Topic: cb350F - odd plug problem

[thirdman]

I have just finished reassembling my '73 CB350F... added Accell coils, Dyna S ignition, replaced defunct floats and did carb kits.  Synced the carbs, too.
(BTW, I don't have a very good idea of how it is supposed to run, as it didn't run for beans before I tore into it.  Bad coils, bad points, gummy carbs, etc.)

It runs ok, but the plugs foul VERY quickly (black and powdery).  I tried dropping the jets a notch, but it still fouls and now it feels "lean" (hesitation, etc).

So is the right answer to try a hotter plug? I am running NGK D8EA right now (the recommended OEM plug), and am looking to go one step hotter to a D7EA.  Dare I do this with the current carb setting, or am I going to end up too lean?

(or am I missing the boat entirely, and the problem is elsewhere...)

[don_m]

D8EA or D8ES are correct.  I use D9EA in my modified 350F.  It's a bad idea to correct a plug fouling problem by using hotter than specified plugs as you could damage the engine.  The air cleaner element is very restrictive & clogs easily.  Clean or replace the plugs & remove the air filter.  The bike should start OK but at feel very lean at about 1/2 throttle or more under load.  Drive it as low throttle, then inspect the plugs.  If they are clean, replace the filter.  If they are wet fouled the problem is likely oil fouling.  If dry, powdery fouled the mixture it too rich.  Check all plugs to narrow down the possible fault.
Don

[thirdman]

.
Thanks for the reply!
Here is some more data: 

• The air filter is brand new... only about 50 miles on it.
• The fouling is dry and powdery.
• I have double checked the timing.
• I have new floats, and have checked the levels twice.
• It was fouling both before and after I stripped the carbs, cleaned them, and put in new carb kits.
• I have new coils and wires and am getting good spark.
• All plugs are fouling equally.
• I have carefully set the plug gaps.
• I carefully clean or replace all of the plugs after every change I make in configuration.
• I have run without the air filter, and it didn't seem to make any difference.  I will try what you said, though.
• It fouled before and after I dropped the main jet a notch (it was in the middle of 5, now at second notch down).  After I dropped it, it feels lean (hesitates).

Any other hints?  I am stumped... I am worried about the hotter plugs as I don't want it to "detonate".

Oh, what octane fuel do you use?  Could this make any difference?

[don_m]

Your symptom is clearly that of a rich mixture.  The float needles on these & most Honda carbs are spring loaded.  If you turn them upside down the weight of the float will compress the spring & give a false indication. To adjust, hold the carb horizontal with the float dangling & just touching but not compressing the needle.  Bend the tang so that the float hinge is parallel to the body.  I can use 89 octane fuel, but my head is modified to a squish band shape & my carbs are re-jetted for the foam element & the increased displacement to 425cc, so your stock engine requirements will be different.  Also, the fuel composition has changed   considerably in the 3 decades since my 350F was stock.  Set points & timing carefully, then synchronize the carbs & experament with fuels.  If you hear a "tinkling" sound at full throttle it is probably pre-ignition, which can damage pistons.

[thirdman]

Your symptom is clearly that of a rich mixture.

I would have bet money on it being too rich, too, but it continued to foul even after I dropped the needles a notch.  Also, it had huge flat spots and hesitated horribly with it dropped.  I put them back to the stock position, and it runs much better... but fouls just the same.  Weird.

Oh, and I did exactly what you said for adjusting the floats.  I even have the "official" Honda float tool... whee.  I even replaced the floats because they were looking a bit raggedy.

I felt a bit psychotic this weekend, so I dropped in the one-step-hotter plugs, and it seemed to foul a bit slower, but still fouled.  Runs maybe slightly smoother at idle.

I also switched from 91 octane to 87, and did not notice any difference in fouling, performance or sound.

I am going to triple check my timing again soon, and re-sync the carbs just for fun.

(current state is: stock jet, hotter plug, new floats (carefully adjusted), 87 octane.)

I'll post again after I recheck the timing.  Thanks for the tips.

[don_m]

On another post to someone I mentioned that the stock filter element is very resrictive & clogs easily.  Also the inlet into the filter box is under the seat & can be blocked by a something there.  (Been there, done that.)  Take your element out plus the tool tray & airfilter cover.  With stock #75 main jet the motor should now be very lean at 1/3 to 1/2 throttle unde load and not want to rev, plus bucking & spitting.
Like most similar carbs there are 4 different stages of regulation.  Idle for low throttle, needle & jet for mid & main for high throttle and slide cutaway for transition from low to mid.  Also a pool of rich mixture at the bottom around the needle jet serves as an enrichment device when you rapidly open the throttle.  Check the sychronization of the choke butterflies.  There is one master controlled by the lever and each one next in line has an adjustment. No matter what others say about synchronizing at full open, be sure that all 4 close fully when the choke lever is up.  Otherwise starting & running with partial choke will be problematical.  Run the bike at different throttle settings to try to pin down the problem.
Where are you?  Don

[Gordon]

Make sure that your carbs have the correct size jets.  Some previous owner may have messed around with the jetting and never put it back to stock.  Happened to me before.

Gordon

[don_m]

Addendum to prior posts, or as I think of things.  Be sure the o-rings on the main jets are in good condition & the jets are right side up.  It does make a difference in flow.  Size code should be visible when installed.  The needle works with the needle jet, of course & regulates mixture in the mid-range, approximately 1/4 to 3/4 throttle.  Above that the needle is pulled out of the needle jet far enough that the main jet is the greatest restriction & controls the mix.
Cheers, Don

[thirdman]

   Success!    The culprit was...... TIMING!  I had to advance my timing a smidge, and voila!  Fouling is gone, plugs are now a happy color, and it runs great!

This kills me as I checked and rechecked the timing earlier in the debugging process...

Thanks for all of the tips!    I am going to add this thread to my permanent collection for future debugging.

[don_m]

Congradulations!  Persistance pays off.  With each cylinder having it's own carb, exhaust etc., & sharing ignition it can be a challenge sometimes.
Don.