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Mercury 25XD not reving out

S

Steve Brown

New member
Hi all. I bought a Mercury 25XD from the 80s I guess to power our raft. As it came with remotes, we also got a steering wheel for it, and are very happy with set up. It runs very nicely up to a certain revs - then starts missing. I've no idea what percentage of total power we're getting, but I suspenct 2/3 to 3/4. Everything else seems fine, it ticks over nicely. It's supposed to be unused from a major service, and although I believe the guy, I've no way to confirm(he seemed dissapointed he couldn't find reciept. Anyway, I've done nothing so far, but will start with new plugs. I think it's electrical rather than fuel as it seems so precise(exactly the same every time) I'll can link to this bit of video, and you will hear the missfire/spluttering - I think it's only on 1 cylinder. You have to forward to 1:50 to hear it though. Basically, we throttled up to where it spluttered, then back a bit. I'm just after ideas of what it may be if new plugs don't fix - which I doubt.



Thanks in advance, Steve.

http://www.youtube.com/watch?v=nUY9no6U8o0
 
Might also be a coil breaking down. Seen a lot of that lately. When it skips, let it for a minute at least, then shut it RIGHT down (fast as possible) and pull the plugs. The blackened one is the bad coil (if I'm right).

Jeff
 
Thanks. I have a manual, and it tells me how to check that I think. Doesn't mean I'll be able to do it though :)

I just took the plugs out. They're BUHW, but the manual says BUHW-2. I've been on NGK site, and that lists the correct one's as BP8H-N-10. Since the manual is old, I presume these have superceded, although the BUHW-2 are still available, and a different design.

One plug is blacker than the other - right in the centre by the electrode. Schoolboy error I guess, because I don't know which cylinder I took that from. Was also going to try super unleaded next time, as I think this is equivalent to old 4*
 
Do a compression check.------Then check to see if you have good strong spark on both.----A good strong spark on those means that it MUST jump a gap of 3/8" or better.--------If it does not then ignition requires trouble shooting.
 
Cheers Racerone. I'm not sure I can get hold of a compression checker, but may be able to. Although engine is old, I don't think it's done much work - it's in great condition under the hood. How would I test it jumping 3/8ths? Would I put a nail or something into plug cap and hold that 3/8ths from engine?
 
I've taken a picture of the plugs in case it helps anyone. Considering it's supposed not to have run since a major service, these sure don't look new. We've only run it for about 5 hours ourselves.

Plugs2.JPG
 
As I suspect, one of those coils is breaking down on you. Clean the plugs, put 'em back in and run the test I mentioned above again. Be careful to NOTE which plug is blacker this time, then replace that cylinder's coil.

Jeff

PS: If you want, switch the two coils and do the test again. The problem should travel to that cylinder if the coil is bad.
 
Thanks a lot Jeff. I've got new plugs on the way, those have been superceded, so getting latest as per NGK website. I don't think that will fix it, so next stage will be your test. I will definitely note which plug is blacker next time - bit of a fool for that one.
 
Looks like your diagnosis was right Jeff. Although with new plugs it will run at max revs(in a bin at least), I checked the new plugs(after only max 10mins running) and one was already black. Was going to swap coils as you said, just to be sure but found this:

coil.JPG


Incidentaly, I can't think of the purpose of the little silver metal tab - the other doesn't even have one. Can you tell me what it does?

Thanks again for your help, I've ordered second hand from USA(3 in fact), as it's £82 for a new one over here. Chance of it being here Saturday is slim I think, and we do intend to take raft out if weather OK.
 
Not much doubt there, Mate!

That metal tab is part of the ground circuit for the coil--some boob must have removed it. Better make one up or you'll need one of those extra coils you ordered.

Jeff
 
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