Jetting and Carbs

[elmos80sg]

I have a 1980 sg. I believe it has aftermarket exhaust becuase my center two headpipes cross. It had the pilot jet plugs, and the mains were 110 on 1 &4 and 120 on 2&3 which is the correct jetting for the year, but since it had the plugs over the pilots i am starting to think that the carbs aren't the correct ones for the bike. How can i identify the carbs. I was having problems with the way the bike ran. I left the plugs out and now it runs ok but the fuel economy is terrible (maybe 20mpg). What can I do to improve this problem.

[TopCatGr58]

Hey there Elmo,

This has been a pet discussion of one of our resident Bike Mech/Guru's Prom, regarding the design of the carbs and which ones had the plugs or not! It seems that Yamaha kinda mixed things up in the 80 models. The 78-79 series are supposed to have the Pilot Plugs, because they have the fuel tunnel joining the pilot and main jet towers, and the fuel is metered THRU the main jets(hence larger 137.5) before again being metered thru the pilots!

The 80-81 series are supposed to NOT have that tunnel between the pilot/main jet towers, and so the pilots are NOT to be plugged, cause they get fuel directly from the bowl, NOT thru the Main, so the Mains are smaller(110-120).

Now, the fun part, some folks with 80 carbs have reported that they found the shared pilot/main tunnel when they shouldn't have one. SO...IF you have the pilot/main tunnel then I think you should use the pilot caps, but your mains would need to be the larger style 137.5 or even a couple sizes larger due to the exhaust!?

IF you DON'T have have the shared pilot/main tunnel, then you should NOT use the pilot caps, and the jets you have are already fairly close to what you need!? However, with your report of poor fuel economy, sounds like you might be running too rich, and that may be due to too much fuel getting into the pilot circuit
WHEN/IF you have that shared tunnel!? The poor performance before removing the plugs may have been due to the mains being too small...again IF you have the shared tunnel!?

Hope this helps more than confuses!?
T.C.

[elmos80sg]

How can i tell if my carbs have the tube between the pilot and main jet circuits. I mean what would that look like.

[skids]

The easiest way is to remove the main jet and look for a drilled hole on the side of the pilot jet side. I am trying to sort out in my mind, how the emulsion tube does not block that cross-hole...you may have to push the emulsion tube (the main jet screws into this) up and out of the way.

Originally posted by elmos80sg
How can i tell if my carbs have the tube between the pilot and main jet circuits. I mean what would that look like.

[pggg]

The emulsion tube needs that side hole to connect to pilot jet.

[TopCatGr58]

Holes in our theories!

As seen in PGGG's photo above, the nozzle also has a hole in it's side, and due to the notch in the nozzle, that hole aligns with the hole that goes to the pilot tower!! Now here are some hopefully interesting photos!

This first one of from the 79SF carbs I cleaned up, and you can see the port thru the Pilot tower WITHOUT having to remove the Main nozzle or jet, I had removed the main jet anyways!

Now below is a carb from a bank I got from BruceGerkin, they seem to be a from an 80SG, I have no history on how they were performing, but it's interesting that they HAD the rubber pilot tower plug inplace, with ONLY 120 main jets, AND as you can see in the next photo.......


It had the supply tunnel going from the Pilot tower to the Mains, so I makes me wonder if these carbs should have the plugs in place, BUT also should have the larger 137.5 mains instead of the 120's !?!?
T.C.

[elmos80sg]

Thanks for all the help guys. I will let you know what I find out. You all have been really helpful.

[elmos80sg]

Ok so now im sure that the carbs are from an older bike. Does anyone have any extra 137.5 jets or the plugs laying around they would sell me? email me at elmo52561@aol.com thank you.

[mro]

Ok so now im sure that the carbs are from an older bike

I'm not 100% positive, but don’t remember seeing any rubber plugs on either my 80 standard or the 80 MNS.

Will check the MNS (carbs sitting on a shelf) although the 80G is running pretty good right now (other than clutch slipping) so don't think I'll check it for awhile.


mro

[TopCatGr58]

Plugged or Screwed!

Hey Elmo,

Not so fast! Did your's have the threaded pilot tower or the smooth tower? And do your carbs have the extra "T" fittings above the fuel "T" fittings for venting the carbs? IF they don't have the extra "T" above the fuel "T", then they probably ARE the OEM carbs!! I just reviewed the Yamaha Fiche for the 80SG, and they show the rubber plug over the pilot jet, but they also mention the 110/120 main jet combo. I checked the carb assbly # and it's the same for the 80SG as the 81SH, and I KNOW that the 81SH carbs don't have that shared tunnel, and I don't have the rubber plugs over the pilot jet tower!!

As for the parts, Bike Bandit sells a set of 4 main jets for $8.99, but they want over $4.00 for the pilot plugs...and they are not listed under aftermarket, you have to find them in the OEM parts listing, and then it's labelled "plug", K&L in their list of parts with their copy of the microfiche, however Yamaha's web site calls it a SCREW!?

MikesXS sells the jets for $6.00 a pair, so $12.00 total, but they also sell the pilot plugs at only $3.00 a pair! You just need to tell them the parts you need, but don't say they are for an XS11.

MikesXS part # 48-0120 = 137.5 large head mains
#48-0122 = 142.5 mains, suggested for your aftermarket pipes!

#48-5002 are the rubber pilot plugs for a pair.
T.C.

[TopCatGr58]

Hey Prom/PGGG?

Hey guys,

Don't you think that IF a carb has that pilot/main tunnel that it should have the larger (137.5)mains, and the pilot tower plug, even though it's the newer series carbs for an 80 or 81 model?!

And, only if it doesn't have that tunnel, then it should NOT have the pilot plug, and should have the smaller 110-120 mains ?!

This sounds right to me! How about your opinions!
T.C.

[mro]

Screwed! Plugged or what?

Checked 80 LG carbs, no rubber plug.
Was in hurry and didn't check jets etc..
Probably should to see if it should have plugs?!



mro

[GNEPIG]

.. if your pilot straw/tube/port or whatever it is called has the passage to the emulsion tube, you would have to have the plug or this hole would bypass your main jet and cause you to run rich
. if your bike does not have the passage and you plug it than no fuel gets sucked up the pilot jet and your bike does not idle
. i have only seen a couple of the 80 style carbs that were the cross between the old carbs and the new, i drank alot back then and cant remember which main jets were in those.. but i would have to say that they should be the larger ones, thats if there are only 3 holes in the face of the ventury. if there are 4 holes that would change things again, also are the needles adjustable or are they the newer ones and the newer carbs also had smaller diameter sliders, all this stuff works together to make them work right
. there were not a whole lot of those style carbs made[the 1980 cross over style], thats why not a lot of people know that much about them.. they can be tuned, you just have to have patiance and be willing to take them on and of the bike alot until you master them
. worst comes to worst you can get another set of carbs from ebay
.. i know none of this helps but good luck anyway

[elmos80sg]

Im gonna try the bigger jets with the plugs. I will let everyone know what happens. Thanks for all your help.

[prometheus578]

"I'm leavin'... on a jet plane..."

Man... all these carb questions and my notes are at work. And that yamaha web site with the parts fiche.... which I can't access from my Flintstone technology computer as home...
Need to use the 'puter at work and print out all the yamaha carb fiche pages for future reference. Some of my carbs have screws, some have plugs, and some have nothing at all.