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CA160 Won't Idle Properly

JimRatti

New Member
Joined
Sep 24, 2020
Total Posts
8
Total likes
0
Location
Middletown, OH, USA
Hello everyone,

Long time lurker/first time poster.

Working on a CA160 (single carb) for a customer. Bike came in with many engine issues due to PO trying to rebuild the top end. Worked through all of that and now have an engine with strong compression, cam correctly timed, valves set to 0.002" intake and 0.003" exhaust. Points gapped at 0.014" and timing set both static and dynamic (timing light) dead on the F mark. Advancer free and working fine. New plugs, condenser and points. Strong spark (for a 160, anyway!).

Carb has been ultrasonically cleaned (three 30 minute cycles) and all jets and passageways checked, double checked and triple checked with compressed air and spray cleaner. Floats set at 20.5mm (spec 19.5 + 1.0mm for ethanol fuel). Floats (brass) are symmetrical and free of holes. Air screw initially set at 1.5 turns out. Reused stock brass (scrupulously clean and in good shape), even though my rebuild kit was Keyster and very high quality. Jet needle is in middle position. Main jet is #95 and pilot jet is #40 (also tried a #38). Carb vent is clear. Carb is correctly assembled with no missing parts per the fiche. O-ring on air screw is new and sealing well. Confirmed no vacuum leaks by spraying brake cleaner on all intake joints while engine was running. Currently running without an air cleaner, but have a NOS filter available to install.

Symptoms are like this: Engine starts immediately. Once warmed up, it revs just fine, but the idle is uneven and misses. Exhaust smells rich to me. Adding choke only makes the problem worse, indicating that mixture is too rich. Tried backing out the air screw to lean out the mixture, but no effect, even if air screw is completely removed. I have to keep blipping (or slightly lifting off idle) to keep engine running. Anything above idle the engine sounds great. No smoke, and immediate, strong revs.

Yes, I double/triple checked the pilot jet air passageways in the carb body (actually, both pilot and main jet passageways).

- Blew air through hole in intake horn to air screw hole, clear
- Blew air into air screw hole and got strong air flow through hole in intake horn, through where pilot jet screws in as well as through the small hole in the base of the carb throat.

I thought the float level might be too high, so I shut off the gas with the engine running and waited. Sure enough, once the fuel got low enough, the engine settled into a nice, smooth idle with no missing. Of course, once the fuel go too low, it'd quit, but I thought this gave me the answer. So, I raised the floats (dropped the fuel level) a few mm. No difference. Tried raising floats some more; still no difference. Finally got to the point that I had the floats so high that when I put the bowl back on, it pushed the floats up, closing the needle valve so no fuel would flow at all.

So I went back to square one and reset the floats to 20.5mm. I then placed a clear battery drain tube into the drain hole in the bottom of the bowl and used it as a sight gauge to see where the fuel level actually was. It came right up to the top of the bowl, just a hair below the body of the carb. That's what I'd expect for proper float height. I shut off the fuel and started the engine. Same symptoms as before. But I watched the fuel level in the bowl and it slowly came down as the engine consumed fuel. When the bowl got to about half-full, the engine suddenly began to idle perfectly. I could rev it up just fine and had good throttle response, at least until the fuel dropped so low the engine wouldn't run anymore. Turning fuel back on got the engine running again, and if I played with the fuel shutoff to keep the fuel level in the middle of the bowl (which seems WAY too low to me) the engine ran great.

So, why would I need an abnormally low fuel level to get the engine to run correctly? What say ye, gurus? I'm not often stumped by carb problems - I do a lot of rebuilds here on way more complex carbs than this and usually have great results. But this one is whipping my butt.

Thanks,
Jim
 
The carb bodies get warped from repeated tightening and distort the slide passage, may seem fine in hand but will drag when mounted and tightened. Filing the ears and that face flat will fix. Suppose it may be possible that the needle not staying seated in slide too.
 
So true! I too have found that to be the case, especially on the inline fours. The bike will idle great but bog down (too lean) when trying to rev up without the air filter in place. They need that little bit of restriction to get the AF ratio just right.

But in this case, I've tried it with and without the stock filter and get the same results. :sad:
 
JimRatti said:
Thanks for the reply. Slide fits nicely in the bore and moves freely, both installed and uninstalled on the bike.
Click to expand...

For future clarity, use the "Reply With Quote" button when you have multiple posts to reply to so each member will know the reply is for them.

Yours is certainly one of the more mysterious problems, and these carbs are so old now that sometimes they have wear or restricted passages that just can't be returned to original functionality anymore.
 
JimRatti said:
. . . I thought the float level might be too high, so I shut off the gas with the engine running and waited. Sure enough, once the fuel got low enough, the engine settled into a nice, smooth idle with no missing. Of course, once the fuel go too low, it'd quit, but I thought this gave me the answer. So, I raised the floats (dropped the fuel level) a few mm. No difference. Tried raising floats some more; still no difference. Finally got to the point that I had the floats so high that when I put the bowl back on, it pushed the floats up, closing the needle valve so no fuel would flow at all.

So I went back to square one and reset the floats to 20.5mm. I then placed a clear battery drain tube into the drain hole in the bottom of the bowl and used it as a sight gauge to see where the fuel level actually was. It came right up to the top of the bowl, just a hair below the body of the carb. That's what I'd expect for proper float height. I shut off the fuel and started the engine. Same symptoms as before. But I watched the fuel level in the bowl and it slowly came down as the engine consumed fuel. When the bowl got to about half-full, the engine suddenly began to idle perfectly. I could rev it up just fine and had good throttle response, at least until the fuel dropped so low the engine wouldn't run anymore. Turning fuel back on got the engine running again, and if I played with the fuel shutoff to keep the fuel level in the middle of the bowl (which seems WAY too low to me) the engine ran great.

So, why would I need an abnormally low fuel level to get the engine to run correctly? What say ye, gurus? I'm not often stumped by carb problems - I do a lot of rebuilds here on way more complex carbs than this and usually have great results. But this one is whipping my butt.

Thanks,
Jim
Click to expand...

The abnormally low fuel level would compensate for an oversize pilot jet. The number stamped on the jet may indicate the correct size, but the passage through the jet may no longer relate to that number. When people clean out blocked jets with old guitar strings, the jet gets enlarged. I'm not saying you did that, but you did mention that you used the old jets. Maybe buy another set from somewhere reputable. In lieu of that, hold your nose and install the Keyster jet that you already have. See what happens.
 
fxray said:
The abnormally low fuel level would compensate for an oversize pilot jet. The number stamped on the jet may indicate the correct size, but the passage through the jet may no longer relate to that number. When people clean out blocked jets with old guitar strings, the jet gets enlarged. I'm not saying you did that, but you did mention that you used the old jets. Maybe buy another set from somewhere reputable. In lieu of that, hold your nose and install the Keyster jet that you already have. See what happens.
Click to expand...
Guitar strings can work ok IF they are smaller than the actual jet sizing. Everyone says the E string but there are several different diameter E strings so you have to be careful which one you use.
The biggest fallacy I see is the "jet cleaners" being sold like this one https://www.ebay.com/itm/1345797302...olVuL02l-CMBP_yf0WC9phtYeZ_ct81Z8J0_a1NUckBhw which is actually an oxy-acetylene torch tip cleaner which resembles a round file. Works fine on dirty torch tips but will definitely bore out a jet.
 
Thanks, fxray. Yes, I thought the jet might be too big too. Stock appears to be a #40, and that's what came out of the old carb. The Keyster kit came with a #38, and it was indeed a smidge smaller than my #40 OEM jet. I tried it, but it didn't make any difference.

And like you, I'm leery of putting wires through jets. I have nearly 100% success using my ultrasonic tank and compressed air afterwards. Sometimes take a few cycles, but it's rare I can't clear one.
 
^^^Well, it is an interesting problem you are dealing with. I hope you share the answer on here once you figure it out. Good luck with it!
 
I was Cleaning jets recently, prior to cleaning someone suggested I use a stand of stranded copper wire. Copper is softer than steel, so less apt to damage or bore out a jet.

Was considering a guitar string, and reconsidered.

Sent from my Pixel 6a using Tapatalk
 
LucasGrams said:
I was Cleaning jets recently, prior to cleaning someone suggested I use a stand of stranded copper wire. Copper is softer than steel, so less apt to damage or bore out a jet.

Was considering a guitar string, and reconsidered.

Sent from my Pixel 6a using Tapatalk
Click to expand...

That's a great idea! Thanks.
Jim
 
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