Which Prop should I be using or is it the Carburetor??

Suggestions, concerns, and what is the correct power plant for your Thompson.

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Which Prop should I be using or is it the Carburetor??

Postby cmassucci » Fri Nov 04, 2011 10:22 am

First you should all be happy to hear that I finally got my 1968 100hp Johnson running and on top of that to actually stay running!!! Ok that is the good part. It idles great, but when I give it power it just lumbers along at about 1/3rd the speed. I would think that she should have more power. She sits on a 1967 21' Thompson Offshore Camper. All four cylinders are firing and compression is about 95-100 on each. I also sprayed gas into the Carb. while she was running at full speed and there was no noticable difference. Two things come to mind - Carburetor High Speed jets/orifices are blocked or I am using the wrong prop???? I rebuilt the Carb about a year ago, but I heard that these can get blocked easily and that the Alcohol in the gas ruins the seals??? I am currently running her on a 12.5 x 16 prop? Is that the wrong size? That is what came on the OB when I got the boat. What size prop should I be using to get more speed? Or any other ideas on why I do not have power? Or... should I just get a bigger and newer OB? I have stuck by this OB for two years and lots of money and I feel so bad she is still not working right. Thanks to all for your input!
Chris
67 Thompson Offshore Camper
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Postby Phill Blank » Sat Nov 05, 2011 7:44 am

cmassucci,

I am not up on these large motors, but I had a problem some years ago with my 1957 35 hp Evinrude with it not running properly at full throtle and the problem was the coils. I was not getting enough spark to run at full speed. It would only reve up to about 2/3 max rpm and the boat powed thru the water and sucked fuel like no tomorrow. Repalced the coils and tuned it up and it runs like a champ.

Check with some one on the Antique Outboard Motor website for some tips. Also I think the fiberglassics website also has an area for motor questions.

Good Luck,

Phill
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Postby txcaptdan » Sat Nov 05, 2011 3:14 pm

I have a 115 Johnson that acted the same way and it was crap in the carbs plugging things up. The seals had broken up inside the carbs into small pieces and would cause that problem off and on. Runs perfect after carb rebuild.
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1965 20' Cruisers Inc. 570 Seacamper
1962 20' Cruisers Inc. 502
1963 16' Cruisers Inc. 202 Seafarer
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Postby JoeCB » Sat Nov 05, 2011 7:41 pm

Yes it could be dirt in the carbs, but also check the throttle /spark syncronization. You should be getting well up the speed curve before the carbs even begin opening.
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Postby john » Mon Nov 07, 2011 7:35 pm

Best way is to find a test prop. They were made by omc to allow full throdle testing without the boat moving. If you obtain full rpm as specified for the test wheel, then the problem is boat or prop.

I did buy one of the first 100 hp Evinrudes when they first came out in 1968. Good engines but everything had to be right, If you pump the fuel line bulb, does it do any better? There are four 7/16 hex head bolts with a slut for a straight screw driver, these are the plugs on top of the high speed jets. If you take these out you can remove the high speed jets for cleaning with a special omc jet screw driver. Not as good but use a wire to ream the jet hole out and spry some Beriman B13? carb cleaner in the jet. This should work, if the carb jets are gummed up.

Try a know working 6 gal fuel tank and hose, if the fuel tank or pickup is bad, you will see a improvement right away. The old built in tanks had a anti sphion check valve that caused a lot of problems, that is still a problem today on some new rigs.

This was the first year for pointless ignition under the fly wheel. Any ones guess as to what can go wrong there. I was a factory trained Evinrude mechaic way back then and if there was a problem with these engines it was ignition issues.

I ran my 100 on a 16' Larson, with a 16 inch and did about 40 mph on a good day. I would guess you need a 14 inch?

Is the boat trailered, if not the boat bottom could be very dirty and might not plane.

Don't give up.
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Postby W Guy » Tue Nov 08, 2011 3:46 pm

Maybe I'm too proper, but I've never used a screwdriver on a slut....... :shock:
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Postby john » Tue Nov 08, 2011 5:36 pm

might be exciting!

how about slot?
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Postby cmassucci » Thu Nov 10, 2011 5:13 pm

Thanks everyone for your suggestions!! I plan on trying them out this weekend.

John - Question on my High Speed Orifice. If I was running at full throttle and sprayed fuel into the carbs wouldn't I get a noticable change in the engine?? I did that and I only experienced a slight decrease in RPM's. With spraying the fuel into the Carbs wouldn't it bypass the High Speed Orifices?? Even though the the High Speed Orifices were blocked?

Could I be runing too Rich?? I idle great but cannot get that top speed?

Again I am learning as I go along? Please excuse my ignorance!

Chris
Chris
67 Thompson Offshore Camper
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Postby john » Sun Nov 13, 2011 5:34 pm

Chris

If you spray gas into a good running engine, it should loose power since it is then too rich.

If you do have a clogged high speed jet and continue to run it, you most likely burn a hole in the top of the piston. If you have been having this problem for a while and been running it, if it is a lean condition, best bet is you would have already burned a piston or two! I fyou are over proped you could burn a piston also, best to find out a solution soon!

What do you mean by 1/3 power, your engine should run 4500 to 5000 rpm, 1/3 power is only 1600rpm wide open? How fast is your boat running, best guess is it should run 25 to 30? What rpm are you running at wot? If you had a hook in the bottom top speed would be way down, although pickup from a dead stop would be ok.

Might be good to find a dealer with a test prop, I have one if you were in Houston, have them for most omc's before 1968!


I would try a lot smaller prop. Tried to find a prop chart for the engine with no luck. I ran both a 14 and 16 on my 16', 14 for sking. I did see omc offered a 10,11,12,14,16 and 18 for the engine. My best guess without current speedor rpm reading would be a 12". Best guess on top speed about 25 mph. Did sell a lot of 20' Wellcraft witht he 100 hp and they ran about 30, can not rember the pitch but most likey a 14"

Are you still able to run? In houston it's "winter" and 80 degrees +.

you can email me directly at john.platou@yahoo.com and then we could visit on the phone.

John
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Postby thegammas » Thu Dec 22, 2011 6:08 am

Realizing this thread is a month and half old....what ever happened? Did you find the problem? If not I'd recommend going over to the www.fiberglassics.com web site. Go to forums, then the JEGO forum. you can get a ton of great advice (in addition to what you have gotten here).

My 2 cents, it's not a prop issue but Fuel delivery or ignition. If it were a prop issue you would get poor performance, but not as you describe. From there I would say its an ignition issue since spraying gas into the carbs had only a slight effect. From there, since it sounds like its an electronic ignition of some sort, I'd say go to the fiberglassics site since I would have no clue :-)
Peter Stransky
1962 Cortland Custom Sea Lancer
Wilmington, Delaware
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Postby cmassucci » Sat Jan 07, 2012 4:47 am

Sorry everyone I have been traveling like a mad man. Unfortunately I went ten steps back. I blew another pulse pack! Uggggg!! Not worth another $280. Actually just picked up a 1982 Mercury 115hp. Very nice motor and it runs!! Anyone know when Mercucry started rating the HP at the prop? The guy I bought the OB from said it was 1982. She is real clean with new power head, rebuilt carbs and rebuilt lower end. Started her on an 8 degree night and she stated right up!! Nice! She should push my 67 Thompson Offshore real nice. After two years, I am ready to see her run! As always, thanks for all your help!
Chris
67 Thompson Offshore Camper
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