Topic: 1975 Honda CB400f Project - Beginner

[RickB]

gjunkie,

On the head gasket leak, I have discovered that that is a common CB400F problem. You'll certainly need to pull the head off and replace that gasket and seals. Have a look at my thread to see how that's done, it's actually quite easy once you have the manuals (Clymer and Haynes) and some courage:

http://forums.sohc4.net/index.php?topic=51957.msg1322662#msg1322662

That photo you show of oil dripping out of the bottom left cover (this is your stator cover) is more than likely coming down from your head gasket. Or you have a crank shaft seal (or similar) leak which would require splitting the cases to replace.

Anyway, you can take that cover off and have a look behind it, it's just a cover. You don't want to seal that off completely as that would just mean oil pools up behind it; isn't meant to have oil in there.

As for the charging issue, I also had that! So have a read of these threads:

http://forums.sohc4.net/index.php?topic=51957.msg981415#msg981415
http://forums.sohc4.net/index.php?topic=87086.0

Basically the charging systems works like this. The stator (basically a magnet spinning inside a copper wire coil) is attached to the end of the crank shaft on the left hand side (this is cover is where you thought you oil leak was coming from).

This feeds AC current to the voltage regulator (the rounded box under the side cover that has a black/white/green sticker on it) that controls how much voltage to send to the voltage rectifier (the finned box on the right hand side near the battery), which then converts that power into DC and sends it into the battery.

More than likely your charging issue is a faulty rectifier. Test it is very easy with a multimeter and I have detailed how to do t in the above threads.

Rick.

[gjunkie]

Rick,

Thank you so much for the detailed response. I'll read these threads thoroughly.

As for the leak/gaskets. You're absolutely right. I've already ordered an OEM head gasket and oil orifice gaskets like the one you pictured (taller than the crappy one). I've already taken the motor apart once, so I feel confident I can replace these parts no problem. My only concern is then setting the timing correctly, which I wasn't able to do the first time I took it apart. I have the manual and followed the instructions, but still wasn't able to get it right. A shop nearby would do it for $75 so I don't mind having a pro do it, its just that I also have to actually get the bike there (rent a tow, find a friend with a car with a hitch, etc).

As for the charging issue, I haven't yet tested anything of the electrical, so I can't say for sure, but I haven't really had issues since the one time it died on me. Still, I'll go through your steps to test it since I've already got a multimeter.

I've also just received a brand new set of floats (I can't believe how expensive they are!) and 4 carb rebuild kits. I've also made my own 22mm (rather than 21mm as per your recommendation) cardboard tool to measure the float height. This is my last attempt at getting the carbs right myself. If after all these new parts I still can't get them right, I'll simply get started on the leak and take it to the shop to set the timing and tune the carbs. I've taken the carbs apart 5 times now. One good thing: I can now take them off, disassemble, reassemble, and remount them in record time

[gjunkie]

Ok! Today I put the carbs back together with the new rebuild kits and new floats. The only issues I had while assembling everything was one of the float needles that came in the kits didn't have as much of a bounce to it as the other three. I opted to use one of the old float needles that seemed to have an equivalent bounce to it. Also, just like Rick mentioned on his thread, I had to adjust the float height by almost completely flattening the float tab. I got all the floats pretty even. They probably have about a .5mm differential that I just couldn't perfect.

Here's a photo of the original floats that I took out... corroded and bent in funny ways... Also, a couple of photos of the carbs taken apart and cleaned as best I could. Notice the one bowl cover has rust inside... also cleaned it as much as I could.







And here they are after I installed everything:







They obviously have to be synced again which shouldn't be a problem as I've taken a shot at it twice now and this time we'll have good parts in there. But the good news is that when I flipped on the fuel there was zero overflow which means the floats are working at the very least somewhat properly. Every other time I've put these back together, there's been some overflow to start.

Also, I had main jets at #80, and the rebuild kits came with #75. I decided to go with the #75 for now. We'll see.

I'll report back when I've synced them (hopefully tomorrow!).

[RickB]

Looking good mate, keep us updated.

[gjunkie]

Thanks Rick,

I do have another question. Since I had to salvage parts for my replacement carb, I had to use the same bowl cover and main jet seat from the corroded carb, both of which are pretty rusted. How will this affect fuel over time? Should I just replace them ASAP?

Thanks guys. Your help is very much appreciated.

[RickB]

Well I'm no expert but rust in your fuel supply, without any filter to catch debris, cannot be a good thing! While you're replacing the head gasket I'm replace that too.

[gjunkie]

Ok. I'm at my wits end.

I took a shot at syncing the carbs and did the best I could. I'm a little perplexed tho as when I tighten the screws to get a higher hg reading, it never really reaches the recommended level, and eventually as I keep tightening it shuts off (naturally). This happens with all the carbs. So the best I could do is get them to about 3-5 in/hg (6.1-9 is recommended).

Eventually revving it to 4000 sounded pretty good. So I decided to take it for a quick spin.... Same old story. Accelerates very slowly. While accelerating, sometimes, around the 25mph mark I'll get a quick jolt of power for about a second as if something clears up. Then continues to be bogged down. At this point I'm willing to bet the problem is deeper than carbs. I should do as originally recommended and do a compression test perhaps. Why the jolt of power??

Here's what I ended up leaving the carbs at:

Honda CB400f sync

Current carb settings:
Air screws: 1 3/4 turns
Main jet: #75
Pilot jet: #40

Another strange thing that was happening: trying to start it up at first I was having issues. Pressing the starter would just produce clicking sounds but not turn the engine. Eventually I used the kick starter a couple of times, then back to the button and it started right up. This happened every time I tried to start it.

[hondamatteo]

The clicking means your battery doesn't have enough juice to turn your starter. Have you checked your charging yet? If its ok then we can help you troubleshoot deeper.

[gjunkie]

The clicking means your battery doesn't have enough juice to turn your starter. Have you checked your charging yet? If its ok then we can help you troubleshoot deeper.

Hi Hondamatteo
I haven't yet checked the charging. The clicking occurred after giving the battery a decent charge, so my theory is that even if the charging is a problem, the battery had a good charge at that time anyway. And like I said, after turning the motor once with the kick start, the button start worked immediately no problem. I know I need to check the charging regardless, I was just hoping to nail down one problem at a time.

Any theories on the sudden, yet brief, jolt of power?

One more thing I forgot to mention. In order for it to start I have to turn on the choke, turn the motor a few times as it almost starts, then turn the choke back off. Only then will it start. If I don't turn on the choke it just tries and tries but never turns on. Could this be rated to the sudden jolt of power? Does this sound like its running too lean or too rich?

Thanks for your help. Without you guys I'd feel completely hopeless!

[RickB]

You are jumping all over the map here mate. Let's focus on one problem at a time and try not to jump at shadows. From my experience, I don't think you know enough information yet to diagnose a bigger issue.

Balancing your carbs is certainly helpful, but usually something you do to fine-tune everything once you have 'bench synced' them by eye, which is usually enough to have the bike run relatively 'normal', at least in my case it was. I didn't experience any 'life changing' performance increases from balancing my carbs.

The pressure level you get isn't as important as having them all read the same. Regardless, put the sync gauge down and troubleshoot. We'll get you there, don't lose hope!

I think you're pretty confident the carbs are rebuilt correctly. You've done it enough now and replaced everything dodgy, so move on to another system instead of second guessing yourself.

Test your electrical system. Follow the troubleshooting guide in the FAQ section of this forum, it is very straight forward.

Be sure to test your coils, condensers (do you have a stock ignition or an aftermarket digital?), plug leads and caps. Whilst you may see a spark, so did I and it turns out my poor performance was related to my plug lead length. So don't assume, test. It's very easy to do and I have documented with photos just about everything you need to test in the threads I linked to at the top of this page.

Report back with your results. I've got a vested interest in you having a faulty rectifier and/or ignition system!

Rick.

[gjunkie]

Thanks Rick,

I'm gonna calm down, test the electrical, and report back. That's just what I needed to hear/read at this point. Thanks for keeping me focused.

[gjunkie]

Ok, I've got some updates.

I've finally done as was suggested before and touched the headers... low and behold, cylinder #4 is not firing. My apologies for being a stubborn newbie!!

So I ran a few tests to try to get to the bottom of it and this is what I've found:

I removed plugs #1 and #4 (my plug socket is too small and didn't fit to take off the other two), and this is what they looked like (plug #1: left, plug #4: right)



The slight blackness at the tip makes me think that this is from the few times I feel a slight jolt of power, meaning some fuel gets through to actually ignite. When I took these two plugs out, #1 smelled of fuel. #4 did not and was dry. Also, the bike used to run too rich I assume, before I rebuilt the carbs, which would explain the darkness of the #1 plug. However, to be sure, I'll replace all 4.

I confirmed that all four plugs indeed have spark. I compared (visually) the spark for #1 and #4 and they both seem just as strong. Just to be sure I decided to do a resistance test on the plug wires. I wasn't 100% sure on how to use the multimeter I have, but I set it on the only Ohm setting it had, which was an automatic setting... there was no option to set it to a specific resistance. Also, I did not take the caps off. This is what I found:

#1 (18"): 5,900 Ohm
#2 (9"): 3,100 Ohm
#3 (15.5"): 4,970 Ohm
#4 (14"): 4,780 Ohm

I assume the differences in resistance is due to the differences in length. #3 and #4 are almost equal in length, and the reading was almost the same. Here they are left to right 1- 4.



I also went ahead and did a compression test on #1 and #4:

#1: 110psi
#4: 100psi

Can anyone confirm what normal psi is?

I started her up and removed the #4 intake filter. I put my hand up to the intake and felt a good amount of suction. If I completely choked #4, hoping it would force more fuel to #4, the bike would just die off. This leads me to believe that the carb itself must be clogged and not letting fuel through... is this a good theory? I'd love any help you guys can provide. Thank you so much!!

[iron_worker]

If you are getting a good spark to number 4 as you say you do and you also have compression then you must not be getting sufficient fuel. I would say that your #4 carb is starving for fuel by the look of that spark plug.

If you were getting fuel but no spark to the #4 cylinder then I would expect that plug to be wet with fuel not dry as it appears. So this also leads me towards a fuel issue.

Likely your "surge" of power is the 4th cylinder getting rich enough to fire again for a brief second and then quickly using up what little fuel is in the cylinder and dying off again.

I, personally, would be looking at the carbs again. Did #4 float height get set way too high somehow? Float upside down? (don't laugh, it's happened lol). Is the fuel port coming into the carb body clogged up? If #3 is firing fine then I would say that your fuel line feeding #3 and #4 is likely fine. Can you see light through the #4 pilot jet? Is the idle screw set correctly?

IW

[gjunkie]

IW,

Thanks for the reply. I've set float heights even across all carbs. Also, when opening up the #4 bowl, fuel does come out so I know fuel is getting at least that far. It never overflows. Also, all jets are brand new as I got rebuild kits for all carbs.

Could the carb be plugged up inside where the jets are seated? Any way to verify if those main or pilot lines are clogged?

[gjunkie]

Today I took the carburetors off to clean carburetor number four. After using carb cleaner and blowing air through the jet intakes, I was able to get air through no problem. I put them back on and at first cylinder #3 and #4 were not firing. I freaked. Then it dawned on me. Could I have the plug wires in the wrong order?

I couldn't find the right order online anywhere, so I switched the cables going from each coil to go to the opposite plugs. I immediately heard a pop from bad timing. So I put them back in their original order and cylinder #3 is now firing. #4 still is not.

What is the correct wiring order? My understanding is that as long as you get the correct coil to the correct pair of plugs, you're fine. The order of plug on each coil does not matter. Is this correct?

If I get spark, air through the carb intake, the jet intakes and jets seem to be clear, why is this cylinder not firing?

[Jore]

Take a look at this:

http://forums.sohc4.net/index.php?topic=49167.msg519696#msg519696

Today one of my coils wasn't firing and turns out the thread i just posted was the solution to that.

[kajtek]

just a quick point, if you are still running points, check your condensers. had a very similar issue where i was going crazy over everything in the charging system, turned out one of my condensers had died.

ashley

[gjunkie]

I will double check both of these just in case! Tho remember, I am getting spark on all plugs.

[iron_worker]

You are correct in thinking that as long as you have the 1-4 coils hooked to the #1 and #4 plug (and vice versa for 2/3) then you're good to go. It is possible to switch your wires around between your points and your coils so the 2/3 points are actually firing the 1/4 coil... but I think the engine will just cough and sputter if you hook it up backwards like that.

Another question that you have to ask yourself is do you have a *strong* spark. It's one thing to have a visible spark but remember that the conditions inside the compressed combustion chamber it is more difficult to produce a spark that will ignite the flame. I believe there are spark testers out there that actually test spark strength by increase a gap more and more until the spark will not jump and then measuring that gap.

Looking into your condensors may not be a bad idea.

IW

[gjunkie]

When I first bought the bike, the previous owner had told me that the ignition system had been upgraded from the old points system to an electric ignition. At the time I really had no idea what he was talking about and that bit of information got lost in all the other things he was telling me about the bike. I took the cover off, and it came back to me. Here's what it's got.







I don't know anything about this particular ignition.

I also got myself an inexpensive spark tester. It's one of the ones with a light inside a chamber and you're able to see if the light is weak, inconsistent, or both. While testing #4 the light seemed to be firing perfectly. I compared it to #3 and they were identical. Strong, steady blinking. The higher I revved, the faster the light blinked.

Here's the current situation:
1: We have what I believe to be a good spark.
2: Carb air intake works.
3: Fuel intake does not seem to be clogged, as I was able to blow air through the main jet intake.
4: Valves and timing were set by a professional.

The only other test that my friend and I thought of would be to put a known-good carb in #4 to see if it fires... but sadly I can't simply swap #3 and #4. I'm inclined to ride it to the local shop where I got the replacement carb and see if they can diagnose it.

I will say, the bike is actually sounding better and better. I can only imagine how nice it will sound once she's firing on all cylinders.

Is there anything else I can check myself before I take it in?

[RickB]

Is it possible you have #1 and #4 spark plug leads confused in your resistance testing posted earlier? You're counting cylinders from the left hand side, correct?

I suggest this because you're problem is almost identical to mine linked to at the top of this page. A weak spark caused by too much resistance in my spark plug leads. This was evidenced by a purple-ish spark. Not white.

Regardless, have you ran those tests on your voltage system? Stator, regulator and rectifier? Are you satisfied your charging system is working as it should.

I say this again, you've checked your carbs a handful of times. I'm not convinced this is a problem with your carbs. They appear to be getting fuel (bowl is full, jets are clear), your air system appears to be working (carbs sucking fuel). Common sense would suggest this is spark related.

[gjunkie]

Hi Rick

I'm certain I have not confused #1 and #4 wires and indeed I am counting left to right while seated on the bike.

This morning I ran through the tests listed in your thread you mentioned above. Here are my results:

I unplugged the alternator and the rectifier from the bike's wiring harness, as well as the green wire from the regulator. Resistance should be less than .5 Ohm on this green connection and this is what I got
At the wire: 0.1 Ohm (good)
At the harness/regulator: 107 Ohm (wow)

Stator AC output at harness at 4,000 rpm:
Yellow #1: 24.9
Yellow #2: 23.5
Yellow #3: 23.6

Stator AC output at harness at 4,000 rpm with the white and black wires from the regulator disconnected and jumpered together. I didn't have a proper wire to jumper them together so I had to use a small removable screw driver rod... Not sure how this affected this test:
Yellow #1: 23.5
Yellow #2: 24
Yellow #3: 24

With the key on does the voltage on the black wire = battery voltage?

Black wire= 11.77 volts
Battery = 12.08 volts

Testing resistance between each yellow wire and ground: 0 on all yellows.

For the next portion of tests I was not sure how to differentiate between resistance and continuity on my multimeter. I believe all of these results are from resistance.

From your post:
http://forums.sohc4.net/index.php?topic=51957.msg981415#msg981415
VI.   Check the five leads on the rectifier.
A.   To perform a forward bias test first connect the red tester lead to the green ground lead.
B.   With the black tester lead test all three yellow wires.
C.   Resistance must fall between 5 and 40 ohms.

Yellow #1: 5.99
Yellow #2: 6.21
Yellow #3: 6.47

D.   To perform a reverse bias test connect the black tester lead to the green ground lead.
E.   With the red tester lead test all three yellow wire.
F.   Resistance must be greater than 2000 ohms.

Yellow #1: 0
Yellow #2: 0
Yellow #3: 0

G.   Connect the black tester wire to the red and white rectifier lead.
H.   With the red test lead test all three yellow wires.
I.   All three must have between 5 and 40 ohms.

Yellow #1: 6.22
Yellow #2: 6.04
Yellow #3: 6.21

J.   Connect the red test wire to the red and white rectifier lead.
K.   Test all three yellow wires with the black tester lead.
L.   Resistance must be greater than 2000 ohms. 

Yellow #1: 0
Yellow #2: 0
Yellow #3: 0

In your post, the resistance tests that must be greater than 2000 ohms (D and J in this list) all came up as error, but 0 for me. I don't know if this is an error or a correct reading. I'm assuming error?

If I did all of these tests correct, it seems like the only one that's totally out of whack is the regulator itself, measuring at over 100ohms at the green lead. What can this cause?

On a side note, I've started noticing what looks like rusty water leaking out of my muffler. At there bottom of it there's a small hole which looks like it is meant for some kind of condensation overflow. The fact that it is rusty probably just means that the muffler is rusted inside... I just wanted to mention it in case it is an indication of something else. This just started happening yesterday.

[puddle pirate]

 I had a problem like this on a Suzuki.  I eliminated the spark question by spraying a wee bit of ethanol car starter fluid into the mouth of the suspected carb, and when it spooled up i knew i had a problem with the idle circuit.  This was with CV carbs but it should do the same with yours.  Also do a valve adjustment before tuning the carbs if you have not done one yet.  Makes a huge difference.

JT

[iron_worker]

You said you can get air through the main jet ... what about the pilot jet?

IW

[puddle pirate]

also for cleaning pilot jets i usually get one small wire from a wire bush, pull it out and stick it in a match stick to run through the middle.  The one i use is far smaller than any sowing needle i have ever found.