'79 Special Carb Issues

[Shuriken]

The mains in this model are 137s. I have 115s and 120s in my 80g carbs.

After a good carb clean, the '79 carbs will not work on the '79 bike - runs terrible but idles good.

Swapping the 80g carbs onto the bike gets it so it will run with fair power but poor idle.

I would like to use the original carbs with the bike because they have adjustable needles.

If #4 cyl does not fire when this bank of carbs are installed, should I check the main jet passages for blockage?

Anybody have a float guage for the '79 special carbs?

I like the special because it steers better and is lower to the ground and it seems the seat is closer to the bars. Comfy. Just can't get the carbs to dial in yet.

Ben

[Bob Falter]

Do a search on this web site. There is a lot of carb info here to help you out. You will find that a lot of people had to clean the carbs three times before they worked properly. I would work with the 79 carbs. Don’t know what you mean by runs terrible but would assume that one, or more, of the carbs still has the mid range, or main jets plugged since the 80 carbs work better. Since the 80 carbs only run fair you may have other problems.

I don’t know if the is such a thing as a float gauge? I use a dial caliper but a small ruler and a good eye will work about as well. The float height is not critical to getting it to run reasonably well. My float heights varied all over the place when I first pulled them apart but the bike ran fine.

[Jdwags]

There is a gauge.. i can't remember who it was that did some up, i have one and its a very useful tool..

it was only like $6 and he had them stamped out ...


anyone else that can remember.. it was from the old list i know.

[Shuriken]

I have one of those guages but it's for my 80g carbs - 23mm, not 27.

I would love to have one for the 79sf.

Ben

[Jdwags]

I know he has the 79sf ones. I have one of those

Steve is the name that rings a bell..

From Texas.

Jeff

[Shuriken]

I've been through carb hell many times. No stranger, here. What I don't want to do is soak the entire carb bodies completely. On my first set of carbs I did that and it ruined the choke circuit seals. In other words, years ago when I got started in all this, I did not completely dissasemble them.

To be more descriptive of the problems with the 79sf carbs:
I can get the bike to idle just fine. But pipe #4 is cold. When I get on the gas and get it into the midrange to full, it pops to life. In addition, I think the circuits are not smooth or overlapping properly. Each circuit is not quite tuned and it feels rough in midrange, too. I dropped the jet needle one position and the bike won't even come off of idle into midrange without choke. That's weird to me, since elev. is 5700 and I'm using the stock jets. I had guessed the needle would have to go even leaner than that....

According to carb tuning theory, I should select the proper main jet first. I'm running 115s and 120s in my '80g and it's still rich. Anybody have ideas what kind of jetting I should try for 5500-6500ft elevation for this year/model?

To complicate things, exhaust has mufflers that don't sound like they have any baffles (OEM were replaced) and I am using stock airbox/filter.

I would honestly rather work from the lean condition to a more rich condition, since everything I have runs rich at this elevation and never seems to dial in. Not severely lean - just lean.

I want to do the bowl fuel level checks but don't know how to get a tube onto the float bowls - brass fitting/nipple somewhere? Seems I once read someone drilled out a bolt or something....

Anyway, the 79sf has richer mains that the later models - which I think I remember was because the emulsion tube or other passages were smaller or something.

So I should try 135s or 132s or something?

Risky getting the proper main jet size at WOT on an unregistered bike. Noisy, fast, and illegal.....

Ben

[Wes Thompson]

Jetting

Try 110's and you may have to go down to 100's. I live a few hundred feet above sea level and I am running 112.5's in a 79 Special.
Wes Thompson

[Shuriken]

I will first try 130s in it. If that doesn't work I have a set of 115s and maybe 110s.

Wes, did you also adjust the jet needle position?-

Ben-

[steelcat]

I'm running the stock 137.5 jets on my 79f. Seems to run preety good. Good color at idle & in town. Leans out a bit on the highway. Elevation here is 5000ft in the valley. I set my float height to the recommended. measured to the float itself not to flange. Next time I'm in the carbs I'll lower the float height 0.25 to 0.5 mm which will richen the carbs full range. I use a small digital caliber for float height setting.

[Czekus21]

Dial Calipers

I use a dial caliper that I also use for reloading ammo. ( mine is made by RCBS ) You can measure inside, outside diameters as well as depth with accuracy. of course I've got to do the conversion from standard to metric. I do think though that they make metric dial calipers. They can be a little spendy though. Depends on how high quality you want to go

[TopCatGr58]

Hey there Ben,

Yeah, your 132.5's should be a good start, that's 2 sizes leaner from Stock. Ben, with #4 being cold at idle, sounds like your pilot circuit is still not properly cleaned and flowing!?

The 78-79 carbs share the supply from the mains thru the pilot circuit, so that's why there are larger, the 80-81 bikes have a separate port and jet for the pilot circuit that is independent of the mains, that's why they are so much smaller.

Wes, if you're running 112.5 mains in a 79sf, then you probably have an 80-81 set of carbs!? Do yours have the extra VENT "T's" on the side above the fuel supply "T's"? If not, then they are not the original carbs!? If they DO, then you've got one strange setup, or you may have K&L jets which are known to be much larger than OEM jets, so their 112.5 may actually be close to 130!?
T.C.