CArbs, go figure...

[Derwat]

I have a 1979 Special which runs great, but gets low fuel mileage.

It gets 30-32mpg most of the time. I got up to 36mpg when I went to Vancouver (sea level) in the spring. She also ran better the lower in elevation I went. This is with imp gallons.

All the jetting, etc is stock as per my manual. The only thing not stock on my XS are the mufflers.

I bought a set of 'stock' exhaust off of ebay, as my stock system keeps cracking. I've welded it several times, but it just cracks again.

The ones off of ebay were the stock headpipes, but some kind of after market muffler. They are about the same size as the stock ones, and upsweep at the back.

When decelerating, I get the occassional 'pop' (loud, like a shot)out of the right side muffler. In the dark it spits flame out the back.

I checked the floats and the outside 2 were set at the stock level and 2 were ~1mm lower (25.7mm and 26.7ish mm) I set them all at 26.7mm.

Should I go lower? I live at approx 3000ft above se level.

The carbs have been cleaned and synched. I have the Rectifier upgrade, 750FD, and updated fuse block.

Any ideas for either issue?

[TopCatGr58]

Hey there Derwat,

You'll get better mileage when you don't go racing Crazcnuk at 75+mph on your 200km runs!!!

Like you said, runs better at sea level where the air is denser, which can help with a richer setting, but can hurt it's performance and mileage at the higher elevation and thinner air!

You've already set your floats 1 mm lower/leaner than stock, have OEM airbox, just aftermarket mufflers, which "might" help to lean things out a little bit?? Popping on Decel is common with other than stock pipes, but you could try tweaking the pilot screws a little more OUT to see if that helps.

Have you replaced your exhaust pipe to head gaskets with all of this exhaust work??

How many miles on engine, what are you compression values?
Perhaps a stronger spark would be helpful..maybe Craz will loan you his MikesXS coils for a trial?

Lowering your floats a little more might also help but might also hurt sustained hi rpm/speed runs?? Might try another 1/2mm, but you'll want to watch your plugs..speaking of which, how to they look?
T.C.

[Derwat]

He was actually following me, my son had to be back for a job interview, and I didn't want him to be late.

It was doing the 'pop' thing before we cleaned, synched and set the idle screws, and it still does it. I've heard lots of bikes where you get the 'poppety-pop-pop-pop' on decelleration, mine does the 'vrooooooom-BANG-vrooooooom' type.

I haven't put new gaskets on, still using the old ones.

There's 35,000kms on the bike, if the odo is accurate.

I was looking at getting new coils, before craz even thought of it, just haven't got the money right now.

I did the 750FD swap, made myself some stainless crash guards, added running lights, and heated handlebar grips. New brake pads, drilled the front rotors, Used Craz's old Windjammer IV, and his stepped seat with high backrest. Not quite right for the 79, but I am getting the stock seat redone.

The bike runs great, pulls hard, smooth, idles fine, just getting low fuel economy, which seems pretty common.

[mro]

For imperial gallons seems your about 20% below average (US) mpg.
For flame to come out the exhaust you may have a cylinder not firing or have an intermitant not firing problem letting fuel enter the exhaust. Weak spark and a cylinder flooding? Have you checked the valve clearences? Could be other explanations but something to consider.


mro

[wgfrie]

do you have a crossover on your new exhaust? I experienced a big mileage improvement when I put a crossover back in my exhaust.

[CarolynS]

Mine too

I get a 'wuff' out of the right side when I let off the throttle, not exactly a backfire, just a low 'wuff'. She'll do it at real low speeds too.
I already know I'm a little lean on the jetting, plus I have the 79 head with 80 carbs on the bike and a 4-2 aftermarket exhaust. All of the exhaust gaskets are new, as are all of the gaskets, but I am going to check and make sure the bolts haven't loosened since the engine has been back together, she's cycled enough now that it may have loosened up. It'll be the first thing I check because the engine was totally torn done and redone.
My car gets better mileage(40+). I figure I'm getting around 22-25. Not too bad, really. If I would stop goosing it, it would probably do better. Nah, I'll just get more gas...
I think you should check those exhaust gaskets, and(when you get the chance)check the jet sizes and maybe go up one or two like I plan to do this winter when I disassemble it again for cosmetics.
Just my 2 cents...

[Derwat]

It never did the backfire thing with the old exhaust. (stock with cracks, and a crossover)

The new system does not have the crossover.

Took the carbs off, again, today, to check the needles. Only had time to check one, before going to work, and it was raised one notch.

I assume they are all up one notch. Haven't decided if I want to raise them to the center spot (spec), or one up (lowered).

So far the carbs have been bone stock, according to my manual, except for the needle being one setting rich.

How much of a difference does each step make?

[mro]

How much of a difference does each step make?

Enough to be the difference between rich and lean

Awhile ago TC had a video showing the slides while engine running.
XSept at WOT and for a very short time the slide rose maybe over half way then came most of the way back down even with the throttle still cranked, so needle position makes a difference.


mro

[mro]

originally posted by CarolynS
just a low 'wuff'. She'll do it at real low speeds too.

reminds me of a blind date I had once....


mro

[Derwat]

took my carbs apart to day. Looks like my needles are ajusted on the rich side. So i am going to move them two spots to the lean side and see how that changes things. Everything else is stock on the carbs so should improve mileage. As far as the backfire goes i will seal up the pipes, that should stop it.

[Derwat]

went for a burn tonight after changing needle settings, now the bike has lots of bottom end great mid range and no top end. Plus it pops and backfires at the high rpm range.
tomorrow i am going on a long run to check mileage.

[egsols]

So i am going to move them two spots to the lean side and see how that changes things

What about "one step at a time". Quite a few people have played with the settings and have usually come back to the middle(stock) setting. I take it you have gone from one above (rich) to one below (lean)?

Here's a thread discussing the middle setting.....http://www.xs11.com/forum/showthread...threadid=19067

[Crazcnuk]

That's what I said. I would have put them in the stock spot, and tried them.

He went from one below (rich), to one above (lean)!

I know, I have to think about it for a while, each time too. But the needle clip was one slot below the middle, and is now one spot above the middle.

I would think he is running a bit lean now, at the top end, but the changes really woke up his bottom and mid range.

His bike always ran fairly well throughout the rpm range, but was a bit anemic in the bottom end. I could usually get a jump on him with my bike, but he would blow by me in 3rd gear or so.

We went up to Miette Hot springs (local twisties) and he was having a blast with the new found low-mid range jam. It wasn't until we hit the highway, on the way home, that he realized he had lost some top end.

[olebiker]

I am surprised that you have enough juice to run heated grips.

[Crazcnuk]

He has heated grips, heated vest and driving lights...

He also has Geezer's RnR unit, and an LED tail light bulb!