Weird Jet Configuration - 1980 G Carbs

[D. P. Larmee]

Pulled the carbs on the 80G for a little spring cleaning and found something kinda weird. Main jets accross starting at #1:

120 - 115 - 120 - 115

Plastic floats

There are drilled ports between the main and pilot jet towers, and they had these weird rubber plugs in the pilot jet towers.

I assume that the PO just got a bit careless putting the mains back in and the 120s should be in the middle two cylinders, but I thought that the 80+ carbs had the pilot jet towers open to the bowl and that they should not be drilled between the towers. I have seen these rubber plugs in carb rebuild kits in the past and never knew what they were for, so I always just through them out.

Were these plugs added by mistake by a previous owner? Should they go back in? I would think that if the pilots were fueled through the mains, the mains should be similar in size to the 78-79 jets sizes (ie, 130-135) instead of the 115-120s. Am I missing something here?

[81xsproject]

First, if the tower are cross drilled then, YES, you have to have those rubber plugs in the ends of the pilots. Also, you are correct that the PO put them back in wrong. The 120's should be in the center 2 carbs. Personally, unless you live somewhere that gets really hot and you don't have an oil cooler, I don't feel that the inner jets need to be that much larger. One size would seem like plenty. Mine came 110/120 combo. I now run all the same jet with no noticable problems. However, I live in a very modertate climate and have an oil cooler.

[D. P. Larmee]

Thanks,

I'll put the plugs back in then. I had just never seen this configuration before. I knew that the later carbs got the smaller mains, but I was under the impression that this was because the pilots had their own fuel supply dirctly from the bowl. It seems that the 115 - 120 combo would be a bit on the small side to feed both the main and pilot circuits.

[81xsproject]

It would be for earlier models, but with the later models that don't share only run 110's. Some of the 80 carbs are really bastardized. I got a set with brass floats once.

[D. P. Larmee]

This bike has seemed like it has run a bit lean as long as I have owned it, but since it ran the stock airbox and exhaust, I never really thought to look at the jetting. Always very cold blooded, requiring full choke even in the summer to start, then a good couple of miles on half choke before she would idle right and not stumble off the line.

I found two 137.5s and two 130s in the garage and I put her back together with these just as a little experiment. It is pouring here today so I have not really had a chance to get in a long ride yet, but she did start up much easier. Did about five miles on her and she was fully warmed up within a block or two and seemed to have a good deal more power thoughout the rev range.

I did a bit of searching around here and it looks like there were a fair amount of '80 standards put together like this, with the capped pilot towers but with the smaller mains. Was this Yamaha's first crack at trying to meet the tougher emmissions standards, just leaning out the jetting compared to the '78 and '79 models?

[John]

If the larger main jets resulted in better overall performance, even in the pilot circuit, then is there any chance that you could go back to the smaller mains, and leave the plugs out of the pilot jet towers. That way the pilot jet will be supplied by it's own source, and will not depend on the tunnel for it's supply.

[D. P. Larmee]

Yeah, I thought about doing that, but unless I plug the tunnel between the towers, wouldn't the main be sucking through that tunnel unmetered?

[John]

Good point!

[TopCatGr58]

Hey Larmee, and John,

There is one point that you need to check, I'm surprised Jessie didn't mention it, because I thought it was with him that WE figured out about the bastardized type of 80 carbs.

The sharing tunnel is one piece of the pie, BUT a more important part is a closer look at the Jet Needle/emulsion tube! Look on the end where the main jet is, and see if there is a hole in the side of the tube where it would be facing that tunnel.

IF it has the hole and it lines up properly with the tunnel, THEN you should cap the pilot, and put in the 137 main jets!

BUT, if it doesn't have that sharing hole in the emulsion tube, then fuel can't get from the main jet to the pilot jet, and you should then keep the 110/120 mains and UNCAP the pilot jet tower.

See this photo to refresh your memory of what you'll be looking for:

T.C.

[D. P. Larmee]

Yep, there is a hole in the emulsion tube and it lines up with the tunnel. She's been running this way since I've owned her, going on 7 years. I never really noticed a problem until I got the SG and saw how much faster it warmed up compared to the G and the SG felt like it had a bit more grunt also. I figured that since I had never been through the carbs on the G, this might be a good time to figure out the reason for the lackluster performance.

[red bandit]

Another thing to think about with smaller mains in the 80-81 vs 78-79, is the 80-81 has fewer cross drilled air holes in the emulsion tubes than the 78-79. In TC's pic the one on the left is 80-81, 6 holes. The middle one is usually 78-79 standard, 16 holes. 79 specials usually had 14 holes for the two inner cylinders and 16 holes for the two outer cly. like the one on the right. I've seen other combinations but who knows what someone else has done in the last 30 yrs.

[D. P. Larmee]

Huh, I didn't even think about the different emulsion tube designs. Six is substantially less sixteen. Well, I do still have the original jets at home.

I took the bike to work today (20 miles) and will check the plugs when I get back home this evening. It does seem to be running noticeably better, but I suppose that this is likely due to the carb cleaning rather than the different jets.

[D. P. Larmee]

Big Mistake

I don't know if the better performance was all in my head or if the unseasonably cold weather we have had over the past few days(30* - 40*) was well suited to the overly rich 130, 137.5 combo, but as soon as it warmed up, she started to run like complete sh*t.

I took the bike to the ball game today; nice day, 70ish. she was running like complete crap as soon as she was warm. As soon as traffic started to back up around the park, she started bogging like I've never felt before. Crappy off idle, crappy mid range and crappy WOT. I guess the 115, 120 combo was in there for a reason. Got home and popped the old main jets in and she is right as rain.

I didn't want someone to come across this post and think that they should follow suit and switch to the bigger jets with the 1980 transition carbs. 137.5 mains in the carbs with the rubber plugs is just way too big.

By the way, I tried the trick where you loosen the screws on the V bracket of the air box and drop the box below the carbs, and wow, what a difference. The carbs slid right out; just as easy as on the special that has pod filters. Carbs out , jets changed, carbs back in, and a test ride all in the matter of a half hour. Before, it would take me that long just to wrassle the air box back onto the carbs. I highly recommend this to anyone that has been doing it the way they show in the manual.

[John]

Instead of loosening the screws you mention, I just bend the front bracket forward until it clears the mount, then slide the box to the rear, and just push the front of the airbox down. That gives you plenty of room to tip the back of the carbs up, and just kinda roll them out of the carb boots.