4hp powerhead rebuild

Ranger Dave · Aug 7, 2003

Ihave a 1979 Johnson 4hp that loses power at high rpms. It bogs down and acts like it isn't getting enough fuel. I rebuilt the carb and it still does the same. I should add that I use this 4hp on my canoe so it runs at higher rpms most of the time. I notice I can only get 10 kph (6 mph) out of the canoe now. It used to have no troubles getting 12.5 kph out of the mighty 4hp (measured by GPS). I checked the compression and both cylinders show 90 psi. I squirted in one pump of oil from my oil can (10W30) and the compression jumped up to 145 psi. I think my rings are shot and this is why the engine runs poor at high speed and sounds sick. I ran the oil mixture at 25:1 and the engine ran better. Any suggestions, comments, thoughts on my problem. Also any suggestions and comments on rebuilding the powerhead? I have a repair manual for it and it mentions the crankshaft bearing alignment is critical. I build gas turbine jet engines for a living so it isn't "over my head" to work on engines.

Hooty · Aug 7, 2003

Re: 4hp powerhead rebuild

Re-ringing will probably do it and the job's no row for a stepper. Check the bore and see if it's still in tolerance. If not, over-bore it and replace the piston. c/6 Hooty

Ranger Dave · Aug 7, 2003

Re: 4hp powerhead rebuild

The bottom bearing is no longer made. Is it normally needed to be replaced? I have not opened the powerhead up and want to have an idea what is for sure needed to be replaced. The needle bearing can be bought new but I don't know about the center beearing on the crank. I am hoping it just needs to be honed and then new standard sized rings. Most 2 strokes seem to wear the rings before they wear the cylinder wall.

Paul Moir · Aug 7, 2003

Re: 4hp powerhead rebuild

Before you get too far into it, you might want to make sure fuel flow is adaquate. I don't know your engine particulairly, but if it's got a fuel filter and it's gravity fed, I would start there. The older 3hp's were notorious for clogging fuel filters causing weak flow. I recently looked over one of these for a guy who had done _everything_ trying to get it to work right. When I disconnected the fuel line, it just let out a slow drip...

alcan · Aug 7, 2003

Re: 4hp powerhead rebuild

Hi Ranger I agree with Paul. There are several thing to try and inspect before tearing into this motor. First as Paul suggests make sure you are getting proper fuel. Try a decarb treatment and see if that helps bring up compression. Check to see if the exhaust is restricted in any way. This may require you to remove the power head, but not disassemble it. I've had tons of small outboards that only test around 90lbs. that run and perform right up to snuff. Hey, if this rascal needs rings, it needs rings. If it were me, I'd really look the situation over first. It has been my expirence (in most cases) that when you loose compression do to worn or damaged rings the motor doesn't want to start or run at all.

Ranger Dave · Aug 7, 2003

Re: 4hp powerhead rebuild

It starts on the second pull and runs fine. It just doesn't have the "snot" anymore. I have replaced the head gasket and cleaned the head. It was fairly clean. I removed the exhaust cover on the side and cleaned the carbon out of it. I have rebuilt the carb. If I pull the choke slightly the rpms pick up. This may just be the added fuel/oil that is dumping into the engine making a better seal. I have added a inline fuel filter since there was never one. It is a light white colour and is easy to see if there is fuel inside it. I can't see how it would run alight, but not great, then to die right out to being a dog. I had removed the filter all together and it didn't make a difference. With the high oil mixture even at low and mid rpms the engine sounds much better. It doesn't sound like its bogging. Its just moving a canoe so it shouldn't be overly dificult. The manual states a 100 psi min but says the compression between the two cylindeers is important. Both have 90 psi but it takes 4 pulls to get it up to 90. The 10W30 oil takes one pull to hit 110 and another to hit 145 psi. This is an interesting problem....... Thanks for your input. Please keep it coming!

P.V. · Aug 7, 2003

Re: 4hp powerhead rebuild

I call for "glazed" cylinders. Any cross-hatch noticed when you pull the head off??

Chinewalker · Aug 8, 2003

Re: 4hp powerhead rebuild

90-PSI on that small bore motor is actually quite respectible. Might be as simple as running some decarbon agent through the motor to clean up the innards. Has anything else changed in the rig? More weight? Different weight positioning? Going with or against the current? GPS only measures your relative speed, not taking into account wind and current. At those speeds, even a small environmental factor can make a big difference... As long as the bearings are not worn too much, reuse them. Slightly loose bearings actually tend to perform better. - Scott

Ranger Dave · Aug 8, 2003

Re: 4hp powerhead rebuild

There wasn't much cross-hatch pattern left when the head was off. The rig is lighter than when I first bought. The captain has lost a few pounds!

I've measured the speed many times this year in different lakes and rivers. The best it would do is 10 kph. It is running slower. If 90 psi was good why does the compression shoot up to 145 with just a bit of oil?

Ranger Dave · Aug 8, 2003

Re: 4hp powerhead rebuild

It ook the powerhead off the leg and there was a fair bit of carbon stuck in a clump in the exhaust. It seems to catch on the gasket. To remove the exhaust pipe took a lot of time because the bolts were seized. The intake had very little carbon. How do you take the reed valve plate off? I may just pick up some new gaskets and see how it runs now.

Chinewalker · Aug 8, 2003

Re: 4hp powerhead rebuild

Carbon build-up in the exhaust tuner can definitely cause the motor to lose power. Basically, what goes in (fuel) must come out (exhaust) so no matter how clean the carb is, it doesn't mean anything if the exhaust is plugged up. The reed plate comes off with the entire manifold (part carb bolts to). Held on by four bolts. - Scott

Ranger Dave · Aug 8, 2003

Re: 4hp powerhead rebuild

The reed plate must be stuck on because the intakes off.

alcan · Aug 8, 2003

Re: 4hp powerhead rebuild

Hi ranger You said when you give it a little choke it seems to pick up a little. This sounds a lot like a fuel shortage problem.

Ranger Dave · Aug 8, 2003

Re: 4hp powerhead rebuild

alcan, I have everything back together now. It is still doing it. It runs just fine and it runs up to full power. Then when I let it idle for a while then accelrate hard the motor bogs and doesn't run up and slowly dies. If I pull the choke slightly the engine picks up and revs freely. It does the same if its at high rpms for a while. I took the carb off and it is submerged in solvent right now. It looks clean. The float bottom sits 1.5" below the lower base of the carb. The needle seems to move easily. I'll leave it in the solvent until tomorrow and try again. With the engine warm, in one full pull on the rope, I could only get around 80 psi. If I pull it several more times it works its way up to 95 psi. The carb is gravity fed but doesn't the vacumm created by the compression draw the fuel into the combustion chamber? This is driving me nuts!!!

DJ · Aug 8, 2003

Re: 4hp powerhead rebuild

Ranger, Nifty little rig you've got there. Decarb. the engine using a decarb product. Believe it or not, ususally pulling the choke indicates your engine is flooding. My 3 hp. (same basic engine) used to do the same thing. I replaced the cork float with a new one and all was well.

MadeCoolCom · Aug 8, 2003

Re: 4hp powerhead rebuild

I had a 3.5 Hp outboard had problem like that and I did everything to the carburetor and when I started it I had the problem ran for a little while and I choke to get from shutting off. I must spent about 1full day working on things cleaned out the carburetor adjusted the float to work better added a inline fuel filter even cleaned the flywheel magnets and still had the same problem. I was about to trash it I only paid $65.00 for it and I second thoughts about ever trying to fix these little outboards for a living. Then next day before I put in the trash can I decided to fill gas tank all to the top. And then I gave it a pull man it ran did it run it ran great. That was the problem I only had the tank 1/4 full and when I started the engine it would vibrate and some how not get enough gas. But it runs great still all that work I did paid off. That was the problem the whole time I didn't need to do all that work all I need to do was fill up the tank.

Ranger Dave · Aug 8, 2003

Re: 4hp powerhead rebuild

Nifty little rig you've got there.

Thanks. I get lots of compliments on it. It was funny last weekend. Boats that cost 15 times more, their owners would come up to me and thought it was a pretty click fishing machine. The canoe sits 5" in the water and motor sits about a foot in the water. Last weekend I managed to hit a rock and it busted off a prop blade. I couldn't believe it. At least I was in a canoe and they are easy to paddle. I thought we were heading out for a hour canoe ride which turned out to be a 3 mile paddle ride back! I found out there is no such thing as a shear pin. They are called drive pins now. they are not meant to shear but bust the prop instead.

I can say without a doubt there is no carbon in the engine. The intake, head and exhaust is clean. I cleaned it by hand.

Chinewalker · Aug 9, 2003

Re: 4hp powerhead rebuild

Hi Dave, The carbon that the de-carboning spray takes care of is the stuff in the piston ring grooves. That's the stuff that robs you of compression as the rings can get sticky. As for the acceleration/bog, try opening up the low-speed needle a hair, perhaps 1/8 turn. The float should sit dead-on level with the carb body when the carb is inverted. You should barely be able to see daylight under the float (again, carb INVERTED). If the float level is too high, it'll flood out. If it's too low, then the motor will run out of fuel... - Scott

Ranger Dave · Aug 9, 2003

Re: 4hp powerhead rebuild

Chinewalker, I did as you said with the float........ It made things worse. It needs the choke on to get up to any power setting above idle. I'm thinking now I'm only getting a trickle of fuel going into the bowl or maybe the flow of fuel can't keep up with the draw from the engine. I pulled out the inline fuel filter and there is a filter in the tank(coarse). I'm going to leave the inline filter out and try to clean the tank with solvent. If this doesn't work I'm not sure what to try next. The carb sat in solvent for 14 hrs last night.

Ranger Dave · Aug 9, 2003

Re: 4hp powerhead rebuild

I cleaned the tank which doesn't have a screan filter. I bought new fuel line and ran it directly to the carb. I started the engine up and it acted up again. I sprayed come GM Cleens into the carb as it was running and it smoked bad. I let it sit for a bit and started it up. It seemed to run better. I readjusted the low speed mixture. When its at an idle and I open it up the full throttle it bogs like it isn't getting enough fuel. The rpms kick up and it appears to being making good power. I'm just running it in a big rubber garbage can. I rechecked the compression with the engine hot and it had 90 psi.

4hp powerhead rebuild

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