1985 Mercruiser MCM 260 Questions

[25thmustang]

Im a “new” owner of a 1985 Regal 255 with a Mercruisers MCM 260, with a Thunderbolt IV ignition and early Alpha drive (all original). I’ve been plugging away on it (project) and took it out a little while back for a shakedown run.

On the muffs it starts great and idles/revs no issues. On the water it started the same. Got out on the water and went along at 1500 RPM or so for a 10 minutes run. After a few minutes there was some hazy smoke coming from both sides of the engine (just under the manifolds). The IR heat gun showed 170-175 at the temp sensor, around 105-110 on the risers, and 175-190 through the manifolds. Coming in to the dock to grab something and as soon as I hit neutral it shut down and would not restart.

At home she starts and runs great on land. I went to check timing and the marks are all over the place at idle. I can’t get a solid reading. Changed the plugs however have not done any other “tune up” items.

Did a compression test and got 118-119 on all cylinders, 110 on one. This was completely cold. I’m getting mixed info on whether this is good or bad. I assumed this was fine and the one “low” cylinder was within 10% which is acceptable. Some say these are all low and signs of issues...

Any suggestions on what could cause the timing marks to be sporadic and impossible to read? Also any thoughts on the compression test looking good/bad?

 

[Scott Danforth]

you did do all the maintenance with your new to you boat that the prior owner neglected for years, correct? such as pull the drive and install a new raw water pump? checked alignment, gimbal, etc.?

your compression is a bit low.
a new motor is 150psi

I would check your gauge, it could be reading low.

if your gauge is accurate, you have a fair amount of leakage which could be from overheating.

 

[25thmustang]

The compression tester is new, but just a Harbor Freight unit. Would it be beneficial to get another one and test again?

I did the gimbal bearing, alignment, bellows and plugs, it had a fresh oil change. Still have to do the impeller and thermostat.

 

[Scott Danforth]

I would test the harbor freight unit. even my milton compression tester reads off by 10 psi...... in its defense, it is 40 years old and was dropped a few times.

 

[25thmustang]

I’ll try and get my hands on another/better unit and test again. Any thoughts on the timing marks being unreadable? I thought I was missing a step, but it appears to be straight forward.

 

[achris]

What was the engine rpm when you were reading the timing? Also, check all your grounds. And if the sensor in the distributor is still the old style (screw terminals and no ground wire), it's worth upgrading... (To the new style)

On the compression test... Cold is not really a good test, also was the throttle fully open?

Chris...

 

[25thmustang]

Engine RPM for the test was 750 or so.

I read mixed reviews on cold verse hot for compression tests. I initially figured I would test cold and if I questioned it, test hot. I heard that cold will indicate an issue that warm may not (but that could be misinformation). Yes I had the throttle wide open.

 

[achris]

750 or so...

It's the 'or so' that is a concern. The advance on TB-IV isn't specced to begin until 1,000, but there are tolerances on the spec, and it can start kicking in as low as 800rpm... You need to have the engine running at NO MORE than 700rpm to check the timing reliably. And no, it doesn't need to be in gear, or even in the water. On muffs is just fine. Idle speed, yes, that needs to be in the water, in gear, and unhindered (not on the trailer or tied to a jetty, out in open water)...

Compression hot/cold. Ok, you can debate all you like, say 'so and so said....', but at the end of the day, every book written by an automotive engineer on the subject says the same thing, HOT.... And the debate ends right there!

Chris..........

 

[25thmustang]

Would a higher (750 RPM) idle cause the timing to skip around, or is that something unrelated? I imagine they are unrelated.

I can do a hot compression test on it and verify what numbers I get if the actual experts say to do it that way.

 

[jimmbo]

Worn bearings in the distributor will cause wandering of the timing, so can a badly worn Timing Chain

 

[achris]

Would a higher (750 RPM) idle cause the timing to skip around, or is that something unrelated? I imagine they are unrelated.....

A couple of degrees is all I'd expect, as would worn dissy bearings or timing chain. If it's jumping around to the point of being unreadable, start looking at the timing light trigger, or the lay of the trigger lead, or if it's running past another spark plug lead. Also check the condition of the leads. I'd more suspect the instrument than the engine, especially if it seems to be running quite steadily. You could also put a vac gauge on the intake manifold and see if that's jumping around. If it's steady, it's very unlikely the timing is actually jumping around....

Chris.....

 

[25thmustang]

The timing light is brand new. I may see if a neighbor has one I can try again with and confirm the same skipping.

Regarding the worn timing chain or bearings in the distributor, would those come from wear over time or wear over use/miles? This boat has less than 300 hours on it (I would have preferred it had a lot more) which it appears to be correct. Not a lot of “use” but a lot of sitting over the years.

It starts and runs steady, no stumbling, no surging.

 

[achris]

...Regarding the worn timing chain or bearings in the distributor, would those come from wear over time or wear over use/miles? This boat has less than 300 hours on it (I would have preferred it had a lot more) which it appears to be correct. Not a lot of “use” but a lot of sitting over the years.

I'm also not a fan of 'low hours' boats. It's not USING a boat that kills it, it's NOT using a boat that kills it.

Wear in parts like distributor shafts and timing chains is only going to come with use, not sitting around doing nothing...

It starts and runs steady, no stumbling, no surging.

The I suggest the timing isn't jumping around, it's the light..... Clip the light to another lead. You might not be able to read the actual timing, but it may show the flashing as more steady.

Chris.......

 

[25thmustang]

Redid the compression warm. Cold verse hot read as such:

1) 118 - 120
2) 119 - 119
3) 118 - 119
4) 119 - 121
5) 110 - 112
6) 119 - 121
7) 119 - 121
8) 119 - 122

I read that the timing on the TB IV should be checked at 1000 RPM. So I did. Here is a video (pardon the messy boat and my poor camera skills).

https://youtu.be/N-Ewhqsco7M

 

[achris]

I read that the timing on the TB IV should be checked at 1000 RPM. So I did. Here is a video (pardon the messy boat and my poor camera skills).

https://youtu.be/N-Ewhqsco7M

Absolutely incorrect. Timing should be read and set at idle speed, 650rpm.

Please obtain a factory workshop manual and read that. Watching YouTube can be very misleading... The are an awful lot of people out there who haven't the first idea of what they are doing...

Chris....

 

[25thmustang]

The timing fluctuations are the same at idle speed (which was anywhere from 680-730 RPM when warmed up). I will get my hands on the manual for this model.