73 4 hp johnson need help

[hobojoe]

hello names joe.my brother bought a 73 4hp johnson(4r73c)last fall.ran ok this spring took out was hard to start or wouldnt start at all. the ball had disintegrated and pluged fuel filter.ok we replaced tank,hoses,ball,fuel pump,filters,plugs,impeller,had carb cleaned.about a month ago found on this site how to set slow speed &fast speed screws started right up,sounds great in the barrel. but when on the lake and top speed will run a minute or two,then bogs down does not die but rpms drop for a min or two then runs great then drops ect. if i adjust the slow speed out a hole turn will run a littlebit longer at high speed but still bogs down.same with the fast speed dial but that i just turn a fraction. i am thinking maybe bad plugs or carb is still not clean?i have rebuilt a car carb once. am a little afraid to try because this is not my motor and if i screw it up worse i cant afford to get fixed and hes handicapped and on disability.is there somewhere to go for step by step instructions? also does it make a diff on size of fuel line?have read some other posts that seam to say it does. if it does i cant find the size it should be on any scamatic.it is pushing a 14ft vbottom boat maybe 600pound (pl boat gear motor) is this maybe just too small a motor?thanks for looking any suggestions or help is appreciated, joe

 

[Rick.]

Re: 73 4 hp johnson need help

Your problems are very similar to a dirty carb. These carbs. are very simple to work on so don't be afraid. Take many pictures of how it is set up before removing the carb. and you can't go to far wrong. I would suggest you get a new kit if you've only ever had it cleaned. Once you set the needles then you need to tighten the packing nut so no air can sneak around the needle. Slow Speed needle should have no affect on high speed performance. Fuel line from pump to carb. is 5/16ths. Take a look at the Top Secret File at the beginning of this forum. There may be something there re: carb. cleaning. There is bound to be something on you tube as well. A 4hp with that load and boat size I thinking somewhere around 5 to 6 MPH would be your top speed but that's just a guess. Best of luck. Rick.
Edit: The gas line size is what I bought for my 71 - 4HP Evinrude. Today I bought new pump to carrb. line for my 6 HP and it looks close to 1/8th inch. I would suggest you cut a piece out and take it to an automotive part place to compare.

 

[hobojoe]

Re: 73 4 hp johnson need help

havnt been over to look at it yet but i know the pump to carb line is same size as tank to pump.also when he first bought it and after running for awhile if you loosen the cap on tank it would psssst,now after running you loosen cap and nothing.cap has no venting,and hose has adapterson both ends.the schamatics iv seen has only adapter on motor end of gas line.could that be some of problem?

 

[TN-25]

Re: 73 4 hp johnson need help

Wait a minute, you were getting a "whoosh" when you opened the cap after running? It sounds like the tank was not vented. That will certainly cause fuel delivery problems.
The second point you made was the connector on the hose; the original-to-that-motor old tank only had a connector on 1 end. As I recall, the 4-horse from that era used the ? height 3 U.S. gallon tank with the hose fixed to the tank (no disconnect at the tank end). As I also recall the original tank had a separate breather; later version of the steel ? height 3 U.S. gallon tank did not have the breather disc anymore and would need to have the knurled breather on the filler cap opened slightly. Modern replacement plastic tanks would also need the gas cap opened slightly as well I believe. I wonder if the motor problems are compounded by the vent not being open on the tank?

P/S, it is not a bad idea to run that motor more oil-rich that 50:1 since those old 5.28 c.i. powerheads were derived from the old 3 and only partially converted to run on the leaner oil ratio.