Changing Plugs On 300DA With 350 Mags

[Stray Cat]

Has anyone attempted this knuckle skinning process? I looked mine over last fall when I winterized...just how difficult is it to get at the plugs? I am at the 200 hr service interval...so it is time. If anyone has hired it done...what was the length of time spent in labor?

 

[Alex F]

I think it's not as bad as it looks. I'm thinking to do mine this spring.

 

[Steve04]

jeff
i have a 04 34 dancer and changed out the plugs this spring, it was a pain but not too bad, you will need extensions for your rachets and a mirror to see under the manifolds, the 34 has little room on the port side of the port engine and the space between the engines isn't all that great but doable. Changing plugs is not rocket science and if you are even somewhat mechanically adapt you can do it. paying a mechanic will just piss you off when you get the bill for labor. Oh by the way DO NOT BUY YOU PLUGS FROM THE DEALER MARINE MAX WANTED 18.00 PER PLUG, I GOT THEM FOR 8.95 AT A LOCAL PARTS STORE.
Steve04

 

[wakeup 19]

Once you learn "where they are" it is not too bad. Iam not the most mech. inclined and I can do an engine in less than an hour on my 300da with 350 merc, I/o and a 10.5 beam. Put the longer extension wrench (rubber boot inside socket) w/swivel socket on them first then put the rachet on after. you are definatley working blind but it is a boat. The starter is the real headache on these. Take your time and do one at a time so you don't cross wires. good luck

 

[Dave S]

Just out of curiousity ........................why do you feel they need to be changed?

I don't have my Merc manual at home but I believe they recommend "checking" them at the 300 hour mark. They are platinum plugs as far as I know and should be good for a long time. I have about 330 hours on my 350 mag and it runs just fine.

Dave

 

[Ididntdoit]

Just out of curiousity ........................why do you feel they need to be changed?

I don't have my Merc manual at home but I believe they recommend "checking" them at the 300 hour mark. They are platinum plugs as far as I know and should be good for a long time. I have about 330 hours on my 350 mag and it runs just fine.

Dave

+1 - do them when you do the manifolds (which, unfortunately, will come up soon enough)

 

[Spookeay]

Yea, I was going to say that too. If you are just having to hard of a time,,,,Pull the manifolds. They are not that big of a deal to take off other than there weight. I'm not that strong and need help to lift them out. They weigh about 60 Lbs I think. But the time it takes to pull the manifolds off will far our weigh the time it takes to blind do any work under them, also the same for the starters. I can't see how any one can change them by not pulling the manifolds.:huh:

 

[wakeup 19]

the plugs aren't that bad, the starter is an upside down job!:smt021

 

[Alex D]

Just out of curiousity ........................why do you feel they need to be changed?

I don't have my Merc manual at home but I believe they recommend "checking" them at the 300 hour mark. They are platinum plugs as far as I know and should be good for a long time. I have about 330 hours on my 350 mag and it runs just fine.

Dave

Here is one reason why ... 360 hours on the mains in my 340DA. Last fall my SB engine developed a strange rpm surging going up and down about 100 rpm without touching the throttles.Then is started backfiring hard at and over 3000 rpm and under full load while getting up on plane ... we pulled the plugs and they looked perfect. SO I got worried. We did a compression test .. 180 psi on all cylinders ... perfect! We decided to put new plugs in to see and all trouble was gone. None of it anymore ... completey cured it. You can certainly run 'em until they croak, but there is no damage in changing them every 300 hours or so.

BTW same happened to my genny. One cylinder ... plug fouled bad and did not run.

Seems to me that age might have to do with it just a s much as hours. Dunno.

 

[Stray Cat]

Thanks for all of the input. After Daves post, I checked my manual...again...and can't find anywhere that even mentions about how many hours for replacement. They are platinum plugs too. If anyone can find an hour interval, that would be nice to know. I'm going on my 5th year with it....the plug threads are probably stuck to the heads.

My right index finger used to have an eye on it...(not really but I could damn near get a wrench on anything) and the plugs on a twin 350's looked like it would be a challenge (with the manifolds in place).

 

[Spookeay]

Here is the rule of thumb. 1 Hr=60 Miles most platinum plugs are good for 90,000 Miles so that adds up to be 1500 Hr's on the HOBS. I like to change them @ just 500 Hr's They are cheep. Also the plug can look fine at the business end but the porcelain can develop hairline cracks. This will most often show it's self as a misfire or backfire under load and at low RPM. Such as torquing onto plain.

 

[magster65]

MPI motors are 'easy' on plugs since they are in a perfect state of tune. With the older carb'd engines, this isn't the case. Like Spook-ster said; 500+ hours is reasonable and not neglecting your engine.
It's a fairly easy job but if you're unsure about it and have the money, get a mechanic to do the work.
Don't wait 'til spring... mechanics are twiddlin' their thumbs right now!