79 carbs on an 80 , match float height to year of bike ?

[DiverRay]

I agree, check the plugs FIRST. If they are off, pull the seat and the bottom off the air cleaner, and try again. If it runs well then, You have a blockage in the intake snorkle of the air cleaner. The second thing you can do is drill about 9 1/2" holes in the bottom of the air filter. These need to be in the middle of the air cleaner, NOT OUT SIDE OF IT!! The middle is the "dirty" air, the rest of the box is clean air.

[TDodge7]

i think i need a new set of plugs , even cleaned these look slightly sooty , i did a few wide open passes today and the plugs came out very sooty . after that i let the bike idle for a while (which is does awesome now , purrs like a kitten , revs up as fast as you can turn the grip ) i thought that might clean the plugs a little and it did start to turn them slightly tan , but they were still pretty sooty from before . but it sat there idling nice and smooth at the correct rpm with no signs of loading up or bogging down , so i think my mixture is set pretty well . now i need to get top end back , on the side stand i can rev it up to about 7500-7700 and it brick walls . under load it hits one wall at 5500 and then if you crack it wide open you can get it to gurgle up to about 7000rpm . i'm wondering if i should mess with float height or needles first . i just bought a new battery and everything is starting to come together , can't wait to ride this thing (aside from in the neighborhood)

[Crazcnuk]

Don't panic on them, if they are working. Just ride the bike around for a couple hundred miles, then check them again.

Check the plugs in a couple of weeks. It will give you a pretty good idea of how it is running in your normal range.

You want to let your changes settle a bit, before you go on to the next phase anyway.

[TDodge7]

then where should i start adjusting to lean up the top end now that i have the bottom close ? needles or floats ? i'm thinking the mid range might be a hair too rich and the same with wide open . if i try to lean it with the floats i might also change the idle mixture right ? if so then just adjusting the needles down one clip would lean up mid and wide open only , correct ?

[Crazcnuk]

You said it's running fine up to 5500rpm.

Just drive it for a bit to let everything settle in, and clean your plugs off. THEN do the chop tests so that you can see if it is rich/lean all over, or rich/lean at the top end only.

This will tell you what you need to change next.

[TDodge7]

well i rode it around a little more and got some plugs . since then i've adjusted the clips on the needles UP one notch which leaned it out a little , but i'm still having issues above 5k . but now when the bike is on the side stand i can rev almost all the way to red line with only a slight gurgle just before it hits red , before 7k was the most i could get . but again on the road it is still pretty bad , gurgling on and off above 5k , if i go completely wide open i can get it to rev up thru the gurgling a little but it's still not right . as soon as it cools down i'll report on how the plugs look . but i have a few more questions .


#1. if i move the clips on the needles up one more position (leanest position) and it's still not lean enough to go wide open on the road should i start messing with the float height or start buying new jets ?

#2. if i change float heights at this point will i have to change the mixture screws again ?

[Crazcnuk]

#1. if i move the clips on the needles up one more position (leanest position) and it's still not lean enough to go wide open on the road should i start messing with the float height or start buying new jets ?

#2. if i change float heights at this point will i have to change the mixture screws again ?


1) You shouldn't have to mess with the needles at all. You should be doing float height first.

2) Yes.

[TDodge7]

yeah , i just tried the needles first because the carbs didn't have to come out for me to change the needle positions , figured it was worth a try . i'll pull the carbs out tonight and lower the floats 2mm ,just so i don't confuse myself , when the carbs are upside down then to lean it out 2mm i need to raise it from 25.7mm to 27.7mm correct ?

[DGXSER]

Maybe its just me, but I am thinking main jet. If it runs fine up to 5k or so (when the mains kick in) but gets rich above there, this would indicate oversize main jet to me.

[TDodge7]

it does seem like that . by 5500rpm would the carbs be on the main jet even if the throttle wasn't wide open ? it seems like it does it even if i'm not wide open , i distinctly remember hitting the gurgling flat spot at 5500 and being able to wide open deeper into it around 7k before it wouldn't give me any more .

[Crazcnuk]

Except have you EVER heard of the stock mains ever being too big?

He's running 137.5s the stock size for 78/79 carbs.

If you say he's running way lean, I could see it, but he's doing everything to lean it out more.

Which is why it's important to TEST things BEFORE making changes, or else you end up chasing your tail in a circle. You should alweays set the carbs up bone stock, then work from there. Lowering the float levels has been instrumental in clearing up a lot of carb issues like low end stumble and mid/high range richness.

You can go through a pretty wide range of mains and still tune carbs fine. Mikuni themselves said that if your mains are within 2 jet sizes (rich or lean) you can still get the carbs to run fine, that a range of 4 jet sizes. I went from 110's to 120's and it made little to no difference in how my bike runs.

Personally, I would have set his carbs up stock, but upped the mains for any mods (according to the chart) then went about tuning. Which incidentally, is what I have done.

Right now I have the 120 mains and the only mod (that affects fuel) is a K&N air filter. I am running ~45-47mpg on the 80G and ~50mpg on the 80SG (imp).

[tarzan]

Amen Crazcnuk

Start at stock.Make it run right.It should run fair, whether rich or lean(it should not be grossly rich or lean if you dont have internal mods).But you need to get there first then if it is lean, you work from there.If those carbs are stock for that year 120 mains should be close.137.5's should cause fouling.
There is nothing like throwing a bunch of variables into the mix.The next thing you know you are throwing wrenches and parts across the shop.

[TDodge7]

Start at stock.Make it run right.It should run fair, whether rich or lean(it should not be grossly rich or lean if you dont have internal mods).But you need to get there first then if it is lean, you work from there.If those carbs are stock for that year 120 mains should be close.137.5's should cause fouling.
There is nothing like throwing a bunch of variables into the mix.The next thing you know you are throwing wrenches and parts across the shop.

well i don't have the stock exhaust and air filter but aside from that it's as stock as it can get without using the 1980 carbs i have , which we've already established are no good . the jets are larger in 1979 carbs because the pilot jet is capped off and fed from the main jet , not like on an 1980 carb where it is open to the bowl . the 79 carbs came off a running xs , i ultrasonically cleaned and synced them , and set them back to 1979 specs .....thats about as stock as it's gonna get . i'll change the needle clip positions back and adjust the floats today .

[Crazcnuk]

Yes, your 137.5s are stock, I think Tarz got confused there between your setup and mine

If you have aftermarket exhaust and air filter(s) you could go up a couple jet sizes. There is a chart around here somewhere that explains how much, and why.

I would think there are a couple of 78-79 carb tuning gurus that can help you with that, depending on exactlywhich pipes and filters you have.

[TDodge7]

ok , well i set the floats to 27.7 (with carbs upsidedown , from the flange to the highest point of the floats , no gasket ) and i set the needles back to stock position . now i it seems to run well at idle and mid range all the way up to 6k where it absolutely refuses to go any farther . it's getting too late here for any high speed passes but tomorrow i'll do a plug chop with new plugs and see whats up . mixtures changed a little but not a whole lot , all are set to 2 1/2 except for carb #2 which wanted to hang out around 1 1/2 , i'm not sure why carb 2 is like that , maybe because thats the carb with the vacuum advance . i'm getting sick and tired of taking these carbs off tho , i've never had a bike so picky about everything .