1975 70hp johnson

[jor593]

ok so i have a 1975 70 horse johnson that ill hit the gas and it will start to rev up like normall and then just bog down and only run at about half speed... any suggestions? ive already replaced all 3 coil packs replaced the gas line and rebuilt the carburetters any input would be great

 

[HighTrim]

Re: 1975 70hp johnson

Have you checked to see if you are dropping a cylinder?

 

[jtexas]

Re: 1975 70hp johnson

it's dropping a cylinder, all right.

All three coils went bad at once? That is really bad luck.

1. First thing to do is observe as someone advances the throttle to full...does the timer base move smoothly without sticking, all the way to its stop?

2. Then take off the airbox cover and make sure that all the throttle valves are closed at idle, parallel throughout their range of motion, and perfectly horizontal (not beyond) at WOT.

3. Make sure all three choke valves stay open unless the choke is engaged.

4. Make sure the spark plugs are champion QL77JC4 in good condition, gapped at .030

5. Compression test. no matter what else you do, it'll never run right with bad compression. Report your results.

6. Spark test (not a "spark plug" test). get an inline spark tester (couple bucks at autozone), set the gap for 7/16", clip it to the engine block, attach the plug leads (one at a time) and look for a strong blue spark while cranking. If it can't jump a half-inch gap in the open air without a spark plug, it's not firing in the hole under heat & compression. (Just because the coils are new doesn't mean you got spark.)

7. Unless by "rebuilt the carbs" you meant "Removed, disassembled, soaked overnight in carb cleaner, blew out all passages with compressed air (or aerosol carb cleaner), reassembled with new carb kits, followed by a link & sync, you aren't finished with the carbs.

Follow these steps you will find the cause of your problem.

 

[jor593]

Re: 1975 70hp johnson

yes that is how the carbs were rebuilt... all three coil packs didnt go bad one did and one had a crack in it but worked just fine so i replaced all three well i was at it... it idles just fine it only does it when i try to go on plane, and its the correct champion plugs correctly gapped and all three flanges are opening at the same time. but i do agree it feels like its dropping a cylinder so why is it dropping a cylinder if i have checked all of that. oh and i had a compression test done last month and the mechanic said all 3 cylinders had great compression for the age of the motor

 

[jtexas]

Re: 1975 70hp johnson

It was running like that when it left the shop? What else did the mechanic say?

"good for its age" doesn't tell us much...didn't he write down the compression readings on your invoice?

Don't forget the spark test.

Motor idling, hold an index card in front of the carb throats, real close. If the card gets wet (I don't mean the occasional mist or tiny droplet, I mean really getting wet), you'll need to pull the intake manifold to inspect the reeds.

 

[jor593]

Re: 1975 70hp johnson

he did tell me the compression i just don't remember it was a month ago i don't think it got wrote down anywhere but i will look around and let you know.
the mechanic did not rebuild the crabs my father did. the mechanic did replace the coil packs though i was having a different problem when the coil packs were replaced the boat ran fine for 2 weeks before i started having this problem and thats when i rebuilt the carbs hoping that was the problem sorry should of said that in the original posting.

 

[jor593]

Re: 1975 70hp johnson

the compression was 150 150 and 148

 

[jtexas]

Re: 1975 70hp johnson

ok, I see.

The motor's been running well for a couple weeks, so something has changed since it's been in the shop.

That means the compression readings from the mechanic don't matter...we need to know what the compression is now.

What you're describing is a classic symptom of clogged hi-speed jets, but cam also be caused by poor compression or bad spark. No point looking at carbs till you know compression and spark are good. You can have a good spark at idle, that fails when you put a heavy load on the motor, is the reason for the spark test as described above.

 

[jor593]

Re: 1975 70hp johnson

ok thanks for the input ill test the spark and have the compression redone and let you know what i come up with

 

[joey nathan]

Re: 1975 70hp johnson

I have '75 70HP Evinrude, too. Same symptoms were caused by the silencer housing on the carbs pinching the fuel line from the fuel pump.

Have to be careful and route the fuel line so it doesn't get pinched between silencer and the engine block. There is a cutout portion in the silencer housing that should allow for the line to snake through without getting pinched off.

Also check where fuel line attaches to middle carb and ensure it is not kinked at right angle. What I did is cut a new line with some extra length to allow for generous routing with minimum bending...

 

[jor593]

Re: 1975 70hp johnson

the old dirty fuel lines were not replaced after the carbs were redone and i just pulled the fuel filter and it was disqusting so my mechanic thinks something got through the filter and is jammed in one of the carbs so i am going to try replacing the fuel lines and then redo the carbs yet again and hope this works!!!!!!!!

 

[jtexas]

Re: 1975 70hp johnson

you didn't by any chance just recently get ethanol-blended gasoline in your area?