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Serowbot's carb tuning tips for beginners... (Read 4624 times)
Serowbot
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Serowbot's carb tuning tips for beginners...
06/28/11 at 00:31:17
 
Note: Please post questions in the Rubberside or PM me...

FWIW... Huh...
Lesson 1... What and why of jetting....

First off,... jetting is never perfect... it's a matter of getting a good average... Because perfect jetting is only perfect for the precise conditions that you are jetted for,.. and this changes hour by hour, day by day, and mile by mile...
What you are looking for, is a good average...

Bigger jets won't make your bike faster... the correct size jets will...

Exhaust pipes will go gold first, then blue, then blue black.  It means hot, hotter, hotter.  Running lean, or rich, can cause it,... so can excessive idling, heavy traffic, or just plain hot weather...  Our big air-cooled thumper makes blue pipes easy...
Once a pipe is blued,.. it won't go away by changing jetting... until the blue is removed with Blue Job or other product, it's there to stay,... so jetting by pipe color is not practical...
If this condition is new to your bike it may be environmental, or may indicate a blockage in a jet...

If you modify your bike in a way that increases or decreases breathing... intake and/or exhaust,... jetting needs will change, and should be adjusted...
It's just common sense,... if you let more air in, more fuel is needed to keep the balance...
How much,.. depends on the how, and muchness,.. of the changes you made...  
Undecided...

In general...

Increased breathing needs bigger jetting, decreased breathing needs smaller jetting...

Then,... it's up to where you live and ride...

thinner air = less oxygen = go leaner
hotter  air = go leaner
more humidity = go leaner
higher altitude = go leaner

thicker air = more oxygen = go richer
cooler air = go richer
less humid = go richer
lower altitude = go richer

Barometric pressure effects jetting in a big way, humidity, air temp, and elevation.  Some places have extreme variations,... and if you live there, your pipe will blue.  I live in the high desert and temps can change 40 degrees f in a couple hours, humidity can vary wildly too.  No way to jet for that.  
Gold and blue are pretty!

A good indicator of overall jetting is the weather...if popping/backfiring increases on humid days, or at higher elevation, that indicates rich....less popping at those times indicates lean.

Drill out the brass plug on the idle mix screw... adjust for smooth and steady idle... if it needs more than 3 turns out, go one step larger on the pilot jet.  The pilot jet will have an effect on MPG's... If you want good gas mileage, don't get carried away there...
CW, in, is going leaner,... CCW, out, is going richer...

The needle jet is adjusted by varying the spacer thickness under the plate that mounts the needle to the slide... this has the biggest effect on acceleration and in the midrange area... Say 60+ mph cruising, and half throttle acceleration...

Quote:
General, seat of pants tuning...
Idle speed...
Set your idle speed with knurled screw on the left side, near the enrichment knob... Set it to be smooth and steady,... not loping or stumbling, (this ain't a Harley, don't try to make it sound like one).. (a slow loping idle is very bad for your engine)...
Idle mix... and pilot jet...
If you can get the smoothest/ highest idle speed, by setting the idle mix screw between 1 and 3 turns out from closed,... you have set your idle mix, and your pilot jet is the right size... (if you have to go more than 3 turns out,.. go up one step on pilot jet)..(if it seems to make very little difference turning the screw, you are probably rich on the pilot, and can go down a step)...
Main jet...
To test the main, accelerate from 30 or 40 mph, in 4th or 5th gear at full throttle for several seconds, then reduce throttle by about 1/8,...if power increases for a second, you are lean on the main jet.  Go up one jet size and test again.
Needle jet...
If your main is good, and your pilot is good,... but you have surging at steady low speed... try pulling out the enrichment knob, one notch (do this with a fully warmed up engine)... if you get some acceleration, set your needle up a notch, or reduce the spacer...
... if it bogs instead of accelerating,.. you are either rich, or good... increase the spacer, or set the needle down a notch, until surging goes away, but you still get some bogging with the enrichment valve pulled out a notch...

There are other ways to test jetting, but this way is pretty simple and seems to get me close... for seat of pants tuning...
Huh...

(keep in mind,.. that changing the pilot, affects the needle and the main,.. and changing the needle setting, affects the main,...
Jets are additive...  (the pilot doesn't stop when the needle starts, and when you are up to the main, the needle and pilot are still serving up fuel)... getting them all balanced takes some experimentation...
Jets are incrementally additive.... starting with the idle mix, then pilot, then needle, then main...  Each jet adds it's volume to the next in line...  Wide open throttle is accessing all of them, so changes in one, adds to next and the next..


Quote:
Noobie tip...  keep your idle speed up,... even though you may think it sounds really cool, the oil pump will not operate well at lower rpm's than 800 to 1000 rpm...  you can cook your cam bearings with too low of an idle...
The idle speed is adjusted with a knurled knob on the left side near the choke knob...


Quote:
A little explanation of how a carburetor works...
A carb is a vacuum operated fuel delivery device... Unlike fuel injection, a carb only works when when your engine sucks on it...
That is engine vacuum...
When the piston in your engine goes down, it sucks fuel from the carb,... the piston goes up, and it blows out of the muffler. Between the sucks and blows, a spark happens that ignites the fuel in your engine, and that explosion makes the piston go up and down.  
The engine sucking on the carb, pulls fuel from the jets...
By turning the throttle, you open a valve in the carb that lets more suck from the engine into the carb... as the valve opens more and more, it accesses more jets. Idle, pilot, needle, and main... Increasingly more fuel,... for a bigger explosion in the engine,... and the piston moves faster, making your bike go faster.
Close the throttle, and the engine pulls less gas from the carb, and you slow down.
By changing jet sizes,.. you are adjusting the amount of fuel that can go into the engine at different speeds... Bigger jet numbers, have a bigger hole in them for the fuel to pass through...


Quote:
Thinking has evolved on this topic... there used to be a lot of advice on eliminating popping and afterfires with jetting,... but that is jetting for a closed throttle...
Under what conditions popping and afterfire occurs, can be used as an indicator of which way your jetting is off,... but you should jet for performance and economy, not quiet deceleration...

Unless, popping is so annoying to you that you will tolerate a bad running bike...

Decel noise is inherent to big single engines... so is a poof at shut-off (it's just unspent fuel from the last engine cycle igniting in the pipe)...
With practice you can learn to control these noises with your riding technique,... it can even be useful in traffic for alerting drivers of your position...

( Grin )...

Jet your bike to run good... not to stop good... Wink...

Ride safe... Wink...

more specific tips follow below...
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« Last Edit: 02/15/15 at 09:59:09 by Serowbot »  

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Re: Serowbot's carb jetting tips for beginners...
Reply #1 - 06/28/11 at 01:56:38
 
Lesson 2...  Where and how of jetting...

The following pics are borrowed/stolen from Essforty, and Savage Wahine's excellent instructional posts on idle mix adjustment and carb cleaning...
Find them in the tech section...

Quote:
Noobie tip... Take care and pay attention to the removal of every screw. From the factory, these can sometimes be boogers to remove.  
The whole key to working on carbs is removing each screw, cleanly, without damage... So don't rush...
Removing screws is not getting to the job,... it is the job...
Wink...



To adjust the idle mixture, the brass plug must be removed...
Essforty has excellent instructions for this... find them...
Here...


... then you can adjust... turn full in CW... then count turns out CCW...
CW, in is lean, or less fuel...  CCW, is going richer, or more fuel...


The slide is under the top cover of the carb...


... the spacer is the white thingy on the big end of the needle...
making the spacer smaller will increase fuel flow at high vacuum,.. mid-throttle... the stock spacer is 0.10"...


the pilot and main jets are accessed from the bottom of the carb, inside the float bowl...
The main jet has a brass washer on it... don't lose it... you need it...



Couple more pics... and a schematic...
... just 'cause they're great pics... Wink...

left side of carb...


View of the jets with the floats in place...








Feeling lost?...  
You are here.....----->... Huh...


keep going,... there's one more lesson... Wink...
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« Last Edit: 08/01/11 at 23:39:47 by Serowbot »  

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Re: Serowbot's carb tuning tips for beginners...
Reply #2 - 06/28/11 at 10:28:37
 
Lesson 3... Jetting a Savage... (blah, blah, blah,... it's about dang time!...)... Huh...

Now,.. for some specifics on the Savage and it's carb...

The Savage uses a round slide Mikuni BS40 carburetor...
Mikuni is a very popular carb, and jets are widely available at most bike shops and sites...  
The BS40 uses a N100.604 large round Main jet ,...
The pilot is,... N151.067 Pilot Jet...

Mikuni jets are numbered in increments of 2 1/2... (ie,... 145, 147.5, 150, 152.5, 155, etc...) These are refered to as steps...
One step, is 145 to 147.5,... two steps would be 145 to 150... and so on...
Each step up, richens the fuel ratio...

The stock bike, in it's current US model configuration, has a #145 main jet, and a #52.5 pilot, with a 0.1" thick nylon spacer under the slide needle...
As it is... the stock bike is jetted slightly lean (smallish jets) for sea level, where it is emission tested for importation...
The stock jetting is pretty good for elevations in the 2500ft to 5000ft range...  
On average, one step smaller in jet size every 3000ft elevation... That's a very variable rule... Temps and humidity make a difference too...
One step larger from stock for sea level, maybe two if it's a very humid location...

Here's a chart of the jetting that comes from the factory,... 95' to present 2011 is the same...



As a basic starting point for mods...
The most common mod to better breathing on the Savage, is to swap to a free flowing air filter (foam or k/n fabric), and install a Harley Shorty muffler...
With this mod, you are allowing the engine to breath in and out a little better, ... and it will in turn, want a corresponding increase in fuel delivery to match the extra air...
If your bike was jetted fairly well in stock configuration for your location,... adding these two mods will want about a two step increase in the overall jetting...
From stock,.. say, from a #145 to a #150 main,... from a #52.5 to a #55 pilot (although,. I did pretty well by actually going down to a #50 pilot... but, I'm at 2600ft) with a little more opening on the idle mix screw (1/2 to 1 turn more)... and a reduced spacer thickness on the needle slide from 0.1" to .066" or 0.50"... (you can sand the stock one down, or buy #4 nylon or brass washers to vary the thickness)...

Note#...#4 Teflon washer from Ace hardware is exactly 2/3 thickness of the stock spacer and will fit perfect.  It comes in white or black.  It is just what you want.  One and only one.  It will measure .066",... stock is .1"... that''s the standard starting point for a Harley muffler w/ free flow air filter....

An even more open pipe, or a cam with more lift, or other more extreme mods,... will require more jetting.

The basic tuning range will go from...
#125 to #160 with the main...
#47.5  to #60 with the pilot...
... and from 0.1" to no spacer at all on the needle jet...

Info on choosing a Harley muffler is available here...
Harley muffler guide...




I hope this info somehow clears things up, more so, than adding to the confusion..
It's not a cut and dry process...  and many experts will disagree with each other...  
... and I'm not an expert... but I have some experience...

So,.. now you know what I know... Huh...

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« Last Edit: 08/25/14 at 08:55:15 by Serowbot »  

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Re: Serowbot's carb tuning tips for beginners...
Reply #3 - 07/07/11 at 23:10:51
 
IT WORKED!! Thanks a ton Serowbot! I pulled out the carb for about the 6th time through this process and took that stupid dynojet kit out and made my setup pretty close to what you had posted. Just in case anyone else has the same high elevation issues here's my specs and what I did...

Ride at about 5,000 ft elevation, 2007 suzuki with HD screamin eagle pipe and K&N air filter...

I swapped out the original 145 main jet for a 150 mikuni, none of this dynojet crap, swapped out the original 52.5 pilot for 55, and took off the original white spacer and added a #4 nylon spacer from ace hardware...

At first since my air/fuel mixture was set to the dynojet it was all screwed up and wouldn't even idle, but after turning the screw all the way back in, then backing it out about 3/4 of a turn, BAM, we're in business...

I still have the noticeable power improvement from before, but now the backfiring is gone. None at high revs, none at downshifting, and hey!, not even any last little poof when I turn it off. Smiley

For anyone else at about this elevation this setup worked like a charm for me when nothing else did.
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Re: Serowbot's carb tuning tips for beginners...
Reply #4 - 05/12/12 at 08:57:44
 
I want to note that the $5 impact screwdriver from Harbor Freight took the two screws from the needle plate out without any trouble at all.  It even comes with a long Philips bit that bites those screws perfectly.  No impact necessary.
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