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74 Johnson Poor idle

mike_lee

Member
"I have a 1974 johnson 50hp &#

"I have a 1974 johnson 50hp (50ESL74M) with an idling problem. I t seems to start and run fine through the earmuffs, but when put into the water takes long cranks to fire and the idle is unstable. Also it wont idle in gear and needs to be slammed into gear to take off. The problem seems to be worse when it has been running a while. The motor had a freshen up with new rings and conrod bearings four outings ago (just before I got it). The compressions are 150 and 152. I have set the timing at about 3 degrees, syncronised linkages and made sure butterflies shut fully on both carbies. I am a motor mechanic by trade, with some two-stroke bike experience. Any help would be greatly appreciated."
 
"Using a spark tester whereas

"Using a spark tester whereas you can set a gap of 7/16" (and with the s/plugs removed), the spark should jump that gap with a strong blue lightning like flame.... a real Snap! Does it?

If so, then I would strongly suspect that the carburetors are fouled somewhat. In which case, remove, clean, and rebuild them using new carburetor kits.

Perhaps they were already reworked. If so, either something was overlooked or that previous work was quite some time ago.

Note that the spark plugs should be Champion QL77JC4 plugs gapped at .040 .

Let us know about the spark test."
 
"Thanks for the reply Joe. I h

"Thanks for the reply Joe. I have just performed the spark test and both cylinders jumped the 7/16" gap with strong blue sparks. One thing I did notice, was when the lower cylinder spark plug boot was placed near the engine casing, a spark jumped from the rear of the boot. Also, the plugs that are installed are L77JC4. I am suspecting that the Q is for a resistor plug, could this be contributing to the problem? I don't think the carbies were rebuilt at the last freshen, as the invoice did not indcate so. Is this a simple process, and what should I be concentrating on? And are kits readily available?"
 
"Michael.... No spark should e

"Michael.... No spark should escape that rubber boot. I'd strongly suggest replacing them.

The "Q" does indicate a suppressor/resistor type plug but that would have no bearing on your problem..... unless the plugs is faulty.

The main cause of powerhead failure is fouled carburetors. A slightly fouled carbuetor causes the related cylinder to run lean which in turn results in piston etc damage.

One should always clean and rebuilt the carburetors when performing a rebuild. Also check and double check the ignition system (you've just done that) but include a visual check of the stator under the flywheel to see if it possibly is melting down, cracking, etc as that component is the beginning of both the ignition and charging system.

If you had a OMC mechanic rebuild that engine and he did not clean/rebuild the carburetors, that could have led to a repeat of your problems.

Have a manual to rebuild the carbs if you're going to do it yourself. It's not difficult but make notes and take pictures before you dismantle. It's impossible to remember everything unless it's a everyday job.

Kits can be obtained at any dealership and also at NAPA at a lower cost.

What goofs you may make can be corrected here."
 
"I have also been told that th

"I have also been told that these motors suffer poor idle due to worn piston skirts. Obviously this wouldn't show up in a compression test. How would I check for this? Would it show up in a leak down test?
Unfortunately I haven't had the time or funds to perform the carby rebuild yet, but will get onto it when I start work in a couple of weeks."
 
"Joe,
I ordered the carby kit


"Joe,
I ordered the carby kits this morning and will get onto the rebuild next week. Last weekend I ran the motor through the muffs and sprayed areostart down the throats at idle. There was no difference in the top carby, but when sprayed down the bottom carby the revs rose considerably. This would suggest that the lower carby is running lean, hinting at some kind of blockage. I will keep you posted.
Mike"
 
"Joe,
I have stripped both ca


"Joe,
I have stripped both carbys, removed gallery plugs (disc shaped ones), cleaned through with solvent and poked wire through all of the holes. There seems to be another ball shaped plug (approx 3mm in diameter)at the end of one of the galleries, does this need to be removed too? I have been reluctant to touch it as it wasn't supplied in the kit. I haven't been able to find any blockages or dirt, is there anywhere in particular that I should be looking? Also, when refitting the plugs is there any kind of sealant required? Cheers"
 
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