Topic: CB650, idles at 6k

[walnuts]

Background:  I while back, I purchased a 1980 CB650, running but in overall poor condition, to rebuild and modify as my first project bike.  As part of the project I removed the carbs, cleaned, rebuilt, etc.  It turns out the carbs are from an '82-'83 CB650.

Situation:  I finally got the drivetrain back together and into the frame, wiring harness all together and attempted to fire it up this weekend.  First try, it fired right up but revved up so I hit the kill switch.  Tried again later and the same thing happened.  It revved up to about 6k rpm with no throttle input and stayed there.  I only let it run for about 5 seconds because I didn't want to cause any serious damage by letting it run so high while cold.

Question:  I know I can trim the idle back a bit with the idle adjust screw and that there is a lot of syncing and tuning to go, but I can't figure out why its "idling" so high.  I know the butterflies aren't stuck (throttle or choke) and I didn't change out the needles from what was in it previously since they were all in good condition and it ran/idled fine before.  Can anyone point me to a thread, resource, or anything where I could start to figure this out?

[goldarrow]

I don't have 650 bike, but would check to make sure the throttle cables adjusted properly and lube, also check idle adjustment screw

[harisuluv]

So you are having carb problems.  I'll say this to you first, these threads pop up all the time, which is fine.  However I will say that the ones that give details about their bike and setup and THEN follow instructions solve their problems.  The ones that don't just try this and that, in no particular order, are told what the problems is and then do something else--these almost never end fixed and just disappear.

So it sounds like you probably have pod filters, yes?

Everyone is going to ask you what air setup you have, stock filter, pods, airbox with aftermarket filter?
What kind of exhaust, stock, 4-1, baffled/unbaffled.

The number/casting of the carbs. 

You have CV carbs, which means that they are much more sensitive than the previous type.  What you describe could be many things, but if your sync is off it could act that way also.  Also you might find yourself trying to turn down the idle with the screw but nothing is happening.  This can happen if they weren't synced properly. 

You never said you actually turned down the idle though...  So you could just do that and... it's fixed.

[walnuts]

Wow, yeah, I didn't include any info, sorry.  Prior to diassembly, there were no filters and a 4-into-1 exhaust and it idled just fine.  Now I have K&N RC-2362 filters installed, same needles/jets and other internals in the carbs as there was before, same 4-into-1 exhaust which I believe is unbaffled (could you clarify that term for me?).

I will try to turn the idle screw down the next time I have a chance to work on it to see if it solves it, I just didn't think that it could adjust far enough out to cause that high of an idle.  As a result of that scepticism I didn't want to keep spinning a cold engine at that rpm, starting & stopping, and potentially cause damage.  I have a vacuum sync setup so do you think it would be safe to attempt syncing the carbs with the engine running that high?  Is there another way to sync them?  I did attempt to "bench sync" them before isntall but maybe I wasn't careful enough.

I will certainly follow any instructions given, step-by-step and track this down and I suppose that begins with trying to turn down the idle screw first.  Thanks.

Note: I just checked again and it would appear the carbs are the CV style from '80-'82 not '81-'83 as I mentioned above.  I did check the throttle cables and it appears that there is no hang-up or adjustment issues.

[harisuluv]

Before you mean there no filters at all?  No stock airbox/filter or do you mean no POD filters?

With the pods you have you drastically altered the airflow on a carb system that is totally reliant on air flow.  To get setups similar to work, people have gone so far as to put electrical tape over about 70% of the filter area to return the airflow back to satisfactory. 

This in combination with a jet increase to the idles and the main jets.  All sorts of fixes and combinations people have tried, not so much on this forum but some DOHC forums you will find some more stuff about what people have claimed worked.

What you're describing is more characteristic of the throttle butterflies just being too open.  Once you adjust that, you will probably notice that when you hit the throttle the RPMs seem to hang and don't come down shortly after. 

First thing's first though, adjust your idle.  You can do that before you start the bike.  I think on that bike turning the screw out should be lowering the idle.  If the idle screw has no affect you might have bench synced it incorrectly.

[walnuts]

It had crappy rusted pod filters with torn boots and holes so essentially in filters at all.  No air box, etc.  I came home a bit early today and checked, yes the idle adjust screw was cranked way up (causing elevated idle, no doubt).  I can't check it running since I found some oil leaks around the oil pan and right crankcase cover that I want to address.  The carb boot clamps weren't tightened down too well either so I "fixed" that by tightening them quite a bit more.

I will report back once I get a chance to start it again and appreciate the input folks.  I'm not expecting it to run well, just. Lumpy idle close to where it should be.  I'll mess with jetting after that.

[cb650]

Find out what carbs you have also.  The 81/82 are way diff than the 83 up. 

[RJ CB450]

Ohnoes, the two things that make my eyes bleed when combined!

"Pods"+"rejetting"

THE HORROR!

Okay.... before I do my favorite rant on that subject, over revving.

Essentially, for rpms to be high, your engine has to be generating x power to meet x vacuum.  at 6000 rpm, your engine is working as hard to draw air in as it produces in power.  Meaning that at least one cylinder is being open a bit.  More open throttle, air draws in faster and in greater quantity, and more fuel=more power.

It doesn't take much.  While I was syncing my carbs after rebuild, adjusting just a single one could cause the rpms to pick up and near 3000 causing constant idle stop adjustment.  That is all it is, a stop screw.  Sets how closed the #2 carb on an 82 set goes. Other cylinders were more closed, so when they opened, I got more power, and #2 had to close up a bit more so it didn't run as much.

Turn down idle with adjuster knob, til it is nice and low.  They say around 1000, but my 650 I have tuned like a top, and once warm, she purrs at a 1400 (on the tach).  Get out the manometers, adjust, adjust and adjust again.

Now for those words I hate....

Nothing wrong with pods, problem is how they are installed.  And rejetting doesn't get a bike working,  It will only get it running.  Your performance will be terrible comparable to the original air intake.  Without going into the heavy science, here is why.

An engine is an air pump.  Plain and simple.  It has a displacement, and it draws a fixed volume in.  When throttle is closed, it draws less than displacement as vacuum (see above about power generated vs used for idle).  For a volume of air, you need a specific volume of gas.  Too little, and you burn real hot, or no combustion, engine goes.  "Mean Lean"  Too much fuel, you get bad combustion, pop and blow black, poor idle, etc. "Pig rich"

Your jets a fuel metering, plain and simple.  By means of the venturi effect, as air velocity increases, so will fuel draw.  You re-jet according to altitude and air temp because these affect air density which means to get correct fuel air mix, need a different jet.

So, does changing an air filter (provided it meets your cfm requirements) change your altitude?  Does it change the ambient air temp? (pods, you want a heat deflector) Does it change the volume of air your fixed displacement draws in?

Answer:  No, so if your air density and volume is the same, then why would you need to change how much fuel it gets?  Reason people re-jet with pods is because they create turbulence which disrupts the venturi effect.... and one other thing.

Inertia

Air has mass, straight up.  So even when engine isn't drawing, with proper tune, it will still have a smooth flow through the carb.  "Tunnel Ram"  That is the whole reason that bikes make use of adjustable venturi's.  When you swap to pods, you lose is resonance.  Air is turbulent, stops and starts, so you do not get fuel draw.  Guys rejet just so that instead of a smooth and consistent flow of fuel, you get big ugly and poorly mixed gobs.  This is why pod bikes almost always are rich in the higher end big time.  You get it revs, air flow smooths and now the jetting is doing what it is supposed to. 

For things like cars and some bikes, you have multiple cylinders drawing through a single carb making it much less finicky because there is always some piston drawing air.  But when a car is set for performance and they don't have the velocity stacks, then they get the bad idle for exact same reason, poor air flow.  In hot rodding, I see it all the time.  Guys put bigger carbs or lumpy cams on without proper ports and matching cam to carb, they just get some big lumpy idle and have less power.  A person can tell when an engine is all bark and no bite for this reason just from the sound.

Hell, nothing wrong with pods, and honda proved it.  The first CV carbs they used on the DOHC 450. Factory pod air filters.  But they had integrated velocity stacks to allow the air flow to be smooth and stable.  My twin (in restore atm) had a perfectly stable 1000rpm idle, and revs to 12000 without any hesitation.  factory pod style filters.

Issue with pods is they do not have that runner.  In fact, this issue of pods and carb mistuning has prompted me to seek out a beat up CV carbed bike for the specific purpose of building a pod racer.  I have never seen it done, but I plan on resonance flow benching and doing test and tune for different horn and ram testing to get proper working pods.

:Edit  The pulsing air also messes up the low rpm operation of the CV carbs, would bounce the puck like a pogo stick....

[JimJamerino]

In your first post, you say your bike is an 80 CB650, and the carbs are 82-83.  This may be the problem.  For one, the CB650 SOHC was made from 79 to 82.  The 79 and 80 models use a mechanical slide carb.  The 81 and 82 models use CV (constant velocity) carbs (which, btw, do not play well with pod air filters at all).  The two styles of carbs are not interchangeable - the spacing on the head is different.  If you have an 80 engine and the wrong carbs, you'll have all kinds of problems.  I've not tried it, but I think you can wrestle the boots to fit the wrong carbs, which would mess with things pretty severely.

I don't know enough to tell you if there is such a thing as 82-83 carbs, but if they do exist, they're probably from the DOHC Nighthawk line, which I strongly doubt is compatible with our bikes.

I'd check the engine numbers to verify which year engine you have and go from there before farting around with the pods vs airbox debate.

[RJ CB450]

Yeah on the 82-83 carbs being dohc.  The 82 and 83 nighthawk 750s were the same.  Easily identified by the diaphragm covers on the side.  650 carbs dont have that.

[walnuts]

Yes, sorry '81-'82 carbs.  Previous owner replaced the whole top end of the engine.  Not sure why.  As I said though, it ran fine when I bought it.

Understand rejetting doesn't get it working and I won't bugger with it if it appears to be getting correct amount of fuel...

I found yesterday after some checking that the cab boots weren't clamped down all the way (tightened clamps) and I had no sealing washers on the screws for the vacuum ports past the butterflies.  More importantly the idle screw was turned quite far in even though I swear I thought I'd set it low before I installed them.  I need to go back over everything I did and see if I can't find any other possible leak sources or screwed up settings.

I apologize if I use incorrect terms or say something that sounds stupid.  This is my first real endeavor into this kind of project and though I've been working on it for about a year now (taking my time to learn each piece as I go) I still have ALOT to learn obviously.

[harisuluv]

No worries walnuts, more important than anything is that you appear to be going through things in an order that makes sense.  It's hard to eliminate possible causes of a problem when you're just throwing darts at a board.

Good work, you will get there for sure.

[TwoTired]

Education for the eyes that bleed... 

If carbs only meter fuel by means of "venturi effect" from the air velocity though them, how do they deliver any fuel before the engine starts?  And how does an entrance "choke plate", which essentially blocks airflow, actually increase the fuel volume entering the carb throat?

Annswer: pressure.
The delivery force which actually gets fuel mass into the carb throat comes from atmospheric pressure.  Yes, that column of air pushing down on the surface of the earth.  To make jets flow, the pressure at each end of the jet tube must be different, and the volume of fuel which flows is directly related to the pressure differential.  This jet tube and it's orifice does not care whether the pressure differential if from venturi effect, or the simple fall of the piston in the cylinder, which provides the source vacuum (differential pressure) to get the whole shebang into operation.

One fall of the piston provides this differential pressure until it is equalized by the inrush of atmospheric pressure. 

Now imagine a tube 10 ft long, and you apply, say, 14 PSI at one end, and 4 psi at the other end.  If you somehow maintain those pressures at each end, you will have a gradient pressure inside that tube as well as material flow within that tube.
For the sake of argument and ease of calculation, let's say it was a linear relationship.  So, the pressure at 5 ft (half the distance) would be half the differential pressure presented to the tube ends. 
The math: 14-4=10 (Total difference of tube pressure end to end.) 10/2=5 (50% position)  5+4=9 PSI at the 5Ft position of the tube.
Now shorten the tube to 8ft, and apply the same pressure to the tube ends, but still take your pressure measurement at 5ft away from the 4 PSI source.  Note that the very same volume of material is transferred from the high pressure side to the low pressure side.  You only shortened the tube/duct.
The math: 14-4=10  (5/8= 62.5% position from high pressure source)   62.5% of the total differential 10, is 6.25 PSI  + the 4 psi basis) = 10.25 PSI at the 5Ft position, simply by shortening the duct. 

If we insert (stub) a jet tube at the 5ft position of a ten ft tube, and supply that tube with the same 14 psi that supplies the entrance tube end.  The jet tube experiences a 5 psi differential from one to the other.
If we simply lob off 2ft of the duct. that same jet tube now experiences only 3.75 psi differential across it.  For any given orifice size within that stubbed jet tube, more volume of fluid will traverse it when the pressure differential is higher.  Same volume of air through the main duct less volume of fluid through the stubbed jet tube means that the mixing ratio changes.  To restore the original ratio you must change the orifice in the jet tube.

Pods shorten the intake duct and allow faster access of the atmospheric pressure to reach the jet exit ports in the carbs, reducing fuel flow of the existing jets. And THAT is only one reason why pods force a jetting change to restore proper fuel mixtures.

The other factor is the pressure drops the filter membranes present to the duct entrance.  Pods can have different filter media and total filter area which changes the pressure differential across them and is then presented to the carb throats.  Change the pressures in the carb throats, and for the same volume of air passing, you must change the jet orifices to restore proper fuel mixture.

Anyone who says changing the stock induction to pods has no effect on jetting, simply is missing some very important factors in the changeover.

See also:
http://forums.sohc4.net/index.php?topic=83904.msg945765#msg945765

[RJ CB450]

Isn't that what I said?  In a carb however need to add the change in velocity through the tunnel creates a strong vacuum.  The choke closes off intake upping the fuel more.  Problem with the logic given is it assumes constant air flow.  I/R carburetion does not have this.  It pulses.  Can be seen with sensitive enough gauges.  What you described is only a small part of the system and if you measured the pressure head over the change in distance, it would be negligible on the fuel intake side.

In fact, if you see a cb650 airbox set up, it is big and wide open.  Actually, my 82 has two airboxes.  It goes filter box, duct, airbox, horns.  The horns themselves are quite short, but they stabilize the airflow.  If we need to get off topic and discuss fuel circuits and trying to jet them to match, but still no help because the main parts of the carbs that make jetting work still wont work proper.

[walnuts]

I haven't disappeared and given up for disregarded the advice given (and found the "Educational" posts quite interesting) but I've simply not had time to work on the bike again yet.  So please, stay tuned for more questions to come.