Low top end power/ Low RPM

[SlumChick]

Hello all,

I'm new to iboats and i'm hoping I can get the right advise here.

I have a 1980 175 Merc o/b mounted to a 17' Invader. Last year I got the boat running after 10 years of sitting. It ran 3200 RPM at WOT and planed quikly because the low end was strong. Also, at 3200 RPM the boat traveled at about 33-35 mph. I Ran a compression check and one cylinder was low. This year I rebuild the engine replacing a sleeve, piston, and six rings sets. Now the boat pulls 4100 RPM at WOT and still has great low end power, but still doesnt travel any faster. Furthermore, The RPM still doesnt fall into 5300-5800 RPM range.

I can get the boat to go full speed with only half throttle, if i give it full throttle the engine just sounds a little rougher and the RPM and speed wont change.

The propeller has never been changed, the fuel pump and fuel lines are new along with spark plugs. Seems like an ignition problem to me. Please help!

 

[CharlieB]

Re: Low top end power/ Low RPM

Get a hold of an adjustable gap spark tester.

Pull all the plugs, rig the wires to the tester and set the gap at 5/16

Crank the motor and observe the spark
Increase the gap to 7 /16 and repeat
1/2 inch and repeat
Blue - HOT Arc Good
anything less and 'Houston, we have a problem'

 

[SlumChick]

Re: Low top end power/ Low RPM

Get a hold of an adjustable gap spark tester.

Pull all the plugs, rig the wires to the tester and set the gap at 5/16

Crank the motor and observe the spark
Increase the gap to 7 /16 and repeat
1/2 inch and repeat
Blue - HOT Arc Good
anything less and 'Houston, we have a problem'

Thanks Charlie for the fast response.

i'll give it a try tomorrow when I get a hold of a tester.
Just one question, should there be a "hot" blue arc at all three of the distances?

 

[SlumChick]

Re: Low top end power/ Low RPM

well I got a adj. spark tester today and 4 of the cylinders have a strong blue arc at 1/2 inch. # 3 & 5 cylinders don't show a spark even at 1/4''.

Now i'm pointing my finger at the coils. The thing is that I replaced The number 5 coil not too long ago.

Where else should I be looking? switch boxes? trigger? stator?

Please help.

 

[SlumChick]

Re: Low top end power/ Low RPM

I messed around with the tester a little more and now # 3 is sparking well. I switched the lead that signals the coil to fire from the # 1 coil to the # 5 coil and I testsed for spark and it works. So that tells me all the coils are GOOD.

However, just to make sure I switched the # 5 lead signal wire to the # 1 coil and it tested negative for spark.

Do I have a bad switch box? what could possibly prevent my #5 coil to fire?

Help!

 

[SlumChick]

Re: Low top end power/ Low RPM

okay I switched the two switch boxes and now everything works well!?

Now i'm really confused? I called my mercury dealer and each box is $248 dollars. i'd be ok with buying it only if I knew for sure that was my problem.


HELP! Thanks