Open pipes and shimming carb - Honda Rebel Forum
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post #1 of 4 Old 08-29-2019, 01:26 PM Thread Starter
Join Date: May 2019
Posts: 61
Open pipes and shimming carb

I cut up an old banged up set of original pipes a few months ago so I could put some 20" slip on pipes on the end. They sound fine and the bike ran fine during the time.

Last weekend I was given a pair of slash cut open pipes, again long pipes at around 20". I put them on, they took me by surprise. Loud and with a nice deep tone. I figured the longer and more bell shaped the pipes, the deeper the tone. Obviously at high rpm they sound a little more ****ty as I appreciate it's a small twin cylinder.

I ran them for a few days, but I don't think the old 450 twin is so happy with them. I have a feeling she is now running too lean, and checking the plugs yesterday they are pretty white.

If I was to keep these long slash cut open pipes, can I just shim the needles with washers to help richen up the mix? Do I need to also replace the jets too? Was hoping I could get away with just shimming the needle as they are easier to get at from the top.

Can anyone advise what size washers I need so they will fit over the needle and are the correct thickness?

Thank you.

1986 Rebel 450, a little bit bobbed here and there!
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post #2 of 4 Old 08-29-2019, 01:47 PM
Join Date: Aug 2008
Location: Monroe, Louisiana
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This should get you close: Jet Kit Calculator Techincal - Jet kits for Carburetors

Keepin' all the left over parts. Gonna use 'em to build another bike!

'01 & '09 Rebel 250, '06 Ninja 250, '89 VN 750
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post #3 of 4 Old 08-30-2019, 12:45 PM
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Join Date: Mar 2009
Location: Chicago, IL
Posts: 1,059
I put fishtails on and after some trial and error ended up with 130 main jet, 40 idle jet and one shim.

She pops some with hard engine braking, but otherwise good! Plugs have a 'healthy char', no chalkiness.

Oh, I sync'ed the carbs and put in new Honda plugs at the same time. This helped when tuning b/c changing the jets requires carb removal and test rides. Took me three tries to get it right. I can now get my carbs out and in in minutes! Yes, you can shim the needles without removing the carbs, just take off the gas tank and use a small phillips to get the caps off. My magnetic one saved me from losing that screw and helped when placing the innermost ones back) Careful that you put the cap and rubber 'apron' back in place correctly.

Go ahead and do the jets, shims likely will not be enough with that exhaust, your bike will appreciate some more fuel to burn! Keep an eye on those plugs too!
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post #4 of 4 Old 09-03-2019, 11:08 AM
Join Date: Jul 2018
Location: Staten Island, NY
Posts: 11
Hi Bob, the lack of back pressure affects your air-fuel ratio making you run lean so shimming the needle jet (which affects idling) alone won't make that go away - Recently, after getting a set of Jardine slash cuts, I replaced my main jet (108 -> 115) and pilot jet (kept it at 38 but wanted to change out everything at once) in addition to shimming my needle jet. She runs very well now with no popping on deceleration and clean plugs. BTW, I also swapped out my plugs switching to NGK Iridium plugs, part nr. CR6HIX. Regarding the jets: as flitecontrol said you can simply dial up Jet Kit Calculator Techincal - Jet kits for Carburetors and plug in your info. They will recommend a kit. I got mine for $20. A good investment. BTW, pulling the float chamber cover and swapping the jets isn't any more difficult than pulling the pan cover and shimming the needle jet. The hard part for me was getting the carb off the bike. I had to remove the side covers and the side cover mounting bracket, then the air filter. This allowed me to pull back the rubber air filter tube that connects the air filter to the carb - giving me enough clearance to pull the carb off of the manifold bolts. Disconnecting the breather tube hoses from the carb I broke my Y-shaped breather tube joint but got a new one for $10 on partzilla. A cheap lesson! HTH.

2015 Honda Rebel CMX250X
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