'79 GS 750 carb help

[Anonymous]

I have a 79 gs 750 that i cleaned out the carbs on and i can't get the bike to start. if i spray starting fluid into them then i can get the engine to fire but it dies right away. The tank is set to prime and the fuel line is not clogged or anything. I took the fuel line off at where it connects to the carbs and shot some compressed air in there and that made fuel shoot out of 3 of the 4 carbs. i think something must be blocking the one that didn't shoot out any gas, but i'm not sure and i would still think that the bike would run, just be really rough. i also tried adjusting the idle air screws on the sides of the carbs. i tried them in a variety of different settings. it seemed to work best when the screw was almost all the way out. if anyone has any ideas i'd love to hear them.

p.s. i'm new hear so if there is a similar post somewhere else please direct me to it. thanks.

[earlfor]

Re: '79 GS 750 carb help

Did you just get the bike? Was it running when you got it? Have you done anything else to the bike besides cleaning the carbs?...anything??
Exactly what did you do to clean the carbs?
Does it have the stock aircleaner box and filter element inside?
Does it have the stock pipes (4 into 2) or an aftermarket 4 into 1 pipe.
Have you put new spark plugs in it. If the old plugs are still in it, what color are the center ceramics on the plugs ?

Earl

I have a 79 gs 750 that i cleaned out the carbs on and i can't get the bike to start. if i spray starting fluid into them then i can get the engine to fire but it dies right away. The tank is set to prime and the fuel line is not clogged or anything. I took the fuel line off at where it connects to the carbs and shot some compressed air in there and that made fuel shoot out of 3 of the 4 carbs. i think something must be blocking the one that didn't shoot out any gas, but i'm not sure and i would still think that the bike would run, just be really rough. i also tried adjusting the idle air screws on the sides of the carbs. i tried them in a variety of different settings. it seemed to work best when the screw was almost all the way out. if anyone has any ideas i'd love to hear them.

p.s. i'm new hear so if there is a similar post somewhere else please direct me to it. thanks.

[Anonymous]

I just got the bike a week ago. no it didn't run. it sat for a few years. it has the stock airbox and cleaner although they weren't on while i was trying to get it to start. i put new plugs on it. the old ones were black. it has a 4 into 1 exhaust pipe. to clean the carbs i used a followed the manual and took everything apart and soaked the pieces in a parts cleaner. the plugs and carb cleaning are all that i did. well i also put a battery in it.

[earlfor]

OK Matt, sitting for a "few years" means we will have to check everything as the carbs are only part of the whole picture and we have no idea of the condition of any of the other components. Jumping around helter skelter just doesnt work out very well. :-)

When you had the carbs apart, did you by chance look on the bottom of the jets to see what sizes are installed in it. Stock size mains should say 102.5 but if the previous owner rejetted for a 4 into 1, they will likely be something nearer 116.

You will have to reinstall the airbox and its filter (at least for now) to give us a base line to work from. A stock system will not run without the airbox and filter.

Under the round cover on the right end of the crankshaft you will find the points. Remove the cover and look to see if it is still a points ignition system, or if its been upgraded to a Dnya S electronic system.

What is your battery voltage at the battery terminals?

Do you have a multimeter or a voltmeter?

Earl

I just got the bike a week ago. no it didn't run. it sat for a few years. it has the stock airbox and cleaner although they weren't on while i was trying to get it to start. i put new plugs on it. the old ones were black. it has a 4 into 1 exhaust pipe. to clean the carbs i used a followed the manual and took everything apart and soaked the pieces in a parts cleaner. the plugs and carb cleaning are all that i did. well i also put a battery in it.

[Anonymous]

I did not check what size jets they were. I don't have the bike here with me (it is at a friends house). the battery is brand new so the voltage should be around 12 volts. i do have a multimeter. i'll have to check on the ignition. why does the airbox have to be on it? why am i checking all of these things? i don't see what you are getting at. the bike is in really good condition for being as old as it is. and everything seems to work on it. I don't believe that the previous owner messed with anything on it, but i'm not entirely sure.

[robinjo]

If you don't have the stock airbox on most GS's run like crap or they don't even start at all if they are not rejetted for it. Voltage is important, if the coils don't get enough it's hard to start, especially with the stock points ignition.

By the way if you do a search for "carbs" or "GS750" you will find lots of info, also check the "old Q & A" section on the homepage.

[Anonymous]

Sounds like the air box.

If you were able to spray starter fluid into the carbs the air box must have been off. Before you try again make sure the petcock is good, and the O-rings between the head and intake boots are new. Pre-balance the carbs, and put everything back on including a clean or new air filter. It should crank right up and run good till you do a final balance of the carbs.

[skip]

I did not check what size jets they were. I don't have the bike here with me (it is at a friends house). the battery is brand new so the voltage should be around 12 volts. i do have a multimeter. i'll have to check on the ignition. why does the airbox have to be on it? why am i checking all of these things? i don't see what you are getting at. the bike is in really good condition for being as old as it is. and everything seems to work on it. I don't believe that the previous owner messed with anything on it, but i'm not entirely sure.

Matt I know it can be frustrating when you have a bike and you know its gonna run but you want it to run now! but you really have two choices, let the experts here help you, answer all their questions and take your time. This will cost you nothing. I have been following this tech forum for months now and have seen dozens of bikes like yours fixed and back on the road after sitting for years with the help fom the experts. OR send it to a mechanic and spend $50 dollars an hour get a bill for $500 or $600 and be on your merry way. Remember the people on this site have all done what you are attempting to do now and are very eager to help you. Ill bet you have the bike running in less than a week.

[Anonymous]

Sounds like if it fires up with starting fluid then it's not getting any or enough fuel. Check your petcock and vacuum lines from the carb (#3) to the petcock. I would fill the carb bowls with the petcock on prime for 15-20 seconds, switch it to RESERVE ot ON afterwards, then try cranking her up. I have worked on my 78 GS750 MANY times without the air filter on. I would NOT drive on the road without the air filter (i learned the hard way- too lean a mixture), but in the garage should be fine. If you have not yet cleaned the carbs out by hand, I would. Our VM26's are super easy to work on, and it gives you a chance to check out what numbers you have for the various jets. I found lots of little critters in all of mine when I opened up the bowls. If you don't have a manual yet, I definately recommend getting one before doing so (Clymer or factory service manual). It will walk you through doing just about everything you need. If it's missing something or blurry in some areas then you always have this EXCELLENT site to turn to. Good luck!!

P.S. My GS was a nightmare when I 1st worked on her. Now it's the only bike I would not get rid of. Hang in there!!

[Anonymous]

three bits of advice

Hey Matt:

Welcome to the club! I have a 79 GS750L that had been sitting for "a few years" when I got it, and the previous owner told me that "it ran when I parked it". I have three bits of advice:

1) A lot of the passageways in the jets are small, and sometimes soaking alone doesn't clear them. Did you blow out the jets with compressed air after soaking? If you are having starting trouble, perhaps one or more of your primary jets is still plugged. I know that some guys recommend using miniature drill bits to clean out the jets. I've never tried this, as I've always heard that it is too easy to oversize the soft brass, but some guys swear by this. One problem with this approach is that you would need to know the size of the jet, so you can get the proper drill bit. Then, simply push the bit through the hole in the jet while rotating it with your fingers.

2) Also, it may be that the holes in the choke tubes are gummed up. The choke tube is a brass tube that is pressed into the carburetor body with a series of small holes drilled into its sides, and I believe it has one hole on the bottom. If some or all of these holes are plugged, you won't get the extra gas you need for startup and warmup. With the carburetor bowls removed, pull out the choke plungers and squirt areosol carb cleaner down into the plunger holes. Observe how (if) the carb cleaner sprays out of the holes in the choke tubes.

3) Listen to Earl. He knows a lot about this bike, and has helped me through some carburetor issues. The more information you know and can post on this forum, the more likely someone will be able to help you.

Best of luck!

[earlfor]

Funny thing about mechanical things Matt. If you dont know for a fact that it is working, then it isnt.

Earl

i don't see what you are getting at. the bike is in really good condition for being as old as it is. and everything seems to work on it. I don't believe that the previous owner messed with anything on it, but i'm not entirely sure.

[TheNose]

Matt, did you pull the plugs on the bowls to see if you had gas in them? I had a problem like this once and it just wasn't getting gas. What I did to fix it was to shorten the fuel line from the tank to the carbs so it didn't go "uphill" and prevent fuel flow. I put a little pressure on the gas tank using compressed air in the fuel fill hole, using a rag to hold the air in. This caused the gas to start flowing and I haven't had a problem since. I may have had stuck floats in the carbs. If you should do this, be very careful and just use a little pressure. You can use the rag as a gasket to hold the pressure in, but let air escape so you don't over-pressure the tank.

Terry

[Anonymous]

I really do appreciate all of your guys' help. like i said if i spray starting fluid in it then it will run until it burns all of the starting fluid up. so it seems like it's not getting enough gas. i did make it so that the fuel line runs downhill. i think that there is probably some stuff stuck in the jets or something. i didn't spray them out with compressed air when i was cleaning the carbs. i'm going to try to work on it tomorrow and see if i can get it to work. btw how do the idle air screws work? (the ones on the side of the carbs) if i screw them in does it make it run richer or leaner? thanks.

[Anonymous]

the idle air/mixture screws on the side of the carb go clockwise to richin...as far as main jets go, i get different info on what sizes are stock, one thing i read says 102.7 is the stocker then elsewear i read its 100`s...im running 100`s with a 4n1 pipe and k&n in stock airbox with a xtra hole cut into it, plugs are on the white side too...anyone have some main jets that are larger?

[Anonymous]

Jet Sizes

My 1979 GS750 came stock with 102.5's, but I did a lot of trial and error adjusting. When I whacked the throttle wide open with the stock jets (stock airbox and MAC 4-into-2 pipes), the plugs would carbon foul, and it would lose power. I replaced the jets with 100's, and she pulls hard at full throttle.