CV carbs, when to call it quits and move on

idhedge

idhedge

New Member
78 Cb400t, kickstart only stock, stock internal, pods with "H" manifold intake to provide a common intake and more vacuum. The internals of the vb21's looks like crap, cleaned them but I think the Cadmium coating is damaged. I can get this thing to start first kick but it has little to no throttle response.

When do you trash carbs and look for a different set?
 
What brand of "pods" are you running. If they are cheap then they will likely flow less air then the stock filter. Jetting could be causeing issues you are having.
 
Cheap pods, but I have run it without any filters through the manifold I built, and I can see the slide fluttering. The air mixture screw has almost no effect, but I'm 2 turns out. I'm running 75/110 jets, do you think that's so small it bogs at 1/4 throttle?
 
Went through the carbs again, no changes. Plug chop is showing either lean or cold - maybe a combination. I believe the manifold is not nearly as effective as I wanted it to be. ...so I am going to shim and go big on the pilot and main jets, block the slow jet bleeds as if I am running with just velocity stacks.
 
What exhaust? and what is the H manifold of which you speak?

It is possible that if you have each cylinder pulling through both carbs that the gas velocity is too low to pick up fuel. The jet signal would potentially be much weaker and would require significantly larger jets - but I am speculating based on what I think you are describing :)
 
If you have no idea what you are doing - dont do it.
All my cv carbs run great with pods, as a general rule you raise the needle one notch, go up 2-4 sizes mains when you run pods and free flowing exhausts. Usually no need to change pilots. If still no closer to running right then they are not clean enough.
 
Teazer,

The exhaust is stock, still has the cross pipe. I made the "H" manifold to replace the airbox, not the intake boots. The idea is to smooth the airflow. My theory is I've been humbled by my plan to avoid big jetting changes. what do you think of 95/125?
 

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A wire strand from a wire brush can clean out those pesky little holes in the pilot circuit, emulsification tube, and the inlet jets behind the throttle slide. If the holes are clean, that's what really matters. Check your compression-throttle wide open, valve clearance, timing advance works and cam timing. Battery voltage, can the spark jump 3/4"? I'm not sure if you have points and condensers or CDI.
 
I like to use a bread tie stripped of the coating to clean my dirty holes :D. Rolling the CDI. Ignition stator has different coils than charging on a cb400t. Battery replaced with a capacitor which currently reads 13.2V.
 
Pod filters are letting in too much air. Bike runs better with bigger jets or more restrictive filters. Headlight isn't getting power though, going to have to address my electric issues before I fine tune jetting. By the way, I replaced my battery with a capacitor with 4700 farad, and then I blew the capacitor :(
 
idhedge said:
Pod filters are letting in too much air. Bike runs better with bigger jets or more restrictive filters. Headlight isn't getting power though, going to have to address my electric issues before I fine tune jetting. By the way, I replaced my battery with a capacitor with 4700 farad, and then I blew the capacitor :(
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when i hear POD filter i think of the smaller clamp on filters,most of them have a cone shape
none of them let in too much air ::) they are all very restrictive and will totally kill horsepower on wider throttle openings even IF you can get the carbs tuned to run smoothly they kill power
do yourself a huge favor put the goddam electric starter back on, at least until you get the bike to run right yer gonna wear-out/knacker the kicker gears and you don't even want to have to fix that
 
idhedge said:
I like to use a bread tie stripped of the coating to clean my dirty holes :D. Rolling the CDI. Ignition stator has different coils than charging on a cb400t. Battery replaced with a capacitor which currently reads 13.2V.
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Thats also a really really bad idea..
Scratch those tiny holes just a little bit and you'll mess up the metering.
 
If you've done any research on this site you should know if you have 2 jet or 3 jet carbs.
If 3 jet, about the only thing you can do is get different carbs or put air box back on
There is nothing wrong with CV carbs for a street bike and many late model 'superbikes' used CV carbs before fuel injection was common
You would be way better fitting a battery to get things sorted out then go back to caps when it's working right.
Restricting filters is just totally stupid, if you wanted less performance it's way easier to buy a smaller bike
;D

Scooter trash said:
A wire strand from a wire brush can clean out those pesky little holes in the pilot circuit, emulsification tube, and the inlet jets behind the throttle slide. If the holes are clean, that's what really matters. Check your compression-throttle wide open, valve clearance, timing advance works and cam timing. Battery voltage, can the spark jump 3/4"? I'm not sure if you have points and condensers or CDI.
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Actually, the wire is way too thick for pilot jet opening, it's probably around 32.5~35 which just happens to be 0.325~0.35mm on the Kei-Hin
You need something around 0.0014" diameter max, 0.012" may be better.
I'm pretty sure one of the bypass drilling's is also less than 0.015"?
It has full electronic advance (low speed and high speed trigger coils)
 
datadavid said:
Thats also a really really bad idea..
Scratch those tiny holes just a little bit and you'll mess up the metering.
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nope
nothing wrong at all with carefully poking a wire thru a jet as long as you don't have to force it
and you aint gonna change the metering with a lil tiny scratch,you would need to be forcing a wire and sawing away at it to remove any material ;)
 
cant go back to the airbox, frame is modified. The pod filters let in too much air and didn't allow the proper vacuum for the engine to run correctly. The "restriction" which is cloth around the pod is not much different than a foam filter and the bike runs fine. The crosspipe on the manifold allows the carbs to pull from a common source so it smooths the airflow, takes advantage of the valve pulses, and fits my velocity stacks. I have found that rejetting and moving the needle on these bikes solves some problems, but creates them in other RPM ranges.

When I fix the wiring issues I may revisit carb tuning, or even get the SMEbike kit and block the slow air jet.
 
xb33bsa said:
nope
nothing wrong at all with carefully poking a wire thru a jet as long as you don't have to force it
and you aint gonna change the metering with a lil tiny scratch,you would need to be forcing a wire and sawing away at it to remove any material ;)
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Thats what a lot of people do xb!
And those little suckers are very smooth inside for a reason, to reduce turbulence and optimize flow.
Just trying to say this is not for the ham-fisted.
Now if you tell some guy here to clean his jets with a bit of wire, the next day he's gonna wonder why his bike runs like shit when he scratched half the hole out of the jet trying to 'clean' it..
I put my jets in a bath of redline carb cleaner, then maybe i poke them carefully with something if there still is some goo inside.
Now i know from your posts that you are an excellent mechanic, and maybe you expect everyone to be as good..
But not everyone can clean out a jet w/o destroying it in the process.
 
datadavid said:
Thats what a lot of people do xb!
And those little suckers are very smooth inside for a reason, to reduce turbulence and optimize flow.
Just trying to say this is not for the ham-fisted.
Now if you tell some guy here to clean his jets with a bit of wire, the next day he's gonna wonder why his bike runs like shit when he scratched half the hole out of the jet trying to 'clean' it..
I put my jets in a bath of redline carb cleaner, then maybe i poke them carefully with something if there still is some goo inside.
Now i know from your posts that you are an excellent mechanic, and maybe you expect everyone to be as good..
But not everyone can clean out a jet w/o destroying it in the process.
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hah right you are
gas welding tip cleaners are perfect for the job as they are specifically designed not to cut or remove any brass
 
xb33bsa said:
hah right you are
gas welding tip cleaners are perfect for the job as they are specifically designed not to cut or remove any brass
Click to expand...
They are the bees knees!
As for the hipster crowd here, they can probably find some guitar strings lying around, also pretty useful..
 
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