18hp Johnson max MPH, motor seems doggy at top end?

Hello-

Can anyone tell me an aproximate speed I should be reaching at wide open throttle with a 1964 Johnson 18hp? I am running it on a 1960 starcraft 14' v-hull aluminum boat. Had it out on the lake today with me and a buddy. Our GPS said 13-13.5 mph and WOT. Tried adjusting tilt of motor, didn't make much of a difference. I have replaced points, coils, plug wires, plugs. I rebuilt carb last year, soaked, blew out with compreesed air. I drain float bowl at end of season also. Compression is over 100 on both cyclinders. I am running 50:1 oil mix fresh gas today. The prop on the motor is a 9X10, part# 592959. PArts book says it should be 9.25X11. Would this make a difference? Took motor cover off and had my buddy at WOT pull link from cam to make sure carb butterfly was open all the way, no change in motor speed when he did this, so carb open completely. I am kind of lost, seems like boat should go faster than 13 MPH. Any thought would be great. Thanks!

Jesse

Re: 18hp Johnson max MPH, motor seems doggy at top end?

Unless you and your buddy are really big, or your boat is really heavy-----I'd think you should be getting over 20mph with your 18hp. Around 23mph or more by yourself.
What is the compression, may I ask?
Also, have you checked your spark with a spark tester to see how wide of a gap it will jump?
Last of all, did the boat plane out, or not?
JBJ

Re: 18hp Johnson max MPH, motor seems doggy at top end?

JBJ-

I did not have a spark tester. The boat does not seem to plane out. The motor sounds like like it is running smooth at WOT. Idles well a slow speeds. Compression was around 110 each. Thanks!


Jesse

Re: 18hp Johnson max MPH, motor seems doggy at top end?

Soaking and blowing out a carb rarely works and carbs don't always stay clean, you need to phsically clean the jets to have them be clean.

I use a stranded copper wire and pull back strands till it just fits in the jet, scrub it in and out a couple times with some Sea Foam and reinstall, a jet can "look" clean and not be, the reason to use copper is it is softer than the jet material and won't bore it out or change its shape.

I "cleaned" my carb, didn't fix the problem so I replaced a buch of parts snd it still wasn't fixxed until I properly cleaned the carb then it ran like duck on fire.

I would also get an inline filter on it tha twill save you form most future plugs.

Re: 18hp Johnson max MPH, motor seems doggy at top end?

Do a cylinder drop test. Run the motor until warm at semi-high idle in neutral. Pull one plug wire at a time w/insulated pliers, or shut down and re-start to test fire each cylinder independently. If it only runs on top (or bottom), reverse plugs and try it again. If it switches, you have a bad plug. If not, you haven't got proper spark to the offending cylinder.

Re: 18hp Johnson max MPH, motor seems doggy at top end?

is it possible that the engine would still run ok with plug wires reversed? Or is it like a car engine if you reverse wires, you get big backfire?

Re: 18hp Johnson max MPH, motor seems doggy at top end?

It will not run at all with the wires reversed. But pulling one wire at a time will tell if you have a weak cylinder or running on only one cylinder. One of those is your problem. It is very unlikely that it is a carburetor problem.

And yes, they will run very well on one cylinder--just lacking horsepower

Re: 18hp Johnson max MPH, motor seems doggy at top end?

I did pull 1 plug wire off at a time on the lake yesterday. Engine ran rough at idle with only 1 plug wire, then when I went to accelerate engine had no power at all. Did the same with the other plug wire, engine acted the same way. Leads me to believe it is something else like carb, but I will try to do a spark test first and also a proper drop test for cylinders. Thanks!

Jesse

Re: 18hp Johnson max MPH, motor seems doggy at top end?

Also need to be sure the link and sync are set right. stator advances all the way and carb opens all the way.

Re: 18hp Johnson max MPH, motor seems doggy at top end?

Well ok, you proved it isn't an ignition problem or engine (compression) problem. So, back to the carburetor. It could be a slightly restricted high speed orifice plug, causing it to run lean. That orifice must be absolutely, positively Clean, Nothing else will do.

It also could be the boat, and not the motor. By any chance, does your boat have floor boards? It wouldn't be the first one to have 50 gallons of water trapped under there. Common in a fiberglass boat, not very likely in an aluminum one.

Re: 18hp Johnson max MPH, motor seems doggy at top end?

The boat does not have a floor, so there is no trapped water in bottom adding extra weight. I will hopefully try and get back to the lake on wed. or thursday this week to try the spray gas down throat of carb when running at WOT. If it speeds up, I will know that it is the carb high speed plugged. I had my buddy this weekend in the back of the boat with motor cover off. When running at WOT, the stator plate was all the way advanced, and I had him pull on the linkage from stator plate cam to carb and it made no difference, so I believe the link and sync is correct (opens carb butterfly all the way). Thanks for all the input!


Jesse

Re: 18hp Johnson max MPH, motor seems doggy at top end?

Would it make a difference if I had the original 9.25X11 prop on the motor? From the part number I took off the current prop (592959) it is a 9X10 prop. Would there be any noticeable difference between the 2 props?

Jesse

Re: 18hp Johnson max MPH, motor seems doggy at top end?

OK, just got back from the lake. I had a chance to retest compression, and test RPM with a laser tachometer this time. Compression was 100 and 105. RPM at WOT was only 3,700 RPM max. My book says it should be at 4,500 at WOT. Cam and carb open completely (link & sync). This would explain why it is slow at top end. But what is causing it. In previous replies we ruled out ignition and compression. Also, put a brand new fuel pump on last fall.

Questions:
1. If I have a plugged high speed jet, wouldn't the engine die off at WOT because it is starving for fuel?

2. When I sprayed gas/oil mix with spray bottle in throat of carb when running at WOT, the engine did not speed up, it acted like I had the choke on. Also, tried pulling choke out a bit ot WOT, did same thing, slowed down. What does this mean?

3. Should the glass bowl with fuel filter be full when running?


I am leaning towards carb, but it runs smooth at the 3700 RPM, just stumped as to why I can't get top 4500 RPM. I am running a 9X10 prop. Any thoughts now? Thanks!


Jesse

Re: 18hp Johnson max MPH, motor seems doggy at top end?

I had the same thing happen to me on my 1959 18hp. I did a carb clean and carb kit. My 12 year old daughter (78#) and I (190#) could only get 13 mph. out of our 14' smokercraft. I cut the middle seat out of this boat and put in a floor and bow mounted trolling motor. Maybe another 100#.

I took it back to the lake and took the cowl off and removed the front plate so I could access the needle valves. The problem was with the lower needle valve (high speed). I disconnected the linkage and turned the needle valve in (leaning) until it found it's right place. This particular motor liked just approx 3/4 turn. With two people and the additional weight of the floor and bow mounted trolling motor I am getting 20.5 mph. The other needle (slow speed) is close to 1 1/2 turn out. You can which direction you need to go with the needle valves by pulling out the choke. If you choke it and it runs better you are too lean. Choke it and the engines stalls you are too rich.

The glass bowl on mine is 3/4 full