CV carbs

A

ahm1

New Member
Ok sorry i know this has been done to death but i dont wanna hi jack someone elses thread.
I have a cb400n with mac 2-1 and pod filters (cant run airbox due to frame mods) with vb series keihin cv's

I installed this kit http://www.smebike.com/carburetor,%20parts,%20jets/jetting-kits/honda-cb250400nt-cb7509001100-cbx1000-cx400500-gl400500-pod-filter-jet-kit
Now I have tried jetting the bike and so far i can get up to 3000rpm but no further I have tried jets ranging 108 through to 128 I have clipped the slide springs 2 turns but still cant get full revs my diaphrams are fine my slides move fine but the bastard just wont go. The best Iv had it run is with stock jets 78/108, can sleeves inside the pods but it cut out at 5000rpm.

Now heres where i'm at, the kit came with a air circuit block off for the idle circuit if i remember correctly (under the kidney shaped plastic clip in the top housing) could this be causing me any issues having this blocked.
Should i drill the slides to help it lift.

What else is left to try I know people have these running but I just cant seem to find the balance
 
Was the bike running well before the changes?

Many think the pods and pipes will help the bike run better, but if was running poorly before the change, it may be some other underlying issue that existed before hand.

You are changing jets. Is it running rich ?
Have you checked plug color?

Your carbs an be dirty. If they weren't right before the pods, they still won't be right. Are the float levels right?

You may have a fuel delivery issue. Is the gas filter/strainer clean in the petcock?

It is good practice (this is for others) to make sure the bike is running well before making significant changes to the engine. New pods won't fix a dirty carburetor or a bad spark plug.


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Have you checked timing (with a strobe) and compression? Sufficiently retarded timing will prevent the bike from revving, especially under load.

Additionally, a weak battery will have similar symptoms. Try hooking up some jumper cables to your car's battery (keep the car turned off) and try again.
 
take your carbs and soak them in a mix of 50/50 diesel and gasoline in say a coffee can, let them sit over night, then in the morning take the can outside and set it on fire. Then get some nice direct pull slide carbs and be happy forever. ;D
 
The bike ran real strong with exhaust and airbox its only been since the pods its fallen on its ass, iv cleaned the cabs thoroughly the floats are set at 15.5mm it just bogs really hard at 3000rpm now, i'm starting to think its a vacuum issuse because I'v tried a large range of jets everything is clean it ran ok with sleeves in it, it bogged at 5000rpm but it was a different kind of bog you could tell it was lean.
 
Im not familiar with your carbs so i dont know if yours is adjustable but I've done a lot of reading about my cv mikuni's. i was having some bogging issues that I finally figured out was my petcock but also the midrange is controlled by the needles in most carbs, not the jets.
Refer to this:
y7yhu4am.jpg

I would recommend taking a look at throttle position when your having the issue and going from there. Plug chop if you can to see what direction to shim or move your clips on the needle
 
Also if you haven't already I strongly suggest taking a look at the carb tuning 101 http://www.motorcyclecarbs.com/carbs101.pdf
 
I think I found the issue one of the carb boots has started to delaminate so ill have to wait for a new pair too continue
 
That is A issue but not THE issue
The basic problem is the carbs, still working on a possible 'cure'
BTW, that carb diagram only works for 2 jet mechanical carbs, 3 jet CV are totally different so it's worthless in this context (plus, it's wrong anyway, they almost always are)
 
Yeah it is deffinatly a issue, Is there any other reasons the bike won't get full revs. I'm pretty certain its not a jetting issue because after trying a huge range it still bogs in the same spot just the amount of fuel smell changes. One thing i did do was remove the vacuum port block kit and put some sleeves back in the filters and I gained an extra 1000rpm so i'm thinking its largely a vacuum issue. Also does anyone know of re production or any new intake boots as I cant find any.
 
ahm1 said:
Yeah it is deffinatly a issue, Is there any other reasons the bike won't get full revs. I'm pretty certain its not a jetting issue because after trying a huge range it still bogs in the same spot just the amount of fuel smell changes. One thing i did do was remove the vacuum port block kit and put some sleeves back in the filters and I gained an extra 1000rpm so i'm thinking its largely a vacuum issue. Also does anyone know of re production or any new intake boots as I cant find any.
Click to expand...

It's a carb/jetting problem
I know how the 3 jet carbs work but haven't had anything like enough time on them to make them work reasonable.
It's a major issue that was only 'fixed' on 900 by drilling carb bodies and fitting internal tubing to change fuel curve (CB750/900 DOHC carbs are almost identical to CB/CM 400~450, there's just 4 instead of 2)
 
have a look at this and check your pods

http://cx500forum.com/index.php?/topic/17865-pods-plug-atmospheric-ports/
 
Its deffinatly not the pods as they have no lip. In the carb kit i got 4 rubber bungs to block off the vacuum port in both carbs but then after looking on another forum a guy only had 1 bung installed in each carb and it worked is this a possibility why mine wont rev?
 
if both ports are plugged the slide wont raise

i would remove both of the plugs try it then once you have it revving try them one at a time to see if you get an improvement

i have not had any luck with completely plugging off the ports and dont do it
 
It's the pods ::)
Those carbs just don't like them
There is a lot of misinformation about the later piston CV Kei-Hin carbs.
The kits being sold hold the slides up higher and block off a couple of passages.
I would like to see a dyno print out with EGA readout
If fuel curve is even close I'll be first to admit I was wrong
 
Yeah, i know they don't like pods but i know of a fair few 400 twins running them without issue (also the back of my frames been cut off so i cant get a box back in there). I removed all the plugs because it wouldn't run right any other tricks of getting the slides to raise better. Without filters the slides flutter really bad and won't raise much so i'm assuming its a vac issue (also i've gone through that many jet combinations I highly doubt its that). All the other bikes running them without issue a running k&n style i'm running uni filters would this affect it.
 
A friend of mine has a cb450 that he put filters on and chopped the exhaust. He got a 6sigma kit with jets and 'instructions' on what to do(basically a bunch of internet links). I dont really know if he's gonna follow through with all of it.. Anyways I wouldn't personally go this far without really making sure it wouldn't compromise safety or longevity but part of it said to drill a hole in the slide and chop the springs a bit. Im sure there's someone around hear that could give you the specifics but like I said I personally wouldn't mess with the spring, I would be too worried about the slide sticking open or something related. That being said when I was researching for my cv's I saw a thread on a harley forum that talked about doing the exact same thing. Pretty sure they are completely different carbs. But definitely keep us updated if you end up doing any of that!
 
ahm1 said:
Yeah, i know they don't like pods but i know of a fair few 400 twins running them without issue (also the back of my frames been cut off so i cant get a box back in there). I removed all the plugs because it wouldn't run right any other tricks of getting the slides to raise better. Without filters the slides flutter really bad and won't raise much so i'm assuming its a vac issue (also i've gone through that many jet combinations I highly doubt its that). All the other bikes running them without issue a running k&n style i'm running uni filters would this affect it.
Click to expand...

The people who say they are running without issues are either fooling themselves or have no clue how bike should be running.
You don't want to hear the truth and are determined to believe it isn't the pods causing problem.
That's fine but I worked on CB250/400 twins at Honda dealers when they were released and there is no way to make those carbs work properly without extensive modification, just changing jets WILL NOT WORK
Even drilling and modifying various parts only improves things slightly
If motor is in good condition and producing 'vacuum' the slides must lift.
Have you checked the carb tops are clean internally so pistons can fully lift?
There is a small hole at top of center column to bleed air as piston lifts, it's not blocked?
Nylon 'piston ring' on slide piston is in good shape?
Enlarging slide lift holes makes slide rise faster, cutting and stretching spring decreases working coils to make it stiffer so slide doesn't rise too fast, kinda puts things back where they were before modifiction
Post some pictures of the modifications to carb ports, I'm interested in seeing what people are doing
 
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