Topic: 78' 550 Idle issues-Not stock

[K_Kalynuik]

 Hi All, I'm going to get right to the point. I'm having some idle issues. The bike runs great above idle. No flat spots what so ever. Starts first try and pulls strong through all the gears. Problem is at idle. When cold, I can start the bike with very little time on the choke. Maybe 20 seconds. Then turn it off. (I didn't touch choke at all when even close to warm). Idle climbs a bit at first start to around 2000-2500 then settles back down to around 1800 when it gets warmer and I turn off the choke. Throttle response is good at this point and this seems to be the lowest idle rpm that the bike can stay at before it wants to die on me. If I turn the idle knob out to lower the rpm, the bike will briefly idle around 1200-1400 rpm but then dies if I didn't keep throttle at it. This is even worse when I come to a complete stop at lights. I seem to have to blip the throttle to even keep the bike running when coming to a stop. After the throttle blip it idles OK but too high rpm because of the low idle issue. If i adjust the idle knob at this point to calm the rpm down to 1400, it just dies.

My bike is a 78' CB 550 with a 592cc dynoman big bore kit and a webcam'A' camshaft (street cam shaft). Very mild porting done to intake runners and ports. All brand new orings at runners and no air leaks anywhere around the carbs (double and triple checked). The intake runners have been matched to the intake ports. Stock PD46 Carbs have been sonic cleaned and rebuilt with all new everything. 110 Mains(too rich at the moment but not the problem), 45 slow jets, Needle third clip from the top and I have experimented with idle screw settings from 1 turn out to 3 turns out. 1.5 turns out seemed to be the closest to best idle but still didn't help. I have the stock Air plenum but have a single large K&N filter attached to the modified air plenum. It has a Mac 4 into 1 header and an open baffle style cone engineering muffler. Dyna ignition and 5 ohm coils. Dyna wires and caps and DR7EA plugs. New battery and charging system has been tested and verified working and charges well. I pulled the plugs after some idling and they are sooty black (not overly sooty) but dry.   

Will running rich on the idle circuit cause my idling issues?


check out my build thread. http://forums.sohc4.net/index.php?topic=121580.0

[Dave Voss]

If I turn the idle knob out to lower the rpm, the bike will briefly idle around 1200-1400 rpm but then dies if I didn't keep throttle at it. This is even worse when I come to a complete stop at lights. I seem to have to blip the throttle to even keep the bike running when coming to a stop. After the throttle blip it idles OK but too high rpm because of the low idle issue. If i adjust the idle knob at this point to calm the rpm down to 1400, it just dies.

It sounds like the float level may be too low.  I don't have a big bore kit, aftermarket camshaft, or a 4-into-1 exhaust, but I did have similar idle issues on my '78 CB550K, and in my case it turned out to be an incorrect float level which was effectively starving the engine of fuel when the throttle was too close to being closed, as the slow jets were above the fuel level.  The earlier carb float level setting is 22mm, but the later PD carb float setting should be 12.5mm, and some manuals don't address this difference.

I wouldn't think that size #45 slow jets would be rich enough to cause fouling of the spark plugs, as size #42 is stock, and that's not that big of a change.  The size #110 main jets are probably too big (as you said), so I would consider trying size #105 or #100 to see if that eliminates the black sooty fouling.

[K_Kalynuik]

Thanks for the reply Dave. I do have the floats set to 10mm instead of 12.5 but dont think it can go up any more. Ill take a look. I didnt think the 45 slow would cause an issue either but maybe it is? who knows. Ill get new plugs, change the slow back to 42 and see if that helps.

Oh and they were PD46C carbs. I forgot to put the 'C' in the original post.

[TwoTired]

I'm having some idle issues.

As in - it doesn't?

The bike runs great above idle.

Hmm, I don't think you know this really, without a an HP curve or some test track times, or some plug reads under full and mid throttle.

No flat spots what so ever. Starts first try and pulls strong through all the gears. Problem is at idle. When cold, I can start the bike with very little time on the choke. Maybe 20 seconds.

This would be more meaningful if we knew what the outside temp was. Or what your definition of cold is.

Then turn it off. (I didn't touch choke at all when even close to warm). Idle climbs a bit at first start to around 2000-2500 then settles back down to around 1800 when it gets warmer and I turn off the choke.

RPM climb behavior during warm up, normal.  Tach numbers are too high to be called idle.

Throttle response is good at this point and this seems to be the lowest idle rpm that the bike can stay at before it wants to die on me. If I turn the idle knob out to lower the rpm, the bike will briefly idle around 1200-1400 rpm but then dies if I didn't keep throttle at it.

Describe how it dies.  Quick cut off, or gradual rise of RPM before "dying".  Or, slow loss of RPM before quitting.

I have experimented with idle screw settings from 1 turn out to 3 turns out. 1.5 turns out seemed to be the closest to best idle but still didn't help.

How did less than 1 turn behave?

I have the stock Air plenum but have a single large K&N filter attached to the modified air plenum. It has a Mac 4 into 1 header and an open baffle style cone engineering muffler. Dyna ignition and 5 ohm coils. Dyna wires and caps and DR7EA plugs.

Aren't the Dyna wires Resistance wires?  And then you added more resistance with resistor plugs?  Do you know the total resistance for each plug lead circuit?  Too much resistance will reduce the spark current cooling the spark somewhat and making rich mixtures more difficult burn completely.

I pulled the plugs after some idling and they are sooty black (not overly sooty) but dry.   
Where is it sooty?  all surfaces? just the outer ring?  What about the insulator?  Can you see any white?

Will running rich on the idle circuit cause my idling issues?

It can.  But they are supposed to run on the rich side at idle, to have proper throttle response.  However since you can't keep idle speed, you can't really test for proper throttle advancement.

Bigger is not necessarily better for everything you know.  Think tumors.
Did you try the carbs with 42 pilot jets?   Were the pilot jets you used from Keihin or after market?
Were the other parts you replaced in the carbs from Keihin?
Did you compare the tiny emulsion tube holes between the original and replacement?

While the 42 -45 change increased fuel volume by 15%, the air jet remained the same dia., and it is now unknown if the emulsion tubes mixed more or less air into the pilot circuit premix due to a size change.

.42 dia > 0.1385442 mm²
.45 dia > 0.1590431 mm²
 Diff    .0204989

Also unknown is how the new vacuum levels at idle compare with the old vacuum levels.  With the same cam profile and displacement increase would deepen the vacuum that pulls on the all the jets, also increasing the fuel drawn.   But, you've changed the cam, and we have no numbers as to how the vacuum a idle has changed.  Many cams will sacrifice idle behavior for upper RPM power gains.  I check the Webcam site for their grind numbers.  But, I don't have the stock cam numbers to compare it to readily available.
What does Dynoman say about idle expectations?

If you are changing jet sizes at random, you can expect random results.  FYI

I do have the floats set to 10mm instead of 12.5 but dont think it can go up any more. Ill take a look.

Why did you set to 10mm?  Don't you realize that makes the entire operating range richer due to the higher fuel level?  It may even make the throttle valve jet leak raw fuel into the carb throats bypassing the pilot circuit.  With all the size increases you've introduced, I speculate 14.5 mm would be a better starting point, as that will lower the bowl fuel level farther away for the carb throat.

I didnt think the 45 slow would cause an issue either but maybe it is? who knows. Ill get new plugs, change the slow back to 42 and see if that helps.

Do get a couple more sets of new plugs to proceed with the tuning process.  Can't tune properly with fouled plugs.

[K_Kalynuik]

Thanks for replying Twotired, your knowledge is wayyyyyyyy beyond me and I thankyou, ill try to do my best with some of the answers. Also, Im new to forums so I dont know how to do that thing you do where you show the previous post and then your response to it. So forgive me on that. Ill figure it out though.

Yes your right, it does not idle to be more precise.

As for my comment on it running great, No I dont know this for sure, sorry. It runs satisfactory for me. This was my first bike build so I was damn surprised when it even started. It seems to pull well but hey, what do I know.

Outside temps have varied in the 3 times I have ridden the bike. All 3 for more then 60 kilometers/37 miles. First time I rode it, it was around 12c/53f. Second time around 20c/68f and the last time it was around 22c-24c/71f-75f. Ill be honest and say that as the weather gets hotter, it seemed to die more at idle. The first time i rode it, the coldest time, it was running the best. Didnt die at idle once at 1400. And idled the smoothest around 1400-1500 rpm(but still high i know).

If I were to describe the way it dies, I can get it to idle at 1400rpm for about 5-10 seconds and then it slowly dies off. it gets into the 'sputtering' stage before dieing. No surging.

I did not try less then 1 turn. 1 turn out was the lowest.

As for the Dyna wires, I had the supplier of the wires email Dyna directly about this issue. I received the wires with no labeling as to what ohm resistance was in the caps. From what I was told, they had no resistance. So they said to use the R series of sparkplug. I was just going by what Dyna told me. Im sure you probably know better though. I will test them this weekend to be for sure. I should have done this in the first place. Things that I learn along the way and thankyou to guys like you for being patient with guys like me.

For the plugs, They are close to what you describe as in they have more soot around the outside and less inside. Its not pure white by anymeans but I can see traces of white on the insulator for sure. Ill take better pics this weekend as well.

For the Jetting and carb bits, Yes they are all Keihin parts.  All the spare main and slow jets I bought are Keihin as well. I made sure of it. The sizes of jetting and clip pos. I used were all recommended to me by other users of the same setup and dynoman. It was what I was using as a baseline as I knew the stock settings would not work. The 110 main was larger then I was told to use but I didnt want anychance of being too lean and burning my new big bore kit up.
The emulsion tubes I did not compare under a microscope and check the diameters of the little holes, but it looked pretty damn close from my eye.
I will try the 42 jets and see.

For the Float heights, no I did not know that it would enrichen the entire range. Again, was only doing what was recommended for the setup. I am very glad to have you tell me that though. Any little bit I can learn is benefit to me. I will set the 14.5 float height first and see where that goes, along with the new plugs. Oh and Ill test the resistance in my dyna wires to confirm.

Thanks again for replying.

[Scott S]

 Do a vacuum synch. I had a similar problem on my '78 550K, also modded but not as hot as your bike.
 Turns out the out of synch carbs (even though I bench synched them) made it VERY difficult to set the idle. That, in turn, made it hard to set the timing, since the advance was already starting to work when the idle went up.

 I'm also learning that these "lean" carbs aren't as bad as everyone thinks. I'm going to end up with stock needle clip positions, a little richer on the IMS screws and a 95 main.....not far off rom the stock settings at all. I, too, went way too rich in the beginning.

[K_Kalynuik]

 Thanks Scott. I actually did a vac sync. helped a bit. ill be spending the summer testing, changing, testing, changing and then testing some more I guess.

[TwoTired]

Outside temps have varied in the 3 times I have ridden the bike. All 3 for more then 60 kilometers/37 miles. First time I rode it, it was around 12c/53f. Second time around 20c/68f and the last time it was around 22c-24c/71f-75f. Ill be honest and say that as the weather gets hotter, it seemed to die more at idle. The first time i rode it, the coldest time, it was running the best. Didnt die at idle once at 1400. And idled the smoothest around 1400-1500 rpm(but still high i know).

If I were to describe the way it dies, I can get it to idle at 1400rpm for about 5-10 seconds and then it slowly dies off. it gets into the 'sputtering' stage before dieing. No surging.

Colder air is denser, therefore containing more oxygen.  More available oxygen allows for more hydrocarbons to "pair off" during combustion.  Therefore, cold air induction can support a richer air fuel mixture.

This data point and your description of how it dies trying to idle, support the diagnosis that existing pilot/idle mixtures are too rich to sustain combustion.

I did not try less then 1 turn. 1 turn out was the lowest.

The PD's Idle Mixture Screw has the ability to completely shut off the pilot circuit.  Can't make the idle mix more lean than that.  As it leans the RPM should increase as it "clears out" the over rich condition. And if after shutting off the idle circuit, it is still too rich, that confirms the throttle valve circuit is leaking way too much.  It shouldn't have much contribution until the throttle is opened to the 1/8-1/4 position.  I should mention that since it cannot operate at idle on the pilot circuit alone, it is probably using at least some of the throttle valve to have enough fuel to keep running.  The throttle calibration will be biased with less total remaining travel than it should have.  It's important to understand this, as the slide are mechanically linked to the throttle, and are RPM independent.  Load factors dominate RPM, and slide positions are a request for power or no power.  The slide position still determines which metering circuits in the carb have a dominant effect.

For the plugs, They are close to what you describe as in they have more soot around the outside and less inside. Its not pure white by anymeans but I can see traces of white on the insulator for sure. Ill take better pics this weekend as well.

You might find this interesting:
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

[K_Kalynuik]

Thanks Twotired. I do find that link very interesting and its going in my favorites. Thanks a bunch.

 So if im getting this, I should set my float height back to 14.5. See how that works. Than if no luck, try turning in the idle screw to 1/2 turn out? If no luck, than try decreasing the size of my pilot jet from #45 to #42. Or anywhere in between that?

[TwoTired]

So if im getting this, I should set my float height back to 14.5. See how that works. Than if no luck, try turning in the idle screw to 1/2 turn out? If no luck, than try decreasing the size of my pilot jet from #45 to #42. Or anywhere in between that?

Sounds like a reasonable plan (I assume you meant IMS instead of idle screw).  Though I don't quite understand why the pilot jet needs to be #45.