Topic: My BOTM winning bike is going to drive me crazy!! Help.....

[StevieMac]

OK, I'm super happy with how the bike turned out but getting it carburate right might just be the end of me.

The bike is '72 CB500 and I'm running a MAC 4 into 1 exhaust.

Originally I started running velocity stacks.  I had stock 40 pilots, mains drilled to 109 and the needle in the #3 grove from the top. The bike ran great in the mid range and up but was lean and hunting for fuel down low.  I installed 42 pilots and that seemed to make it run just horrible everywhere above initial throttle.

In the interest of saving my engine I decided to swap the velocity stacks out for UNI foam pod filters.  Now I'm really confused.
I started by running the 40 pilots, 109 mains & needle in the second slot from the top. The bike seemed super rich.  It wanted to bog badly down low and once going it would accelerate but only so much.  Opening the throttle all the way would barley increase the speed of the bike at all which I've always known as a rich condition.  I did a plug check at WOT in 1st gear and the plug looked ok, not rich, hmmm.  Mid plug chops looked rich.

So I went back to the stock jetting, 40, 100, needle in the second notch from top.  Bike still seemed rich.  Same symptoms above but sliiiightly better.  I moved the needle all the way to the leanest setting and tried again but really there was no improvement.

I don't get it.  Why the heck should the thing be rich (maybe I'm just lost...) but I've tried all sorts of settings and the bike just seems to run like crap now no matter what I do.  I've had the carbs apart everyday for weeks now and I'm getting nowhere.

The carbs are clean and like I say it did run pretty good with the velocity stacks once I got past the initiall throttle openings.  Any guidance on what to try next with the pods is appreciated.....

[StevieMac]

Nobody.......??

[ohiocaferacer]

How many turns out are your airscrews???

....also check your float heights.

That stumble off idle or during throttle changes can be your carbs out of balance.....have you balanced them??

Later,
Greg

[scondon]

In my experience, once the stock airbox is replaced ,with either pods or stacks, all bets are off in getting the carbs tuned to run great in all throttle ranges. Sounds like you were set pretty good with your initial "stack" settings since they are used for racing(mid,open throttle) and aren't much good for putting around town.

    Makes sense that you'd want to put some type of filter in place but not sure that Uni's are working well for you at all. I've had limited experience with them but found them very restrictive, easily clogged and,well, just plain don't like'em. Don't know if K&N makes pods to fit the 500 but they'd be a much better choice. Also, you can fit some fine mesh fabric over the velocity stacks that should keep a lot of the big, engine damaging, bits from getting through.

    Wish I could help with the carb tuning but I'd really just be guessing. I have found that getting a good "plug chop" reading requires that the bike be fully warmed up and that the plugs you will be reading needs to be very clean or new. I usually ride the bike to an open stretch of road, put in a clean plug and ride for 1/4 to 1/2 mile at the throttle range I want to read. Usually in 3rd or 4th gear since there is very little load in 1st or 2nd and I have gotten confusing results in the lower gears.

    All of this is from my trials of tuning a 750 with engine/exhaust mods and K&N filters so may not apply directly to your situation. Hope some of it helps.

[StevieMac]

How many turns out are your airscrews???

....also check your float heights.

That stumble off idle or during throttle changes can be your carbs out of balance.....have you balanced them??

Later,
Greg

Haven't checked the float height yet.  Yes I've balanced the carbs but not since making 100 changes in jetting.  If I could ever get it close to running good again I'd redo it but it seems pointless at the moment.

Man, I've tried the air screws in every position possible too.  I think they are 2 turns out at the moment.

I'm not going to buy K&Ns sorry, I've spent good money on velocity stacks and now the UNIs because in the past they were recommended as being better than K&Ns and other pods.  Maybe that's not the case but that's what on there and I just can't afford to buy something else at the moment.

[StevieMac]

Sounds like you were set pretty good with your initial "stack" settings since they are used for racing(mid,open throttle) and aren't much good for putting around town.

 

Yeah, maybe I'll have to go back to the velocity stacks.  Maybe I'll try that tomorrow.....
I'll check the float heights and balance the carbs as well.  Maybe that's just the best it's going to get.

I'm disappointed about the UNI pods though.

[cben750f0]

mate if it was ok in the middle and up top, dont you just lift the needle a clip position, to number 4 from the top?, this would richen up the bottom, and a little bit in the middle, but would leave the top as it was.... just a tought.. anyone else...


peace

[StevieMac]

mate if it was ok in the middle and up top, dont you just lift the needle a clip position, to number 4 from the top?, this would richen up the bottom, and a little bit in the middle, but would leave the top as it was.... just a tought.. anyone else...


peace

Interesting thought.  God I'm sick of moving that needle!  Gotta be done though...

[cben750f0]

just another though, windeing the screws out on  750 makes it leaner..... is it the same on the 500?... just read that you have yours out 2 turns, thats a long way out........

peace

[StevieMac]

Yeah out is leaner.  It just seems to be too rich everywhere with the UNIs.  I guess I'll go back to the velocity stacks and try again.  I'll let you know how it goes.  Thanks guys

[Noel]

'73 500, stacks, Supertrapp with the baffle removed. Stock needle one clip from the bottom. Stock slow jets. Mains drilled to 130. Your airscrews are WAY too far out, IMO. Mine were at 1/2 from closed with the baffle in the pipe, now fully closed and idling slightly lean, with the baffle out. For you to be running stacks, aftermarket pipes, and barely-richer-than-stock mains, something is effed up if you're two turns out and still rich.

ADJUST THE FLOAT HEIGHT! I've never played with one of these bikes that didn't have the floats way out -- and you will get nowhere but the loony bin trying to jet a bike with wonky float heights.

Then, IMO, you need to run some WOT plug chops on clean plugs -- and, also IMO, first gear chops won't cut it. The thing only does 100 MPH in top -- I'd find a freeway or something and have at it.

BTW -- I rarely dick with the needle position. That's just moving the "map" up and down, and is fine tuning: something to do after the fueling is 95% sorted.

Bottom line: You tried a free flowing exhaust and a relatively free-flowing filtration set-up with stock carb settings and got... rich. Which means something's screwed up. My guess is float heights. If that's not the trouble, it may be worth checking ACTUAL jet ID. Just because it SAYS 100 doesn't mean somebody with a drill bit didn't get there first. And as for the 109's that you drilled? Are you sure they're actually 109? If you made a mistake in bit selection, or if you used a power drill and a few passes "just to be sure", you may have 1??s. (No offense, I hope. Just trying to toss out idears.)

[StevieMac]

Much appreciated Noel.  I will report back.....

[StevieMac]

I think I've got it figured out!


It seems the problem was way too much packing jammed around the baffle in the exhaust.  Engine breathing MUCH better now!

[Noel]

Huh. Just when you think you've heard it all...