Trying to rap my head around this.... Jets?

[Roidfingers]

I've read a lot about people going to 38 pilot jets. Especially if they are running modified exhausts. I have a modified exhaust. Very open an relatively no back pressure I would think. ( see pic). I have tried turning out my mixture screw to 2 3/4. Has eliminated some popping and back firing. But in the mean time I get a lot more fuel in the exhaust. Don't want that. So if I were to go to 38 jets from stock, What exactly is that doing? Less fuel at idle? and will the popping come back. I am not that concerned about HP, if I lose 4 or 5, she's fast enough for me already. I really like the look of the exhaust , I just want it to be running right.  Any thoughts will be appreciated. TIA

[98valk]

http://factorypro.com/

"As far as the pilot... Look at it this way...

There are 4 outlet holes for the pilot mixture. ~3-4 at the butterfly and one "downstream" of that (for 75% of the idle mixture).

1 hole is controlled by the mixture screw and with the other 3 or 4, max flow is limited by the size of the pilot jet.

At idle, 1 hole (metered by the fuel screw) and 1 un metered hole are open.

At cruise, when the "butterfly" is just "cracked", all 3-4 unmetered holes + the 1 metered one are uncovered -

So.....

At cruise, you get those 3 unmetered holes + the metered hole... So, at least 75% of the fuel delivered at cruise is limited by the size of the pilot jet.

At idle, you get 100% of the trimmed 1  hole + a butterfly valve-trimmed amount of the other 3-4 metered holes.

Rule of thumb....
If you go 1 size larger or smaller on the size of the pilot jet, you will change the fuel screw ~1.5x richer or leaner to retain the original idle mixture -

Example:
You have a #40 pilot jet installed (with the proper main, needle height and fuel  level already done)  and to get best idle, you are 4.0 turns out (from lightly bottomed out).
Cruise seems lean....

So, I'd expect that I could richen the cruise with 1 size larger (42) pilot jet (size of pj is 75% of cruise mixture) and the "trim" the fuel screw "in" for best mixture for best idle (size of pilot is 25% of idle mixture).

To go back to ~ the same idle mixture, after going from 40 pj/4.0x to the 42........ try 2.5x

40 pj / 4.0x = our reference idle mixture
42 pj / 2.5x = ~ same idle mixture
38 pj /5.5x = ~ same idle mixture (this is a "stretch - after 4.5x, not much changes)

(Using another "rule of thumb", if you have the correct pilot jet, the fuel screw will end up at between 1.5x and 3.0x when set for best idle.)

Marc"  http://factorypro.com/

[Roidfingers]

Yes I read all that already. So if I go 38's , which are larger pilot screws, all I'm really doing is eliminating how far out my mixture screw is. ????? so whats the point???

[Patrick]

Most folks that have gone to the 38s have done so to help eliminate varnish issues if the bike sits for any period.

[WintrSol]

Yes I read all that already. So if I go 38's , which are larger pilot screws, all I'm really doing is eliminating how far out my mixture screw is. ????? so whats the point???

The point is, after adding larger pilot jets, if you adjust the pilot screws to being the idle mixture back to a good idle, the mixture will be richer at any throttle position above idle. Mostly in the first 1/4 open range, but it will still contribute at wider throttle positions, and especially so when going from a more open throttle to just above idle. Your fuel mileage may drop, especially if you usually cruise with the throttle nearly closed; that comes down to riding style, and how much drag your bike has.

[98valk]

Yes I read all that already. So if I go 38's , which are larger pilot screws, all I'm really doing is eliminating how far out my mixture screw is. ????? so whats the point???

exactly. During one of my phone cons with Marc. He said, "it is always what the engine wants"
and using the larger pilot will have an affect throttle smoothness 3/4 to WOT.
u always tune for best Main Jet first and then go from there.

http://www.mikuni.com/tg_backfires_in_exhaust.html
link suggest to also ck ignition system, all connections. ck ohms of each plug wire, the internal resistor sometimes goes bad, very rare though.
  But like I said in another thread u have, if u go really rich on the pilot that will help eliminate the backfire. The larger PJ might help a lot, cause there will be more fuel at idle. Don't go larger than the 38s. I tried 40s because of my air box mods and viking exhaust and the leaness caused by ethanol fuel. MPG and power dropped, and I could fully close the idle mixture screws and there was zero change in the idle. I have 38s, just dyno'd with 4 gas exhaust and it was in the right CO range should be at idle, although a tad to the CO rich side. from 50% to WOT I was pig rich with 120MJ. So I put in 112s runs much much better, and mpg went up. didn't touch anything else. So when I estimated the 120s I was 3 sizes too rich if I remember correctly.
hope this helps.

[Roidfingers]

Ok, read this

1)  When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.


So according to this I go with 38's then reduce mixture screw. That way on decal when it pulls a vacuum the 38's will provide enough extra fuel from open throttle to idle to eliminate dumping fuel to exhaust?

[98valk]

Ok, read this

1)  When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.


So according to this I go with 38's then reduce mixture screw. That way on decal when it pulls a vacuum the 38's will provide enough extra fuel from open throttle to idle to eliminate dumping fuel to exhaust?

yes, will not eliminate but greatly reduce and most likely will be lower volume. and u would be able to open up the mixture screws more if need be.
u can open up the OEM to about 4-4.5 turns might be all u need to do. wouldn't hurt to try.

[Roidfingers]

Ok, I've got a better understanding. Thank you all so much. And from shoptalk, it can be done without pulling rail. So I'm ordering 6 38"s. Cheap. more time then money. Thank you all so much.

[pancho]

Also, keep in mind that any pilot adjustments at idle speed are pretty much meaningless unless you have a good carb sync first. (butterfly resting place/idle air) 

Just a bit of information on this, a carb sync is determined by balancing the intake vacuums, which is determined by the butterfly opening.....  it's value will remain constant weather the cylinder is running or not. You can test this by removing a plug wire while doing a carb sync, if the RPMs stay the same, the flow will be the same.  This tells me that a good sync is the foundation to any mixture adjustments.

[Roidfingers]

Just sync them yesterday. Bons. Had 2 that where 4 out. all at 1 or non now. But will do again after new jets and mixture screw adjustment. So any ideas where I should start at with mixture screws?

[Roidfingers]

So, if you have no vacuum leaks, no exhaust leaks , everything is sealed up tight, and you have backfiring and popping, it is not so much to rich, it's more of the fact that when you throttle back you don't have enough fuel at idle/ throttle back to provide combustion from the amount of vacuum. Hence , 38 jets that will provide the fuel at idle/throttle back to create combustion. ??????

[pancho]

Just sync them yesterday. Bons. Had 2 that where 4 out. all at 1 or non now. But will do again after new jets and mixture screw adjustment. So any ideas where I should start at with mixture screws?

Once you have a good sync, the best idle mixture can be obtained by the old method of adjusting the pilots individually for maximum engine RPM,, (there is a procedure in the manual) however, weather the just off idle mixture is going to be good with your exhaust flow is another story.. all carbs are designed to meet certain flow characteristics.  I think you are just going to have to experiment to see what gives you the best all around results.

I had seen it mentioned in a video by a SS carburetor guy, that you can't fix an exhaust problem with carburetor mods.  I'm experimenting too.

[Roidfingers]

Yea I read that too. Just a pain. Guess that what I get from installing exhaust. LOL. Got my 38's ordered.

[Roidfingers]

Want to thank everyone for there replies. I now have an understanding, in laymans terms. Just keep it simple.  I never read anywhere that its just because the pj wasn't dumping enough fuel to fire the combustion. when its pulling the vacuum on decal. Always thought it was from to much throttle/ gas dumping in / to rich. Now I get it. But lets see if the 38's help first.

[WintrSol]

There is one more thing involved - the air cut-off valve. When the vacuum rises enough, this valve causes the mixture to get richer; although I can't find a detailed description of this carb, I expect it reduces the air flowing into the pilot circuit, hence the name 'air cut-off'.

From the FSM:
Afterburn when engine braking is used
- Lean mixture in slow circuit
- Faulty air cut-off valve
- Faulty pulse secondary air injection (PAIR) system
-- Faulty PAIR control valve
-- Faulty PAIR check valve
-- Clogged hose of the PAIR system
-Faulty ignition system (section 17)

If your bike is 'desmogged', ignore the PAIR stuff.

[Roidfingers]

It's desmoged and didn't have issue with other exhaust so that 10 out of 8

[pancho]

This post and the links in it are a treasure trove of information on some of the carbs circuits.

http://www.valkyrieforum.com/bbs/index.php?topic=40414.0

[pancho]

Hey Roid,, after thinking about this for a minute, I would think that the most effective thing you could do to reduce the problem and keep the exhaust would be to open the pilot screws to 3 1/2 or 4 turns open, and put the emissions system back on if you still have all the pieces. Opening the pilot screws  will address the area that is causing the problem more than going up a step in jet size (although I think they will help) to my thinking. I believe this is what CA was saying also.

Doesn't cost anything to give it a try.

[WintrSol]

It's desmoged and didn't have issue with other exhaust so that 10 out of 8

Often, when more open exhaust is installed in place of stock, complaints of afterburn begin. It is usually because the afterburn was already there, but the stock mufflers were quiet enough that it wasn't heard. Open pipes let all that noise out. Was the 'other exhaust' quieter than your current set?

[Roidfingers]

I had the cobra 6 x 6 with now baffles. Mixture at 2 1/4. installed new exhaust and had popping at decal. turned them to 2 3/4 , it helped but to much fuel in exhaust. Apparentley from what I'm gathering is that with 35 jets, its not enough fuel on decal to provide combustion so it dumps it in exhaust. Because at shut/ idle throttle its only the pilot supplying fuel. There for 38's will help.

[WintrSol]

Don't forget the air cut-off valves; if the diaphragms have hardened or are starting to leak, you would get the same symptoms. The vacuum lines to them could also be failing. Could just be a coincidence that they are failing at about the same time the exhaust was changed.