Eng. Not getting the right R.P.M.'s

[tywos]

I have a 1966,Merc 650 (65 hp) S/N 1897689 motor, with a 14in. 15Deg. prop on it at the moment.<br />Installed my new Tiny Tach. and went WoT for 3850 rpm's. manual say's i should be more around 5200 rpm's. Could it be i need a smaller Deg. prop.? or maybe i should check compression of my cylinders. Eng. seems to run fine, no missing or hesitation that i can detect.<br />Any suggestions would be greatly appreciated.

 

[Hooty]

Re: Eng. Not getting the right R.P.M.'s

The props we deal with aren't measure in degrees. That 15 you're refering to is 'pitch' and is measured in inches. In other words that 15 is 15" of pitch. It means that if the prop were in jello it would move forward 15" in one revolution.<br /><br />It might be that your prop has to much pitch or it might be a problem with the way the boatis rigged or it might be an engine problem. You're probably on the right track starting with a compression check. It's as good a place to start as any. <br /><br />A word of caution though. I wouldn't continue running your engine like that. If the prop is oversized and the engine is lugging, (working to hard at that lower rpm) it'll destroy it.<br /><br />c/6<br />Hooty

 

[rickdb1boat]

Re: Eng. Not getting the right R.P.M.'s

Unless that motor is on a large boat, the 15" pitch prop should get higher than 3800 RPM's. If it seems to run well and has good top speed, I would be suspicious of the Tiny Tach set-up. Re-check your tach set-up and make sure you're on the correct wire. Cheking compression would be a good idea to prove that the motor is in top shape..

 

[RICKRICK1]

Re: Eng. Not getting the right R.P.M.'s

TYWOS--FIRST GET A MANUAL. Can not stresssssss that enough. Check your compression. Next check for proper link and sync of carbs to throttle movement. If this is good, rebuild your carbs with new needle and seats, rebuild the fuel pump, and check timing.<br /><br />I have a 67 65hp and had some of the same problems. I found out that my carb to throttle linkage was not right, after I rebuilt carbs, fuel pump and timing. Also if you have any build up on the carb filters it will starve the motor of fuel.

 

[quantumleap]

Re: Eng. Not getting the right R.P.M.'s

Are you saying that the prop is a 15 pitch with a diameter of 14"? If so it sounds like the prop diameter is too big for a 65 horse motor. My 125 uses a diameter of 12 3/4 to 13 1/4 depending brand.

 

[mercuryblues]

Re: Eng. Not getting the right R.P.M.'s

ok guys this boat is sort of mine and tywos.The motor is a little cold starting but after initial start it runs like a top,no hesitation or miss.Like he said according to tiny tach we are getting 3850 at wot and also 27 mph and this is in a 15 ft v-hull fiberglass boat.Does it sound possible that we are only getting 3850 rpm's and still that speed?Also one more question,there is an air vent fitting,little plastic piece,that is on the magneto/distributor housing.Can anyone tell me what the function of this is/what it is,etc.The reason I ask is because on this motor the rubber hose is connected to the fitting mentioned above but it does not go to anything else it is just hanging out of the little hole the control cables come in through to open air and it just does not look right to me.I do have a clymer manual and I do read it continuously but I am not a mechanic and and I have seen this air vent mentioned but no diagram of vacuum lines etc to tell me where it should go.Thank you all in advance for your time and information.

 

[bryan1]

Re: Eng. Not getting the right R.P.M.'s

glad to see you got your comp. working again MBlues.<br />i think your prop is a bit big. and as far as a compression check ..that's something that i feel needs done once every year.<br />keep us updated on the check. hopefully they are within a few psi of each-other, if not isolate the low ones and go from there.<br />GL.<br /><br />Bryan1

 

[rickdb1boat]

Re: Eng. Not getting the right R.P.M.'s

Prop selector shows a 13" X 17" for that size boat. I think you are right in the area where you need to be as far as propping. I had a 1967 15' Wellcraft with the a 67' 65 HP merc. I ran a 13 X 15 on mine and it was a heavy boat. You are losing RPMs for some other reason than the prop. I think your best bet is to make sure you have real good fire on all cylinders, check the compression, Have a good strong battery(As they need all they can get to start them) and do the lync and sync. I would at least pull the carbs and clean them out, if not put rebuild kits in them anyway. I ran about 33-34 mph on my boat with the 65 HP @ 5300 RPM's if that is any indication. I don't recall the plastic tube, but it does sound familiar now that you mention it. Wish I could help more, but it has been along time since I had that motor. Maybe RICKRICK1 could help you with the tube.

 

[RICKRICK1]

Re: Eng. Not getting the right R.P.M.'s

I would bet you are not getting a proper reading from the Tiny Tach. I had one and it fluxuated constantly replaced it with a magneto type RPM gauge used on automibles (older hotrods all had magnetos). If you are pushing that boat at 27 mph you are about right. Memory of Merc Manual states 27 to 32 MPH with a 15 pitch prop. If your link and sync is good you are about right where you need to be. On my fiberglass 16ft bassboat I run 32mph at 5300 rpm. 15 pitch x 13.25 prop. The air tube connects from the magneto to a fitting that snaps into a fresh air box located at the front base of the cowling. If you have pulled the front support this air box is where the lower mount nuts are, the fitting slipps into a hole just left of the mount bolt on the magneto side. I dont think you have anything to worry about if its not connected.

 

[mercuryblues]

Re: Eng. Not getting the right R.P.M.'s

Thanks alot everyone who replied.I will do as instructed and post results of compression check as soon as I have them.