Topic: Idle problems? Or is this normal.(and how I setup 750 carbs on a 650)

[Soos]

Ok...

My '79cb650 idles nicely at 1300-1400 rpm, but is erratic and dies anywhere lower than 1200 rpm. Bike has 40+K miles on it.
My '81cb650c idles REALY nicely at 1000 rpm, will even idly ok down to about 600 rpm. Bike has 14K  miles on it.

The '81 is bone stock except for the main jets. I dropped them 1 jet size.
I did a full tune up about 2-3 months ago, have never dona a carb synch however.
CV carbs.

The valve clearances on both bikes are 0.002 intake, 0.005 exhaust.


I was running my '79 with dyna 3 ohm coils, '73 cb750 carbs, '84 nighthawk voltage regulator, '79 750DOHC CDI units... everything else non-stock has nothing to do with the motor or electrical system.

I did a full tune up including a synch about 2 weeks ago.

Tonight I tried swapping back to the original electrical bits that are stock to the '79, and no positive change at all. Seemed to need higher idle by 500 or so rpm.
I also re-checked cam chain tension,  timing, valve clearances, spark plug gap, all wire connections and anything else i thought might even affect the idle.
I did not swap back to the stock carbs. They are dis-assembled at the moment.
Cylinder pressures range from 150 to 160 all 4 cylinders.


Is the higher mileage a possible explanation why the  '79 requires a higher idle?
Valves no longer seating properly?
Could this be due to the cam chain being stretched over time?
And what is the life expectancy of a cam chain?


l8r

[Soos]

I ask because last year and earlier this year my '79 would idle at 1200 without problems.

But the past 2 months of riding, it seems i have had to up the idle 2 or 3 times.

l8r

[eurban]

Perhaps your pressed in idle jets have become plugged.

[scondon]

 I'm not familiar with the cb650's, but am curious what it took to fit '73 750 carbs to yours. Are the outlet diameters different between your 650 and 750 carbs and, if so, are you using different carb rubbers to avoid leaks?  Do the 750 carbs fit in your stock airbox, or are you using pod filters? Did you do any cleaning or rebuilding of the 750 carbs?

   Sounds like you have ruled out most everything else and need to check out your carbs.

[Soos]

Perhaps your pressed in idle jets have become plugged.

On the '81??

The '79 cb650 has '73 cb750 carbs, and the slow and high speed jets are screw in type.

l8r

[Ibsen]

I agree about the carbs. Since the engine has been running good and idled properly earlier this year, I would check for air leaks in the intake manifolds first, and the pilot jets and pilot passages secondly. And the carbs could also be out of synch.
Btw, according to my book, the valve clearances for all the SOHC 650's should be intake .002in/0.05mm  and exhaust .003in/0.08mm.

[Soos]

I'm not familiar with the cb650's, but am curious what it took to fit '73 750 carbs to yours. Are the outlet diameters different between your 650 and 750 carbs and, if so, are you using different carb rubbers to avoid leaks?  Do the 750 carbs fit in your stock airbox, or are you using pod filters? Did you do any cleaning or rebuilding of the 750 carbs?

   Sounds like you have ruled out most everything else and need to check out your carbs.

The '73 carbs literally bolted right on.
Ok, I had to modify the location of the hanger that holds the push/pull cables(the pull side in particular) to get it to clear the motor.(bent the heck out of it, but it works without a problem)
Another thing I had to do to get the 750 carbs work is (I hate to admit I did this) I cut some off the side covers to allow the lift bar to clear them.
Unless you remove the covers you can't see the cut out area, but I know it's there....
The pull cable is from a '81cb650c, but the push cable is a stock '79cb650's.
I now believe my '79 is past the point to be able to do a full restore to truly stock anymore...

Anyways....
The OD of the motor side of the 750 carbs is 1/2mm(or less) larger than the stock 650 carbs, but basically negligible as to instillation purposes.(maybe seals better due to this?)

Spacing is nearly identical as well.(center to center of the carb bores) Nowhere enough to cause any difficulties installing.

The only tight fit was the intake side of the carbs.
The stock 650's are actually BIGGER!
Mechanical linkage(non-CV) 650 carbs almost look as the intake side is set up like a velocity stack, they bell out quite a bit.

I was hoping I would be able to use the pod filters I had been toying with when I had the stock carbs on, but I couldn't squeeze the rubber on those small enough.
The stock air box intake boots are thin enough and flexible enough I was able to use smaller clamps and get them to have a good seal though.


The reason I wanted to try running the 750 carbs is they have a larger bore diameter venturi. (and a full rebuild on 750 carbs -$70.00 the 650 carbs - 170.00!!! did I mention i'm cheap??)
Also they have a 1 or 2 mm bigger throttle piston.(can't remember offhand how much bigger)

I'm taking off the 750 carbs today, and will try to remember to take pictures to show these differences.

And cleaning... yes.
LOTS and LOTS of cleaning.(nothing too bad, but there was varnish everywhere)
Completely torn them down, used carb dip, and compressed air and a few micro fiber cloths.(no drills or wires)
When I put the carbs back together, I used new rebuild kits(aftermarket).
The kit had almost everything,new float valve assemblies, new jet needle, jet holder, main jet, slow jet, all gaskets.
As well I replaced all o-rings and hoses with new parts.
I am using a "T" for the dual gas line the 750 carbs, my petcock has a single output line only.

Oh, I also had to turn my petcock at about 25-35 degrees off to make clearance between the gas line and the lift bar of the 750 carbs.



Oh well, it runs like a bat with it's a** on fire though! 

The 750 carbs have shown a marked improvement on in-town gas mileage compared to the stock carbs.
The pull from low rpm is smoother.
The choke lever is pretty much non-damage-able !!!(and it looks cooler on the side of the carbs)
As well as the 5 minuite jet changes. (god I LOVE that!!).
The length(motor to airbox) of the 750 carbs is shorter, making removal easier as well.
I have yet to find a negative side to these 750 carbs.


l8r

[scondon]

  Good info.  Sounds like you have been paying attention and been very methodical doing this mod

   After reading everything you've done, and hearing that this is a symptom that has developed over time I would suspect either a leak at one or more of the air filter boots, or that the air passages of the idle screws have become partially clogged. My guess would be the latter.

   If one or more of the carbs is running leaner/richer than the other at idle it can really affect the speed at which the engine can idle without dying. My method for setting the idle mixture screws(71-76 carbs) is to get the engine good and warm, then turn all mixture screws out to 1 1/2(lean). I'll turn them in, one at a time until the rpm's start to bog, then turn it back out 1/4 turn from that point. Doing all four carbs this way, if one or more carb requires 1/4 turn more/less than the others then I'll check that idle circuit, provided that I'm confident the synch(and all other tuning procedures-ignition timing, valves, etc..) are good.

   I admit that I am a little lazy and will try the easy route first. In this case it would be to drop all the float bowls and remove the idle jets. Then remove all the mixture screws(4) and spray copious amounts of carb cleaner through the screw ports and idle jet holes. Also, check for white chalky substance that can develop on the steel? mixture springs that contact the aluminum carb body.

Hope this helps

[Pinhead]

This is extremely interesting. Especially the part about your "marked improvement" in gas mileage and torque. What kind of mileage are you getting, BTW?? And what jets are you running?

[Soos]

I dunno about torque, I was mainly referring to smoothness of acceleration from low RPM's as compared to stock carbs.(lack of hesitation)

And the "marked improvement" on in town milage(30mph, lots of stop and go driving) was in the upper 30's low 40's MPG with stock carbs, With the 750 carbs, I get an average of 44mpg, and have hit upper 40's doing town driving.
5-10mpg is a "marked improvement" to me.

Mind you my best MPG I get now is down with highway driving though.
Stock carbs gave me (once) over 60MPG on highway speeds, and averaged in the high 40's to low 50's for MPG.
Now (with the 750 carbs)my MPG at highway speeds is between 47-50 mpg.(54 is the highest I have recorded so far)

the 750 Carb body # is 7A's, the 650 carb body # is PD-50A.
In the 750 carbs I am using 40 slow jets, mains are 100's, but I am changing them to 98's today.
The throttle needle clip is at the second from the top slot(making them run leaner up to 3/4 throttle?) I am going to move the clip one position richer(middle slot on the needles) while I am changing to 98 mains.

I'm working on downsizing pictures at the moment, but I can throw out some numbers I measured of the differences between the '79 650 and '73 750 carbs.

Intake side (OD)
650 - 1.804/45.82mm
750 - 1.578/40.08mm
motor side (OD)
650 - 1.395/35.43mm
750 - 1.455/36.95mm
length (from face of intake side to motor side face of the carb body)
650 - 3.930/99.82mm
750 - 3.475/88.26mm
slide diameters (of slide itself, not bore)
650 - 0.943/23.95mm
750 - 1.020/25.9mm

I was incorrect earlier on which cable mount I had to bend...
It was the push, not the pull is the one I had to seriously bend in order to clear the motor.

And after looking at my '81 frame, even with a '79/80 motor(non-CV carb head) I think the '79 frame is the only one to allow enough clearance for the pull cable holder.
The '81 frame dips down too far for clearance.(IMO, i have not tried this)
If you were to find a set of 750 carbs that had a more compact push/pull cable holder it MIGHT give enough clearance, or made up a custom one it might work on a newer frame.


Hope this info might help some other nut get a base setting for doing this to another '79 one day.

l8r

Gonna have to change the title of this thread.....

[Soos]

here are a few pictures if of areas i measured... sorry, i couldn't get the caliper measurments to show well. Good think i manually entered them anyways.

[Soos]

and yet more pictures.

[Soos]

And the last three pics I got for you all.
2 of the bent up pust cable hanger, and a re-assembled pic of my 650 with the 750 carbs.
It looks even better now I have those little black boots on the lifter arm...

l8r

[Soos]

And... no I don't clean my engine too often.

l8r

After taking daisy around the block a few times with the kids, and re-setting the idle screw a bit, The idle has not changed really.
(the idle adjustment screw is a PITFA to get back in AFTER installing the carbs!!!)

And to whoever found you can adjust the clip without having to re-synch... YOU'RE A GENIUS!!!
I went ahead and hooked my manometer to see how far they were off after adjusting the needle clips.... not worth adjusting IMO.

I removed all the slow jets, and all still look brand new, no varnish yet, and no bits of floaties in the bowls either. But thats expected, my tank has a liner in it... better to check anyways while i was in there.
Hoping the leaner jets, and one notch ritcher jet needle either keep the MPG i have, or increase it a bit.



Anyone have any ideas as to why my idle has needed to be raised to keep from killing my motor? I'm open to anything.(if i can figure out how to test it)
Checked the battery, and at 5000rpm, a voltmeter reads 14.7 volts.

Beyond that, i'm out of ideas for now.

l8r

[paulages]

good to hear your 750 carbs are working well for you. i haven't figured out which i'll run with my 718 ('79 head), but i did fit up a set of cb750 PD carbs and also found that they fit fine. i like the idea of having an accelerator pump, but like the adjust ability of the early series. considering the amount of tuning i'll be doing, it'll probably be the latter.

[Soos]

Found the problem.(or at least i think it was the problem)

I tried testing the resistance of the spark plug boots, and cyl #3 was reading resistance of about 12K.

put on a gook 5k resistive plug boot... and it idles properly again...

I didn't think that the boots would go bad, but i guess they do from time to time.
Idles nicely at about 1200 now.

l8r