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Discussion Starter #1
Seems to be a common problem? I changed my plugs to irridium when I pulled the old middle plug out it was sooty, so 200 miles on new plugs I took new plugs out to check condition, SOOTY, the outer plugs are perfect. Ballanced carbs,changed air filter, swapped coils eg. left to center etc. Still sooty center. PVL coils, Mikuni carbs, standard pipes, :confused: Bike starts easily runs great no misfires? Bike has done 9,500 miles. Before I start pulling carbs apart any ideas?
 

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Seems to be a common problem? I changed my plugs to irridium when I pulled the old middle plug out it was sooty, so 200 miles on new plugs I took new plugs out to check condition, SOOTY, the outer plugs are perfect. Ballanced carbs,changed air filter, swapped coils eg. left to center etc. Still sooty center. PVL coils, Mikuni carbs, standard pipes, :confused: Bike starts easily runs great no misfires? Bike has done 9,500 miles. Before I start pulling carbs apart any ideas?
Check the idle mixture on the middle carb and make it a little leaner. I'm not sure which way is leaner for the idle mixture screws though, I think screwing in (clockwise) but not 100% sure.

I have a Gunson mixture sightglass kit, but it's almost impossible to do the middle cylinder. I must work on bodging something for that!

Also check the tiny pilot air inlet jet at the rear of the carb inlet mouth, there's 2 holes at the bottom, the right side one is the air inlet for the pilot circuit. Give it a good clean, if that's slightly blocked I guess that would be restricting air to the idle circuit making it richer.

Here's a pic showing what I mean -

Pic1
 

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Discussion Starter #4
Thanks for sugestions, looks like the choke is returning, I suppose the carbs are going to have to come off any way!
 

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Discussion Starter #6 (Edited)
I thought mixture screw is for tick over and small throtle openings?
I've since read articles saying needle jets can wear out in under 10k on Mikuni's? It ticks over fine and I've given it a good hard ride up and down mountains, center plug still sooty??????
 

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I thought mixture screw is for tick over and small throtle openings?
I've since read articles saying needle jets can wear out in under 10k on Mikuni's? It ticks over fine and I've given it a good hard ride up and down mountains, center plug still sooty??????
But the colour seen on plugs is just the last bit of running before the engine stops. Which is usually on slower running before parking up.

There are some subtle workings on the Mikunis which I think Arfer (?) has posted on before with a detailed description of the Mikuni (pilot?) air circuit.

Sounds like this might be the issue as the more usual blocked fuel circuits tend to give lean conditions. That is, aside from the known needle jet wear situation which will usually have an accompanying misfire when picking up from a closed throttle.
 

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Discussion Starter #9
O.K. Raining again in sunny south wales so bke stays in garage!
I,ve taken carbs off and checked mixture screws, outer ones 2 turns out, middle 1.5 turns out! Could it be as simple as that? Rarely is with me, if I didn't have bad luck I wouldnt have any at all.

I'll update when I've put it back together and given it a run.

Thanks for the help in anticipation!
 

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Hmm...no, I don't think 1/2 turn on the pilot screws would make that much difference. Mostly, the general opinion here is to go to a richer base setting around 2.5 turns for better starting. All the plugs are DPR'8' grade not 9s (9 was an early spec from Triumph & persists in the aftmarket a bit, & can carbon up more)?
 

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Discussion Starter #11 (Edited)
NGK IRIDIUM DPR8EIX-9. They are less than 100 miles old. Cant see it being a plug problem as outers are perfect? The plugs I removed were NGK DPR8EA-9 and had the same reading. When I removed them I thought that the lazy git that fitted them may not have changed the center plug as its a bugger, but seems not to be the case.

Been for a ride took plugs out, YEP same!!!! I think I'll leave it at that reading other posts with people spending a small fortune on jets needles etc, etc, to no avail. A friend said to me if its not boke dont fix it! I said if it's not right it is broke? But its running fine starting fine so I may stop being so OCD about it.
 

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Did you have a look at those jet holes at the rear when you had the carbs out of the bike?

And you're right, it will cost a fortune to rebuild them :eek:. Mine cost around €300, but I must say they've been great ever since.

And it would also bug the snot out of me if one was running rich, even if it wasn't a problem, in fact that's why I rebuilt mine. #3 was running black and I was getting lousy mileage. When I pulled mine apart I discovered that the idle mixture screw in the middle carb was corroded solid, I ended up buying a set of 4 cylinder trophy (or was it sprint?) carbs and adapting one to fit.

It cost me about €500 all up with tools and stuff, but now they're perfect, I have a complete second set as backup, and all the tools so I can fiddle to my hearts content :D.
 

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Discussion Starter #13
Hi Mike, jet holes looked good in fact all looked good but as we all know looks can be decieving!! At only 9,500 miles it cant be to serious? CAN IT? I realy cant afford to buy parts on a hit and miss basis and yes it does bug me but I think I'm Going to have to live with it for the time being. I did have a thought that it may be the HT lead breaking down but I guess that would include missing?
 

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Hi Mike, jet holes looked good in fact all looked good but as we all know looks can be decieving!! At only 9,500 miles it cant be to serious? CAN IT? I realy cant afford to buy parts on a hit and miss basis and yes it does bug me but I think I'm Going to have to live with it for the time being. I did have a thought that it may be the HT lead breaking down but I guess that would include missing?
I think that's Arfer you meant to reply to there?

I can't recall all the details but I do remember Arfer getting stuck into the Mikunis & turning up some interesting stuff. But if the fuel economy is ok & it's running ok, I can understand leaving off a major carb teardown for now. Poss a winter job and/or if it does start affect running?

The Iridiums may well be helping there & yes, they are all the correct ones specced later (slightly hotter, cleaner running grade).
 

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Discussion Starter #15
Sorry Arfur / Mike, must be all the petrol fumes making the grey matter spongy!!

Yep my theory now is if it's that bad it will get worse and manifest itself into something obvious, but at the moment unless I did the plug swap I would have been blisfully unaware. Petrol consumption seems to be about 45mpg and as I've said before runs and starts great. I realy do appriciate your time and trouble, Arfur and Mike. Sorry I got you mixed up! THANKS!!!!
 
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