Suddenly misfiring popping and banging

[bazz]

Having recently finished an xs650 for my girlfriend she tells me it's popping and banging, misfiring and juddering she's pretty sure it blew a flame out the pipes at one point,and difficult to pull away but cleared up when she was doing 65-70mph
I had done around a 150 miles on it without issue it actually ran spot on, revved out fine ,idles fine apart from when I very first rode it when it did have some of the symptoms she described
But it was only for a few hundred yards and I'd assumed it was just clearing itself out
I shall be taking a look at it in the next few days hoping that it's something simple
I'm suspecting it could be the coils as it's fitted with a Boyer Norton black box and a pair of 6v Lucas coils wired in series as Boyer recommend
But the coils could be 50 years old
What else could cause the symptoms described?
I'm thinking it's probably not fuel supply as it's running fine at 70mph
I'd ridden it down to hers the day before that's around 40 miles and it ran perfect
The next day it runs like crap!!

 

[5twins]

Check the negative battery cable to make sure it's tight. I had somewhat similar symptoms on one of mine and that's all it was. Of course I didn't discover it until AFTER I pulled and cleaned the carbs (didn't help). And it wasn't sloppy loose either, just loose enough that I could move it if I tried.

 

[bazz]

Check the negative battery cable to make sure it's tight. I had somewhat similar symptoms on one of mine and that's all it was. Of course I didn't discover it until AFTER I pulled and cleaned the carbs (didn't help). And it wasn't sloppy loose either, just loose enough that I could move it if I tried.

I'll be going there Friday
I'll check the battery cables and earth
I'll take a jump lead with me and power the CDI directly from the battery to rule out dodgy connections
Cheers

 

[bazz]

Very slight update I got a few minutes on the bike this evening
I started it up and it's just the right side cylinder that's misfiring
So tomorrow I'll try swapping the leads and plugs etc to see if it swaps from one side to the other

 

[bazz]

Looking at the bike this morning in daylight I can see that the carbs have moved back
Either caused by the bracket I made at the top to prevent this sort of thing happening
Or maybe it stopped them coming completely off
How do folks stop their carbs from coming out of the rubber boots?

 

[5twins]

You need to make the clamps really tight for starters, and if the heads on the original screws are buggered up at all, that may not be possible. I change them to Allen heads then tighten them until the two clamp halves touch (basically as tight as they'll go, lol). Also check the timing. If it's off a little bit, that can lead to backfires out of the carbs when starting, and that will "blow" the carbs out of their holders. This happens mostly when starting a semi-warmed up machine.

 

[bazz]

Further to this
I took a look at the new inlet rubbers I had bought and realised the rib inside is in the wrong place
They were never going to tighten properly
I can't remember where I bought them but they are clearly not made properly

 

[5twins]

Get the "ARS" brand if possible. They were the original supplier. They will be labelled .....

 

[bazz]

Get the "ARS" brand if possible. They were the original supplier. They will be labelled .....

View attachment 355882

View attachment 355883

Ok

Get the "ARS" brand if possible. They were the original supplier. They will be labelled .....

View attachment 355882

View attachment 355883

Ok brilliant thank you

 

[Its been a long time]

Ok

Ok brilliant thank you

I found that inlet rubbers sold as Tourmax were made by and marked ARS. They're working great.