Difference between a 1984 or 1985 Johnson 5 hp and 6 hp?

[WarthogMan]

I just purchased a 1984 or 1985 (the plate is missing so I don't have the model #) Johnson 5 hp short shaft and can't find it listed by parts suppliers. It looks to be pretty much the same as a 6 hp of that same vintage (same size power head, leg, and prop) and I found a reference that suggested that the only difference between the 5 hp and 6 hp was a blocking plate between the carburetor and the manifold. Does anyone know if that is the case or if there are other differences? I am interested in upgrading it to the 6 hp configuration and am wondering if it is as simple as pulling the carburetor and removing the plate. I found another reference that said the limited run of 5 hp versions was simply a change in the number on the hood to help folks who had hp restrictions on their lake. Any help with this would be greatly appreciated!

 

[racerone]

Marketing had a lot to do with that.

 

[ondarvr]

You need to look at the parts diagram and compare part numbers to be certain of the differences. But if you can't find one for the 5 you my need to compare each part in hand. If it is a blocking plate (no idea if it is), the jetting may be different, timing too.

 

[jimmbo]

I'm not sure where you are getting your info, but in 1984 or 85 there was no 5 hp, only a 6 hp. Unless OMC was making some oddball classified Military motor.

In those years they also made an 8hp with the same displacement as the 6hp.

 

[hardwater fisherman]

If you look on page 2 of OMC recreational outboards 1980-1999 they list a 5hp from 1984 and up.

 

[Vic.S]

I'm not sure where you are getting your info, but in 1984 or 85 there was no 5 hp, only a 6 hp. Unless OMC was making some oddball classified Military motor.

In those years they also made an 8hp with the same displacement as the 6hp.

If you look on page 2 of OMC recreational outboards 1980-1999 they list a 5hp from 1984 and up.

Certainly was on the UK market. Presumably made in Belgium. Oddball European model!

Sorry cannot help with the differences between that and and the 6 hp. If there is a restrictor plate between the carb and the manifold remove it. Also check that the throttle opens fully, not limited in some way.

 

[jimmbo]

I was wwwwrrrooonnnn... mistaken.:embarassed: My apologies:embarassed:
As mentioned earlier, differences would most likely be restrictions in the induction, and possibly in the exhaust.

 

[WarthogMan]

Thanks for all the help on this. I see from Vic.S's post that the Propeller shaft power is 3.5 KW for the 5 hp and 4.5 KW for the 6 hp which may indicate that there is probably a restrictor plate or throttle limit. I'll look for the throttle limit tonight since that would probably be the easiest thing to bypass. Any suggestions on where this could be located if there is one or how much advance should normally be expected?

 

[Vic.S]

Thanks for all the help on this. I see from Vic.S's post that the Propeller shaft power is 3.5 KW for the 5 hp and 4.5 KW for the 6 hp which may indicate that there is probably a restrictor plate or throttle limit. I'll look for the throttle limit tonight since that would probably be the easiest thing to bypass. Any suggestions on where this could be located if there is one or how much advance should normally be expected?

I have looked at my 6 hp ... it is not obvious how the throttle opening could be restricted. I suggest you start by checking the max throttle opening. If it opens fully then this is the wrong tree to be barking at.

Maybe a different main jet in the carb, but without a parts list for the 5Hp it is difficult to know, ( 6 and 8 hp have different jet sizes , although there are other differences too. so quite possible the 5 hp is limited by a smaller jet size.)

 

[jimmbo]

The carb could be a different size mounted on an intake with a smaller inlet, with smaller or fewer reed valves. Reeds might be limited by different reeds tops.
I doubt they limited the timing advance, but it is possible.

 

[ondarvr]

It is never just the jetting, the jets are different due to other differences between the two, just changing jets does not increase power.

 

[racerone]

They do amazing things with outboards.----In the 1960's they marketed a 5 hp , non shift motor.----A lower price.-----It was the 6 hp powerhead.--The carburetor body was from the 3 hp at the time.

 

[WarthogMan]

Thanks for all the inputs. I checked the throttle travel and advance and couldn't find any restrictions. I decided to boat test my 5 hp 'as is' versus a friend's '89 Johnson 8 hp. Physically, they look the same except the '89 has a primer versus a choke on the '84. The 5 hp topped out at 11 mph on a 14' aluminum fishing boat (approximately 500 lbs for boat, passenger and gear) on a lake at 5500' altitude. The 8 hp topped out at 14 mph but had an 8 x 7 altitude prop on it versus the 8.5 x 9 standard prop on the 5 hp. So.... I figured with a lower pitch prop on the 5 hp I could probably get 12 mph and I'm happy with that comparison. My conclusion is that there is probably not much difference between the 5 hp and the 6 hp and it may have just been a decal change on the hood. At any rate, these are great motors and would love to do a side by side comparison with a 6 hp - I don't think there would be much of a difference if any.

 

[jimmbo]

By dropping pitch the engine will run faster and the prop too will turn faster, but for each turn it travel less, so you may not see any difference in speed. You really need a tachometer to tell you what rpm the engine is running. If the engine is not getting into the WOT rpm range, it needs a lower pitch prop and will probably speed up the boat. If the engine is running in that range, going to a lower pitch will raise the rpms but the boat speed may go up, down, or stay the same.