Carbs rebuilt, synched, annoying hiccup

[MGordon]

Hi All,

I just acquired this bike and the PO had the carbs/intakes rebuilt. The bike was running quite well, though quite rich. I had a motorcycle tech have a look and he reset the idle mixture screws back to 2 turns. I got the bike back and put some Techron in the tank.

Next day washed it, and then put some Iridium NGK plugs in it (old plugs were oil fouled). I went to do a carb sync on it and accidentally adjusted one of the mixture screws on #2. The bike started missing on that side really badly, backfiring. I got it down to only an occasional sputter. I thought maybe I either dislodged some crap in the tank/lines, or more likely got the plug wires wet.

The bike starts on 1/2 choke, warms up pretty fast and idles really smoothly when warm. However, at low rpm off-throttle or idle, it occasionally pops/hiccups on the left muffler. I've adjusted the mix screws by rpm and it seems to settle down only to sputter again intermittently. Today I put brand new vacuum hoses on the petcock lines, which made no difference.

Any deductions? The fact it got better after drying out from a wash seems suspicious...Also, I'm not entirely sure that turning the screws back to 2 might have done it, or the fuel cleaner...Like I said, only affects idle..pulls great.

Thanks

[cywelchjr]

Could be any number of things. Iridium plugs are often not something that works well with the stock coils, so if they stock coils are still in place they might not be giving a hot enough spark to keep them running hot enough. It also could be that water is getting into the wire harness down by the neutral switch where the splice is for the input coils for the ignition which can cause a problem. I would also be concerned that the previous plugs were oil fouled, that may point to a problem, and iridium plugs are probably not going to like that. Unless your running aftermarket coils I would stick with the OEM or direct cross plugs for the bike, as the iridiums really want a significantly hotter spark than the stock system provides.

[MGordon]

Thanks for you insights. I wish I knew about that before I dropped $40 on Iridiums! The fouled plugs might have had to do with the fact that the screws were turned out 3.5 turns and the bike really smelled rich. I'll pull them and have a look.

Since the problems started after I washed the bike, I'm inclined to agree with the water getting into the splice at the neutral switch--is that accessible with wd-40 or contact cleaner?

[MGordon]

Just pulled my plugs. All but #3 cylinder are pretty sooty. Maybe the floats aren't right, or the screws are still out too much?

[bikerphil]

I'd make sure all the wires are dry first, let her sit overnite for that. The air filter isn't dirty/plugged up is it? As for floats, they should be 25.7 mm ± 1MM, upside down on the bench with the gasket removed. How did the bike run before you washed it and mistakenly turned an idle mixture screw? Use new plugs (regular type) to re-test, might want to clip 1/4" off the end of each plug wire, the caps unscrew.

[1981XS]

I had the exact same miss, pop,burp and fart problem after the first wash i did to my bike after purchasing it, after chasing after several things and never really finding a problem i ran into it again on a nice hot day in the middle of nowhere and found that my outside 2 cylinders werent firing (leather gloves dont protect from shock if you pinch the wire too hard though so be careful ) turned out one of my coils had a crack running the entire length of it and well it was letting moisture in and we all know how well that works lol, just an idea of something to look at that isnt too horribly hard to get too if youve got an hour on your hands, mine also only missed at idle when it was wet (Ie rain storms or at the car wash) Rev it up and itd purr like a kitten

[foreda]

Irridium Plugs

Did you replace the spark plug caps. I am pretty sure the Iridium plugs have the resistor built into the plug. That means you would need to replace the spark plug caps with the non resistor type. Someone one here confirm this!

[natemoen]

It really does not matter if you have resistor caps or not. The resistor just gets rid of radio interference created by the ignition system.

[cywelchjr]

The biggest problem with the iridiums is that the gap is too large for the stock coils and because of the brittleness of the material cannot be changed. They work great with high output replacement coils, but reportedly don't work well with stock coils.

[MGordon]

Fixed

Trimmed back the sparkplug leads on the outside pair #1 & 4 by 1/4 inch and presto! First I checked for vacuum leaks and resistance issues for ignition/pickup coils. Re-synched and set the mixture screws (this time with petcocks on prime--doh!) and she purrs, no spitting, backfiring!

Thank you for all your help. Sometimes it is the simple things...I had a feeling it was timing/spark related...I guess washing the bike really brought out the plug wire insulation issue....Funny, I had ridden it home from purchase in a downpour, but i guess it still doesn't get under the tank....completely like soaking with a hose..Off for a ride.

[MGordon]

Still popping a bit on down-throttle...the bike has slip on mufflers, with no gaskets. I wrapped some rescue tape around the joint which sealed it up to hand-feel for leaks, but probably a small exhaust leak, enough to make it run a bit leaner post-header...Still runs and idles great.

[cywelchjr]

Still popping a bit on down-throttle...the bike has slip on mufflers, with no gaskets. I wrapped some rescue tape around the joint which sealed it up to hand-feel for leaks, but probably a small exhaust leak, enough to make it run a bit leaner post-header...Still runs and idles great.

Any exhaust leak can cause popping on deceleration. This can also be caused by being a bit lean on the mixture setting, not so lean it doesn't run ok, but a little on the lean side. As a matter of fact, the 80 and 81 machines in many cases popped when new because they were tuned very lean from the factory.