18hp Johnson lacks power.

[Hanr3]

This engine has been challenging.
Top speed according to my GPS is 9mph, used to be 7.3mph.
What I've done so far.
new points, coils, condensers, plugs, plug wire, rebuilt the carb, bought a used carb on e-bay, rebuilt it. Compression 105/95, spark from both plugs, pull one plug and the engine rpm's change, pull the other and teh same thing happens. there both firing. I aslo machined a new cam follower from bronze. The old follower had grooves worn into the rubber.

The ebay carb increased my mph from 7.3 to 9.


Question- the flywheel appears to have only one coil, the other side appears to be for balance purposes only. reason I say that, is only one side of the flywheel is megnetic. Should both sides be magnetic?

What else can I check?
What am I missing?

 

[F_R]

Re: 18hp Johnson lacks power.

Start by telling us what year your 18 is. And what kind of boat and load you are shoving. 9mph is way to low unless you are way underpowered. You aren't even getting it up on plane, are you?

 

[Hanr3]

Re: 18hp Johnson lacks power.

sorry for the delay, one of my dogs at my pc power cord.

1957 Johnson Seahorse.
Pushing a 14' Alumacraft boat.

Ive been strugling with this motor all summer. It seems like it just doesnt have any top end.

No I dont get on plane. I did get to 10.5mph while riding the top of a wave and a 15mph wind at my back. Drop off the wave and my speed dropped to about 9mph.

Could it be a severe case of carbon build up?
What other possibilities would casue it to lack top end speed/power?

 

[steelespike]

Re: 18hp Johnson lacks power.

Try a good decarb with some sea foam.You might pull the exhaust cover and look around.You might see what wot does with one plug at a time un hooked.

 

[bktheking]

Re: 18hp Johnson lacks power.

2 different carbs, spark on both plugs, piles of compression and the only thing missing is timing, my 15HP evinrude pushed me, my buddy, and 50+ extra lbs to 18mph the other night, 10.5??? Should be doing 25+ with just you?????

I'd have a look the link and synch to make sure the carb is opening all the way at full throtlle, pop the flywheel and check 0.020 gap on the points. The flywheel should have no coil, you should see 2 under the flywheel, i'm thinking you are talking about the magnet.

The other got ya and it's "got me" before is a missing gasket in the carb. 2 things happen 1- the carb is missing the boss gasket between the float chamber and body at the time of rebuild, you don't know it goes there so it never gets installed or
2- you rebuild the carb and forget to install it. This gasket creates a tight seal to allow fuel to be pulled up into high speed nozzle at a metered rate, if not it won't run right, it will get to half throttle and not go any faster. It's #73 on the parts list.

Hope this makes sense.

 

[Hanr3]

Re: 18hp Johnson lacks power.

Checked the sync. Carb is at WOT (butterfly laying flat) when the engine is at WOT. I checked, thought maybe the cam follower was bad, it had grooves wore into it, so I had a new cam follower machined from bronze.

Yep, I missed that gasket on teh first rebuild, the new carb has the gasket, I checked.

I did syncronize the engine to carb via the manual instructions. Butterfly starts to move just as the cam follower gets to teh line.

Yea, I keep reading it should be pushing me aobut 20+ mph, Im only getting half that. Ill keep plugging away at it. eventually Ill figure it out. Open to more suggestions.

 

[F_R]

Re: 18hp Johnson lacks power.

You suggested carbon. That is a possibility, but would be extremely rare. The possibility is the exhaust ports are plugged with carbon and exhaust can't get out of the cylinders. It is common on two-stroke lawn mowers, but rarely happens on outboards for some reason. If you want to follow through on the idea, you will have to pull the exhaust covers off the side of the powerhead for a looksee.

 

[Hanr3]

Re: 18hp Johnson lacks power.

what else could it be than?

How many magnets are in the flywhell?
1 or 2? Mine appears to only have one.

 

[F_R]

Re: 18hp Johnson lacks power.

what else could it be than?

How many magnets are in the flywhell?
1 or 2? Mine appears to only have one.

What else? I have no idea. Well ok, a lot of ideas, but so far off the wall I won't mention them. I gotta wonder about the boat or load.

It isn't the magnets. Just doesn't happen. Besides, it wouldn't run at all.

 

[asdasc]

Re: 18hp Johnson lacks power.

What type of prop is on it? Does it sound like you are getting good RPM? Perhaps a slipping prop hub? Also, do you have the motor at the right height on the transome? Trimmed properly?

Thanks, and Good Luck!

 

[1946Zephyr]

Re: 18hp Johnson lacks power.

I would pull the hand starter and remove the little cover plate from the flywheel. Then you should be able to see the condition of your coils. It sounds like you're running on one cylinder. If your coils are cracked, then I would replace them.

 

[cajuncook1]

Re: 18hp Johnson lacks power.

what else could it be than?

How many magnets are in the flywhell?
1 or 2? Mine appears to only have one.

Have you tried adjusting the tilt of the motor? If you have in the 1st hole, then your boat maybe plowing. Try adjusting the motor's tilt to the 2nd or 3rd hole position and see if your boat planes and get better speed. I know, it sounds silly, but sometimes we forget the simple stuff.

Have you set your carb's high speed needle correctly.

http://forums.iboats.com/showthread.php?t=167352

 

[bktheking]

Re: 18hp Johnson lacks power.

I would pull the hand starter and remove the little cover plate from the flywheel. Then you should be able to see the condition of your coils. It sounds like you're running on one cylinder. If your coils are cracked, then I would replace them.

Top of the post states it's all new

 

[jbjennings]

Re: 18hp Johnson lacks power.

Here's what I would do:

Check the lower unit for good oil. Turn the flywheel with it in gear and make sure there's no binding and it's nice and smooth.

If so, then take the cowl off, run it on the lake with the air silencer off and have a spray bottle of fuel/oil mix on hand. With it running full throttle, spray some fuel/oil mix in the carb throat and see what effect it has. If it's fuel starved, it should pick up speed quickly, if it immediately bogs, I would think that fuel is fine or too rich, or spark isn't good on one cylinder. Most of the time, it's spark from my experience. BUt the fact that pulling both plug wires has an effect pretty much makes me think it has good spark. If it picks up rpm's when you spray the fuel to it, the problem has to be a clogged or incorrect carb.
Just some ideas,
JBJ

 

[1946Zephyr]

Re: 18hp Johnson lacks power.

Top of the post states it's all new

Oops. my bad. I would still check to see how well the two wires are firing.