Topic: 1975 Honda CB400f Project - Beginner

[gjunkie]

Puddle: all jets are brand new.
IW: I just took off the carbs to confirm air/fuel flow through the pilot jet. I dropped a bit of carb cleaner through that pilot intake and it was coming out of the carb through the "outflow". I put it all back together, and now #3 and #4 aren't firing. I haven't changed anything electrical wise. So strange.

[iron_worker]

Have you set the timing?

What is your battery voltage?

IW

[gjunkie]

I had the timing set by a professional, and I tested the battery at 12.08 volts on a full charge.

[puddle pirate]

ok, so if when you try to start it, headers 1 and 2 coming from the left side while sitting on the bike, both get slightly warm and three and four do not, and you can see that your getting spark on 3 and 4. I would think it's still a fuel problem.  Somthing to check or look at, how many volts do you have at the coils, especially while turning the bike over.  Because of the age and condition of the wire, it is possible that your getting a very week spark caused by low voltage at the coil, while your at it check the voltage of the battery while you turn the bike over 12.08 seems pretty low to me.  I would still advise you to pop a quick spray of ethenol car starter down the throat of 3 and 4 when you try to start it and see if it heats the headers/fires a bit.  If it dose heat the headers on 3 and 4 you have a fuel/carb problem.  If it dosnt fire then I would think its electrical.

my 2cp

[gjunkie]

Puddle:
I sprayed starter fluid in both #3 and #4. The bike started super fast and #3 got warm. #4 did not. Once it burnt through the fluid #3 remained a little warm but didn't get hot. So it does sound like its a fuel problem... ?

I should also add I did a compression test on all four:
1: 110psi
2: 105psi
3: 110psi
4: 100psi

Shouldn't they be closer to 130psi?

[puddle pirate]

Should it be closer to 130?  That all depends on what type of compression tester you have.  In my opinion if they are all within 10-15 lbs thats good.  I would work your problems out and ride the piss out of it.  So per your discovery i would agree that number 3 has a fuel problem.  If 4 did nothing.. you have either a really fouled/black plug or some type of an electrical problem.  If the plug in number 4 is fouled i would dry it, sand the contacts real lightly with 220 or higher sand paper and redo the ethenol test.
If that dosnt change i guess i would suspect the wire/sparkplug boot.  I have never heard of a coil going bad on only one lead.

[gjunkie]

Well the spark plug looks almost new. To test if it was indeed the spark plug, I switched the plugs between #1 and #4. #1 continued firing just fine with the switched plug, #4 still nothing.

[puddle pirate]

Man that's a weird problem.  If the spark plug looks good i'm at a bit of a loss.  I would think that if low power was the issue you were having as far as the coils go, #1 would not fire as well or would not fire at all as it's a longer wire with more resistance in it.  Kinda long shots here, did you do a resistance check from the bottom of the spark plug, not the tang but the electrode to the end of the lead?  I hope swan gets on here soon and straitens us out.  Is #4 spark plug still really clean?

[gjunkie]

I have not done a resistance test on the plugs. #4 spark plug is still really nice and clean, If you looks a few posts back I posted a photo of the difference between #1 and #4. Here's a photo of plugs #2 and #3. #3 looks a bit too clean I think which would make sense since it only seems to fire sometimes. I'm gonna get a new set of plugs since these were installed before I did the carb rebuild. I need to see how even/uneven the truly burn now.

#2:





#3:

[puddle pirate]

My opinion,  2 looks a bit rich, but thats to be expected with 40 idle and pods.  #3 look lean in my opinion.  How dose 4 look? dont need a picture just curious if its more like 2 or 3.

edit:  Have you swaped the position of lead number 1 and 4 on the green dyna coil?  I dont know if one side can go bad and the other not but that's a pretty easy check i guess.

[gjunkie]

Here's #1 and #4:
http://forums.sohc4.net/index.php?topic=116070.msg1331938#msg1331938

Again, I don't trust how they look simply because I've done a lot of work on these carbs after riding on these plugs for a while, so I think the color is misleading at the moment for #1.

EDIT: I think I did switch the leads between #1 and #4 and got the same result, but at this point I don't remember I'll have to recheck that.

[puddle pirate]

Something to keep in mind about plugs is that what your looking at when you remove one that has been replaced in the last few months is pretty much the condition of the fuel/air mixture at that moment.  If a plug gets fouled you can brush and clean the spark gap area and it will work just fine.  The dielectric in the spark plug will break down over time but pretty much there is no reason to replace the spark plugs all the time unless you find something wrong with it.  Later down the road when you get all this sorted you will have to do plug chops to evaluate your jetting unless you pay a shop to do it.  When you do your plug chops you will have to kill the engine at speed in order to see how it is burning on your main jets at that moment in time. 

My point is that even though 1 and 2 are really black right now, that is not as concerning at least to me as #4.  Have you looked at the #4 plug sense the last time you cleaned everything out?
I would still at some point do a continuity/ohm check from the bottom of the electrode that sticks in the engine to the end of the lead where it plugs into the coil.  If that checks good and swapping the 1 and 4 leads on the coil doesn't change anything I'm about out of ideas.

I would still be interested to know how many volts you are getting at the coil.
I had to put a relay in on my GS650 because i was loosing about 2 volts between the battery terminal and the coil and i was getting really weak spark and it would die at stop lights because the battery was old and at low RPM's it wasn't making enough power to overcome that.

sorry for the ramble.

[gjunkie]

Puddle:

Thanks for helping me out. You're not ranting at all, this is good info. The #4 plug has not changed in appearance at all. Just a little bit of black from the "sometimes" jolt of power. In the next couple of days I'll check the continuity from plug to cap and report my findings on all the plugs.

I've been looking at how to check voltage at the coil, but I have not found any instructions on how to do so. Any chance you know how to? I'd love to try that while I'm at it. Might as cross everything off the list that I can.

Thanks again!

[puddle pirate]

I have no idea how to check the actual output of the coil.  However if you check one of the input wires, one should go to your Dina timing ring and the other should have voltage coming to it from the battery.  You migh have to roll the motor over by hand or kick start a few times to get a good reading at both coils but at any rate you want to compare the voltage on the input side to what you have at the battery and see if you have a difference in voltage.
Swan, Rick B, if i'm missing anything feel free to chime in.

[iron_worker]

If your battery really is 12.08v with a fresh charge then your battery is crap.

http://www.energymatters.com.au/renewable-energy/batteries/battery-voltage-discharge.php

IW

[gjunkie]

I think the 12.08v might be inaccurate. I'll retest that.

[gjunkie]

I tested all four plug wires with the plugs connected. From coil connector to tip of plug, these were my resistance results:

1: 6.11k Ohm
2: 3.02k Ohm
3: 4.86k Ohm
4: 4.72k Ohm

Also, starting it up I noticed that header #3 was getting a little warm, but nowhere near as hot as #1 and #2. #4 was still cold. I sprayed a little starter fluid in the #4 carb and started it up again. This time #4 got a little bit warm making me think that indeed the issue is fuel.

The bowls all get fuel, and the #4 main jet allows air to flow through. The pilot jet cavity is a little more questionable because it's hard to blow air through it. However, at speeds of about 25-35mph, shouldn't the main jet kick in anyway? Essentially giving me problems at low speeds, but not high speeds? #4 seems to not fire regardless of RPMs.

[puddle pirate]

Alrighty bud, your not going to want to hear this but I think a complete carb dip and strip is in order at the very least on #4.  My opinion is if your going to do one you should do them all.  Do a Google search for " CB400 CarbV02 " and you should get a carb rebuild document written by Tim Boughen.  Sorry i could not find the direct link on my work PC but i do have the document at home and if you can't find it Message me your Email and i'll attach it and send it.  Highly recommend it at any rate.
Someone correct me if I'm wrong but your main jets have nothing to do with speed, but everything to do with RPM.  If I recall correctly up to around 1/3 throttle your idle circuit to include your air screw/slow jet are what run the bike.  From really 1/4 up to 3/4 throttle your needle taper and clip position on the needle come into play.  from 3/4 to WTFO is your main jet.  So.. i cant tell you what your bike should be doing at 35 mph because that is all dependant on what gear you were in and what your RPM is at.  If you have not re jetted and are running anything more open than stock intake you will have to re jet it.  If i recall you will be looking at 38's for your idle and 80 for your mains with the clip moved up one spot but every bike is different and you will have to do throttle chops. 
If you have not done so it might be a good idea to take your battery that's in your bike, check it again.  If its still reading below 12.5 than throw another car battery inline with jumper cables and quickly check and see if it runs any different.  I'm willing to bet though that your main issue is a plugged idle circuit in your number 4.

JT

[Mikkel_H_ Denmark]

Looks good there Gjunkie !
these old CB400 can give you quite a bit of trouble as i have experienced my self, but still fantastic bikes.
great thread you have here i will definitaly check back soon. hope there will be some cosmetic Work as well some time

hope you will check out my CB400F project ?
http://forums.sohc4.net/index.php?topic=120231.0

Mikkel

[gjunkie]

I was on a 2 month hiatus with little time to tend to the bike as I was getting married. I finally scheduled a mechanic to come out and help me as I was running low on options. We finally found what the problem was, and quite quick. It turns out that carbs #3 and #4 were blocked... unblocked them, viola. This is when things took a very unexpected turn.

It is with great sadness and true disbelief that my return to my dear CB400f has to end in this way.

The mechanic took the bike for a short spin to make sure it wasn't running too rich, etc. when we noticed a strange sound. Turns out that when I had put the motor back together and mounted it back onto the frame I had apparently not tightened all the bolts onto the frame tight enough so it was slightly shifting. Well, the mechanic was there already, so I asked him to tighten them for me. The motor was very hot as we had been revving it, letting it idle, etc, while trying to fix it. When we got to the last bolt we heard the worst possible sound I could've ever heard....

She broke her back.

The case cracked, oil spilling onto the ground as if she had just been stabbed in the gut. It's hard to tell now if the reason for the original oil leaks were due to small cracks and this just sealed the deal or not. Regardless, she will never ride again. And I never even got to ride it...

I could try to find someone to weld it, etc, but after so much time and money spent on a bike that I never got to ride, it's hard to justify...

I didn't dare to look at the full extent of the damage, so for all I know it might be a tiny crack. Regardless, you can imagine my sense of loss right now...

That is all....

[SOHC4 Cafe Racer Fan]

That's heart-breaking.  With that much effort into the bike, you could repair or replace the crankcases.  Hopefully, your new better half will support your quest to finish the product and enjoy the fruits of it.  It would be a shame to can the project.

[Powderman]

Bummer. Replacement cases can be had for very little money. Of course tear down and rebuilt is time consuming. Or you could just sell it to me for $5.00.

[gjunkie]

Ok... I admit I was a little over dramatic in my last message. Yes, it sucks. But I did find a new case for $115 on eBay and decided to go with it. Besides, 5 days earlier I had also paid for the registration on the bike along with a nice %100 late fee.

Now that my carbs are in working order (for reals this time), all I need to do is open up the bad case and move everything over. If anything this will also give me a chance to take a close look at every component and see what needs replacing.

[Bankerdanny]

Now that my carbs are in working order (for reals this time), all I need to do is open up the bad case and move everything over.

Oh, is that all?

[gjunkie]

@Bankerdanny ha! yeah, should be like 45 min, right?