GS750 carb on a GS850

[gs850cmc]

Hello,

Have a 1980 GS850G, replaced carb three (Mikuni CV)(broken float tower) with a GS750 CV, I replaced all internals (jets, needles floats etc.) with the 850 parts. Also completely rebuilt all four, dipped, rep all rubber..the works. Yet when I run it, cylinder three is barely firing and pipe is barely warm. I took the rack off again and opened the float bowl on #3 and noticed that the brass tube that extends into the float bowl had a much smaller opening than the original. I observed that the tube has an insert to make the hole smaller, so I removed it now same as the rest, carb reassembled and again cyl three not running right...barely warm. Compression on each cylinder:

1:130
2:118
3:118
4:130

I really appreciate advice on this one, it has me completely stumped

Thanks

[mike_of_bbg]

Hmmmm, questions:

Did you bench sync the carbs before you reinstalled them? Did you vacuum sync the carbs after installing them? Are you running just at idle as the pipe does not warm up? Does #3 warm up if you open it up a little for a minute?

[gs850cmc]

Thanks for the quick reply, I did do a bench sync, and the first time around I also did a vaccum sync with a motion pro gauge. When I took them off this time I re-bench synched them because I suspected that maybe the vaccum sync did not go right, now with the latest results I think that the vaccum sync was fine. Negative on the warm up, the bike currently will not run w/o choke, so this time around I choked it pretty good and ran at high rpm 3500 for about a minute or two. 1, 2 and 4 were hot, though 4 not as hot as 1 and two, three barely warm. Is there some other component or compatibility issue I could be missing on the 750 carb? The other thing that concerns me is that I replaced the needle valves and seats. The new ones did not come with the screens, they installed fine and there appears to be no leaks. in fact with the carbs off an on a level surface (last night) I let my tank run on prime into the carbs, initially some fuel did come out of #3 (vent tube, actually between 3 and 4), but it stopped after I tapped the the carb, I think the float may have stuck for a moment. But then I primed again for some time and definitely no leaks. I laso drained each bowl and observed the same amount of fuel removed from each bowl. Just really confused by this

[bwringer]

Did you reset float height when you installed the new needles and seats?

Also, are the little spring-loaded nubbins in the center of the new needles strong enough to hold the floats up before the springs start to compress? Some aftermarket needles have low-quality weak springs.

This may or may not be related to your original problem, but it's worth asking.

[SqDancerLynn1]

The brass tube is part of the choke circuit.. Try swapping 2&3 plug wires. if three is still not hot recheck the carbs again

[chef1366]

If you spray carb cleaner into the pilot air jet does it come out the pilot jet? If you do the same to the main air jet does it come out the main jet?
Is the pilot jet clear and you can see light through it?
Did you replace the idle mixture screws o-ring? Is it clear in there?

[bwringer]

The passage in the float bowl (I believe it's for the choke circuit) is also very difficult to clean.

Just throwing that out there.

[gs850cmc]

Thanks for all the comments! Pilot jets and main jets are all clear, I meticulously cleaned these parts and replaced the small orings on the mixture screws, also used small wire to make sure that mixture screw passage is clear. I did not reset the float height on any of the carbs, I went with the relative fuel levels in each bowl as my check. Also I tried to use a basic mm ruler to measure the heights---from the gasket seat to the correct area on the float (I read about this on BassCliff's site) but I'm unable to get an accurate measurement. Is it because you can't use a ruler for this measurement? Do I need a caliper? Anyway when I used the ruler I get someting less than 20mm, they couldn't have changed this much from standard. The one other item that I didn't change out on #3 was the diaphram and its needle but I don't think this should be an issue since they looked exactly the same.

[gs850cmc]

Update: Took carbs off...again and rechecked float heights with a digital caliper. This worked quite well, my floats were at about 21mm, reset them to 23.3mm. Bench synched the carbs and reinstalled. This time it started up and I didn't have to leave the choke on but for a minute or two. Vacuum synched the carbs allowing for a little more vacuum on 1 and 4. Buttoned everything up and now it runs much better but it takes a while for pipes on three and four to get hot like 1 and two. When I get it on the road and open up it feels pretty good. But when it comes back down to idle after going 30 to 40 mph, she seems a little rough, I have to feather the throttle to keep it from stalling. I think this is related somehow to my original issue of three and four not getting hot enough on idle...maybe. The other thing is that I'm using this 750 CV (just carb #3) on my 850. As I said earlier I changed out everything on the 750 with my original 850 parts. The one thing that is different and this is a little hard to explain is that along the float bowl mating surface, where the float bowl gasket sits on the carb body, normally for the 850 there is a round rubber gasket that sits in place in a slightly recessed surface, then the bowl cover goes on and covers this up. However on the 750 carb the recessed surface is there but it is not as deep as the 850 carb and it doesn't and won't allow me to use the round (doughnut like) gasket. Now when looking at this from the front of the carb body into the throat where the butterfly is you can see this same cylindrical insert but on the 750 carb this piece appears to be deeper than it does on the 850 carb, which stands to reason given that the gasket won’t seat all the way when looking at this from the bottom of the carb. Well I hope someone out there can follow this, now for my question: what does this arrangement do to the operation of the carburetor and could this be a compatibility issue that is causing my problem.

Thanks,

Charlie
80 GS850G…

[Larry D]

If the bike is completely warmed up and running moderately well and it wants to die at idle or when you blip the throttle, it is running rich on the idle circuit.
Where are you idle mixture screws set ? 2-2 1/2 turns out from lightly seated ?

Search for Highest RPM Method. Doesn't seem to me to be a problem with mis-matched carbs.

[chef1366]

Update: Took carbs off...again and rechecked float heights with a digital caliper. This worked quite well, my floats were at about 21mm, reset them to 23.3mm. Bench synched the carbs and reinstalled. This time it started up and I didn't have to leave the choke on but for a minute or two. Vacuum synched the carbs allowing for a little more vacuum on 1 and 4. Buttoned everything up and now it runs much better but it takes a while for pipes on three and four to get hot like 1 and two. When I get it on the road and open up it feels pretty good. But when it comes back down to idle after going 30 to 40 mph, she seems a little rough, I have to feather the throttle to keep it from stalling. I think this is related somehow to my original issue of three and four not getting hot enough on idle...maybe. The other thing is that I'm using this 750 CV (just carb #3) on my 850. As I said earlier I changed out everything on the 750 with my original 850 parts. The one thing that is different and this is a little hard to explain is that along the float bowl mating surface, where the float bowl gasket sits on the carb body, normally for the 850 there is a round rubber gasket that sits in place in a slightly recessed surface, then the bowl cover goes on and covers this up. However on the 750 carb the recessed surface is there but it is not as deep as the 850 carb and it doesn't and won't allow me to use the round (doughnut like) gasket. Now when looking at this from the front of the carb body into the throat where the butterfly is you can see this same cylindrical insert but on the 750 carb this piece appears to be deeper than it does on the 850 carb, which stands to reason given that the gasket won’t seat all the way when looking at this from the bottom of the carb. Well I hope someone out there can follow this, now for my question: what does this arrangement do to the operation of the carburetor and could this be a compatibility issue that is causing my problem.

Thanks,

Charlie
80 GS850G…

Same carb. That is not the issue.
Take out the idle mixture screws on the colder pipes.

[gs850cmc]

Thanks for the replies! As for the screws, they are sitting at 2.5 turns from lightly seated. I should have mentioned this but when I adjust for highest idle I get a response from #1 a barely detectable change from #2 and nothing from 3 and four (when I turn the screws) and I'm certain that everything is clean. I had them apart and can see through the little holes, new orings on the screws, carbs totally rebuilt.

[gs850cmc]

I ran it again today and from a cold start I have to choke for about two minutes. Then it idles fine, when I blip the throttle it hesitates a little, when I roll the throttle it sound much better. All pipes get pretty hot except for 3. Next time out I will remove the mixture screw from #3 and see what happens

Thanks again for the help

[chef1366]

Did you change the o-rings behind the intake boots?