1986 Johnson 90hp Rev Limit?

[poop]

Hey guys.....Love the Forum. Allright here is my problem this month. I have a 90hp Johnson {86'}, yesterday the water was complete glass on my way back toward my channel. So I thought I would see what she had and run wide open for a few. When I ran wide open I was at 36 - 37 mph on my GPS (Garmin) and 5500 rpm on the Tach and right away I started noticing a slight drop in Rpm's every couple seconds. Not like a fuel bogg situation but just a small resistance feeling and then right back up to the 5500rpm.( a loss of 2mph or 100-300 rpm for a 2 seconds or so) IS this my motor hitting its rev limit?? Maybe my tach is off because I thought my rev Limit was like 6000ish. Please note that the fuel pump,carbs,hoses,plugs,wires etc...are all new....within 2 months. If I backed down just a little for example to 5200rpm it ran fine no slight or random cut in speed.<br />I look forward to hearing from our FORUM. Thank you in advance....

 

[reeldutch]

Re: 1986 Johnson 90hp Rev Limit?

5500 is not bad.<br />what kind of boat and prop.<br />what are the compression numbers.<br />dont worrie 5500 is not close to overreving.<br />would try to get 5800.<br />how does it sit on the transom?<br /><br />reeldutch

 

[poop]

Re: 1986 Johnson 90hp Rev Limit?

-reeldutch.....<br />16 foot 10" Flats boat. Transom is straight across and the motor mounts on that. compression as of last week was 120psi on all 4. Which was amazing. The prop is stainless 3 blade. I do not know the pitch and size...I will find out. Thanks.

 

[G DANE]

Re: 1986 Johnson 90hp Rev Limit?

As I remember the 90 HP Crossflows had no rev limiter. Be sure it gets the fuel it needs or problems can occur. How does the plugs look like ? Was the carbs cleaned recently ?

 

[poop]

Re: 1986 Johnson 90hp Rev Limit?

G Dane....<br /><br />fuel pump is new...the carbs cleaned and rebuilt with factory kit (OMC) from my local mechanic. the plugs are only 2 months old and when I pulled them they just looked a little dark but they all looked the same color. Dark as in oil/carbon. They are the flat end Spark Plug, with no gap to set, recommended by a local dealer. I did have a fuel issue last month and i'm wondering if maybe that roasted either a coil/plug or whatever.....but it seems to idle/start/get up fine. Sunday I am going to check the compression again.

 

[G DANE]

Re: 1986 Johnson 90hp Rev Limit?

Sounds good. I would check that every hose clamp is tight, and no lines cracked ( rare though ) Tank vent and tank pickup filter/screen. Will squezing primer bulb make the high speed surging better/worse.<br /><br />Good you have had your carbs done - reason for most one cylinder failures. <br /><br />Can you tell a little more about the fuel issue ?? and do you have an inline filter.

 

[reeldutch]

Re: 1986 Johnson 90hp Rev Limit?

Try running wot with a portable 6 gallon tank and fresh 87 (50:1 if you dont have vro)let us know if it still hapens.<br /><br />reeldutch<br /><br />but everything sounds good

 

[poop]

Re: 1986 Johnson 90hp Rev Limit?

I was running it with a brand new 6 gallon 50:1 with fresh 93 octane.

 

[poop]

Re: 1986 Johnson 90hp Rev Limit?

Well......ran the boat today. The prop size is 13 3/4 x 15 stainless. It seems to run fine but max rpm not matter how I trimmed it up or down just varies between 4800 (trimmed down) 5200,5300 rpm porpoising (trimmed way up. People say I should be able to get 6000 out of it....but I can't. SHould I just leave it alone because it seems fine? Compression on a warm engine today was <br /><br />[120] [115]<br /><br />[120} [115]<br /><br />Please advise oh great masters of the boating world..

 

[Dhadley]

Re: 1986 Johnson 90hp Rev Limit?

First start with the right fuel. 87 is whats recommended. Then lets look at the tach. Be sure its set on position 6. With that small of a boat you should be running a much larger prop. <br /><br />Also look at the power packs and tell us about any and all numbers on them.<br /><br />While your at it make sure it has Champion plugs in it.

 

[G DANE]

Re: 1986 Johnson 90hp Rev Limit?

Not being one of the great masters, I think your tach is accurate, at 37 MPH at 5500 RPM with a 15" pitch, you would have 8 % slip, sounds reasonable. If your boat isnt specially heavy , you ought to be able to turn a 17" up to 5800, I think. Compression is brilliant.

 

[reeldutch]

Re: 1986 Johnson 90hp Rev Limit?

5300rpm at wot is not so good.<br />dont leave it like it is i am a little puzzled but lucky for you, dhadley is on your case.<br /><br /><br />good luck

 

[poop]

Re: 1986 Johnson 90hp Rev Limit?

I am very glad Dhadley is on it, he has always helped my friend Keysrat. But I also get a lot of help from everyone. I think I need to run 87 Octane like Dhadley said.Also I am using no gap champion plugs that went through a bad carb problem when I could only plane up with the choke pushed in until I rebuilt the Carbs. Even though the plugs are only 3 months old they might be toasted. Also.....I have not changed the coil packs but after using a spark testing tool it seems as if all is well....but who knows what its doing at 5500rpm's. I still have not changed the port side head gasket and water jacket seal. The starboard side has been replaced due to an overheating issue. I will keep on rebuilding and posting then hopefully I will post some pictures of this restored project.<br /><br />Thanks to all.....

 

[Solittle]

Re: 1986 Johnson 90hp Rev Limit?

No need to replace the port head gasket with your compression.