Author Topic: Carb problems cb650 (Read 6389 times)

nate sere · « **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?

scunny · « **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.

nate sere · « **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.

Retro Rocket · « **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....

curemode2002 · « **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.

nate sere · « **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.

phil71 · « **Reply #6 on:** October 13, 2011, 09:30:44 PM »

A little too aggressive? What does that mean?

nate sere · « **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.

Pinhead · « **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...

nate sere · « **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?

Pinhead · « **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...

nate sere · « **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.

Pinhead · « **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...

nate sere · « **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.

Pinhead · « **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...

nate sere · « **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.

grasscutter · « **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.

chopped550 · « **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.

nate sere · « **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

trueblue · « **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

nate sere · « **Reply #20 on:** October 17, 2011, 09:22:56 AM »

we did add a small baffel to the open header....

trueblue · « **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

nate sere · « **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.

Pinhead · « **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.

grasscutter · « **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.

nate sere · « **Reply #25 on:** October 20, 2011, 10:31:07 PM »

I traded the 650 carbs for the 750 carbs so thats not a choice. I need to figure these out. I am running pods, open headers, and keihnin 750 carbs. We put 110 jets and advanced the timing and it helped, but its still not right.

I was thinking about dropping down to 102.5 jets. I am looking into getting some type of muffler but hate to alter the look of the bike. Its exactly what I want right now.....

The stock 650 carbs came with 90's I believe.

Will the 102.5's help?

Pinhead · « **Reply #26 on:** October 21, 2011, 12:21:55 AM »

With pods? The 650's carbs are a cast-iron #$%* to tune since you can't raise the needles or change the idle jets. The 750 carbs are a bit more tunable.

In my '79 with the stock airbox I run 102.5's since I removed the air filter cover. With just pods you'd probably have to go much larger than that. I believe a member with the 650's PD carbs went all the way up to 120 mains but I could be mistaken. I'm sure there's a thread or two out there with regards to the 650 and pods...

nate sere · « **Reply #27 on:** October 21, 2011, 12:34:16 AM »

but im using 750 carbs now. is that what you are using?

phil71 · « **Reply #28 on:** October 21, 2011, 01:05:29 AM »

you're on your own bro. You've introduced so many random elements here, that nothing any of us say is anything more that speculation. Keep dicking with it till it runs okay, and eventually it will. but nothing we say matters at this point... you have a one of a kind, and tuning is your problem now.

nate sere · « **Reply #29 on:** October 21, 2011, 01:36:23 AM »

so i'm the only person on this board who owns a cb650 custom, with pods and 4-1 open pipes who is using cb750 carbs?

trueblue · « **Reply #30 on:** October 21, 2011, 05:40:52 AM »

Quote from: nate sere on October 21, 2011, 01:36:23 AM

so i'm the only person on this board who owns a cb650 custom, with pods and 4-1 open pipes who is using cb750 carbs?

You are the first I have heard of, but I can't be certain

Retro Rocket · « **Reply #31 on:** October 21, 2011, 02:48:28 PM »

Quote from: nate sere on October 21, 2011, 01:36:23 AM

so i'm the only person on this board who owns a cb650 custom, with pods and 4-1 open pipes who is using cb750 carbs?

You aren't really listening to people that know mate, that combination is going to be extremely hard to get even close to running well.....
750 carbs hate pods at the best of times and it is well documented here, add in those pipes and you are going to spend a lot of frustrating hours trying to get it right...... Good luck

scunny · « **Reply #32 on:** October 21, 2011, 03:50:52 PM »

nothing wrong with tinkering Mick. anything is possible nate. you are sure giving yourself an uphill battle tho.

Retro Rocket · « **Reply #33 on:** October 21, 2011, 03:55:26 PM »

Quote from: scunny on October 21, 2011, 03:50:52 PM

nothing wrong with tinkering Mick. anything is possible nate. you are sure giving yourself an uphill battle tho.

I am a born tinkerer mate, but some things just make life a lot harder.....

Pinhead · « **Reply #34 on:** October 21, 2011, 05:39:49 PM »

Quote from: nate sere on October 21, 2011, 12:34:16 AM

but im using 750 carbs now. is that what you are using?

Nope, I have no experience with that Carb combination so I'll probably be of no help at this point.

nate sere · « **Reply #35 on:** October 25, 2011, 06:38:30 PM »

I thought that I was getting too much fuel from the CB750 so I went down from 110 to 102.5 and it was much worse. I went the other way and went to 120's and it ran like a whole new bike. Pulled very hard all the way through the rpm band. Slight hesitation on very bottom end and very top end. I am going to try 125's tomorrow just to see if it makes any difference, but I am pretty sure that the 120's are going to be the best solution.

phil71 · « **Reply #36 on:** October 26, 2011, 01:59:05 AM »

are those numbers honda, or dynajet? that hesitation out of the hole is probably the vast mismatch when the pilot jets hand off to the mains. At least you're getting closer.
125s seem awfully big for a 650 with no internal mods tho.

nate sere · « **Reply #37 on:** October 26, 2011, 02:26:57 AM »

no known internal mods.

It was really fast and pulled the front tire a little in 1st.

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Author Topic: Carb problems cb650 (Read 6389 times)