120 mercruiser wont pass 2500 rpm ?? help please!

[recycledboat]

Hello all,
I have a 1989 mercruiser 120 2.5l gm that wont pass 2500 rpm . My local
mechanic said it sounded like points, so i installed a pertronix ignitor ii kit
because i hate messing with points. now it runs better , but still only to about 2500 rpm . If you throttle past that it all goes downhill. I took the flame arrestor off just to see if it was getting enough fuel at 2500 rpm on the water, & when you throttle beyond that point vapor actually comes out of the carb . runs great in driveway to 4800 rpm or so hooked to garden hose.
This year doesnt have an ignition module & new plugs didnt help either? Any suggestions would be greatly appreciated
and might bring you good karma or something like that.
Thanks
Damian

 

[4winnsskipper]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

compression test, and change plugs. but im thinking fuel issue

 

[recycledboat]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

I havent done a compression check, but no smoke and very low hrs. I did try plugs and no real change. What fuel possibilities? weak pump??
Thanks 4 winns

 

[Don S]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

Here is a list of possibilities.

 

[recycledboat]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

thanks Don S for all of the ideas. There are several I had not thought of and could very well be it!

 

[Don S]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

When you get to testing the boats fuel system, which is a common problem for this type problem, try this.

HOW TO: Test your boats fuel system

 

[Lyndy]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

I took note that the Engine was run up to 4800 RPM with Water Hose connected. Is that not a good thing to do or is it actually OK thing to do?

 

[Haut Medoc]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

As long as the hose doesn't collapse......
I wouldn't do it for any extended period anyway....
You can rev it all day long with no load & it won't tell ya anything.....​

 

[recycledboat]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

Thanks everyone, I am ordering a vac / pressure gauge today and plan to work on it this weekend. I will post results! The 4800 rpm was only for a burst while checking throttle...Probably still a bad idea though.

 

[Don S]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

You really don't need to order some special gauge, just pick one up from your local auto parts store. You will also want to get some fittings and hose to adapt the meter.

 

[robjen1019]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

Same exact problem. I went ahead and replaced my points with an ignitor II ignition and a flamethrower II. I set the timing as stated in the manual and I lost 700 rpms off the top end (idled like a dream). I got so fed up, I finally went out and bought a new set of points and it ran like it used to. I think it has something to do with the timing. What you could try is to take it out and runn it at full throttle and have someone adjust the timing as you are going to see if it will pick up.
I would try it, but I had to send my initor II in becuase i burned it up. rotor caught it and shorted it out. Make sure you tape those wires down.
If you ever figure it out. Let me know. Im going to experiment more once my replacement comes.

Good Luck

 

[Coors]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

I don't think that trying to squeeze the last 700 rpms out of your engine equals longitivity.

 

[recycledboat]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

could a 4 blade prop have anything to do with this? It is a 14.5 x 18 p
prop and it was on the boat when i got it. I also should give a little history,
the boat did not run when i got it , it had a cracked block center of the middle two pistons and blown head gasket. I pulled and replaced the engine complete with everything including exhaust system from a 1989 low hr boat.
the prop has me wondering because the original would have been a 3 blade
and i dont know the history as to wether this prop ever got that boat on plane prior to my owning it.
I did the exhaust check and found no rags or anything in system. Also bought a gauge and am currently trying to round up the tee and adapters.
Thanks guys!

 

[sabastianunf]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

I have a 120 hp mercruiser with a 16" pitch 4 blade cupped prop on an old 18' cuddy cabin that is currently hitting 4000 rpms at wot. I think that you prop is part of the problem but there is also some other issue going on. Your problem sounds simmilar to what I ran into when my fuel filter would get clogged by sediment from my old fuel tank. Have you replaced your fuel filters lately and have you had your carb rebuilt lately?

 

[recycledboat]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

Hello Everyone! I have an update... Always make the last thing you would check , the first thing you do check! I ran a compression check today and
have no compression on the middle two cylinders.. Just so everyone knows
a 4 cylinder will run suprisingly well under these conditions! I would have never guessed it. I ordered gaskets and guess i will be pulling the head as soon as they get here. I will also be taking the head to be checked out. I really do appreciate everyones help up here and hope if anyone has suggestions they will post them and maybe save me a mistake. Don's checklist is now my computers wallpaper and has been for a while now.
compression was the one thing that I really ***-umed to be fine. Live and learn! Thanks again everyone!

 

[recycledboat]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

ok , here goes the next chapter. The compression readings from front to rear are 120-0-0-90. If i put oil in the middle cylinders I get 30-40 , but only till the oil goes away. I called the guy I bought it from and he said that he thinks the rings are stuck. He said it sat for a couple years, but was winterized by him (mechanic) and maintained by him. He said he really couldnt believe it would be something major. He told me to try shooting penetrant
or wd-40 in the cylinders and bump it... repeat....repeat .. Anyway, Has anyone here had this before ? any recommendations ?? I have ran this boat on the lake and they havent broken free . This is why people buy NEW boats! Thanks again!
Damian

 

[bjcsc]

Re: 120 mercruiser wont pass 2500 rpm ?? help please!

This is why people buy NEW boats!

Maybe, but it's definitely why people check compression before they buy.

30-40 with oil doesn't sound like rings to me...and 90 is no good either. If you're sure you checked it properly, you may consider a leak down test or taking a look under the head given the adjacent cylinders...