Honda Rebel 250 Carburetor Re-Jet / Needle Shim - Page 3 - Honda Rebel Forum

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Old 01-21-2011, 01:49 AM   #21
 
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Jetting is a larger or smaller hole, shimming raises the pin out of the hole so more fuel flows.
Shimming makes the carb run richer and makes it more responsive. It is a VERY easy mod that does not take a complete disassembly of the Carburetor.

Hmm I bet you need more information.. BuickGuy, you want to do the Honors?
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Old 01-21-2011, 07:54 AM   #22
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Thank you Mx5 for the vote of confidence. I will try my best.

Quote:
Originally Posted by mysteriouslyraptured View Post
So what is the difference between jetting and shimming?
To best understand each, you'll need to understand what is going on inside the carb.
Simply put, the carbs used on bike are really several different fuel squirtguns blended in to one unit. Each part has its job in the throttle range. The trick is to get the flow of fuel vapor optimized for all throttle positions and all rev ranges. The bike comes from the factory with all these things carefully calculated out, taking in to account airflow through the intake, engine specs (displacement, valve timing, valve opening, compression, exhaust relief) and ehaust velocity through the muffler system.
If you look at the usage of each part of the carb, you start to see where each component does its job. The pilot system is only for idle up to about 1/4 throttle. Changing the pilot jet, pilot air juet or adjusting the pilot screw will alter the mix there.
From 1/4 to about 1/2 throttle the throttle valve (butteryfly, plate) does the job of metering mixing from the pilot system to the needle.
from 1/2 to 3/4 throttle, the needle and its nozzle do the mixing. The taper of the needle determines the mix as does the nozzle.
from 3/4 to WFO (wide full open) the main jet is what is metering fuel.

Changing one or a combination will affect the mixture in those ranges.
To fully tune the carb is to carefully calculate out what is needed in each part and make sure the transitions are smooth.
This can be done by trial and error but it is time consuming and can get costly.
Shimming the needle is done instead of replaceing the needle with a different taper. Replacement is better but not all carbs have the parts available to fully tune for modifications so raising the needle in the nozzle a bit thereby having the taper move sooner can get that part of the tuning close.
Changing a pilot jet will tune that porting of throttle response.
Changing a main will affect the wide open running.

Quote:
Originally Posted by mysteriouslyraptured View Post
What is the difference between their purposes?
With all the stuff I threw at you in the previous treatise, basically each jet, the throttle plate, and the needle/nozzle have their portion of the throttle response to take care of.

Quote:
Originally Posted by mysteriouslyraptured View Post
When should they be added in a mod, or what are the tell tale signs that it is needed?
The signs of improper jetting come in by either seeing too rich a mixture or too lean a mixture.
When to do this if if you have significantly changed either intake airflow, exhaust velocity or altered the cams/valvles/compression.
It is particularly evident when multiple alterations have been made.

With the Rebel, major engine mods aren't that possible (no hot cams, big bore kits, or oversize valves) since there just isn't a call for those aftermarket parts.

The most likely mods will be dropping the airbox and a higher velocity exhaust.
That will lean things out and then carn work would be in order.

Before you travel down that road, consider that there is little to gain in usable power by changing your airbox and exhaust. Really nothing that will make a difference. Jetting(carb tuning) will be hit or miss, with some trial and error. Engine smoothness, fuel economy and longevity will be affected with smoothness and economy being the most notable.


Here are some diagrams that sort of show the first part I talked about.
The cutaway drawing is of a first generation Hitachi carb used on early Viragos but the princples are the same.
Attached Images
File Type: jpg Hitachi.jpg (166.5 KB, 90 views)
File Type: jpg carb_jet_use.jpg (34.3 KB, 58 views)
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Old 01-21-2011, 09:11 AM   #23
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Most informative.
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Old 01-24-2011, 05:19 PM   #24
 
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Yeah, what he said above, da stick in da hole meters the go go juice to the cam thingy to make da wheels go faster, by chain drive, I think.


Great write-up BuickGuy.. and believe it or not, all of what BuickGuy said is true.
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Old 04-09-2011, 10:44 PM   #25
 
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I had to resort to shimming after troubleshooting a problem where the bike would idle but hesitate during acceleration. I tore down the carb, cleaned it out, tried Seafoam, etc. I found this suggestion and went through my hardware bin for my radio control aircraft. I got the prefect washer, slid it on the needle as described. Reassembled and the bike was a whole different animal. It runs like a champ now!
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Old 04-10-2011, 09:01 AM   #26
 
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Many times a modification can disguise and unresolved problem. My bike had the same symptoms until I got the fuel orfice on the low speed emulsion tube completely clear. The orfice (at the bottom of the emulsion tube) was so small it reuired me taking a needle and sharpening the tip with a file to get the obstruction out of the orfice.

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Old 04-10-2011, 10:28 AM   #27
 
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Quote:
Originally Posted by Badger View Post
Many times a modification can disguise and unresolved problem. My bike had the same symptoms until I got the fuel orfice on the low speed emulsion tube completely clear. The orfice (at the bottom of the emulsion tube) was so small it reuired me taking a needle and sharpening the tip with a file to get the obstruction out of the orfice.

regards
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Thanks Badger. I'll check out the emulsion tube as well. I should have qualified my experience with this note that the "stock" exhaust appears to have been modified as it is quite louder than the stock Rebels normally are. That lead me to believe the jetting was an issue. I bought the bike used and also found the air filter element missing which I replaced. I thought this would be enough to resolve the issue but the exhaust mods were still not addressed. Hence the shim.
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Old 04-10-2011, 10:50 AM   #28
 
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To confirm your mod is correct do various speeds and then stop and pull a spark plug to check its color. Sould be a light brown to medium brown with no carbon deposits of any significance.

If you are running too rich it can dilute your oil and cause problems. To lean can cause other problems. Either way you should get it running at the proper mixture, if it is not already doing that.

It took me 3 tries to get my carb right. My mileage ranges from about 70 at Interstate speeds to 85 on my local routes at average 38 MPH.

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Old 04-10-2011, 10:57 AM   #29
 
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Thanks again Badger. Once I get some ride time to fiddle with it, I'll check the plugs.
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Old 06-08-2011, 12:04 AM   #30
 
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I am looking to shim my carb, but don't know how to get clearance to get the needle out. Do I need to pull the carb to do this?!? I read that wasn't the case!
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