Bike starts up idles on choke, dies with choke off our when warm

Hello Group,

Picked up a 1980 XS650SG with 5100 miles from the original owner that had it in storage for 31 years. New tank, new fuel system and carbs have been cleaned and rebuilt recently with stock internals. No upgrades just bone stock carbs. New battery, replaced round fuses with contemporary plug in fuses. Bike finally runs on the choke but runs like crap. Rough on idle, when I tap the throttle bike dies. When I take off the choke bike dies. If I leave the choke on and let the bike get warm it runs for a bit and then slowly dies. When trying to ride it hesitates and then jumps to life and then hesitates, sounds like left side backfires. Sounds like its not getting The proper spark. And the plugs look completely different for each cylinder. The left spark plug is fouled, has a black film on it and smells of un burnt fuel. The right one is fairly clean and electrode on end is white in color. Where in the world do I start on this project. I'm not mechanically brilliant I'm just stubborn, won't quit, understand most shop manuals and some combustion basics with cars. I mainly bought the bike because of what is was being from the original owner, garaged for 30+ years and I wanted a winter project. PLEASE HELP and thanks in advance.

Evan

The choke is designed to only provide a rich fuel/air mixture for a short period when the engine is cold. Within the first 20 or 30 seconds the choke should come off and the required fuel/air mixture is then supplied by the pilot circuit. Your pilot circuit is obviously not doing its job.

Everyone always says they have cleaned the carbs, but many really don't understand how the carbs work, or what to look for.

The pilot circuits must be proved clear by blowing carb cleaner and compressed air through them. The pilot jets themselves must be checked that they have no blockage............they have a very, very tiny orifice, which is easily plugged. The mixture screw must be set to the correct starting position, and tuned as required.

Make sure the rubber plug is in place covering each pilot jet.

Read the Carb Guide in the "Tech" section.

Welcome to the site evans.
Sounds like you have all that required to get it running.
The being stubborn part I mean. She'll come around with some loving.
What retiredgentleman said can't be stressed enough.
There are some wee little holes the fuel goes through and knat shit is big enough to clog them. You will also want to check the choke feed in the bowl. It is jetted and NOT replaceable so be weary with poking wire in there.
Look for 5twins posts in carb threads. He has some really good pics explaing things.

Hi Evans - loads of carb cleaner and an airline, grab a manual or the carb guide in the tech section and clean clean clean

Try spraying some carb cleaner around the intake manifolds and throttle shafts while it's running. Also plug off the LH vacuum line and run the petcock on prime see what happens.

I think you are correct here based on what I have read in the forums but that is why I am confused. When I first cleaned the carbs I did a rebuild kit, unscrewed the pilot jet completely, blew carb cleaner through it, through the hole in the housing, then came back with an air hose at 80+ PSI and blew it through the Pilot Jet again as well as the housing. I also replaced the rubber plug over the Pilot Jet with a brand new one. The bike wouldn't run at all before going through this. Now it runs when cold and dies when warm but the choke has to be on. I understand what you are saying about the Choke Circuit. It opens an additional channel allowing more fuel into the carb and after it has started it then can be shut off as the Pilot Air Jet system takes over for the idle. It would make sense this is why the bike is stalling.

That being said, I understand that everyone may say they have cleaned the carbs but I do believe I did clean them properly. However, I may have missed something and I will give it a look again if necessary but based on this information I have provided do you still think I need to go through it again? They must be CRAZY CRAZY CRAZY sensitive.

As for the mixture screw I also understand how it works and that in the 1980's bikes a sheet metal screw must be used to get the metal plugs covering the Mixture screw out. Factory adjustment I believe is 1.5 turns out after being screwed in all the way. However, I have adjusted mine to 2.5 turns as xscafe.com said this can help to get a more fuel rich circuit that will help with idle. It did make a difference.

What did you set your float height to?

I'm with Gary on checking for air leaks. 2 ways to go lean. Too much air, or too little fuel. Since you say ya know your way around a carb cleaning, look into the air leaks. The shaft seals may be an issue, or the flange boots on either end of the carb. Install brand new plugs too, since they are not in the same condition, and of unknown age. These bikes don't make the strongest spark in the world, and like good fresh plugs. Make sure your battery is well charged too.

The BS34s usually like about 3 to 3.5 turns out on their mix screws. At 2.5 turns, you may still be too lean. The fact that 2.5 turns improved the way it was running is telling you something. There never was a factory setting given for the BS34s since they were set at the factory and capped. You're not supposed to mess with them because "big brother" knows what's best for your bike, lol.

littlebill31 said:

What did you set your float height to?

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You will probably laugh at me but I didn't know there was a setting. Although I can picture the fittings in my head now and I'm thinking you can just pull the seat out a bit and that rubber washer will hold it in place. This should reset how far the float valve needs to go into the seat as the bowl fills up and the float raises the valve. Does this sound about right? I have the float valve seats pushed all the way up as far as they can go into the hole they fit into with the rubber washer on the end holding them in place.

jd750ace said:

I'm with Gary on checking for air leaks. 2 ways to go lean. Too much air, or too little fuel. Since you say ya know your way around a carb cleaning, look into the air leaks. The shaft seals may be an issue, or the flange boots on either end of the carb. Install brand new plugs too, since they are not in the same condition, and of unknown age. These bikes don't make the strongest spark in the world, and like good fresh plugs. Make sure your battery is well charged too.

Click to expand...

I will look into the air leaks tonight. Although I did check the boots holding the carbs into the manifold of the engine block. Not sure what the shaft seals are you are referring to? Also I installed brand new NGK Iridium plugs and it did seem to run a bit better but I still had the same problem of it dying. The battery is brand new and well charged as well.

5twins said:

The BS34s usually like about 3 to 3.5 turns out on their mix screws. At 2.5 turns, you may still be too lean. The fact that 2.5 turns improved the way it was running is telling you something. There never was a factory setting given for the BS34s since they were set at the factory and capped. You're not supposed to mess with them because "big brother" knows what's best for your bike, lol.

Click to expand...

Yeah, just like Big Brother knew best not to tune the Nissan Skyline GTR any better and Nissan was saying that tuning wasn't possible when the car came out in 2009. Now aftermarket tuning companies have them tuned up to 1100 HP on Race Gas or well over 800 hp on 91 pump gas. Go figure.

In thinking back. I actually set mine to 3 not 2.5.

Well, no, that is not how you set the float height. I think you are talking about the float valve and float valve needle. The float has a tang on it, which depending on the angle of adjustment, allows the float valve needle to close in the float valve. It is basically a plunger. The bowl fills, the float rises and the valve needle presses up into the float valve, shutting the fuel supply off.
An '80 has brass floats, which need to be set at 27.3mm. If they are the foam floats, ('81 and up) they need to be at 22mm+/- 1mm on both. This is a very exact measurement, very exact!!!
Here's a little video to help.

The shaft seals are the seals where the butterfly pivot shafts go through the carb body. If 5twins is right (He usually is!) sounds like you might be pretty close. Based on what you have said, If I were you, I'd check the float height, run those mixture screws out 3.5 turns, and put it all back together and give it a whirl. Check the boots going from the carbs to the airbox as well. either end of the carb leaking can mess with you. At idle, the vacuum is very high, and the effect of a leak can be considerable. Stick with it, and accurately report your steps as you go, and the old hands around here (I'm not included, I just know the basics) will get ya running like a sewing machine.

Ok so here goes.
#1- I went through and adjusted the float height. One carb was a couple mm off and the other was off quite a bit. I think the book calls for 27.3 mm on the 1980 Brass floats and one of mine was like 40 mm. So that explains that.
#2- Then I took the carbs apart again and cleaned the entire Pilot system again
#3- Hooked everything back up, pulled out the choke 1 click and it wouldn't start. Pulled the choke out two clicks and it started right up and rpm's jumped to around 3500 rpms and kept climbing so i pushed the choke back into to the first position again and the bike died. I went through this process about 3 or 4 times and then I tried starting the bike with no choke. Back to life it came and then died again. Then I pulled the choke out again to the first setting and the bike started up and idled at about 2000 on the tach and started backfiring like crazy.

I took off the tank and readjusted by air/fuel mixture screws from 3.5 back down to 3 and still had the same problem.

Would it also be helpful to mention that when I put everything back together I'm not hooking the air box back up? I'm not sure if that will make a huge difference because the box does restrict air flow but cars run without an air box on them just fine when your trying to see if they run.

Oh and I went through the entire manual by 5twins. Good Info in there but I'm still where I was at before. Bike runs rough on start up and then when I blip the throttle just a little bit the bike dies again. Then after a while it won't start at all. Basically it's start, putter putter die. Start putter, putter, putter die. Start,vrrrooooooooooom and idle won't slow down then I push the choke back in or take it off and the bike won't start.

Any more ideas? I'm probably just being an idiot and it's right in front of me. I haven't been able to get the bike to stay running for more than about 30 seconds to a minute this entire time.

Ah I posted after your update.
Have you blocked the vacuum barbs on the intake manifolds and are in the prime position on the petcock?
If you still have the vacuum line hooked up check it for cracks at the ends. Also check the blind plug(rubber cap on the off side barb).
The seals are located on the shaft that opens the butterfly.

evans_search said:

Would it also be helpful to mention that when I put everything back together I'm not hooking the air box back up? I'm not sure if that will make a huge difference because the box does restrict air flow but cars run without an air box on them just fine when your trying to see if they run.

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You must have the air filters on. It will get way too much air without them. This will cause an extremely lean mixture. If the bike is running with the choke on (plain raw fuel) and then you blip the throttle, killing the bike, that tells you that there is too much air.
Put the filters back on and try again. These carbs are very tricky if you miss a step.

Filter! There's the missing piece of the puzzle! FYI, with the choke on, these babies will jump to a pretty high idle, even when everything is right. 3-4k rpm is not unusual, but you should be able to get it to idle with the airbox on. After that, valves, timing and sync.

Just make sure you don't have mice or something in the air box , that happens .

jd750ace said:

Filter! There's the missing piece of the puzzle! FYI, with the choke on, these babies will jump to a pretty high idle, even when everything is right. 3-4k rpm is not unusual, but you should be able to get it to idle with the airbox on. After that, valves, timing and sync.

Click to expand...

Ok so I put the Air box back on tonight and buttoned everything up. Pulled the choke out 1 click and tried to start...nothing. Pulled the choke out 2 clicks and the bike jumped to life. Literally jumped to about 4100 RPMS. Now that you told me this is fairly normal, I just let the bike idle there for a while and rpms gradually started to drop down lower. Eventually I got to around 1500 rpms and I was still hearing a lot of poping so I stopped the bike. Went back to the Air/Fuel Mixture screws and turned them counterclockwise another half turn (from 3.0 to 3.5 turns from Zero).

From there I was able to get the bike started again, twist the throttle a bit and see the response. It was extremely delayed. I would twist the throttle and then count...one one thousand two, then the rpm would come up a bit and it was also slow to drop back down again. I was able to do this a couple of times and then as the bike got warmer the idle continued to drop lower.

Finally when I started twisting the throttle again the bike would just die and I couldn't get it started again. This time it ran for around 5 minutes though.

Oh also, while it was running around 1500 RPM I was able to take some carb cleaner and spray around all the boots, hoses and fittings. Didn't see any response. However, when I sprayed the carb cleaner in the hole at the air box the idle jumped up to 4000 and stayed there for quite some time before dropping back down to around 1200 RPM. Then the bike died and I couldn't get it started again.

Is is possible my tank contaminated the Pilot Jet again? These carbs are sparkling inside, and aside from putting them in a jewelry cleaner I don't know how else to clean them any better. Should I buy new ones? Float bowls have been adjusted. What would cause this kind of behavior? Common sense based on what you all have told me tells me its still the Pilot System? I've read the carb guide and I guess I'll read it again. Thanks in advance for all your help thus far. This is such an awesome community and site.