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Spark plug inspection
- Thread starter 1980 Special Twin
- Start date
dps650rider
XS650 Addict
78 and 502's
dps650rider
XS650 Addict
I made some comparison measurements of an OEM 502 and the aftermarket 502 using a drill gauge and the depth gauge on my caliper. I did this by inserting the needle into a hole and then measuring how much the needle protrudes from the face of the gauge, I did this using the 1/16, 5/64, 3/32, and 7/64 gauges. In the 3 smaller holes the OEM needle protruded about 0.5 mm more than the aftermarket needle, in the largest hole the aftermarket needle protruded about 0.25mm more than the OEM needle. This would make the aftermarket needle a bit leaner in the lowest range of throttle position and a bit richer everywhere else. This agrees well with my observations, at the settings I was using I had a slight flat spot at part throttle and was a bit rich above that.
Great diagnostic and inspection work--simple and effective! I wish we weren't stuck with distributors who contract for Chiwanese repops at the cheapest price. The Chinese are capable of as much precision and QA as the buyer is willing to pay for. For some reason the suppliers assume that we won't pay for quality.
If you're still not happy with the state of tune the original parts give you, I have found an alternative needle for the '78-'79 carb set that works pretty good, the 5Z1 needle from an XS400. It's the same length as the 5O2 but it's taper differs. Actually, it has a dual taper, a little fatter or leaner through the midrange but it also has a sharper tip .....
I've found this needle gives snappier off-the-line performance when taking off in 1st and when you whack the throttle open at low RPMs. I think the sharper tip is responsible for this. Although sustained slide lift doesn't happen until late on these CV carbs, say at about 4000 to 4500 RPMs, the slides do bounce open momentarily when you grab a handful of throttle. I think that's where that sharper tip comes into play, giving the bike a richer shot of fuel.
I've found this needle gives snappier off-the-line performance when taking off in 1st and when you whack the throttle open at low RPMs. I think the sharper tip is responsible for this. Although sustained slide lift doesn't happen until late on these CV carbs, say at about 4000 to 4500 RPMs, the slides do bounce open momentarily when you grab a handful of throttle. I think that's where that sharper tip comes into play, giving the bike a richer shot of fuel.
Also the needle and NJ are in play through the slide cutaway, even at idle.
dps650rider
XS650 Addict
Put the oem needles in today. I have stock air boxes and internally modified special stock exhaust and kn filters. The kns have always seemed to richen the mix a bit compared to oem filters. Engine is 700cc with a mild megacycle cam. Today I went for a ride with 27.5 pilots mix screws at 3 turns 135 mains and needles in the second notch. Ran great flat spot gone and throttle response excellent. Still need to try swapping mains but am very close. Oh and 59 mpg.
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Congrats! That low fuel consumption from a performance modified motor indicates seriously high efficiency. Kids, pay attention: this is what happens when a whole lot of things are done right.
dps650rider
XS650 Addict
Good mileage cruising has multiple benefits. With a small tank it means increased range, at 5 bucks a gallon I save a little money. The other less obvious benefit is you get less carbon buildup in the combustion chamber. When it is clean it rarely detonates but once the carbon builds up it can be a real problem.
1980 Special Twin
XS650 Enthusiast
I put the oem needles back in and it was the best day of work i put into the bike in recent memory. I researched the boards before making all my mods, i don't know why I went and put those useless long kit needles in the carb. It's nice and smooth now!
