Jetting - Check my math

[Brak]

Ok, so i figured out why my bike runs like carp... when the PO "rebuilt" the carbs, he apparently forgot to reinstall the Pilot Air jet in #1 carb, which prompted me to open them up and check the other jets...

according to the thread here, stock jetting for my 80 should be as follows

Main Jet - 110/120 (1&4/2&3)
Pilot Jet - 42.5
Start Jet - 25
Main Air - 140
Pilot Air - 185

Currently everything appears correct, except for the main jets, which are:
1&4 - 120
2&3 - 112.5
they seem to be reversed...

To further complicate things, I will be running without the airbox, so this is where the math comes in...

This Thread says:

+3 jet sizes for individual filters
Add up all the jet size increases and subtract one. (Remember they go in steps of 2.5 for each jet size)

Under a mismatch condition, like individual filters and stock exhaust or 4 into 1 header with stock filter and air box, subtract an additional 1 jet size.

since I have stock exhaust, i guess i only need to bump up one jet size?

Can i just swap the existing main jets to their proper carbs, and call it good?
the 112.5 jets would be the proper +1 for #1&4. and 120 on #2&4 is already pretty rich...

according to my math, +1 would be as follows:
Main Jet: 112.5/120
Pilot Jet: 45
Main Air: 142.5
Pilot Air: 187.5

Correct?

[TopCatGr58]

Hey there,

Well, the first thing that most folks will say is what do your plugs look like?

The manual and specs sheets are partly wrong in that Yamaha learned early that the staggered jetting sizes in an attempt to provide better cooling affect for the center cylinders wasn't really necessary, and they did away with them and just put the same size mains across all 4. Also, with increasing pressure from the EPA they kept making the jetting leaner for emissions and mileage. The 81's had 110 mains.

I have both Pods and 4-1 pipes, and am running 117.5's in my 81SH with decent color on the plugs. I didn't check to see if your bike was a special or standard, but there were differences in jetting and slide needles, and so the 120 may be good for your setup, and so you would only need to acquire 2 of them to have them the same across all 4.

But you'll still then need to see both how it performs as well as what your plugs look like.

T.C.

[motoman]

IMO, with stock exhaust and apparently pods instead of stock airbox, 112.5 mains across all four and a SLIGHT chance of one step richer for all pilot jets. Double check that ALL four float levels ARE EQUAL at 23mm. Then first run and adjust idle mixture CORRECTLY along with a carb sync......ride for a 10-20 mile at a constant 4K rpm and then pull sparkplugs for a burn inspection.

[motoman]

BTW Brak, as I'm sure you know, in order to access idle mixture screws tank has to be removed. You can either use another fuel source or flip tank around backwards setting it in place where seat goes and then hooking up fuel lines.

Idle mixtures are to set using 'lean drop' method. With bike idling, back out idle mixture screw a few turns, then slowly start back in with it. When you hear an rpm drop(exhaust changes tone), stop and back idle mixture screw out 1/8 of a turn and absolutely no more than a 1/4 turn out! Go to next carb and repeat the same thing till all four carbs are done. Then using mercury manometer or 4 vacuum gauges hooked to each of the four brass ports on intake boots, sync the carbs. The three screws at topside of linkage is where this is done. Start with third one on right side matching #4 carb to #3carb. Go to first screw on left side matching carb #1 to carb #2. Then go to second screw in middle and match the two carb pairs together. Definitely use a box cooling fan in front of motor during ALL this tuning process! After that, shut bike off and flip throttle several times to seat the screw threads of the three screws that were just adjusted. Re-start bike and re-check any differences of readings and make ever the slightest adjustment needed to match up all four vacuum draws. Done,..... replace tank with all correct hose hook-ups and go for a rest ride. What it amounts to, if any internal changes were made(jetting, float levels, etc.) and no idle mixtures nor carb sync were re-done, any initial changes made were an act in futility!

[motoman]

BTW Brak, as I'm sure you know, in order to access idle mixture screws tank has to be removed. You can either use another fuel source or flip tank around backwards setting it in place where seat goes and then hooking up fuel lines.

Idle mixtures are to set using 'lean drop' method. With bike idling, back out idle mixture screw a few turns, then slowly start back in with it. When you hear an rpm drop(exhaust changes tone), stop and back idle mixture screw out 1/8 of a turn and absolutely no more than a 1/4 turn out! Go to next carb and repeat the same thing till all four carbs are done. Then using mercury manometer or 4 vacuum gauges hooked to each of the four brass ports on intake boots, sync the carbs. The three screws at topside of linkage is where this is done. Start with third one on right side matching #4 carb to #3carb. Go to first screw on left side matching carb #1 to carb #2. Then go to second screw in middle and match the two carb pairs together. Definitely use a box cooling fan in front of motor during ALL this tuning process! After that, shut bike off and flip throttle several times to seat the screw threads of the three screws that were just adjusted. Re-start bike and re-check any differences of readings and make ever the slightest adjustment needed to match up all four vacuum draws. Done,..... replace tank with all correct hose hook-ups and go for a rest ride. What it amounts to, if any internal changes were made(jetting, float levels, etc.) and no idle mixtures nor carb sync were re-done, any initial changes made were an act in futility!

Also, any jet change if needed HAS TO BE Genuine Mikuni with the Mikuni logo stamped into them.......no aftermarket K&L or RD jets allowed!

[Brak]

Thanks for the feedback...
This bike was mostly disassembled when i got it, so it's been a path of discovery figuring out what is and isn't correct...


The mixture screws aparently have never been touched since they left the factory, as the block off plugs are still intact...

one thing i noticed is that the 120 main jets are smaller. they physically match the pilot air jets...



here they are side by side



am i correct in assuming they are not interchangeable?


Also i found, all but one of the carbs are missing the rubber plugs on the side

[motoman]

From pics, looks fairly clean and decent internally. Main jets and pilot jets are NOT interchangeable. IIRC, that's just a blind casting hole anyways. Initially, just do as I suggested.

[MindWebs]

pilots and mains.....

well..

they should be used in the correct locations..

but the threads are the same and they will seal against the main washer.

the size they are marked is all based on the same "scale" so technically is you have a big enough (right size) pilot jet you could use it for a main jet.

but if you want to swing toward the "purist" side, the bigger headed mains should be used.

i am more of a as long as it works correctly, let the big dog eat.

besides, fuel milage goes out the window at 100+ mph anyway.

if you want i should have an extra airbox laying around. PM me and we'll figure it out.

[Brak]

The reason i asked about interchangeability, is that the jets that fit in the front (pilot air), are listed as "main jets" on just about every website.

Either way, The correct jets should be here tomorrow...

MindWebs, Thanks for the offer, but i think i'll pass, for now

[Brak]

Well, since swapping the jets, I was able to get them tuned, Synced, and generally running well, except for #1 carb randomly pissing gas all over the place

I've checked everything, swapped floats, and needles around between the carbs and the issue remains on #1, so i guess i'm replacing the seat...

My question is, i've seen several mentions of a plug for the pilot jet passage on 80 and up bikes... I don't currently have them, Should i?

Since i have to order a new valve needle & seat, should i pick up the plugs too? are they necessary?

Yeah, it's only $11 for the set of four plugs, but all these "only $10" purchases add up fast.

[motoman]

Well, since swapping the jets, I was able to get them tuned, Synced, and generally running well, except for #1 carb randomly pissing gas all over the place

I've checked everything, swapped floats, and needles around between the carbs and the issue remains on #1, so i guess i'm replacing the seat...

My question is, i've seen several mentions of a plug for the pilot jet passage on 80 and up bikes... I don't currently have them, Should i?

Since i have to order a new valve needle & seat, should i pick up the plugs too? are they necessary?

Yeah, it's only $11 for the set of four plugs, but all these "only $10" purchases add up fast.

The stock main jets are different between the 80-81Standard and Special. Those carbs having the staggered main jetting is relavent to 80Standard, which also has a different number/letter stamp in side of metering rods. Where as the 80-81/Specials had a tad shorter and quicker angle metering rods. Initial response was also a tad quicker with the Specials('bar-hopper').
So, if your bike is in fact an SG,(80Special) and if having the original carbs, main jet size stock is 110's across all four. To decifer which carbs, unscrew hold down screws of vacuum caps, pull slide out(don't mix them up). Then a clippy removed where metering rod attaches. Unfortunately, it has to be removed(metering rod) in order to read designation data#'s on side of upper portion of metering rod. At this point, the last pic. designate them as 80'carbs.......which metering rods as to Special or Standard would be the question.....a look-see would have the answer.

[TopCatGr58]

My question is, i've seen several mentions of a plug for the pilot jet passage on 80 and up bikes... I don't currently have them, Should i?

Since i have to order a new valve needle & seat, should i pick up the plugs too? are they necessary?

Yeah, it's only $11 for the set of four plugs, but all these "only $10" purchases add up fast.

Hey Brak,

IF you have the sharing tunnel between the main and pilot jet towers, then the carbs need the caps. If yours doesn't have the sharing tunnel, then you do NOT need the caps, the carbs and pilots need to feed directly from the bowls, since there isn't a tunnel for the pilot to get fuel from/thru the main jet source.

T.C.

[Brak]

Hey Brak,

IF you have the sharing tunnel between the main and pilot jet towers, then the carbs need the caps. If yours doesn't have the sharing tunnel, then you do NOT need the caps, the carbs and pilots need to feed directly from the bowls, since there isn't a tunnel for the pilot to get fuel from/thru the main jet source.

T.C.

Ok. i'm not familiar with the "tunnel"

how do i identify it?
do you have a picture?

I appreciate the help