Author Topic: CB400F Alternator question  (Read 3336 times)

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Offline Simon

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CB400F Alternator question
« on: April 25, 2010, 12:01:39 PM »
Quick question please

Should the resistance between the 3 yellow wires from the generator coil stay the same between a cold motor and a warmed up motor?

CB400F 1975

Cold motor, resistance measured with a multimeter set to 200 Ohm. testing all combinations between the yellow wires = 1.0 Ohm on each combination

Hot motor, resistance measured with a multimeter set to 200 Ohm. testing all combinations between the yellow wires = 1.5 Ohm on each combination

The Hayns workshop manual (1995 edition) states this :

The resistance between any two of the three yellow generator coil leads in turn should be 0.61 - 0.69 Ohms for the CB 400F.

Looks like I'm at double the prescribed resistance.

Bike is good and strong when cold. When it's warmed-up, it's fine at idle but hesitates severely in the first 1/8 throttle. It's strong and steady at higher throttle.

Opinions, suggestions appreciated.

Simon

Offline Simon

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Re: CB400F Alternator question
« Reply #1 on: April 25, 2010, 08:47:48 PM »
TTT for the late night people.

S.

Offline scottly

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Re: CB400F Alternator question
« Reply #2 on: April 25, 2010, 09:06:17 PM »
You can't accurately measure such low resistances with your meter. If you suspect a charging problem, measure your voltage at the battery with the motor running at 2000RPM. 
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Offline camelman

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Re: CB400F Alternator question
« Reply #3 on: April 25, 2010, 11:46:02 PM »
I doubt it is your alternator.  What have you done to tune the bike up lately?

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Offline TwoTired

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Re: CB400F Alternator question
« Reply #4 on: April 26, 2010, 12:05:28 AM »
You have far more faith in the meter and your test technique that warranted.

On a 200 ohm scale, a meter with a 1% error allowance would read any value plus or minus 2 ohms.  If you are reading 100 ohms, a 2 ohm error isn't much concern, usually.  A 2 ohm possible error when reading 0.7 ohms, is a misapplication of test equipment.

I've seen meter leads and probe tip connections have .5 ohms, without adding in any meter measurement error.

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Offline Simon

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Re: CB400F Alternator question
« Reply #5 on: April 26, 2010, 07:55:53 AM »
Thanks for the reply gentlemen.

I do not have any charging problems. I was just testing the electrics (voltage drops, resistances, etc.). I have been cleaning all the connectors and switches and in the process, getting to understand the electrical part of the bike. I thought it would be best to test the components while they are good and note all the readings in my book so I could compare them to eventual readings when a problem occurs. I'm not quite there yet as I am still in the learning curve.

I understand now that the multimeter I am using is not as precise as one would want to do this test. However, although the actual Ohm numbers can not be considered accurate, the readings are different between a cold stator and a hot one. Would my $25 multimeter be accurate enough to at least measure a difference? or am I on a wrong path here and should just forget about it. Maybe this is completely normal too, I don't know. BTW, there is continuity between the yellow wires and no continuity between the yellow wires and ground when cold. I read that this is good. I have not checked when warm.

If my battery gets full charge, would this mean that the whole alternator (stator and field coil), and all the ignition components are OK and that my 1/8 throttle problem can only be carburetor related? Or have you ever experienced bad ignition only when the engine is warm?

I have connected the ignition coils directly to the battery with the solenoid trick because even though everything was disassembled and cleaned, the kill switch robed me almost 3 Volts. I did not cut anything on the original wiring harness and plan to replace the kill switch soon as a permanent solution.

This modification did give me an easier start and better idling right away and my plugs (D8ea) are now dark coffee instead of the dry black sooty I used to have. I know it's still running rich, even though the carbs are all to specs, clean and synchronized. Valves are right on, timing is dead on the F at idle and in between the 2 lines at higher revs, new NGK 5 Ohm plug caps, new AMG battery, stable at 13 Volts, Regulator from Oregon Motorcycle charging in the high 14 Volts at 3000 RPMs.

Still, the motor stumbles in the first 1/8 throttle, only when warmed-up.

I was kind of wondering if by any chance my problem could still be electric, hence my question. All the tests so far were done at start up, un-choked but not completely warmed-up.

I am still learning here and I try my best to be methodical but before all this crystallizes in my head, I will poke around and probably be wrong in the troubleshooting department.

Thanks to the moderators for this forum.

Feel free to comment.

Simon
« Last Edit: April 26, 2010, 08:15:28 AM by Simon »

Offline jessezm

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Re: CB400F Alternator question
« Reply #6 on: April 26, 2010, 08:45:15 AM »
Simon, I would suspect the Carburetor, but I am way less experienced than others, so they may want to chime in.   Based on the little bit of info you've provided, I would try setting the mixture screws a half-turn in and see if that makes a difference.  you also want to check the float levels.  See steps 3 and 4 below:

• 1. Top end (full throttle / 7.5k to redline -
Best Main Jet must be selected before starting step 2 (needle height)!

Offline 754

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Re: CB400F Alternator question
« Reply #7 on: April 26, 2010, 09:41:12 AM »
Usually ignition weakness shows at high rpm , not low.

 It sounds like your carbs low speed circuits/jets, did not get cleaned out.
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Offline jessezm

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Re: CB400F Alternator question
« Reply #8 on: April 26, 2010, 11:11:13 AM »
Oh yeah, definitely, definitely, definitely, if you didn't do it when you cleaned your carbs the first time, remove the emulsifier tubes from the carb bodies to clean them thoroughly (they usually have to be pressed out from above after taking the slides out) This doesn't sound like it's causing the issue you currently have, but still, it's worth doing if you hadn't yet.  I had broken down my carbs completely and dipped the whole bodies in carb dip, but i still forgot to take out these tubes to clean them.  I finally convinced myself to do it (I was stalling because I didn't want to take out the slides again cause of those stupid lock tabs...), and boy am I glad!  one of them was completely full of crud.

Offline Simon

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Re: CB400F Alternator question
« Reply #9 on: April 26, 2010, 02:42:38 PM »
All good advice here. Thanks

The carbs have been cleaned, jets and all the removable components, emulsifiers included. The jets are «newish» since I overhauled the carbs a few years ago trying to solve this same problem. That's why I tend to want to look into the electrics since I feel I have done all I could do carbwise. Last year, I lowered the needles one notch to no avail. The jetting is still stock, the floats are of the synthetic type. I tried lowering them and solved nothing really. The spark plug tips got real white surrounded by dry black soot. All the sparkplugs have the same aspect. Still got that 1/8 throttle hesitation. Played a lot with the idle screws, out 1 turn, 1 1/2, 2, 2 1/2, 3 and so on, giving me a variety of crappy results.

One question that's in the back of my head, could there be a internal air passage, one that can not be inspected easily, blocked in some way? I have forced carb cleaner and compressed air into every hole I could find and all passages seemed clear. I put a small amount of gas resistant gray gasket sealant around the main jets to assure me that the O ring holding them in place would not let unmetered gas pass by them. I did this a few times before and I have never found any gasket sealant blockage, debris or contamination anywhere in the carb upon subsequent carb cleaning.

I will probably unmount the carbs, again, check every thing out, again, and hope to find the problem, this time....

BTW, I call this bike, The SPHYNX.

All suggestions, insight is appreciated.

Simon