Quick carb tuning question/opinion

[Johnny Glass]

1982 xs650
stock carbs
#45.0 Pilot Jet
#142.5 Main Jet
2-1 XS Performance Headpipe Set
Part #07-0041 taper muffler
K & N stock looking air filter

Thats my set-up. Here is what my plugs look like after about 300mi:

Here is what my headpipes look like as well:

Now it backfires a little bit mostly on deceleration a bit while idling. I thought that meant a rich condition. Which the plugs support that thought, and I think the head pipes do as well. But when I adjusted the fuel mixture screws to be leaner it backfired worse. So I am a bit confused? Power seems pretty good across the board if anything it's has a bit of sluggishness on the lower end of throttle. Wich makes me believe it's the idol mix or pilot jet to big?

So my question is this: What is the most likely cause of the backfiring and how do I fix it?

Thanks in advance.

 

[5twins]

Backfiring and popping can be either from a rich or lean condition. Usually it's a lean condition but in your case, the mains may be too big. I would try maybe a 137.5 and see if it improves things. Popping on decel is usually a lean condition. Set your mix screws at 3 to 3.5 turns out. Anything less on these 34s will usually cause that popping. Your pipes are aftermarket and single wall. They're going to turn colors no matter what, even with the correct jetting. Your plug pics don't help much. You want to look down inside them, at the porcelain, not at the sides .....

Ideally, you're looking for a ring of color at the bottom, that's all. If the porcelain is colored most of the way up, you're too rich .....

 

[Johnny Glass]

The mains? I really didn't think it would be them. Here's a better pic of one of the plugs, they both look the same. don't mind the sink, bathroom has best lighting in a pinch.

 

[5twins]

Well, your plugs don't look really bad. They're not totally black or anything. I still feel the 142.5 is a bit large. If it's all you've tried besides stock, you really can't jet carbs that way. You've taken one shot in the dark and are hoping it's right. You should try several sizes in succession to find what works best. I would start stepping down (140, 137.5, 135). Also check for air leaks and verify the float levels are correct. If the floats are off, no jetting will work right.

 

[Johnny Glass]

ok thanks, i ordered up a couple sizes down to see how they will work. should get them in by the end of the week.

 

[XSLeo]

You mentioned a stock looking K&N filters. Do they install in the stock air boxes?
And have you read the carb guide?
Leo

 

[Johnny Glass]

Yes the k&n install in the stock air boxes, and yes I have read the carb guide. It just that it seems to me that the carbs are close. This is the first time I have rejetted carbs, so I wanted to get some opinions on it to see if i'm heading in the right direction. It's really not backfiring bad, I just turned down the mixture screws ever so slightly yesterday and it helped some. But again, I have not done this before and also this is the first time I've had the bike running so I have no frame of reference on how well it should run?

 

[xjwmx]

5Twins, how long should the plugs look like you're describing? I notice his have 300 miles on them.

 

[5twins]

With only 300 miles, his haven't fully colored yet. Once they do, I think they should remain that way for the life of the plug. The plugs in my pic have probably 1000 to 1500 miles on them. The bottom of the threaded portion is carboned over, which is normal, but the side electrode remains burned clean, colored a light gray or tan. The porcelain center remains almost white. There is a smoke or mixture ring at the bottom of the porcelain but you can't see it in the pic.

On Johnny's plugs, that mixture ring hasn't formed yet but should soon. If it never does, that would indicate a too lean condition somewhere, like maybe an air leak. It's possible for your settings or tuning to be too lean in one area, too rich in another. The fact that the top of his porcelain is coloring brown is what makes me think his mains may be too large.

With today's gas, it takes a while for plugs to get fully colored. That's why I feel traditional plug chops don't work as well as they used to back in the days of leaded fuel. Sure, they can show you if you're way off but so does just looking at the plug as a whole. For fine tuning, I use a combination of overall plug color, smoke ring height, and actual test riding. You can only tune a bike so far in your driveway. You need some on-the-road testing to get the best final tune.

 

[XSLeo]

With no frame of reference, I might start with the stock settins and as described in theguide, test it, then change tuning as the testing indicates.
Do one change then test, do another change, test.
These carbs can be overjetted and still run ok. The vaccum operated slides will only open as much as the engine needs. If over jetted the slides won't open fully.
That's why you need a wide range of jets for tuning. Start at stock and increase as needed till adding size gains nothing.
It's trail and error, no set recipe for jetting. No two bikes have the same engine condition, air intake, exhaust, altitude. All these things effect your tuning.
Leo

 

[Johnny Glass]

Ok, I think I get it. Try some different jet sizes and see how it goes. since i'm starting likely to big try the next size down if it runs better, try the next size down until it doesn't run better. or try the next size up, if your starting to small, until it stops running better. I should be getting some in real soon, going to check to see if the local yamaha dealer has any pilots in stock. Didn't remember to order any of them.

Guess I was just hoping for that magic formula!

 

[5twins]

One up on the pilots like you've done already is what usually works for the idle circuit. You may not need any more of them. Hopefully trying some different mains will fix the rest up for you.

 

[Johnny Glass]

Hope so to, I guess what was making me think it was the pilots was the backfiring on deceleration mostly.

 

[Moody]

lean exhaust on deceleration will cause this popping!!!!! looks like your heading in the right direction, up the pilots and adjust idle mixtures accordingly

 

[5twins]

I like to start at the top with the mains and work my way down in succession through the other circuits. The mains can have an effect on all the other carb circuits so I like to get them at least close before doing the others. If you do the others 1st and then the mains, chances are you will need to go back and re-do them.

 

[Johnny Glass]

Thanks on all the replies, should be able to take the carbs out soon.

I guy I know took a look at the plugs and said they looked pretty good, maybe a little lean. Which surprised me? He thinks it is likely an air leak. So i'm checking that first and going from there.

 

[Moody]

i agree that the plugs looked pretty good, this is why i suggested up the pilots alittle and adjusting the idle mixtures.... possible that air/vacuum leaks could cause similar issues though, at idle spray brake parts cleaner around carb and intake system and look for rises in rpm's which would indicate a leak... on more modern fuel injection bikes when guys throw aftermarket exhaust their bikes with out blocking off the pair emission valves will create a severe lean condition on deceleration resulting in backfiring on deceleration. anyhows, good luck my friend!

 

[scabber]

The best advice I can give you is to sell that exhust system IT is Not designed for a street bike If you look at the dyno chart the stock exhaust out performs that setup by a long shot. That system builds power and falls on it face in the RPM ranges where most needed on a street bike the horse power is gained at 6500 RPM to 7800 RPM their claim I doubt it. Back to your jeting the 45 poilet should take care of of idle stumble shown on the dyno chart a fall on face your main a 137.5 would be big for a 650 motor. The fact that your plugs are not all black and wet with gas with the 142.5 main jet what leo said or vacume leak you might have blown off a cap off a barb. out of sync. carbs can cause back fire. The Heiden needles another problem along with the exhaust the stock needles might work better and less quess work. A set of 1 1/2 inch 2 into 2 pipes shorty emglows stock needles 45 piolet 132.5 or 135 main works for shure and perform better Good Luck

 

[pahako]

I have a similar situation. I recently switched to pod filters, and opened up the stock exhaust by removing the back plates, and am now noticing popping on decel. I have an 81 with bs 34's, and put a 45 pilot jet and a 140 main jet in to compensate for the mods. Mike's says to use a 142.5-147.5 main for these type mods. Mix screws are at 3 turns out, cam chain and valves adjusted, and carbs in sync. Last time I checked there where no air leaks but will check again. My question is: Could I still be running to lean with a 140 main? And yes, I read the "carb guide" quite frequently and realize it has the info I need but, it seems to help when I can get input from others too.

Thanks

 

[5twins]

There's no set recipe for jetting. What works for you may not work for my bike. All you can do is experiment. I suppose you could still be lean. The only way to find out would be to try a 142.5 main to see if it improves things. If it makes it worse, go down to a 137.5. What is a fact about jetting is that you need to try several sizes in succession to properly dial things in. And sometimes just that one more size makes all the difference.