RPM Issues (Among other things)

[kmuir2108]

First time boat owner here. I bought a 1984 Mercruiser 120 IO on a 17' Glastron. The engine runs very good and we set the idle rpms to 500. My problem is I cannot get any WOT rpms.

It seems to max out at around 1800 or 22 mph. When I trim a bit I can get maybe 24. I have alot of trouble getting it to come out of the hole due to the lack of power. Takes a good minute. It did have old gas which I syphoned out and replaced. Checked the cylinders and all are operational reading 110-120. My oil pressure reads around 50 at 1800 rpms but dies down to almost zero when trolling at idle speed. I pulled the engine to replace the coupler over the winter and replaced all bellows etc. Aligned the engine, replaced the spark plugs, checked the points. Checked fuel filter and it looks clean. Put carb cleaner through it. Would rebuilding the carb make a big difference? Any suggestion would be appreciated.

Also if anyone can suggest a good tune up mechanic around Winnipeg, Manitoba???

Cheers.

 

[kmuir2108]

Re: RPM Issues (Among other things)

Thats a good question. I took it off and it has no #'s on it anywhere. How do I measure it?

 

[Don S]

Re: RPM Issues (Among other things)

Props have nothing to do with rpms only reaching 1800 rpm. Here is a list of possible causes. If it ran right before, and now it doesn't, then you can eliminate things like wrong gear ratio or wrong prop. And with 1800 max.

Engine Won't Reach Operating RPM. Check

• Fuel condition. Type and Octane possibly old fuel
• Propeller pitch or diameter, damaged blades
• Restricted fuel pickup tube or anti siphon valve Fuel System Test
• Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
• Marine growth on hull and outdrive
• Wrong gear ratio in outdrive
• Restricted carburetor air intake (clogged flame arrestor)
• Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
• Poor cylinder compression Compression Test
• Carburetor defective, or wrong type.
• Fuel pump pressure and vacuum
• Boat overloaded, improperly loaded, or improperly trimmed.
• Engine Overheating
• Engine timing and ignition system operation
• Remote control cables and linkage for proper travel to open throttle plates fully.

 

[Coors]

Re: RPM Issues (Among other things)

What Don said, then his:
http://forums.iboats.com/showthread.php?t=167561
Then to carb; Start trouble shooting by elimination.

 

[Coors]

Re: RPM Issues (Among other things)

And swap in a temporary mechanical oil gauge, then your helm unit is wrong.

 

[Don S]

Re: RPM Issues (Among other things)

What Don said, then his:
http://forums.iboats.com/showthread.php?t=167561
Then to carb; Start trouble shooting by elimination.

I already posted that link, read the 3rd item of the list (Fuel system test)

 

[Coors]

Re: RPM Issues (Among other things)

Missed that, Don, you have made the perfect check list.
But people seem to want to skip through it......

 

[kmuir2108]

Re: RPM Issues (Among other things)

Thanks Don S! Here is what I have done and will do.

1. Fuel condition. Type and Octane possibly old fuel - Replaced Fuel with regular
2. Propeller pitch or diameter, damaged blades - Will have to get it tested
3. Restricted fuel pickup tube or anti siphon valve Fuel System Test - Not sure how to test this. Did replace the fuel line to the filter. Will check the manual.
4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse - Oil level shows good on dip and was replaced
5. Marine growth on hull and outdrive - boat is clean
6. Wrong gear ratio in outdrive - Not sure how to test this or how to find out. Again, will check the manual
7. Restricted carburetor air intake (clogged flame arrestor) - Will rebuild the carb
8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive - I will check what this is in manual
9. Poor cylinder compression Compression Test - tested and cylinders read from 110-120
10. Carburetor defective, or wrong type. - I believe it's stock so I will rebuild
11. Fuel pump pressure and vacuum - will check. Not sure how to test
12. Boat overloaded, improperly loaded, or improperly trimmed. No overload. Installed new trim/tilt sensors and cables and set as per mfg instructions
13. Engine Overheating - Guage does not work but seems to be fine. Checked for water circulation and OK.
14. Engine timing and ignition system operation - Seems to run well. Will check timing. Points were set and plugs replaced and gapped.
15. Remote control cables and linkage for proper travel to open throttle plates fully. - Checked carb at full throttle and plates were fully opened. Set linkage as per manual.

Sorry if my mechanical terminology is not up to par as it is my first boat. Will update as I try the rest of the list.

 

[novicetech]

Re: RPM Issues (Among other things)

How much time have you had the boat the water? You say you siphoned all of the old gas out of the tank are you sure you got it all out? Have you cleaned the fuel filter in the glass bowl? Removal of this bowl could tell you if you have any water in your fuel.

Good luck!

 

[kmuir2108]

Re: RPM Issues (Among other things)

I ran the boat for probably a total of 12 hours on the lake over the weekend. Burned about 1/2 tank of fuel. Not sure if there is a different filter or not but I don't remember any type of a glass bowl at the engine side. Is there another one in the tank? I bought the boat last year and the first time out the coupler blew up in about 10 minutes on the lake. We rebuilt it over the winter so this was really the first time out for any extended period of time.

 

[kmuir2108]

Re: RPM Issues (Among other things)

Ok! As I'm working my way through the list, (Carb rebuilt, checking fuel pump) I found out what prop I have. It's a 14 1/2x21. Any suggestions if I should go to a 14 1/2 x 17 to get out of the hole faster?

 

[Maclin]

Re: RPM Issues (Among other things)

Figure out the engine problem first. The 21 is ok for now, most 17 to 19 footers with the 3.0 can and do run a 19-21 pitch. Going from a 21 to a 17 is a BIG jump and should not be necessary in my opinion. I would spend the money, time, and effort on the engine debug.

 

[chiefalen]

Re: RPM Issues (Among other things)

go thru the list one by one.it's tedious but must be done.or u can throw money at it and still not find the problem.

 

[kmuir2108]

Re: RPM Issues (Among other things)

Thanks guys. It was #2 on the list so I just wanted to make sure it is the right sized prop for this boat. I was not sure if it was just put on and sold or if it was the proper one. I will leave it for now and maybe buy a 19 as a backup.