Back fie out carbs...

[Joe699]

Hi all,

Just to check my thinking:

The bikes a '72' and the carb mounts are quite worn and I'm going to replace them and clean the carbs.

With the choke out there's not much of a backfire, push the choke in a notch and it appears, push the choke all the way in and the bike will die unless you work the throttle which results in backfiring through the carb.

I figure the worn/cracked mounts are creating the issues through air leaks?

any advice appreciated

Thanks

 

[xjwmx]

It's not physically possible to backfire though the carb unless the spark comes at the wrong time or the intake valve is open at the wrong time. I presume you've checked your ignition timing and done your valve adjustment? In that case, then it's only physically possible to backfire through the carb if the cam timing is off or the intake valve isn't sealing right.

 

[Sundie]

it's not possible... Unless you've upgraded to a electronic ignition.

 

[Joe699]

I didn't really think that through before posting thanks

The first thing I will do is check timing as there does seem to be some sort of back pressure at different times coming out (?) the carbs, sounding like small backfires?

 

[twinsarehot]

http://www.xs650.com/forum/showthread.php?t=1872

I having similar issues but with the later models. Take a look at that thread (about 2/3 of the way down) and see if it helps at all.

When you say backfiring, you mean puffs of unburned gasoline is blowing out the intake right? Not flames....

 

[inxs]

- cam timing? valve seal? carb air leak? exhaust pipe inner crushed?
- retension your cam chain, set your valves, check and set your points..go from there

 

[twinsarehot]

It's commonly caused by a lean mixture or vacuum leak.

Fuel starvation could do it too, I'm in the process of cleaning my tank and replacing the line filters. Hopefully it helps.

 

[Joe699]

Thanks again all,

Yeah thats right Twinsarehot, not flames - thankfully

The previous owner said he had issues and I've ended up with an extra 3 sets of carbs from various years that he tried. The set on there are from a later single cable set up.
The motors had a "tune" up apparently by someone that knows there stuff but who ever has worked on other areas has been a bit rough and ready.

 

[xjwmx]

It's commonly caused by a lean mixture or vacuum leak.

Fuel starvation could do it too,

Please describe for us HOW those things cause it?

 

[Sundie]

Fuel is too lean to fully ignite in the chamber. Exits via exhaust, where it gets to sit in the nice hot headers and a couple of puffs will reach a mix that flashes. Pop.

 

[twinsarehot]

http://www.mikuni.com/tg_backfires_through.html

 

[bassgoat1979]

The timing would have to be WAY advance to fire befor the intake was closed. It would backfire every stroke as well. It is almost always lean or a weak spark. (the fuel doesn't burn all the way, you can get exhust back fires, sometimes both.) Weak spark can make it ignite too. If you timing was so advance that it fired at the bottom of the compresson stroke, it would even run at all, think about it, there would be no cropression. A bad or misadjested valve would be consitant every turn too. (probably wouldn't run either)