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Replace switch box or coil

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dogditcher

New member
"Hi All
I have a 1994 20hp me

"Hi All
I have a 1994 20hp merc model 1-02020ind
serial OD209393 -compresion checks at 120 each cylynder -replaced fuel line and installed new ngk plugs -fuel is fresh - so i kind of thought that its not fuel related -so heres what I am dealing with -motor seems to run well with no hesitation but i recently kinda felt that it was not running at top speed -I have one of those tiny tacks wrapped to bottom plug wire and usually it would run around 4900 rpm at full throttle but now it runs at about 3000 rpm's -I decided since it didnt really show any hesitation just less power I switched the tach to the top plug wire just to see if there was a difference and that shows it at 4900 rpm's seems like a switch box or coil failing -also it seemed to drop rpm's the longer the trip when it was on the bottom plug wire -any help or ideas would be great-thanks Steve

heres a pic of plugs bottom is on the right
220563.jpg
plugs
 
"check for a consistent spark

"check for a consistent spark with a timing light while running, compare it to the top plug. Looking at the plugs, I would say its not getting fuel. Which means a reed valve or seal problem, lets hope I'm wrong."
 
"JB thanks for the reply
hook

"JB thanks for the reply
hooked timing light today and really couldn't see a difference in strobe-I did move the the tach back down to bottom plug and it was at correct rpm,s for idle - this was done in a tank so maybe the switch box fails as it get hot -going to take it out this weekend and see if it acts up -is there a way to test switch box to eliminate as the problem and if it is what you suggested is there a test to check reed valve -thanks"
 
"re-check spark while under wa

"re-check spark while under way when warm, pump fuel bulb if engine acts up to see if it helps. There isn't a test for the switch box without special equipment, its a process of elimination. Normally they either work or not. The reeds are veiwable behind the carb adaptor plate. Reeds would cause the engine to spit fuel back through carb while running if stuck open. You can try pouring oil down carb to temporary seal the bottom seal. Does each cyl go poof when the plugs are removed and engine spun over? Perhaps Graham Lamb will help us out here, small engine expert."
 
"The switch boxes on these are

"The switch boxes on these are pretty solid - if you are getting any spark at all I wouldn't suspect the box. They almost never ever fail, but when they do, it's totally toast, no intermittent issues.

If you are losing spark when the motor warms up, and only on 1 plug I would concentrate on the trigger or coils. If you swap the coils and your nice clean plug comes out not so clean after running for a bit, probably a bad coil. If there is no difference I would check the continuity on the trigger.

You can also do that without running the motor. Just take a hair dryer to the flywheel and warm it up for 5 minutes or so - any issues with the windings will become apparent.

However, based on what you have noted so far I would suspect either stuck reeds or a blown lower crank seal (somewhat common - my upper seal on my 15 horse is "going" - evident by an oily substance all over the the top of the powerhead below the flywheel). Have a look in the base pan and if you have (oil) pooling - the mixed gas evaporates and just leaves the oil - then you may have a shot seal..."
 
"[b] motor seems to run well w

" motor seems to run well with no hesitation but i recently kinda felt that it was not running at top speed -I have one of those tiny tacks wrapped to bottom plug wire and usually it would run around 4900 rpm at full throttle but now it runs at about 3000 rpm's -I decided since it didnt really show any hesitation just less power <u>I switched the tach to the top plug wire just to see if there was a difference and that shows it at 4900 rpm's</u>

Question,
If the tack usually ran around 4900 RPM and then you kinda felt it was not running at top speed, (as suggested by a drop to 3000 RPM?), then how is it that you got 4900 RPM on the top? A loss of power will always result in a drop in RPM at a given load. So when the tack is hooked to #1 and it reads 4900 RPM where is the loss in power? Something just isn’t right here."
 
Thanks Graham -Jb -i do apprec

Thanks Graham -Jb -i do appreciate taking your time and helping me out
Allan
I here what you are saying -My thought is that it is an electrical problem as that top cylinder shows correct rpm' and bottom shows the lower rpm's -so my thought is it could be a bad coil- bad wire -bad switch box-or trigger since it seems to be a be a reading of the tach that shows the difference and not mechanical as to the running of the motor-both Graham and JB are top notch and there thought of it possible being a reed or seal issue may be spot on but just trying to eliminate the other possibilites-I did not notice any pooling of oil in base pan and not noticing any fuel spitting of gas out the carb-will be testing tomorrow and may switch coils while im out there if weather allows-will post my results-Thanks Again
 
"My guess was based on the con

"My guess was based on the condition of the new plugs. If not a seal or fuel problem, could also be a water intrusion problem, that one plug looks to clean. Still hoping I'm wrong and it is electrical."
 
"Just for the sake of argument

"Just for the sake of argument let me posit the following:
Since we are dealing with two cylinders, each cylinder contributes to one half or 50% of the power output. If cylinder #2 shows a drop from 4900 RPM to 3000 RPM that is a loss of 1900 RPM or appoximately 40%. Since power output is shared equally that is a 20% drop in total power available. Since RPM is a direct function of horsepower at a given load and 20% of that power has been lost, how can the tack read 4900 RPM on the top cylinder? Any corresponding loss in power must be accompanied by a loss in RPM of both cylinders.
Can anyone enlighten me?"
 
My thought is that the since t

My thought is that the since these tach's pick up a pulse from the ignition wire the top cylinder coil or switch box is working correctly but the when it is on the bottom cylinder the switch box or coil is not firing everytime so the reading would not be the true rpm's but it is also causeing a loss of power to the total operation my thought of why the bottom plug would be cleaner because it is not always firing as much as the top - just my thinking
 
Ok update
Took it out today a

Ok update
Took it out today and ran it with tach on bottom cylinder registered at appx 3000 -switched coils and still registered at appx 3000 rpm's so I switched tach to top cylinder and it was at appx.4600 rpm's- so it looks like the coil is not the issue and thinking it is either switch box or trigger or possibly a bad ignition wire or maybe its just all in my head !!
 
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