won't rev over 3800 RPM

If you've got a good prop shop around, put this in their laps and see what they can come up with.<br />I'm stumped!<br /><br />c/6<br /><br />Hooty

 

[SCStripers]

Re: Twin 115 mercurys won't rev over 3800 RPM

I dont have dual engines but i did have the same style problem with one engine, 78 85hp evinrude. started and ran great, went to wot throttle and had no real good rpm. As I was in wot and i would pull back on the throttle i would get about a 2 or 3 burst of rpm! checked the plugs and one was always very wet, changed plugs all that crap...still ran slow..Here is what I did...i completly tore down the carbs and flushed them out, cleaned floats and all. Now it runs at full speed. If you just bought the boat and have no idea how long the gas was sitting in those carbs I would clean them. Just the smallest eye lash sucked in could screw things up. Just my 2 cents worth, good luck and let us know