Author Topic: Motorcycle Carburetor Theory 101  (Read 62368 times)

Offline TonUpSoldier

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Re: Motorcycle Carburetor Theory 101
« Reply #20 on: Nov 29, 2011, 20:10:39 »
CB160 carbs are very simple.  Take teh slide out again and see why it's sticking.  It may be rotated out of position or the needle may not be going into the needle jet correctly.  It should be fairly obvious if you take your time to look.

Also, if you've used a dip type substance to clean the carb (which you should. ALWAYS assume the carb is dirty if youve never cleaned/rebuilt it. And be sure to replace any and all Orings inside...they dryrot, deteriorate from ethanol, etc..they're sealing devices) and didnt rinse it thoroughly, the residue left from the dip will cause the slide to stick. If you left it in the dip to long, depending on the chemical make-up of the dip (the crap they sell now is WEAK SAUCE...stupid EPA) can cause a oxidizing type reaction in the slide bore (cheap alu/zinc alloy...god help you if it ever sits in water for long periods of time...) which will cause stiction. Clean the bore out well with some 000 or higher steel wool...make sure you blow it out well with compressed air or the like dont leave that wool remnants in the bore. Should rectify your problem...
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Offline VJ400

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Re: Motorcycle Carburetor Theory 101
« Reply #21 on: Jan 04, 2012, 18:54:27 »
THis answers sooooo many questions for me. HAHAHA!!!

Offline bajabill

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Re: Motorcycle Carburetor Theory 101
« Reply #22 on: May 02, 2012, 20:04:05 »
looking for carb diaphrams go to jbnindustries  they are just 19.00 each shipping is 4.00 hope this helps all you at dtt

Offline bajabill

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Re: Motorcycle Carburetor Theory 101
« Reply #23 on: May 02, 2012, 20:07:48 »
sorry they are 19.50

Offline sham

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Re: Motorcycle Carburetor Theory 101
« Reply #24 on: Jun 17, 2012, 10:22:43 »
I also came across this article, which deals more with carb tuning, but it seems quiet good to me (I haven't done any of this).


Offline Sonreir

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Re: Motorcycle Carburetor Theory 101
« Reply #25 on: Jan 12, 2013, 18:59:43 »
Topic moved to the Engines section.
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Offline sxecafe

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Re: Motorcycle Carburetor Theory 101
« Reply #26 on: Mar 06, 2013, 13:02:21 »
Ok guys... I'm stumped on something. I've been blasting around on the scoot ('74 CB550) and trying to tune things up. I am running pods, dual 2-1s that have been chopped down. I have stock jet sizes and around 1/2 throttle I'm loading up heavy and can barely pull out of it to get any power back. To me... with those modifications, I would feel like I would need to jet up but I pull the plugs and they look like this:

1 & 4, respectively.

They're black as Wesley Snipes! How could that be? Float levels maybe?

Offline teazer

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Re: Motorcycle Carburetor Theory 101
« Reply #27 on: Mar 06, 2013, 13:52:51 »
Those are black and wet but that is just a place to start from.

How long were they in and how were you riding during that time - hard, nice and easy, redline all the time. How it was ridden is important.

It could be float levels but it could also be a weak ignition on that pair.  I'd start with the points/coils.  Then I'd check float levels and after those are checked/corrected, then I'd look at the air jets, needle jets and needles. But if the problem is just 1-4 and not on 2-3 I suspect weak sparks on that pair.


Offline sxecafe

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Re: Motorcycle Carburetor Theory 101
« Reply #28 on: Mar 06, 2013, 14:15:26 »
I had it rev'd fairly high for about 20 sec and then hit the kill switch. I just pulled 1-4 because they were easiest to get to. What would be wrong with the points coils? I have new Dyna ign and coils.

Offline cqyqte

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Re: Motorcycle Carburetor Theory 101
« Reply #29 on: Mar 06, 2013, 15:09:23 »
Best jetting post I have read.

However, my bike is giving me problems, and I can't figure out where my problem fits in all of this.

When I bought the bike it had sponge-like unifliter pods on each carb of my 82 CB650.
I installed K&N pods instead.

The bike runs pretty good, i have never ridden a bike of this size so I can't be sure. It idles fine, and seems to have good power. I have gotten it up to the ton no problem with more rpms to spare.

HOWEVER, today I finally put insurance on it, and plated it, and got ready to go on a road trip this weekend and I took it out for a ride. It was moderately windy (more wind than this bike has seen in the short time i've had it). It ran great going with the wind. No problem. When I turned around to go against the wind the bike ran like crap. Whenever a gust of wind hit me the bike bogged down so bad I almost had to pull over. I tried running with the choke half way and it seemed to fix the issue and it ran pretty decent.

So what is my issue? I suppose the bike is running lean since the choke helped.
Why does the bike run fine with lots of power as long as there is no wind?
I want to fix this problem fast for the weekend. I might try to tape up the air filter pods to limit some air and see if that helps.
Which jets would I be looking at changing in this case?

I think I'll post this under it's own subject too.


I has to do with the turbulence that pods cause in the throat of the carb.  The stock air box creates a space of quiet air for each carb to draw from.  Wind direction and gusts have little affect on the air in the stock box, however eliminate the stock box and the mouth of the carb is now susceptable to the wind turbulence as the bike travels down the road.  The stock box also had short smooth tubes that connected the carb mouth to the box, which infact lengthened the throat of the carb creating a laminar flow of air in each carb.  Removing this allows turbulence to play havoc with the drawing affect of the air over the jets (main, and pilot jet opening) changing the metering affect, which in turn causes the air fuel mixture to bounce all over the place.

Now that is a lay person explanation and I am sure there will be others that will jump all over what I said with text book explanations.  I am not here to argue with them, they know better and I admit that.  I have tried some ideas that in some way mimock the OEM air box set up yet still appears as if I am running pods or individual air filters, and they have resulted in more stable situations, but not perfect.

I have made myself velocity stacks that extend the carb horn some 2" out of ABS plumbing couplings.  I have flared the inside throat of the ABS fitting to make a smooth transition using my lathe and then made the opposite end large enough to fit over the carb horn tightly.  Then I went to the K&N filter book and found filters with the opening flange large enough to slide over the ABS fitting.  I then place the ABS fitting on to the carb slid the filter over the fitting and tighten the ring clamp down so the filter and the fitting are held tighly on the carb horn.  The filter completely covers the homemade velocity stack and because the inside diameter of the filter is larger that the outside of the ABS coupling air can be draw through all of the filter material, albeit more easily from the part of the filter that extends beyond the end of the ABS coupling.

A severe cross wind will still cause issues but I have quieted down the turbulence affect significantly.  Adding the side covers back onto the bike would also quieten the air around the pods but then if you are going to run with them why not just go back to the stock air box.
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