Old 15th September 2019, 01:07 PM #1
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Honda CV Carb - anybody know there way round them ?

Long story - kind of stuck on getting the Cb450 running right. She's a '72 bike;, with the original Keihin CV Carbs. Complex bloody things - made worse by 45 years of wear and tear. I've stripped the carbs and cleaned them - many times - and rebuilt them with new gaskets and jet kits.

Where they're at currently is the bike starts and runs ok, pulls really well from the midrange and on up - but - the air mixture screw still isn't working properly on one side. Makes it hard to get a smooth idle, and to me, the bike feels lumpy and generally unhappy pulling off idle. Not the worst problem in the world, but it's bugging me that it's not right,

According to the manual:

"Next, adjust your “air/mixture screws”. Usually, 3/4 turn out from full in will be a good place to start. Adjust back and forth for maximum rpm, on each carb. If you end up at some extreme, like all the way in or 3 or 4 turns out, you have a problem somewhere in the carb - probably indicates a good cleaning is in order. These carbs must be kept scrupulously clean to work properly."

One side works as described above - but on the other, the screw does bugger all until it's almost all the way home, and then you get maximum rpm when it's fully closed.

I've swapped the needles around, put the old ones back in, tested the damn thing for air leaks by spraying carb cleaner on the outside, but can't make any headway at all - the screw on that side just doesn't seem to be adjust the mixture the way it should.

Anybody any pointers or steers on anything else to try from here ?

Last edited by -alan-; 15th September 2019 at 01:09 PM.
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Old 15th September 2019, 04:30 PM #2
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Re: Honda CV Carb - anybody know there way round them ?

Get them in an ultrasonic cleaner,you need to put the entire carb bodies in and it needs to be done correctly,degassed etc.

I did these 2 weeks ago,from an Italian v8....they had been to various places for rebuilding etc and it still didnt run right,30 mins at 80 degrees and they now function the way they should,plus they are minty clean




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Old 15th September 2019, 08:28 PM #3
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Re: Honda CV Carb - anybody know there way round them ?

I would start by making sure that the valve clearance’s are correct also do a compression test.The 3/4 turn out is usually too lean as they tried to pass emission tests in the US.
As response above on the ultrasonic cleaning.
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Old 15th September 2019, 08:37 PM #4
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Re: Honda CV Carb - anybody know there way round them ?

Thanks guys.

I've ultrasonically cleaned the damn things twice at this stage, but just with a little home-cleaner. Could well be down to some fairly resistant crud in there alright

@rdtz - not a bad assumption, but I'm pretty sure the valve and compression is ok as I switched the carbs left to right, and the problem moved with them. I took it from that that the issue has to lie with the right hand carb rather than anything ignition or mechanically related. Thanks for the tip all the same.
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Old 15th September 2019, 09:38 PM #5
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Re: Honda CV Carb - anybody know there way round them ?

A proper industrial Ultrasonic cleaner with the correct solution is the way forward Alan,Id say youve a blockage in the carb...also the major thing I found is if the fluid is not up to full temp in the cleaner youd be as well to piss on them for all the good its do
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Old 15th September 2019, 09:38 PM #6
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Re: Honda CV Carb - anybody know there way round them ?

A proper industrial Ultrasonic cleaner with the correct solution is the way forward Alan,Id say youve a blockage in the carb...also the major thing I found is if the fluid is not up to full temp in the cleaner youd be as well to piss on them for all the good itd do
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Old 15th September 2019, 10:02 PM #7
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Re: Honda CV Carb - anybody know there way round them ?

I cleaned them first with petrol, and then industrial strength vinegar as the solution (all in the kitchen, much to the better half's delight.)

Anybody you'd recommend to take the carb to who'd do it right Davy ?
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Old 15th September 2019, 10:04 PM #8
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Re: Honda CV Carb - anybody know there way round them ?

Originally Posted by -alan-
I cleaned them first with petrol, and then industrial strength vinegar as the solution (all in the kitchen, much to the better half's delight.)

Anybody you'd recommend to take the carb to who'd do it right Davy ?
If you strip them down Ill put them through the cleaner in work
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Old 15th September 2019, 10:21 PM #9
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Re: Honda CV Carb - anybody know there way round them ?

I'll definitely ake you up on that sire.

Take me a couple of days to get peace to get at them, but probably only about 10 minutes to pull them apart at this stage, I've done the bl**dy things so often. I;ll pm you Davy - and see if I can get them round to you, thanks a million :)
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Old 18th September 2019, 02:16 PM #10
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Re: Honda CV Carb - anybody know there way round them ?

Sounds like an air leak to me .
Are inlet manifolds ok ?
Have you tried swap emulsion tubes/jets/needles/Air screws over to see if problem shifts too.is the oring on air screw ok?
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Old 18th September 2019, 02:18 PM #11
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Re: Honda CV Carb - anybody know there way round them ?

Originally Posted by -alan-
I'll definitely ake you up on that sire.

Take me a couple of days to get peace to get at them, but probably only about 10 minutes to pull them apart at this stage, I've done the bl**dy things so often. I;ll pm you Davy - and see if I can get them round to you, thanks a million :)
Ive a 5 gallon ultrasonic if you need it
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Old 23rd September 2019, 08:45 PM #12
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Re: Honda CV Carb - anybody know there way round them ?

Ah thanks for the offer Chop - got them cleaned properly with a bit of help from DavyH (Thanks Davy :) )

Haven't got them back on the bike yet, but a very slow and methodical build post cleaning revealed some other suspects for the dodgy running.

Update:


All but one of the little felt-washers that seal the end of the throttle shaft were well and truly shot (or missing). I made some new ones out of an A4 sheet of kiddies craft felt - in striking Royal blue - to match the 450. Left them to soak in some oil whilst I replaced all the jets and brass componentry.

Having read up on the dangers of bent throttle shafts, I took a fair amount of time dry fitting the throttle plates and locking screws and checking out how well the plates fitted at the last part of their arcs when they closed down the Venturi. Despite putting the plates back exactly the same way they were when they came out, I couldn't get the things to close down properly - ie all the way in front of the two little fuel bleed holes in the lower front of the throat. 20 minutes worth of disassembling, flipping, rotating, swopping and then swopping back the plates, and polishing the edge of the rim with cutting paste and a cloth - I eventually got the plates to close smoothly to a snug fit. Screws back in place and thread locked just in case / throttle plates and idle screws synchronised.

There is evidence somebody's been at that them before, with an oversize screw on one of the plates, and it may well be that they weren't fitted correctly, leaving one slightly twisted out of the correct plane and not sealing uniformly - and yes, on the problem carb. Maybe not - but either way, they're in properly now, as they should be.

So in summary, the issues (or potential issues that could have been) at play discovered to date are:

(i) Missing throttle shaft felt seals
(ii) Possible residual blockages in the internal carb passgeways
(iii) Incorrectly fitted throttle plate(s)
(iv) A rung thread on the carb top possibly causing an air leak/impacting the vacuum over the sliding piston.

Plenty of suspects for lousy running there all told. Carbs are reassembled at the moment, but I won't get to put them back on for a few more days. Genuinely curious to see if it's - finally - going to behave..

(If not btw, and one of the carbs is still off, I'm pretty confident the mechanical bits of the carb are now working as they should - so as Chopper suggests, will focus on the jets, swapping them around to see if/when the problem moves.)

Can't let the bloody thing beat me at this stage :)

Last edited by -alan-; 23rd September 2019 at 08:46 PM.
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Old 23rd September 2019, 09:38 PM #13
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Re: Honda CV Carb - anybody know there way round them ?

Hopefully that sorts it out
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