CB400F 3 hot cylinders 1 cool/cold one.

D

Dingo

peace and grease
I have very limited time to wrench around, and also limited tools, today I was playing around with the bike and noticed that 3 of header tubes are really hot (like burn your skin off hot) and the other the right one is cold, you can touch it bare hands and it ain't even warm compared to the others. The bike is running rich.

I took off the spark plug wire and the bike seemed to idle a little slower, although the idle has always been rough since I have it, also I haven't even ridden the bike since I brought it home.

What do you guys think?
 
You need to determine if that cylinder is not firing due to ignition or carburetor problems.

First, pull the each plug (one at a time) and check the burn of the plug to see the color of the plug. After verifying, re-install plug and move onto next one. Ideal color is tan to light brown. White is too hot (possible lean condition). Black is rich. There are many different colors variations between the three listed...this is just a beginner's list.

After verifying the notes on all four, see if the one causing problems is different from the rest (my guess would be that it is a little darker).

Then, pull the plug in question, install it back onto the plug wire, and *IMPORTANT*, ground the electrode onto some metal (engine, bolt, or other place that is a good ground). Hit the starter button to see if the there is spark jumping to the electrode.

The reason it is important to keep it grounded when cranking/running the engine is that if it is not, it can cause problems with the ignition system.

If the spark is bright blue, then it is good. If it is yellow, then it is weak.

If it's weak, start looking into changing out wires and plugs.

These are easy checks, but I'm going to guess the most likely culprit is your carburetor. However, you have to verify you have good spark,and it's easier to do than pulling apart the carb.

Also, double check the spark plug gap, the valve gap/settings and all the electrical wire connections.
 
Thanks, I'm actually planning on a major tune up, chaining all the consumable parts, and also work on a leaking gasket, my plan is to have it running before xmas.

i'm hopping that clears most of the bugs from the bike.
 
Almost certainly a clogged jet in that cylinder's carb. Are you running stock airbox? Then it's easier to just leave the carbs on and drop the float bowl using a z-shaped philips screwdriver or a small ratchet tool. If you're running pods, it's easier to take the whole carb rack off, but since it's only one carb I'd still just pull the bowl. Pull the jets, unclog them (thin wire, carb cleaner, look through them to make sure you can see through them and they're not clogged), and you should be good to go. I've done this quite a few times -- I run fuel filters in my lines but it happens every once in a while. If you've run the bike on three cylinders for a while, check to make sure the spark plug is still good and that the tip hasn't fused. It could be a clogged jet first, leading to a messed up plug.

It's not your coil if the other cylinder is firing, but it might also be an arc in your plug wire.
 
I'm running the stock air box without the filter. Thanks for the advice, I'll send it to the mechanics shop because I don´t really have time to wrench on it. And really want to get out ridding ha!
 
Well... ok. But seriously it'll take you 20 minutes or less, and cost nothing, with no need to buy any parts. Bring it to a shop and you're walking out paying 50 bucks or more. Lead a horse to water and all that.
 
carnivorous chicken said:
Well... ok. But seriously it'll take you 20 minutes or less, and cost nothing, with no need to buy any parts. Bring it to a shop and you're walking out paying 50 bucks or more. Lead a horse to water and all that.
Click to expand...

Thanks mate I'll definitely give it a go, so basically I just have to poke around the main jet right?

carbbot1.jpg
 
Dingo said:
Thanks mate I'll definitely give it a go, so basically I just have to poke around the main jet right?

carbbot1.jpg
Click to expand...

You'll find it's not difficult. No time like the present to learn. Shit like this may seem intimidating if you've never done it, but this is a snap. So yeah, the diagram has it basically correct, although that's not a CB400F carb (at least not a stock '75-'76, USA CB400f). No matter, principle is the same. Get a small ratchet screwdriver (google: small ratchet screwdriver, and it's the one that looks like a flat socket wrench, not the ones that look like ordinary screwdrivers). Undo your bowl screws (you're also lucky that it's one of the outside cylinders, not one of the inside ones, as that makes it a bit tougher). Pull your float bowl straight down (disconnect the overflow tube if you have one hooked up, before you do this). Set the bowl aside (and make sure nothing drops out -- one of the things that's missing in that photo is a clip that fits onto the main jet below the floats). Unscrew your pilot jet, and pull out your main jet (main jet is pressed in on these, not screwed in). Try not to wiggle it back and forth -- there is an o-ring on the jet and you don't want to muck it up. Hold the jets up to the light -- can you see through them lengthwise? I'm betting not... Get as thin a wire as you can that's still stiff (I pluck one out of a wire brush) and poke them out. You can spray carb cleaner in there to help. You don't want to jame the wire in there to damage the jet or get stuck, just to dislodge whatever is blocking it. At the same time, look at the pilot jet from the side. There should be a few holes that go all the way through -- make sure these aren't jammed as well.
When you do this, your floats shouldn't move enough so that the valve above them comes loose and falls, but make sure of this.
Make sure the oring for the float bowl is in decent enough shape -- it doesn't really hold gas in under normal situations -- the valve and floats are supposed to make sure that the gas doesn't get above the height of the float bowl. But gas splashes around when your riding, and the o-ring helps that from leaking out the carb. Also, clean out the bottom of your float bowl.
 
redwillissuperman said:
Your coil is dead...happens all the time. By two new coils, keep old good one as an emergency spare..
Click to expand...

Sorry, but three cylinders firing is not a dead coil. These bikes have two coils that each fire two cylinders. One dead coil would mean two cylinders are dead. Coils on the SOHC fours rarely give out, in my (20+ years) experience. Don't buy expensive replacement parts until you're sure you need them. Coils can also be tested for spark to determine if they're good or bad.
 
I had a few minutes to wrench today and changed the spark plugs around, the plugs are coal black, but the one in the non firing cylinder seems to be dryer. So as carnivorous chicken said, it's going to be a clogged jet. Hope to get some time tomorrow to mess around with that.
 
Coal black plugs also mean it's running a little rich; try backing out the screw a 1/4 turn or so, and take a wire brush to those plugs. That is if they're sooty. If they're oily you've got bigger problems.
 
It felt/smelled more like gas than oil. But I do have an oil leak from the valve cover or so it appears to be.
 
carnivorous chicken said:
Sorry, but three cylinders firing is not a dead coil. These bikes have two coils that each fire two cylinders. One dead coil would mean two cylinders are dead. Coils on the SOHC fours rarely give out, in my (20+ years) experience. Don't buy expensive replacement parts until you're sure you need them. Coils can also be tested for spark to determine if they're good or bad.
Click to expand...

You need to get out more:) I run three sohc race bikes and the coils die all the time. Usually one post of one coil goes cold first, stays that way for a while, then both drop out suddenly (usually as far away from the pits as possible). You pickit up on the plugs where one cylinder starts misfiring and the plug darkens. I carry a spare coil all the time and can swap it out in about 5minutes. I have used the Dyna coil, Harley and stock Honda ...all have the same life. I've started checking the pipe temp in the hot pits to see if I can trend the failure. The first instance is usually intermittent failure but you do see a general trend to a colder pipe on the cylinder going away.
 
But if the coil was indeed dead, then why does the cable spark when I disconnect it from the plug?
 
Dingo said:
But if the coil was indeed dead, then why does the cable spark when I disconnect it from the plug?
Click to expand...
Excellent question my friend.

*** Whoa, I read that wrong. You spark plug is sparking outside the cylinder when against the engine? Or your cable is sparking without a plug?
 
redwillissuperman said:
You need to get out more:) I run three sohc race bikes and the coils die all the time. Usually one post of one coil goes cold first, stays that way for a while, then both drop out suddenly (usually as far away from the pits as possible). You pickit up on the plugs where one cylinder starts misfiring and the plug darkens. I carry a spare coil all the time and can swap it out in about 5minutes. I have used the Dyna coil, Harley and stock Honda ...all have the same life. I've started checking the pipe temp in the hot pits to see if I can trend the failure. The first instance is usually intermittent failure but you do see a general trend to a colder pipe on the cylinder going away.
Click to expand...

I was guessing that the OP is talking about a street bike. And he's got spark on all four cylinders. And I've been working on SOHC fours for 20 years and rarely do stock coils go bad on street bikes. Frequently, however, people report a cylinder not firing and someone says "replace your coils" when that shouldn't be -- due to cost -- the first step of a diagnosis. But hey, everyone has their own experiences with these things. You might be right.
 
carnivorous chicken said:
Excellent question my friend.

*** Whoa, I read that wrong. You spark plug is sparking outside the cylinder when against the engine? Or your cable is sparking without a plug?
Click to expand...

If I disconnect the cable and put it near the engine you can see (and feel dumb me) the sparks, I haven´t tried the same thing with the spark plug outside the cylinder, should I try it?
 
Easy check to find out if its ignition or carbs.

Find the cold cylinder and swap the plug leads. Cylinders 1&4 and cylinders 2&3 share the same coil on a CB400F.

If the cold cylinder follows the plug lead swap then it is ignition, if it doesn't its usually the carbs.
 
You must log in or register to reply here.

DTT Light or Dark

  • DTT Light
    • www.jadusmotorcycleparts.com
    www.lostapostle.ca/
    www.sparckmoto.com

    Spread the Love

    Back
    Top Bottom