Backfiring Exhaust Sometimes

T

Take Warning15

XS650 Addict
Messages
436
Reaction score
2
Points
16
Location
Philadelphia
Hey guys, I finally got my carbs sorted out but now I'm having another problem that I'm hoping someone can point me in the right direction on.

The bike is a '75 with '79 carbs. I went for a ride and the bike ran great for the first 7 or so miles. Then it started to sound like it was going to quit for a while and finally it did quit. I suspected clogged idle circuit but put new plugs in on the side of the road and got it lit. It was tough to start but it did run. Again, it idled but sounded like it wanted to quit.

Here comes the problem. From this point on the bike started to backfire out of the right exhaust when ever I closed the throttle and the engine was between 2400 and 2900 rpm (i.e. slowing down for a light, etc.). I tried to adjust the mixture screw slightly but ended up with it back where it was. I also checked timing with a light when I got home and it checks out. I lubed the auto advance rod today too. Finally, the spark plugs were pretty white when I pulled them but not sooty. I can post pics if that would help. Any suggestions on what to do next? Weak spark/ compression?
 
Mine does that when a grain of grit gets into a pilot jet. Not uncommon after carb work, gets jostled in the tank somewhere and finds itself in carb bowl. The white plug means lean. Drop the bowl, see what's in there...
 
Here's a general question for you. Is it necessary to remove the carbs from the engine to clean the idle circuit? In other words, will any obstruction to the pilot circuit most likely be in the jet/ bowl?
 
Out of sync can cause back fire. If you haven't synced do it. I have lost a cap off a vacuum barb and it causes a backfire condition.
 
I have bs38, I just drop bowls, clean bowls and jets, leaving carb body on. I can shoost carb cleaner up into body if necessary. Most obstructions I've come across involve the pilot jets. Next most common is bleed air obstruction, but that shows as rich condition.

A quick test/band-aid is to blast some air and/or carb cleaner into the mix/pilot screw opening. If spec dislodges, bike runs fine, temporarily. At least you know where the problem is.

Something else to check is the fuel delivery: fuel thru petcock, fuel thru float valve. Empty bowl, slow flow can do what you're experiencing.
 
Check your battery cables to make sure one hasn't come loose.
 
Okay, I tightened battery terminals, cleaned my carbs (again) with carb cleaner and air, and made sure fuel is flowing well into both carbs. All checked out okay. The carbs are synched as well as I could get them. I'm still getting backfire though only after I've been riding for a while. It also runs poorly when cold unless I have the choke on, in which case it revs on its own up to 4,000 or so. because my plugs seem a bit too white, may the backfiring be because my needle is a step too lean?
 
It does sound like rejetting may be needed. If/when you get the pilot jets out, you can check to be sure the orifice size is correct by checking with the reverse end of micro drills (#61-#80) as a guage. The pilot jets are stamped with the orifice size in 1/100 mm. Ex: #42.5 is .425mm, which is .0167". Sometimes a thin ring of varnish in there makes the orifice smaller, and a very thin coating makes a big difference. Takes forever to dissolve out. I sometimes add 'Sea Foam' to the gas to prevent this.
 
Would I need to go smaller for this? Isn't around 3,000 rpm out of pilot jet range? Would I then be looking at adjusting the needle? Or is that a range of some over lap? Unfortunately I don't have drill bits here that small. I did look at the jet I believe it is a 27.5(?)
 
The area of influence of the various fuel delivery systems in the carb are more tied to throttle opening than rpm. If the throttle plate is near fully closed, just the idle port is involved. But if you've got an air leak, combined with something that would normally read as rich, then that's like having an artificial more-open throttle.

The 'choke' isn't really a choke, it's a fuel enrichener, so the engine should properly rev higher with it on.

A spark advance with weak springs can make the engine rev too.
 
That all makes sense. Thank you. The thing that's throwing me is that it is only backfiring on one side. Wouldn't you imagine it would be both since both carbs are jetted the same?
 
You betcha, still got some work here to do.
Lets try the systematic approach, isolating individual fuel delivery systems.
You know now how to read plugs, this is what you will use to figure this out.

Start with idle. If you're satisfied with idle, move on to next test. Else, fix idle.

Next is transfer ports. These are used just off idle. They handle the transition from idle to needle jet cruise zone. They are controlled by the pilot jet. With clean plugs, ride around awhile just off idle, avoid any other throttle position. Then shut it down, coast to a stop, pull and read plugs. If satisfied, move to next test. Else, fix pilot jet/circuit.

Next is low part of needle (actually the upper part of the needle). This would be at about 40-45 mph. Do the same runawhile/shutdown/plug pull routine. If satisfied, move on. Else try adjusting needle.

Next would be mid-needle, maybe 55-60 mph.

Next would be needle jet (the long tube the needle run in) maybe 65-75mph.

Last is main jet, wide-open.
 
On the '78-'79 carb set, if you drop (lean) the needles a step, you'll need a larger pilot.
 
Thanks, I'm going to go play with it now. I'll let you know how it looks. If the leak down test checks for diaphragm compromise then yes, I've checked. I lifted them and covered the hole in the bell and they stayed in place, maybe falling slowly.
 
What mods are you running (pods? pipes?) and what is your jetting now? If you have done the usual "pods and pipes" mods then yes, some re-jetting will probably be called for.

The 3000 RPM mark is sort of in the transition area between the idle and midrange circuit. That means both are influencing what happens there. Pilot jet size, mix screw setting, and needle position all need to be right for this transition to go smoothly.

There are several threads going now about idle and low speed running problems but no one is mentioning the mains in them. You really should sort those 1st because they can dictate all your other settings. Yes, there are overlaps in your carb circuits and when you change one, it effects the one next to it to a certain degree .....

Carb_Circuits.jpg
 
Excellent pictorial 5twins. Years ago I had a Mikuni tuning manual that had a graph pictorial of fuel delivery volumes for each subsystem vs throttle opening, showing the overlaps. I wonder if that manual is online...
 
Yes, I think it is on-line and I have seen that pic (I have the book of course, lol). Mine is a somewhat simplified version but gives the general idea of what happens. You have to realize this is happening in order to tune carbs, and many don't. What it boils down to is you usually can't just change one thing, like a pilot or a main, without having to tweak another circuit to compensate for that change.
 

Similar threads

M
Backfiring and rich
Replies
6
Views
334
bosco659
bosco659
Prisondawg
1979 XS 650 idles fine , falls flat just off idle, carb is clean.
2
Replies
24
Views
751
Honda600rr
H
Batterylizard
‘79 xs650 won’t start ..backfires out of left cylinder on kick
Replies
8
Views
815
5twins
5twins
Gpaws1968
Inconsistent Idle
Replies
5
Views
348
Gpaws1968
Gpaws1968
abert
TCI system worth replacing?
Replies
11
Views
1K
sparky4123
sparky4123
Back
Top