1977 Johnson 70HP Gas comes out of bottom carb

[Matthewgordon]

I have a '77 johnson 70HP. I have had the motor running, but gas comes out of the bottom carb. It comes out when engine is off and I squeeze bulb and when engine is running. I took out the spark plug and nothing changed. There is no spark on the bottom plug. WHen I get it started, and it takes a while, it runs high RPM, revs, then goes down to idle, which is terrible, then stalls. It varies, sometimes it stalls immediately, sometimes it will run for a few minutes, but it always revs up then down, like too much fuel is building. I was going to take off the card to inspect but I can't get to the bottom one easily without really dismantling them motor. There is low compression in that cylinder, about 90 lbs aposed to the 120 lbs in the other two. Before I take it to the boat shop and leave it there forever and then get billed like I'm bill gates, is there anything I can do to narrow down what the problem might be. My guess would be the float stuck or obstruction of seat, but I am by know means a mechanic. It was winterized and then sat for 4-5 years. I can't spring for a rebuild now and would just like to get it in the water a few times before the end of the season. THanks for any help.

 

[Thompy_04]

Re: 1977 Johnson 70HP Gas comes out of bottom carb

Well I've only been here a few months but after reading all the posts and actually running into this myself your compression sounds funny. Should be within 10lbs of each other. Take off the head and look for scoring cylinder walls, pitted piston heads, and pitted or burnt spots on the head. As for the spark, look for cracks in the HT leads or bad coils (cracked) You could be getting erratic spark. The gas coming out of the carbs sounds like a bad float, needle, and seat. Dissassemble the carb and rebuilt it with a OMC kit with a float. Check all fuel lines in there for cracks also. Just some suggestions.

 

[Chinewalker]

Re: 1977 Johnson 70HP Gas comes out of bottom carb

I am guessing that the bottom carb needed to be rebuilt about 30 psi ago. That cylinder ran lean andhas since cooked the corresponding cylinder. Each cylinder relies in it's carburetor for lubrication and when one carb goes down, it can take the cylinder with it - which is why it is NEVER a good idea to just run it that way...<br /><br />I would pull the cylinder head and take a peak at the bottom piston and cylinder walls. My gut feeling is that you won't find good news and you'll need a rebuild...<br /><br />- Scott

 

[reeldutch]

Re: 1977 Johnson 70HP Gas comes out of bottom carb

i agree with chinewalker.<br />but to get a few more monts out of it defenetly clean out your carbs an put new needle and seat in.<br />clean the high and low speed jets very wel.<br />with 90 psi you can still run it. the question is<br />will you do more damage to your internal parts.<br />if you break a piece of your rings and it ends op in one of the bearings you could trow a rod and destroy your block.(worst case)<br />so running the motor in this condition is possible but you are taking risks ending up with more damage ore loosing the whole block.<br />if you do deside to run it any way i would not run it on 50:1 but 25:1 to give your stuck broken rings more lube so they might not break .<br />good luck.<br />after you get your carbs clean and have the motor running ok, decarb the moter with a few cans of omc engine tuner en check the compresion you might be lucky.<br />reeldutch

 

[shadetreemarine]

Re: 1977 Johnson 70HP Gas comes out of bottom carb

the golden rule: 10 % differance between cylinders is the tolerance. So if one was 90 psi and the other was 120, then 10% of the highest cylinder is 12psi. So 12psi from 120psi is 108psi.Which is a differance of 18psi between cylinders which is unacceptible.Recommend rebuild,new carb kits,new fuel pump, and any other parts that vary from specs.

 

[Matthewgordon]

Re: 1977 Johnson 70HP Gas comes out of bottom carb

Thank you all for the help, unfortunetly I was hoping for slighly better news, but I definetly don't want to kill the motor, or even risk it. I have never had work done on a boat motor at a shop before, is it possible that JUST the BAD cylinder can be repaired, possibly saving some $$, or do they generally rebuild the whole thing. I have a '72 merc also that needs a rebuild for sure. I'm weighing out my options, if I spend a grand getting the motor fixed I think the only time I'll get on the water will be if my wife pushes me in it with a cinder-block tied on.

 

[Chinewalker]

Re: 1977 Johnson 70HP Gas comes out of bottom carb

Hi H3B1F,<br /> Yes, it is possible that the motor could be fine with a simple piston replacement, honing and new rings all round. Won't know until you pull the cylinder head. Those OMC triples are very good motors and IMHO worth putting some money into it if all else appears to be in good shape.<br /> At the moment, you may only have a lightly scuffed cylinder wall that might clean up with the hone enough to be put back in service. Honing the other two bores is recommended, as well as new rings on all pistons. If the budget is really tight, a good used standard bore piston shouldn't be hard to find, as many marinas will punch out all three bores in a rebuild, leaving one or two good used pistons leftover. If you're mechanically inclined, it's not a terribly difficult job and can be very satisfying. Make sure the carburetors are rebuilt in the process as I believe they were the cause of the initial problem. A factory service manual is your best bet for rebuilding, although some people do fine with the aftermarket ones.<br /> Good luck!<br />- Scott

 

[Matthewgordon]

Re: 1977 Johnson 70HP Gas comes out of bottom carb

Well I took it to my local guy, and he rebuilt one of the carbs. Apperently I have no idea how to test compression because he said it was 120psi, 120psi, 115psi, and the motor runs sweet. Theres a little rattle in the top piston but I still have some good use out of the motor. I took her out and she runs quiet and smooth, great idle. Thanx for all your help, now lets see if I can get the merc working on the hydrostream.