Mercruiser 250 inline 6 rpms intermittently dropping

[Cbastuan]

Hey guys,
Having an issue with my 1981 mercruiser inline 6 and wondering if I can get some opinions from people that know more about motors than I do.
What the problem is, is when I am cruising at around 1500-2300 rpms, sometimes a little lower/higher, the rpms will drop a few hundred then pick back up again, it will do it a few times, then run fine, or it will carry on for a couple minutes and runs fine.
It seems to run better after it has warmed up/ran for 8-10 minutes.
The work I've done to it this year so far is as follows.

-New stainless gas tank, moved to the bow of the boat, properly vented.
-new gas
-new 3/8" fuel line from tank, all the way back to the carb.
-new fuel filter/water separator
-new fuel pump
-new plugs (ngk's)
-new pertonix ignitor electronic ignition
-new cap and rotor
-rebuilt carburetor

So, I'm at the point where I might take the carb apart again and try rebuilding it? That seems to be where there could be a problem, just seems odd that it is intermittent and only runs poorly sometimes, and usually within the first 5 minutes of being underway.
If it was a carb problem would it not normally be running poorly all the time?

If anyone else has any suggestions or anything else I could troubleshoot or replace it would be greatly appreciated.

Thanks.

 

[NHGuy]

Is this before fully warm? Perhaps the choke is acting, you could wire it open or prop it open with something and give it a run. I don't know the distributor on those engines, but if it uses weights for centrifugal advance they can be tired or rusted sticky. The rpms you are talking about are right in the range where advance is occurring.
If there is any kind of vacuum hose to the distributor i'd replace it, they dry and leak over time affecting advance.
Edit, does the pertronix include a distributor? or just the electronic module with coil setup?

 

[Cbastuan]

Thanks for the reply,
Yes, it seems to be before fully warm, I only use this boat for fishing, so the main motor is really only ran for about 10mins at a time, I use a kicker for trolling.
Next time I take the boat out I will keep an eye on the choke while it's running, the distributor does use weights and springs for the advance, when I installed the electronic ignition I sprayed some lube on everything under there and it all seemed to move freely - when I set the timing after I installed the electronic ignition and gave it a bit of gas the light would smoothly advance forward.
No vacuum hose on the distributor.
Could the electronic ignition be the culprit?

 

[Scott Danforth]

May still be a bit of dirt in the carb. Could be the choke needs adjusting

 

[Cbastuan]

Yeah, I'm thinking it could be dirt or some crud in the carb, I'm not too familiar with carbs but if it was a carb problem with dirt or crud would it not be a consistent problem instead of only happening sometimes?

 

[stonyloam]

Try running non ethanol gas if you can get it in your area, 89 or 91. http://www.pure-gas.org My old 470 runs WAY better on non-e.

 

[Cbastuan]

Ok, next time I fill it up I will put in premium

 

[alldodge]

Ok, next time I fill it up I will put in premium

IMO that will just cost you more money, won't help your current issue.

I think you have a loose connection somewhere, may even be in or part of the distributor pick up module or plug wire

 

[Cbastuan]

IMO that will just cost you more money, won't help your current issue.

I think you have a loose connection somewhere, may even be in or part of the distributor pick up module or plug wire

Ok, I will double check all the plug wires, what is the distributor pick up module?

 

[alldodge]

what is the distributor pick up module?

The pertonix module bolted on the inside of the distributer.

 

[Cbastuan]

The pertonix module bolted on the inside of the distributer.

Ok,
I will double check that this weekend, check the connections, and make sure the module is getting a full 12v, will report back my findings.
Thanks

 

[Fishermark]

While you are checking the distributor, check the side to side play of the main shaft. I know I have had issues with straight six engines in the past of worn bushings causing the dwell to fluctuate. I don't know how it would affect the Pertronix, but it is something to check.

 

[Cbastuan]

While you are checking the distributor, check the side to side play of the main shaft. I know I have had issues with straight six engines in the past of worn bushings causing the dwell to fluctuate. I don't know how it would affect the Pertronix, but it is something to check.

Ok,
If the shaft of the distributor does have some side to side play in it is a new distributor in order?

 

[Fishermark]

Ok,
If the shaft of the distributor does have some side to side play in it is a new distributor in order?

Well, one thing at a time. But yes, if the distributor shaft bushings are shot, then a new distributor is probably in order.

 

[Cbastuan]

Good to know, thanks for all the responses, should make it out to the boat later this week, will report back with my findings, thanks

 

[Cbastuan]

Update,
Was on the boat today troubleshooting the ignition.

First had to replace the accelerator pump, the skirt kept slipping off the pump, (which I thought was odd cause I just rebuilt if with a kit from mikes carburetors, but it seemed to be missing the garter spring that nests under the skirt to prevent it from slipping off) so it wasn't working at all, hard to start, etc. Put a new pump in and or seemed to fix the hard starting and poor throttle response.

Wasn't able to take it on the water due to weather so was only able to troubleshoot with muffs on in the driveway. So I'm not sure if the intermittent hesitation while under load is fixed or not, wouldn't think the accelerator pump would fix that or cause the issue of hesitation/surging.

So, put a new coil in cause the one that was currently installed had 1.5ohms of resistance and I saw that I should have a coil with 3.0ohms of resistance, again not sure if that could solve the problem or not.

Checked the voltage on the + side of coil that the pertonix is hooked up to and it is 12.5v, so no resistor wire.

Checked the plug wires, they are newer Sierra ones, look to be in decent shape, and snapped onto the plugs and into the distributor cap tightly. However I have not replaced these so not sure on age of the wires.

Checked distributor shaft, no side to side play.

So, it idled fairly rough compared to earlier this year. A few months ago when I replaced the plugs, installed the electronic ignition and set the timing, the engine idled very smoothly like it was brand new.
Now it idles ok, but seems to stumble and miss the odd time and shudder a bit here and there.

Anyways, pulled the plugs in every cylinder to check them, every one was fouled with dry black soot, so I believe that means it is running/idling rich?

I'd like to get it idling/running a little better before I take it out on the water, the choke is functioning and opening fully once the motor warms up, both idle mixture screws are about one turn out.

So, I am going to try:
Putting a new set of plug wires on.
Taking the pertonix off and putting a new set of points/condenser on to see if it is the module that is the issue
And if neither of those things work, rebuild the carb again.

Does this seem like the right order of operations or should I be trying or looking at something else?
Thanks

 

[Bondo]

Ayuh,.... I'd start with the carb,..... The ignition won't cause a rich condition,.....

 

[alldodge]

Agree, go back to the carb
Now if it's just soot then fuel, but if its a darker and building up then maybe valve seals. Are you burning any oil?

 

[Cbastuan]

Yeah, I was suspecting the carb, now is there anything I should watch out for or pay really close attention to when I rebuild it again? What would cause it to run too rich?

Every plug had a thin coating of black soot, it didn't look like it had built up but the plugs are relatively new, now should I buy all new plugs or clean these up and keep using them.

I keep a pretty close eye on the oil, the level hasn't changed all season so I don't believe I am burning oil.

I am using NGK br6fs plugs

Thanks for the help

 

[alldodge]

A carb this old could be worn out but still worth making sure. As fuel goes thru the channels things wear. The most common cause for running rich is the carb float level. Then the idle mixer screws, can they be adjusted in and have the motor stumble.

Other things that can cause running rich is low compression and a vacuum leaks. Leaks can come from the carb base, linkage and intake manifold