Topic: One cylinder running cold

[Johnny5]

73 CB350F

I have cylinder 4 running super rich compared to the other 3. The bike is all tuned to spec. Carbs cleaned, floats verified at 24mm. New plugs. Timing is spot on. Valves done. Only thing I can think of is the coil might be petering out. How can I switch coil 1 and 4 to verify if it's the coil or not? If the problem goes to cylinder 1 then that should narrow it down.

[Pat_at_APE]

Just unplug them and swap.....  Both leads should be about the same length since they come from the same coil. 

Pat at APE

[Johnny5]

Cool, that's what I thought but didn't know if I had to swap 2 and 3 as well to not throw off the timing.

Just unplug them and swap.....  Both leads should be about the same length since they come from the same coil. 

Pat at APE

[calj737]

Cylinders 1&4 are on the same coil already. And if 1 is firing, then 4 is too.

You say 4 is running "super rich". Is the plug dry and sooty or wet and black? If it's wet and black, then I'd suspect your plug caps not the coil. Replace any fouled plug to continue your diagnosis after you measure the resistance of the actual plug cap.

If the plug is dry and sooty then I'd suspect too much fuel despite your recent rebuild. It's not uncommon for floats or jets to become dislodged when assembling.

[Johnny5]

Hey Calj,

Yeah, I first thought fuel too. And correct, it's black and sooty, not wet and oily. It looks like a rich symptom. I took off the float bowl and checked the slow jet and floats (again). All is good. Also checked to make sure the choke wasn't closed for any odd reason on that cylinder. I've tried three new plugs all resulting in a rich mixture.

I even messed with the air screw and had it turned out 2 1/2 times to really lean it out and it was still sooty. Odd. So that's why I was narrowing it down to spark?

Cylinders 1&4 are on the same coil already. And if 1 is firing, then 4 is too.

You say 4 is running "super rich". Is the plug dry and sooty or wet and black? If it's wet and black, then I'd suspect your plug caps not the coil. Replace any fouled plug to continue your diagnosis after you measure the resistance of the actual plug cap.

If the plug is dry and sooty then I'd suspect too much fuel despite your recent rebuild. It's not uncommon for floats or jets to become dislodged when assembling.

[Johnny5]

Just measured caps 1 and 4.

Cap 1 measured 11,300ohm
Cap 4 measured 9,200ohm

Hmm, according to Hondaman, these are both close to out of spec.

"The SOHC4 bikes all came with "resistor caps" for the sparkplug connections.
On the bikes prior to 1976, the ohms value of the resistor inside was 7500 (7.5k) ohms from the factory. Over time, they slowly burn out and increase in value: at 9500 (9.5k) ohms, they are considered to be "burned out" and can cause several problems, including hard starting and fouled sparkplugs."

[markreimer]

I had a very similar issue on my 74 cb750K. I'd foul #2 every 60-100 miles. I chased my tail for a long...long...time, until I replaced the plug caps and plug wires (used a NGK splice kit with original coils) and I haven't fouled a plug since.

[Johnny5]

Going to definitely replace all the caps. But what's strange is that cap #1 is worse than #4, but it's #4 that's showing the worse symptoms. Will report later when I swap these out.

I had a very similar issue on my 74 cb750K. I'd foul #2 every 60-100 miles. I chased my tail for a long...long...time, until I replaced the plug caps and plug wires (used a NGK splice kit with original coils) and I haven't fouled a plug since.

[mrbreeze]

Check your bowl levels with the clear tube method. Measuring with a ruler doesn't ensure that the fuel levels are even.

[markreimer]

Check your bowl levels with the clear tube method. Measuring with a ruler doesn't ensure that the fuel levels are even.

Any suggestion on how to do that if you don't have the bowl style where the screw opens a drain, but where the screw IS the drain?


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[danyo]

Check your bowl levels with the clear tube method. Measuring with a ruler doesn't ensure that the fuel levels are even.

Any suggestion on how to do that if you don't have the bowl style where the screw opens a drain, but where the screw IS the drain?


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Small diameter tube (can't recall exact dimension, sorry) after preheat will screw in into the hole exactly like drain screw making good seal. Another way is to buy some spare drain screws and modify them to fit clear tube (more hassle imho)

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[Johnny5]

I'll check the clear tube method tomorrow but don't think that's the issue. Honda manual states floats at 21mm and these are set at 24mm. Gas isn't coming out of the overflow tube. The condition just seems pretty extreme. But Ill rule it out for sure.

Thanks for the suggestion.

Check your bowl levels with the clear tube method. Measuring with a ruler doesn't ensure that the fuel levels are even.

[PeWe]

Fuel level is very important. Not too much nor less. General rule of thumb is 2-4mm under the float bowl gasket.
Too low can cause lean running cylinder. High fuel level will soot the plug at idle and low rpm, can cover up for a way too lean jetting.

[Johnny5]

Makes sense. I'll report my findings tomorrow. Looking forward to getting this sorted. Bike runs fantastic regardless and I know once this cylinder is cleared up it'll run even better.

Fuel level is very impartant. Not too much nor less. General rule of thumb is 2-4mm under the float bowl gasket.
Too low can cause lean running cylinder. High fuel level will soot the plug at idle and low rpm, can cover up for a way too lean jetting.

[calj737]

Just measured caps 1 and 4.

Cap 1 measured 11,300ohm
Cap 4 measured 9,200ohm

I'll bet a dollar to a donut this is your problem because you state plugs are dry and sooty. If it were fuel, #4 would be wet with too much fuel.

[Johnny5]

Gonna swap the caps to see if the problem switches cylinders today as well. Got some new NGK caps on order anyway.

Just measured caps 1 and 4.

Cap 1 measured 11,300ohm
Cap 4 measured 9,200ohm

I'll bet a dollar to a donut this is your problem because you state plugs are dry and sooty. If it were fuel, #4 would be wet with too much fuel.

[Johnny5]

Yep, switched the caps and the rich symptom followed. All should be well with the new NGK caps in a few days!

Thanks for the help, all.

[calj737]

Yep, switched the caps and the rich symptom followed. All should be well with the new NGK caps in a few days!

I'll bet a dollar to a donut this is your problem...

Bavarian Creme filled please, chocolate dipped of course. 

[Johnny5]

Bavarian Creme filled please, chocolate dipped of course. 

+1 !!!

[calj737]

Bavarian Creme filled please, chocolate dipped of course. 

+1 !!!

The wife just surprised the kids (and Dad) with a dozen from Krispy Kreme (election day, schools out). Brought home a caramel flavored creme filled, salted caramel drizzled thing. Geez, I had to leave the house lest I eat a dozen. Heroin addicts ought to switch to these things. Cheaper and easier to get.

[Johnny5]

Bavarian Creme filled please, chocolate dipped of course. 

+1 !!!

The wife just surprised the kids (and Dad) with a dozen from Krispy Kreme (election day, schools out). Brought home a caramel flavored creme filled, salted caramel drizzled thing. Geez, I had to leave the house lest I eat a dozen. Heroin addicts ought to switch to these things. Cheaper and easier to get.

Nice wife!!!

[Johnny5]

Hey Mark,

I checked the outside carbs today while they were on the bike on the centerstand, just to double check. I found a random small plastic hose barb that I wrapped with some duct tape to seal enough and twisted it into the drain hole. Here's a picture. The fuel is about 3mm below the gasket line which I'm happy with.

Check your bowl levels with the clear tube method. Measuring with a ruler doesn't ensure that the fuel levels are even.

Any suggestion on how to do that if you don't have the bowl style where the screw opens a drain, but where the screw IS the drain?


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