Spitting Carb & Popping Pipes

Almost finished restoring my '81 XJ 650 Maxim, but I'm still having problems with at least one of the carbs. I've taken them apart and cleaned all of the nozzles & holes I could find, except for the front air jets which I've only now discovered. The bike starts and runs, after a shot of starting fluid, but the outside right carb spits back through the air intake, and I get backfiring pops out of both tail pipes. It is usually while the bike is idling or under gentle accelleration. It seems that only the outside right carb is doing the spitting. At speed, the bike runs OK, although I haven't taken it much faster than 30 MPH yet. Can anyone suggest what is causing this? Should I be looking at the floats, jets, or do I have to take it to have the carb synced with the others?

 

I'm gonna leave this one to the experts, I'm still learning about carbs myself.

 

While I'm not sure if this is symptomatic of bad sync, I do know that you shouldn't have to take it anywhere to get your carbs synced, except maybe to another XJer's house.

Syncing is easy, and the Morgan Carbtune Pro is, while a bit pricey, a "must-buy".

That being the case, I'm sure there's someone in/near Chi-town who has one and would either bring it over or let you come by.

 

If there is someone in/near Chicago willing to do so, please let me know. I'd love to have someone knowledgeable to look over my bike, bark orders at me on what I have to do, and anything else that might be useful.

 

Off hand I suspect 2 thing. First is that you may have missed the pilot mixture screws so they may not be clean. Second is that poping thru the carbs is usually due to a lean mixture. Turning the mixture screw out should help but a Colortune is the best way to remedy and diagnose the problem.

 

If I understand correctly, the pilot mixture screws are like little copper colored plugs with small holes bored through the center. You are refering to either the ones inside the float chamber or on top of the carb under a removable plate. I have removed and cleaned both sets of these. I am unclear as to whether these are meant to be adjusted in or out, or if you simply screw them back down as far as they go. I have also just found that there are two more inside the air intake which I have not yet inspected.
I don't know where the mixture screw is on the carbs or what a Colortune is. The Haynes book I have is vague on a few points and usually says to leave certain adjustments alone unless you are a certified Yamaha mechanic or some other such.

 

Yea the Haynes doesn't cover setting the pilot mixture screws. They are the ones on top of the carbs near the choke plungers. They usually have a brass plug that seals them. Plug is installed by the factory. Since you cleaned them I would set them at three turns out from bottom. Be careful turning them in so yu don't damage the delicate tip. Front air jets usually don't clog up so if they looked clean they probably are. Here is one of the definitions I wrote for reference on Colortuning.

Colortune:
Refers to the Colortune plug and it's use. This
is an instrument which replaces the sparkplug for
tuning. It has a clear veiwing port which allows
you to see inside the combustion chamber while
the engine is running at idle. While veiwing you
can adjust the pilot mixture screws (which control
fuel flow into the idle stream) and see the
color of the fuel burn. A bright blue is ideal. Lean
mixtures yeild a light blue or even flashes of
white which are the sparkplug firing. Rich mixtures
yeild a yellow color. A clockwise turn of the
screw from yellow to blue is optimal. Too lean a
mixture can damage vital engine components.

 

Backfire and spitting can be few things:

Mixture related ~ Lean ~ Tweak-able.
Jet Related ~ Clogged ~ Passage Clogged
Air related ~ Leak ~ Causing a Lean ratio.

Valve related:
Tight ~ Correctable ~ Shim
Burned ~ Correctable ~ Repairs ~ Valve Job
b

Ignition related ~ Timing ~ Not likely ~ Testable ~ Timing light and look at the rotor when #-1 is firing.

 

It sounds like you have the same thing happening when I first got my maxim, and it was the mixture screws mentioned above. Once I turned them out everything was fine. I still don't understand why they were only 1 half turn out to begin with though.

 

Because some people think that those screws are Everything BUT what they really ARE ... and, they mess with them looking to make the bike run on LESS Fuel. Trying, as they may ... to get more Fuel economy out of an already economically advanced system.

I've seem people think they are the Sync Screws.
I've seen others think they are the Idle adjustments.
One guy brought me his Carbs with those Bottomed-out and stripped-tops thinking they were mechanical stops for the Butterflys when he really just needed to adjust the Idle Rod.

Other people just call them "Mystery Screws" and "Play" with them for the fun of it!

 

I'm getting a little confused by some of the descriptions everyone is giving.
I'm looking at my Haynes book now and see that on top of the carb, under the Diaphragm/Valve/Needle assembly are a Main Air Jet & Pilot Air Jet protected under a removable plate.
At the bottom of the carb in the float bowl are a Main Fuel Jet & Pilot Fuel Jet. Under the Main Jet is the long cylindrical Needle Jet.
There is also a Cold Start Plunger Assembly which I have never removed.
Lastly there are also Air Compensator Jets in the Carb intake which I have also never removed.
I have just read that wire should not be used to clear the fuel jets because it might enlarge the hole, but I have already done this. Perhaps I ruined the size of the hole while I was cleaning the gunk out of it. A small bit of wire was all I had that would do the job. I have some spare parts left over from the original carbs (I have some ebay replacements on now) so I will try swapping the jets from the spitting carb with the spares. If worst comes to worst I'll see if I can find some new clean ones.

 

Or am I way off base here? Are there adjustments on the outside of the carbs that I'm missing? The book doesn't show anything but mentions that a Pilot Screw may be recessed and covered or set in place with thread locking compound. This seems to be what some of you are refering to. According to the book the Pilot Screw is above the Throttle Valve, and from what I can see on my bike, it is covered by a little brass or copper disk.

 

Found it on Page 128. Pilot Mixture Adjustment & Mixture Screw. DO NOT ATTEMPT ADJUSTMENT.
The screws on my carbs have this little metal disk covering them, so I can't adjust it anyway.

 

Yamaha put the anti tamper plugs in place to meet with emissions regulations. Old bikes really need to have some adjustments made to get them to run correctly.

Sure you can, just like the rest of us.......

http://www.xjbikes.com/Forums/viewtopic ... screw.html

 

This takes us back to step one. If the plug is in there then the pilot screws were not removed and therefore the passage for them has not been cleaned. I would still try to set them just as Nick suggested. If it works then thre is no need to go back into the carbs. If not then go back in and clean that passage.

 

Between everyone's suggestions, the Haynes manual, and a careful examination of the carbs, I think I have a clearer idea of what to do now. I'll tear into this problem again either tonight or tomorrow.

 

If you think it's a Fuel related issue in the Pilot System ... try:

Running the Bike.
Adjusting that Pilot Screw OUT and listening to the Idle.
As long as the Idle increases ... keep bringing it OUT.
Eventually, there will come a spot where the Idle does not further increase.
Continue bringing it OUT until the Engine coughs or becomes unsteady.
Gently return the Screw in until you have the good Idle; again.

That will let you know it WAS that Pilot Screw.

If Nothing happens ... that whole Passage needs to be flushed clean.

 

I replaced the pilot jet on the caughing carb and drilled the little covers off the pilot screws and marked their original position. They were all between 2.25 & 2.5 turns out. I spoke to a local dealer who suggested turning them out to 3 turns which I did. I did pull one completely out, and it appears to be perfectly clean, so I'm guessing they're all OK.
The one carb stopped caughing, but the backfiring has gotten worse across the board. Since there is a connector between the exhaust pipes, the backfire comes out both pipes, so I can't really tell which cylinder/carb is the problem, and I can only get at the two outer carbs as long as the tank is on. How exactly can you adjust the inner carbs on a running bike with the tank in place?
I've called around looking for a Colortune kit but no auto parts store, or even this local Yamaha dealer has ever heard of one. I've seen them around the web for about $55, so I guess I'll have to order one to find out which of the four jugs is giving me the problem.

 

You make a Tool.

Rip-off the Pocket Clip on a small Craftsman Screwdriver.
Hacksaw off the Plastic Handle right at where the shank ends in the plastic.

Grind or Hacksaw off the blade end.
Chuck it into a drill.
Spin the tool at high speed and sandpaper a nice new top on it.
Cut the lower shank off half way up.
Grind a new blade on the end using a Pilot Screw to measure for a precision tip on the new tool.

Leave the tank on and tweak, tweak, tweak!

 

Well, I took the tank off, set it on a portable work bench & ran a line to the carbs so I could run the bike and get at all the mixture screws. The whole set up looked like the bike was on life support.
I did narrow the problem down to the #4 piston when I started pulling the spark plug cables and found that although it was sparking, it wasn't firing. Coincidentally the backfiring stopped completely too. I checked the fuel level in the bowl by hooking a clear tube onto the bowl as the Haynes book described, and found it was a little low. I pulled the carbs off again and shot carb cleaner through every opening I could find on #4, and flushed some rust out of the fuel inlet line. I also swapped out the float with a spare from my other carb set and went over the jets again. The mixture needle was as clean as could be.
I put the whole thing together again, but I had to run the mixture needle out 3 1/2 turns before #4 started to backfire and spit again. Six turns out seems to have gotten it to fire quietly now. I'll keep tweeking it now until I'm happy with it.
One thing I did notice, and I'm not sure if it's related or not, but while the bike was running, I saw smoke drifting out of the plastic intake manifold assembly. I haven't put the rubber connectors back on between the manifold/filter assembly and the carbs yet, but it seems to be coming from the hose that runs from the gearbox up to the intake manifold. I just gave the bike a complete oil change a few weeks ago, so I thought that it just must be from the new oil heating up. But then I thought, what if I'm getting blowback through the piston rings, particularly #4 which needed so much adjusting at the carb?
I'd like to check the compression on the cylinders. Does anybody know what the proper compression reading should be?
Tony

 

Tony:

You can't get an accurate ColorTune without the Carbs hooked-up to the AirBox with the Boots connected.

Your bringing in too much AIR without the restrictive flow from the AirBox.

I thought you were aware of that?

 

I haven't bought a colortune system yet. I've been keeping the boots off because until recently I still needed to deliver a shot of starting fluid to get the bike running. However last night I put everything back on the bike and made a few more adjustments to #4 carb. Everything seems to be running much better as I rode it around the neighborhood. All four jugs are firing without any popping or back spitting. By now I've lost track of how many turns I have the #4 carb mixture screw set to, but it's something like 5 turns out. Any less and the cylinder quits firing.
Now I have to give the tank and side panels a new paint job, replace the seat, and finish polishing the chrome.
Thanks for everybody's help.
Tony