Jetting for altitude

[Ivan]

Is there a linear relationship to altitude and required jet sizes? I kitted my carbs and they are running a tad rich, sputters on decel and the plugs are black, but not oily. Puffs a bit of black smoke when I rev it.

I put the new 110 main jets in that came with the kits, and they were marked the same as the ones in the carbs. I would guess that they are set up to work at sea level and are a bit big for 2500 feet. So is there a rule of thumb like go down a size for every 500 feet or such?

[DiverRay]

The "kits" probably have the "wrong" jets! I ONLY use mikuni jets, NO OTHERS . There IS a reason for this, and you have found it. I would say put the "old" jets back in, ALL of them, and the bike will probably run well.
The MOST the carbs need are new needle and seats for the floats now and then.

[wingnut]

Hey Ivan, what MRO said. I've done 3 or 4 sets of carbs and never had to replace the jets unless they had to be drilled out. The jets that come in the kits look close but aren't. The pilot jets seem have the biggest differences. If you can use the OE Mikuni jet do it. I tried the kit pilots a couple of times and every time went back to the Mikuni's to get the bike to run right. The original 110s should be fine.

[Ivan]

I'll give that a try. Although I suspect the original Mikuni jets are long gone. The new jets looked almost exactly the same as the old ones. The same branding stamp and the same stamping font etc.

If nothing else, I can use some gage pins to determine the size and solder up the holes and remachine one set of the jets.

[TopCatGr58]

Hey Ivan,

You've got the 81SH, and you say the new kit jets had the Mikuni marks, so in that case, they should be good. What did you set your float heights to?? The manual says to use clear tubing and to have them set upright, connected to fuel, and view the level in the tube next to the float bowl=PITA!!!! The book height setting is ~23mm for the 80-81 series.

Secondly, did the kits have the O-ring for the float needle seat? A worn O-ring can cause leaking and flooding etc.. Did you find the brass caps on the pilot jet SCREW towers on the top front fo the carbs, or had they been removed by the PO?? IF they were still there, AND you didn't remove them, then you weren't able to clean the pilot circuit effectively. Just needs a small drill to penetrate them about 1/4" so as not to damage the pilot screws beneath, then using a sheet metal screw into them to be able to grasp with pliers and remove them, so you can have access to the pilot screws. There are small metal and rubber washers down in the pilot screw holes beneath the screws!

Due to your elevation, you "MAY" need to adjust the float height a little higher(upside down) towards the 24mm mark to help lean it out a little, but reading plugs will be your guide. There was a thread discussion altitude effects, but no specific jetting guide has been made. The CV carbs usually adjust well with atmospheric variances until you get to extreme changes, is. 5k feet and above, and the 110's are already lean due to EPA setting and tuning by Yamaha.
T.C.

[wingnut]

If you need new you can try these folks

http://www.motorcyclecarbs.com/

[Ivan]

The floats were all set at 23mm, very uniform, so I didn't mess with them. I can drop them a bit and see about that.

The pilot needles were all replaced as well, the caps were gone, one of the heads was stripped and wouldn't come out, took some creative grinding.

I used a piece of wire to synch the carbs mechanically, but haven't done it with the vacuum gage yet. The engine is very smooth, to I would bet I am close to the mark. Exhaust pipes are all within 20º of each other which would indicate an even loading of the cylinders as well.

I don't know if the marks on the jets are mikuni or not. it looks kind of like a square with another square in one corner.

[tarzan]

Hey Ivan,
I rebuilt my carbs a few months back and set the floats to 23mm.Well to make short story long.I ended going back in and
I found out I had a couple floats that were a little cocked side to side,so it measured good on one side but not on other.I made sure all were straight and then set floats about .015-.02 lower(about .92).No more running rich.I am sure I could probably go back and set em back to .906 and they would work fine,but why bother it's in spec and it runs great.Anyway I would bet your float level is probably more likely the culprit than jet size.You did replace the o-rings under the seats from the kit right?Good luck

[mitch]

Kit

Was it a K@L kit if so go back to stock as they are known to be bad kits
Doug

[tarzan]

I used k&l kits and they work fine ,it is up to you to confirm they have the correct components,i.e.jets and other parts.If they are not the same then I wouldnt use them of course.JMHO

[Ivan]

I think they were K&L kits. I can swap the jets easy.

I did drop the float level from 23mm to 24mm, and it helped quite a bit, but it still pops on closed throttle deceleration and plugs are still sooty. Throttle response is better though.

I will swap jets soon and report back.

[TopCatGr58]

Hey Ivan,

The main jets in the kits aren't usually the problematic ones, it was the Pilot jets! Also, can't remember your intake/exhaust setup...stock...modified??

Some of the individual pod filters are quite restrictive and cause excessive richness.

Decel popping is more noticeable on aftermarket pipes, but not uncommon with stock. Were the exhaust pipes removed and put back on...old gaskets not sealing?? It's usually due the overly lean condition that occurs when the throttle butterflies are slammed shut, and not enough fuel is drawn in. However the black sooty plugs hint at an overly rich condition!?

You didn't say anything about the Pilot SCREWS and brass cap??
How many turns out on the pilot screws??
T.C.

[Ivan]

The pilot screws are about 7/8 of a turn out from closed. I took the idle down as far as possible and tweaked each one for maximum rpm, and kept necking down the idle and re adjusting. Now it idles at about 750-900 and very smooth.

the jet in the front of the carb intake was not included in the kit. I am guessing this is the jet that controls idle air. These were cleaned and reinstalled.

The smaller jet that parallels the main jet was new with the kit. I am not sure what the purpose of this jet is.

There were no caps over the pilot screws, they had been removed by a PO. Are there other caps that I am not aware of?

My exhaust is a 4 to 1 with a conical muffler. I cant find a brand label on it, but I have seen a plethora of them in pictures. I am guessing it was fairly popular on these bikes.

My intake is a stock air box with a KN replacement filter. I think the snorkel is a bit restrictive, but I haven't figured a way around it. I drill the air boxes on my cars, but it doesn't seem to be possible with this motorcycle due to the configuration. It has enough power anyway, probably won't mess with it.

After dropping the float level, the plugs went from sooty black to a powdery grey, with much less residue. It took a few miles to get them to change, but I think I am getting close. The popping is much less pronounced on deceleration now.

Thanks for the help and suggestions.

[TopCatGr58]

The smaller jets that were next to the MAINS were the Pilot jets.

Glad to hear you're getting a better color on the plugs. You could try turning out the pilot SCREWS a half turn each to further reduce the decel popping, and can also help in the low rpm throttle response. The air box can be drilled, just remember that the snorkel flows into the MIDDLE of the filter, it filters from the inside ring outwards to the BOX, so you drill in the area that fits within the inner ring of the filter, look at the microfiche if needed.

Those brass caps on the pilot jet screws were the only ones on the later carbs, the pilot JET gets it's fuel directly from the float bowl, no caps on the jet tower.
Keep at it!! T.C.