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Discussion Starter · #1 ·
I built a drop dead gorgeous CB160 cafe racer and the carburation is driving me nuts. It has a stepped header exhaust with Dunstall style mufflers, slightly raised compression and stock carbs with pods or open (makes no difference).

At idle is is way too rich to the point that air screws adjustment makes no difference. Carbs have been cleaned multiple times in the sonic tank. I have sprayed cleaner through all the drillings and compared the results with spare carbs.

The exhaust have a large diameter perforated tube inside a megaphone shaped outer shell and a flat piece of perf plate half way down the main tube to deflect sound waves and attenuate the sound. The mufflers have an awesome, mellow sound but are much larger diameter than teh headers so I made up some stepped adapters to make it all work.

End result is that the bike starts on the first push of the button without any choke and as it warms up it gets wet and soggy and revs drop because it is so rich and the plugs are black and wet.

Normally I would try smaller pilot jets but not surprisingly, jets for these old bikes are not readily available in any size especially smaller than stock. I am pretty sure it would run without the airscrews so that leaves me limited ways to lean it out.

I will remove the center plate obstruction in the muffler but that's unlikely to make much difference and I could possibly drill larger air holes in the emulsion tube part of the pilot jet or I could throw the carbs away and buy a pair of VM22 Mikuni carbs (or copies) and jet from scratch for each circuit.

I have boxes of larger Keihin carbs and stock carbs and jets for Mikuni and Keihn carbs but I think I am running out of ideas.

Any ideas?
 

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Discussion Starter · #3 ·
Thanks.

They are 20mm Keihin carbs. Stock are #38 IIRC and I have some that measure .35mm and are marked #35 as you might expect. Some of the stock jets are way larger than spec, presumably from POs cleaning them out with a blunt nail. CB175 (PW18 Keihins) use a similar carb with different air bleed holes.

If I end up buying a pair of VM20 or VM22 Minunis, I may take you up on that offer. Larger carbs might lean things out a bit with lower gas velocity, but at idle it shouldn't make much of a difference.

I do have sets of PW24 from a CL350, PW26 from a CB77 and a complete set of "PW28s" from a CB750K0 but what I really need. I tell you that what I really really want are a pair of PW22 from a CB72 and I gave a pile of those away when I sold my last CB72.
I have never needed smaller than stock jets before. This is frustrating.

I wonder if I have an intake reversion issue. maybe making the intake longer might help.
 

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You should be able to find the jets you need on Jets R Us or Treatland.tv.

I ran chinese knockoff PE24s on my CL175 with no issues other than the cheapo pe carbs used pw jets. The jets cost more than the carbs.
 

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Discussion Starter · #5 ·
Thanks Al,

I have teh smallest (#35) from JetsR Us but I didn't think about Treatland. I did order a pair of cheap, imitation Mikuni VM22's off ebay that the seller claimed used genuine Mikuni jets, so I asked them to confirm that and all of a sudden they cancelled the order. Not a surprise.

I didn't consider a pair of knockoff Keihins though. Interesting idea.

We ran Cb77 26mm keihins on our CB160 AHRMA race bike and Mikuni VM26s on the 240cc CB160 without any real issues. The 175cc Cb160 made 21 rear wheel HP on a good day. Fun little bikes for sure.
 

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Lower the float height in the bowls. What is likely happening is the fuel level in the bowls is too high, and flooding the engine from overflow.

OR, your float needle & seat may not be sealing good enough, and not shutting the flow off, and overflowing and flooding...
 

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I had an issue with jetting a cb160 I put together sometime back. What I ended up doing to get things working was turning some velocity stacks that took pod filters. If I recall I made the stacks the same length of the stacks set into in the stock filters. These were not funnels but long and narrow just flared at the ends. The outside had a step to take a pod on each. Shit I think a piece of pvc might even fit on them
 

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Tin is correct, that 6Sygma will likely have jets for it. I don't think jetting is your current issue as of yet. Jetting will not flood you out at idle, unless someone did some really silly shit in there.
 

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Discussion Starter · #13 ·
Thanks for the input guys. Checked and adjusted fuel level to around 3mm below the gasket surface. Carbs have been in the ultrasonic tank and blown clear.

6Sigma sent me a kit and none of their jets fit and all they sent were pilot jets up one size and two sizes of main jets plus needle shims. needle jets were quite different to OEM and the wrong thread diameter. Main jets were ISO threads (1.0mm pitch) which are fine for a Cb750 but not so much for older bikes with JIS 0.8mm threads. So that was a bust. Not blaming them or bad mouthing them. There can't be a lot of demand for CB160 jet kits and who knows what "stock" bike they had to set up the kit.

Oddly enough I used a pair of the same OEM 20mm Keihins on a budget race bike years ago and they worked fine. No issues at all and it ran fine from low down to the very top end. Not as fast as our 175 motor with 26mm crabs but faster out of bends.

I tried different filters and no filters and long intake tubes (radiator hose) and nothing is making much difference yet. I know a couple of guys used long straight inlets on their AHRMA bikes but I used CB750 inlet stacks on 26mm carbs and they were fine when they were dialed in.

Next trick is to drill out the air bleeds from 0.7mm to 0.8mm and see if that works and if not, I will block a pilot jet off completely to see if it still runs at small throttle opening. It shouldn't run with a blocked pilot jet so let's see what that does. I suppose I could fit a wideband bung and wire it up to read the air:fuel ration but with black/wet plugs and the stink of unburned fuel in the exhaust I don't think that's necessary.

From 1/4 throttle up it runs fine and sounds mean AF. it's just too rich at idle. It's not oil as far as I can tell. Compression is OK on both sides, sparks come form a new coil and dyna. I could try more initial advance and see if giving it more time to burn cleans it out. That's an easy change.
 

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Have you checked the timing? I had an issue with fouling plugs and rough starts and it turned out my timing
was too retarded.
 

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Discussion Starter · #15 ·
Funny you should say that Al. That's the next thing I will check - before I drill out the air bleeds in a set of JetsRus #35 pilot jets. I have small diameter drill bits and if I manage not to break one, I can open up the air bleed holes from 0.6mm to 0.7 or 0.8 - all the way to 1.0mm if I have to but timing is next on the list.
 

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Just make sure EVERYTHING else is correct, before making irreversible changes like drilling. I am still betting that the lower float heights will be an improvement. If so, but still rich, lower them some more.
I am assuming you do not have a manual with all the correct specs?
I think I saw in another thread you mentioned you have one from another country, which may have different specs, and could even be completely different carbs.
One other thought, you may not even have stock carb for that bike on it now?
 

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Discussion Starter · #17 ·
The timing was more or less spot on the timing marks at around 5 degrees BTDC but there's some free play in the automatic A/R, so I will tighten the springs slightly and reassemble that and see if it makes any difference. Spark form the Dyna ignition and new coils and wiring is really strong, so that's not the issue.
Onward and upwards as they say
 

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Have you verified the slides are opening and closing properly? I suspect you have a slide that is sticking open and flooding the cylinder. Are these cv carbs? If so inspect the diaphragms for pinholes or tears, make sure the needle isn't bent or binding, etc. Etc. Make sure someone didn't shim the needles so high that they fail to seal with the throttle closed. Make sure the choke linkage is adjusted properly and the butterflies are not covering the pilot circuit with choke off. Lastly find out if there are supposed to be plugs under the idle jets that prevent them from drawing raw fuel from the bowls (keihin is famous for this bullshit and they fall out and are easily lost when the previous owner is "rebuilding" the carbs....)
 

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Discussion Starter · #19 ·
Carbs are stock and I already tried replacing the bodies with a spare set to no avail. They are simple carbs. No hidden passages or missing plugs. Slides are opening perfectly well and are more or less synched. They don't have vacuum connections and are too small to use with the old Edelbrock synch tool.

One thing I tested last week was to spary WD40 into the air jet inlet and turned the air screw in and out and it made a difference to flow out of the pilot jet outlet. Then I tried replacing teh stock jet with a drilled pilot jet (air bleed holes enlarged) and that increased flow as expected. I will try them in the bike next week when I have some time.

Some early keihin carbs had a small slot/notch in the front face of the slide and I might even try that next.

Thanks again for the input.
 

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They should have vacuum ports in the top caps. Yours don't? That's where you would connect your vacuum sync tool. But... I don't think that sync is your issue. I still think you need to lower your float height if you are running rich on idle. Are you measuring their height properly? Or just eyeballing them? This is very important, as it IS the main factor in the idle to just off idle mixture adjustment. The screws are just for fine tuning the fuel input on idle.
 
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