Carbs backfiring, plugs rich, stalling at idle

[azr]

Stock 79 GS850 with stcck VM26SS carbs. I've been working on this bike for sometime with no avail. Airbox is rebuilt and sealed tight. Valves have been adjusted. New points. Timed. New Reg/Rec. Carbs have been dipped and rebuilt (several times..arrr). New boots to air box. New rings for intakes. New petcock. Checked the needles on the carbs and they are all stock and at stock settings. New plug caps. Carbs are set to factory specs, synched properly. As far as I could tell I checked the float levels properly and they were all good. Compression is good on all four. All four pipes heat up when running, evenly it seems. When I start her up she runs fine until she warms up. She wants to die at idle no matter how much I put the main idle up. The plugs are black soot coated (all four are rich), and occ. I have fuel run out of #1 (at the bottom overflow). This problem is driving me nuts!! I read on another post something about removing the float bowl vent tubes, but I'm not sure where those are. I thought maybe the clips on the jet needles had been moved but they are at #2. The bike seems to idle as if there are no vacuum leaks. I'd really appreciate some more help with this as I've been chasing what seems to be the same problem for far too long. Thanks in advance.

[chef1366]

Check the voltage at the coils

[azr]

dumb question but exactly how to I test the coils?

[chef1366]

Get a multimeter if you don't have one.
Take off the tank
Unplug coil
Turn on ignition with headlight off or disconnected
Harness side of the coil plug check DC volts (set mulitimeter at 20V) on the plug. One side of the plug is hot and the other ground. If it runs you will get a reading if not you aren't doing it right.

[azr]

before turning ignition I get 10.5 while ignition is on 8.5 (thats with lights off). I tested the ohms across the plugs and got 20'000. I"m guessing this means both my coils are no good or at least not getting enough volts to them. It's odd that both coils are exactly the same, that makes me think there is a problem getting full voltage to the coils and not necessarily the coils themselve. Am I correct in thinking that? if that is the case then a coil relay modification (as described on Bikecliffs site) would work in this situation?

[chef1366]

You should get very close to volts you get at the battery. If not do a coil mod relay.
Do a search for "coil mod".

[atchbo]

Have you checked the needle valve? If you have fuel leaking out, that should be the culprit (especially if your petcock is new). Then check the float height or the floats themselves.

I know some people talk down on them, but I bought a Morgan Colortune on ebay to check for rich/lean conditions, and it seems to work. I can't say it will get your mixture dialed in to perfection, but you can clearly see yellow vs blue flame happening inside the cylinder, which tells you if you are running lean/rich respectively.

[azr]

Finished the coil mod, the output is the same as the battery now, BUT the bike is still backfiring!!!! reset the fuel and air mixture screws best I could with the best idle method and resynched the carbs but she is still backfiring into the carbs. This is driving me nuts! I thought maybe I accidently changed the clips on the main jets but they are all set at #2 as is the stock setting. About the only thing I haven't checked (or rebuilt/replaced) is the stator but I'm not having any charging problems. Atchbo, when you say "needle valve" to which needle valve are you refering to? I did check the floats last time I had it apart and they were all fine (so I thought). I can pull the carbs apart again and check them but I've done that so many times now I'm a bit tired of it. I really have no direction to go in now, it just doesn't make sense to me.

[Steve]

The "needle valves" are the ones that let fuel into the float bowl. What was the measurement that you used to set yours? I believe the VM carbs use a setting of 24 mm. If you looked at the spec for the CV carbs (22.4 mm), and set your carbs to that, that could explain why everything is running rich.

Are you sure that the second groove is the proper setting for the needle clip? I have not taken the time to actually look it up in the manual, but I seem to remember that most, if not all, bikes with adjustable needles used the middle groove (third one) as the stock settting. Also, when you say "second", are you counting from the top or the bottom of the needle? The second from the top will lower the needle, leaning it out, the second from the bottom will raise the clip, richening the mixture.

.

[azr]

the second setting for the clip is the manufactures setting for stock, it's in my manual. It it the second from the top, which is the correct one as far as I know. As for the carbs I did set the floats to 24 (as far as I could tell) but it was the first time I played with the floats. As the bike is running rich and I've done everything else I might aswell pull the carbs apart (again..arrr) and check the floats again. In the manual it talks about measuring just as the needle begins to be touched by the float not when the float is resting on it on its own. I'm wondering if that is true. In order to measure just as the needle is being touched I have to actually hold the float up a bit. If I let gravity work on its own the needle is actually depressed a bit. Is there a way I can fill the float with fuel and actually check the float height to see what the actual true hight will be?

[chef1366]

Sorry the coil mod didn't fix the problem. It did need done though!

[bmac]

Okay try this!!!

If that didn't do it try this.Ape heavy duty valve springs, 83 1100 head with 1.5mm oversized SS intake valves, APE cam chain tensioner, Vance and Hines balanced clutch hub, Star Racing high volume oil pump gears, Kerker oil cooler, Dynojet stage 3 jet kit, 36mm GS1150 carbs, K&N pods, Honda R/R, Progressive rear shocks and fork springs, Dyna S, Dynatek green coils and Kerker 4-1 exhaust. Sorry Bill, just kidding. It was a good stab at it though.I feel the need for rechecking the carb o'haul. bmac

[renobruce]

Set the floats with the float resting naturally. don't hold it up. Measure with the gasket removed, and measure to the highest spot on the float.

Sounds like you may be measuring while raising the float, which would cause you to set the fuel level too high.

[azr]

No worries about the coil mod chef, it was needed, it was easy and another great chance to learn something about the bike and bmac, thanks for the tips but I'm frustrated enough at times already and I'm not sure I'm ready for that 'little' performance increase....yet. I know, I know about rechecking the carbs but I've pulled those things apart so many time now it's really not funny. Although I am going to pull them just so I can recheck the floats. One question though. Throughout the carbs are these steel balls that are set into the carb, all over. What are those for? some of them look awfully seized, if they are supposed to rotate, I don't think so. I do have a bank of carbs from a 79 GS750 I may just put together and just swap my fuel needles over. If I do that do I need to swap all my jetting over? (main jets, air mixture needle, fuel mixture needles, float needles, basically everything).

[renobruce]

The steel balls are just plugs. They shouldn't move. If you swap carbs, you will need to use the same pilots and mains. Everything including the needle is the same, but you would need to be set in the same notch.

Your problem sounds like your gas level is too high. Just re-set your floats and I'll bet you will be good to go.