Spark plug inspection

1980 Special Twin

1980 Special Twin

XS650 Enthusiast
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I'm tuning my carbs and re-jetting (pipes and pods, other funny business), it might be a bit of a rolling project for awhile. Is it necessary to use brand new spark plugs for a visual inspection or does it suffice to clean them?
 
No, you can just clean them. But just be aware, reading plugs today with the cleaner burning fuels we have isn't as easy as it was years ago. You may not ever get the nice brown plug color many say is best. Instead, you'll be dealing with shades of gray. But yes, you'll still get a black plug if you go too rich, and white or no color if too lean. You'll want the side strap to burn clean and be a light gray color, and the porcelain to remain relatively clean with just a ring of dark color down at the bottom. If that dark color extends more than half way up the porcelain, you're a bit rich .....

Plug Color.jpg


What you're "reading" on the porcelain is the plug's "smoke" or "mixture ring". Ideally, you don't want it to extend more than about 1/4 of the way up the porcelain .....

Smoke Ring.jpg


Also, you won't get an accurate plug reading just after starting with the choke. Use of the choke will blacken the plugs and make you think you're too rich. You'll need to drive the bike a few miles to burn them clean again. So, maybe having a spare set of plugs isn't a bad idea. Cold start the bike with one set, warm it up, then swap in the other clean set. Honestly, you should have spare plugs anyway.
 
If you want to know how to read a plug accurately, go here: https://www.strappe.com. Click on the Tech button, click on the spark plug icon, and read the best thing ever written on plug reading by Gordon Jennings, a master of both tuning and the written word. (Warning: clear your mind of preconceptions before you read. Urban myths, erroneous bench racing BS, and fragments thereof are prone to detonation. Cognitive damage can result.) And thank you, Jacques Strappe, whoever you are!
 
Dang, mine are reading a clean bill of health. Jets and pilots are correct, carbs are sync'd, advance is good. But it has a cough and sputter just before WOT, which happens to be just where a comfortable cruise at the speed limit is. Aw shucks, guess I gotta pin it and blast off. Or rather, guess I should re-read the carb guide.
 
If the throttle is almost wide open at highway speed limit (~70 mph?), you have a serious power loss. Could be carburetion, could be something else entirely. The combustion tripod stands on three legs: ignition, compression, and fuel. Troubleshoot in that order.
 
While many run “conventional” plugs in their bikes, others run more expensive plugs with iridium tipped electrodes. I’m used to using these plugs in engines with a high output ignition system, but would there be any advantage to running them in a conventional points / condenser engine?
 
I guess i don't mean "WOT." I mean it's coughing a bit between the midrange and the top circuits. And that tends to happen when traffic is moving at 25-35 mph. I have a decent knowledge of the mechanics but I still struggle to call each part or function by its correct name!
I have a feeling it has something to do with the needles. I have aftermarket ones that i think came with the jet kit i got, don'trememberwhich kit or where from. They're installed properly and i drilled the hole a bit even though people say it doesn't matter. I don't remember if they are adjustable, but if so I will crack them open and try to drop the clip a slot. Last time i checked the diaphragms they held their vaccuum, and the pods arent in the way.
 
Sounds like your needles are from that Heiden Tuning kit. Reports on them aren't very good. They're a few MM longer (leaner) than the stock needles and designed to run with quite large mains (in the 140s). They were also designed to use with Heiden's 2-1 exhaust. So, on a somewhat "normal" 650 with your basic "pods and pipes" mods, they don't work too well. Best you put the original needles back in if you still have them.
 
Jet kits? Oh, my. We've posted many warnings about those. Keyster is simply junk, and the rest contain bits that are not specific to any application and dead wrong for most, no matter what the kits are listed for. Genuine Mikuni brass costs more up front, but it will save you from a world of frustration. The only Chiwanese brass I can recommend is repop needle jets, and those only because Mikuni doesn't offer them. Niche Cycle Supply offers both Mikuni and repop parts, and they clearly indicate which is which. Go to https://www.amckayltd.com/carbguide.pdf and spend some time with it. You'll find year-by-year jetting specs and much more.
 
grizld1 said:
Jet kits? Oh, my. We've posted many warnings about those. Keyster is simply junk, and the rest contain bits that are not specific to any application and dead wrong for most, no matter what the kits are listed for. Genuine Mikuni brass costs more up front, but it will save you from a world of frustration. The only Chiwanese brass I can recommend is repop needle jets, and those only because Mikuni doesn't offer them. Niche Cycle Supply offers both Mikuni and repop parts, and they clearly indicate which is which. Go to https://www.amckayltd.com/carbguide.pdf and spend some time with it. You'll find year-by-year jetting specs and much more.
Click to expand...
Interesting comment about the Keyster carb kits. In the GT750 world they are the kit that’s recommended. Granted there are few other options available for these old bikes. I looked on Jets R Us and they listed a rebuild kit for my GT750. Did a reverse look up on the part number and looks like a kit distributed under the K&L brand name. Any intel on this manufacturer / distributor?
 
Don't know about Keyster water buff kits, but what I've seen from that outfit is crap. Maybe good for the GT750, maybe not, but you wouldn't go wrong to pay a little more and order individual Mikuni brand parts. K&L is a major distributor of aftermarket and OEM parts. Most of their products are excellent--ARS seals and carb mounts, for example, are OEM, sold under the Tour Max label. I don't trust repop kits of any provenance, and I'm not alone in that. This from personal experience: K&L kits for any OE XS650 carb will contain much useless generic junk that is not appropriate for the application and will cause trouble if installed. The only items found in rebuild kits that can usually be relied on as usable in any XS650 kit are the float bowl gasket and the float valve, and it's not unusual for either of those to give trouble--holes in gaskets not punched through and burrs in the float needle seats are pretty common. IMO the best way to avoid trouble is to buy Mikuni brand parts. Mikuni's rebuild kits for their aftermarket carbs are excellent, and for newer bikes you may find good rebuild kits made by Mikuni and sold by the bike manufacturer.
 
Griz, do you know what Mike's sells for individual needle jets and jet needles? I have given up on dialing in my bs38's with them and planning on putting the originals back in tomorrow.
 
Don't know what parts are giving you grief (needles from kits?), but Mike's sell repops of stock needle jets and needles for all versions of XS650 BS38s except TX650A and XS650B. AFAIK they're the only source for those very critical parts, which are long term wear items.
 
Yep, they do wear over a long period of time, why I replaced them. Down to 132.5 mains and needles in position 2 and plugs still dark. Suspect the needle taper may be off and they are 1mm short. Swapping them now...
 

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