running rough at low revs

[bradski2]

just purchased a pride ski boat with a 1985 200hp oil injected mercury outboard. have had it out twice now and have noticed that anything under 2000 revs and she runs abit rough, anything above this and she runs fine. While being towed behind the boat i noticed that once we slowed down it started to smoke alot and run rough we think it may be getting to much oil at low revs just wanting to get some thoughts as i cant get anyone to look at it this side of christmas and am prepared to have a crack at it myself. I also just replaced the plugs but it made no difference. thanks Brad

 

[j_martin]

Re: running rough at low revs

Welcome to Iboats.

It's obviously missing at low speed. First thing to do is a compression test. If it's bad, that needs to be fixed first. If it's good, then we can go hunting for the trouble.

A maintenance manual and systematic diagnosis is by far the fastest and least expensive route to the fix.

hope it helps
John

 

[bradski2]

Re: running rough at low revs

thanks john just did a compression test and got 150 psi on each cylinder i am assuming this is good???? thanks brad

 

[j_martin]

Re: running rough at low revs

If your gauge is accurate, you need to be running at least 92 octane fuel. The PO may have CC'd the heads.

Stock compression is somewhere around 120 Psi.

hope it helps
John

 

[bradski2]

Re: running rough at low revs

what do you mean by po and cc'd the heads sorry for not knowing but this is my first boat brad

 

[j_martin]

Re: running rough at low revs

what do you mean by po and cc'd the heads sorry for not knowing but this is my first boat brad

PO is Previous Owner. CC is a reference to the volume of the head space in cubic centimeters. Heads may have been cut.

A common mod is to mill the heads to increase compression. 145 - 150 is about the limit for pump premium. Way to high for 87 octane.

hope it helps
John.

 

[bradski2]

Re: running rough at low revs

thanks john, that sounds llike a possibility for and old engine with such good compression. i will drain the fuel and replace with a higher octane and see how that goes thanks again brad

 

[j_martin]

Re: running rough at low revs

Brad, when you up the squish like that, they are usually a little uppity at idle. For skiing, where much of your time is spent putsing around, and half throttle pulling a skier, the best bet might be to get a set of stock heads and throw them on, bringing the compression down to 120 or so.

If full blast is yer mode of operation, live with the idle.

hope it helps
John

 

[bradski2]

Re: running rough at low revs

well just got the boat back from the mechanic he found a voltage problem with one of the power packs so that has been replaced. he also replaced a couple of cables that were abit ****ty and gave it a service. just got back home after a test drive and i am very pleased. the motor ran alot smoother at idle and under 2000 revs. so very pleased just in time for the holidays thanks again john for all your help
Brad