Topic: Coil testing for dummies

[Patrick]

I found this thread in the FAQs:

http://forums.sohc4.net/index.php?topic=5752.msg8383#msg8383

It is a good thread. Well written. Very informative. Unfortunately, it assumes a level of electrical expertise and understanding I have not yet attained. Ths is my situation:

My CB750 K5 starts very easily. It runs very well when cold. It idles smoothly at 1200, again when cold. When the bike gets hot, it idles badly and usually dies it I don't keep revving it. It runs fine, even when hot. But the lack of a reliable hot idle is pretty annoying. This is what I've checked:

Three days cleaning electrical connections. I could serve a formal dinner on this connections, they are so clean. I can no longer disconnect and coupling without a call from the FAA accusing me of blinding the pilots in passing aircaft. I'm talking shiny here. Bike seems to be charging correctly now.

I have reset the intake valves. They are spot on.

The points are clean and unburned and only have a few hundred miles on them. Dwell is at 92-93 degrees and my well tach indicates it is as exact matched as a dwell/tach orginally bought used by Fred Flintstone can measure. Condensors were new with the points. 

The plugs are a tinge dark, so the bike is running slightly rich, but not black or even dark, dark brown. The timing, checked with a timing light, looks right on the mark on both sides. I've pulled the air fliter and all the choke plates are pulling up completely. Stock air box. The air filter is a K&N and it was new two months ago. No fuel filter, and fuel flow seems to be fine through the petcock. The bowls on the carbs are filling appropriately. That brings me to the coils.

I am starting to strongly suspect that my coils, originals that they are, are starting to believe they have worked hard long enough and are ready for for stud service and pasture. I may be projecting here. At least about the stud service thing. Before I invest in new coils, however, I want to make sure I'm not chasing shadows. So I searched the FAQs and I found the string listed above.

Good God.

Can someone dummy it down for me a bit? What setting do I use on my Craftsman multi tester? Continuity? It that, like, voltage or resistance? Or should I be looking fr a "C" somehwere. And which wires to I attach the tester to? The blue and the spark plug wire and then the yellow and a spark plug wire? Or perhaps pick a couple of spark plug wires?

I apologize in advance for my Rainman-like simplicity. But I have to say I think I come a long way on the electrical investigation thing over the last couple of months. Until a breif few weeks ago I would get all excited and want to call my mother when I threw the wall switch and the lights came on - especially if I didn't shock myself. Now look at me! Electrical investigations! And moderate levels of success at that!

While I wait for replies, I have to call mom....

Thanks, y'all.

Patrick

[TwoTired]

Continuity: in electrical parlance it is the abilty to conduct electricity.  Zero or near zero ohms resistance is high conductivity or continuity.  High values or readings of resistance nearer to the infinite is considered low connectivity or no continuity.

However, your problem description of running well cold, then poorly when warm, seems to point toward fuel mixture issues rather than spark issues.

Two suggestions:
Temporarily remove your air filter to determine if your hot performance improves.  It adds restriction, making the carbs run slightly richer.  Removal should lean things up.  It's a diagnostics tool, so you can chase the proper cause.

Review/check your Idle Air Screw settings.  Book values? Turning them out makes the idle a bit leaner.

Are any of the head pipes colder than others?

Have the carbs been synchronized in your ownership history?

Cheers,

[Patrick]

Hi TwoTired,

I will take the air filter off and see if it makes a difference. In reference to your other questions, all the header pipes get very hot and the plugs are similar in their burn characteristics. I synched the carbs early this spring. I have to synch them again since I adjusted the intake valves, but the hot no-idle problem is subsequent to the initial  synch. I only have a mercury tube synch tool, but it seems to get them pretty close.

I'll let you know about the air filter removal and its impact if the rain will ever stop. I never thought I would be hoping for it to stop raining in Texas. We usually have the opposite problem.....

Patrick

[Patrick]

And sorry about the double post of this inquiry. When I tried to post the string I got an error message saying the server could not be found. I hit post again and the string appeared twice.....

[Klark Kent]

my bike started dying at stops when hot today.  did you get to the bottom of this particular symptom?  my situation seems very similar except i ride a 550K1.
thanks for anything you happen to remember.
-KK

[kettlesd]

My local bike shop will test coils for free. Takes two minutes to hookup, test and disconnect on the apparatus they have.

[TwoTired]

my bike started dying at stops when hot today. 

Did you switch to reserve?

[nteek754]

Hey all I would check out your intake carb boots  Im thinking once it warms up the intake boots get kinda plyable and may allow unwanted air in the cylinders an easy check would be to wrap all 4 with tape then try it like you did before hope this helps seven fifty four ever Ceaig in Maine

[mlinder]

Or spray starting fluid at your boots while running. If rpm increases, you've got a leak.

[kghost]

I'm curious when you say that the idle problem is subsequent to the carb sync and the valve adjustment.

Just a hunch.....

Are you sure you did not get those valves too tight?

If your off and the valve clearance closes as the engine gets hot the valves are held off the seat slightly and Idle goes all to hell and gone as the engine warms up.

Did it idle before?

[Klark Kent]

i think i figured out what did it to me...

no, i hadnt switched to reserve, but it was the longest run i have had since i last adjusted my air bleed screws.  i had adjusted the carb slides as close to synch as i could a while back and the machine was running sweet, but i noticed my number one pipe was hotter than the others shortly afterstartup.  so i leaned out the other pipes by loosening the screw and tightened the number one screw in an effort to make it run a little cooler like the others.  i put them back the way they were and i think my idle is back.

but i havent been on a really long run to find out yet.

-KK

[TwoTired]

The reserve setting fills the carbs from the bottom of the tank instead of up at the stand pipe level when "on".

As the fuel tank level approaches the top of the stand pipe, fuel flow to the carbs becomes sporadic.  When the carbs begin to starve for fuel the fuel levels lower, (Duh).  The main jets receive fuel from the bottom of the bowl and are the last to starve.  The slow, or idle jets are fed from higher in the bowl and these stave first as the fuel level falls in the carbs.

When the idle jets starve, the engine won't idle.

Sometimes, switching to reserve will solve an idling issue. 

There are certainly other reasons for poor or no idle, as well.

Cheers,

[Patrick]

When I had the issue with my K5 750 in June I finally narrowed the problem down the the carbs. The needle jets in my carbs were very worn and I couldn't get the fuel mixture adjusted right. I had one air screw out one turn and I had one out four turns and the others were somewhere in between. After I replaced the needle jets the bike idled (and idles) very nicely. It's like a new bike. It's been running very well ever since and the plugs are the prettiest shade of tan I've ever seen - except, of course, for the skin tone of that lovely young hula girl I encountered on Kawai last year. Now THAT was a gorgeous tan. So pretty, in fact, that I can't swear for a fact that she had a face....

Patrick

[Klark Kent]

patrick-
all of that is good to hear.
ive cleaned my carbies lenty, and replaced all the ruber bits, but i never went ahead and bought the kits with new jets.  they look to be original and my air screws tell the same story yours do, so i am pretty sure what my next project is.  thank god too, i thought i was down to wheel truing.
thanks for the help
-KK

[TwoTired]

There are a lot of similarities between the 750 and the 550.  There are a lot of differences, too.  I've seen worn slide needles and needle jets in 750 carbs (not that many, really.  But, it not hard for me to believe it is a common problem).  However, I have never seen worn slide needles or slide needle orifices in the 550 carbs.  I don't know exactly why that is, though.
I've found cb550 carbs where the slides and slide bore was worn and sloppy.  But, even then, the needles were still undamaged, which seems odd.

Has anyone actually seen Cb500 or 550 carbs with worn needles?

Just curious...

Cheers,