Why am I running lean?

[Incubus]

Austin prime riding season is just around the corner and I am trying to get Incubus back to the bike she once was. She is coming off a period of enforced idleness she endured while I was putting a stake through the heart of my working life. Just because I retired doesn't, though, mean she gets to. Quite the opposite, in fact.

Right now she is running lean and missing the top of the range. I think I may have an idea about what the root of my problem is, but I was hoping for some thoughts about things I may be overlooking. I haven't done much wrenching on my bike for a couple years.

My '79 XS1100 Standard is almost running real well. It starts quickly and has a good strong, steady idle at about 1000 rpm. Once warmed up, runs quite well in the 3000 - 4000 range. Above 5000 the engine loses torque and sputters. I pulled the plugs and the bike seems to be running lean on three of four cylinders.

I've had the stock carbs off multiple times and I am convinced they are clean. The jetting is stock, the air filter is a K&N and the exhaust is an aftermarket 4-1. The bike used to run well through the entire range with this same setup. I once had it at 8000 rpm in fifth and was still pulling when I tried to find the next gear. I am not sure how fast I was going as the lack of the additional gear scared me just enough to immediately begin decelerating. I didn't check my speed until it was slowing past 100.

If there is something in the carbs beside jetting that can cause this, please tell me. I have never had a diaphram fail, for instance, so I don't know what those symptoms would be. Everything else was pulled and cleaned and compressed air was forced through every orifice. The slides seemed to be sliding smoothly and easily with a good spring back.

As for now, I am suspecting an air leak on three of four cylinders. The carb holders are relatively new and uncracked, the vacuum lines to the petcocks are uncracked and still flexible and both additional sync ports have new rubber caps, so I think it must either be where the carb holders meet the heads or the clamps around the carb necks.

I've been looking for an old post that gives the year and model of car that used a thermostat gasket very similar to the same of our carbs holders. Does anyone remember what vehicle and gasket I am talking about? That could solve the issue if it is at the heads. Any suggestions for replacing the 35 year old carb clamps that may or may not still be round?

Any other ideas? I'm riding the bike the way it is, but I would really like to get it perfect.

Thanks.

Patrick

[DiverRay]

Check the fuel flow from the tank.
Check the vent tubes that plug into the airbox.
check float level, too low OR high.
Pinto 2.3 therm. gasket fits the carb holders. I just used the grey non-hardening sealer on the new ones I installed. Also, check the vacuum caps on them! If they will rotate, they are bad! You should be able to get new plugs from an auto store or online. DO NOT get "new old stock", as they will ALSO be hard after a VERY short time.

Just my three quick places to look.

[Incubus]

Float level. I forgot float level. Thanks, Ray.

I am pretty sure I used a good sealant when I installed these carbs holders when I bought them new. I was hoping that would not be the cause of the problem. But I did not check the float level the last couple times I pulled the carbs. Low fuel level at high RPMs could certainly be rooted there.

One thing to check.

[skids]

Try runnng with the petcocks on prime just to make sure the fuel feed is OK.

[Incubus]

Now you both have me thinking fuel starvation rather than an air leak. That actually seems to tie the clues together a little better. Plus, this way I start with the tank, so save the 12 minutes it takes to get the carbs on the bench. I am still way too practiced at pulling these things.

But it's always the carbs.

Patrick

[dbbrian]

... I have never had a diaphram fail, for instance, so I don't know what those symptoms would be. ...

Don't worry, if THAT fails she will have all the symptoms and will surely let you know

Sorry, couldn't resist, since you got some serious answers

[skids]

i have had my sx11s cut out with a constant 60mph due to the petcock, where if I switch to prime (left side on mine) the problem immediately disappears.

I used to use those fuel filters that contain a porous stone element. They are too restrictive. I have since gone with Fram 3515 in-line filters. Those are small diameter and last a very long time, and they are metal.

Some carbs have dome-looking screens at the intake of the fuel valve. They can get blocked with rubber bits from fuel hoses and petcock valve gasket (I assume). Someone on the list even found a dense spider web in there!

If the head pipes leak at the gaskets, you can usually hear it as a "tick-tick-tick." I suppose you could bubble-test it when still cold.

The condition of your 4-into-1 exhaust can deteriorate. Is it noticeably louder? I had troubles with a Jardine 4-1 where the end cap came loose. Also the muffler innards can corrode into nothing. These two possibilities will make it fuel-starved at high throttle.

If you check for leaks at the intake manifold boots, have some kind of extinguisher handy. I had a small flame doing that once and I was able to blow it out.

If your carbs and jetting is clean and unchanged, you still ride at the same elevation where it ran great before, I would at least give the above some consideration.

[Schming]

Jat

Have you tested the butterfly shaft seals ?

[Incubus]

I plan to check the fuel system from the tank to the float to confirm flow. There are other good suggestions to pursue.

I tried to do butterfly seals on a set of carbs several years ago. I will never do it again. I will spit gasoline into the intake before I mess with those damn seals again.

A man must know his limitations. Mine is somewhere this side of butterfly valves.

[MPittma100]

Lean

These engines were set up to run lean for emissions and fuel mileage. Maybe you have one cylinder too rich rather than three too lean?

[donebysunday]

Now you both have me thinking fuel starvation rather than an air leak. That actually seems to tie the clues together a little better. Plus, this way I start with the tank, so save the 12 minutes it takes to get the carbs on the bench. I am still way too practiced at pulling these things.

But it's always the carbs.

Patrick

Here is another thing I had once, from sitting the muffler baffles can rust and swell before they disintegrate and blow out the back in an orange cloud of dust.
When swelled they can restrict the exhaust flow and actually stop/slow the fuel flow into the carbs, this reduces the power and the engines ability to power up under load and reach what used to be attainable rpm.. It can also accentuate unnoticed problems into noticeable problems. JAT

[Incubus]

I have the carbs on the bench right now. The floats were spot on - at least as far as I can tell. The inline fuel filters seemed a bit restricted, so I replaced them. I hope it was just the fuel filters.

The exhaust on this bike is of unknown vintage and make, but it still appears outwardly to be OK. I replaced the exhaust gaskets and added a layer of muffler tape inside the joint between the header and the muffler. I still got popping on deceleration after that, but I attributed that to the lean conditions.

Sadly, I sold my parts bike a few months ago. It had an exhaust on it I could have tried, but that pipe was very loud and required more muffling steel wool. I thought it too free flowing. The exhaust I am using is still comparatively quiet, so I hope that is not the issue.

Patrick

[Incubus]

OK, my problem may be solved. I am not absolutely sure yet, but a test drive around my neighborhood demonstrated that I have smooth and consistent acceleration up through 5,000 rpm. I cannot say for sure it is OK above that as my engine was still cold and I was in my neighborhood. For some reasons, the the mommies around here get all het up when I cruise down our residential street with my engine running at 8,000 rpm. It doesn't matter what speed I am actually doing or how many wheels I have on the ground. Reactionaries, I tell you.

I will not say to what I traced the problem. Let us just say is was operator error and I - nor anyone else - probably should not be allowed to actually ride motorcycles I work on.

On a completely unrelated topic - so unrelated it should be in another post - so unrelated I am not sure I can type them in the same language - so unrelated that I may be violating federal law putting them on the same page.... if anyone wants to know what happens if you fail to properly torque the covers above the diaphragms, let me know.

Patrick

[3Phase]

Nah, it's not important and it's not as if the covers should be tight anyway, those diaphragms will just wear out and start leaking if there's enough vacuum to make 'em move the slides up and down all the time so it's actually saving money and fuel.

[Incubus]

I am pioneering new fuel economy methods. Cool.