Topic: '79 CB650 Carbs

[Pinhead]

Stock carbs, stock ignition, stock exhaust, pretty much everything is stock. The only thing that's going to be changed in the near future is the rotor for the alternator. Here's my problem...

No matter the temperature, no matter how long the bike has sat, in order to start the bike I have to pull the choke completely out. That's no big deal. I have to drive with the choke closed until the bike warms up. This will keep the bike "idling" at around 3k rpm. No biggie. Now, the problem comes into play when the engine is good and warm and I try to actually drive the thing. It has NO torque below 3000 rpm. I have to either rev the engine to around 3500 or push the bike to get it to go. Once I'm going it runs alright, up to about 3/4 throttle. Around that point (regardless of engine RPM) the engine begins to cut out. The bike won't go much faster than 65 mph, since that's a little under the 3/4 throttle position. If I crank it to WOT at 65, it'll miss, slow down to about 5500 and then finally clear out and accelerate. Once it's cleared out, it'll run great down the highway as long as I keep it above 3/4 throttle. Obviously this isn't possible, because I'd be driving 90mph everywhere. If it slows down below 5500rpm I have to go through the "clearing" process all over again.

I've taken the carbs off the bike 4 or 5 times and cleaned as well as I could. All of the jets (that I can see and remove) are clear and I was able to spray WD40 through all of the holes and note where the spray comes out. The needle valves all work perfectly, the floats float (no leaks) and all 4 get gas.

The one thing that I do know, is that this thing is rusted pretty badly, and the rubber is pretty stiff. I was able to make it move freely and cleared the holes that I could see, but the thing will need to be replaced.



Could this little thing (called an Air Cut-Off Valve, IDK what it's function is) be causing my problem? I thought it was an accelerator pump when I first put it back together, but the name seems to suggest otherwise. If that thing isn't the culprit, where should I look next?

Edit: I forgot to mention that after the engine is good and warm it'll idle beautifully around 1500 rpm.

[scunny]

if it's on the bottom of #2 carb it's the accelerator pump I think, my Haynes suggests 'little can go wrong with the pump apart from cracked or perished diaphram, but if it's completely dry it may require priming

[cb650]

Do you still have the POS vacum gas shut off ?



                     Terry

[Pinhead]

I don't believe so... Is that on the fuel petcock? If so, there's no vacuum lines going up to it(and nowhere for a vacuum line). If it's somewhere on the carbs, I'm not sure.

[cb650]

Its mounted on the carbs.  Fuel line goes to it then the carb feed.   A vacum line goes from it to #2 or 3 cant remember top side of the carb by the head. 



                     Terry

[TwoTired]

Take all this with a grain of salt.  650s aren't my forte.  But, perhaps an analytical approach may serve you.
Assumptions are that you have mechanical slide carbs and everything inside is clean and well formed, IE. no damaged jets.

Air cutoffs on my CV carbs of the CX500.  Enrich the mixture on deceleration to eliminate backfiring in the exhaust.  They "cut off" the low speed or idle circuits air supply when very high vacuum exists in the intake tract.

The slide carbs have three fuel delivery metering devices, the slow jet, Throttle valve needle / jet, and main jet.  They determine mixture for the throttle setting you select on the twist grip.

I think the major piece of the puzzle missing at this point is what the combustion conditions are while you have your "problem".  This knowledge can be gained by equipment that "sniffs" the exhaust for chemical composition.  Or, by reading the spark plug deposits while operated in this problem area.  No or light deposits are lean.   And, black sooty deposits are too rich.

It is possible to have the slow; rich, the midrange Throttle Valve (TV); lean, and the Main; rich again or any combination of rich/lean/ just right applied to each metering device.

Therefore, a plug chop, where clean plugs are inserted and the engine operated in problem mode, then suddenly stopped, preserves the combustion conditions that existed during the noted throttle position.  This information can be used to address the correct aspect of carb metering and adjusted for beneficial effect.  An aid for reading plug conditions can be found here:
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

And idea of the area to adjust can be observed with this basic chart:

[Pinhead]

The problem with doing the "plug chop" is if I run it at WOT (where the engine starts to cut out) it'll cut out for a while and then clear. I don't think it "cuts out" long enough to show the deposits on the spark plugs. And when it finally does clear out, I'm running 85 to 90mph... Not the best scenario when I'm concentrating on figuring the bike's engine out instead of concentrating on crazy drivers around me.

If I let the engine get good and warm, put it on the center stand, and hold the throttle wide open, it'll go up to 6k rpm to 7.5k rpm and waiver between the two. If I pull the choke, it'll crank to past redline (won't do this when I'm going down the road though) How long should I hold the engine there (7000rpm to do the plug chop)? I don't want to blow the thing up, as 7,000 rpm is pretty dang fast if you ask me.

Edit: It doesn't have the vacuum shot off. There's nowhere for a vacuum line on the head/intake at all.

[Bob Wessner]

TT,

What is the difference between "Throttle valve needle / jet," and "the main jet?"

[Pinhead]

I'm looking at my Haynes manual and the pic I have is actually the wrong part. It is the accelerator pump that I noted was rusty/stiff. I didn't take apart the air cut-off valve. It doesn't backfire when decelerating, so I don't think that has any problem with it.

[TwoTired]

The problem with doing the "plug chop" is if I run it at WOT (where the engine starts to cut out) it'll cut out for a while and then clear. I don't think it "cuts out" long enough to show the deposits on the spark plugs. And when it finally does clear out, I'm running 85 to 90mph... Not the best scenario when I'm concentrating on figuring the bike's engine out instead of concentrating on crazy drivers around me.

If I let the engine get good and warm, put it on the center stand, and hold the throttle wide open, it'll go up to 6k rpm to 7.5k rpm and waiver between the two. If I pull the choke, it'll crank to past redline (won't do this when I'm going down the road though) How long should I hold the engine there (7000rpm to do the plug chop)? I don't want to blow the thing up, as 7,000 rpm is pretty dang fast if you ask me.

Edit: It doesn't have the vacuum shot off. There's nowhere for a vacuum line on the head/intake at all.

I can only tell you what I think needs to be done.  The engine needs to be run under load conditions as it takes more fuel to make power than when it is simply overcoming it's own internal friction on the center stand.  This means a test track or a dyno, your choice.  I recommend checking the main first.  A quarter mile run at WOT with immediate shut down after should show plug conditions for that setting.  Mark your throttle so you know what part of the carb is working.  Read your plugs, and tell us what you find, or post a pic. 

Drag racing is rough on clutches.  You rev for RPM, Hold it, and then dump the clutch and go WOT at the same time.  Lean way forward to keep the front end down.
Put a little pressure on the up shift and touch the clutch lever when you get up near red line.  It should jump to the next gear without releasing WOT.   In 2 and 1/2 blocks hit the kill switch and pull in the clutch, coast to a stop, pull the plugs and read them.  The indication will tell you if your main jet is correct.

Before you race do a preliminary check for lean conditions that can kill your motor.
Pull ALL the plugs to make sure all carbs and cylinders are working evenly.  Have you EVER looked at the plug deposits?  What did you find?

Using public streets as a test track is frowned on by local police usually.  But, the only viable alternative is to use a Dyno or a bona-fide test track.

It seems very strange to me that a stock bike in stock configuration and stock settings doesn't run as Honda intended.  Something is not as it was when it left the factory.  Plug chops or a dyno will probably tell you what aspect of the carbs to focus your attention.  But, I expect you will find that something in your carbs is not set, adjusted, or properly cleaned.

Post a pic of general plug condition.  But, if choke improves performance,  I'm guessing your TV is too lean.  Raise the needles, raise your float level or find out what crud inside the carbs is restricting the fuel flow while in TV operation.

But, I could be wrong...

Cheers,

[TwoTired]

TT,

What is the difference between "Throttle valve needle / jet," and "the main jet?"

The Throttle Valve (TV) is the slide and the needle assembly along with the orifice that the needle pokes into.  The main jet is at the bottom of the carburetor and supplies fuel to the TV.  When the TV is fully withdrawn at WOT, the TV needle and its orifice are cross sectionally larger than the main orifice. So, the main becomes the fuel restriction for the engine's max power demands.  As the slide closes, closing off air passing through the carbs, the tapered needle inserts into the TV orifice, restricting fuel entering the carb bore in a (hopefully) perfect ratio to that of the air being restricted.  At some point during TV closure the metering ability becomes too coarse to properly control the cylinders. These fuel demands then transition to the slow jet circuit, as the TV is mostly shut off at this low throttle setting.

Is this explained well enough?

Cheers,

[Pinhead]

The Throttle Valve (TV) is the slide and the needle assembly along with the orifice that the needle pokes into.  The main jet is at the bottom of the carburetor and supplies fuel to the TV.  When the TV is fully withdrawn at WOT, the TV needle and its orifice are cross sectionally larger than the main orifice. So, the main becomes the fuel restriction for the engine's max power demands.  As the slide closes, closing off air passing through the carbs, the tapered needle inserts into the TV orifice, restricting fuel entering the carb bore in a (hopefully) perfect ratio to that of the air being restricted.  At some point during TV closure the metering ability becomes too coarse to properly control the cylinders. These fuel demands then transition to the slow jet circuit, as the TV is mostly shut off at this low throttle setting.

Is this explained well enough?

Cheers,

Makes perfect sense!

[eurban]

Hey Pinhead,

Based on at least part of your symptoms, I would guess that your idle jets are not clear.  The pressed in jets should be gently pulled out with a twisting motion and then cleaned chemically and mechanically with something like a single strand of copper wire.  A clogged idle circuit may dramatically effect driveability so if they are indeed clogged, fixing this may improve many of your symptoms.  The accelerator pump is pretty critical to the proper functioning of these carbs so you should at least confirm that it is working properly.  The brass nozzles  connected to the pump poke up into the intakes bores of the carbs near the choke plates.  Take off your air fliter box or remove the carbs and (making sure there is ample fuel in the bowls) whack open the throttle and observe if there is a good stream shooting from each nozzle.  Freshly rebuilt carbs do take some priming for the pumps to work but if your bike has been in use and fuel levels in the bowls are good then there should definitely be a stream.  Look for damaged parts, frozen linkages and or clogged nozzles if no stream.  Determine if your bike has the the vacuum operated fuel shutoff (safety device designed to keep fuel from dumping on your garage if a float hangs open)  It would be mounted on the carb body: it would have the fuel line from the petcock connecting to it, have a fuel line going to the carb body and a vacuum line connecting to it.  If you do have this it may not be functioning properly.  Its presence usually means longer cranking times from engines that have been sitting as fuel will not flow until vacuum is applied to the valve so it takes a while for the bowls to refill.  Also it may simply be limiting fuel flow if it is not workind as well as it should.  Easiest test is to remove it from the system by connecting a new fuel line directly from the petcock to the inlet on the carbs.  Good luck.

[Bob Wessner]

Is this explained well enough?

Yup. I've thought of them as one, but you have dissected its operation, so I will hereafter trash my generalization. 

[Pinhead]

Hey Pinhead,

Based on at least part of your symptoms, I would guess that your idle jets are not clear.  The pressed in jets should be gently pulled out with a twisting motion and then cleaned chemically and mechanically with something like a single strand of copper wire.  A clogged idle circuit may dramatically effect driveability so if they are indeed clogged, fixing this may improve many of your symptoms.  The accelerator pump is pretty critical to the proper functioning of these carbs so you should at least confirm that it is working properly.  The brass nozzles  connected to the pump poke up into the intakes bores of the carbs near the choke plates.  Take off your air fliter box or remove the carbs and (making sure there is ample fuel in the bowls) whack open the throttle and observe if there is a good stream shooting from each nozzle.  Freshly rebuilt carbs do take some priming for the pumps to work but if your bike has been in use and fuel levels in the bowls are good then there should definitely be a stream.  Look for damaged parts, frozen linkages and or clogged nozzles if no stream.  Determine if your bike has the the vacuum operated fuel shutoff (safety device designed to keep fuel from dumping on your garage if a float hangs open)  It would be mounted on the carb body: it would have the fuel line from the petcock connecting to it, have a fuel line going to the carb body and a vacuum line connecting to it.  If you do have this it may not be functioning properly.  Its presence usually means longer cranking times from engines that have been sitting as fuel will not flow until vacuum is applied to the valve so it takes a while for the bowls to refill.  Also it may simply be limiting fuel flow if it is not workind as well as it should.  Easiest test is to remove it from the system by connecting a new fuel line directly from the petcock to the inlet on the carbs.  Good luck.

My bad, I forgot to say that after it's good and warm, it idles beautifully below 2000 rpm. I usually set it to 1500. I'll look for that vacuum actuated shut off. I never noticed it before, and it usually starts up pretty well when choked (I don't have to "crank it" for long before it starts) but I'll check it anway.

[Ibsen]

Terry, do the 79 CB650 US model have a vacum petcock? My 1980 Euro Z have got a gravity fed petcock.

And Pinhead, the picture in the first post shows an air cutoff valve. It is operated by the intake vacum, and it is there to enrichen the fuel flow to the carbs, and prevent backfiring, when you close the trottle, but it has nothing to do with the petcock. If the valve diaphragm is holed, or fails to seal around the edge, you will have an air leak in the intake system or the fuel deliver might be disturbed. That could be one reason why you have a power loss at lower rpm.

[cb650]

The petcock is gravity feed and normal.   It feeds into a separate vacuum shut off that is mounted to the carbs.  then a line goes to the carbs.   It is very easy to remove.   3 screws i think and plug the vacuum from the carb and a longer fuel line.




                      Terry

[Ibsen]

Are you sure the 79 US model PD carbs had the fuel shut of valve on the top of the carb rack? They have the air cut off valve, but in the manual I can't see the fuel cut off valve assy used before on the 81 CV carbs?

[cb650]

OK the only 79 I bought was a basket case and maybe it wasnt on it. Sorry.   Maybe it was mid 80 item cause I have taken it off a 80 but just looked at a 80 parts bike I have here and it doesnt have one.   But you never know what has been done to these over the years.
What manuall you looking in?



                            Terry

[Ibsen]

I have checked the Clymer, 2. endition, and the Haynes manual. I also have the manual for the European Z model, but not for the US Z model.

As you and I have discussed in the past, you know that I have made a few mistakes too regarding the difference between the US and European models. 

[cb650]

That and what POs do to them.  I have seen one with a drum front brake.




                    Terry