Gas consumption is way too HIGH, Mileage too low

[RichV]

i finally got the carbs synced and I've been out on the bike seeing how it rides. All is well but the gas consumption is crazy low. We're talking 10-15MPG.

Also, if I accelerate fast the engine hesitates and "bogs down".

So I checked the fuel lines and all is well. I checked the advance and it seems to be working okay. I checked the wiring from the pick-up coils and I can't see any problems (no breaks). This is an 81 so I can't do much about the timing.

The carbs were rebuilt and the floats adjusted correctly. I tested the bowl levels all seems well.

It idles good and runs. It just sucks gas and feels like it's flooding out when accelerating.

Any suggestions? I'm running out of ideas.

[IanDMacDonald]

Main jet size?

[RichV]

I replaced the jets when I rebuilt the carbs. The main is #110 and the small jet is #42.5. At least that's what they said.

My air intake, carbs, manifold, displacement, and exhaust pipes are all original equipment. The muffler is a Harley Davidson muffler I had in the shop off a stock 2009 FLSTC. I've seen where others on the forum have done this.

If you think it's the jets, what do you suggest for the correct size for both?

[BA80]

First, be sure the jets are genuine Mikuni parts, aftermarket jets don't play well in these carbs.

You still have a carburetor issue it sounds like to me. how do the plugs look, black and sooty?

[RichV]

First, be sure the jets are genuine Mikuni parts, aftermarket jets don't play well in these carbs.

You still have a carburetor issue it sounds like to me. how do the plugs look, black and sooty?

I replaced them with parts from a kit so I'm sure the current ones are not genuine.

The plugs are:
1 - gray
2 - black and sooty
3 - black and sooty
4 - gray

[GLoweVA]

I replaced them with parts from a kit so I'm sure the current ones are not genuine.

The plugs are:
1 - gray
2 - black and sooty
3 - black and sooty
4 - gray

2&3 share the same coil, so it looks to me that that coil might be part of your problem. Either the coil or the plug wires or caps. Try measuring the resistance from plug cap to plug cap, compare that to 1&4. If it's high, then you're getting weak spark there and not good combustion.

[RichV]

2&3 share the same coil, so it looks to me that that coil might be part of your problem. Either the coil or the plug wires or caps. Try measuring the resistance from plug cap to plug cap, compare that to 1&4. If it's high, then you're getting weak spark there and not good combustion.

One of the advantages of having two bikes is spare parts. I have another complete set of coils.

I can't get to the bike until tomorrow but I'll at least swap out the coils to see what happens when I get to it.

I did check the resistance on the current coils before I installed them a while back. I can't remember what it was or if it was within tolerance. I did cut 1/4 inch off the ends of the wires and reseated the caps.

The backup set of coils measured at 12K ohms on the secondary and 3 ohms on the primary (without caps). The spec is 11K +-20% and 2.5 +- 10% for the 81 XS11SH. That puts the secondary in specs and the primary slightly higher than spec. I would think at that low of a resistance it shouldn't matter on the primary.

This is a good direction to go. Thanks!

[TopCatGr58]

Hey Rich,

Yes, you want to rule out electrical issues first....BUT essentially ALL of the aftermarket carb kits use either K&L or other GENERIC jets that are NOT sized the same way as GENUINE MIKUNI, and especially with the PILOT jets, and have cause many members lots of tuning grief...black sooty plugs, poor throttle response, poor mileage, etc.!

You can play/test the coils if you want to....but I would HIGHLY suggest you get some GENUINE MIKUNI pilot and main jets from any source you want...the local Yamaha dealer, Dennis Kirk, etc, just make sure they are GENUINE MIKUNI, and not just "like mikuni"! This will go a long way in getting your carbs tuned right....you will probably need to retune/resynch them once you put in the new proper jets!

JMHO!

T.C.

[MPittma100]

Carb Parts

The kits also come with replacement needles. You should use the originals if they are Mikuni. If they are not, get some. Also need to be sure that the needle e-clip is in the stock (center) slot. Recheck and recheck again your float settings being sure that both floats in each carb are the same height. I've had the best results by setting them at the lowest level of the spec. Your fuel mileage certainly suggests a carb problem. Check your oil for fuel contamination before you run the engine again. Change oil and filter if fuel is present.

[jetmechmarty]

I have to agree with previous comments. Weak spark could very well be a contributor. The kit parts have messed you up, too. Jets-r-us is a gook source for correct parts. My mileage is awful since rebuild as well. I'm pretty sure it's the float needles from the kit. Those need to be genuine as well. I think the kit float needles are shorter that the Mikunis. Maybe your original ones are fine and should go back in.

What Mike says too. The kit slide needles are a no no. I don't think the position is adjustable on yours. The kits have messed you up.

[RichV]

Hey Rich,

Yes, you want to rule out electrical issues first....BUT essentially ALL of the aftermarket carb kits use either K&L or other GENERIC jets that are NOT sized the same way as GENUINE MIKUNI, and especially with the PILOT jets, and have cause many members lots of tuning grief...black sooty plugs, poor throttle response, poor mileage, etc.!

You can play/test the coils if you want to....but I would HIGHLY suggest you get some GENUINE MIKUNI pilot and main jets from any source you want...the local Yamaha dealer, Dennis Kirk, etc, just make sure they are GENUINE MIKUNI, and not just "like mikuni"! This will go a long way in getting your carbs tuned right....you will probably need to retune/resynch them once you put in the new proper jets!

JMHO!

T.C.

Should I buy the original jets (#110 and #42.5) or something different.

The bike is stock except for the HD mufflers.

As a final check, the jets are the N100.604 and N151.067 if I'm not mistaken. Is this correct?

[jetmechmarty]

Yes. Original jets. Original everything. I'm sorry, but those part numbers mean nothing to me.

If you were running slightly bigger main jets, (I'm not recommending that) you wouldn't really be getting such bad fuel economy. All those K&L parts are working against you. I think running them means reinventing the wheel.

[TopCatGr58]

Hey Rich,

Okay, with just OEM headers and Harley slip ons, and OEM airbox, you should be able to use the STOCK jetting parameters of 110 main and 42.5 pilots.

HOWEVER, in 81 Yamaha jetted the bike fairly LEAN for both EPA and mileage. Yet the 81 Standard/Venturer IIRC had 120 mains. My 81SH came with 110 mains stock, I ran them even with a Mac 4-1 header/muffler system I put on it just a year or so after I had the bike(YOUNG & STUPID ME!!), and it seemed to run just fine, plenty of power, redlined in 4th....never got to try it in 5th..but seemed a little weak up there, did several long distance rides, like from here to Ft. Benning, GA and back, never burned any holes in pistons! But this was also with the earlier mid 80's quality fuel BEFORE Ethanol!! After my big bore and Pod filters, I went up to 115's and 45 pilots, and my dyno run showed that I could have gone up to 117.5's, and just a couple of years ago, I did just that.

SO....if I were you knowing what I know now, I would probably put in 115 mains but stay with the 42.5 pilots with your setup. The N100.604 is the right number for the MAINS...but can't find a reference to match the 2nd number....but the PILOT is the BS30/96 type/style.

Here's the Dennis Kirk link to them (pilots) sold in pairs!
https://www.denniskirk.com/mikuni/oe...rd/1760425.sku

Here are the 115 Mains: https://www.denniskirk.com/mikuni/10...rd/1725115.sku

T.C.

[GLoweVA]

One of the advantages of having two bikes is spare parts. I have another complete set of coils.

I can't get to the bike until tomorrow but I'll at least swap out the coils to see what happens when I get to it.

I did check the resistance on the current coils before I installed them a while back. I can't remember what it was or if it was within tolerance. I did cut 1/4 inch off the ends of the wires and reseated the caps.

The backup set of coils measured at 12K ohms on the secondary and 3 ohms on the primary (without caps). The spec is 11K +-20% and 2.5 +- 10% for the 81 XS11SH. That puts the secondary in specs and the primary slightly higher than spec. I would think at that low of a resistance it shouldn't matter on the primary.

This is a good direction to go. Thanks!

Something i just thought of, you didn't mention anything about the plug caps. They should be 5kohms each. I've seen the insides corrode up and increase resistance.
Total resistance from cap to cap should be around 20k.

I DO agree that the jets HAVE to be mikuni. See this thread
http://www.xs11.com/forum/showthread.php?t=42075

[donebysunday]

Mpg.

How many miles on the carbs ?
Could be worn emulsion tubes/main needle seats ?
I was getting 30 and thought that bad but your mpg. is washing your pistons and cylinders dry !!!!!/ or where is that fuel going, in the air cleaners or on the ground ?
How many miles have you put on it ?
Do a real test, fill it, put miles on it fill it, see what it's using, could get real scientific and use a dip stick to make sure your filling it to the same level. Not to full though, hate getting fuel all over my paint.