Topic: Runs good! Sometimes.....

[Stev-o]

Get purses and jewelery if she's that type of gal.

Jewelry works for me!

[Deltarider]

I rode my bike (CB400F) to college 10 km's, then rode it home. It took me a semester to fill it up with carbon deposits (i believe so). I would describe it as lagging on a cylinder. After checking everything and cleaning carbs over the winter, i tried to just ride 40 km's. And the last 20 were great. Then i rode another 35 km and they were all great. I believe, (but i dont know) that it was to many short rides that caused my problems. especially since the school rides were low rpm's. I am contemplating to use additives regarding all my short rides.

Our CB Fours don't like short distances. They're coldblooded, so they need time to warm up. On short distances they won't and so will build up carbon and sparkplugs will foul. Then there's the risk of turning your oil in mayonaise and your muffler will rot away in no time. Tip: use a bicycle or 50cc moped for short distances. Whenever I rode my bike on the highway at 130 km/h + for a longer period of time, I was always amazed how lively and responsive my bike was when I left the Autobahn and continued on countryroads.

[cb750f-2010656]

I hear ya.  Don't really wanna open her up till I've exhausted all options, but that weak 2nd gear sure gets on my nerves.

[cb750f-2010656]

Changed the oil today and found rubber bits in the filter housing.  I have let her sit for a few days at a time and my oil level in the tank never drops below full.  I mention that because I have read a topic on this forum about a bad seal in the oil pump.  Maybe a deteriorating oil line?  I dunno, but I think I just need to quit riding her until I figure it all out.  Just need piston rings, gasket set, and carb kits and I can ride the GS500F for a while and take the Old Girl apart.

[cb750f-2010656]

So I took the tops off of the carbs, pulled the slides and checked needle clip position.  I understand that the middle or third from either end is stock.  Mine were and still are set one notch richer.  I left them as they were because I was running out of daylight and didn't wanna undo anything in the dark.  I wasn't able to check #4 because the screws are rusted and trying to strip out. 

Now, I know that cylinder #2 is happy.  The medium Brown tip of the spark plug tells me so.  But the other spark plugs are still sooty black, and it still wants to load up with fuel at very low throttle.  I'm gonna check the float operation after work today.  My carbs have the nipple coming off of the bowls.  Just gotta unclog them and buy some clear hose.

I'm hoping the needles and seats are just fugged up.  I have carbs 1,3,4 mixture screws like almost 2 turns out to give it some kind of driveabilty.  Will post again later today.

[cb750f-2010656]

I still need to modify some screws to go in the bowls before I can check my actual fuel level within.  I drained the fuel from each carb and got some murky gasoline with bits in it.  Small pieces of rust getting by my fuel filter I assume, because my tank is getting there. 

[flybox1]

I still need to modify some screws to go in the bowls before I can check my actual fuel level within.  I drained the fuel from each carb and got some murky gasoline with bits in it.  Small pieces of rust getting by my fuel filter I assume, because my tank is getting there.

PVC vacuum line connectors from the auto.hardware store are great for clear tube test connectors.
Drain all the gas from your tank.  Correct the rust problem with prepnetch or the works before re-filling.
If rusty gas has gone into your carbs, they should be cleaned again.  Spray through everything.  pull all the jets and emulsion tubes.

[cb750f-2010656]

There is some stuff that mixes with water and won't harm paint.  I was told it was developed for aviation fuel tanks.  Anyway it eats the rust and leaves a black, rustproof paintable surface wherever the rust was.  Anyone know what it is called?  I am not a fan of coating the tank as that stuff tends to flake off over time.

Hey flybox1, do i still need to drill out some screws for the fittings to go into?  I wish I could find SOMETHING with a nipple that would just thread in there.

[flybox1]

There is some stuff that mixes with water and won't harm paint.  I was told it was developed for aviation fuel tanks.  Anyway it eats the rust and leaves a black, rustproof paintable surface wherever the rust was.  Anyone know what it is called?  I am not a fan of coating the tank as that stuff tends to flake off over time.

Hey flybox1, do i still need to drill out some screws for the fittings to go into?  I wish I could find SOMETHING with a nipple that would just thread in there.  PVC vacuum line connectors from the auto.hardware store are great for clear tube test connectors.

Just screw these into the drain screw hole.



I prefer Kleen Strip Prep n etch.  Its phosphoric acid, eats rust, wont hurt paint, leaves a nice grey phos coating in the tank.  little to no flash rust.

[cb750f-2010656]

Ok!

[cb750f-2010656]

Checked oil pressure today and it's good.  One less thing to worry about.

[cb750f-2010656]

Been working with the old girl again.  I've installed new resistor caps, new points and set them with dwell meter, set timing, advancer springs are working as they should, checked float heiggt again, confirmed the needles arent leaking, cleaned and synced the carbs again...  Also checked connections all through the primary ignition circuit.

It just runs rough once it has had a good warm up.  I guess ill throw a set of coils on it when i get some cash.

[cb750f-2010656]

I finally drilled out the stripped screw holding the #3 slide rod to access the slide needle.  It was one position too rich.  I moved the clip back to the middle slot and synced the carbs again and that helped a lot with the bucking problem I had.  Then today I got out the vacuum gauge and set the mixture screws for most vacuum, and synced the carbs yet again (what a PITA, by the way).  I know that the book says best vacuum then in -100rpm, but my bike doesn't idle that smooth.  It still has that flat spot just off idle where it just coughs smoke and tries to die.

I would think if the mixture screws were too far out I would have a lean condition.  However the soot coming from the muffler tells me I have unburned fuel leaving the combustion chamber, hence why I believe I need a coil.  While in this flat spot it sounds like a couple of cylinders are firing and a couple are choked.  Oh, well.

I suppose I could have the carbs sonic cleaned for once.  If anybody knows of something else I should try before I bite the bullet and spend $120 on coils I would certainly appreciate knowing about it, if anyone even looks at this old thread anymore.

[cb750f-2010656]

The last time i was in her carbs i noticed i have 110 mains.  After some research i learnt that 105 mains are stock.  Could be why she's loading up on me in that flat spot.  I reckon the Jardine muffler doesn't flow enough to require rejetting, as i still run a stock airbox and paper filter.  Does anyone have four 105 mains for the old round top carbs i can try out?  Thanks.

[cb750f-2010656]

So with the mixture screws 1 1\4 turns out I have an acceptable level of driveabiliy.  The hesitation and surging is only an issue at low rpms, around 3000.  She runs pretty d@mn good for as far as i go on the first trip from cold.  However, after a 20 minute rest she still runs like crap.

[flybox1]

I finally drilled out the stripped screw holding the #3 slide rod to access the slide needle.  It was one position too rich.  I moved the clip back to the middle slot and synced the carbs again and that helped a lot with the bucking problem I had.  Then today I got out the vacuum gauge and set the mixture screws for most vacuum     and synced the carbs yet again (what a PITA, by the way).  I know that the book says best vacuum then in -100rpm,  but my bike doesn't idle that smooth.  It still has that flat spot just off idle where it just coughs smoke and tries to die.

I would think if the mixture screws were too far out I would have a lean condition. Correct thinking  However the soot coming from the muffler tells me I have unburned fuel leaving the combustion chamber, hence why I believe I need a coil.  Its not a coil, just incorrect A/F mixture While in this flat spot it sounds like a couple of cylinders are firing and a couple are choked.  Oh, well.

I suppose I could have the carbs sonic cleaned for once.  If anybody knows of something else I should try before I bite the bullet and spend $120 on coils I would certainly appreciate knowing about it, if anyone even looks at this old thread anymore.

Its not coils.  Your ^^^ process is incorrect.
now that your carbs are loosely vacuum sync'd, you need to dial in idle mixture....complete an idle plug chop.

[flybox1]

Leave your mains alone.
too may adjustments at once and youre just chasing your tail.

Idle plug chop with new plugs.
Follow up with posting a closeup picture of their tips.  one picture with all plugs lined up 1-2-3-4.

[cb750f-2010656]

I got around to buying the new plugs.  I just must ask:  how would idle mixture play a part in an intermittent problem?  Sometimes she runs very smooth with good power.  Stop to go into a store or at a friends, come out, and she just runs like @$$.  Will buck till ~3700, smooth out through ~4200, then begin to rough up over that.  Sometimes, she'll run great 'till the ~4200, then rough up.  But very seldom, Just smooth consistent power.  I'm getting kinda frustrated today.  I need to hurry up with this Rebel so I can retire the Ol' Girl again.....

Also, she has developed a charging system issue now (oh GAWD, I'm bad at this).

[TwoTired]

The idle mixture fuel path ( also called the pilot mixture circuit) operates in parallel to the mains and throttle valve fuel path.  This means they are somewhat interactive at all throttle settings, even if dominant at a specific throttle setting.

An over rich mixture at any throttle setting, can form deposits on the spark plug's internal electrode insulator. With enough build up the carbon path shunts away spark energy from the electrode tips.  The bike will run like ass thereafter until the carbon deposit either self clean or the spark plugs get attention.  Replacing spark plug in the scenario is only a temporary fix. As the root cause is still a mixture delivery problem.

When your bike runs like ass, stop the engine immediately and look at all the spark plug tips to determine combustion run conditions and if insulator fouling has been added to cause list.

Cheers,

[cb750f-2010656]

I pulled the plugs and they were all sooty but no 1.  It was sooty but had a dark brown tip.  I installed the new ones and had no time to chop as it was dark already.  Ran the bike with the mixture 1 3\4 turns out.  Still runs poorly once warm.  Pulled those plugs and 1 and 3 are fully sooty and 2 and 4 are sooty except for where the center electrode meets the insulation, there is a white ring around there.

Above 4800 rpm it runs but sounds like a cylinder or two are choking back.  Once it is warm low throttle is still shaky.  If you slowly open the throttle in neutral it will climb steady to about 5000 rpms.  At that point it will begin to miss and cough black smoke.

Im charging the battery now and will vacuum sync the carbs a gain tomorrow as well as buy new plugs to do the idle chop.  After that I'll do a full throttle pull and check a again.

[TwoTired]

It won't ever run well with sooty plugs.
Got to figure out why its getting more fuel than it can burn.
May not be jetting.

Cheers,

[PeWe]

Changed the oil today and found rubber bits in the filter housing.  I have let her sit for a few days at a time and my oil level in the tank never drops below full.  I mention that because I have read a topic on this forum about a bad seal in the oil pump.  Maybe a deteriorating oil line?  I dunno, but I think I just need to quit riding her until I figure it all out.  Just need piston rings, gasket set, and carb kits and I can ride the GS500F for a while and take the Old Girl apart.

This can be a bad sign. Oil orifices in the head can be clogged and cam will not get lubrication on either side 1-2, 3-4 or both. The oil transport is via the oil filter then direct to head.

[cb750f-2010656]

I really beleive this to be an ignition problem.  I wish i had an ignition scope.  Also, sometimes when i rev it past 5000k the headlight will flicker.  Not on and off, but dim out like there is a huge load somewhere.

[Jerry Rxman Griffin aka MuthaF'er]

How's your battery and charging system now that you mention this?? Load test the battery then run voltage diagnostics at the increasing rpm ranges.

[cb750f-2010656]

I dont have a way to load test it...  But it turns out that I drained it while resetting the points and timing and I guess there just wasn't enough voltage for that field coil to properly supply the stator.  After charging the battery the votage gets to 13.5 ish with the high beam on so its cool.  I was freaking out a little though.

I set the points perfectly the first time but after a few weeks the dwell angle increased to abiut 106.  I figure parts wearing in, so i do it all again.  Two weeks later my dwell hasn't moved but my 1-4 timing was WAY retarded.  My 2-3 breaker had a sharp peak and wouldnt fire the coil but every once in a while.  I filed it down, set dwell and timing.  Now heres the kicker:  my plate is advanced almost all the way to time 1-4 and my subplate to time 2-3 is fully retarded, leaving 2-3 still firing a touch before F.  Advancer is still working.  Wonder what could be upsetting my timing.