Misfire after sitting a few months-not fixed...

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jamesp

jamesp

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Hi, after sitting a few months, my 850 ran poorly when started. Misfire occurs from idle on. Motor won't idle, misfires up to 3500 for sure, some back firing.

It was equipped with Boyer, carburetors have been sleeved and rebuilt within last two years. Prior to sitting it ran great. Since this started, this is what I have done: changed plugs several times, pulled Amal bowls, checked float levels and made sure jets were not plugged, switched Boyer out for new Trispark, using their coils and spark plug leads with their NGK end caps. Have also changed the gas, tested battery charge, took out idle adj. screws, checked for debris. None of this has helped. Any suggestions where to look next would be appreciated. Thanks.
 
Re: Misfire after sitting a few months-looking for suggestio

Sounds like a battery gone bad. Even though it's showing full charge, it could be weak when under load. A very quick test of this is to put a meter up to the battery leads, turn on the key/ignition, and hit the kill switch a number of times repeatedly in very quick succession. This will fire the Boyer and you'll be able to watch the meter and see if the voltage drops appreciably.
 
Re: Misfire after sitting a few months-looking for suggestio

Holmeslice said:
Sounds like a battery gone bad. Even though it's showing full charge, it could be weak when under load. A very quick test of this is to put a meter up to the battery leads, turn on the key/ignition, and hit the kill switch a number of times repeatedly in very quick succession. This will fire the Boyer and you'll be able to watch the meter and see if the voltage drops appreciably.
Click to expand...

Does that work for Tri-spark(?) as they can still function brilliantly way down below 9VDC (from real life experience).
PS: he has fitted a Tri-spark.
 
Re: Misfire after sitting a few months-looking for suggestio

Hmmmm. I confess I read the first few lines of the symptoms and also saw the word "Boyer" in the text, and immediately thought of Boyer/weak battery syndrome and didn't read further. I didn't even see the part about the Trispark! My mistake.

In any case, it could very well be a weak battery, but to be honest I don't know if the kill switch will fire the ignition on a TriSpark as it does as a Boyer. Wouldn't hurt to try, and hopefully rule out the battery.
 
Re: Misfire after sitting a few months-looking for suggestio

Once started, even with a weak battery, the charging circuit energises the electrical systems and the battery is effectively redundant until again called upon at restart.

To clarify my statement above: this is with a Tri-spark I currently have fitted, my experience with Boyer Bransden from the 80's onwards is that it needed a fully charged battery to start and then to maintain function (or they pop and fart and backfire all over the place).
 
Re: Misfire after sitting a few months-looking for suggestio

Hm, even low volts sensitive anal-og byah's get plenty of brain juice after 2000 rpm. Ya covered the easy no brainer stuff so suspect distal bad connection usually a red wire return path but could be anything in condition path, key terminal cluster, bat terminals, fuse holder or fuse itself [even if looking good outwardly], kill switch to internal copper breaks that conduct badly until rpm vibes reconnect enough to for smoother running. Just for laughs check into the tail-brake light. Picking poking jerking-bashing leads and connections after dark sometimes worth while. Of course may really imply ya just ain't mechanical man enough to own a fully fettered Cdo.
 
Re: Misfire after sitting a few months-looking for suggestio

jamesp said:
Hi, after sitting a few months, my 850 ran poorly when started. Misfire occurs from idle on. Motor won't idle, misfires up to 3500 for sure, some back firing.
.
Click to expand...

If you have changed the entire ignition system, then it doesn't sound like its electrics. !
(Unless the kill switch, or battery etc has a dodgy connection )

Do you have a choke system fitted ?
Have you tried giving it full choke for starting ?

What happens if you tickle the floats while its doing all this coughing and spluttering ?
Can you actually get it above 3500 rpm if you give it the big handfull.
Sure sounds like its starving for fuel.....
 
Re: Misfire after sitting a few months-looking for suggestio

No, I haven't tried tickling the carbs while motor running. I agree it seems like a fuel problem, but don't see any fuel delivery problems. I have the floats pretty much level with top of bowls, and the float needles looked good, are almost new. Jets were clear. I also pulled the idle air screws and poked around with some wire, found no debris.

I was reading in Trispark trouble shooting guide that problems could be in headlight wiring, tailight wiring. But I never had problems here before. I should check those areas when I have time.

Thanks for all the suggestions, but not sure about the not being man enough comment. Am I missing anything here? Thanks.
 
Re: Misfire after sitting a few months-looking for suggestio

To clarify your 'float level' comment - the 'fuel level' should be 5mm from the top of the bowl when the float has full shut the float needle.

Ignore manliness comments but do make sure you unfetter your bike before you go for a ride.
 
Re: Misfire after sitting a few months-looking for suggestio

I'm jerking your leg on man enough as similar to my efforts on show stoppers - tested to my wits ends after all suggestions followed and double checked, so I do question my capacity to care for Commandos. Often I don't know even after solving mystery if it was my fault or not my fault but either way a depressing loss of limited time to do something more useful-desirable. Besides reasoned trial-error persistence the man hood element for me includes ability to kick start, push uphill a ways for gas or load after a break down, pick up drops while injured or take a smackeroo from riding hazard and bounce back with money enough to burn. Such a wholesome hobby on the surface. We a-wait another smooth sounding video solution soon.
 
Re: Misfire after sitting a few months-looking for suggestio

hobot said:
I'm jerking your leg on man enough as similar to my efforts on show stoppers - tested to my wits ends after all suggestions followed and double checked, so I do question my capacity to care for Commandos. Often I don't know even after solving mystery if it was my fault or not my fault but either way a depressing loss of limited time to do something more useful-desirable. Besides reasoned trial-error persistence the man hood element for me includes ability to kick start, push uphill a ways for gas or load after a break down, pick up drops while injured or take a smackeroo from riding hazard and bounce back with money enough to burn. Such a wholesome hobby on the surface. We a-wait another smooth sounding video solution soon.
Click to expand...

You're right, this hobby is not for the faint of heart, or weak of leg, or cardio vascular system.
 
Re: Misfire after sitting a few months-looking for suggestio

needing said:
To clarify your 'float level' comment - the 'fuel level' should be 5mm from the top of the bowl when the float has full shut the float needle.

Ignore manliness comments but do make sure you unfetter your bike before you go for a ride.
Click to expand...

I had read before that the floats should be slightly below top of bowl. If they are closer to top, but not above, will that keep needle open? Or put fuel level too high for running? Thanks.
 
Re: Misfire after sitting a few months-looking for suggestio

jamesp said:
If they are closer to top, but not above, will that keep needle open? Or put fuel level too high for running? Thanks.
Click to expand...

If the fuel level is too high, it will run excessively rich, and could well cause coughing and spluttering and misfiring.
Drowning in fuel, as it were.

Which is why I asked if you had a choke system, or had tried tickling it while running.
If choking/tickling makes it worse, then it is already too rich.

Maybe try shutting off the fuel tap before trying to start it, and seeing if the running improves as the fuel gets lower in the float bowls...
 
Re: Misfire after sitting a few months-looking for suggestio

It does sound like a fuel issue but I can't think how the float height would spontaneously adjust itself while the bike was sitting unused. I would suggest pulling the carbs and getting them ultrasonically cleaned - Amal concentrics are notorious for having idle passages that are hard to get to and there are tiny drillings that a piece of wire just won't get to.
I bought an ultrasonic cleaner for £16 (around $25) in an Aldi supermarket - I'm sure you could get a similar deal somewhere like Walmart. Not only is it cheaper than paying for it to be done but you can also use it to clean the wife's jewellery thus earning brownie points approximately equal to heating a crankcase in the oven (but not quite enough for a full kitchen table engine rebuild).
 
Re: Misfire after sitting a few months-looking for suggestio

pev said:
...Amal concentrics are notorious for having idle passages that are hard to get to and there are tiny drillings that a piece of wire just won't get to...
Click to expand...

Trouble is jamesp says "...Motor won't idle, misfires up to 3500 for sure, some back firing..." which takes the carby through idle circuit, cutaway, needle height and onto needle jet. The two constants through that range is the air and the fuel - so blocked aircleaner, incorrect float height, fuel quality, joint air leaks, loose connector hose, etc are the fuel system suspects. Oh, knowing the variation in compression for both cylinders would be handy to quantify.
 
Re: Misfire after sitting a few months-looking for suggestio

Thank you for all your suggestions, you guys are great. I'm going to yank the bowls, readjust the floats. Also check floats to make sure they have no openings that could fill them up, hold needle open. Failing that, I will pull the carbs and try the ultra sonic cleaning. Unfortunately, can't work on it until next weekend...
 
Re: Misfire after sitting a few months-looking for suggestio

"Prier to sitting (three months) it ran great". You correctly wonder how a float level can self adjust with just sitting, it can't and I doubt if you'll find that to be the problem. You did state it misfires and pops and backfires to 3500 RPM. My bet is on a blocked idle mix circuit. Others have said it's tricky to clear yet easy to clog. Make sure the wire your using to clear it with is small enough and make sure it goes all the way through follow up with a shot of WD or something and then a shot of air. The wire size is .016, a single wire brush wire is a bit smaller and will work well if you have one long enough.
Another poster stated 3500RPM takes the carb out of the idle circuit, I'm guessing the misfiring you are describing as well as the non idle issue is all taking place in neutral while your standing next to the bike. No load 3500 RPM here is almost still wholly on the idle circuit. Slide cut-away, needle position/jet hardly play at all. RE CHECK IDLE PASSAGES! Some mung grew in there while your carbs were sleeping.
 
Re: Misfire after sitting a few months-looking for suggestio

jamesp said:
Hi, after sitting a few months, my 850 ran poorly when started. Misfire occurs from idle on. Motor won't idle, misfires up to 3500 for sure, some back firing.

It was equipped with Boyer, carburetors have been sleeved and rebuilt within last two years. Prior to sitting it ran great. Since this started, this is what I have done: changed plugs several times, pulled Amal bowls, checked float levels and made sure jets were not plugged, switched Boyer out for new Trispark, using their coils and spark plug leads with their NGK end caps. Have also changed the gas, tested battery charge, took out idle adj. screws, checked for debris. None of this has helped. Any suggestions where to look next would be appreciated. Thanks.
Click to expand...


Make sure your battery is in top condition, the boyer doesnt like weak battery....That will cause your symptoms. If it ran great before and the carbs are clean, there is nothing wrong with carbs


As I was told on this forum...90% of carb problems are actually ignition problems.
 
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