Cylinder down? 99 90HP Merc

[zacros2020]

How can I tell if a cylinder is down on an engine if I do not have a tach? Boat comes on plane, but seems to take a little while. 17' Key West tops out at 35 mph at WOT with 19 pitch prop (two people, no gear 25 gal of fuel). Does this sound about right? Seems a little slow to me.

 

[Silvertip]

Re: Cylinder down? 99 90HP Merc

Check for spark on all cylinders. Spark testers are cheap and readily available at any store that sells auto parts.

 

[zacros2020]

Re: Cylinder down? 99 90HP Merc

Thanks for your help. I will try it out this weekend.

 

[Silvertip]

Re: Cylinder down? 99 90HP Merc

If you do have good spark on all cylinders, a compression test is in order.

 

[zacros2020]

Re: Cylinder down? 99 90HP Merc

Finally got a chance to try the spark tester today. All three cylinders seem to be firing. Guess a compression check is next. I gave read before that these Mercs use only 2 cylinders at idle. Is this true? If so, could this cause the hesitation that I am seeing from time to time? When I was checking the spark, I could definately tell the engine sounded different when testing the bottom two cylinders. However, when testing the top cylinder, there was no change in the tone of the engine (when compared to having all three cylinders online). Any thoughts?

 

[Silvertip]

Re: Cylinder down? 99 90HP Merc

You 99 Merc is a three cylinder and it fires on all three at idle. It's the 115/125 four cylinders that idle on two cylinders.