Keihin CV34 Tuning and Balancing after Rebuild. May Have Broken Air Fuel Needle

ALIVE! Started, sounds rough like 3 of 4 cylinders are firing maybe. Then it revved up to 5k as I went for the box fan. I dashed back and flipped the choke on and it really revved 6-7k. Smoke from down under, shut off. (only happened first couple times)

I removed the vac lines 2016 to troubleshoot. All are well plugged to spec. New K&N Air filter, I still have the old one. Carbs rebuilt but not tuned. From seated position left to right, I spec'd temp on the exhaust around 100, 130, 100, 90 Celsius. The video is of the second run, with much the same effect. Throttle cable was too tight, but that's not the issue. Above 2k rpm it hits another level of fire goes to 6k rpm, and it will not return to idle.



START 2: Bike starts up fairly easy. Throttle cable was too tight. No happy medium, it either dies or wants to fire to 5 or 6k. After dropping throttle I was able to drop choke. No smoke out bottom. Sounds rough like 3 of 4 cylinders. This time temps were
130 120 130 110 by memory. The hotter one got cooler.
Order me if wrong...
1. Pull plugs check fouling?
2. Test floats again on bike.
3. Attempt to balance.
4. Air fuel screws only roughed in from soft bottom out.

!ALSO! ONE of the carbs might still have the broken A/F needle tip from PO in the passageway leading to the throttle valve.

START 3: Ran again with minor adjustment to looser throttle cable and it purred a lil rough at 1k! Gave it gas and it spiked to 6k rpm. Doesn't return after it spikes any of these times and sounds rough. STARTS WITHOUT CHOKE OPEN.
As described above sitting on bike left to right:
93 61 98 90 Celsius

START 4: Adjusted throttle cable. Runs rough 1k rpm, have to hold throttle once a touch warm, okay to 2k rpm then goes haywire and jumps up to 5k 6k rpm. . I know you know what that means, and my searches aren't turning it up.
Now the pipes are more distinct. STARTS WITHOUT CHOKE OPEN
Ride seat left to right
100 50 105 80 Celsius
The 50 corresponds to the carb that had the air fuel needle broken (by PO rebuild ) and down through the passage into the carb butterfly engine side where those 3 little holes emerge on the top of the outlet riiiight where the plate stops. There is a slight impression where it hit on the throttle plate.. I'm thinking it's time to buy a new rack or individual carb for a final rebuild. Is there an efficient way to test this? Open air/fuel screw first?
Thx.



Pulled Spark Plugs
From seated left to right
1. Blackened
2. Cleanest, reads lower temp than others.
3. Browned a bit.
4. Browned a lil.

Start # 6?
Adjusted fuel screws left to right seated on bike:
3/8, 0, 1/8, 1/4 turn in
Temp at shutoff
70, 50, 50, 55 Celsius..

An improvement and it was longer around 2.2k before taking off to 5k 6k rpm. Keep going? Thx.

EDIT: Morning after... reading this article, and here are my notes even though this isn't specifically for CV34. Still searching for the article I found on CV34 rebalance.
www.cycleworld.com/story/bikes/kevin-cam...to-tune-carburetors/
1. Idle screw - general idle from throttle cable
2. Set idle: Air screws (fuel screws on CV34) . My KZ idles semi-well at 1k, so my messing with the screws last night was semi pointless. It takes off at the 2k rpm after some other passage kicks in...
3. Needles on the slides. I think in the rebuild they just screwed in to the plunger, and thus there is no setting of the c-clip to determine height. The article describes this as 1/8 to 1/4 throttle, which may be what takes me to the 2k point and things are good here.
4. Slide - unsure if there are different slides for the CV34, but this and next seems to be where the engine spirals out of control.
5. Checking Mixture on the Needle - unsure where I clipped if there were options on the rebuild kit. I may go dig the old parts out to see. I have to be too rich here. Outta control 5-6krpm,
6 - Needle Jet Tuning - if down to slide needle clip #1 and it could still improve, adjust needle jet. Not here yet.
7. Slow roll tune, Snap response tune, then road testing to see if the Main Jet is proper. I shouldn't need to do any of this adjusting since this is stock.

Alternate tests I might run today is switching the spark plugs of (1&3?) to see if the 50 Celsius exhaust starts firing before chasing other issues. And again, don't forget, I could have a broken fuel needle in carb 2nd from left seated on bike that is still stuck in the passage. Can I borrow a set of carbs?

The spiking idle is most likely an air leak. Did you replace the carb boots?

Pilot screws should be set at 2.5 turns open from lightly seated as a starting point.

Be sure to sync the butterflies or the idle won't be steady.

If you think there may be a broken pilot screw tip in the passage use a light and check. You can spray carb cleaner though the pilot circuit to check as well. If there is a broken tip you can usually push it back out using a pick tool. I've done it a number of times with good success.

Good luck

I swapped spark plugs, temp checked the tubes, and the same one is 50 Celsius - not firing at idle at least. I was thinking of letting it spark up to 5k 6k to see if it gets fuel from another channel after the idle circuit. If it gets hot then, it confirms my fuel needle tip fear maybe? Is the fuel needle an idle circuit?

EDIT - I attempted to check float levels on stand, and it seemed like that carb was a little low. Right under the lip of the bowl by memory, but they were all at the head of the allen bolts, a good 4-5mm under the top, and especially that one around 6mm..

Nessism wrote: The spiking idle is most likely an air leak. Did you replace the carb boots?

Boots replaced and sealed well everywhere except the airbox, which I buggered a little trying to get the old crusty rubber boots back on the 10+ times I've removed the carbs. When done I was going to use a sealant there or try to buy a nice old airbox. Do you think the suction is drawing around those? Spray test me fresh boots? Trying a smoke test with sage. Also vac exhaust rebreather lines removed.. well blocked.
Rule - Buy new rubber boots and carb holders first thing!

Nessism wrote: Pilot screws should be set at 2.5 turns open from lightly seated as a starting point.

Very sure I did so to spec, and they all lined up proper. Notes taken on adjustments.

Nessism wrote: Be sure to sync the butterflies or the idle won't be steady.

Engine-side butterflies balanced really well for opening same time and amount, visually.

Nessism wrote: If you think there may be a broken pilot screw tip in the passage use a light and check. You can spray carb cleaner though the pilot circuit to check as well. If there is a broken tip you can usually push it back out using a pick tool. I've done it a number of times with good success.

This sickens me, as I spent a lot of time trying to get that confirmed before rebuilding. I found my post and the pictures are gone. Refinding the pics for repost. I never got a good answer of where it should come out other than the fuel screw hole..

EDIT: Swapped 2 & 3 spark plug cables after swapping plugs and letting it cool. It ran what seemed a higher idle, same 50 Celsius on the same pipe. This time it revved and returned to idle slowly, so I did what any reasonable person would do and took her for a ride!!! WOOOOO HOOO! Upon returning, the temps were, from seated left to right,
150, 120, 170, 150
Having detuned the fuel screw as mentioned above, copied below, but no adjustments this time.
3/8 in, 0, 1/8 in, 1/4 in (turns)

This result has to mean the carb works fine under load, just missing the feed from the idle circuit and shows a little less activity. If I can get that brass tip out... time for some Brass Monkey! :woohoo:

Unacceptable still, so removing carbs after we move to chase the idle issue. Until then, thank you Nessim, you are a gentleman and a scholar! Look for updates next week.

I can’t help you with your carburetors I don’t know your version. But I can say you should be able to tell if a tip is broken off... that idle screw tip would be blunt. They should be tapered to a sharp point...

And yes the “fuel” needle is part of the idle circuit

Keihin CV34 rebuilt, discovered one of the needles was missing the whole tip from the screw head down, and found it at the throttle butterfly sticking part way through a hole in the ceiling. Opened all circuits, blasted up through, inverted in my lap, and never found the offending tip. It is likely stuck inside the channel still, blocking the idle circuit, or the float is waay off, or both.

I've had float nightmares, and a neighbor bench fidgeted them last. If I remember, I ran my tubes on it and they looked pretty good. My attempt today shows them a little low. Running the test again when I take them off bike again. It runs enough to get it to the new home. Check back, I need all the help I can get!

Here's what I use to push those broken tips out (use the shank end, and hold it with needle nose pliers). One of these will be the right size.

www.harborfreight.com/high-speed-steel-m...set-30-pc-61526.html

ETA: you may need to shorten the bit.

Highway Magician wrote: Here's what I use to push those broken tips out (use the shank end, and hold it with needle nose pliers). One of these will be the right size.

www.harborfreight.com/high-speed-steel-m...set-30-pc-61526.html

ETA: you may need to shorten the bit.

Spiral drill in pliers, shove thru? The tip of fuel needle didn't want to be pulled out by pliers as it stuck thru the hole at throttle valve.

Scored a full body rack... advice here:

Scored a rack of Keihin 34s, then saw a post (edit copied below this paragraph) where there are diffeerent Cv34 carbs... some with differing jets and passages. Any help confirming I'm good here? Nessim's keen hawk eye is always good.

kzrider.com/forum/3-carburetor/613898-81...arb-tuning-questions
"Do you have the same CV34's as me? I found out on here that there are two CV34 models. One with pilot and main jets and one with pilot mains and secondary jets."

www.kzrider.com/forum/3-carburetor/608501-rebuild
Wired George said, "there are two "main jets" in a 1980 Keihin CV34 carb; one is large/fat and if you unscrew this jet, the "emulsion tube" you refer to is screw into the well. Take care unscrewing the primary main jet atop it as they strip easily."

Throttle plates frozen, bout to drip household ammonia to break the cold weld after confirming if ammonia is safe on brass etc. (edit: the jury is out, some stating ammonia makes brass brittle while others say only pre-industrial brass metals are affected.)

QUESTION: Disassembling for cleaning. Should I keep this rack whole and swap my rebuild pieces over, or insert a single new body in my original rack? I am curious how free the throttle line will be after being either cold welded or gas lacquered stuck shut.

Bump. Nessim, your eyes have saved me before. Can anyone confirm Wired George's statement on the 1980 Keihin CV34 carbs being slightly different from other model years of the same CV34? I've compared them visually and they seem the same to me. Nothing to reference on the other years of CV34 carbs.

Maybe they should have named the new one CW34? :dry:

There are 2 types of Keihin CV34.

a) CV34 with 3 jets: pilot #35, primary main #62, secondary main #125, needle N01A
b)CV34 with 3 jets: pilot #35, primary main #62 main #90, needle N10A

Any idea if the two specs were done for near sea level and at altitude cities?  Thoughts?

Replaced the single bad carb tonight. Finishing it up tomorrow!

wrote:

Any idea if the two specs were done for near sea level and at altitude cities?  Thoughts?

Replaced the single bad carb tonight. Finishing it up tomorrow!

Not related to altitude.  Most likely a cost cutting measure.