Rpm for my 78 85 hp Johnson

[clemsonteg]

I have a 1988 24ft pontoon with a 1978 85hp johnson motor. In neutral it will rev to 5500 rpm but I notice when we are going down the lake it only gets about 3300-3500 rpm. Is that just because of tHe load trying to push the boat or should I be concerned? I currently have a 13 3/4 x 15 prop and plan to go to a 13 3/4 x 13 ( recommendation from the local boat shop) this week as well as my prop is a little dinged up on the edges

As of today:
Compression check showed 110 psi on average. Spark tester shows that all coils are firing well over a half inch gap. The carbs were rebuilt before the beginning of the season. With it out of the water it will rev in gear to 5500 with a set of muffs on. All sparks plugs were replaced today. One was a little wet but the other 3 were dry and brown

 

[jayyyy]

Re: Rpm for my 78 85 hp Johnson

You should'nt rev that engine past two or three thousand RPM in neutral you can scuff a piston doing that. Your boat probably is not meant to plane, four blade low pitch props work good in your application. A 13 or even an 11 pitch should get your RPM's up. Figure about a 200 RPM increase with each inch decrease in pitch.

 

[nwcove]

Re: Rpm for my 78 85 hp Johnson

if you like your motor.......dont rev it without a load on it! id go with a smaller diameter prop , with the same pitch. it depends on what you want.....top end speed.....or accelleration.

 

[clemsonteg]

Re: Rpm for my 78 85 hp Johnson

Thanks for the info this is my first boat so it's good to have. As far as what I want from the boat I use it just for cruising the lake. It's a pontoon boat so I know it won't be a speed demon or be able to pull a skier up. My main goal would be decent fuel consumption and maybe around 20 mph if possible. Right now I think I'm around 16 mph and I can go around 12 miles on 6 gallons of fuel. Overall I just want the boat to be reliable so I try to keep up on maintenance. I installed a tiny tach because the guy at the boat shop said to re-prop it I should know what I'm running as far as rpm. He mentioned the 5500 rpm so when I couldn't get more than 3500 rpm I got a little worried. The motor fires up great and runs smooth so I couldn't really figure out why it would run a lower rpm unless there was something I was missing or if the load was just being pulled down due to trying to push a 24ft pontoon boat with small toons on it

 

[Solittle]

Re: Rpm for my 78 85 hp Johnson

You do have a problem. The 5500 rpm is for wide open throttle under load. I'll bet you are dropping a cylinder.

 

[clemsonteg]

Re: Rpm for my 78 85 hp Johnson

You do have a problem. The 5500 rpm is for wide open throttle under load. I'll bet you are dropping a cylinder.

That was my original thinking. I used the adjustable spark tester and the fire was jumping about half an inch and it was blue. I did have the one plug that was a little wet so maybe the coil on that cylinder is a little weaker. The only other thing I could do is get a new coil and see if that solves the problem.

 

[clemsonteg]

Re: Rpm for my 78 85 hp Johnson

I ran the boat in the yard tonight since it was dark and didn't find any spots where it was arcing so I'm still stuck between there being no actual problem or outside my range of expertise. Because all of the plugs were nice and brown I would assume I getting plenty of fuel but maybe I'm only getting enough to run it 3500 rpms.

 

[Rscardina]

Re: Rpm for my 78 85 hp Johnson

My suggestion would be to make sure all spark is strong on all cylinders.. after that.. you may want to check to see if your butterflies are opening full range when your throttle is up,, meaning that in neutral that plates on the carb are closed...and when you up push the lever full forward ..the plates rotate to wide open. (do this without the engine on!!....There is a term "link and sync"...which is basically what i just described. If your butterflies are not wide open at full or closed when in neutral then you may have some adjustments to make on your linkages.. I would look under the "sticky files" for a description and direction. Also, if you plan on doing any of your own work you might consider buying an OEM epair manual for your engine. not the SELOC brand.. Will come in very handy...

Last but not least...each engine like its own spark plugs..you may want to be sure your plugs are the correct ones for your engine as many plugs have different heat ranges..

 

[clemsonteg]

Re: Rpm for my 78 85 hp Johnson

My suggestion would be to make sure all spark is strong on all cylinders.. after that.. you may want to check to see if your butterflies are opening full range when your throttle is up,, meaning that in neutral that plates on the carb are closed...and when you up push the lever full forward ..the plates rotate to wide open. (do this without the engine on!!....There is a term "link and sync"...which is basically what i just described. If your butterflies are not wide open at full or closed when in neutral then you may have some adjustments to make on your linkages.. I would look under the "sticky files" for a description and direction. Also, if you plan on doing any of your own work you might consider buying an OEM epair manual for your engine. not the SELOC brand.. Will come in very handy...

Last but not least...each engine like its own spark plugs..you may want to be sure your plugs are the correct ones for your engine as many plugs have different heat ranges..

checking the butterflies is actually something I checked before the first time I even put the boat in the water. I work on a lot of mowers and weed eaters with my father in law so I find quite often that can cause a problem from the start. I did install a new set of plugs, I installed NGK bh7s recommended by the local marine shop based on their catalog. I believe originally it called for a champion ql77jc4. I am considering putting in the old plugs vs. the new plugs because I did not check this. I am wondering if the one that was wet was dropping out under compression. It got a spark in open air and the coil fired well under open air, but if the plug is a little old it could be that under the compression and heat it failed. I might be able to pick up a small difference based on the rpm at idle. I can try this tomorrow with the muffs on in the yard. I also read the article on de-carbing which I may do as well, on the old plugs though.

 

[clemsonteg]

Re: Rpm for my 78 85 hp Johnson

The other part I forgot to ask was about the carbs. If I were to rebuild them I would use something like 18-7046 available here on iboats is that right? Would I need anything else besides some carb cleaner and compressed air? I'm fairly competent with a carb on a mower so I don't see a reason I couldn't do the boat. I just don't want to cause the plugs look so good

 

[clemsonteg]

Re: Rpm for my 78 85 hp Johnson

Just a quick update. I decided that I should put the old plugs in and see how it ran with the muffs on since I never checked that. I got more rpms and a smoother idle on the new plugs so there must have been an intermittent misfire on one of the plugs. It wasn't a total break down because it would still drop if I pulled a plug wire off any plug but it ran better with the new plugs.

I followed that with a de-carb to see if there was any junk inside the cylinders and I got plenty out. It should run better now I would guess.

I spoke with the guy at the marine shop again today and he thinks I'm over-propped and that is bringing down the rpms as well. He suggested an 13 pitch or maybe an 11 based on my rpms, the fact that it's a 24 ft boat and it has the smaller toons on it. I also found out that when I check it for max rpms it should be on a light load which meant just me and some gas and I tested it with myself two other adults and my 11 year old nephew and the coolers so that brought down the rpm as well

I'm going to try and get in the water one day this week so i can test it again with just me in it.

 

[Tim Frank]

Re: Rpm for my 78 85 hp Johnson

Just a quick update. I decided that I should put the old plugs in and see how it ran with the muffs on since I never checked that. I got more rpms and a smoother idle on the new plugs so there must have been an intermittent misfire on one of the plugs. It wasn't a total break down because it would still drop if I pulled a plug wire off any plug but it ran better with the new plugs.

I followed that with a de-carb to see if there was any junk inside the cylinders and I got plenty out. It should run better now I would guess.

I spoke with the guy at the marine shop again today and he thinks I'm over-propped and that is bringing down the rpms as well. He suggested an 13 pitch or maybe an 11 based on my rpms, the fact that it's a 24 ft boat and it has the smaller toons on it. I also found out that when I check it for max rpms it should be on a light load which meant just me and some gas and I tested it with myself two other adults and my 11 year old nephew and the coolers so that brought down the rpm as well

I'm going to try and get in the water one day this week so i can test it again with just me in it.

What RPM were you getting on the muffs?
The plug references are completely confusing.
Are you you calling the old plugs new?...or are you switching back from the old-new ones to the new-new ones????