Running Rough Merc 165 L6

[brodie123]

Hi All;

Mercruiser L6 165 HP

Engine is running rough after winter storage. It seems to 'miss' periodically and bog a tich. Was fine in the fall. Other than the odd miss, seems to run well. While running, it will run smooth but then once in a while 'rattle' and you can here what sounds like a miss.

Here's what I have done:

- replaced spark plugs and when I re-checked them, they were coated lightly but black carbon
- Re-did the timing
- Cleaned the rotor and cap - looked fine. Only a couple years old. I upgraded to electronic ignition a few years back using a conversion kit for the distributor.
- re-did the mixture screws(2) - "seemed" best at 1.25 turns
- compression is good. Engine was rebuilt a few years ago

The fuel tank had about 40L of fuel in it (with stabilizer) since last fall. I put around 25 liters of fresh gas in.

Any suggestions??

I'm headed for the water next weekend and would like to make sure I'm good prior.

 

[brodie123]

I don't know if this helps - but here's some additional info.

- switched to fresh gas, same issue
- Plugs were gapped at .035
- this 'miss' is more evident at higher RPM and appears to become worse.

I'm open to any suggestions.

 

[brodie123]

Any suggestion at all? All I can come up with is to put in some fresh gas and take it for a run.

If it was bad gas, how long would it take to clear itself out?

 

[Bondo]

Any suggestion at all? All I can come up with is to put in some fresh gas and take it for a run.

If it was bad gas, how long would it take to clear itself out?

Ayuh,..... Have ya checked/ changed the fuel filters,..??

Addin' fresh gas to bad gas, just makes more bad gas,....

I'm thinkin' the carb needs to be Rebuilt,.....
Yer symptoms say it's goin' Lean,.... that Rattle ya hear is detonation as the motor comsumes itself, 'cause of a Lean condition,....
The soot on the plugs says it's rich at other times, which indicates the carb is AFU,....

 

[Scott06]

I was gonna chime in with fuel related as well. sound like filter and carb rebuild time. may want to look in the tank as well. Given your engine has to be an older boat, might need to inspect the entire fuel system. Start with the filter and anything but clean gas and you've got to clean things out.

 

[brodie123]

Thanks. I sure will do. The carb hasn't been rebuilt so worth doing in either case.

Curious: Will leaving 20-40L in a 150L tank over the winter actually gain enough condensation/water to spoil the gas? I can appreciate that in that time the gas goes a little stale and not have as much power, but would it actually affect it that much??

If it sat for a year+ I could see it. I'm just surprised is all.

Thanks!! I do appreciate it.

 

[Scott06]

Hard to say diagnosing over the internet. Probably not- only time I had this issue was in a 30 year old boat so I suspect it's an accumulation of debris over the years, carb never rebuilt etc. rebuilt carb cleaned out fuel tank and lines ran like new. I assume your fuel lines aren't rated for ethanol exposure they could be breaking down on the inside ( assume your boat isn't ore 1982 as I think that's the last time merc made a 165) and clogging up carb. Only way to tell is to look. If you find junk in carb only way to fix it is to keep working back to tank until u find good clean gas.

 

[brodie123]

Thanks. Fuel lines were replace around 5 years ago.

I had the carb rebuilt at a shop. Replaced the fuel filter. Let her run at 2000 RPM for a while. Still getting a lot of black carbon on the plugs and the odd flutter. (rattle)

fyi, new plugs and gapped at .035. plug wires/dist cap etc. replaced 2 years ago.



Suggestions????

 

[brodie123]

Perhaps I should be looking at the plug heat range?? I recently installed AC Delco R42TS and admittedly this is when the problem became apparent. Not sure if it was there prior or not.

I had NGK BR9ES but experienced hard starting. Mind you, the timing was a little retarded and was corrected when I changed the plugs.

Should I be replacing the plugs?

The manual calls for Champion (no thank you) RBL8

 

[brodie123]

Ok. Installed AC Delco MR43TS (heat range of 3 vs 2) and same problem - I did let it run.

Problem is worse at higher RPM.

Bad Gas????????

 

[kenny nunez]

Try to check the dwell when the engine starts to run rough. You may have a worn out distributor.

 

[brodie123]

It's an electronic distributor.

 

[kenny nunez]

Just be sure that the rotor shaft is snug and that the centrifugal advance is in good order.
I once had a 3.0 with a full throttle miss that we were checking on my shaft dyno, the problem was fuel starvation because the power valve spring was rusted and not strong enough to hold the valve open.

 

[brodie123]

I am amazed of how the gas made a difference. I'm not sure how much was in the tank but it probably had a 1:5 of old and new gas. Once that ran out and used 100% new gas, much improved.

I do think I'm going to replace the distributor - it's old and was converted to electronic. The coil is quite old. I would tend to think the coils weaken over time???? or at least intended for points vs. electronic?

I have the same engine on another boat and I went with an HEI and that performs really well. It's worth the upgrade.

Would you agree?