Rough first start for the season

Finally got a chance to start my 1974 850 Commando. Bike ran well last Fall when put away for season. Bike started OK this year and idles well. However, when revved to about 2500-3000 rpm, the bike starts to loudly backfire from the right (timing side) exhaust. Flame actually shoots out the right muffler end at times! First time anything like this has ever happened. Ease off the throttle and idle returns to normal. Rev up again and backfire starts up again. Cleaned the air pilot (no noticeable blockage), drained the carbs (no debris), pulled the plugs (left plug a little sooty, but right plug was soaking wet). I intend to check the points this AM to see if there is anything obvious going on there as a first step and then remove the carbs (Amal Premiers) for examination. Hopefully, the right carb has an issue. If nothing is obvious, then I can reinstall the old OEM carbs that were on the bike before the switch to Premiers. They are not in great shape which is why I upgraded to Premiers but they might give an indication of what is going on. The situation is a little perplexing as I did not do any significant work on the bike this off-season other than replace the severely degraded rubber bullet junction box under the tank and replace the headlight ear rubber o-rings to stop an annoying vibration. I am certainly open to any suggestions regarding further troubleshooting. Thanks in advance.

Sounds more like losing spark than a carb issue. Battery is good and all connections? Check the points and my guess would be a bad condenser.

Check all electrical connections on the right side, make sure the coil connections are good and tight both LT and HT. Fit new spark plug and solid copper HT leads.

Also, check the kill switch. Vibration at a certain RPM may be causing issues.
Probably a fouled plug and stale gasoline.

Have you put in fresh plugs? Since the right one is obviously fouled, that is where I would start It could be the cause rather than just a symptom.

Carb jet unwinding out the base?

Did you drain fuel out of carbs during storage?

Float needle, seat or float itself not closing off fuel when full?

Update. Took the carbs off and dismantled them. Float height was spot on and carb parts were pristine, which is probably to be expected for low mileage Premier carbs. Reassembled carbs and checked both plugs against head for spark. Spark on both cylinders. Figured I try a start. Bike started but running worse than yesterday. Right cylinder not firing at all. Swapped right plug out. Bike started right up and ran like a top. I have to sheepishly admit I didn’t follow my own advice I give my son; try the simple things first. My excuse, lame as it is, is that there was spark on both plugs when laid against the head so I was kind of blind to electrical issues. A mechanic friend said that the plug against the head test is not always 100% accurate. You might have spark in a low pressure situation but not enough spark when trying to fire under pressure in the combustion chamber. Who knew? Kudos to those who recommended a new plug. And thanks to everyone for their advice. Test ride tomorrow. I’ll update again.

WEM said:

Update. Took the carbs off and dismantled them. Float height was spot on and carb parts were pristine, which is probably to be expected for low mileage Premier carbs. Reassembled carbs and checked both plugs against head for spark. Spark on both cylinders. Figured I try a start. Bike started but running worse than yesterday. Right cylinder not firing at all. Swapped right plug out. Bike started right up and ran like a top. I have to sheepishly admit I didn’t follow my own advice I give my son; try the simple things first. My excuse, lame as it is, is that there was spark on both plugs when laid against the head so I was kind of blind to electrical issues. A mechanic friend said that the plug against the head test is not always 100% accurate. You might have spark in a low pressure situation but not enough spark when trying to fire under pressure in the combustion chamber. Who knew? Kudos to those who recommended a new plug. And thanks to everyone for their advice. Test ride tomorrow. I’ll update again.

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Always try the simplest, cheapest fix first.

Here I go again with my regular dull sermon…

Folk often think a fouled plug just means it’s dirty, this isn’t so, a fouled plug malfunctions badly and can make the engine behave in ways you can’t possibly imagine a plug could cause.

Folk also often think that plugs either work or they don’t. Again this is not so, plugs can degrade gradually, or have hot symptoms or cold symptoms etc.

The ‘plugs sparking on the head’ test is a good test, but it does little more than confirm the circuits are good. It doesn’t confirm spark power or ability to spark under compression.

Folk also often think that new plugs must be good. This is not so either, duff plugs out of the box have caught me out before.

Counterfeit plugs are also an issue. To put the odds in your favour re counterfeit plugs, always buy from a trusted sources, I always buy from The Green Spark Plug Company and never from unknown eBayers or autojumbles etc.

And buy a box full, you’ll use them, and always having good plugs on hand will save time when trouble shooting.

Change them regularly. Changing plugs with your oil is a good rule of thumb. Does it really matter if you’re changing them before they fail ?!?

And always, always, change the plugs FIRST when testing for ANY running issues.

Twice I have had an issue with backfire, hard running on one cylinder, barley made it home.
Both times it was loose plug wire at coil. First time port side, second time, starboard.

WEM said:

A mechanic friend said that the plug against the head test is not always 100% accurate. You might have spark in a low pressure situation but not enough spark when trying to fire under pressure in the combustion chamber. Who knew? Kudos to those who recommended a new plug. And thanks to everyone for their advice. Test ride tomorrow. I’ll update again.

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In the Navy, we had a spark tester that we connected a compressor to, and pumped it up to 120PSI, 50% of plugs would fail.

Completed first ride of the season. No problems. Bike ran well. Thanks again to everyone for their advice.

I couldn't agree more with the advice on plugs-

Way back in the 60s, I had an old mechanic tell me to take a box of 8 new plugs, put them to the test- and at least a couple, will show weak spark under pressure. And I KNOW that flooding plugs, running rich, low electrical currents, extended stalled traffic conditions- will foul plugs- and all the cleaning and wishing in the world will never get them back to 100%. I wouldn't even consider troubleshooting a bike without fresh plugs. And gas.

My Boyer ignition system fires both plugs simultaneously with a double ended coil from a CB750 Honda. If one plug fouls the motor would stop. And there are fewer electrical connections. It is always better when problems are found quickly.

acotrel said:

My Boyer ignition system fires both plugs simultaneously with a double ended coil from a CB750 Honda. If one plug fouls the motor would stop. And there are fewer electrical connections. It is always better when problems are found quickly.

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Every twin cylinder engine I know of utilizes "waste spark" ignition. Even uneven firing Harley Davidsons. Spark is wasted on the exhaust stroke.

grandpaul said:

In the Navy, we had a spark tester that we connected a compressor to, and pumped it up to 120PSI, 50% of plugs would fail.

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In aviation it's called a bomb tester, part of routine cleaning/gapping. A resistance check is another normal check usually between 500 ohms and 4k. I look for all 12 in my engine to be about the same as one another and will change one that's way out. They're expensive so it's not something I take lightly. 100LL gas despite the Low Lead name actually has lots of lead so that's the primary reason to clean.

Single fire ignitions options (such as Ultima) with no wasted spark are a common upgrade on old Harleys.