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Discussion Starter #1
A friend of mine has a '76 900ss (lucky b*stard) and it has run fine since he bought it, but last week it began coughing as thought the vertical cylinder was running lean, he limped it home and gave me a call.

I went to look at it tonight and fiddled with the carbs for about an hour before retiring home to think about the problem. Here goes, when you start the bike, you have to tickle the carbs and it runs fine (what sweet music thrugh the Conti megaphones) for about a minute, then the left tailpipe starts popping and eventually shoots flame before the bike dies. Tickle the carbs and it runs fine for about a minute. I took the carb off and checked the te floats for proper adjustment, all of the jets are clean, needle and seat are functioning properly, accelerator pump works, no air leaks between carb and cylinder head, reset the mixture screws to a baseline that matches the other carb (they weren't more than 1/4 turn different) and reasembled. Fired right u[ and ran beatifully for about a minute. tickled the carb with the engine running and it claered up for awhile and then went ti sh*t again. I rechecked that the floats were adjusted properly and not sticking, but it appears to not be getting any fuel into the bowl while running. The plugs appear to be burning about right, a little dark, but there is no load. BTW it has the 40mm Del'Ortos as factory for an Australian bike.
 

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I would check the fuel pump and fuel lines to verify that there's not something gunking them up. It sounds like there's just enough fuel getting in to keep the bike running, barely. How old are the fule lines? Has the bike been sitting at all? You might also want to check compression on the problem cylinder.

-dj
 

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i know you say jets are fine, but are you sure? all the low speed circuits are clean and blown free of crap.
 

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Any chance he might want to switch to a set of Hitachi carburetors from a Monster. I happen to have a pair available.
 

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I think I'd keep that bike stock myself. And if I would change, i'd use a Mikuni just for parts availability.
 
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Definitely don't change those carbs! That bike is a keeper and is gaining in value everyday. Keep it stock. Since it was running fine and then developed this problem rather suddenly you can rule out jetting - the jetting didn't change. Since it runs when you tickle it you can assume the left carb is running out of gas. This is likely caused by: a) the float is sticking and/or sinking; b) the fload needle is sticking in its seat preventing the float bowl from refilling with fuel; c) the fuel line or fuel petcock is clogged and not refilling that carb with fuel. If I remember correctly, the tickler on that carb simply depresses the float and allows fuel to flow through the float needle and into the float chamber. Since it runs when you do this you can probably rule out the float needle, but I'd pull it and check for a wear line on its surface and probably replace it just as a precaution. Take the float out of the offending carb and set it in a cup of gas to make sure it actually floats. If not, you've found your culprit. Now disconnect the fuel line to that carb and turn on the petcock. Ideally you should check the flow to both carbs and determine if they're equal. One way or another you'll find the problem. Good luck!
 

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Discussion Starter #8
Thanks for all of the help everybody. I'll get a chance to look at it again on Saturday and I'll check the floats, again. Correct me if I'm wrong, but if the floats are sinking, then it would be getting too much fuel in the bowl, right? It is definitely getting fuel flow through the vlave and the lines are clear. I'm thinking that there might not be enough ventilation through the filler cap, but why would that affect ONLY the rear carb?
The bike was fully restored in Australia 18mos ago and then shipped to the USA. All of the hoses are nearly brand new, and when I open the valve with the float bowl off, it getts PLENTY of fuel through the needle (I still smell like gasoline after 2 showers). What really puzzles me is the that this came on almost overnight. Rob rode the bike one weekend, parked it, and on Tuesday it started running like dog feces.

Keep the ideas coming and I'll check back. Charles
 

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When they restored the bike, did they reline the fuel tank? You might want to verify that there's no rust in the tank. Even a tiny little peice of rust can clog the petc0ck, fuel line, or any of the internal ports of the carb.

I ran into this on an old Honda that I was working on. Ran great till it just started acting up. Turned out some tiny peices of rust got into the fuel inlet on the carb. Taking the carb off and using a kigh-pressure solvent spray to clear out the ports might help.

Good luck!

-dj
 
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