80 SG, what jets at altitude?

[Bonz]

Hey guys,

I have the carbs out of my 80 SG for cleaning. Main jets are 110, pilot's are 42.5. Other than K&N airfilter, the bike is 100% stock. Had the bike for a year and a half, carbs had never been off to my knowledge, but they look pretty good for being 30 years old inside... Definitely got a good bit of fine debris out, but nothing stood out as a major blockage, etc. so that helps explain why the bike runs pretty good for the most part. 14,500 original miles.

I can't find a specific recommendation for jetting at the altitude I live and ride at, between 6,000 and 7,500 feet of elevation. I get 36 mpg day in and day out whether on the interstate at 75 mph, cruising around town or anything in between. Overall, I don't have a reference to know if my bike could benefit from dialing back on the jets a size, or if it really is a pretty good runner and not worth the chance it would run worse, relatively speaking.

What is the consensus out there on going down a size in main jet size based on high elevation that I live at? OR what's the next step to adding more air, i.e remove snorkel, hole in airbox, etc?

[tarzan]

What do your plugs look like?It sounds to me from your description that it runs pretty well the way it is.
I sold an 80SG to a friend of mine a few months back.
He lives in Denver and it has the 110 mains and stock exhaust with a k&n air filter.It gets about 40 miles per gallon.

[tw1980]

With CV carbs I don't think altitude should require rejeting.

Terry

[CatatonicBug]

The vacuum advance on these carbs makes it so there is no need to adjust for altitude. If you are getting 36mpg, and the engine is running well, I'd say to just leave it alone! You're doing pretty good! The only thing I've heard of to improve from there is to replace the emulsion tubes and slide needles. They wear after 30 years of sliding up and down, and tend to waste fuel a cruising speeds. I'll be doing this next week myself, for exactly that reason. Another option is a dynajet kit, that does basically the same thing, but also includes a few other parts and adjustment possibilities,

[Bonz]

The plugs look great, 1500 miles on them since I got the bike and the center insulator is white (I am used to a nice light tan from days of old, but I guess no more with the gas formulatiions these days?). The headers are still shiny as can be with no discoloration so the bike isn't running lean (hot).

I have to respectfully disagree with vacuum timing advance negating the need for re-jetting at altitude. Timing advance has nothing to do with how much air/fuel is mixed and sucked into the engine. As we know, timing is critical to the overall performance of the engine, and ours was ahead of it's time in this regard. But timing advance never was nor will be able to compensate for lack of oxygen in the air, and the subsequent surplus of fuel through the fixed jets in a carburetor at altitude.

With regard to parts wearing out inside thecarb, there are but 14,500 original miles on the bike so I am hesitant to say parts may be wearing.

I'm picking up stainless allen bolts to replace the screws on the carb, and am getting ready to put the 3rd coat of plati-dip spray on the intake boots. Definitely looks better, and appears the cracks will be filled/covered up after another coat or two.

When I removed the intake boots, there wasn't any gasket, per se, simply the metal surface on the boot that mates to the engine. Is there something recommended to use, such as high temp sealer to really seal it up well? Or does this just tighten down and make the seal?

[dbeardslee]

If the center insulators are white you're probably already on the lean side. The 80/81 carbs were factory jetted anywhere from 110 to 125 mains, with the 80 SG being factory jetted with 120's on #1 & 4, and 125's on #2 & 3. It appears someone has already jetted it down. By your plug color you may want to go up a size or two.

On your carb boots, they came from the factory without a gasket - just the rubber seal. If they aren't leaking I wouldn't mess with them. You can tell easily enough by spraying some starter fluid around the boots while it's running (have a fire extinguisher handy). If the rpms rise then you have an intake leak. Ford 2.3 liter thermostat gaskets can be cut down to fit, or you could use permatex ultra copper to seal them (just make sure you let it cure fully before firing it up.)

[Bonz]

NO, the 80 SG has 110's across the board, unequivocally. That's what are in the carbs, that's what the service manual calls for. The richer jets in the middle cylinders were supposed to help with cooling things, but the benefit is debateable, as Yamaha went to the same size jet in each carb on the 80SG and there has never been an issue. I don't know where anything other than 110's is called out in literature for the 80 SG.

I don't think it's running lean, especially at 7000 feet with stock jets! I am fairly certain the insulator is whitish because of the different gas formulations than when this critter was a new bike. The plugs I took out of it a year and a half ago did have a tan color to them, but they had been in the bike for almost 30 years of which the bike hadn't seen more than 200 miles in the last 20 years. So when all I do is change plugs and nothing else, a white color doesn't mean lean in this case IMO.

[dbeardslee]

I don't know what kind of a manual you are looking at, but I suggest you take a look at the Yamaha parts fiche. That states very clearly 120's and 125's. I also suggest you take a look at this and compare your plug condition to the chart. I doubt seriously that the type of gas is the cause.

[Bonz]

The factory service manual states 110's. It's not a big deal, but I feel comofortable that it is correct if anything is to be correct. As well, in this forum under "maintenance"-"carburetors"-"carburetor facts- models and jetting" there is a jet chart that shows 110's for the 80 SG/LG. Don't know why the parts fiche is different than the original manual or other sources on this forum.

Yes, plugs should be a light tan and I've heard of others who have similar plugs to mine and have no lean running characteristics as well.

As well, the headers are perfect chrome, no discoloration what-so-ever, and I deliberately ride on some hot days where it has had plenty of chance to discolor. Here is a quote from elsewhere in the forum

"The black carbon around the base of the plug with white insulator is decent plug condition, these newer cleaner unleaded fuels don't necessarily give as good of coloring to the insulators like the old gas of the 70's".

Once I get the carbs back on the bike after this cleaning, I'll get another read and see if anything's changed. The K&N filter could/should be passing more air, but I ran the new plugs for a few hundred miles with the cleaned stock filter before going to the K&N, and they were the same white colored insulator and black center electrode.

We're debating the little stuff here...

What model do you have?

[dbeardslee]

I've got a '79F, but I've worked on a bunch of them - 80SG's included. If you're confident it isn't lean, and want more air, the other thing you can do is drill the airbox. The thing is, if you adjust one side of the engine (intake) you generally need to address the other side (exhaust) to get the full benefit. Course that usually takes you back around to jetting. I think you've got the right idea to check your plugs again after cleaning. Plugs don't usually lie.

[Bonz]

Yes, it's a dynamic set of variables for sure. Thanks for all the info thus far! I had a 97 Bandit 1200s that was running 11.5:1 on the A/F with stock jetting and a Kerker exhaust. Most guys said to go smaller on the jets, but adding air would accomplish the same thing, so I drilled a 2" hole in the airbox (snorkel already removed by previous owner), shimmed the needles, put in a K&N, adjusted the air screws and it became a huge beast of a motor!

Quick question... It makes sense the air screws are under the brass plugs on top of the XS carbs, yes? I assume they are just like they were on my Bandit 1200s.

[dbeardslee]

Your mixture screws are under the plug, and they're notorious for getting crud under them where the o-ring and the washer live. Gotta be real careful drilling the plugs out, as the top of the mixture screws are pretty close to the plug. You don't actually have to drill all the way through the plug to get it out. The plug is about 3mm thick, so if you use a drill bit that's around 4mm in diameter, and drill down until the tip of the bit has penetrated to a depth of 3mm, they should lift out easily using a dental pick or similar tool.

[Bonz]

Last time on the B1200s, I took a small metal screw, and used a drill to screw it in EVER SO SLIGHTLY and then pulled the plug out with the screw as the "handle". Felt like there was less chance of "screwing up" compared to using a drill bit.

Is there a recommended number of turns on the air screws to start with, or just count where they are now and start from there when it's all back together?

[dbeardslee]

You start at 1.5 turns out and go from there. I personally like to use a colortune, but you'll get conflicting opinions on that tool - some people swear by them, others swear at them . The screw method should work too for plug removal, although you'll probably have to enlarge the hole a little - it's pretty small. They're not in there that awful tight. I think it's funny that many of the carb rebuild kits include new plugs. I guess they figure it's so much fun removing them, that you'll want to do it again the next time you want to adjust your mixtures.

[Bonz]

With the carb intake boots do I need to put a thin bead of high-temp sealer on the mating surface when I put them back on the bike, to seal things up, or do they do fine as-is after cleaning them up? I could detect no leaking before I took them off, but they didn't inspire confidence with how they appeared to be sealed when I took them off.