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Honda CB650 (Carb)

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Old 08-28-2007, 04:18 PM   #1
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Default Honda CB650 (Carb)

Looking for some answers on the Carburators of my 1981 Honda CB650 (not custom, 4 cyl)

A real quack owned it before me and messed up the carbs really bad. I have no idea what he did but when I got the bike the whole inside of the carbs and jets were all clogged with liguid gasket. Anyways, now that gunk is all taken care of and I got it to run, Im having some choke/idleing problems.

It runs too hot (One side even runs hotter than the other) It wont run without the choke 1/4 or sometimes even 1/2 way out unless your at high RPMs. And as you can imagine it really bogs out with the choke out around 7k-9k rpms.

The seals are on tight on both sides so i know no extra air is coming form there. Theres a main air flow jet on the bottom of each carb and as you can imagine ive tried putting those in all the different places to no avail.

Having to run it with the choke on causes it to idle around 3k-4k rpms, wont run any lower. Even messing with the idle screws doesnt help.

I'm thinking about cleaning them out again (Maybe running gas through the old tank caused stuff to get stuck in there etc)

But that really isnt what I had in mind considering 4 vaccuum carbs are no fun to assemble.

Jets looked fine last time I took it apart, nothing major.

Any ideas? Thanks in advance.


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Old 06-07-2010, 02:42 AM   #2
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Default Re: Honda CB650 (Carb)

I have seen this, and on a 160 twin, this issue melted a piston and wrecked a cylindar and the piston, requiring a new sleeve.
You still have a blocked passage in at least one of these carbs.
Also, the cb650 carbs can leak severely when the float needles appear in good condition from the accelerator pump adjustment. Must be .004" + clearance between the pump shaft and the actuator linkage
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Old 06-07-2010, 08:35 AM   #3
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Default Re: Honda CB650 (Carb)

Op, this is the aircooled motor? If so, how do you know that the motor is running hot?

In as far as your idle goes, you are going to need to try and find out what the OEM jet size was, this rpevious owner MAY HAVE changed jets and unless you get back to OE specs you will be chasing your tail around in circles.

Running hot, white Spark plugs, is consistent with a lean condition.
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Old 06-07-2010, 09:37 AM   #4
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Default Re: Honda CB650 (Carb)

Did you actually pull all the carbs apart and rebuild them or just clean up the gunk sitting on the outside and down the venturi? How does the rest of the bike look? Any leaks, timing correct, etc?
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Old 06-07-2010, 12:40 PM   #5
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Default Re: Honda CB650 (Carb)

I forgot to mention... you need to inspect the rubber manifolds for cracking and leakage. that may have been the initial reason that the original owner used Rubber sealant.
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Old 05-27-2012, 04:56 AM   #6
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Default Re: Honda CB650 (Carb)

I see that this post is a bit dated, but I had a very similar problem with a friends 650 recently and thought it might be worth posting in case anyone else runs across this thread. The symtom was that the bike was hard to start (took a lot of choking) and once running seemed starved for fuel. One cylinder ran hotter than the others and back-fired as well. I won't drag out the post with all that we tried, but what it came down to was the automatic fuel shut off valve. This valve is operated on engine vacuum and is located between the tank and the carburators. The diaphram was dryrotted and leaky thus only opening the valve partially. We bypassed the valve and man what a difference. The bike ran great. We still have a carburator fuel leak which is why I cam to this site in search of answers. Anyway, the fuel shut off valve is no longer available, but we purchased an aftermarket valve that doesn't mount to the carburator like the old one, but works just fine. I hope this post is helpful to others. I see a lot of posts about carburator rebuilds and if you can avoid it by testing this valve, it's worth the time.
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Old 12-04-2012, 11:06 AM   #7
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Default Re: Honda CB650 (Carb)

nice one, thank you very much, your post helped me a lot ))
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Old 08-24-2014, 07:34 AM   #8
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Default Re: Honda CB650 (Carb)

I have a 81 Honda CB650 and it starts fine but it chokes out at 4.75-5 rpms. Carb has been thoroughly cleaned. The people I bought it from received it with the carb having 2 diff size needles and we are unable to adjust them. PLEASE HELP! !!!
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