The great carb jetting mystery

[Crazcnuk]

Looking through the manuals makes one wonder what the heck Yamaha was doing.

If you look at the carb specs for 80-81, there are essentially 2 carb setups used.

80-81 std uses 115 mains in 1&4 and 120s in 2&3

80-81 spl uses 110's and 120s.

stds and specials use different needles.

Some of the carbs seem to have come with all 110's.

We know why the bigger jets in the center carbs, but we still have some coming with 110's and some with 115's.

My question is if they put everything from 110-120 in stock, when jetting up what do you use as the starting point?

I also wonder why they would jet the std and special differently. The only difference between a standard and special, insofar as the motor is concerned, is the different size rear wheel, for a slight gearing difference and a weight hit for the std.

I have tried everything from 110-120, so far and found little or no difference in top end performance, and no soot on my plugs.

[Hawkins]

Any difference in the torque and hp curves for the std and spl?

[Barnfresh]

TC suggested the inner cylinders ran richer for cooling reasons. I think it was a funny time when all manufacturers were detuning and trying to pass fuzzy federal emission standards... With factory dual exhaust they were measuring what comes out each tailpipe - one cyl slightly leaner on each side to pass the import emissions test. I can't help remember my poor old '79 chevy 305 wheezing down the highway...

[Crazcnuk]

"Any difference in the torque and hp curves for the std and spl? "

I'm not sure, would be interesting to see, though.

Yes, I am wondering if the REAL jets were the center two, and they leaned out the outside ones for emissions, knowing they could cool better?

The reason I ask is, when we use the jetting chart, I wonder if we should be starting with the LARGER jets as our base?

For ex. For an 80SG it says the jets are 110 outer and 120 inner. With the 4-2 turnouts and a K&N filter it says I should go up 2-1 or 1 jet size.

I had 110s across the board, originally with #45 pilot jets, and some 'Canadian' needles.

Since I couldn't readily get 112's, I used 115 jets. It made no difference, on 2 different bikes.

I am wondering if I shouldn't start with 120 + 1?

I have 120's in it now, and it makes no difference in how it drives. I am going to drive it for a bit to see if the plugs darken up.

My 80G came with SG carbs on it, also with 110's across the board and #45 pilots. It was even worse for top end power. According to the guide, the 80G should have 115's outer and 120's inner, and different needles than the special, but not sure where the 'canadian' needles fit in, as both sets of carbs have the same needles.

Probably the best place to start is with a poll for each series of carb, to see what runs best.

[Ivan]

Here's my take:

Somewhere along the line, some fruitcake jerk trying to get a bonus from his boss at the newly formed EPA used the most complicated piece of equipment he could find to measure exhaust gasses. This was probably a 3 gas analyzer, which, by todays standard is obsolete. He measured the three gasses abd found the least amount of emmissions generally occured at 14.7 to 1 fuel ratio. This was more than likely on an engine with a huge bore and small valves, which gives great torque but wastes a lot of power. He got the politicians to believe his magical number which means nothing, so it is gospel now.

On a gasoline engine with relayively good volumetric efficiency, best power occurs at about 8:1, and best economy is around 19:1. On more modern engines that can effectively burn more fuel, I am pretty sure minimum emissions isn't at 14:1. Now lets ponder this point for a minute: It has been proven that an engine can be coaxed to run well at extremely lean mixtures. The early honda civics ran this way, somewhere around 30:1 IIRC. There is speculation that with modern computer controls this could be pushed to 50:1 in a steady state cruise. So, we could possibly see some 100mpg cars...BUT the EPA stands by its 1980 line of thinking that 14.7 is best for "the environment". So, whats better on emissions? A car that can drive 100 miles on a gallon and produces a few ppm more NOx, or a car that goes 20% of that and puts out more CO and HC?

So, we are stuck with bikes jetted to make the EPA happy, but neither for best power or economy.

So, is the jump from 8:1 to 14.7:1 about the same as the jump from 110 to 147 (which is the size of the early mains IIRC)?

Well, 14.7 / 8 = 1.8375, and 147 / 110 = 1.3363. Pretty close if you ask me. I am sure the older carbs were running a bit lean from the 8:1, since they didnt want them to look like a diesel truck going down the road.

Thats my conspiracy theory for today.....

[greengoon3]

Yamaha Jetting ... ?

I have a set of 79 CARBS I am working on. Anyone have any idea WHY THEY HAD SCREWS Blocking 2 of the idle jets in the bowls ???
Also the carbs had 2 plastic jets in the same bowls
Blew my mind when I saw that.
Thanks,
George

[catdna]

I have a set of 79 CARBS I am working on. Anyone have any idea WHY THEY HAD SCREWS Blocking 2 of the idle jets in the bowls ???
Also the carbs had 2 plastic jets in the same bowls
Blew my mind when I saw that.
Thanks,
George

Now, I'm a carb-newbie and slowly learning through the huge amount of info on this forum.... but I wondered the same thing.

Both my sets of carbs have screws covering the pilot jets - why ? how does the jet operate under those conditions ? the manual mentions that the pilots are located under the screws so I'm stumped how they actually work ....

(... and they say the only stupid questions are the one's not asked )

[John]

78-79 carbs should have screws or plugs in all four pilot jet towers. There is a small passageway between the main jet tower and the pilot jet tower, and a corresponding hole in the emulsion tube that aligns with this passageway. That is how the pilot jets are supplied with fuel, through the main jet. The 80-81 carbs did away with the screws/plugs and passageway by leaving the pilot jet tower open and shrinking the main jets, as they no longer supply the pilot circuit. The 78-79 carbs shoud have all brass floats, and the 80-81's have plastic floats. It almost sounds like you have a mix and match set with two carbs from each year type. Can you post some pics? That would help immensly.

[petejw]

from what ive read the '78 was the fastest,
other than the carbs, the head (valves), the exhaust cam and the piston crowns were the only things that were different on the early model engine compared to the later ones.

my 81sports came with 110 mains and 40 pilots, with adjustable needles.

George,
does sound like youve got different model carbs on the rack, the 78 -79 had exposed mixture screws with the later model ones the mixture screw is sitting in a tower,
are the mixture screws different on the carbs.

[DGXSER]

Now that would be VERY interesting, mix and match carbs in a rack. The 78-79 carbs also had a vent from the float bowl that was a second T between the carbs, a lower one for fuel and the upper for vent. The 80-81 carbs had a fourth hole in the inlet bell of the carb that is the vent, no more second T. Along with the idle mixture screw buried in the tower as stated, and the exposed pilot jets as also stated, and the plastic floats as also stated. So if you were going to play carburator garanimals or however you wanna refer to it, it seems you would have to have either 1 and 2 or 3 and 4 matched due to the vent T requirement.

[Crazcnuk]

78-79s are different, but they all came set up the same, according to the manuals I have.

137.5 mains and 42.5 pilots. The E had a larger starter jet (40) which I think is just more choke.

Comparing the 137.5 to 110 in the different series doesn't mean anything as they work differently.

I can see where keeping the pilot jet smaller, and the needles adjusted will definately help the cruise economy. But having the larger mains will only have a real effect when you open the throttle up.

All the pollution crap started when the US gov't mandated lower hp cars, to battle the muscle car boom that was going on at the time. Detroit, instead of designing smaller, more efficient motors, kept making the big V8's, but detuned them to the point that most the fuel was going out the pipes unburnt. Almost all the emission controls are about reburning unburnt fuel. it's ture that most gas engines like around 14.7 air to 1 part fuel, but they aren't efficient enough, so most are set somewhere around 12:1. 19:1 would melt your pistons. 8:1 you would look like a diesel on acceleration.

The only difference between a 185hp V-8 (what most of them were in the late 70's) and a 425hp one was the cam. You can add at least 100hp to any V-8 that came in the common cars by simply changing the camshaft. funny part is economy goes up too.

[greengoon3]

I can look in the bore for the main jet and see where it is cross drilled to feed the pilot jet.
All 4 of my carb bodys are drilled the same.

I did buy this rack off of Fleabay for ~ 50. When I got them #1 had a air screw broken off in the carb body. While wrestling trying to get that out I droppen the damn thing on the shed floor and broke one of the float posts off( presently resides on my desk at work as a conversation piece)
I got another #1 from another guy off of fleabay that one even had the commonly broken choke lever in tact.

I also got lucky and scored all new floats, the brass type like was in it when I started. I also got new kits that incluude everything, bought pilot jets off of MikesXS, Main jets were the same as Solex so I got them off of Fleabay as well from someone in England. Carb kits were Fleabay as well. Bought the wrong ones the first tie after being told they were off of a 80 model. The kits they sent had press in fuel valves? The seller graciously exchanged the kit for the earlier model which I neede. Kits came with everything jets, gaskets, needles, real happy with them.

I the process of disassembling them, I of course lost the choke rod balls and springs (that's what I get for working on them on the deck). A real learning experience....

But now I am down to a few final details before trying them out. Beginning to see the light at the end of the tunnel... (probably an oncoming train.

Starting to get a couple of hundred dollars in them now. I sure hope they perform as I am planning.

Thanks for all the comments and advice. I need it.

Want bike to run in the QUARTER IN 11.5.

George

[Crazcnuk]

I've been thinking about this some more and it seems to me that the main jets only control WOT.

The only time the motor will flow (more or less) the full flow of the main jet is when the needles are as far out of the tubes as they can get.

So, as far as the mid range is concerned, going to different sizes of mains will only change where you have to hold the throttle grip at a given speed.

Larger mains will require less wrist twist than smaller mains to maintain any given speed between idle and WOT.

Even Mikuni says that you should be able to properly tune these carbs if your within +/- 2 main jet sizes. That's quite a range.

This means that the mains really have very little to do with economy, unless you run flat out a lot.

I still think our gas is 'lighter' than it used to be when these bikes were made. It would be interesting if it were possible to get a sample of 70's gas to check the specific gravity.

They are adding up to 10% alcohol these days, plus no lead, add MTBFE (or whatever it is) and upped the octane of regular pump gas. This should account for having to modify the float height to get proper fuel level in the bowls.

The needles and tubes should be what we would be using most of the time. Which means they are critical to the behavior of the bike.

[Big Daddy Funk]

Dyno Believer

Hey Craz, For what it's worth, I had my bike dyno'd and I was surprised by the numbers. Especially the A/F ratio. My numbers won't help you much as I've done Performance Mods. The suprising thing is I went up two sizes on the main jets and kept the stock pilots. My A/F was averaged 14.5 across the whole pull. I will do further jetting changes to get the bike to 12.0 A/F. That will be a little harder on fuel, but more power and a cooler engine.
My guesstimate is it will take about 2 or 3 more sizes up on the mains and 1 size over stock on the pilots.
If you can get a decent deal on a dyno pull it can save a ton of trial and error. I paid $ 70.00 cash for two pulls

[scalded dog]

All this about jetting has my curiosity up, been working on an 80 special, got it running smooth, but drags a little when picking up speed, idles great, but running quiet warm, it has 4-1 headers, also dosn't have the top-end it should have, of course I am comparing it to my 79 standard, but I can be cruising around 50-55, shift to 4th, and try to pass another vehicle, and it takes awhile to get around it, on my standard, i'm around it and gone, in no time, I am pretty sure it is not getting enough fuel, so I thought about trying the 137.5 out of my standard, any thoughts? Later 'Dog