Topic: cb400f 2 and 3 cylinders not firing but getting gas and spark

[81shovel]

I guess I will start at the beginning.  I am working on a coworker's cb400f.  A few weeks ago she took a passenger for a ride that may have been a little too big.  She said the bike had a hard time getting up to speed. She figured it was from the added weight.  The next day she jumps on the bike to go to work and the bike still didn't have the power that it normally does.  She limps the bike to my house and we see what I thought was oil dripping from the exhaust where the muffler connects to the collector (she has some pinholes in collector).  The head gasket has been leaking for awhile but not bad enough to effect performance.  I figured she finally blew the head gasket with the added strain on the motor.

I replaced the head gasket, started the bike up without the muffler connected and fluid started coming out of the exhaust again.  Now I can tell it is gas.  I guess the gas was sitting in the collector and mixing with the soot in the exhaust and came out black like oil. 

So I started looking for the cylinder that wasn't firing and found that cylinders 2 and 3 were cold.  I pulled the plug, grounded it to the motor and cranked the engine over.  I have good spark and fuel but its not burning.

So, what caused a good running bike to drop two cylinders?  I figured it was the points or coil since cylinders 2 and 3 run on the same coil but I have spark.

I am stumped!  It does seem like there is a lot of fuel running into those two cylinders.  Enough to make it out of the exhaust after the bike only running for 30 seconds or so.

any ideas?

Thanks,
Dario

[Don R]

Strange as it seems sometimes a plug will fire in open air but not in a wet cylinder with compression.  Without a better idea, I'd try to dry the cylinder, new plugs, check the float levels etc. compression or leakdown test?

[TwoTired]

Are the spark plugs carbon fouled?  If you don't have new or know good ones to try, swap them for the ones in cylinders that do get hot.

If you are getting gas in the exhaust (I suspect it is really only condensation), gas can only get there via the piston chamber.  If that is filling with gas, then the oil supply is also diluted, making very high risk of engine mechanical damage.  So, check the oil.  If too full or smells like gas, new oil is a mandate.

Anyway the gas can only get to those points via the carbs and the most likely culprit is the float valve system.  This is either gunked up, worn, or having rust particles block the valves closure.

Does your rider know to turn off the fuel tap when parked?

[81shovel]

Are the spark plugs carbon fouled?  If you don't have new or know good ones to try, swap them for the ones in cylinders that do get hot.

If you are getting gas in the exhaust (I suspect it is really only condensation), gas can only get there via the piston chamber.  If that is filling with gas, then the oil supply is also diluted, making very high risk of engine mechanical damage.  So, check the oil.  If too full or smells like gas, new oil is a mandate.

Anyway the gas can only get to those points via the carbs and the most likely culprit is the float valve system.  This is either gunked up, worn, or having rust particles block the valves closure.

Does your rider know to turn off the fuel tap when parked?

Thanks for the replies.

The plugs are not carbon fouled, they are actually pretty new.  I put new plugs in it at the beginning of spring.

I ran home at lunch and the oil has gas in it.  I will drain the oil tonight.

soo.... is it either too much fuel or bad spark from the coil?  Is there a way to test the coil?  I don't want to start throwing parts at it since it isn't my bike.

I also switched the blue and yellow wires to the coils and then switched the spark plug wires.  I can get the bike to sputter but won't stay running.

I hope I can get this sorted out so I can save her some money.  I don't want her to get ripped off at a mechanic.

Dario




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[81shovel]

Strange as it seems sometimes a plug will fire in open air but not in a wet cylinder with compression.  Without a better idea, I'd try to dry the cylinder, new plugs, check the float levels etc. compression or leakdown test?

I will check the float levels tonight.

[TwoTired]

The plugs are not carbon fouled, they are actually pretty new.  I put new plugs in it at the beginning of spring.

Not clear from your response if you have looked at them since spring.  It only takes a thin layer of carbon on the center electrode insulator to shunt spark away from the gap.  I've seen it happen to brand new plugs the same day as installed, if the conditions support that.

I ran home at lunch and the oil has gas in it.  I will drain the oil tonight.

Check the gas tank for rust and verify the filter in the tank is still intact.

soo.... is it either too much fuel or bad spark from the coil?  Is there a way to test the coil?

Sure, got a multimeter?  Check the FAQ or the shop manual.


If you have gas in the oil, bad spark won't cause that. Sooo...

[81shovel]

I have a multimeter.  I will check the FAQ when I get home tonight.  If there is gas in the oil, does that mean the rings are bad?   I will drain the oil tonight to confirm.  Is the next step to do a compression check?

I will also check the plugs again tonight.

thank for the help

Dario

[TwoTired]

I have a multimeter.  I will check the FAQ when I get home tonight.  If there is gas in the oil, does that mean the rings are bad? 

It's not something to conclude yet.  Kinda depends on how long it was run with diluted oil.

I will drain the oil tonight to confirm.  Is the next step to do a compression check?

Compression check won't hurt if you do it right.  There's a FAQ entry about that, too.  Newbs often make the wrong assumptions when using equipment meant for much larger displacement auto engines.
You can get a good "feel" just by putting your finger on the spark plug hole.  With electric start, it ought to blow your finger off the hole, unless you put 150- 200 pounds of weight on your finger to hold seal.

[81shovel]

I have a multimeter.  I will check the FAQ when I get home tonight.  If there is gas in the oil, does that mean the rings are bad? 

It's not something to conclude yet.  Kinda depends on how long it was run with diluted oil.

I will drain the oil tonight to confirm.  Is the next step to do a compression check?

Compression check won't hurt if you do it right.  There's a FAQ entry about that, too.  Newbs often make the wrong assumptions when using equipment meant for much larger displacement auto engines.
You can get a good "feel" just by putting your finger on the spark plug hole.  With electric start, it ought to blow your finger off the hole, unless you put 150- 200 pounds of weight on your finger to hold seal.

Thanks for the quick responses, I will try the finger method tonight. 

Its really cool that these forums exist. 

[81shovel]

I found the culprit.  Taking your advice I dropped the bowls on the carbs.  The main and needle jet were laying in the bowls.  My next question, why did they fall out.  I was surprised that they don't screw in.  My only carb experience is with the butterfly and CV keihin carbs that are on Harleys.  If I put the jets back in, what is going to stop them from falling out again?  Are the O rings supposed to hold them in? they seem dried out. I will search the site but if someone has a quick answer, please let me know.

[MoMo]

You are missing the metal retainers that hold the jets in.  Check a microfiche for your bike and you'll see what is missing...Larry

[81shovel]

You are missing the metal retainers that hold the jets in.  Check a microfiche for your bike and you'll see what is missing...Larry

Boom!  just checked the book. I am missing the leaf springs.  Not one of the carbs had a leaf spring holding the jets in.  Supposedly a mechanic tuned the carbs for her a couple years ago... she has been riding this bike without anything holding the jets in for years.

I will do some googling but is there a good site to order parts from?  Or, is there anyone on the forum that sells the leaf springs?  I would rather give my money to someone on the forum than a random website.

[TwoTired]

Harisuluv sells carb leaf springs.  But, I don't know if the ones for the CB550 are the same as those for the CB400.  Could be.

[MoMo]

http://forums.sohc4.net/index.php?topic=101803.0


Also, check this clever repair.  I bookmarked occasions such as this...Larry

[81shovel]

http://forums.sohc4.net/index.php?topic=101803.0


Also, check this clever repair.  I bookmarked occasions such as this...Larry

Looks easy enough, I will head to Lowes tonight.  I let my coworker know that I found the problem, she is ecstatic.  I should have her back on the road by the weekend, although it is supposed to rain but I don't think that will stop her! Thanks for all of the help.

Dario

[harisuluv]

Harisuluv sells carb leaf springs.  But, I don't know if the ones for the CB550 are the same as those for the CB400.  Could be.

Yes they all use the same leaf spring, and yes I do sell them for $40 a set shipped.