carbs

[78GS550]

i have a set of 750 jets and was wondering if it would be a bad idea to put them in a 550 carb for the bigger jet size and should both the mains and the secondary or pilot fit also? all the advice and help is greatly appreciatted

[earlfor]

Jet size required is dictated by air volume and velocity. Intake velocity between the 550 and 750 should be aproximately the same. However the volume of the 750 is 50% greater. In other words, unless you have pods and a performance exhaust system, it will be running (if it will run) WAY too rich.
Bad idea is the answer.

You need to maintain a fuel air mixture around 13 to 1. If you increase jet sizes, you MUST increase air volume. Many seem to think the way to get more hp is throw in more fuel. It doesnt work that way. All that will do is flood the engine (assuming the mixture was correct in the first place), cause it to run crappy and kill your gas milage. :-)

E.

i have a set of 750 jets and was wondering if it would be a bad idea to put them in a 550 carb for the bigger jet size and should both the mains and the secondary or pilot fit also? all the advice and help is greatly appreciatted

[78GS550]

what if i were to add more air to by putting on pods and my exhaust is already done so putting pods would make it work u think thank you for the help

[KEITH KRAUSE]

what if i were to add more air to by putting on pods and my exhaust is already done so putting pods would make it work u think thank you for the help

As Earl said, you don't just install larger jets without good reason. But now you say you may run pods, and, you already have a 4-1 pipe??
If that's what you want to do, then I'll offer some jetting advice.

[78GS550]

yes i have pods and a 4 into 1 already i just was wondering i have 750 jets which would be cheaper then buying new ones so would it work or no go all ur help is greatly appreciated thank u

[KEITH KRAUSE]

yes i have pods and a 4 into 1 already i just was wondering i have 750 jets which would be cheaper then buying new ones so would it work or no go all ur help is greatly appreciated thank u

Just saw this as I'm leaving for work. Will reply about jetting tonight.

[KEITH KRAUSE]

Sorry to be late if you were waiting.
Actually, you're very close, or maybe even spot on, with wanting to use the 750's main jets. But I believe the 750's used a 100 main and then a 102.5 during the 70's. Not much difference but still not the same when trying to allow for your mods and guessing the jetting right the first time.
You didn't say what brand of pipe you have or the filters. I'll just assume it's a decent pipe and the filters are "Emgo" type/cheaper ones?
I'd probably try a minimum 3 full sizes (full size= 5) for those pods and two full sizes for the pipe. If I'm right, that would be a minimum increase of 25 over your stock 550 mains which are 80's. So 105's or possibly a little larger. If you have 102.5 mains in those 750 carbs, they might work but I'm just giving my honest opinion. As always, it depends on your parts. A quality pipe and K&N's would most likely require even richer jetting. Test at full throttle.
As for your jet needles, use the 550 needles. I'd raise them at least two positions. I believe your stock needles have the e-clip in the middle, 3rd position. I'd lower them to the bottom position on the needles. Replace any spacers in factory order. Hopefully, the stock needles will work. Test at 1/3 throttle position.
I'd try the stock 550 pilot jets, should be 15's. Then initially set the pilot fuel screws underneath to about 1 1/2 turns out from lightly seated. Initially set the side air screws to 1 3/4 turns out.
Bench synch the carbs well.
REMOVE the two floatbowl vent lines and leave the ports open.
Be sure ignition timing and valve clearances are correct.
Start up, warm up fully and fine tune the side air screws usung the highest rpm method. Test the pilot circuit at minimal throttle position, such as cruising around on level/uphill road a few miles after full warm up at about steady 35/40 mph in 4th/5th gear. Chop off. and read.
The bike should be vacuum synched before getting plug reads/getting accurate test results. Don't know if you have access to the tool?
Test all three circuits at the various throttle positions. I'll try to get back for your results.

[KEITH KRAUSE]

Are you still there? Any progress?

[KEITH KRAUSE]

Are you still there? Any progress?

Guess not. I don't know why I bother with these "hit and run" posters.
I take the time and they can't even say thanks.

[Jarrod]

I am new here and you have helped me a bunch just by reading
your posts and you have pointed me in the right direction with
my GS.
I know it can get on your nerves with the hit and run peeps
but as for me thanks for all of your help!!

[earlfor]

At least he didnt come back and tell you that now his bike wont run and its all your fault. LOL

E.

Guess not. I don't know why I bother with these "hit and run" posters.
I take the time and they can't even say thanks.

[KEITH KRAUSE]

At least he didnt come back and tell you that now his bike wont run and its all your fault. LOL

E.

That's true. Once in awhile I get a little irritated about it and I felt like saying something. He could of course be busy and come back later and make me feel stupid.
I'm confident about my jetting suggestions and I know the owner will at least be in the ballpark. If basic plug/performance tests are done right, the jetting will be done without too much trouble. We all know how frustrating it can be, so when someone offers you a chance to do it right, the least you can do is say thanks.

[KEITH KRAUSE]

I am new here and you have helped me a bunch just by reading
your posts and you have pointed me in the right direction with
my GS.
I know it can get on your nerves with the hit and run peeps
but as for me thanks for all of your help!!

Thanks for that. I hope your bike runs great when it's done.

[78GS550]

im srry for not getting back to u earlier im not a hit and run poster just a really busy one ive been working to so i can buy all kinds of fun toys so im srry for that and thank you for the jetting advice and all the adjustments to make i am in the process of putting the carbs on the bike and getting it tweaked for may cause shes gotta beat a harley 1200 at the strip i got money down so hopefully shes running sweet
also i have a question is there a sproket and chain kit i can buy that i can raise a few teeth and make her reallly quick i was thinking raise 2 or 3 teeth in the rear but i was wondering what ur thoughts were
and once again thank you for all the help

[KEITH KRAUSE]

Beat a 1200 Harley? Hmmm. Is the Harley stock? What model/year? I wonder what the factory 1/4 mile (the race will be 1/4 mile I assume) times are for that Harley and what your 550 is? I'm just guessing a stock '78 550 is in the mid 13's??? Seems like the Harley is in that range? I'm only guessing.
I'm guessing you MAY be able to shave off around 3/4 second with a good pipe/pods and correct jetting?
I'm no expert at gearing and I can't truthfully advise on what's the best gearing to get the quickest 1/4 mile time out of your 550. 2 or 3 teeth on the rear sounds like the right idea but I can't say what the exact effect would be. Lots of guys here seem to like to do the math on these gearing questions and I bet they can tell you what to expect if you do this or that to the gearing. But you must get the jetting right first.
Anyone??
We'd all love to see the race results. Maybe a video or something?