1986 gs750es backfire really sluggish past 2nd gear?

[magillamike]

i just got a 1986 gs750es,it has problems starting and oonce you get it going it backfires and well under load i.e in gear first and second gear seem responsive but when going up any further in gears it will not rev past 5000 rpm or so,is it possible the vacuum hoses are misplaced or vacuum in tank petcock could affect this?the guy i got it from had carbs off and cleaned but never could seem to get running right with out the bog ,it seems like even in lower gears doesnt want to rev e out like it should,almost like its not gwetting enough fuel,any ideas would be appreciated ,and if anyone has any idea why i can not find a new petcock or rebuild for this year of bikes petcock or what may fit from earlier or other model gs bikes may work

[BassCliff]

Greetings and Salutations!!

Hi Mr. magillamike,

Try Z1 Enterprises for a petcock. If not, an OEM should still be available from most online Suzuki vendors. There are lots of vendor links in your "mega-welcome". These bikes need a lot of TLC up front. Many of them have been abused and neglected for quite some time. In your "mega-welcome" are a couple of maintenance lists that will guide you to the path of GS nirvana, if you take no shortcuts. If you stray from the path you will be cursed with hard starts, stuttering performance, dead batteries, and much wailing and gnashing of teeth. You will be left stranded on the side of the path instead of riding briskly down the path. Let's get started.

I just stopped by to welcome you to the forum in my own, special way.

If there's anything you'd like to know about the Suzuki GS model bikes, and most others actually, you've come to the right place. There's a lot of knowledge and experience here in the community. Come on in and let me say "HOoooowwwDY!"....

Here is your very own magical, mystical, mythical, mind-expanding "mega-welcome". Please take notice of the "Top 10 Common Issues", the Carb Rebuild Series, and the Stator Papers. Now let me roll out the welcome mat for you...

Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike!

Thanks for joining us. Keep us informed.

Thank you for your indulgence,

BassCliff

[chef1366]

Are you running the stock airbox?

[magillamike]

yes stock airbox ,but seems to be sealed ,will usually start if i cover airbox hole and create suction almost like a extra choke ,

[chef1366]

I would check the compression.

[banditom]

check list

Hi,

This sounds as if it may be a fuel feed type problem, as in too little being able to be fed to the engine. Start with the easiest thing first.

After checking the compression and the workability of the petcock or replacement of petcock as advised above, check your fuel cap for venting, check fuel lines for kinking, check any filter in tank, inside carb inlet or in the fuel lines for plugging, and then check carbs for proper float level. Start with the easiest first, if none of these proves to be the culprit, check your pipes for temp from cold startup to see if one or two dead or cooler running cyls is a problem, all pipes should warmup similarly. Be sure to let us all know what it was when you find out please.

Feedback always helps keep the helping ones stay enthused about helping others here. If none of this helps, jump back on and say so, there are more ideas, not just from me, but from others I am sure.

[banditom]

more yet

Hi Mike,

Regarding the petcock, if the petcock will flow fully, and that is the big question here, you can install an inline petcock to be sure youre able to shut off fuel. My local napa has cheap plastic ones for about six bux that can be put in line along with a five dollar inline filter. I cant be sure that this would work for fuel flow for a setup for a single line type 750, but mine works ok on a double line yammi eleven, or maybe, not, since it balks at about seven thousand, hahaa, never ride there anyways.

You also may want to change your fuel line, if its really old and stiff, Ive seen it be kind of odd shaped from being in a different position during another previous installation. I had really good luck with tygon see thru line, although see thru is kinda ugly as installed, you can see the fuel running thru it, which helps when the need to trouble shoot comes up. I got my tygon from an ole lawn mower shop for about a buck a foot.

[magillamike]

ive got good compression,i noticed the right side middle pipe did not get as hot as fast,and its same side as backfiring so im trhinking carb is dirty there, but as for testing petcock how can you tell if the vacuum is working?

[magillamike]

does anyone no if other older gs750 bike carbs weould work as i can not seem to find any parts for anything older than a 1983 gs750es almost like the 86 gs750es did not exsist,heres a picture of my last 78 gs750 i did up for fun

[banditom]

vacuum gauge

Hi Mike,

Have you torn apart and cleaned your petcock yet? If not, those screens need it prolly. Until you can prove that gas is flowing out of the fuel line, in suffficient quantity, I wouldnt spend much more time with anything else, at least that is more complicated.

A vacuum gauge will tell you for sure if you have vacuum, a vacuum pump will give you external vacuum that should allow gas to flow if all else is well with the petcock n all. Holding a finger over the end of the vacuum line that goes on the petcock,with the engine running , should allow you to feel a bit of suction on that finger tip indicating that there is some vacuum present. At the very least try that, also take the petcock apart and be sure to clean every little bit, check a manual to be sure its reassembled properly. Who knows what po may have done with that too. Also, re the carbs, I think that if one of them is bad, the others must be questionable. Let it cool off, and double check your exhaust heat again to be sure its number three. I am assuming that youve checked your plugs to be sure they are firing. It may be best to change the plugs since in missing they can also form invisible tracks that wastes the spark under compression. Definitely change number three, and check the new one to make sure it at least fires when out of the bike.

Hopefully, in doing all of the above suggested work, do one thing at a time, and see if it helps. There is no fast way to troubleshooting these different things. The basic steps have to be taken or you will be frustrated more. I wouldnt mess with different carbs until you are sure of whats wrong with your own ones that you have. There isnt much point in doing one thing, and then coming back to the list, unless you ve done all of those one things, that entire above list would take me more than one full day to do, but I work slowslowslowwwwww.

[salty_monk]

Have you changed the O rings behind the carb boots? That's your first port of call. This is a big thing that most people never do. They look good but probably aren't!

Make sure the airbox is well sealed.

Also pull the carbs off, clean & dip them, change the O rings, check float levels etc.

Get O rings here:

www.cycleorings.com

Dan

[tlepac]

Coming from one who has already done it--don't muss with the old petcock. Just order a new one from Z1 and be done with it. Then the next thing I would do is to make 100% sure that the airbox is sealed. The fact that holding your hand over the intake on yours helps tells me that it is not. See old threads or BassCliff site for the proper way to seal the airbox. It helped me a lot! Then next in line is to see if all of the carbs and getting gas to their respective cylinders. If not then you need to dip the carbs---again see BassCliff site---Then, assuming you have recently checked your valve shims and know them to be in spec, then you can get to the carb synching and fine tuning

Doing any of these out of the proper order only gets you frustrated and you will have to start all over.

Good luck