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Author Topic: Carb questions  (Read 1844 times)

LarsBloodbeard

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Carb questions
« on: October 18, 2012, 07:35:07 PM »
Carb jetting, adjusting, intake, exhaust flow... this whole system has always been not unlike a dark art to me.  Without some way of measuring something, looking it up in a table, etc. I'm at a total loss.

I have a 2002 bullet (Indian), which I have upgraded to 535 with a forged piston and 2mm spacer, and I plan on changing the exhaust from stock to classic.  Also, at some point the air filter was moved from the tool box to that rectangular box.

First question: The carb is a Mikuni 28 (I think stock for this bike).  I ordered a set of jets which are 1 step richer than stock.  Is there any way I can determine, scientifically, what jets I actually need?

Second question:  I live at about 2,000ft, and I regularly ride up to 10,000ft, and down to the beach (0ft).  Would I be better off getting a CV carb?  Or learning to tweak my carb as I ride?

ace.cafe

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Re: Carb questions
« Reply #1 on: October 18, 2012, 11:35:20 PM »
If you want the scientific information, you need to get the Mikuni Carburetor Super-Tuning Guide. It has all the sliding scale info there for temps and altitudes.

I can tell you from experience that if you live at 2000' elevation, and you have a free flowing air filter and free-flowing exhaust system, you will likely need a 27.5 pilot jet, P0 needle jet(Mikarb specific needle jet, Mikuni needle jets don't work in the Mikarb), stock needle, and 127.5 main jet.

When you go up in altitude, you will need to lean it out. When you go to the beach, it will probably be ok at sea level, but maybe a little lean.

A CV carb will be a lot easier to deal with if you change elevations alot.
The Mikuni BS29 CV carb is what the Electra X came with, and probably would work pretty well. I have no info on tuning that carb, though.

BTW, your air filter was always in that rectangular box. They didn't start putting the air filter into the tool boxes until much later.
And the air filter that is inside that rectangular box is about as good flowing as if you had a roll of toilet paper in there. Very poor airflow thru that OEM filter system.

« Last Edit: October 19, 2012, 12:48:58 AM by ace.cafe »
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LarsBloodbeard

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Re: Carb questions
« Reply #2 on: October 19, 2012, 06:58:23 AM »
Interesting... I'll look for that guide.

Just double checked and it says Mikcarb vm28.  I think the jets I ordered are the size you recommend, so that's cool.  There are 2 external adjusters.  One thumbscrew which seems to only restrict how far the slide can go down, and a phillips screw which I assume is mixture.  So to adjust for elevation, I need to adjust the phillips screw?

I thought the filter had been moved because the toolbox appears to be designed to hold an air filter.  The previous owner must have swapped that toolbox in then.  Does anyone know of a free flowing filter that will fit into that rectangular box?  I like the filter being in there (out of sight, plus I like the look of the box).

AgentX

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Re: Carb questions
« Reply #3 on: October 19, 2012, 11:19:55 AM »
The air screw will only change the mixture at really low RPM (idle) when the pilot jet is active; the jet needle (size/position) and main jet are going to control mix as the venturi opens up.  So there's no one-stop shop for adjusting across the board.

Does the Mikarb vm28 also have an interchangeable needle jet?  (The Mikarb vm24 does not, just main and pilot, and that's all I've used.)


Ace, how do you feel about CV carbs in a performance application?  I might experiement with one on my bike when I'm back in the US, since riding in the mountains is my favorite thing to do...  Would probably be willing to sacrifice a tad of crispness at the throttle for a proper mix at all elevations.  Any recommendations on one I might try to replace the standard Fireball 34mm flatslide?

ace.cafe

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Re: Carb questions
« Reply #4 on: October 19, 2012, 11:45:22 AM »
Interesting... I'll look for that guide.

Just double checked and it says Mikcarb vm28.  I think the jets I ordered are the size you recommend, so that's cool.  There are 2 external adjusters.  One thumbscrew which seems to only restrict how far the slide can go down, and a phillips screw which I assume is mixture.  So to adjust for elevation, I need to adjust the phillips screw?

I thought the filter had been moved because the toolbox appears to be designed to hold an air filter.  The previous owner must have swapped that toolbox in then.  Does anyone know of a free flowing filter that will fit into that rectangular box?  I like the filter being in there (out of sight, plus I like the look of the box).

The needle jet is the only jet in the Mikarb which no Mikuni jet will fit. It must come from NField Gear or Hitchcock.

The thumbscrew is the idle speed adjustment screw.
The little screw that's on an angle is the pilot mixture adjustment screw, air-bleed type.

For small changes in elevation, the pilot screw will help to correct starting and idling adjustments.
However, for any larger elevation changes all the jets must be changed to keep correct mixture. Obviously, most people do not do this, and they simply run the bike rich at higher elevations, so that when they come back down to lower elevations, they are okay. As long as you don't bog down or foul plugs during the ride, I suppose that's acceptable. You are going to lose power at elevation anyway, and your compression will be lower, and your timing will be off too, so you just deal with it as best you can.
At 10,000 feet, you'll be lucky to have half the power the bike is supposed to have.
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ace.cafe

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Re: Carb questions
« Reply #5 on: October 19, 2012, 11:54:36 AM »
The air screw will only change the mixture at really low RPM (idle) when the pilot jet is active; the jet needle (size/position) and main jet are going to control mix as the venturi opens up.  So there's no one-stop shop for adjusting across the board.

Does the Mikarb vm28 also have an interchangeable needle jet?  (The Mikarb vm24 does not, just main and pilot, and that's all I've used.)


Ace, how do you feel about CV carbs in a performance application?  I might experiement with one on my bike when I'm back in the US, since riding in the mountains is my favorite thing to do...  Would probably be willing to sacrifice a tad of crispness at the throttle for a proper mix at all elevations.  Any recommendations on one I might try to replace the standard Fireball 34mm flatslide?

Personally, I don't like them in a performance application. But they will perform the necessary actions for handling the large elevation changes. They were used in some popular bikes, and the Mikuni BS32 or BS34 came as OEM equipment on some Yamaha XS models.
I'd probably try to get a BS32 or BS34, if going that direction.
Don't ask me how to jet it. I have never tried one of them.
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LarsBloodbeard

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Re: Carb questions
« Reply #6 on: October 19, 2012, 03:56:45 PM »
For small changes in elevation, the pilot screw will help to correct starting and idling adjustments.
However, for any larger elevation changes all the jets must be changed to keep correct mixture. Obviously, most people do not do this, and they simply run the bike rich at higher elevations, so that when they come back down to lower elevations, they are okay. As long as you don't bog down or foul plugs during the ride, I suppose that's acceptable. You are going to lose power at elevation anyway, and your compression will be lower, and your timing will be off too, so you just deal with it as best you can.
At 10,000 feet, you'll be lucky to have half the power the bike is supposed to have.

I've lived at the base of this mountain all my life, so going from 2k to 8k-10k regularly is simply normal to me.  Would you consider that a major elevation change?  It sounds like I should be switching to a CV carb perhaps... does anyone have experience with the BS29, 32, or 34 on a setup similar to mine?

ace.cafe

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Re: Carb questions
« Reply #7 on: October 19, 2012, 04:35:20 PM »
The Royal Enfield AVL-engined bikes are equipped standard with a clone of the BS29.
Check in the AVL forum with the Electra X owners.

Yes, 2000-10000 feet elevation change is a very severe change, not only for carburetors, but for the whole engine. It's do-able, but you are going to lose a lot of power at that high elevation, and also be much richer than you need to be, due to the thin air.
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noisymilk

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Re: Carb questions
« Reply #8 on: October 19, 2012, 05:48:09 PM »
Just as a note, I will be TRYING to dial in a Keihin CVK32 I've acquired from (I believe) a KLR250 takeoff. This will be driving an 03 Iron barrel with an ACE 535 piston and barrel, stage 1 head, ACE aircan, and free-ish flowing exhaust. I, too, am looking at maintaining stability for large altitude ranges.

Also managed to acquire an old KLR250 throttle, so I don't have to screw around with custom throttle cables.

Soon as the head comes back from Ace, and I get things reassembled, I'll start trying to dial it in. But I am hoping this is the solution I am looking for. Frequent rides from Phoenix to Flagstaff (1200ft to over 7000ft) would make a CV optimal for me, especially as I am not looking at a performance machine, but a strong reliable build.

Anyways, I'll be updating in my meandering thread down in the Campfire section as I go along.

LarsBloodBeard...if you come up with anything successful, please let the community know. We all like to have alternatives. Many minds and all that. Be safe.
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barenekd

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Re: Carb questions
« Reply #9 on: October 19, 2012, 09:12:01 PM »
You need to be talking to the Indians. They commonly ride to elevations higher than 10,000 feet, although it's usually on 350s. Those guys usually changed jets once on the rides to 17,000 feet at about 11,000.
I have ridden several CV carbed bikes to 8000 on the Crest and they do alright. The GB500 worked great up there.
Quite honestly, you are really making too much out of the whole thing.
When you get the carb set right for 2000 feet you really won't have any trouble going up the mountains. My Amal equipped Nortless (Matchless) did fine up there. Yes, you will lose power, but I don't care waht kins of carburetion or FI you have, it's going to lose power. I have never fouled a plug up there in 30 years of riding these mountains. I have had bikes that were jetted too rich at sea level stumble occasionally at high altitudes, but they keep on truckin'.
So don't worry too much about it, just get the jetting right for 2,000 and it will easily handle any of the altitudes around here. If you get up to 10,000, it will probably still be OK, If not, change the main jet. No Biggie.
You guys out there really put to much worrying about the abilities ofthese carbs. If all the parts are working properly, the carbs are to going to go wicky-wack in one session.  And certainly the jetting isn't going to up and stop working if it's been working for months or years. Very few running problems are carb related. Or if they are, they should be quickly found. they are usually found in; clogged idle air holes, crap in the float needle, sunken floats, main or needle jets coming loose, needle clips falling off, and if it's an old slide, worn out. Here can be come secondary spots like the enrichening circuit being worn out, but those usually let you have some early warning.
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LarsBloodbeard

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Re: Carb questions
« Reply #10 on: October 19, 2012, 09:27:43 PM »
I was taking a closer look and most of the spots I frequent are around 8k.  So it might not be that big of a deal.  Still, I'd just like to have the right tool for the job.

barenekd

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Re: Carb questions
« Reply #11 on: October 19, 2012, 09:42:52 PM »
The old slide carbs will work, the CVs will work a bit better. The whole thing is to get either to work right for the engine at your base altitude. The rest will take care of itself.
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coolgoose2

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Re: Carb questions
« Reply #12 on: November 04, 2012, 10:38:08 AM »
Hi

I took my Standard CI-350 to Leh last year from New Delhi via Manali(2050 m), Sarchu(4200 m) and (Tangang La pass 5360 m) to Leh (3400 m) and returned back via Srinagar to Delhi. 

This is what I did. Mine had Mikarb VM-24 carburetor with 90 stock jet.

From Delhi to Manali, it ran in stock jet with no adjustment. On finding the mixture rich at Manali, leaned out the airscrew and rode till Keylong(3350 m).  I dropped the needle of the carburetor at Keylong by one notch and again leaned out the air screw.  Also advanced the timing slightly (removed the timing cover, loosened the screws holding it and adjusted till RPM peaked. Even though I was carrying smaller jets (up to 70 are available in India), never bothered about it as it was running fine; a little rich though. It was never possible to run the bike in full throttle during any stretch.

For safety, changed plug at Sarchu. Changed the needle back to normal at Sonamarg on the return trip (we had come back via Srinagar) and adjusted air screw once again.  As expected, there was definite loss of power. But bike worked with lower/medium throttle openings during the ride.

ride safe

 :)


tooseevee

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Re: Carb questions
« Reply #13 on: November 04, 2012, 02:36:09 PM »
The Royal Enfield AVL-engined bikes are equipped standard with a clone of the BS29.
Check in the AVL forum with the Electra X owners.


            Just for info: My 2008 AVL Classic has the BS-29 & front drum brake. It's not only the Electra-X that used it.
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