I surrender. Carbs, you win.

[hobbsy84]

Well, it took quite some time, but I've finally lost patience with this bike. I think I have had the carbs off and cleaned about 14 times now, and all it's managing to do is frustrate the crap out of me. As nice a this bike is, I will never re-build a 4 cylinder bike ever again. Four times too many things to buy, four times as many things to go wrong. But thats enough ranting.

The main problem (this time) is that after removing the carbs and cleaning them today, the bike is starting, and idling, but the No. 1 cylinder is backfiring sporadically. And loudly. Yesterday the No. 1 and No. 2 cylinders weren't backfiring, but when the bike was taken for a run, it would hesitate, misfire, eventually stall out, and blacken the plugs on those two cylinders. Cylinders 3 and 4 were running great.
I removed the carbs and carried out another good cleaning with carb cleaner (even used pipe cleaners in all the damn little orifices) and once again came back with nothing really out of the ordinary coming out of them. So back on with the carbs, and now the backfiring on cylinder 1 has appeared. I should say re-appeared, because the day before yesterday the carbs were taken off and cleaned because cylinders 1 and 2 were both backfiring (rather loudly).
Prior to all this, I was having no problems with backfiring at all, but I was having trouble with the revs rising during idle. I replaced the intake boots (after about 4-5 removals and cleaning of the carbs) and fixed that problem. But the fouling issue with cylinders 1 and 2 appeared around now.

I'm at the stage now where I'm about to start looking at alternative carbs to run on this bike, because quite frankly I've had a gutful of the ones on it. From reading about I'm obviously not the only one to have problems, and certainly won't be the last. Are there any decent alternative carbs that people have been known to use on these bikes that aren't a hard change over?
Or failing that, am I missing something that will help to fix the carbs I already have? I can't see why it would be anything other than a carb problem, since everything else has been replaced. Cheers all, hope you guys can help, I'm at the end of my tether with this one.

[Ralphy]

ahh, New fuel, new plugs, synchronised carbs, valve timeing? bet ya its something so simple your not even looking at it, walk away take a breath and come back in your own time.

[03mach1]

Float level too high and check the synching of the carbs? I went the thru the cleaning and rebuilding of my '78 and had the same problem the first and second time with backfiring, not running correct, hestiating, I was getting frustrated too. I had the float level too low (Rich) and had to raise them up and the synch was way off.

Then using a vacuum gauge (with a canister to absorb the pulsations) we were able to synch them in about 5 minutes, adjust the idle/mixture screws and it runs great now.

Hang in there, like they said, you have to take it one step at a time and check everything.

[boyat68]

After 13 times through because of fuel leaking after rebuild, I discovered that I let the floats drop too far down in the bowls and this caused them to wedge stick on the bottom sides of the bowls at the rounded transition line. Those floats only have to drop a millimeter or two in order to fill up the bowls so I bent the back tangs on the floats to just allow the floats to drop a couple of mills. and that problem was solved. That problem also caused my plugs to foul as that extra fuel was getting sucked back from the air box. A couple changes of plugs, a decent synchronization and fiddling with the pilot screws brought everything in line. Now that I know what to look for, no fear no retreat.

Just don't give up the ship.

[DGXSER]

The idle up issue and this could be synch for sure. Did you do the bread tie synch to start with? That will typically get you close.

[egleaves]

I learned not to get too caught up with cleaning and adjusting the carbs until other things have been diagnosed and/or fixed. Dbeardslee can vouch for this, check and clean/replace electrical connections to make sure you have a healthy spark.

I was fed up with tuning the carbs over and over without solving the problem so I finally started looking at my ignition coils and as it turns out, they are near death's door. I've messed with two sides of the triangle without making sure the third side was reliable; fuel and air are nothing without the spark.

I'm done with the carbs until I get new coils.

[Guest]

carbs

hey 84 I've been going through the same thing on my 81 project for a month and just borrowed a elec tester and found a bad coil also starting to wonder about my vac diaphragm? take a break from it, these bikes are worth it lol

[Dragon Tamer]

You didn't mention vacuum syncing the carbs, if you didn't do it after reinstalling them you WILL get a lot of backfiring until it is done.
This is not to say that you might not have another problem also but is very hard sometimes to track two problems at once.

[81xsproject]

The carbs on these bikes are very simple. If all the parts are in the condition they should be, are the right type of parts (not aftermarket replacement garbage), the carbs are clean and set to stock specs THEY WILL RUN. If you've done all these things, my guess is that you have something else wrong.

[hobbsy84]

Well, I've picked up the tools that went flying across the garage earlier on today, and had a think. After reading some responses, I think I'll have to go back in and re-check the float heights. I did check them 2 carb pulls ago and they were all at around 26mm, but I might have sat the carbs on the floats or something silly. I guess it doesn't take much to bend a brass float tang. Time to find my ruler...

I know for sure the coils aren't the problem, I installed a set of MikesXS coils to be sure of that. Before all the backfiring began (about 2 carbs pulls ago as well), I was able to get rid of the high idle issue, so I know vacuum leaks aren't going to be the problem either. The intake boots were replaced to ensure that one. Might go grab another set of plugs to be sure as well, current set is fair fouled.

I know this thing'll get there eventually, but after 3 months of carb problems, it just seems like the light at the end of the tunnel is moving further and further away. Fingers crossed I get somewhere tomorrow. I'd like to be riding this thing instead of dismantling still. My poor little RD250 2 stroke is a good bike to go to work on, but nowhere near as comfortable as the XS11...

[TDodge7]

i hear ya man , i have a set of 1980 carbs that just refuse to work . i've been thru them 50 times , adjusted everything , synched them , ultrasonic cleaned them . everything is clean , every circuit clear , they are clogged with magic . yet the dirty , way out of synch set of 79 carbs i have with popped brass floats makes the bike run pretty well ....go figure

[dbeardslee]

Egleaves was right on the money. You can diddle with the carbs 'till the cows come home, but if that's not the problem you're still going to be scratching your head. If it's backfiring you might check the resistance on your pickup coils at the TCI. Out of spec valve clearances will make 'em backfire. And yes, the synch will do it too.

[Crazcnuk]

I had the same problem. I had mine off about a dozen times. Still wouldn't run quite right.

Put the carbs on a different bike, and they ran fine....

Backfiring at idle is usually an ignition miss, loud, or a lean idle mix, pop.

You can easily see if it's the idle mix by simply running the idle mix screw on each offending carb from one end of the adjustment to the other, slowly. You can even pull the screw right out, while it's running and see what happens, but that shouldn't be necessary. The idle should go from rough, (popping) (in) to smooth, then to rough as the screw is run out.

If that does nothing for you, then look at whether the problem is happening in pairs. Ie if the two cylinders on the same coil are backfiring, then it is likely an ignition issue. if only one, then maybe a sparkplug is wonky, or a bad sparkplug wire. You can test a sparkplug by moving it to a different cylinder.

[81xsproject]

You CAN'T be sure you have no vac leaks until you have tested with something highly flammable. I recommend a propane torch. There are other places to get vac leaks besides the boots. Butterfly seals synch port caps are two. I didn't see anything about you fixing the PU coils (even if they look fine and test fine).

[hobbsy84]

Well, after months of losing sleep (and hair, I'm sure of it) I think I'm finally getting somewhere. Managed to take the bike for a quick spin around the block this afternoon, it's running really well in the whole rev range pretty much. No stutter when starting off either!

I ended up sitting down the other day after a bit of a break from it and thought about things, after I'd been having the constant backfiring from the #1 cylinder. I switched over the spark plugs from #1 and #4, and who would have thought it, the backfiring transferred across! After cleaning up the plugs a bit (used brake fluid and some fine sandpaper along with compressed air) I tried the bike again, and the backfiring was gone. After a bit of a sync, and some fiddling with mixture screws it seems to be going well.

Only a couple of issues left to deal with... The idle is steady upon start-up, but after running around the block, the idle seems to deteriorate and stalls out. Looking at the plugs, the electrodes aren't fouled up, so I'm pretty sure that can't be the problem. I was thinking perhaps the mixture screws need adjusting, but does 3-4 turns out sound like too much?
Other than that, the front brakes (which had the pads changed on it during the re-build) are squealing big time, to the point it's kinda painful on the ears to apply the brakes... Does this mean the pads aren't in properly? Or perhaps the rotors are glazed?

I'm excited about getting this bike on the road, I can't believe how comfortable it is on the road. I reckon they had seating positions down pat back when these bikes were made. The light at the end of the tunnel is coming...