Author Topic: CB 350 F - fouling plugs, backfiring... (Read 5866 times)

NapraP · « **on:** February 22, 2014, 02:07:40 PM »

Hello,

I spend maybe 200 hours tryed to run this CB 350 F and still nothing...

What I try:

Engine overhaull, timing chin new and set right, valve clearance 0,05mm intake, 0,08 exhaust
Carbs ultrasonic cleaned, emulsion tubes too, new gasket, synched
New ignition coils, new spark plugs, ignition set with stroboscope lamp

here is my problem:

New/cleaned spark plugs, cold bike and it starts great, run great. After warm up idle good, above 1/6 throttle run great but above 6000 rev´s start back firing. Between idle and slight open throttle no power and try to die.
After 5-8 miles idle gone bad, backfiring is worse, problem idle-open throttle is worse too. Spark plugs gone black. I try to lower float level with no progres. With different air screw setting is better/worse but not good.

I am helpless...please if anyone could help...

Thank you very much!

tomkimberly · « **Reply #1 on:** February 22, 2014, 04:09:10 PM »

Inspect the spark advance and make sure it is not stuck.

Tom

NapraP · « **Reply #2 on:** February 23, 2014, 02:12:55 AM »

Spark advancer is good idea. I checked ignition timing on little warm engine not on really warm so maybe it gone hot and sometime stuck.

Thanks - i look on this and write how it turned out...

NapraP · « **Reply #3 on:** February 23, 2014, 11:17:46 AM »

Ignition is 100% allright. Today I had a chance to borrow carbs from another CB 350F. I changed carbs - no synch, no air mixture settings, only swap. Bike run like new, nice sound, excelent power,throttle response - all was OK.

Borrowed carbs had air screw 1/2 and little bit...with this setting my carbs don´t work (too rich), my carbs works better with 1,5-2 rev. I tried float height 21,22, 23, 24mm, borrowed carbs had 21mm. Tomorrow I will try to synch my carbs again (this work is pain) and I will see...

In my carbs I tried aftermarket jets, original jets, needles are clipped in half...all seems good, but does not work

Any idea?

Thanks!

tomkimberly · « **Reply #4 on:** February 23, 2014, 01:36:31 PM »

Your air screw setting on the carbs should be 7/8 +/- 1/8 turns out from lightly seated.

Tom

flybox1 · « **Reply #5 on:** February 24, 2014, 07:56:31 AM »

what is your intake configuration? airbox or pod filters?
if you have an airbox, what is the condition of the filter? new/old? paper/oiled?
what exhaust do you have? 4 into 1, 4 into 2, or 4 into one?
23/24mm is the best float height
+/- 7/8 on the air screws is correct.
what jets do you have?
pilots should be #35, mains #70

NapraP · « **Reply #6 on:** February 24, 2014, 12:10:31 PM »

Quote from: flybox1 on February 24, 2014, 07:56:31 AM

what is your intake configuration? airbox or pod filters? airbox
if you have an airbox, what is the condition of the filter? new/old? paper/oiled? new paper
what exhaust do you have? 4 into 1, 4 into 2, or 4 into one? four into four
23/24mm is the best float height both tried
+/- 7/8 on the air screws is correct. too rich
what jets do you have?
pilots should be #35, mains #70 the same

I thought about it and if on the other carbs it works with aprox. 6/8 air screw and with my carbs only with minimally 1.5 on air screw and all other is the same (float height, genuine jets, needle setting...) so reason of my problem will be gas leak around main jets (second new o-rings now). Because this carbs are very simple this is only way where it can get rich and that is what i "fixed" on idle with air screw...

What do you think?

Thanks a lot!

harisuluv · « **Reply #7 on:** February 24, 2014, 01:01:56 PM »

Have you tested for vacuum leaks and afterwards tried a vacuum sync?

By the way the stock main for both 350f and 400f is 75.

Did you do all the carb work yourself? Just wondering if you know firsthand what was done or if you are taking some other person's word for it.

NapraP · « **Reply #8 on:** February 24, 2014, 01:51:16 PM »

All was my work - engine, carbs.... Jets size was my mistake (70/75) - both carbs had the same size like in Honda manual.

What do you mean - vacuum leak?

flybox1 · « **Reply #9 on:** February 24, 2014, 02:20:06 PM »

yes, a bad/old o-ring around the main jet can allow fuel to enter. if you've replaced, its probably OK.
check to be sure you dont have anything covering the inlet (snorkel) to the airbox restricting airfow.
do you have a modified seat that might be restricting it when you sit on it?

if no, then....try running the bike with the top cover of the airbox off, how does it run?

if it is a little better, try running it with the air filter out, but put the airbox lid back on.

NGK D8EA or Denso X24ES-U plugs, yes?

NapraP · « **Reply #10 on:** February 24, 2014, 02:41:34 PM »

After rebuild before carbs setting I inspect whole air inlet (looking for loose parts, rubber freeplay, craks...) so snorkel is OK and the seat is standard (the other CB carbs were fitted to the same boots, airbox, seat so it is carbs problem). I tried two new air filters. I tried also "exhaust tube method" for mixture setting with no seat in garage and 1,5 rev air screw was optimal...7/8 air screw and engine die with wet plugs...

edit: spark plugs NGK D8EA...lifetime aprox 20km

flybox1 · « **Reply #11 on:** February 24, 2014, 02:47:43 PM »

is it running well now at 1.5 rev or still rich?

NapraP · « **Reply #12 on:** February 24, 2014, 03:15:56 PM »

not running well. idle ok, from idle at neutral throttle response relatively ok, under load after slight throttle open almost die, when open throttle fast and more, for second die then revs and run good, above 5-6000rev/min start backfiring and slowly go to highest revs...

This power hole behind idle and open throttle can be caused by mixture disbalanced by fuel leak around main jet and additional air by 1,5 air screw. After little throttle open mixture goes lean because idle system is lean due to balance aditional fuel from around main jet and in low rpm and open throttle is small amount of vacuum to suck fuel through main jet and around. In high rpm is vacuum good so mixture is rich again. (Sorry for my english I hope you will understand)

This is now my theory...

CB750F2 · « **Reply #13 on:** February 25, 2014, 01:30:13 AM »

I have just had a quick read of the replies and I don't see any mention of checking that the chokes are open. Pat

NapraP · « **Reply #14 on:** February 25, 2014, 02:03:01 AM »

Choke are set and are ok.

flybox1 · « **Reply #15 on:** February 25, 2014, 08:32:18 AM »

Have the slide needles been altered or their clip position changed from stock?
Have you done a vacuum sync of your carbs?

NapraP · « **Reply #16 on:** February 25, 2014, 12:48:00 PM »

Needles are original and clipped in the middle like in the other cb carbs. Vacuum synch done. Today i try new o-rings at main jet without success. I try main jet from borrowed carbs without success too. Main jet tower looks smooth with no cracks or pits. New gasket under top covers, bowl breathing clean. Tommorow i will try start spray to the slides shafts...
When i made test ride engine made this: neutral, idle -slow opening the throttle and rpm goes normally up to 3000rev then engine start dying then black smoke from all exhausts then run quite normal. Backfiring is still with me.

This bike is realy cursed...

flybox1 · « **Reply #17 on:** February 25, 2014, 01:03:31 PM »

emulsion tubes. the ones under your main jets.
pull them again and clean them. do this to your pilot jets as well.
clean all the tiny holes.

tomkimberly · « **Reply #18 on:** February 25, 2014, 01:04:36 PM »

When you cleaned the carbs, did you remove and clean the emulsion tubes?

Tom

NapraP · « **Reply #19 on:** February 25, 2014, 01:29:22 PM »

Yes I cleaned emulsion tubes and whole jet tower and (I don´t know this word in english) air inlet to emulsion tube. All tiny holes are clean and 15 minutes in ultrasonic made them almost new...No problem to try emulsion tubes from borrowed carbs...
I have idea about my bad cleaning of idle system - maybe is my 1,5 rev at air screw only 0,3 rev due to dirty air channel in carbs (this bike not run for 10 year so the carbs were full of white deposits, rust and other ugly things before i ultrasonic them).

Really thanks for helping me!

flybox1 · « **Reply #20 on:** February 25, 2014, 02:05:26 PM »

your mention of white deposits makes me ask if its possible that the carbs are beyond use? $:-\$
If left along too long without proper care, they can deteriorate.
how about posting up a picture if the carbs, inside and out, next time you have them off....
thanks

NapraP · « **Reply #21 on:** February 25, 2014, 02:38:38 PM »

White deposits were oxidated aluminium i know, but slides are without freeplay. Jets are right and functional. Main jet tower looks good, smooth and functional too. Emulsion tubes are little pitted and coroded (tommorow i will swop it with borrowed carbs). Pilot jet and pilot channels can be cleaned (amount of fuel is dependent on pilot jet and amount of air is depends on air screw)i hope. These carbs are pretty simple so i still trust them. Maybe my will to repair it lays on dificulty of buying these carbs for me.

After my tommorow trials which will give me more depression i will ultrasonic them again so i make some picture for you. Do you want some specific pics?

flybox1 · « **Reply #22 on:** February 25, 2014, 02:49:26 PM »

just some general pictures of the carbs inside, and how about a picture of this 350F you're working on

NapraP · « **Reply #23 on:** February 25, 2014, 03:21:52 PM »

Ok, you got it

My clock shows 0:20 am so pics will be soon...

CB750F2 · « **Reply #24 on:** February 26, 2014, 04:20:44 AM »

A couple of suggestions. Ensure that all of the fuel and air passages are clear. To do this remove all jets and emulsion tubes and squirt fluid through the passages. You may need a diagram showing the passages to assist you. Also, ensure that the float chamber vents are clear. If these chambers are blocked or partially blocked the fuel level can rise above normal levels - even flooding. The overflow tubes in the bowls also act as vents but there should be vents at the top of the chambers as well. My comments are of a general nature because I own a CB750 but I think the carbs on the CB350 are similar. Pat

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Author Topic: CB 350 F - fouling plugs, backfiring... (Read 5866 times)