Bike backfires and pops during start up

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Quilty

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This may or may not be related to carbs but I'll throw this out there.
I've just finished a rebuild of a stock 72 motor. It has 76/77 carbs on it which had one size up on pilot and one size on mains which ran fine before the rebuild.
Since the rebuild she is running rediculously lean. She pops and backfires on startup then after a minute or two will idle fine except for a flat spot just off idle. So I went another size up on the pilot and raised the needle one space 2.5 turns on the screws. No more flat spot ,grand, but she's still a bitch to start. I've checked for leaks can't find one. The choke doesn't seem to work aswell anymore either. Should I have to make her richer given the new compression of the engine or am I compensating for something else?
Bike has new cylinder sleeves and bearings
 
I've cleaned the enrichener circuit, took float bowls and fuel screws out and blew air and carb cleaner in every orifice. Bike starts better but still running lean. Atu is good and correct and working properly. Not sure what else to look at
 
The choke system wasn't working because the screw in the linkage arm of the second carb was missing so I was in effect only enrichening one carb. Choke fixed but still popping throught carbs for first few mins of riding then overheating slightly after about 10 miles. Can't find the leak. I've got a set of NOS 74/5 bs38s but they are missing some brass bits I can't find to replace.
 
In stock form the 76/77 carbs used 1 1/2 turns out on mix screws, 2 1/2 is way to much.
Over heating is often a lean condition, I might go through the carbs again.
Have you read the carb guide? If not it may help with your carb issues.
Leo
 
Read the carb guide back to front and back again. I'm scratching my head because i didn't have these carb problems before the rebuild. I replaced the r/h crank bearing,all of the cam bearings and the cylinder sleeves.
 
Ok since fixing the choke I seem to be running very rich now. There's quite a bit of soot on the plugs but the business end of the plugs are white so lean pilot circuit rich midrange. I will move the needles back to no 3 slot On the way to work today the bike started surging in rpm and wouldn't come down until I shut off the engine. I know that this points to the Atu but the springs are fine and seem to be doing what they are supposed to.
Thanks for all your help.
 
you really need to be meticulous and deal with the various aspects systematically rather than all at the same time.

I have recently gone through the same problems with my 79 and its frustrating to say the least. What I have learnt to my cost is that 'near enough' simply isn't 'good enough' with these carbs or you'll be redoing stuff like I did.

I would start from scratch and remove the carbs , inlet stubs and rubbers.

Dress the stubs flat on wet and dry then refit both the stubs and the rubber inlets with new gaskets and some yambond liquid gasket to both sides of the gaskets .
From this point on you can disregard these and be reasonably certain that its unlikely there would be an air leak on either of the inlets .

Next I would check both carbs for cracks holes splits and missing plugs ,bungs and parts that could cause an air leak. I would remove both choke bodies/assemblies and renew their gaskets or apply a little Yambond .Even a slight air leak here would make it impossible to get a stable idle or easy starting.

Check the seals on both choke valves to make sure they are seating properly and not leaking. Sync the choke valve levers and operating arm.

Back off both throttle stops so that both butterfly valves are fully closed and check using a torch that the discs are not passing any light. If they are, loosen the butterfly disk screws and centre both disks by repeatedly opening and releasing the throttle arm.
When there is no light showing past the disks hold the disks firmly closed and retighten the screws. Carefully adjust the centre sync screw so that both valves are in perfect sync.
Then adjust both valves slightly open together for initial idle using the throttle stop adjust screw from that point on . To check your in sync at any time just put each venturi to your mouth in turn and blow like a trumpet. You can tell quite accurately by the back pressure if they are open equally :thumbsup:

Refit the original pilot and main jets unless you have since fitted pod filters in which case you'll almost certainly need to increase pilot from 27.5 to 30 and go up 2x sizes on the mains .Your air screw adjustment range will tell you if our pilot is right or not.

When you refit the carbs pay particular attention to the carb inlet clamps to make sure they are centred all round and have even clamping pressure on the necks of the carbs.

At first concentrate only on getting the idle spot on when warmed up using the air screws. Until your idle is set correctly it is pointless messing about with needles and main jets because your pilots have a profound effect throughout the rev range and on deacceleration.

Once you have it starting easily and stable on tickover you have a sound stable base to test the mains and needle settings .

If you still can't get a stable idle or it rises by itself then using the thin plastic applicator tube and WD40 squirt around all 4x throttle bushes, the air screws and the choke assemblies etc to track down the air leak
 
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:doh::laugh:
I did that last week . it ran for about 100 yds and conked out ! I thought bloddyell here we go again....then I remembered to turn the taps on:bike:
 
Thanks for the help peanut one again you have come to my rescue. I did what you said. I removed the carbs and boots, wet an dry paper on both the engine side and boot, put new gaskets on with liquid gasket between the boot and the gasket surface. I have a set of brand new 74/75 bs38s with dual throttle cables that I've installed after making doubley sure I have set everything up correctly.
Now tuning. I have set the carbs up as stock. Mine are European bs38s with the number 533-00 stamped on the carb body. It took a lot of research but I identified the as 74/5s. 120 mains and 45 pilot. The bike started but missed and hesitated sounding v lean so I've upped the needle one slot and mains to 126 rides and idles way better but I think she's now 8 stroking but Ive e never heard a bike 8 stroke before. Kind of louder and faster. Not sure would apreciate any advice
 
Let's say I have a set of European model bs38s circa 1974/5 dual throttle cable barrel style cable loops. They had 120 mains on it and 45 pilots. There were parts missing off the carbs like jet needles so I bought new ones from mikes xs. My bike is stock 72 us import with paper filters from what I've read the 74/5 models have wet foam filters is there a difference between the two? Should I rejet the mains to suit the U.S. carb specs for that year?
 
can't help much with the specifics for those carbs as the only limited experience I have is with the later BS38's but referring to the carb guide in the tech section it looks like the main jets should be 127.5 not 120 but the pilots would appear to be stock at 45
Check you have the right needles from the guide .http://www.xs650.com/forum/showthread.php?t=43


You need to concentrate on getting your idle right before you go mucking about with the needles and main jets. They have nothing whatsoever to do with the idle or coming off idle and throttling up to1/4 +throttle This part of the throttle range is controlled by the slide (throttle valve) diaphrams ,pilot jet and air screw.

The main jets are not really effective until 3/4 throttle to full throttle and it is impossible to tune your needles needle jets and main jets unless you have your pilot circuit sorted first as the pilot circuit is active right through the throttle range from closed throttle to near full throttle

You need to read the carb guides and look at the throttle range from closed to fully open and understand which parts of the carb effect which part of the throttle range .
 

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The main jet on European models differs from U.S. counterparts. my conclusion on the identity of my carbs came from this site http://www.xs650.fi/pohja_files/fi/XS650carbs.htm as far as I can tell the main jet is the only real difference between the two. My needles are correct and correctly set at no 4 clip position as per manual. The bike idles fine but died as soon as you put it into gear. I put the larger mains in and it runs better but I don't think that's the cure. I think maybe the sync is out
 
Quilty said:
The main jet on European models differs from U.S. counterparts. my conclusion on the identity of my carbs came from this site http://www.xs650.fi/pohja_files/fi/XS650carbs.htm as far as I can tell the main jet is the only real difference between the two.
t
Click to expand...
ah I didn't know that :wink2:

Syncing is I imagine a little trickier with twin throttle cables. You can't bench sync as you can with the single cable.

Next time you have warmed the engine up and have the idle stable try putting on a fraction of choke and then snap the throtle open. See if a little extra fuel improves the throttle uptake off idle which would indicate that you have a lean idle mixture or possibly a problem with your diaphrams maybe.

A little bit of choke can be a very useful fuel mixture diagnostic from idle right through the throttle range.
 
The left carb was not working as well as it should have so I pulled it and found that the floats were set at 27 mm so I corrected it. while I was looking at the bike without the carb on I noticed that the right breather tube was leaking quite a bit of oil onto the choke crossover tube. Is this any cause for concern??
 

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