80 SG Pilot Jet Tower

[dbeardslee]

So I'm rebuilding a friends 80SG carbs yesterday, and when I removed the pilot jets I noticed there was nothing covering the top of the pilot jet tower. I just checked the fiche (probably should have done that yesterday ) and it shows a screw in that location. Since they weren't on there originally, and it was running fine before the carbs were overpowered with gunk, I put them back together the way they were. The bikes running great, but I did have to adjust the mixture screws in pretty far (1/8 turn out) to get good color. My question is this - what is the function of the missing screws, and could this be why I had to put the mixture screws in so far?

[TopCatGr58]

Hey DB,

The 80 year carbs were during that transition time that Yamaha had 78-79 bodies, but were going to the 80-81 type innards. IT's kinda late now to check, but you could have looked at the main and pilot jet towers for evidence of the sharing tunnel between them. IF the tunnel was there, AND the Main Jet Needle/Nozzle/Emulsion tube also had the hole in the end of the nozzle that would allow the fuel to flow from the main to the pilot jet tower, THEN you should have installed the rubber caps that are part of the kits for the 80-81 carbs.

Having to turn the pilot screws down to only 1/8 out kinda indicates that they probably had the sharing tunnel, and so with the tower caps off it was getting dual access to fuel, thru both the main jet tunnel and directly from the bowl!

T.C.

[dbeardslee]

TC - I thought the kits should have had rubber plugs, but they didn't. Looking at the fiche it showed screws, not plugs. I wish I had them apart now so I could check for threads in the towers. I thought it was kind of strange that I had to adjust the mixture down so far when it had 110 and 115 mains. That would explain it. I'm sure I'll be in those carbs again someday as that was the first cleaning. Next time I'll make sure I put the appropriate device over the tower. Now if I could only talk her in to getting rid of the stock airbox. Pod filters have spoiled me . Thanks for clearing up the mystery, TC.

[Crazcnuk]

Both my sets of 80SG carbs only have the idle screws out that far too, and mine are together correctly, no cross tunnels, etc.

I have run 42.5 and 45 pilots, and anywhere from 100-120 mains, with different needles, and never have I had to open the idles more than 1/2 a turn.

[DGXSER]

If they had the "tunnel" would they also have the vent Ts and the exposed mixture screws? These definitely have the mixture screw in the tube and no vent Ts. (I know the friend also and have seen the carbs).

Doug, when you put the needles in the carbs, were they adjustable like your 79s or just a plastic "washer" with a metal washer on top and a spring? If the latter, and you put the spring in first instead of last, they will run very rich and need to be turned in like that...DAMHIKIJK

[dbeardslee]

It's not the mixture screws that's the issue - they're properly assembled. Take a look at this picture, particularly item #54 (item #53 being the pilot jets). The diagram of the 80SG carbs is identical to Wendi's 80SG's carbs with the exception of the fact that screw #54 was missing. It was also obvious that somebody had been in the carbs before - evidenced by the screwdriver marks on the main jets, and the absence of metal mixture screw plugs. I looked down in the pilot jet 'holes', and I don't remember them being tapped for screws, but I wouldn't swear to it. I'm guessing either the PO forgot to put the screws back in, or it had rubber plugs that weren't reinstalled. Colortune revealed that the mixture was OK at 1/8 turn out, but it was somewhat sensitive to rapid application of throttle suggesting that what TC said is correct. The good news is I think I can take the bowls off the 80SG carbs without removing the carbs - a real PITA with the airbox attached. Once I've got one off I can use a mirror to look inside the hole to determine what is needed. Should be a lot easier than taking them off.

[dbeardslee]

I just checked carbkitscapital.com and they're kits for 80/81 include the rubber passage plugs. They also sell the plugs separately for $8.25 for a set of five. Looks like they're worth a try.

[red bandit]

If you have 45 pilot jets in on 80 carb that's probably why your mixture screw is only out 1/8. If you put 42.5's in you will probably need your mixture screws out about 2 turns and won't be so touchy. This is if you have 185 air jets. If you want 45's go with 210 air jets and I think it all comes out the same.

[red bandit]

If it runs fine with out rubber plugs, adding plugs is going to make it not run good enough to drive.

[TopCatGr58]

Hey DB,

As Red said, IF it doesn't have the tunnel, then the caps are NOT needed. You didn't state what the size the pilot jets were, I think Red was assuming from your signature, but that's for YOUR bike, not your friend's bike!

But it does make sense regarding IF it has 45 jets instead of 42.5 in only needing a 1/8 turn out instead of the usual 1.5 or more.

Here's a photo I borrowed from 81XSPRojects thread, he had a 80 Standard with the tunnel and brass floats, but an 80 SG without the tunnel and plastic floats, and NO caps on the towers! The photo shows the tunnel between the pilot and main jet towers.



So...you don't want to just arbitrarily put in the jet tower plugs unless you have confirmed the tunnel, AND the hole in the end of the Main Nozzle that would allow the fuel to flow from Main to Pilot thru that tunnel. But if it had the 110-120 mains in it, then it probably doesn't have the tunnel, and the caps are not to be used!
T.C.

[dbeardslee]

TC - The pilot jets looked to be the stock 42.5's. I looked down in the holes, and I didn't see any holes in the main towers. I had the emulsion tubes out and I cleaned the passage with a round nylon brush, and I then showed the owner where the pin is for the emulsion tubes. I would have remembered a hole in the passage. Looks like they're correct. The other thing that might have been causing the short adjustment on the mixtures is the new o-rings. They're completely uncompressed, so they may have been holding the mixture screws up a little. I'm not one for cranking them down 'till they get stuck, so.... Thanks again, TC.

[tarzan]

FWIW,I just assembled a set of SG carbs today to put on my E and they dont have screws or rubber plugs.In fact the carbs on my SG are the same as these and the two other sets of SG carbs I rebuilt before were the same.I would be surprised to find out you need to plug the pilot tower.
Rick