300XS miss at low speed

[rocketman6965]

300XS-1E052618

Motor misses at low speed making it hard to plane sometimes. Runs 6000 RPM at top end. I ran a compression test. Results below. Plug #6 looked fouled. Photo of plugs attached. I also copied a you tube link. Hope it works.
I rebuilt this motor a while ago and everything worked well. When I installed the reeds, I noticed some of them had some separation from the block but it didn't seem enough to affect operation. I just read a post about oil bleed check valve causing oil to run into intake? I swapped #6 plug with #4 after spraying brakekleen to all of them. It ran better but the miss was still there just less noticeable.

Compression:
#2-128 #1-128
#4-126 #3-124
#6-133 #5-132

FYI I started from bottom up 6,5,4,3,2,1 with a fully charged battery, throttle opened fully, warmed up engine after cleaning my strainer from earlier fishing trip.


https://youtu.be/lHI58VELpfY

[Savage]

Plugs look good. Perform a spark test as the problem is occurring.

[rocketman6965]

Thank you Savage. I know pictures sometimes don't show everything. I oriented the plugs in line with engine. The bottom left plug (#6) was totally different than the others. Very brown and wet. Ironically, this is the cylinder with the most compression. I will check spark and report back.

[EuropeanAM]

Joe- how full have you traditionally been filling your remote (boat) oil tank?

Maybe it's just the audio... but that engine sounded AWFUL. Almost like an old "hit and miss".

[rocketman6965]

Never ever full. I add a gallon when it says I’m low.
As far as a visual, between 1/3-2/3 full from low to high.

[EuropeanAM]

When you rebuilt it, what parts were replaced (cylinder wise)?

[rocketman6965]

Okay. Here is the humiliation part. I tried the “o ring” fix for the oil pump. I’m still not convinced that’s what made it pop. I only lost #6 cylinder. Upon rebuilding and diagnosing, I found the crankcase check valve was no good. Did it fail before or after the metal fragments were deposited into the crankcase? The rest was in good shape. Here is what we did:
Sleeved cyl #6. Pinned, honed and cross hatched all cylinders
Replaced piston #6 and all new rings
Replaced all bearings for crankshaft, rod journals and wrist pins.
Replaced oil pump and check valve.
Had all injectors checked, cleaned and flow tested.

FYI the compressor has no movement whatsoever.

Edit: And of course all new rod bolts, head bolts, gaskets, etc.

[EuropeanAM]

Joe- understand these are honest questions, and not any reflection on you:

-Are you pretty confident that the DISH on the crown of the new #6 piston matched the ones in #2 and #6?

-Are you extremely confident that the piston was a PORT piston?

-What reeds are you running (and are the reed blocks still rubber-coated)?

-Have you tried a "cylinder drop test" to see if you can confirm the irratic power/running is coming from "X" cylinder?

-And have you tried another KNOWN GOOD spark plug in place of the one that was in #6? Just to be sure it's not as simple as a bad plug...

[rocketman6965]

see replies in quote

Joe- understand these are honest questions, and not any reflection on you:

-Are you pretty confident that the DISH on the crown of the new #6 piston matched the ones in #2 and #6?
YES

-Are you extremely confident that the piston was a PORT piston?
YES

-What reeds are you running (and are the reed blocks still rubber-coated)?
STOCK. Yes blocks were rubber coated and I was afraid to lap them when I saw the “slight” separation on some of them.

-Have you tried a "cylinder drop test" to see if you can confirm the irratic power/running is coming from "X" cylinder?
NO

-And have you tried another KNOWN GOOD spark plug in place of the one that was in #6? Just to be sure it's not as simple as a bad plug...
As in my first post, I swapped 6 with 4 and ran on muffs in the driveway. It sounded better but less knocking. I thought I had an extra plug to run but I don’t. Thanks for all your help.

[rocketman6965]

I also have approximately 50 hours after rebuild with a textbook break in procedure
I’d say this has been slowly getting worse and for quite some time.
It happened during break in I think but it got better when I changed plugs and running normal oil amount but I do believe it started again shortly after that.

[rocketman6965]

When I said it ran better after swapping plugs, it was on land in gear at idle.

Here is a video link:



You do hear some prop chatter and I believe a "slight" miss but much better which leads me to believe it is not a reed?

[EuropeanAM]

Probably not a reed. Let's clarify your comment about "slight separation". Are you referring to a Reed GAP at the end of the petal, or are you referring to separation of rubber material that is bonded to the aluminum reed block?

As broken a record as this seems- I started out WAY back when we had "diagnostic flow charts". One of the very first steps was confirming that a COMPLETE ignition tune-up was done, then retest and determine whether the condition still exists.

I would absolutely replace the plugs at this point- if you want to save 5 of the ones coming out for later, that's understandable.

Also- what is the throttle plate air-gap (currently)?

[rocketman6965]

Don,

A reed gap. No rubber is coming off the blocks. I always do a complete maintenance at one time; plugs, fuel filter, pump, lube, etc. I will try to get the air gap but most likely it will be when I get back from vacation around July 9th. Thanks again for all your help.

[EuropeanAM]

[rocketman6965]

300XS-1E052618

BTW What is the throttle air gap for this motor?

[EuropeanAM]

.093"

[rocketman6965]

.093"

[rocketman6965]

It’s .093. Never measured before or have the right tools for this so I used a 3/32” (.09375) drill bit. It fit “snugly” on the back side of the butterfly. It almost went in on the front side.

[lpugh]

That looks like may be to much oil for #6 between the darker brown and a glazed black color on the electrodes
Maybe that is why Don questioned the oil tank level ?
I heard the same thing Don did

[rocketman6965]

I know that’s what I think. Too much oil. Hopefully it is just a foiled/fouling plug. Have a new set and one extra on the way. Will change out the suspect one and see. Hope that is all it is.

That looks like may be to much oil for #6 between the darker brown and a glazed black color on the electrodes
Maybe that is why Don questioned the oil tank level ?
I heard the same thing Don did