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Offline nate sere

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Carb problems cb650
« on: October 12, 2011, 07:25:57 PM »
I have a 1980 CB650c with the keihin carbs, pods, and open headers.   I replaced the stock 90 jets with 110's.  It runs great with the choke half way out but bogs down at low throttle.  Its almost impossible to get off the line unless I dump the clutch at high RPM's.

I have no desire to replace the air box or exhaust.   It seems that my fuel/air mix is perfect at half choke, should I step up to 120 jets and see what happens?
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Offline scunny

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Re: Carb problems cb650
« Reply #1 on: October 12, 2011, 09:41:54 PM »
I have a 650, same carbs, pods, open 4/1 and running 125. even when on the 110's never had the problem you are. are your carbs clean.. including the emulsifier tubes below the mains, have you pulled the slow idle jets and cleaned them. what setting are your slow idles set to, my are at 2 1/4 turns out from lightly seated. have you adjusted your valves, timing and synched your carbs. sounds like a lot but it all has to be done. 
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Offline nate sere

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Re: Carb problems cb650
« Reply #2 on: October 12, 2011, 11:05:46 PM »
Mine are set at 1.5 turns out.

Valves are adjusted, carbs synced, timed, carb is clean.   
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Offline Retro Rocket

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Re: Carb problems cb650
« Reply #3 on: October 13, 2011, 01:06:59 AM »
Quote
I have no desire to replace the air box or exhaust.

Sorry Nate but that just about rules out a good running bike mate. Get a better pipe and you may have half a chance....
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Offline curemode2002

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Re: Carb problems cb650
« Reply #4 on: October 13, 2011, 10:46:16 AM »
Also these carbs are a bear when changing your intake or exhaust I have been tinkering for a month or more trying to get my MPG to something respectable for the size. But as Scunny said keep an eye on timing and such mine was off about .5 degree and it really hurt my idle. Another thing to check is the pilot jets mine were a little to big I went down from a 35 to a 32 and my idle is much more reliable. You may need to go up or down see if you can get her to idle a bit and pull the plugs. If you can't adjust the lean or rich out using the air screws you need a different pilot jet size.
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Offline nate sere

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Re: Carb problems cb650
« Reply #5 on: October 13, 2011, 08:46:02 PM »
Well today we went and cleaned the carb, set everything to factory specs, tried to rejet, adjusted the air screws, etc and nothing worked. 

We managed to get a set of 750 carbs to fit and it fired right up and was meaner than ever.  No choke and it purred from the start.   Its a little too aggressive right now so we are going to try to jet it down to 110 and see what happens.   Should be able to take it for a test run tomorrow.   
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Offline phil71

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Re: Carb problems cb650
« Reply #6 on: October 13, 2011, 09:30:44 PM »
A little too aggressive? What does that mean?
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Offline nate sere

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Re: Carb problems cb650
« Reply #7 on: October 13, 2011, 09:40:29 PM »
good question....

Its shooting flames about a foot out of the headers right now.   starts right up with no choke and sounds really pissed off.   Need to tone it down a little and it should be scary. 
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Offline Pinhead

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Re: Carb problems cb650
« Reply #8 on: October 13, 2011, 10:56:05 PM »
Quote from: Retro Rocket on October 13, 2011, 01:06:59 AM
Quote
I have no desire to replace the air box or exhaust.

Sorry Nate but that just about rules out a good running bike mate. Get a better pipe and you may have half a chance....

Even if you don't add a muffler (I wouldn't), adding an 18-inch length of pipe to the end of your collector will net a marked improvement in performance...
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Offline nate sere

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Re: Carb problems cb650
« Reply #9 on: October 14, 2011, 08:26:50 AM »
Quote from: Pinhead on October 13, 2011, 10:56:05 PM
Quote from: Retro Rocket on October 13, 2011, 01:06:59 AM
Quote
I have no desire to replace the air box or exhaust.

Sorry Nate but that just about rules out a good running bike mate. Get a better pipe and you may have half a chance....

Even if you don't add a muffler (I wouldn't), adding an 18-inch length of pipe to othe end of your collector will net a marked improvement in performance...

I'm not following you here.   I'm all for performance as long as it doesn't take away from the overall look of the vehicle.   

Anyone have any tips for setting up a cb750 carb on a 650?
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Offline Pinhead

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Re: Carb problems cb650
« Reply #10 on: October 14, 2011, 10:15:29 AM »
You will have a very difficult time getting the bike to run correctly, regardless of carb settings, without proper exhaust scavenging. Did you know that the exhaust pulls harder on the intake charge during overlap than the piston does through the entire intake stroke?  Without at least a little length on your collector, you have reduced or eliminating that scavenging effect...
« Last Edit: October 14, 2011, 11:08:09 AM by Pinhead »
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Doug

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Offline nate sere

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Re: Carb problems cb650
« Reply #11 on: October 14, 2011, 10:20:24 AM »
What is a collector and will this extension be visible?    Sorry,  but this is my 1st custom bike build.   
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Offline Pinhead

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Re: Carb problems cb650
« Reply #12 on: October 14, 2011, 11:06:24 AM »
Quote from: nate sere on October 14, 2011, 10:20:24 AM
What is a collector and will this extension be visible?    Sorry,  but this is my 1st custom bike build.   

The collector is the portion of your exhaust where the exhaust headers come together. It "collects" exhaust pulses and reflects them back to the opposite cylinder's open exhaust valve. Generally, the longer the collector, the more torque your engine will produce (generally and only to a point; a 14 foot collector probably won't produce any more torque than an 18" collector, as an example).

I'd start off with at least a 10" pipe of the same diameter as your collector (thus extending the length of the collector). You can use your butt dyno to tune the length from there.

HERE is a good starter...
« Last Edit: October 14, 2011, 11:09:56 AM by Pinhead »
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Doug

Click --> Cheap Regulator/Rectifier for any of Honda's 3-phase charging systems (all SOHC4's).

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Offline nate sere

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Re: Carb problems cb650
« Reply #13 on: October 14, 2011, 08:28:58 PM »
thanks for the info.  the 750 carbs seemed to fix the issues we were having.  Its running great right now, but I have not been able to get it out for any extended rides.   0-40 it is as faster than any Harley I have ever owned. 
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Offline Pinhead

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Re: Carb problems cb650
« Reply #14 on: October 14, 2011, 09:48:58 PM »
Quote from: nate sere on October 14, 2011, 08:28:58 PM
thanks for the info.  the 750 carbs seemed to fix the issues we were having.  Its running great right now, but I have not been able to get it out for any extended rides.   0-40 it is as faster than any Harley I have ever owned. 

That's what I noticed when I first chopped my exhaust, too. Torque from idle to about 3500 rpm was amazing. The bike pulled like a train. Much above that, though, was just noise without much more kick...
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Doug

Click --> Cheap Regulator/Rectifier for any of Honda's 3-phase charging systems (all SOHC4's).

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Offline nate sere

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Re: Carb problems cb650
« Reply #15 on: October 14, 2011, 09:54:07 PM »
I will take it out on a better ride on Sunday.  Not sure that I want to go on the freeway with it yet, but I am sure I will once I get comfortable on it.   I probably have 5 miles on it so far. 
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Offline grasscutter

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Re: Carb problems cb650
« Reply #16 on: October 16, 2011, 12:00:59 PM »
Although it may seem that you've got the carb issue fixed by swapping the 750's on.. your original 650 carb set was simply not cleaned well enough. 

Mains have lots to do with WOT, and minimal to do with idle of 'off the line' issues you described.

Idle circuit, pilot circuit, emulsion tube... all work together when transitioning from idle and throttling up from a stop. 

Got back through the carbs.  And really do it this time.  Soak. Clean.  Soak.  Repeat as necessary.  Then vac sync the carbs. 
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Offline chopped550

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Re: Carb problems cb650
« Reply #17 on: October 16, 2011, 02:16:55 PM »
Quote from: nate sere on October 14, 2011, 08:28:58 PM
thanks for the info.  the 750 carbs seemed to fix the issues we were having.  Its running great right now, but I have not been able to get it out for any extended rides.   0-40 it is as faster than any Harley I have ever owned.

I have the same open 4-1 exhaust setup on my cb550.  It's stock jetted, doesn't run lean, and is a little ripper well into high RPM's. 
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Offline nate sere

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Re: Carb problems cb650
« Reply #18 on: October 17, 2011, 01:13:05 AM »
I took it out for an extended ride tonight and its still not right.   Can't go over half throttle without it bogging down.   Could only get up to around 60 on the freeway.   Runs great from idle to half and then its game over.....

should I try to move down on jets on the cb750 carbs?   I'm running 110's now
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Offline trueblue

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Re: Carb problems cb650
« Reply #19 on: October 17, 2011, 04:17:15 AM »
Quote from: nate sere on October 17, 2011, 01:13:05 AM
I took it out for an extended ride tonight and its still not right.   Can't go over half throttle without it bogging down.   Could only get up to around 60 on the freeway.   Runs great from idle to half and then its game over.....

should I try to move down on jets on the cb750 carbs?   I'm running 110's now
Listen to what pinhead has to say about adding a length of pipe to your collectors
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Offline nate sere

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Re: Carb problems cb650
« Reply #20 on: October 17, 2011, 09:22:56 AM »
we did add a small baffel to the open header....
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Offline trueblue

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Re: Carb problems cb650
« Reply #21 on: October 17, 2011, 02:07:51 PM »
Was that baffle inside your header in the pic or in addition to what you have in your pic, if you havn't done it already try adding a length of pipe to the headers it will aid in scavenging of the cylinders at higher rpm ;)
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Offline nate sere

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Re: Carb problems cb650
« Reply #22 on: October 17, 2011, 02:23:58 PM »
It didn't add any length to the header.  It's internal.   I'm going to go for another ride in a few and see if I can figure anything out. 
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Offline Pinhead

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Re: Carb problems cb650
« Reply #23 on: October 17, 2011, 06:42:30 PM »
Forget back pressure (I assume that's why you thought a baffle would somehow make the bike run better). An engine does, NOT need back pressure to run correctly.

Length is what you need. The extra length of pipe will help "suck" the exhaust out of the headers.
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Doug

Click --> Cheap Regulator/Rectifier for any of Honda's 3-phase charging systems (all SOHC4's).

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By the way, I'm going for the tinfoil pants...so they can't read my private thoughts.
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Offline grasscutter

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Re: Carb problems cb650
« Reply #24 on: October 17, 2011, 07:03:35 PM »
Quote from: nate sere on October 17, 2011, 01:13:05 AM
I took it out for an extended ride tonight and its still not right.   Can't go over half throttle without it bogging down.   Could only get up to around 60 on the freeway.   Runs great from idle to half and then its game over.....

should I try to move down on jets on the cb750 carbs?   I'm running 110's now

Because carbs are too big.  Jetting down will attempt to go in the correct 'direction'..  But unless you are a carb tuning master, time is better spent on doing the 650 carbs correctly. 

**And yes, the bogging is likely from being too rich. 
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