Backfire on Deacceleration

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Increase the throttle stop setting to hold the butterfly or slide open more. Adjust the idle mixture to suit engine idle speed. Problem solved.
 
jsouthard said:
Mighty Norton Knowledgeable,

Recently my 74 850 has started backfiring on normal deacceleration. I haven’t changed anything (this week).

Everything else is great. Idles fine, Pulls great, starts in one kick etc. etc. Point me in a good starting direction?
Click to expand...


It is almost always carburetion. Usually it is caused by a (rich) almost liquid mixture (at closed throttle) hitting the hot exhaust. Sometimes opening up the air screw and resetting the idle solves the problem, sometimes not.
 
So i increased the Pilot jet to a 45 and the backfiring is gone!

However, since i did this i have found a unique Idleing issue. When i first start up my bike she gets up to idle very quickly. Then after about 10-15 minutes of riding the idle starts to lower and i need to tighten uip the air screw to keep the idle at 1,000. It continues to get worse the more i ride. Then the next time i start her it idles between 1500 and 2000 RPM's. Bring it back down to 1,000, start riding and and will need to go through the process again.

1 - The original backfire issue came out of nowhere. I never had any issues with my carb then suddenly the backfire issue?
2 - i never had idling issues before i switched to the bigger pilot.
3 - There is not a vacuum leak. I have sprayed carb cleaner all over the 2-1 manifold & exhaust headers with no effect.
4 - The carb is clean and the air screw still very much effects the idle.
5 - Timing is set.

Could it be electrical? I'm wondering if my original Lucas alternator may not be supplying the boyer with enough power? Any other ideas?
 
Larger idle jets cured your problem, which was that the idle circuit wasn't delivering enough fuel when throttle was closed on the overrun. Classic cause of this symptom.

Perhaps your original idle jets are a bit gummed up. Did the bike sit a bit longer than usual between runs?

But right now your idle circuit is too rich. Your original idle jets weren't flowing quite enough gas, but the unclogged, larger idle jets are flowing too much. Clean the old jets and replace them (or better yet, buy some new ones as cleaning old ones is a bit hit and miss) and I'd think you'd be good to go.
 
Hehe danced a gig on ya hehe. As stated pad the stand and rip past 5000 to see where the indicator stops. Advance should not rise past 5000 rpm or oh oh.
One possible not mentioned is bad Boyer terminals or broken wires in the trigger leads, fine until vibes hit resonance or slack hits on closing throttle to misfire/back fire. Might try flicking at those and other wires with a plug out to listen/watch for a spark with key on. Also low voltage can do very similar. Depending on size and charge of bike battery, often its better to power the light off its own stable 12v source. Generally best timing method is to over advance till back fires on starts then retard till it just don't - then put a light on to see where its lands to set it in the future.
 
Mine was backfiring not too long ago and carbs were out of sync! Even though it idles fine doesn't mean they're in sync! The method that works best for me is taking all the slack out of the throttle cable at the twist grip adjuster till either cylinder holds idle with one plug attached. Match each cylinders idle then back off both carbs equally. The balance method of using your fingers or drill bit to check when slide engages doesn't account for cable stretch accurately!
 
Syncing the carbs is as easy as holding it full open, reaching in the throats and making sure both of the slides are at the top at the same time with the cable adjusters. JBA explains this very easily. The drill at the bottom is just to adjust the idle stop, not to sync them. At least that's how I read it.

Your mileage may vary and I wave my right to further consecutive interpretation.

Dave
69S
 
jsouthard said:
So i increased the Pilot jet to a 45 and the backfiring is gone!

However, since i did this i have found a unique Idleing issue. When i first start up my bike she gets up to idle very quickly. Then after about 10-15 minutes of riding the idle starts to lower and i need to tighten uip the air screw to keep the idle at 1,000. It continues to get worse the more i ride. Then the next time i start her it idles between 1500 and 2000 RPM's. Bring it back down to 1,000, start riding and and will need to go through the process again.

1 - The original backfire issue came out of nowhere. I never had any issues with my carb then suddenly the backfire issue?
2 - i never had idling issues before i switched to the bigger pilot.
3 - There is not a vacuum leak. I have sprayed carb cleaner all over the 2-1 manifold & exhaust headers with no effect.
4 - The carb is clean and the air screw still very much effects the idle.
5 - Timing is set.

Could it be electrical? I'm wondering if my original Lucas alternator may not be supplying the boyer with enough power? Any other ideas?
Click to expand...

If your bike was running OK on the smaller pilot before the backfiring, then wrong size jet is not the problem. It would be worth checking tappet clearances, and compression readings, as well as removing intake manifold, exhaust headers and making 100% certain there are no leaks. However the Boyer ignition systems are not particularly great, and are made using cheap shoddy parts, so I would think the ignition is almost certainly going to be the cause of your problem, but if this is the case things will generally get worse if the load on the electrical system is increased (all lights on etc).
 
Make sure that tappet clearances are correct.

With an exhaust valve held slightly open on the over run you can get the popping in the exhaust and it costs nothing to check !!!

PS if you strobe check timing on the side stand it doesn't jump around but the engine appears to sink as you rev it :)
 
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