Carb Main Jets

[Snow]

Hi all,
I am running what I *believe* to be 80 or 81 special carbs. When I pulled the apart, they had 110 main jets all the way across, which leads me to believe they are from an '81 special as illustrated here:
Parts Fiche 81 SH

The '80 special had 120-125-125-120's as illustrated here:
80 carb parts fiche

Now, I've got my original 79 standard motor back in her (thank goodness for tax refunds, neh?) and it is running quite hot. I have what I believe to be a MAC 4into2 exhaust system and a K&N stock style air filter. So, being the brainiac I am, I decided to put 140 main jets into the carbs last night...well, she ran great but a lil on the cool side while on center stand, but barely ran once I got on the road, she was bogging down badly. I also have 127.5 jets in my toolbox (from playing with my GS750T a few years ago) and she ran ok on the stand on those, but that doesn't really mean anything on the road.

Two questions:
1. How can I be absolutely sure of what carbs I have (pretty sure they are either 80 or 81 special) besides what jets were in her.
2. What main jets should I run?

[Bill__Woods]

The 80 and 81 carbs are the same except for the jetting. The earlier 78 and 79 carbs have screw in Float Valves (needle & seats)., the 80 and 81 carbs have push in float valves.
If you go up in main jet size you may have to change your pilot jets, too keep it from running rich. On the 80-81 carbs with uncovered pilots, the pilots get their fuel from the float bowls. On the 78-79 carbs (with covered) pilot jets get the fuel from the Main jet.
May want to try a set of 120 or 125 main jets.
Let us know the results.

[Snow]

Thanks for the info - I've been thinking about trying anything from 112.5-125 and I think I'll try 122.5 as the local shop has 5 in stock, and I need to pick up a couple parts that came in this morning.

After getting everything back together, I'll have to figure out a place to do the "plug chop".

Another somewhat related-but-not-really question: I have two sets of carbs, the "Special" carbs I have on her now that run great, aside from the jetting problem, and also the original set of '79 standard carbs. If I were to get the standard carbs all cleaned up and rebuilt, which would offer better all-around performance, assuming both sets are perfect? (note - when I say performance, I mean not only just consistent power over the entire RPM range, but also good gas mileage)

[Snow]

Well, she runs a lot better with 122.5's than 140's, still feels a bit rich, but I havent really got on the throttle too heavily as I want to be gentle with this motor for a lil while.

[chopper]

hi snow, 122.5 is the size of jet i would use on your bike if i were running seperate filters so i think its safe to say its rich.too much fuel washes the oil off the bores by the way,if you have seperate filters try them or rejet to suit your filter(should be close to stock) if you use seperate filters raise your needles one notch or shim them with a tiny washer

[Snow]

With all the work I've been doing with the carbs lately, I am half tempted to get pod filters - aren't there bad side effects to running those? Like something with the venturi being messed up?

[chopper]

on the xs i find they work fine,on modern multi valve,variable valve timing,pressurised airbox style bikes they are a bitch to set up,but i find xs's etc real easy to set up. apart from what i said in my last post i recommend you set your pilots with a colourtune or a gas analyser

[TopCatGr58]

Hey there Snow,
I'm running an 81 Special, with 4-1 pipes and Indy filters, and I'm only running 117.5's with nice tan color on plugs, good response thru entire rpm range. I started with stock all 110's also. So I would think that your 127's are quite rich. The indy filters work great, especially on the 81 carbs cause there aren't any vent tubes to worry about. My bike also has the 1179 big bore kit in it, which gives it more compression than true stock, so that may be another reason why I can run a modest main jet size, and I haven't changed my pilot or Air jets. Good luck tuning!

[Snow]

Ok, just ordered 2 sets from the local shop , I'm getting 115's and 112.5's - should be in by Thursday. I'm going to try the 115s first. I wish I had all sorts of various tools to check scientifically, but seeing how she "feels" and plug chops after she breaks in will have to do as all of my tax refund is blown now...but at least I have a motor that has all 5 gears and a fully functioning starter gear! A nice little side effect is that she is also 1196cc now

I'll see if I can get the pix from the rebuild uploaded at my parent's house when I go out there next - it was quite the...learning..experience, I learned a bunch of new words from my brother and I created some of my own