Topic: Carb bewilderment

[1timduke]

Okay, here it goes...

#3 cylinder does not heat the header @ idle.   Works when above about 2500 rpm (about when auto advance kicks in).

Tested for spark at plug w/ plug on head, good spark.   Tank vent open.

Timing, points all good, tappets adjusted w/ engine COLD, check.

Stock airbox + exhaust.

Tore off carbs 5 billion times (give or take a few), clean as a pin, verified float levels, air screws 1.75 turns out, clean new jets, blah, blah, blah.

Bench synced (I know, it's not perfect) then adjusted #3 carb @ idle w/ no improvement in performance. (moved set screw at .25 turn increments several times in both directions, then returned to original 1/8" setting.

Recent top end rebuild (less than 1500 miles on the rebuild) w/ good compression at 500 miles.   And the idle was GOOD, no problems.

Here it is, can it be anything more than a fricking idle jet problem?   Is there anyway that the #2 lead is not firing w/ the #3 lead and that somehow improves at higher rpm?   Any other possibilities?

I've posted this before but it started working and I thought I fixed it (nope).

Is there a passageway under the idle jet that I can flush or probe that I would be able to see if it was clear?   I should've looked at this before while they were off, but I thought I must've surely gotten it this time  

'75 cb550, stock.

-Thanks in advance,

-Tim

[TwoTired]

If the compression is good, the spark is good and occurs at the right time and the fuel mixture is right, then you have a running engine.

Something is missing and perhaps the combustion deposits on the spark plugs may give you a clue?

With what you listed, I'd lean toward an idle circuit or the carbs badly out of vacuum sync.


You say the problem left and then came back?  Fuel contamination?  Have you drained the carbs a examined the contents for debris?  Clean carbs only stay that way until they're fed nasty fuel.  It only takes a single particle larger that 0.016 inch to plug a slow jet.  It is also possible that the slow circuit passageways drilled into the carb body have corrosion flakes inside, that move about doing their nasty deeds.

Cheers,

[KingCustomCycles.com]

+1 on TT.  Try flowing the idle jet passage with carb cleaner or compressed air from a rubber tipped nozzle and visually verify flow to the venturi area.  Sounds like classic idle jet/passage plugging to me.  Details, details.

[1timduke]

Okay, I guess it's what I figured, just wanted to see if there was any other possibilities I didn't think of.   Thanks guys, I'll pull 'em off again (I'm actually getting pretty good at it, I can get 'em off in under 10 minutes now!)   I'll post again w/ the results.   

Thanks again,

-Tim

[1timduke]

Idle passage clean, blew out w/ plenty of flow, no debris in bowls.   Swapped #3 and #4 idle jet (just in case), swapped #3 and #4 spark plugs.  Compared spark on #3 and #4 plugs while holding on case (identical).   Rechecked tappet clearance (good), gave #3 and #4 carbs a good inspection/cleaning of lower portion (removed floats, main jet, float valve, idle jet, blew through all passages, looked good).   

All to no avail...(sigh...)

I got a sync gauge and balanced all 4 carbs, it did settle the idle a bit, but #3 never fired.   I adjusted #3 to the same vacuum as the others.   Idle RPM @ 1000.   Sprayed WD40 around boots, no leaks.

Again, to no avail...(sigh...)

I'm thinking my top end rebuild may be suspect, it had good compression at 500 miles.   I have no smoke or smell from the exhaust, and no excessive pressure or vapors from the valve cover vent tube.   No leaks around head or base gasket.   No oil on plugs.

When I ride, I can feel the engine gain power as I move through the throttle (duh, what I mean is a little extra power as the #3 cylinder decides to fire at what seems like random throttle positions other than idle).   It gives a bit of a jerking effect to the bike.

Compression test is next...   Any more suggestions would be appreciated.

Also, is it possible that there could be a mix up somewhere in the ignition?   Perhaps a capacitor or fiber washer not in the right place, maybe causing the #3 to fire at the wrong time, but overcome by auto-advance as RPM increases igniting residual gas/air mixture in the chamber?   I've played with the ignition a time or two trying to set timing w/o a timing light (I have one now, makes it much easier).   Maybe I messed it up?

Is there any way this could be a float needle valve issue?

I'm nearing the end of my rope, and when I get there, I'm going to tie the rope to the bike and drag it up the stair to my 3rd floor apartment and break it down into a million pieces in the living room.

So, what takes motor oil out of carpet???

Thanks,

-Tim

[Spanner 1]

Does #3 plug stay dry or get wet/sooted when you run the bike ?

[1timduke]

Plugs show even tan, no soot/ oil.

So here's what I did...

Turn fuel off, started bike @ idle, ran until engine sputtered, shut off motor, observed fuel level in bowl #3 and #4 and levels were the same.   #3 header never heated.

So, that tells me it's not the carbs.

I thought, "perhaps an air leak causing too lean syndrome", but this is ruled out by this test and plug condition.

We all thought "plugged idle jet, classic symptoms". Again, i believe this is now ruled out.

I checked the wiring of the points/condensers, it's properly wired (at least 1/4 and 2/3 look the same).

The last thing I can think of is to check the compression, and I hope I'm wrong.   I'll pick up a kit tomorrow.

Q:   Does spark intensity (amps?) increase as RPMs increase?   I tried to switch my 2 and 3 leads but the cables are too short.   I'm wondering if the spark is too weak?   It looked pretty good when I held it on the head (looked like #4).   Also when I connected the timing light to the cable it worked at idle speed.

Any other ideas are welcome at this juncture.   

Thanks,

-Tim

P.S.   Someone please suggest something so ridiculously obvious that in my search for a problem I've overlooked the most simple explanation

[Spanner 1]

Good move, bud...at idle with the slider closed ( down ) I quess the air supply must be thru' the idle air passage...and 3 and 4 used the same amount of fuel until run-out and stall...ah ha....now what?....wait now ,both used fuel by the same amount but only #4 has the air supply to go with it to fire.....were back to plugged idle-air tube on #3 !?, but #3 does not get wet ?

[1timduke]

Ahhh..... I checked the plugs after a ride, but not after the last idle test....   I'll run down and check.   But if it is the air passage, should I back out the idle mixture screw entirely (out) to test?   I've adjusted it out 1 turn more (and then returned) than the rest before w/ no improvement.

-Tim

[Bill Vaughan]

Just another thought to add to the mix:  Have you replaced the sparkplug boots (noise suppressors)?  They do go bad and usually on an intermittent basis. 

[Spanner 1]

Re-read this post and I see where you have swapped just about everything possible but I agree with Bill I didn't see where you swapped the #3 plug cap ?? At 'rest' testing a plug out of the motor is the easiest time for the coil to make a spark ( loads of dwell time ), but at idle the plug is needing to fire 8 times per second, a totally different ball game...so quick swap #4 and #3 cap and try..........

[1timduke]

Spanner1 is on to something!!!

Pulled the plugs after idle, #3 has a slight gas odor, but not wet (didn't run it long).
 
Pulled out air mix screw and spring and drowned in carb cleaner, after a little sputter it started to fire intermittently!   I then switch the choke to all off (more air) and got a slightly hotter pipe, it seemed to want to fire most of the time and I got a hunting idle.

I did check the spark at idle and it looked good, however, the suppressors are a bit old.   Where could I find replacement NGK pieces online?

So now methinks as follows...

-I'm going to pull carbs (sigh) for hopefully the last time and do an 'every damn bit of it' teardown.   I really thought I had gotten then clean enough, I guess there's no 'clean enough', just clean or not clean!

-Replace suppressors.

-Vacuum sync again.

I think that should cover it.   I really appreciate all you guy's input.   Hopefully w/ this little bit all of my frustrations will be over!!!

Thanks again.

-Tim

[WHALEMAN]

I have seen the internal register of the spark plug cap corrode. Unscrew the spark plug cap-connectors and swap from 2 to 3 and I will bet your problem moves. I have seen where it will spark with the plug just set on the engine but not under compression. I beat it open with a hammer and it was all corroded inside. The register looks like a fuse. Dan

[WHALEMAN]

Should use spell check. I meant the internal "resistor" Dan

[WHALEMAN]

www.denniskirk.com has the connectors cheap. Dan

[Bill Vaughan]

The suppressors are still available from Honda, but they are pricey.  I just bought a set for my CB550 from Service Honda for around $50.00.

[1timduke]

Mystery solved!!!

And the SOHC4 member of the day is...Everybody!

As it turns out the idle jet passageway was partially clogged, I discovered this during the full on carb assault that took place on the patio table today.   The carbs were also tuned (again) with the vacuum gauges.   

After reassembly, I got nearly the same result (a really loud swearing marathon)... so I thought, better check the plugs.

Remember how I swapped #3 and #4 plugs and also the plug connectors and tested both plugs on the head w/ the same result?   I do, each time hoping for a glimmer of what the #$@! was going on.   Only to be disappointed.   I even unscrewed the plug end (little silver kinda threaded thingamabob on the end of the plug) and looked for corrosion.   

No dice

Frustrated, I tried the simple thing...

I put a new plug in cylinder #3...

Fired up and idled smooth...

Turns out #3 and #4 plugs were both going bad, but for some reason I got a better spark out of #4 than #3.   Methinks the plug wire on #3 may be going bad soon.   Unfortunately, my leads are too short to switch #2 and #3 to test this theory.

So, everybody gets a gold star!   I really appreciate everybody's input on this, it's been frustrating to no end.   But at least my carbs are REALLY clean now!!!   

Thanks,

-Tim