1971 johnson 100hp carb issues I think

fire0501 · Jun 16, 2013

Hi Guys. I recently bought a used 1971 Johnson 100hp and found out that the carbs on the motor are not the stock carbs they are from a 125 hp motor. They do not have the slow speed adjustable needles that this motors carbs are originally equipped with and so it runs a little rich on the bottom. My problem is when I get past about half throttle the motor sputters and will not rev up. I think it might be a rich issue but I am unsure how to correct this short of buying new carbs. Can I just replace the high speed fixed jets? and If I can does anyone know what size I should start with? I live in Northeast PA I think Altitude is about 1200 above Sea Level. Thanks in advance for any and all info.

HAPPY FATHERS DAY!!

fire0501 · Jun 16, 2013

Re: 1971 johnson 100hp carb issues I think

Anyone?

fireman57 · Jun 16, 2013

Re: 1971 johnson 100hp carb issues I think

I am not a tech by any stretch, I have just worked on Johnnyrudes as a hobby for the last 30 years because that is what I have owned. the slow speed jets don't have anything to do with it not revving up when you give it gas except that they have to make sure you have enough fuel before the high speed jets take over. If you have the OEM manual it will tell you which size jets should be in it. The difference, I think, between the 100 and 125 is just jetting them. I could be wrong because I am not looking it up as I am typing. There are a couple questions that need to be answered first: Is this the first time you have run this engine or have you had it a while and it is just starting to do this. Second is have you taken the carbs off and cleaned the properly. Your altitude is not enough to make a difference.

fire0501 · Jun 17, 2013

Re: 1971 johnson 100hp carb issues I think

Hey thanks for the reply. Yes I just got this motor and this is the second time out on the water I had it out for 5 hours. I did take the carbs off and clean them properly. I have taken compression test several times and they have been the same all the time I forget the exact numbers off the top of my head but they were all above 120 and about 10% of each other. I do have the service manual and it is the johnson Manual. Good thinking I forgot to even look at the manual to see what jet size Is needed with the original carbs. I would think I could just put those in these carbs and they should function in these carbs just as well as the ones that are supposed to be on this. I will check the manual today and see if I can get those jets asap Thanks!

fire0501 · Jun 17, 2013

Re: 1971 johnson 100hp carb issues I think

The model carbs I have are from I believe a 1972 125HP Johnson. The high speed jets are a 72C as per a Diagram stock. And the Jets that are supposed to be on the 100hp are a 59C Could anyone tell me if I can just order the jets for the 100hp Carbs and put them in the Carbs that I currently have installed on this motor?

fireman57 · Jun 17, 2013

Re: 1971 johnson 100hp carb issues I think

check ishopmarine dot com and see if the carbs are the same part number. You can look up all your engine diagrams there

fire0501 · Jun 25, 2013

Re: 1971 johnson 100hp carb issues I think

Ok so Here is what I did I bought the carbs that are supposed to be on the boat and Cleaned and installed them. I took the boat out Sat and Sunday and it ran a whole lot better....but It still has the high rpm cut out. Is it possible bad connections from battery would cause this? It runs excellent under like 4500 RPM'S If the tach is right and occasionally it will run over that good but not for long. Have you ever heard of the battery being the cause of this?

fireman57 · Jun 25, 2013

Re: 1971 johnson 100hp carb issues I think

No but have heard of a bad rectifier causing this. First make sure that your timer base is going all the way forward when you have it full throttle. If it doesn't move all the way forward where it should it will not advance your spark to meet your fuel needs. Also, make sure that your high speed jets are totally clean.

fire0501 · Jun 25, 2013

Re: 1971 johnson 100hp carb issues I think

HMM Ill have to check the rectifier The carbs are absolutely clean I cleaned them myself before I put them on the boat and I have cleaned many carbs so I am not new to that. The timer base is moving freely all the way to the stop with no issues I took care of that when I first got the boat. perhaps the timing is off and that will be my next thing to check.

fireman57 · Jun 25, 2013

Re: 1971 johnson 100hp carb issues I think

Usually the timing is not off so I wouldn't worry about that yet. Does the timer base move all the way forward when you are under a load? Not just sitting there on land. Sometimes heat makes them stick. You can check your timing with an old fashioned timing light but the best way to set it is Joe Reeves method. It might be at the top of the page in a sticky note or you can do a search for it. he has posted it many times.

fire0501 · Jun 25, 2013

Re: 1971 johnson 100hp carb issues I think

I have the Joe reeves Method and No I haven't Checked it under load. I will do that I had the flywheel off and distributor off and greased everything as well but I will have to check while on the water and the prob happens. Thanks for the tip

emdsapmgr · Jun 25, 2013

Re: 1971 johnson 100hp carb issues I think

Years ago I put 125 carbs on my 1972 100 hp engine. Initially, the larger carbs overfueled the engine. Would hardly run. After trying a number of sets of new high speed jets, I believe I settled on .064 or .065 jets and it ran fine. Keep in mind that the 100 hp block is 92 cubic inches, the 125 is 100 cubic inches. Not unusual to have to rejet the carbs to get them dialed in. After the carb switch, the engine picked up about 200 rpm's, enough to have to go to a higher pitch prop.

1971 johnson 100hp carb issues I think

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