carb question

[Eddie]

I have ordered new main jets and pilot jets for my 74 gt750, it was running slightly lean i beleive, I have not got the parts in so I raised the needle and put colder plugs in. The question I had is that I usually run at 1 quarter to half throttle, so I was thinking I probly need a slightly bigger pilot jet, (which i ordered) but also I was readin my clymer book and I just noticed that it says when adjusting the pilot screw it should be 1 quarter turns out after seated, then it says at alttitudes high than 3,000 ft you should turn it out an extra turn? I have had mine at 1 quarter so now I guess I should turn it out to a half? my elevation is 3,800 ft, but this is the only book that says to turn it out an extra turn, what do you guys run yours at , at high altitudes? Im asking because I thought at high elevations the bike should run rich? and backing this out would make my bike leaner and it already is lean....

[GT750Battleship]

I have ordered new main jets and pilot jets for my 74 gt750, it was running slightly lean i beleive, I have not got the parts in so I raised the needle and put colder plugs in. The question I had is that I usually run at 1 quarter to half throttle, so I was thinking I probly need a slightly bigger pilot jet, (which i ordered) but also I was readin my clymer book and I just noticed that it says when adjusting the pilot screw it should be 1 quarter turns out after seated, then it says at alttitudes high than 3,000 ft you should turn it out an extra turn? I have had mine at 1 quarter so now I guess I should turn it out to a half? my elevation is 3,800 ft, but this is the only book that says to turn it out an extra turn, what do you guys run yours at , at high altitudes? Im asking because I thought at high elevations the bike should run rich? and backing this out would make my bike leaner and it already is lean....

Eddie,it's the other way round mate! On these carbs "OUT" is RICHER, IN is LEANER,I think you need to throw that Clymer Manual in the bin! The vacuum carbs are 3/4 quarters of a turn out,or around there,wouldn't worry with the altitude bit!! Please have a look at the "Pink Possum" Web Site,tells you everything about the Carbruetors.....These bikes are not as straight forward as some people would have you believe! I've had three in 30+ years & I'm still having to keep on top of the servicing to get the best out of the "old girl" If you want to ride them everyday without any problems you have to know how they tick & their idiosyncrasies!! I think far to many people get old motorcycles & do a quick face lift,paint,chrome etc,& expect the machine to ride & run like new.These machines are now well into their 30 + years!! Suzuki never intended them to still be around today!! It's only by dedication & proper restoration & tender loving care by their owners,that these bikes are still around in reasonable numbers, Suzuki built around 68,000 GT750's over the six different models,which was a very small production run!! By comparison to Honda's CB750 Four....something around 680,000 bikes! a long production run & god knows how many models....never fancied one myself !!!!
Cheers,
GT750Battleship.

[Eddie]

Thank you so much!!!! I have three different books and all have different recomendations on the carbs! all say something different, but the clymer takes the cake! It says at elevations over 3,oooft to turn screws one extra turn out, the book calls for 1/4 turn in all my books (even the suzuki carb book) except the clymer has that elevation exception. I was not sure on the pilot screw setting (in for lean or out for lean) because of the book, and you would think at higher elevation the bike would be rich, so when the book says to turn them out an extra turn it sounds like its saying out is lean? I have seen so many contradictions in the 3 books I have its not even funny. I have a 1963 superhawk, I restored it about 17 years ago, I love that bike , I have been inside of the motor and tranny several times just to check stuff, and replace stuff that I was not able to in the past (before internet) I was unable to locate some parts, but now I can, the only problem I have had is the head has a leak now, which it never had before, anyway I love the gt750 but this is my 1st one, so far i rebuilt the forks, put tappered bearings in , rebuilt carbs, had to recheck the float level though, replaced the water pump, put new swing arm bearings in, i rebuilt the oil pump and cleaned the lines,the list goes on and on, but
but thank god I have not had to tear the motor apart! (yet) I hope the crank seals last and I can just enjoy riding it for now.. but since i just said that it will probly take a dump on me now hope not, thank you for your help, I got so confused because of the different manuals I have!!!

[Eddie]

my phone takes crappy pics but these are some of the manuals i have, I also have 2 more that are put away so I didnt get the pics of those, not sure if you can read the pic, but thats the clymer book that says at 3,ft turn screws out an extra turn, which makes it sound like out is lean , I have found little things like that in the whole book!!!! the idle mixture screw or pilot screw setting for my gt750 is 1/4 turns per the book mine is a 74 model, i dont think you can see that in the pic?

[tz375]

That Clymer reference is correct, but only for 72/73 where the mixture screw is an air screw. On those OUT = more air = leaner.

The way I approach a new carb is if the mixture screw is on the filter side, it changes air supply, so OUT=Lean and if the screw is on the engine side of the main body it changes fuel, so OUT=Richer

At higher altitude, the air is thinner ie less oxygen and therefore it needs less fuel, but it takes more throttle to make the same power, so it runs harder. Bottom line is that every bike is different and if yours runs better with the mixture screws 1/4 turn in from stock, then do it. If it runs best at stock or the other way, that's OK too. See what your bike responds too and a little rich is wasteful, but safe. Too little fuel is potentially more of a problem.

[Eddie]

Thanks for the help! ya the clymer book is talking about the early style carbs, but then when you read the bs40 part (later model carbs) it says to reference this table which says this altitude thing, and apparently this goes for both early and later carbs the way the book is written, which is really my complaint. I have my screws out 1/4 from seat, which is what the books says, I warmed the bike up and messed with the screws to see where it runs better and the further turns out it drops the idle. I cant really tell where it runs better , either 1/2 turns out or 1/4 turns out but my bike is kinda lean right now so maybe 1/2 turns out will be ok. I got confused at 1st because it was lean and usually stock jetted bikes here are rich, but I forgot about the pods I have on it and as far as the needle and the mains im not sure which way to go yet, I need to ride it more at different throttle postions and check the plugs, if you read the clymer book , when you get to the bs40 part it says to reference table 4 which is for the airscrew setting, so tabke for is for both stlye carbs according to the book unless im getting senile which could actually be the case

[tz375]

That reference is not correct. Do you have a copy of the Carburetor Manual from Suzuki? It's better but not great and of course includes the bizarre adjustment technique. Haynes isn't too bad but not as clear as it might be.