Mileage - Beat it to Death

[Tired Old Man]

I have seen a posted problem with milage that is almost identical to a problem I am having with a 79 XS 11 Special. I bought this bike from the origianl owner who left it stored for 19 plus years with a tank of gas! The bike had slightly over 12K miles on it. As you can imagine it was a "mess." I had the tank boiled and resealed and the carbs rebuilt. I got it running and about the second tank of gas one of the needles wasn't seating and my petcock was shutting the fuel off so I had a leak on #1. Took the carbs back and the guy went back thru them and re-cleaned them. I did have kits put in the first time. On the first tank of gas I got about 34 mpg. I installed a pair of inline fuel filters to help with the dirt problem, cleaned my tank again and put it back together. I have pod filters, K&N, a 4 into 1 header pipe. Everything is working the way it should except poor mileage. No dragging brakes or slipping clutch. I was getting about 17 mpg after the re-cleaning. Took the carbs off, again, and set the float heights to factory spec. They were off but I think it was from the cleaning. Put them back on and synced the carbs again and my milage went up to about 22. Sometimes you can smell raw gas but I don't know if it is from the pod filters or what. It isn't going into the crankcase. Where is the overflow on these carbs? I am about ready to give up but it has to be something simple, I hope so I am asking. The compression is great on all 4 heads, plugs look good, no carbon or smoke on the exhaust. I ordered a color tuner that should be here this week. ANYONE - Please HELP!! Sorry this is so long but I wanted to try and give a complete history.

[Dennyz]

We need to know more about the carbs jetting and the type of "pod" filters. There is a brand of pod filters that block some critical ports.

I have found that fuel economy will suffer if the carb jetting does not match the intake and exhaust modification.

It is my opinion you should be running at least 45.0 pilot jets (up on size) with "pod" filters and probably 47.5s.

To best track down the problem, resist making multiple changes. It is very easy to swing from lean to rich, missing the sweet spot where mileage hits it's peek.

I am running 45.0 pilots, 140.0 mains, K&N air filter with drilled air box and a 4:1 header with a low resistance can muffler. I am getting 38-39 mpg on a 'F' with 140K miles on it.

[Tired Old Man]

Thanks for the input. I am upsizing my jets this weekend and will color tune and re-synch the carbs as well. I am sure it is something dumb, or at least I hope it is. My pod filters are K&N. Is it possible one of my needles is not fully shutting off when running and I have excess fuel being dumped? That is why I asked about the overflows on these carbs.

[Dennyz]

It is always possible that a float needle is not closing off the fuel. I would think it would show up as a darker plug color for that cylinder.

There are rebuild kits that are junk. I have had particularly bad results with K&L kits.

There is a tool some of us have made that would help track this down. By removing the float bowl drain plug and installing a tube nipple and a piece of tubing, the float lever can be watch in real time. The '80-'81 (and probably XJ) carbs have this built into the float bowl.

[Tired Old Man]

Thanks for the help. It baffles me why I started out with fairly normal mileage until the carbs were cleaned the second time. At one point I ran out of gas at 60 miles on the tank! I just hope fellow XS riders with more experience can offer suggestions. I am open to anything reasonable. It does run good enough I put a Sportster in it place.

[79XS11F]

Poor fuel mileage can be anything or any combination of: carb jetting, valve and ignition timing, valve to seat seal, valve clearance, cylinder compression, oil weight, ignition component condition including, plugs, plug gap, plug caps, plug wires, coils, auto advance condition ....
Start with the basics and make sure the jetting for the air filters and pipes is not too rich, If that's good start testing the electrical components and auto advance units. Do a compression test and attack the problem one system at a time.
Rob

[Tired Old Man]

Thanks. Will do, plan on starting by tearing the carbs down one more time to make sure there is no dirt that may be holding one needle from seating, re-sync the carbs, color tune and then check timing and go from there.

[randy]

Do yourself a favor, check/adjust timing before synching carbs.

[egsols]

The overflow is through the air jets out the back of the carbs and into the airbox. Check your air box for any gas, see if the filter is wet, that will tell you if its leaking out the back. I usually just run my finger under the drain hole in the bottom of the box. The little foam filter in there stays wet for a while. Depending on how the bike is sitting and how bad carbs leak you can also get fuel in the crankcase.

The floats can hang and the valves are finicky, both of which can cause intermittent leaks.

You say you adjusted the floats...I take it you set set them at 25.7+/- mm from top of float to carb base (without gasket)?

The float height is critical and a little can cause too rich or too lean. Some of the aftermarket float valves and seats aren't the greatest so you may have to play with the float height.

Since you were getting 34 mpg before the second cleaning and float adjust I'm assuming that either your floats are leaking or they are set too rich, or a little from column A and a little from column B.

[Tired Old Man]

Ok thanks for the advise. I'll let you know how things progress.