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GS 750 Jet question

  • Thread starter Thread starter polyestermojo
  • Start date Start date
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polyestermojo

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Hey everyone. I just scored a 78 GS 750 on craigslist. The previous owner was gonna make a chopper or something. so I decided I would make her into a cafe racer, since he didn't bob it out just cut the frame loop. Anyway, I got it cheap with a buncha parts. Namely, 4 pod filters and a 4 into 1 exhaust with a vance and hines muffler. My question is I know I'm gonna have to rejet the carbs, it also came with a ton of jets btw, but what size should I go to? I know its gonna be getting a ton more air now, so any help would be greatly appriciated. I'm having trouble getting it to fire up, I know it has spark, and compression. I've tried spraying starting fluid in the carbs and still nothing, so if you have any ideas on that too once again, greatly appriciated.
Thanks Guys.
Mike
 
Assume the carbs need rebuilt for starters. There's an excellent tutorial on this on BassCliffs website (wholl be along shortly to welcome you)
Jetting is viewed by some as black magic, and while it takes some experimentation and some decent knowledge on how they work, it's really not that tough but you need to be thorough, and start from a known good baseline. That will involve tearing the carbs down, soaking the bodies, jets and float bowls in dip for 24 hours, and replacing all of the Orings inside, as well as behind the intake boots (the things the carbs mount to on the engine side)

This MUST be done or you'll just be wasting your time. A valve clearance adjustment is also a really good idea, as it will have some bearing on how your engine breathes, which will effect jetting set up.

To your pod filters, if they're those silver mesh screened EMGO pieces of junk, just throw them in the dustbin cause they're nothing more than garbage. They don't filter, and they choke down on the inside and don't flow very well. I'd suggest spending a little dough on some real K&N filters. Spend a little more now for filters you'll never have to replace. Those Emgo ones are simply junk and I can't stress that enough.

If you don't have one, download a service manual for your bike. I've brought more than a couple of these 750s back from the dead, and they're easy to work on (as are all GSes) but you'll want to go through and service all of those things that have been neglected throughout the years.

Also, Several members have listed the things that you'll want to attend to, and most of it is necessary for both good running, and safety issues. Please don't ignore it.

As to your jetting set up:
With GOOD pod filters, and a Vance and Hines exhaust, youll probably need to be in the 117.5-122.5 range on the main jets, with the needle clip moved to the second clip up from the point. The pilot jets should probably be ok at the stock #15.
These are ball park estimations cause no two bikes are alike and some trial and error, plug reads and tuning will be required. But remember to start from a level playing field! That means all the basic maintenance needs to be addressed which includes clean, rebuilt carbs, sealed intake boots, a valve adjustment and checking of the timing, as well as assuring the charging system is good (a known weak point on the GS).

Good luck!
 
It's unlikely that "its gonna be getting a ton more air now". The air flow through the engine will not vary to a great degree unless it has been rejetted to match air filters which are very restrictive and you replace those with decent ones. Go with the recommendation regarding better air filters and start close to stock jetting as TheCafeKid has recommended.

A "ton more air" would require higher intake pressure which isn't there.



polyestermojo said:
Hey everyone. I just scored a 78 GS 750 on craigslist. The previous owner was gonna make a chopper or something. so I decided I would make her into a cafe racer, since he didn't bob it out just cut the frame loop. Anyway, I got it cheap with a buncha parts. Namely, 4 pod filters and a 4 into 1 exhaust with a vance and hines muffler. My question is I know I'm gonna have to rejet the carbs, it also came with a ton of jets btw, but what size should I go to? I know its gonna be getting a ton more air now, so any help would be greatly appriciated. I'm having trouble getting it to fire up, I know it has spark, and compression. I've tried spraying starting fluid in the carbs and still nothing, so if you have any ideas on that too once again, greatly appriciated.
Thanks Guys.
Mike
Click to expand...
 
Thanks, yeah. The pods aren't K&N I'm guessing they are EMGO, so duely noted lol. I need to check the plugs and see if they are fouled, I know he had the firing order wrong when I got it 2 days ago, I just haven't had time to mess with it with work and all. He had said he had like 150's in there which seemed rediculously large to me, but I'm new to bikes so eh. but anyways, if they were that big and your recommending something in the 125ish range that couldda fouled the plugs right? which may explain my no firing issue. I will get a manual for the bike, and I figured when I rejetted I would go ahead and rebuild the carbs because honestly the guy I got it from was high as a kite when I picked it up, and the paint scheme he started makes me think that was the usual lmao. I'll have to post pics. well, thanks for your advice though and I'll hopefully get started working on it tuesday afternoon. *crosses fingers* But yeah. this thing has been neglected a bit atleast. I think part of the chain is seized up cause it will go back or forth about 5 or 6 feet and then the rear wheel locks up and the chain goes taut. so thats on my replace list too.
 
Yikes! That doesn't sound like a chain, that sounds like something internal. I'd get a hold of that manual asap and start going through that bike. Hopefully you didn't get screwed on a bike with some severe internal damage. Fingers crossed for you, but until you figure out what's binding up, I wouldn't even attempt to start that bike anymore.
 
cafekid, Yeah, I don't know what the deal is. it moved fine when I got it. but I went to push it inside and it started doing that. I hope there isn't anything too serious wrong there either, because, well that would suck. only reason I guessed chain was a buddy of mine at work said one of his bikes did that when the chain messed up and it couldn't get around the sprocket so it just locked.
 
polyestermojo said:
cafekid, Yeah, I don't know what the deal is. it moved fine when I got it. but I went to push it inside and it started doing that. I hope there isn't anything too serious wrong there either, because, well that would suck. only reason I guessed chain was a buddy of mine at work said one of his bikes did that when the chain messed up and it couldn't get around the sprocket so it just locked.
Click to expand...

Well maybe you snicked it into gear by mistake? You'll figure it out but flying blind isn't something ya wanna do. The manual will answer a lot of questions.
 
I agree, flying blind isn't too bright. Also, haven't tried to start it since the lockup, I figured even if it is just the chain hung up if it trys to go thats gonna tear stuff up in the tranny and motor. I'm gonna look for a manual tonight, this is my first streetbike so, I'm a total newbie but I've worked on a bunch of cars, mowers, go karts etc. so I like to hope I can figure some of this out with a little help here and there. thanks a bunch for your advice.
 
If he told you he had 150 jets in the carbs, you might want to look to see if he may have done a DynoJet kit modification. The DynoJet main jet numbers are way different than stock Mikuni jets.

My 850 has 155 mains, but they are DynoJet, so are actually the smallest of the stage III jets for my bike.

When you pull the carbs, look for some tell tale signs of the kit. the biggest indicater would be if it is a made for American market bike, the needle (sticking out the bottom of the slide) will not be adjustable. (take the slide apart and look at the needle, if it has a series of clip grooves, it may well be a DynoJet needle. If so, 150 is not out of line, since it would be a DynoJet main jet.
 
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