Backfire on decel now

brianis

brianis

XS650 Enthusiast
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So I rode the bike to a car show today when I started the bike to leave while working the throttle to warm it up. I was getting a lot of backfires through the carb I know it’s not running lean because I checked plugs the day before. Also synced the carbs yesterday so got home checked the points and all checked out good so I went to check timing have a Hughes Pma no marks on it. I brought it to tdc and made a mark so that I could check timing turns out the left cyl was advanced more then the right so fixed that. Rode it down the road and now it’s backfiring on decel if I let the bike slow me down I have the ya mamma exhaust which has no baffles. And I’ve got the carb mixture screws a half turn out on the round slide carbs
 
Your exhaust tight? Only held on by exhaust bolt flanges on that pipe and if from vibration it loosened a tiny bit for air leak it maybe source of backfire.
 
What do you mean by low speed jet needle. The air fuel mixture or the slide needle
 
Last edited:
I don't have my manual with me, but I'm talking about the very small needle on the side of the carbs. That, I call a low speed jet. It adjusts the air/fuel mix at idle and slightly above. It's funny, but while at the Vintage Yamaha Rally, this past weekend, Maxpete recommended this to me because of my backfiring problem. It seems that my low speed needle tends to slowly turn in over several thousand miles.
Anyway, it idles smoother and faster, (had to turn it down), and the backfiring is almost none existent. I didn't bring my tiny screw driver today, so I'll have to experiment with different settings when I get home. Oh, and before I adjusted it, it would occasionally pop back thru the right carb.
 
fuel for both idle and deceleration is supplied by the pilot jet, air screw and slide .
Popping on deceleration is generally caused by a lean idle mixture ( incorrect mixture screw setting, inlet air leak , insufficient idle fuel delivery , partial blockage ,wrong size pilot jet etc) or an air leak into the exhaust which is igniting unburnt fuel in the exhaust,

If your mixture screw is only 1/2 a turn out it would suggest that your pilot jets might need changing up a size
You don't mention what carbs you have fitted or the pilot jet size
 
Marlin72xs said:
I don't have my manual with me, but I'm talking about the very small needle on the side of the carbs. That, I call a low speed jet. It adjusts the air/fuel mix at idle and slightly above. It's funny, but while at the Vintage Yamaha Rally, this past weekend, Maxpete recommended this to me because of my backfiring problem. It seems that my low speed needle tends to slowly turn in over several thousand miles.
Anyway, it idles smoother and faster, (had to turn it down), and the backfiring is almost none existent. I didn't bring my tiny screw driver today, so I'll have to experiment with different settings when I get home. Oh, and before I adjusted it, it would occasionally pop back thru the right carb.
Click to expand...

the part you are referring to Marlin is variously referred to as an 'idle mixture screw' , 'pilot screw. ' or 'air mixture screw' .Most people understand what part you are referring to if you use any of those terms. Depending on the carburettor type , the description doesn't always describe the function and leads to much confusion.
your post was spot on and very helpful too........ unlike some contributions :)
 
Marlin72xs said:
I get it. But you didn't answer my question. How's the weather out there?
Click to expand...
apologies I missed the question.
Its been pretty fair here today in South West England The sun has been out most of the day and it has been quite warm.
If I wasn't trapped in a chair with back spasms I would have been working on my 64 Lambretta and riding my 78 XS650
 
I'm pretty sure Brian has Mikuni VM slide carbs. The mix screw on them would be an air regulating screw. Turning it out would add more air and make the idle mix leaner.

You said you marked TDC and set your timing. TDC isn't for timing, only for setting valves. Idle timing is set at 15° BTDC. If you've set it at TDC, that could be the cause of your problem.
 
peanut said:
apologies I missed the question.
Its been pretty fair here today in South West England The sun has been out most of the day and it has been quite warm.
If I wasn't trapped in a chair with back spasms I would have been working on my 64 Lambretta and riding my 78 XS650
Click to expand...
Sorry to hear that and hate to rub it in, but just got back from a 90 mile ride. This weekend i took 4 passes at the Dragon during rhe Vintage Yamaha Rally in N. Carolina. Now i want to go back. We're having a late summer here in Georgia. All this week high 80'sF. Hope ur back heals.
 
Hey 5twins they are the mukuni round slides and how do you going about figuring correct timing marks with the Hughes Pma setup no markings for different cyl on the stator
 
And I understand what the mixture screws do being on the backside of the carb they control air more air leaner less air rich. Not my first ball game. Not having timing marks and such on my stator timing setup is a first
 
Even the stock set-up doesn't have different timing marks for the different cylinders, you use the same marks for both. You'll only be checking one cylinder at a time. Check/set it at the timing mark then check/set the other cylinder to the same timing mark. Hugh has a sticker with timing marks for his PMA that is applied to the side cover. I'm pretty sure he sells them .....

GBmaijd.jpg


You find TDC then mark the PMA rotor in line with the TDC mark on the sticker. Then the other timing marks on the sticker will be valid as well. Yes, it's pretty silly-stupid-crazy-idiotic to install a PMA without timing marks, but guys do it all the time, lol.
 
Mine is exactly like your photo so when at tdc I mark the tdc spot on the stator. When I start the bike and use the timing light where should the tdc mark that I made be when running
 
The way I set it was marking tdc and when I’m running the bike I am setting the tdc mark on fire when running if you could walk me through this it would be appreciated
 
I also moved the points plate to the same cyl to get the tdc mark when running to sit at the fire mark while running
 

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