Popped through the intake.

[cyclops222]

How much leakage can really happen when the cylinder is firing 12 times a second ? At 600 times a second ?
I would run it as is.

 

[Pmt133]

So I just ran an hour sustained 3800... no issues. So I'm more confused than when I started.

 

[Lou C]

Old boats with carbed engines are just prone to these kinds of things, can't hook up a scanner and read live data!
I agree I would run it, if the missing comes back, do a cylinder balance test....
Is the carb a 4bbl? Weber or Edelbrock? In which case it is pretty easy to take off the air horn to see if there's anything floating around in there...

 

[Pmt133]

Old boats with carbed engines are just prone to these kinds of things, can't hook up a scanner and read live data!
I agree I would run it, if the missing comes back, do a cylinder balance test....
Is the carb a 4bbl? Weber or Edelbrock? In which case it is pretty easy to take off the air horn to see if there's anything floating around in there...

1 jet 2bbl mercarb. I have a 4bbl weber sitting on the bench to go in... once this is figured out for good.

 

[cyclops222]

You are aware that true Weber carbs used to require a LOAD of different liquid & air jet sizes to get them tuned correctly. It is a slow & tedious procedure. Unless things have changed.

Weber's make great paper weights.

 

[Pmt133]

You are aware that true Weber carbs used to require a LOAD of different liquid & air jet sizes to get them tuned correctly. It is a slow & tedious procedure. Unless things have changed.

Weber's make great paper weights.

Its a merc weber off the same engine set up to the factory specification. Having had them all, for this application I'll take the weber.

 

[Lou C]

I thought the Mer-Carb 2bbl was a modified Rochester 2GV (sure looks like one) whereas the Weber 4bbl is an update of the old Carter 4bbl used on many Chrysler products and the Edelbrock is very similar if not identical to that....
I have a Quadrajet on mine now that needs a cleaning and rebuild and I also have a Holley 4160 calibrated for this engine sitting on a shelf in the garage, and believe it or not I just bought an Edelbrock 1409 as well. So if one carb gets gummed up and can't be made to work right due to dang ethanol gas I have options.
BTW, here's an idea for those of us in draconian states that force you to use E10. When winterizing, how about filling the fuel/filter separator with straight gas like VP Racing, etc, and then run the engine on that to fog it? That way the E-10 isn't sitting in the carb all winter. The ethanol related deposits and the deterioration of rubber parts seems to be what causes most of the problems with carbs and E10. My fuel is always totally clear when I dump the filter though.

 

[Lou C]

You are aware that true Weber carbs used to require a LOAD of different liquid & air jet sizes to get them tuned correctly. It is a slow & tedious procedure. Unless things have changed.

Weber's make great paper weights.

The Merc Weber 4bbl was not a traditional Weber but a refresh of the old Carter 4bbl. I know of the ones you’re referring to lol!

 

[Pmt133]

I thought the Mer-Carb 2bbl was a modified Rochester 2GV (sure looks like one) whereas the Weber 4bbl is an update of the old Carter 4bbl used on many Chrysler products and the Edelbrock is very similar if not identical to that....
I have a Quadrajet on mine now that needs a cleaning and rebuild and I also have a Holley 4160 calibrated for this engine sitting on a shelf in the garage, and believe it or not I just bought an Edelbrock 1409 as well. So if one carb gets gummed up and can't be made to work right due to dang ethanol gas I have options.
BTW, here's an idea for those of us in draconian states that force you to use E10. When winterizing, how about filling the fuel/filter separator with straight gas like VP Racing, etc, and then run the engine on that to fog it? That way the E-10 isn't sitting in the carb all winter. The ethanol related deposits and the deterioration of rubber parts seems to be what causes most of the problems with carbs and E10. My fuel is always totally clear when I dump the filter though.

It is a Rochester, just with one idle jet instead of two. But I also have the 4bbl and intake ready to go. Sorry for any confusion.

Honestly, I've only had 1 ethanol related issue on any of my boats and it was caused by being laid up for 2 years. Splash of stabilizer at the end of the season on the last run and when fogging the engine purging the bowl vent and filling with fogging oil has kept me from needing to touch any of my carbs for like 20 years now. Same with the lawn equipment. It is an issue, but much like anything else, proper preventative maintenance goes a long way.

 

[Scott06]

Its a merc weber off the same engine set up to the factory specification. Having had them all, for this application I'll take the weber.

Agreed the webber edlebrock is easy to tune if you have some background in it. If you have the factory webber put that on and see if the issue goes away.

It is a Rochester, just with one idle jet instead of two. But I also have the 4bbl and intake ready to go. Sorry for any confusion.

Honestly, I've only had 1 ethanol related issue on any of my boats and it was caused by being laid up for 2 years. Splash of stabilizer at the end of the season on the last run and when fogging the engine purging the bowl vent and filling with fogging oil has kept me from needing to touch any of my carbs for like 20 years now. Same with the lawn equipment. It is an issue, but much like anything else, proper preventative maintenance goes a long way.

The rochester with one idle jet (screw?) is a mercarb. Merc made their own after GM stopped making 2 jets in late 80's or 1990 ish. Obviously a common lineage to the 2 jet.

I have had them all except a holley (only rebuilt one on my buddies VP). They all work until they dont... most of the complaints you hear mean the guy complaining doesnt know WTF he is doing...

I think the AFB/webber/edlebrock design is nice because you can do a lot of tuning by just the step up rods. While certainly not as tuneable as some of the higher end holley or holley derived carbs they are forgiving to the avg mechanic...

I also have had no issues with ethanol fuel. I run non ethanol in the boat but fill my sea doo, classic car, and lawn stuff with E10 out of convenience. Put stabil in in the fall, The only one I gotta watch is my generator that sits ....

 

[Pmt133]

So I got it to do it today. Firstly, I went out for 4.5 hours. Spent 90% of that time between 3200 and 3600 RPM. No issue. Last leg I decided to open it up. Hammer down boat comes up to about 4600 then starts breaking up within seconds. Back off and it went away. So. I pull into a cove and do some testing. Noting the engine is idling fine. Firstly, I don't believe this is a lean sneeze but a possible slightly lean condition?

Now... Temperature with IR gun, everything is 140-160F. Coil is 170 and feels hotter than everything else. So, I pull it and ohm it out. Now on my fluke meter (same one for both tests) I previously got .80 ohm on the primary winding and 11.38 kohm on the secondary.... mercruiser book is .60-.80 and 9.4-11.70. After it acted up I got 1.2 ohm on the primary and 13.24 kohm on the secondary, so out of range. Obviously heat will drive up resistance but... I don't know that it should drive it that far out of spec.

Am I right in assuming that there may be a coil issue too? Two different things contributing to a common problem?

 

[Lou C]

Hard to say but it seems that the spark is breaking down under higher rpm, try ohming out your plug wires, coil lead, and check the cap and rotor carefully. Think the plug wires shouldn't exceed more than 1,000 ohms per foot....

 

[Pmt133]

Merc doesn't specify plug wire resistance. But... There is a note in service manual 18 to replace if it exceeds 30,000 ohms. so...

1. 14,620 ohm
2. 12,790 ohm
3. 15,550 ohm
4. 13,160 ohm
5. 12,400 ohm
6. 36,500 ohm
Coil wire: 6,610 ohm

So I'd say 6 is to be replaced... so they'll all get replaced.

Then I pulled the cap and rotor. Also getting replaced... or at least scraped with a screwdriver for testing purposes... That cap is only a year old. I broke a terminal off swinging the engine last year and replaced it before I went out. I will also add that the pickup seems worse for wear. Though being magnetic, they either work or don't usually. Just looks bad.

Photos here

 

[Lou C]

I think my # was from my OMC shop manual but I've see it in others to, 1000 ohms per foot. Many years ago my wife had a 2 year old Toyota Camry 4 cyl twin cam that all of a sudden started sounding like a 2 stroke! This was about 1990 or so. Very surprising to me, I ohmed out the plug wires and two were really bad. A new set of plug wires and that car ran up to 150,000 miles before we traded it.
So yeah your #6 is the outlier.

 

[Pmt133]

I was thinking also, #6 wire being like that may also be the reason why there is so little coloring on the plug itself.