Carbed Mercruiser 260 5.7L Missing Above 4000 RPMs

[Rick Stephens]

Anything over 150 is a crap reading. Your gauge is not right.

 

[Kola16]

I did another dry compression test after running some seafoam through the engine to try and de-carbon cylinder 8 and got:
1 - 170 PSI
2 - 155 PSI
3 - 168 PSI
4 - 167.5 PSI
5 - 150 PSI
6 - 170 PSI
7 - 167.5 PSI
8 - 195 PSI
Obviously did not de-carbon it much if at all. I do agree that the numbers are inaccurate, but they are most definitely precise. It is not a coincidence that I have 3 tests on different days that pin cylinder 8 as the high, and 2 and 5 as the lows. That equates to a 25-30% difference on all of the tests between my highest and my lowest. That's okay? The internet seems to think it is not, but I have no clue.

I did verify the firing order and it is correct.

The carbon fouled plug is way rich I suspect that may be yoyur issue, although as weve seen hard to diagnos over the keyboard.

So you think I should start changing the rods and jets?

 

[Kola16]

How much is the plate for the choke on the carb supposed to open? It seems like it should be wide open to me? I will upload a picture when I get home, but it was not vertical after it had been running for 15 minutes or so...

 

[nola mike]

How much is the plate for the choke on the carb supposed to open? It seems like it should be wide open to me? I will upload a picture when I get home, but it was not vertical after it had been running for 15 minutes or so...

Yes, nearly vertical for sure. That could definitely do it. However, if you had these exact same symptoms before with the old carb...

 

[Kola16]

Yes, nearly vertical for sure. That could definitely do it. However, if you had these exact same symptoms before with the old carb...

I'm coming to think they were not the exact same symptoms with the old carb. The old carb wouldn't do it right out of the gate usually. It would take it a little bit. And the old carb was more sporadic, and the idle was more rough with the old carb.

I have been looking at videos of adjusting the choke, and the choke on mine does not seem right. I see people tapping the choke plate on Youtube of their Edelbrocks and it moves like it did on my old Quadrajet, but when I touch the Edelbrock, the choke plate does not move at all. I think it is stuck. Possibly corroded in place, but not positive. I will check it out tonight and do a lake trial with the choke plate open once I figure out how to get it open...

 

[Rick Stephens]

If you are going to calibrate a carburetor then you need to start at idle and work your way all the way up to WOT, calibrating each stage on the way up. Can't just address one portion since all the lower throttle settings effect the more open settings. And any carburetor purchased that wasn't calibrated for and on your motor needs calibration.

 

[Kola16]

GOOD NEWS (kind of) UPDATE! The choke was rusted solid! The lever that the spring pushes was so corroded after the first time in saltwater, that I had to tap it good with a hammer to disassemble it to clean the corrosion out so it could move freely again. I didn't realize that an electric choke is basically the same thing as an automatic choke The arm that I have my thumb on is what was rusted stuck and could not be moved, which is located on the back side of the choke.

Anyway, so i take the choke apart and clean all the corrosion and stuff and took it to the lake. No more missing!!! Got it up on plane too!

Unfortunately my cruising and top speed are significantly lower. 3-4 mph at 3500 RPMs and who knows top speed I didn't want to try. I'm going to start a new thread on it since the engine is not missing anymore! One problem solved!

 

[Scott06]

GOOD NEWS (kind of) UPDATE! The choke was rusted solid! The lever that the spring pushes was so corroded after the first time in saltwater, that I had to tap it good with a hammer to disassemble it to clean the corrosion out so it could move freely again. I didn't realize that an electric choke is basically the same thing as an automatic choke The arm that I have my thumb on is what was rusted stuck and could not be moved, which is located on the back side of the choke.View attachment 344085

Anyway, so i take the choke apart and clean all the corrosion and stuff and took it to the lake. No more missing!!! Got it up on plane too!

Unfortunately my cruising and top speed are significantly lower. 3-4 mph at 3500 RPMs and who knows top speed I didn't want to try. I'm going to start a new thread on it since the engine is not missing anymore! One problem solved!

ok good to know it usually the easy stuff. I had a sticking choke on my mercarb would foul the plugs pretty quick. You can force it open to test but if opening it made it run worse you are probably lean in the carb

 

[Kola16]

Another good update! Just for future reference if you ever hear of high compression in a newly rebuilt engine. (High in this case being relative since my compression tester reads inaccurate but precise.) I put a lot of seafoam in the cylinder with high compression. Let it soak and retested compression. No difference. So i took off the valve cover and looked at the valve for cylinder 8. The intake rocker nut on cylinder 8 was completely backed out. Not touching the rocker at all on any stroke. So i lashed the intake rocker on cylinder 8 and compression in that cylinder is the same as the others after lashing. And my boat runs back up to speed like it normally did. Just another thing to look for in the future. I could hear the lifter tapping as can be heard in one of my videos, but i just assumed it was a leaky gasket. I'll have to inspect the lock nut and threads further to see why it backed out in the first place. It seemed like it was gripping enough when i was tightening it down though. Hopefully it didnt wear down the cam but it was lifting and falling so hopefully it was just a faulty lock nut.

Also after fixing the stuck choke and cleaning the spark plugs, they all look normal after 4 hours of run time. No more sooty plugs!

Boat is finally running at 100%. Now that halibut season in Washington is over....