sileightygx Posted January 8, 2010 Share Posted January 8, 2010 Hey guys, My 71 still has the original SU carbs on it, and ive run into a bit of a interesting issue..which im sure maybe 90% of you know about hehe. So I popped off the air cleaner cover and air filter to take a look because I've never opened it before since I just got the car a little while ago. I noticed when I removed the cover there was gas pretty much everywhere and dripping all over the place. Upon closer inspection (engine running) I noticed it was coming out of two holes, one by each carb opening. On the backside of the cover (facing the carbs/engine) the hole has a rubber fuel line going back to a smaller "module" on each carb. I have looked at a '72 manual but could not seem to find the name of the part, but i believe it to be part of the choke system. Can anyone confirm this or point me in the right direction in the FSM. Yes I am new to carbs and the only other carbs I have had was on my CBR600. Either way one of those holes is leaking massive amounts of gasoline while the other is dry. Any help in indentifying the part/function would be really appreciated. Thanks! Quote Link to comment Share on other sites More sharing options...
at-jefft Posted January 8, 2010 Share Posted January 8, 2010 I believe that module you are talking about is the fuel bowl. And if it is overflowing then it is more than likely it is going out the fuel bowl vent into the air cleaner. (the vent hose is normally connected to the air cleaner when it is in place.) As for reasons why it would be doing this, I'm no expert but I would say that either the needle valve is stuck open or the float is busted/maladjusted. I realize that both these probably don't make any sense to you but I'm sure you can search and figure out what they do and how to adjust/replace them. Quote Link to comment Share on other sites More sharing options...
sileightygx Posted January 8, 2010 Author Share Posted January 8, 2010 you hit it dead on! i was playing with it earlier and with the choke engaged all the way, it is dumping fuel out of the vent hose into the air cleaner only on the carb closest to the dizzy. when put the choke back into normal position it slowly stops coming out of the vent hose. Quote Link to comment Share on other sites More sharing options...
at-jefft Posted January 8, 2010 Share Posted January 8, 2010 Does it stop when the choke is closed? I would expect it to come out of both if they were synchronized properly and the chokes synchronized as well. Quote Link to comment Share on other sites More sharing options...
sileightygx Posted January 8, 2010 Author Share Posted January 8, 2010 yeah it stops completely after the choke is closed. strange enough it only pours out of one of the vent hoses...i literally mean pours out Quote Link to comment Share on other sites More sharing options...
ezzzzzzz Posted January 14, 2010 Share Posted January 14, 2010 One or two things are going on here. First, there is no choke on a SU. You're referring to the fuel enrichment. The nozzle is lowered allowing more fuel to pass the needle at idle than would normally flow. That helps the engine run until it warms up enough to properly vaporize the fuel in the intake system. The two rubber hoses are connected to the vents of the fuel bowls to equalize pressure therein. If the needle/seats become clogged with debri the fuel flow isn't cutoff at the preset fuel bowl level. That fuel continues to rise in the bowl until it flows out the applicable rubber hose and into the air cleaner. Here it sits vaporizing and drawn into the intake via the SU horns. You should be smelling gas to the high heavens! Moving on. The other culprit could be an aftermarket pump applying too much pressure and blowing the needles off the seat in the fuel bowls. The SU fuel bowl top can be removed to check and clean the internals. It has a thin gasket so proceed with care. I'd look into fixing that issue before concerning myself with carb adjustments. I'd also remove all the rubber hoses feeding the fuel rail and SU's and blow the fuel rail out really well with air hose to be sure the return line isn't blocked. You'll never get the sync'ing right with the present problem. Quote Link to comment Share on other sites More sharing options...
Daeron Posted January 15, 2010 Share Posted January 15, 2010 Look up "z therapy" and visit their website. They have excellent breakdowns on how these carburetors work. A Youtube search will also yield many significant results: SU's are very simple little machines, and everybody who has ever played with them either loves them to death or can't stand them. My opinion is that some people simply aren't made for SU carbs. They operate on a few very simple principles, and have only four moving parts (IIRC: throttle blade, needle assembly, vacuum damper, and float bowl assembly) and are extremely versatile. If you get them, they are almost as useful as a Swiss Army Knife. Quote Link to comment Share on other sites More sharing options...
sileightygx Posted January 15, 2010 Author Share Posted January 15, 2010 thank you guys for the very detailed info. the gas smell was my first hint something was wrong. since i had just got the car i havent had time to check everything out yet. hopefully this weekend when i get some time i can pop those caps off and take a look for debris. i also notice that it only leaks out when the fuel enrichment level is pulled more than 3/4 of the way. i'll need to check adjustments for the cable, is it also possible that it is pulling too much? either way i still am going to check for debris and cloggage to remove those possibilities. Quote Link to comment Share on other sites More sharing options...
Quetzalcoatl71Z Posted December 6, 2010 Share Posted December 6, 2010 I am having the same exact problem. Except on my 71, the fuel pours out of BOTH vent lines into the air cleaner assembly. I mean there is gas everywhere. Should I unscrew those 2 needle guys at the top of the bowls to check for debris? Or just take the whole shibang apart and start cleaning? Quote Link to comment Share on other sites More sharing options...
Quetzalcoatl71Z Posted December 6, 2010 Share Posted December 6, 2010 Well I took the float/bowl assembly apart, as well as where the needle valve is in that weird main part. Cleaned everything up and all. didn't really see anything majorly dirty in either places though. I only did thi on the SU nearest the sidewall since more gas was coming from there than the front one. Unfortunately i ripped the gasket on the float/bowl assembly, so I guess I am on my way to order new gaskets for everything. All warmed up, the as seems to have stopped coming out of the vent nozzles completely. Maybe the float was stuck and not letting gas back through the return line? I dont even know if anything besides a physical clog affects the return line, but it all seems ok for now. Going to get a new air filter now that mine is soaked in gas, lol. This thread was very helpful. Quote Link to comment Share on other sites More sharing options...
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