1976 75hp johnson troubles

[75-JOHNSON]

hello all,
i recently got a boat with a 1976 75 johnson engine from my uncle, and the boat had been sitting for 15 years without running. It started up fine and after replacing the fuel we took it to the lake and tried it out. he says the engine never did run right and sure enough it would not open up. the throttle linkage appears to be opening all the way but it wont get up on plain and the max rpm i can get is only about 2000. however out of gear with the warm up leaver it will get to about 3500 rpm. i felt the tops of the three heads and they all seemed to be the same temperature which leads me to beleive all three pistons are firing, or could this be just because of the circulating warm coolant? any help, information or thoughts about where i should start would be greatly appreciated.

thanks

 

[Bifflefan]

Re: 1976 75hp johnson troubles

Start with a compression check and go from there. all need to be 120 or better and within 10% of each other.

 

[Les Robb]

Re: 1976 75hp johnson troubles

After that many years, and if compression is good, might have gum in the carbs unless it was really prepared for storage. Unscrew the bowl drain on one of the carbs and if you have gooey black gunk flow out or no flow at all a carb cleaning is probably gonna have to be done. If all the fuel had totally dried out the gunk would harden.

 

[Boss Hawg]

Re: 1976 75hp johnson troubles

http://forums.iboats.com/showthread.php?t=299680

 

[wvmedic]

Re: 1976 HP johnson troubles

Re: 1976 HP johnson troubles

I agree with Les Robb, it is probably going to be a carb issue. I just bought a 90 HP Yamaha that had been put away for four years or so, it did the same thing. I was certain the carbs where in need of a rebuild and like Les Robb said, when I took the drain screws out the only carb that had good flow was the top, the middle had a little trickle and the bottom carb was dry.

I agree you need to check you're compression this is the first step in checking the health of you're motor, I'm not sure that you need 120+ psi though. I believe if you have 100 psi and are within 10% on all three you will be good to go, as long as you don't have drastically low compression, the most important factor is that the compression is close to equal across you're cylinders.

Jeff.

 

[75-JOHNSON]

Re: 1976 75hp johnson troubles

alright thanks guys...ill start with those few tips and probably will require more assistance at a later time.

THANKS

 

[wvmedic]

Re: 1976 75hp johnson troubles

75-JOHNSON, keep us posted. There are some good knowledgeable folks here that can guide you in the right direction.

Jeff.

 

[75-JOHNSON]

Re: 1976 75hp johnson troubles

alright so before i was going to clean out the carbs i hooked up the garden hose to the engine with that special device and ran it. after a few minutes it opened right up to 6000+ rpm according to the tach, with the propeller engaged. it seemed to run perfect with no dead spots so i checked the compression, all had between 105 and 115. put new spak plugs in and put it in the lake. as soon as the propeller is enganged and the engine is put under a load it just boggs down and only reaches about 2000 rpm like before. yet with the engine tilted out of the water it opens up perfectly. im not sure of the size or pitch of the prop but could that be a problem or does that have nothing to do with it? ill include some pictures.

 

[Boss Hawg]

Re: 1976 75hp johnson troubles

alright so before i was going to clean out the carbs i hooked up the garden hose to the engine with that special device and ran it. after a few minutes it opened right up to 6000+ rpm according to the tach, with the propeller engaged. it seemed to run perfect with no dead spots so i checked the compression, all had between 105 and 115. put new spak plugs in and put it in the lake. as soon as the propeller is enganged and the engine is put under a load it just boggs down and only reaches about 2000 rpm like before. yet with the engine tilted out of the water it opens up perfectly. im not sure of the size or pitch of the prop but could that be a problem or does that have nothing to do with it? ill include some pictures.

NEVER NEVER NEVER run more than 2000 rpm's on muffs

 

[75-JOHNSON]

Re: 1976 75hp johnson troubles

Note taken!! but does anyone have any idea why it runs so well without a load but as soon as its put in water it boggs down and barely does anything? i went to clean the carbs today and the when i removed the drain screw from the top one it was dry, but the other two were full of fuel. would this have been the problem? i was reading on an old post about how for this year of engine there was some engineering flaws which mayu have contributed to this bogging down under a load..has any1 ever heard of this?

 

[wvmedic]

Re: 1976 75hp johnson troubles

75-Johnson, You need to follow a certain protocol when trouble shooting you?re problem. I believe when you do this and report you?re findings, you will get a better response because there is a more complete picture as to what is going on.

Start with a compression and spark check, you will need an ignition spark tester or three. You should have at least a 7/16 gap for you?re spark.

Then check to make sure you are getting good fuel, and you will also need to do a link and sync to make sure the timing advance and fuel flow are working in unison.

After all of this is checked and confirmed to be good, or if not in spec others can help you sort the problem out.


Jeff.

 

[Les Robb]

Re: 1976 75hp johnson troubles

The key to your question was load. How fast can you run 100 yds and how fast can you run with a 200 pound weight on your back? There's a ton of people on this site willing and more than experienced to help you. Just follow the basic troubleshooting techniques as suggested in the last thread.

PS: 3 cylinder engines run so smooth on 2 it's hard sometimes to even detect the fact. Empty float bowl, where did the fuel go to?

 

[Bifflefan]

Re: 1976 HP johnson troubles

Re: 1976 HP johnson troubles

I agree you need to check you're compression this is the first step in checking the health of you're motor, I'm not sure that you need 120+ psi though. I believe if you have 100 psi and are within 10% on all three you will be good to go, as long as you don't have drastically low compression, the most important factor is that the compression is close to equal across you're cylinders.

Jeff.

I do agree with you that as long as it is within 10% and at 100 or better you should be good to go. But i have been politly complained at for using that low of a number in the past. 100 on a well used engine is acceptable. they cant stay new for ever.

 

[wvmedic]

Re: 1976 75hp johnson troubles

bifflefan I understand, also I meant no disrespect in my comment.

Jeff.

 

[75-JOHNSON]

Re: 1976 75hp johnson troubles

biffleman, you are right, it is only running on 2 as i can remove the top spark plug wire and it runs the same. but the top cylinder does have spark and the plug appears to be wet so i assume its getting fuel. could it possibly be spark at the wrong time? something with the timing?

 

[countvlad]

Re: 1976 75hp johnson troubles

biffleman, you are right, it is only running on 2 as i can remove the top spark plug wire and it runs the same. but the top cylinder does have spark and the plug appears to be wet so i assume its getting fuel. could it possibly be spark at the wrong time? something with the timing?

its normal to remove one spark plug wire and the motor to run the same... try to leave the top in and remove the second spark plug wire and see what happends? if everything works right you probably wont notice a diffrence.. if theres a problem then you'll know...

if there was no fuel in the bowls then you probably have a leak somewhere... look around find the leak.. dont run the motor until you find it.. the vapor from the fuel could ignite..... does it smell like fuel?

im confused? have you rebuild the carburators, when was last decarb done???

 

[TN-25]

Re: 1976 75hp johnson troubles

Just a thought, but in addition to the other suggestions, check the lower unit. It might have some issues causing it to seize up. When was the l.u. lube last changed? Was it the right type? How tight is the nut on the prop? Just some other things to check.