plugs dark-ish, still backfires on decel

[ann0yed]

So after running up many jet sizes and then going back down, I've got my bike running pretty well. I've got the mixture screws like 3.25 turns out (where the bike always seems to like them despite the combination of jets I've tried) and I've been listening for backfire popping when decelerating to tell if I'm running too lean. The mixture screws were adjusted until the popping stopped, but when I look at the plugs, they aren't that optimal light brown they should be, they are dark around the rim. It seems I'm running too rich but if I turn in the screws .25 the popping starts. My question is this, is it ok to have some popping on deceleration? Is it really lean or is it just the setup I've got:

79 Special:
XS Performance Stainless Steel Headpipes - 1.75" (45mm.) OD
XS Performance Reverse Cone Megaphone Mufflers
XS Performance torque inserts
XS Performance pod filters
Mikes green high output coils

Pilot -1 size over stock
Main - 1 size over stock

Thanks!

-j

 

[xjwmx]

Turn your carb balancing screw in 1/8 turn increments in both directions until the popping on deceleration stops or becomes rare. It will probably take less than a quarter turn in one direction or another from where it measured balanced.

 

[ann0yed]

The balancing screw? I just balanced my carbs with a CarbMate earlier this summer. How can my bike be too rich and also backfiring on deceleration?

 

[cooltouch]

If you have a leak somewhere in your exhaust that can cause the popping on trailing throttle. My '81 SH with a Mac 2-1 did this some, and honestly I didn't worry about it.

 

[jayel]

what brand/# plugs are you running? do you have adjustable needles (can you raise the clips one notch to lower the needle into the main jets?)

 

[bloodthirstysystem]

i have bs38's, up one size on both pilot and main jets, pod filters, straight pipes and my mix screw is 2.5 turns.

did you set the idle with the dead cylinder method? make sure you ground the plug if using anything but points..

 

[5twins]

Plugs being dark or black around the rim or edge by the threads is normal. The part you "read" is the bent over electrode and the porcelain insulator inside.

The stock mix screw setting for the '78-'79 carb set is 2 1/4 turns out. You should end up within maybe 1/2 turn either way even with re-jetting. 3 1/4 turns is too much and would indicate a larger pilot is needed. Mix screws should be set for best idle (fastest and smoothest). If it still pops at that setting, increase the pilot size. Mix screws should be reset after every pilot size change.

Mikuni now makes a 32.5 sized VM22/210 pilot. It may be what you need but might be hard to find as it's rather new. You'll probably end up getting a 35. If that proves to be too rich, you'll need to seek out that elusive 32.5.

This carb set is right on the verge of being too rich in the upper midrange because of it's large stock main (135). Sometimes even one size larger on the mains will require leaning the needle setting.

 

[ann0yed]

Hey thanks for all the replies! Good stuff! I'm using Iridium plugs. If I'm only supposed to pay attention to color of the electrode and the insulator, then all is well since the insulator looks clean and the electrode is light brown. Does it really matter how many turns out the mix screw is if the bike runs well? Theoretically, if I go up a size in the pilot, I can go in a few turns on the mix, correct?

-j

 

[bloodthirstysystem]

your plug sounds good. the first few threads will be dark on a tuned engine. you really should set the mix screw with the dead cylinder method. it allows you to hear one cylinder at a time and you can really dial in that idle. to the point where 1/8 turns really sound and feel different.

with points - unplug one plug wire, same side remove the hose or cap on the carb holder barb. set the mix screw to stock setting: 2 1/4 out. from there play with it until you find the best idle - fast and smooth. then do the other side.

 

[5twins]

A general rule of thumb is about 1/4 turn less on the mix screws for every size you go up on the pilots.

 

[xjwmx]

You're 3 1/2" turns out on the next size up pilot jet. Do you really think you're lean?

 

[xjwmx]

The balancing screw? I just balanced my carbs with a CarbMate earlier this summer. How can my bike be too rich and also backfiring on deceleration?

Did it pop before the CarbMating? If not, then putting the balance where it was before will fix it. Needing to be off from exact measured balance might have to do with difference in slide springs or something. I don't know. More gas going into the combustion chamber isn't the panacea it's made out to be, but that's just my personal opinion.

 

[ann0yed]

Hey, thanks again for all the help. After reading all of your interesting responses, I conclude that the bike is in fact not running rich, I've been reading the plugs wrong - the electrode is light brown and some soot around the threads is apparently ok. I was trying to get the whole plug end that light brown color, thus leaning it out to the point of backfiring while never reaching that goal. So now the only issue appears to be that the mixture screws are turned out too far and to get them to an acceptable range of turns, I should go up a size in pilot. That is what I shall try. Thanks!

-jonathan

 

[bloodthirstysystem]

backfiring could be from running such a rich mixture that unburned fuel is leaving the cylinder in the exhaust stroke and exploding in the pipes..

here is a good spark plug chart http://www.dansmc.com/spark_plugs/spark_plugs_catalog.html

 

[bloodthirstysystem]

with your set up you could probably get it to run really well with the stock jets, i would not go up any more on your pilot or main.

 

[ann0yed]

Wow, you think so? I think I'll try that then and see how it runs.

-j

 

[retiredgentleman]

ann0yed;
bloodthirstysystem is giving you some good information. I would also recommend using the "Dead cylinder method" to adjust your mixture screws.

Use a fan to cool the engine. Adjust the rpm of the running single cylinder down to around 300 rpm, using the throttle speed screw. The low rpm makes the mixture screw very sensitive and easy to adjust. Always have a path for the high voltage on the non running cylinder, such as an additional plug that is grounded to the engine.

 

[ann0yed]

Awesome, I'm going to try that. So I attach a plug to the sparkplug wire and just rest it on the motor somewhere, any place in particular? I can't imagine my bike running on one cylinder at 300rpm that sounds improbable!

-j

 

[bloodthirstysystem]

next to the cylinders or case.

with electronic or pamco ignitions that plug has to ground to something or you will fry some shit. with points it's doesn't have to but i still do it. you can make a clamp that holds the plug electrode around 5-6mm from the motor.

also pull off the hose on the vacuum barb on the carb holders. this way the cylinder won't be sucking in gas when the wire isn't on the spark plug

look in the topics section under carbs. it tells you how to do it all

 

[retiredgentleman]

Here's a pic of the spark plug I use for "Dead cylinder method". Just clip the alligator clip to an engine cooling fin.

Oh yeah, if your carbs have clean jets (correct size also) and correct float level, your engine will idle down on one cylinder to 300 or 400 rpm..............that's where you adjust the mixture screw.