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Changing main jets

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jchesshyre
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Changing main jets

Post by jchesshyre on Sun 07 Sep 2014, 9:37 am

My CB is stock except for a Fuel mini end can (baffle in). Although Fuel say it shouldn't require a re-jet, my plugs are consistently white instead of tan. I'd like to try slightly larger main jets - can anyone suggest what kind of increase to try first?
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toffeeroffles
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Re: Changing main jets

Post by toffeeroffles on Wed 10 Sep 2014, 1:26 am

Size number #125 main jet and one (extra, as there is already one)shim washer under the needle. Thats what the pros(racers)recommend. Thats running with a standard air filter, a K&N could probably do with more. HM Racing fitted me with a #126 Dynojet branded jet, equal to a #128 Keihn or pattern one.

It might be worth tinkering with your fuel mixture first though as the coloring on the plug could just be from a weak pilot circuit, closed throttle overrun popping is a symptom of this but CB500s do it alot with an aftermarket can anyway Smile

I cant remember the stock pilot screws turns out off the top of my head, 2.5 maybe but K&Ns start up at around 3.25 turns. Thats where mine was set to.

What airfilter do you use?
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jchesshyre
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Re: Changing main jets

Post by jchesshyre on Thu 11 Sep 2014, 8:17 pm

Thank you very much toffeeroffles, this is a very helpful answer! I use a stock air filter – I own a K&N but find it runs too lean with that and in particular stifles the top end power.

I will try your suggestions when I have some time. Although I've been doing all my own maintenance for the last six years and 150,000 miles (!), I've never opened up a carburettor so I'm a bit daunted by it. HOWEVER, I've just made a very interesting discovery which has pretty much solved the lean running problem, even though I know that adjusting the jetting will probably help even more. I'm going to put a post about it in the General section because it's very interesting... My plugs are now a nice grey-brown.
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jchesshyre
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Re: Changing main jets

Post by jchesshyre on Fri 12 Sep 2014, 2:33 pm

So, I would still like to change the main jet and add a washer under the needle as you suggest. (by the way I've already richened the pilot mixture).

Where is a good place to get these parts? Am I best off getting a Stage 1 Dynojet kit?
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toffeeroffles
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Re: Changing main jets

Post by toffeeroffles on Fri 12 Sep 2014, 4:08 pm

Interesting results with your fuel change. My CB loves the V-power but it is an old '95, they have a slightly better advance curve on the ignition timing so actually may make use of the extra octane. Well I notice the difference anyways.

I would suggest that it only highlighted your lean condition and you would defo benefit from larger jets still.

I wouldnt bother with a dynojet kit of any kits to be honest. Too expensive for what they are and from what I have gathered, they usually get the set up wrong.

I purchased some through AllensPerformance via email. 2x #125 jets and a few pairs of shim washers. All came to under a tenner which is miles better than any kits you may be able to find.

You can also find lots on ebay but most will be pattern and may have varying quality.

You could also try any local dyno tuners and see what parts they carry.

I would also recomend getting yourself a stainless allen bolt set to replace the crappy stock screws for the carbs. It makes tinkering so much easier after the effort getting the carbs out in the first place.

I'll go find ebay links for the bolts.

Edit*
http://www.ebay.co.uk/itm/HONDA-CB500-CBF500-1994-2008-Carburetor-stainless-allen-screw-kit-CB-CBF-500-/190915841626?pt=UK_Motorcycle_Parts_13&hash=item2c73788e5a the kit was'nt perfect. One pair of bolts didnt seem to be the right size or something or I just didnt use a few. Replaces all the fuel bowl and top caps perfectly though.

http://www.allensperformance.co.uk/
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jchesshyre
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Re: Changing main jets

Post by jchesshyre on Fri 12 Sep 2014, 5:16 pm

Thank you thank you thank you!
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jchesshyre
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Re: Changing main jets

Post by jchesshyre on Fri 12 Sep 2014, 5:16 pm

Will report back when I've done it.
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jchesshyre
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Re: Changing main jets

Post by jchesshyre on Fri 12 Sep 2014, 6:03 pm

Are there any other bits, gaskets etc., that I should get before doing the job?
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toffeeroffles
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Re: Changing main jets

Post by toffeeroffles on Sat 13 Sep 2014, 1:22 am

You're quite welcome Smile

I purchased a carb seal kit from ebay. Though I ended up using just the bowl seal in the end, just to be on the safe side, my ones where a bit manked in a few spots and had shrunk a bit. I also used the ones that fit one the needle holder but after fuel got to them the swelled up and made it very difficult to remove, so I went back to the original ones.

You could probably get away with out the kit and save yourself £15. Theres only one way to check them though and that would mean a potential few days waiting for post if it turns out you do need them.

http://www.ebay.co.uk/itm/Honda-CB500-96-02-Carburettor-Carb-Repair-Kit-331628-/151000315053?pt=UK_Motorcycle_Parts_13&hash=item232851f4ad

Looking at the pic, I remember using the one with the tabs too. Its for the stack holders I believe, my ones where also a little old looking and gummed up so I replaced for the hell of spending £15 and only using one pair of seals lol.
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jchesshyre
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Re: Changing main jets

Post by jchesshyre on Mon 15 Sep 2014, 1:39 pm

Cool. Is it possible to do all this with the carbs on the bike, or is it better to take them off?
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toffeeroffles
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Re: Changing main jets

Post by toffeeroffles on Mon 15 Sep 2014, 3:39 pm

You can tinker with the needles and shims with the carbs still in place but to change the main jets you have to pull them out and flip them over. It can be quite a pain.

You have to pop the aibox off the inlets, try to keep the rubbers in place as they have some factory glue on them that helps seal. Don't worry too much if the do pop out, just clean them up and use some sealant when refitting them. You can leave the airbox in situ but it takes a bit of Tetris like wiggling Smile

The choke linkage also needs disconnecting I believe, its been a while but I remember having to do this every time.

The hardest part for me is getting the carbs out of the head side rubbers but its just takes some welly and persistence.

The clamps on my rubbers where also a bit old and manky so I replaced them with some stainless ones.

This is also probably the best time to replace your fuel hose if you where even remotely thinking about it. It can be done with the carbs still in place but it had me struggling for about 45mins and left me with some cuts on my knuckles. Those air/fuel screws are like cheese graters!
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jchesshyre
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Re: Changing main jets

Post by jchesshyre on Tue 29 Sep 2015, 2:47 pm

I never got round to ordering these #125 jets and shim washers, as other things came up like moving house etc...

I've decided now to go ahead and make this adjustment, and it turns out I already have the bits I need. My bike came with a Honda restrictor kit (unfitted) which includes a pair of #125 jets, and I have the carbs from my retired CB from which I can steal the shim washers. So when I have a free weekend I'll be getting the carbs open and making this swap. I'll report back on how it goes.
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Beresford
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Re: Changing main jets

Post by Beresford on Tue 29 Sep 2015, 4:48 pm

The easy way to see whether the bike will run better with richer jets is simply to apply the choke a bit when the bike is warm. That might save a lot of faffing about as changing jets etc is really only of value if the work can be checked on a rolling road. Without that in other words, if there is an improvement it's going to be because of luck not judgement.
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jchesshyre
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Re: Changing main jets

Post by jchesshyre on Tue 29 Sep 2015, 4:56 pm

Does the choke richen the mixture at all throttle openings? I would have thought that turning on the choke for 3/4 to wide-open throttle (the area under consideration here) wouldn't tell you much, because the choke restricts (or chokes) the air intake and so would limit power at these throttle settings in its own way, unless things were seriously lean up there which I don't think they are.

The purpose of this re-jetting, as above, was to compensate for the slightly freer-flowing Fuel exhaust. Fuel say that a re-jet is not required, and of course they don't want to put people off buying their exhausts, but they also say 'you should not need to rejet or remap your bike when fitting one of our exhausts however on the odd occasion, some bike models may require some slight adjustments to the standard set-up. As a general rule, when fitting aftermarket exhausts, a dyno test is recommended and if you do choose to rejet or remap, you will see further power gains and improved throttle response.'


So far I've just been going by the fact that after a hearty ride my plugs are grey-ish, not tan coloured.
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eternally_troubled
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Re: Changing main jets

Post by eternally_troubled on Wed 30 Sep 2015, 12:51 pm

@jchesshyre wrote:Does the choke richen the mixture at all throttle openings?

I think it does. Mainly because I think it isn't a 'choke' but probably an 'enrichment circuit'.

The cable from the lever pulls a plunger in the carbs which (probably) opens an extra air passageway which contains a jet through which more fuel is sucked into the air-stream, richening the mixture (it may be more crude than that - the plunger might just open a hole down to the float bowl through which drops of petrol are sucked).

My slight hesitance is because (thankfully) I haven't had to take the carbs apart on my CB5 (have done on a few other bikes!).

IME a lot of carb bikes have something like this, rather than an actual choke which reduces the air flow.
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jchesshyre
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Re: Changing main jets

Post by jchesshyre on Wed 30 Sep 2015, 4:07 pm

If what is written here is true of our carbs too then the enrichment circuit is only active when the throttle is closed.

I suspect this 'air from the front of the carburettor' is what the sub air filter is for btw.
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eternally_troubled
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Re: Changing main jets

Post by eternally_troubled on Thu 01 Oct 2015, 1:01 pm

@jchesshyre wrote:If what is written here is true of our carbs too then the enrichment circuit is only active when the throttle is closed.

I suspect this 'air from the front of the carburettor' is what the sub air filter is for btw.

I don't quite see what you are getting at - the diagram on that page shows a slide bypass drilling with a plunger to control its opening - the bypass drilling goes from the airbox-side of the carb, past the slide and re-emerges on the output side.

Anyway, yes, the effect of the bypass drilling will drop off pretty quickly once the throttle is open (it is usually pretty small, so the the throttle doesn't have to open much to reduce the pressure across the tube enough to stop it working.). BTW - I hadn't thought of that in my previous answer (I'm not changing my mind - honest! :) )
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jchesshyre
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Re: Changing main jets

Post by jchesshyre on Thu 01 Oct 2015, 1:43 pm

Yes, it's gone off on a tangent a bit. My point is that I don't think turning the choke on and seeing the effect it has on full throttle behaviour is a reliable way of telling whether a slightly larger main jet would be beneficial. 


I'm going to change the stock #122 jets for the #125s I have, and raise the jet needles with an extra shim washer, to compensate for the slightly lean condition I have with my less-restrictive-than-stock Fuel exhaust, as toffeeroffles originally suggested.
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jchesshyre
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Re: Changing main jets

Post by jchesshyre on Sat 17 Oct 2015, 11:11 am

Well, last weekend I finally got round to it: changed the stock #122 main jets for #125s and added an extra washer under the top of each jet needle, to compensate for what I suspected to be slightly lean running caused by my Fuel exhaust. 

I then did Chester to London and back in the same day (around 400 miles total) so had a good chance to see what difference it's made. 

I haven't looked at the plugs yet, but the change seems to be absolutely for the better. The bike's running cooler (the temperature gauge moving a bit too much after spirited riding and then slowing down was one of the things, along with grey spark plug tips, that made me suspect leanness) and it feels torquier at all revs and generally happier. Very, very pleased with this change. Although Fuel say that a rejet is not required it would seem that it is nevertheless beneficial for optimum running (which they do also say). Even better, it's cost me nothing as I nicked the shim washers out of my old bike's carbs and had a set of #125 main jets that were part of the OEM restrictor kit that came with my new bike.
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Celt500
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Re: Changing main jets

Post by Celt500 on Sat 17 Oct 2015, 10:58 pm

How's your fuel economy? Curious to know if it would change noticeably.
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jchesshyre
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Re: Changing main jets

Post by jchesshyre on Sat 17 Oct 2015, 11:33 pm

As expected it's gone down a little bit. But I have also been caning it since I've made the mod because I'm enjoying the torque. So it's hard to say. I've last switched to reserve at 183 and 192 miles whereas I normally see nearly 200. But I'll report back after I've done a longer and slightly more relaxed run!

I'm expecting to see slightly lower oil consumption because of the cooler running at mid to high revs.
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jchesshyre
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Re: Changing main jets

Post by jchesshyre on Fri 30 Oct 2015, 6:52 pm

I can confirm that there's been no noticeable drop in fuel economy since I changed my main jets and shimmed my jet needles. Last two tanks I've switched to reserve at 192 and 199 miles, and that's after fairly short (c. half hour) journeys on A and B roads involving hoonage.

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Re: Changing main jets

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