Topic: Fueling, pilot jets, and drilling

[Noel]

Apologies if this has been gone over before (ARG! Yet another carb thread!) but I did a thorough search and couldn't find what I'm looking for.

With velocity stacks and a SuperTrapp with 12 disks, my bike was popping and my plugs were bright white. Been working my way up through mains; am at 120 right now. No more popping, plugs are slightly off-white. Will keep working my way up until I've gone too far, so then I'll have a good idea where to compromise.

The remaining problem is popping at idle while warming up. My previous SOHC did not do this and I assume that despite the bike's reputation for cold blood, this is not normal and should be corrected. I believe I have ruled out most problems, as the bike is thoroughly gone through -- all the standard tune up bits. I do have some concern about my kill switch, which seems overly sensitive, and of my coils and HT leads, which are simply old. The bike runs perfectly off idle, and idles perfectly when warm, so I'm thinking that the problem is fueling.

So that's question #1: is this a logical conclusion, or am I making a mistake with my assumptions?

#2: Assuming it IS fueling, I believe I should bump up the pilot jet. Yes or no?

#3: I have had good luck drilling mains, although I do intend to replace the drilled jets with factory ones after I am reasonably comfortable that I know what to get. I have never tried drilling pilots, nor do I know anyone who has. Does anyone have any opinions as to whether drilling pilots is any more difficult or failure-prone than drilling mains?

#4: If I do decide to drill, the only option I have yet come up with is a #77 twist drill, which should take the pilots from 40 to almost 46. Is this a large jump in the world of pilot jets?

#5: (Last one, I promise) I would love to find a source for Keihin pilots. Does anyone know the part numbers for the CB500 pilots? I see several sites that offer Keihin pilots, but there are quite a few variations. Is my best bet to simply measure up my existing pilots and attempt to find the same thing from the parts suppliers?

Sorry for the long-windedness, but I hope I got all the pertinent facts down. Thanks for any help.

[Noel]

Oops, there is one other key point: The popping at idle seems to have dissapeared with the air screws at 1/2 open (that's less than half of the standard setting for the 500). I say "seems" because I have not run it in enough various weather conditions to know for sure.

So I may be asking for trouble by messing with something that, while not exactly perfect, ain't exactly broke either.

Having said that, I want the bike to fuel perfectly. So I guess question #6: is chasing perfection here silly and more trouble than it's worth? Should I let the bike run with air screws at 1/2 turn and the occasional backfire while warming up? Or should I really start chasing after pilot jetting the way I have with mains, and get that airscrew up where it belongs?

[Noel]

Okay, so the long version didn't fly.

Short version: Anyone ever drilled pilots? Did it work? And does anyone know where to buy richer pilots for the CB500?

[scunny]

I think if you are happy drilling your mains go ahead with the pilots. I have used solder to fill in over zealous drilling in my lada carb, don't know if that's any help.

[KB02]

So, let me see if I understand, this: Your air bleed screws are 1/2 a turn out from seated? Is this correct? And with the screws at this position, the popping goes away?

What happens at a full turn out?

With running the stacks, you are letting in allot more air than a stock configuration, so your air screws (logically thinking) can be turned in a lot more than stack to compensate, thus running the stock jet with out a problem. I would say fiddle with the screw before you try drilling anything. You might b closer than you think.

Personally, I never liked drilling jets. They are cheap enough and easy enough to change that I don't see a need to drill and possible screw up the jet, BUT, that's my opinion.

As for your main, I say try one more step up in size and see what that gets you. Sounds like you're pretty close, there.

[Noel]

So, let me see if I understand, this: Your air bleed screws are 1/2 a turn out from seated? Is this correct? And with the screws at this position, the popping goes away?

Correct.

What happens at a full turn out?

Popping at idle, until fully warmed up.

With running the stacks, you are letting in allot more air than a stock configuration, so your air screws (logically thinking) can be turned in a lot more than stack to compensate, thus running the stock jet with out a problem. I would say fiddle with the screw before you try drilling anything. You might b closer than you think.

You may be right. My thinking was/is that the solution to lean running is not less air but rather more fuel. This may be an unimportant distinction at idle, however.

Personally, I never liked drilling jets. They are cheap enough and easy enough to change that I don't see a need to drill and possible screw up the jet, BUT, that's my opinion.

Yes and no, IMO. It's not that cheap to buy half a dozen sets with which to experiment, so I think drilling has its place. At any rate, I haven't yet found a source for pilots -- or at least I don't think I have, as I'm not entirely sure what pilots the 500 needs -- which is why I mention drilling in the first place.

As for your main, I say try one more step up in size and see what that gets you. Sounds like you're pretty close, there.

I simply cannot get my plugs to change colors. Extended idling won't do it. Slogging around town at 3000 RPM won't do it. I'm up to 130 mains now and that hasn't done it. I am beginning to wonder if drilling does become less effective at some point; there is now more "hole" in the jets than "funnel", if that makes any sense. I don't know if it's any different with factory jets. I guess I do need to order a few sets; maybe 120 and 130, and see if I get any more fuel flow. As of now, the bike seem to be telling me that 130 is too lean, and that's a bit hard to swallow.

Thanks for your help.

[crazypj]

drilling pilots is much harder as you have to use much smaller drills (and they break real easy)

[eldar]

Instead of going up in mains, have you tried lowering the clip on the needle? This will make things richer on the mains without having to go way up in jet sizes. I dont know if they go much over 140 on the 500s. i am just assuming your 500 has slots on the needles like the earlier 750s did.

[TwoTired]

With running the stacks, you are letting in allot more air than a stock configuration, so your air screws (logically thinking) can be turned in a lot more than stack to compensate, thus running the stock jet with out a problem.

Slight conceptual problem.  Stacks only flow more air WOT (potentially, anyway)
At the same idle speed, the air flow through carburetors is exactly the same, whether stock air filter or stack.

The reason why fuel mixture changes are needed at this speed, has to do with pressure, not flow.  The fuel orifice (jet) exists on the border between two environments, normal outside atmospheric pressure on one side (the carb float bowl air vents pushing on the fuel level in the carb bowl where the jet is immersed) and the carb throat.  The intake stroke of the engine cylinder, lowers the pressure in the carb throat.  the differential pressure then present at the jet causes the fuel to flow through it in relation to the magnitude of the differential pressure.
Air filtration media presents a resistance to air flow.  Overcoming this resistance and providing the same amount of air (remember same idle RPM), creates a larger pressure differential across the air filter media.  And, this become a deeper barrometric depression inside the carb throat that enables more fuel to flow through a given orifice.  Stock jets are selected to match the restrictive nature of the stock air filter.

Changing the resistance of air filtration media, thus effects idle mixtures provided by the carburetor.

The idle air bleed screw on the Cb500 controls the amount of air premixed with the fuel by the emulsion tubes before enry into the carb throat.  It has a limited range however, as the tip of the screw adjuster is hollow with a bypass hole.  This make is impossible to close off the air completely.  Which is a good thing, as complete closure would eliminate emulsion of the fuel before enry into the carb throat and diminish the extent of atomization of the fuel.  This increases the likelyhood of fouling plugs over prolonged idling periods. 

One more point. 
The carb passageway in the slow circuit between slow jet and carb throat outlet is filled with emusified fuel.  The viscosity of this emulsion changes with the fuel density or mix ratio.  As the throttle is opening, this emulsion is drawn into the engine to enrich the mixture (no accelerator pump).  So, slow jet selection and air screw setting can effect engine pickup response from low speed.

Hope this helps.

[Noel]

drilling pilots is much harder as you have to use much smaller drills (and they break real easy)

Yeah, that occured to me. I took a look at the appropriate sized drill at work, where we use them to to add pull strings to tiny hearing aids that fit completely within the ear canal. The thought of trying to drill metal with those things gives me real pause.

So do you have any first-hand experience with this?

[Noel]

Instead of going up in mains, have you tried lowering the clip on the needle? This will make things richer on the mains without having to go way up in jet sizes. I dont know if they go much over 140 on the 500s. i am just assuming your 500 has slots on the needles like the earlier 750s did.

  They do, and I have. I'm at the second slot from the bottom and plan on moving it all the way down. My impression was that needle height changes affected the curve rather than making things generally richer. Thoughts?

[Noel]

TT, I'm always glad to see your name. Your stuff requires careful reading, but that effort is usually rewarded.

So your post makes me wonder: With the air screws at half a turn and the choke fully on, the bike kicks from cold, first try, every time. Pickup from idle is instant, and better, IMO, than mechanical slide carbs have a right to be. I'm beginning to think that my "problem" is imaginary. What's your opinion?

And as far as being unable to get my plugs to color, I'd be interested in hearing your opinion on that as well. Seems to me that 130s should be showing rich on the plugs -- especially the eights I'm running. Is it really possible that I'm flowing that much air?

[techy5025]

Where does one procure main and pilot jets?

Jim

[Noel]

Mains are available from http://www.crc2onlinecatalog.com/misc_carb_parts.htm#Main%20Jets at least for my bike.

As for pilots, I hope someone can answer that.

[Bodain]

I'm curious. When you say it pops at idel cold.

Are your refering to a pop from the exhaust pipe or a pfff, pfff backfire type of thing in one of the carbs?

[techy5025]

The reason for my question is that I will be using a '72 carb rack on
a '69 bike.  My assumption is that if I select the correct needle valve,
main and pilot jets, they will have the same characteristics as the K0
carbs.  I have the K0 carb set and could swap out the parts but I'm
not sure the K0 needle valve will work in the later carb set. 

I would rather just buy the correct K0 parts.

Jim

[TwoTired]

Mains are available from http://www.crc2onlinecatalog.com/misc_carb_parts.htm#Main%20Jets at least for my bike.

As for pilots, I hope someone can answer that.

Not a definite answer but,
http://www.sudco.com/keihinjets.html

The pilots for the Cb500/550 carbs are 28mm long. Threads are 3.5 mm with a 0.60mm thread pitch.  They look physically similar to N424-22-XXX.
Call sudco and ask if they can work in your carbs.

Cheers,

[Noel]

Thanks TT.

Are your refering to a pop from the exhaust pipe or a pfff, pfff backfire type of thing in one of the carbs?

It's the piff piff thing from the carbs, accompanied by a blast of bluish white gas. When it happens right next to your face you don't like it.

And I think maybe I'm going to not fool with it. The mains still aren't right, but with the air screws at half a turn I get no popping at all. Just to kind of prove something to myself I just walked downstairs, turned on the petcock, closed the choke, screwed down the idle knob, and kicked a stone cold bike into a beautiful, even roar with one try. Everyone should have that kind of trouble, eh?

So I think maybe I'll keep playing with the mains until the plugs are spot on and then call the mechanical side of things finished.

[cben750f0]

Noal, are you running leaded fuel, or unleaded?, i have been running high octane unleaded in my 750, and the only time my plugs get colour on them is if they foul, other than that, i cant tell the difference, apparently its b/c there is no lead to give a colour indication.... ended up at 142's on the mains drillin, b4 i got on here and went back to 125's nd lowered the clip on the needle, i have also found that i get that poping at idle you talk about, and that i get rid of it when i turn the bleed screw in 1/2 a turn, hope this helps...peace

[Noel]

I'm in California. I believe you can get the death penalty for using leaded gasoline here.

Those are interesting observations though. Seems to me the main idea is that you are looking for free carbon deposits on the plugs. Carbon formation wouldn't be dependent upon the presence of lead, I wouldn't think.

At the same time, you could hardly have been lean with 140 mains, eh?

[dusterdude]

I'm in California. I believe you can get the death penalty for using leaded gasoline here.

Those are interesting observations though. Seems to me the main idea is that you are looking for free carbon deposits on the plugs. Carbon formation wouldn't be dependent upon the presence of lead, I wouldn't think.

At the same time, you could hardly have been lean with 140 mains, eh?

i wouldnt be a darn bit surprised

[Bodain]

Thanks TT.

Are your refering to a pop from the exhaust pipe or a pfff, pfff backfire type of thing in one of the carbs?

It's the piff piff thing from the carbs, accompanied by a blast of bluish white gas. When it happens right next to your face you don't like it.

And I think maybe I'm going to not fool with it. The mains still aren't right, but with the air screws at half a turn I get no popping at all. Just to kind of prove something to myself I just walked downstairs, turned on the petcock, closed the choke, screwed down the idle knob, and kicked a stone cold bike into a beautiful, even roar with one try. Everyone should have that kind of trouble, eh?

So I think maybe I'll keep playing with the mains until the plugs are spot on and then call the mechanical side of things finished.

Oh your chasing the Holy Grail..

Take a look at this thread.......http://www.sohc4.us/forums/index.php?topic=6195.0

Or you can search on Holy Grail. <Grin>

I worked the Grail for weeks.  Never did find it.

[Noel]

Heh heh, "Holy Grail". Yeah, that's a good name for it.

The thread was an interesting read. I know the o-rings on my mains aren't as good as they should be. I'm going to order a few sets of new mains tonight and see what that does.

Interesting to read about increasing valve clearances for open pipes. Does anyone else have an opinion on adding a thousandth to both intake and exhaust valves with open pipes?

At any rate, I notice that you did try stopping-down your idle air screws to 1/2 turn. As far as I can tell, that has solved the problem entirely, on my bike. I still have not tried it in weather cooler than about 65 degrees, but so far I haven't had a single pop from the bike since I made that change.

So in a way my Grail quest is sort of silly, as my bike runs perfectly. I'm just obsessing over the fact that my air screws aren't near the factory position and that my plugs won't darken. If I knew less about this stuff I'd be perfectly happy.

[Bodain]

Heh heh, "Holy Grail". Yeah, that's a good name for it.

So in a way my Grail quest is sort of silly, as my bike runs perfectly. I'm just obsessing over the fact that my air screws aren't near the factory position and that my plugs won't darken. If I knew less about this stuff I'd be perfectly happy.

No you wouldn't... <GRIN>   

I offered assistance to a guy, down the road having problems with his CB 750.

His question to me was.... " Are you a turn-key biker?   I think his assumption was. I might have a CB 750, but possibly just bought it running and never touched a wrench.  I can't imagine owning a new or old bike and not wrenching it. The only thing that keeps from being a turn-key biker I guess , is money.
In the last 3 years I've purchased 6 bikes.. The only one running was the 2001 Kawaskai ZRX..

[Bodain]

Heh heh, "Holy Grail". Yeah, that's a good name for it.

The thread was an interesting read. I know the o-rings on my mains aren't as good as they should be. I'm going to order a few sets of new mains tonight and see what that does.

Interesting to read about increasing valve clearances for open pipes. Does anyone else have an opinion on adding a thousandth to both intake and exhaust valves with open pipes?

At any rate, I notice that you did try stopping-down your idle air screws to 1/2 turn. As far as I can tell, that has solved the problem entirely, on my bike. I still have not tried it in weather cooler than about 65 degrees, but so far I haven't had a single pop from the bike since I made that change.

So in a way my Grail quest is sort of silly, as my bike runs perfectly. I'm just obsessing over the fact that my air screws aren't near the factory position and that my plugs won't darken. If I knew less about this stuff I'd be perfectly happy.

Your discussion of air screw settings was on my mind this morning when I was tinkering with the CB 750.

The minor irritating problem is in the commonly known clutch rattle of the 750. It's never noticable except at idle and often makes idle erratic.
Syncing the carbs does a world of good, but I could never get the idle I desired. The manual states we should be able to achieve an idle of 950 rpm.
My idle because of the clutch rattle is at about 1300 to 1400. It's the best I could get to have the bike idle without dying.

It should be noted that stock airbox is absent and I'm using Pods.
Standard #40 Pilot Jet and #120 mains.

I was intrigued by the fact that you turned your air screws all the way in and saw improvement. Whenever we put pods on these things we screw them all up. The manual states the screws should be 1 to 1/25 out on the 750. I was running them at 1 turn out.
I screwed them all the way in and started the bike to let it idle.  Hmmm
I now backed them out one half turn and started the bike again.  This is getting interesting
And finally turned them out 1.5 turns and observed the idle.

Bike idles best with zero turns out... I set them at 1/4 turn out and can now achieve a solid, reliable, smooth idle at 1000 rpm.
I'm gonna go out and do another sync....

Your oddity is a great tip for my situation... thanks.