Misfiring/Backfiring on Low RPM's

[mfrias2nd]

Hey guys,

I hope you can shed some light on this problem I came across today. I finally changed a float needle out of the right hand side today. It's been leaking for a while and it may or may not be related to my misfiring/backfiring issue but I went on a 10 mile ride today and after it warmed up it started to bog down on low RPM's. I would be high on the RPM's and then when i had to let off it would start to bog down and give me terrible throttle response. After another couple of miles it started backfiring on the low's then when i pulled hard on the throttle it would kick in sweetly and run back to the mids and through the highs without a problem.

I came home and decided it was time to re-sync my carbs which I did quite nicely. I got the bike to purr sweetly by doing the carb adjust but I still have a little of the backfiring and poor throttle response in the LOW's and my left plug is coming out very sooty. My right one is fine. My peers tell me it's getting too much fuel and there's a carb issue but The carbs should be fine and this wasn't an issue for the last few months after I rebuilt my carbs. I switched plugs and it's still the left side.

I read this entire article http://www.bmwr65.org/cgi-bin/yabb2/YaBB.pl?action=print;num=1307801301 and I have a few ideas to go on but based on my story can someone give me a few more tips. Keep in mind the bike has been running fine until today. So it's a next-day problem.

Oh one more thing. Keeping in mind I did the carb sync and the idles are matched and low...after I really get the bike going when I come to a stop it's idling crazy high. I can't imagine lowering the idle adjust any more without the bike not staying on...this enforces my peers to tell me there's too much fuel going in the (left) side. Related problems? I don't know. Like aways much appreciated for your time to read. I hope you guys can help.

[Bob_Roller]

It sounds like part of your problem, the high idle speed at idle once warmed up, is performing a carb synchronization on an engine that isn't up to operating temperature .

If you had no problems before you replaced the float needle, you need to go back into that area and recheck your work .

Not that you did anything incorrectly, but is the small wire clip holding the needle to the float arm ?

By chance did you bend the 'tang' on the float, altering the adjustment there, maybe causing a low fuel level in the float bowl .

Is the float pivot pin fully inserted into the arm or leg of the carb, serrated end inserted in the correct position ?

[mfrias2nd]

Well the bike was ran for at least 10 miles before heading back home for the carb sync. I started from scratch. pull the idle screws all the way out and half turning them in once they touched the throttle. I can try going for a longer ride and rechecking the idle screws. That can account, possibly, for the high idle while warm.

Any ideas about the sooty left plug? it's an '82....too much fuel, electronic issues? possibly ok on fuel but intermittent spark? Coil? Wires?

[Bob_Roller]

If the plug has a dry soot on it, it's running too rich .

A not too uncommon problem that arises from time to time, is the 'choke'/enrichener, if it doesn't close all the way when the choke lever is in the off position, it can cause a rich condition on one plug .

An improperly adjusted float level can cause a rich condition, if it's too high .

As far anything else, a bad ignition lead is a possibility, or a corroded contact area in the coil .

[mfrias2nd]

ok...

Bottom line. The bike rev's too high even though the idle screws are set pretty close to nothing. I will pull another 10 mile ride and re-adjust. I'm still getting some backfires/misfires in low RPM's but when throttle is given it clears up and runs smoother than ever. I could be in 4th if i let off for just a couple seconds when I try to pull the throttle again it roughens up and backfires/misfires. I tried swapping the plugs and determined it's the left side. Wires? Coil? Also...when backfiring/misfiring my tach is jumping like crazy...leads me to believe the carb is not the culprit. Electrical?

Let me know your thoughts.

[tvrla]

It sounds like a tuning issue to me. Go back over the carb synch procedure and I'll bet a step was missed somewhere. Was the float level set correctly? Why were the mixture screws run all the way out and then back in? They're supposed to be screwed in till they lightly seat, then back them out a turn or so. The idle speed is controlled by an external screw on the butterfly shaft. Were the cables given enough slack when first starting the procedure? Are both butterflies open a half to one turn at the beginning of the synch?

[mfrias2nd]

Thank you for your response.

Yes the throttle cables were slackened. The idle screws were drawn out then in together going half turns until they equally keeping the bike on. Then I synched the cables and they're taught and pull equally. The carbs themselves were rebuilt about 3 months ago and until today they never gave me problems. After the sync I did today the bike runs even better than before but I'm still having that misfire problem. And since the tach is jumping when it's backfiring it leads me to believe it's got an electrical issue.

[Barry]

And since the tach is jumping when it's backfiring it leads me to believe it's got an electrical issue.  

That's a reasonable assumption bearing in mind that the tacho is driven by sensing the number of ignition sparks per minute so when it misfires and the spark is absent it would cause an erratic tacho reading. Problem is what's causing the misfire? You said it was definitely the left side even after swapping plugs so that does sound like coil or lead but to be sure it's not engine/mixture related can you make it move to the right side by swapping the plug leads over.

[montmil]

Appears to be a bit of miscommunication regarding idle adjust screws and idle mixture screws. High percentages that the excessive idle speed is due to incorrect adjustments of the idle mixture screws. These rascals are the first items to "screw with" during a carb synchro job.

I would definitely re-visit the float and float needle repairs, particularly as said, the bike was running OK prior to that work.

Monte

[tvrla]

How familiar are you with tune ups on these bikes - and specifically, with the Bing carbs?

Reason I ask is because the mixture screws aren't adjusted by screwing them out and then back in! The setting is the number of turns from lightly seated.

It's ok if you're new to these things - Letting us know the actual condition of all factors involved will make it easier sorting it out. And knowing what exactly has been done helps a lot.

[mfrias2nd]

Hey guys,

There is some confusion. I haven't touched the mixture screws. I've only ever meant the idle adjust screws. I will say that the bike runs really smooth now and idles quite nicely. It's only when taking off or letting off the throttle and giving it throttle again where it's popping and roughening up. Maybe I let it cool off too much before adjustment. I'll re-check.

Regarding switching the leads. YES. this was my next step. I heard for the later coil where it's the 1-2 style (NOT the individual coil style) will not work period if just one side is having problems. If I'm wrong please correct me. If I'm right this pretty much narrows my problem to the leads, which is my next test.

[Bob_Roller]

What year bike do you have ?

You can add it to your profile, so we know what 'generation' bike we're dealing with .

Makes a bit of a difference electrical  troubleshooting wise, if the  bike is a'78-80, or '81 and later bike .

[mfrias2nd]

It's a 1982 R65 about 30k miles. Strong Engine, good compression. Never had problems. Purchased with corroded carbs did the rebuild myself no problems since then except for some leaks (the rebuild kit didn't come with the float stop needle) purchased them later and replaced them yesterday. No more leaky carbs.

[Bob_Roller]

I'll agree with the tachometer indicating very erratically, that you have some sort of electrical problem .

No easy solution here, about all you can do, is take apart all of the electrical connections, check them for security, cleanliness (corrosion, etc.) .

The ignition control unit (ICU), it's located under the fuel tank and has a heat sink attached to it, should have a heat sink compound applied between the ICU and the heat sink, it's available from Radio Shack for $5 or so for a small tube .

Though I doubt it's your problem, if the ICU gets too hot, it can 'act up' and cause problems .

Again probably not the source of your problem, but if the ignition coil on your bike is a black and grey dual output type, it would be in your long term interest to replace it .

It's commonly referred to as the 'crack-o-matic' coil, the plastic case cracks at the ends, moisture gets inside and causes ignition problems, most commonly during damp conditions .

[Barry]

If you are brave try this to test for a weak spark.

Remove one plug cap and fit a spare plug.

Now I'm not liable if you have a heart attack so wear rubber gloves and hold the spare plug by the cap so that the earth electrode touches the top of the plug still fitted in the cylinder head.

You now have 2 spark plugs in series. Start the engine and observe the spark from your spare plug it should be healthy blue/white in color.

Rev the engine which will increase cylinder pressure which is more demanding in terms of the voltage needed to create a spark. The spark should not change color. If it turns orange you have a weak ignition coil.


Caution

Besides the electric shock risk you must keep the spare plug connected to the firing plug at all times while the engine is running or you risk frying the coil if it's output is open circuited. Perhaps a 2 man operation is called for. Better still it's not beyond the wit of man to devise a permanent connection between the spare plug earth and the running plug. I've done it with a length of cable and strong crocodile clips.

A lot of fuss you might say and if you don't understand the risks then don't do it but this is a brilliant diagnostic which is showing you how the spark behaves under load.  It's way better than just cranking the engine with a plug lying on top of the cylinder head.

Actually I think you can buy a properly designed spark plug tester that fits between the plug cap and the plug.  Where's the fun in that though.