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Idle and low rpm problems

O

oneal

Member
"All HELP NEEDED AND APPRECIAT

"All HELP NEEDED AND APPRECIATED!!!!
1975 25hp Johnson 25R75B
New seals & o rings in lower unit.
New water pump. (pumping)
New carb. kit. (including gaskets).(clean)
New points and condencers. (@ 20)
New plugs. (4's) Champions (@ 30)
All is great from a cold start idles great nice and smooth.
Excerlatetion smooth/ mid smooth/ top smooth.
As motor WARMS / 1 to 2 hundred yard run.
Top end stays fairly smooth and crisp*(note 1)
Mid is hampered a small bit (not real bad).
Idle will sputter, or sneeze, or spit, (jerking)then dies (as if too lean).
Mixture set @ 1 and 5/8 OUT. Trying many different settings.
Warmer it gets, the worse it gets, but motor does NOT get hot.
Confusing several good mechanics.. (me also).
Clean fuel 50 to 1 ratio. (No water).
Clean fuel lines. (No air leaks). (tight fittings).
All orfices in carb clear.
Water passages clear.
Thermostat removed (to eliminate suspicions).
Compression 125 (both cylinders).
Good spark at plugs. (Checking while not running) NO timing marks.
All is set at specs since parts are new.
The note* was at top end (wot) it is shearing prop shear pins...????? and I'm not hitting any thing at prop.
Really 2 issues that are Really confusing.
I'm wondering can it be vapor lock/ or maybe timing or.....?????????
Will greatly apreciate all imputs and suggestions.
Please help put (little ole me) on my fishing hole with no worries.
THANKS!!!
DAVE."
 
"Some tips:
Change plugs to J

"Some tips:
Change plugs to J6.
How does the color look like at the plugs after running (warm engine)? Post a pic if possible.
If that engine has been running since 'new' with same head gasket and exhaust cover gaskets, change them!"
 
"Morten Thanks!
Motor was sup

"Morten Thanks!
Motor was supposedly rebuilt 2yrs ago.
I will order plugs, and gaskets today!
Thanks again!"
 
"One reason for my comments is

"One reason for my comments is that old gaskets dries, bolts gets stuck With temp expansion of head bolts and dried out gasket, minor leakage may occur. Exhaust side the same. You may probably be able to see from 'outside' and feel if bolts are loose or 'stuck'.
The plug recommendation was made many years ago, mainly due to low speed and idle problems. The J4 ran too cold.
If removing head, check that all passages are open and circulation good in both block and exhaust cover.
And get the thermostat back!"
 
"Oops sorry,
PLugs are just a

"Oops sorry,
PLugs are just a tad darker than what they are supposed to be.(More toward brown).
Top plug electrode is more blackish.
Pictures on the way."
 
"First mechanic said he tighte

"First mechanic said he tightened up those bolts around head and exaust, so -- I agree that they need replacing.
I'm also going to replace plug wires and boots.
Still waiting on that thermostat.
Thanks!"
 
"i might add,.....that 1 5/8..

"i might add,.....that 1 5/8.....is prob too far out....try 1 turn out,and fine tune from there."
 
jwbmarine Thanks!
I've be

jwbmarine Thanks!
I've been wondering if I should change the intake manifold gaskets also.
Would it draw air as the motor warmed if it had a crack?
 
True.
I've been hearing t

True.
I've been hearing that the older Johnsons were
notorious for overheating.
Any truth???
I wonder because I had a 74' 20hp and it acted the same way now that i've been thinking about it.
 
not at all...in my opinion....

not at all...in my opinion....just like any other motor......it all depends on a good water pump...which...is a flimsy lil impeller.
 
"David there might not be much

"David there might not be much wrong with your engine, if the head gasket and side cover gaskets were changed two years ago they will be fine as OMC recomended every 5 years.

The carby low speed needle valve has a seal which pushes in first then the needle is screwed in after that.

If this seal is deteriated you will suck in air between the thread of the needle valve.
The setting of the low speed needle was 1 and a half turns out from the seated position and you may find do to elevation that you might have to screw it in or out to eliminate your problem.

Cheers from Australia"
 
Are the points open (to ga

Are the points open (to gap setting) at TDC?
If so then the other piston would be bdc. RIGHT?
I have (set) in writing in the lobe the points ride against.
But I noticed the piston would be at TDC at a different area than the set mark.
And each piston different than the other.
Please go into detail on answer...
Many Thanks!!!
 
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