3.0 mercruiser missing on top end

[scottn83]

I have a 1983 3.0 alpha 1 on a 17ft cheetah runabout prop is a stainless steel diameter 14" 19 Pitch

First of all the problem is at about 2800 rpm's(30mph) sounds like it was missing, rattling defiantly a change in the sound of the motor if i push through it will settle back to normal around 3200(32 MPH) seems to be the sweet spot then starts the noise agian at about 3500 rpm's. also WOT is about 3500(34 mph). Was expecting to have WOT at about 4000 to 4200 rpms and speed closer to 40MPH. I guess that makes two issue one WOT and Two the change in engine noise

Before going to the lake checked the timing had to adj slightly found the disto cap loose tighten as well. Checked compression on a cold engine. 90psi in number 1,2,and 4 and 120psi in the 3. I understand that is 10psi low in 1,2,and 4 but will this cause the problem I am experiencing, Or should i be looking somewhere else

Thanks in advance

 

[alldodge]

Re: 3.0 mercruiser missing on top end

Low compression is going to cause a bunch of issues. Three out of four cylinders low is a rattle can. Max WOT can never be reached if the engine is not producing max HP. SOunds like you need to rebuild or replace

 

[scottn83]

Re: 3.0 mercruiser missing on top end

i have no blow buy have about 15 hours on an oil change no need to add and no smoking could it be an issue with the head or valves

 

[scottn83]

Re: 3.0 mercruiser missing on top end

investigating another issue found the valves in cylinder 3 was way out of adj the rest a little loose. Haven't had a chance to check it out if see if it fixed the problem but running up to 3000 rpms in the drive way doesnt have any rattles at all but it isn't under load either

 

[scottn83]

Re: 3.0 mercruiser missing on top end

Valve adj has fixed the pinging. I ran it today for the first time today after changing the lower unit (which had alot of issues) removed the plastic fin installed on the out drive. Now she runs out at about 42 mph at about 4100 WOT. No pinging.

One question though if i have the vavle adj to tight will it make it hard to start after the motor has warmed up? I am wondering if i didnt get the lifters pumped up enough before I adj. them.

Asking because after spending about an hour running around the lake on a test run ideling into the marina engine dies just as we got to the dock and wouldnt start had to pull it up on the trailer.

 

[tabc]

Re: 3.0 mercruiser missing on top end

Yes it would be hard to start after warm. A tight valve gets even tighter when hot due to expansion causing the valve not to seat properly, causing compression to drop. That would make for a hard start and rough running when hot

 

[alldodge]

Re: 3.0 mercruiser missing on top end

Valve adj has fixed the pinging. I ran it today for the first time today after changing the lower unit (which had alot of issues) removed the plastic fin installed on the out drive. Now she runs out at about 42 mph at about 4100 WOT. No pinging.

One question though if i have the vavle adj to tight will it make it hard to start after the motor has warmed up? I am wondering if i didnt get the lifters pumped up enough before I adj. them.

Asking because after spending about an hour running around the lake on a test run ideling into the marina engine dies just as we got to the dock and wouldnt start had to pull it up on the trailer.

Agree with tabc as for the hard to start if the valves are adjusted to tight, but your engine should have hydraulic lifters, right?

If they are hydraulic they are adjusted down till they touch and add another 1/4 to 1/2 (1/4 is usually enough). If they are solid they should be I'm thinking (have not looked them up) .010 on intake and .030 on exhaust.

 

[scottn83]

Re: 3.0 mercruiser missing on top end

according to the 3.0 service manual you pump up the lifters then at #1 compression TDC you adj certain valves until the push rod just tighten up enough that you cant turn it with your fingers then 3/4 of turn clockwise from there. Then one revolution back to 0 on the timing mark you adj the rest of them the same way. That is how i adj them.