Converting 25hp Johnson 1990 with primer to 35hp carb with choke

[patricio]

Hi!

I got hands on an intake manifold and carb from an 35hp, the thing is that the carb comes with the choke system so i'm planning to go for it and convert to the choke system at the same time as i upgrade to 35hp.

But there is one difference, on the intake manifold there's a nipple that right now has a hose connected to it that goes to the primer pump, but the motor i took the parts from that hose went to another nipple on the bypass plate, to the left of the fuel pump.

On my engine that bypass plate doesn't have that nipple, where do you think i should connect the hose from the intake nipple, or could i just plug it?

Hope anyone can help, i post a pic showing the intake manifold nipple i'm writing about, this i how it looks right now, the 25hp carb.

Thanks, Patricio

 

[racerone]

In my opinion it is much simpler to convert the 35 hp parts to the primer system.------Just open up the hole in the 25 hp manifold to the same size as the 35 hp manifold and install a nipple for the primer on the 35 hp carburetor.

 

[patricio]

Thanks for the reply, i installed the manifold and carb today and i feel a great difference, it has more power and i gained like 4 knots in speed (we measure speed in knots in Sweden)

I did not modify anything and plugged that nipple, i tested it open and then it spitted out small drops of oil/ fuel.

The only thing is that when i go from full throttle down to idle is missfires and makes a sneezing sound. is it getting to little fuel/ to much air?

Runs perfect on idle and open throttle.

Thanks!

 

[Bosunsmate]

Sneezing usually from lack of fuel, you could try backing out the slow speed needle valve half a turn or so- its in that bronze looking sleeve in your photo, back top left of carb, flat screw head.
Must be nice and cool conditions there in Sweden at present, the suns come down under to here

 

[patricio]

I will try to adjust the needle valve, but doesn't that one only affect the idle?

Well Bosunsmate, we're in late autumn now in Sweden, had some below zero nights this week but back now to 5-10 degrees celsius again, it's wet and cold all the time, but as an angler it's prime time for pike fishing until the winter comes and all waters freezes

Thanks again for the replys!

 

[Bosunsmate]

Those pike look huge.
Im not sure but since you are throttling down, the idle circuits become whats in charge. As you back off the throttle the timing will retard and the butterfly closes leaving just the idle circuits working to keep the cylinders primed as rpm slow

 

[patricio]

Those pike look huge.
Im not sure but since you are throttling down, the idle circuits become whats in charge. As you back off the throttle the timing will retard and the butterfly closes leaving just the idle circuits working to keep the cylinders primed as rpm slow

Yeah, pikes can get pretty huge

I'm going to try to do a Link N Sync and see what happens, if i manage to do it, never done it before

 

[restornator]

How did you get to 35hp when Johnson didn't make a 35 in 1990?

 

[racerone]

Propshaft POWER and crankshaft power.---This is an age old discussion topic that will never go away.--------------What used to be rated at 35 hp is now rated at 30 HP.------It is only a decal on the motor actual power may vary !!!!

 

[patricio]

How did you get to 35hp when Johnson didn't make a 35 in 1990?

http://www.*****************/parts/jo...ear=1990&hp=35

According to marineengine they did

But yes, the carb is from a 35hp before they started measuring on the prop shaft, so when that carb was new and sat on a brand new 35hp johnson it probably was only 30hp on the shaft, so ok, i'm converting to 30hp.

I think that they started measuring the power on the shafts in 1985 so if you get hands on a carb and intake manifold from a motor newer than 85 it should be a prop shaft 35hp setup.

 

[patricio]

Update, the sneezing came from the bypass plate that had the three lower screws loose, so the gasket had gone bad and burned oil and fuel were coming out from it, as the noob i am i hadn't notice it, i saw it when i was out running and saw the smoke puffs coming out from that place, strange thing the engine even ran as i probably must have had a real leakage on compression having that plate "open".

Made my self a new gasket using the old one (used real gasket material of course) and it fitted perfect, the sneezing is gone, i still have the misfires on the way down from full throttle but the sneezing is gone, i think the misfires comes from the retainer plate not returning in synch with the carb butterfly, i found out the solution for the plate retrievel when i had the boat on the trailer again so i will try it out the next time i have the boat in water again and come back, hopefully it'll run as new again

 

[patricio]

Update.

The motor is after the upgrade stronger and revs up faster to WOT, but i still have problems with lean sneezes (misfires), i'm creating a new thread for that.

The problem is a fuel delivery problem as it disappears if i spray fuel/oil mix into the carb while it happens, it only happens at mid throttle around 2000rpm.

Thanks for all the answers in this thread!