Topic: Pulsing carb on CB650

[Hush]

OK someone is going to know or have had this happen to them, I have never seen anything like it so I'm in the "HELP" mode.
My CB650 which has had a few days rest as I hunted for a petrol tap is doing something peculier, the #1 carb is pulsing like a real human pulse in the carb isolator, it's almost like a back fire but it is regular.
Also every now and then #2 shoots a small amount of gas out through the overflow tube?
The bike is running like crap and considering it just had a $130 carb sync I'm not too impressed.
It ran fine when I picked it up from the bike shop and great when I rode it but a few days sitting and this!
I've had the plugs out, #1 was a bit carbon fluffy but as I've been idling the bike while I work on it I kind of expect that, #2 was wet, I cleaned it up, tested it and chucked it back in.
The bike mechanic said when he went to sync the carbs #2 wasn't firing at all so he did the old take out the plug and clean refit and it worked again.
It's got me scratching my head and with the beautiful Spring weather finally here I don't want to miss too many riding days.
That blue stuff over the house is cloudless pure blue riding day sky, something we haven't seen here for a few months.

[Gordon]

Did you happen to put on new carb boots recently?  I've never had brand new ones before, but from what I've heard the new ones are soft enough that they do actually pulse as the engine runs. 

The #2 problem sounds like either a float level or float valve issue.  Letting too much fuel into the bowl causing it to drain some out of the overflow tube and fouling out the spark plug. 

[Hush]

Think you are right on the second part Gordon, I've been slaving away in carb Hell all day, thought I had it licked but nah it failed again.
Seems that the #2 carb is flooding and then flooding #1? is this how they work!
Anyhow after much swearing and the loss of a few gallons of gas I think that's the problem, #2 is hanging or sinking and back filling #1.
I thought I would be riding today but no!
I have a full spare rack of carbs so I'll take a float from there and redo #2 tomorrow, now I need some sleep before my night shift.

[honda750k]

it sounds as though you need to check float height in your carbs very thoroughly, and ensure the #1-2 carbs are set properly.

[Hush]

Yeah well I've never had any trouble with my float heights, I have replaced 1 float when it sank, I'm guessing they are all doing the Titanic thing.
What amazed me was the backfilling from carb to carb, if one floods it runs to it's neighbour and then you have two cylinders not running.
The mechanic who did the sync mentioned the #2 plug was wet and he did not know why but told me to watch it for the future.
Well he was right on the money and I guess the #2 float was begining to sink then.
I did find something out, my bike starts beautifully on 2 cylinders and even idles?
I had 2 plugs out and was just turning the bike over to check for spark when it fired right up.

[Spanner 1]

Hush, fer Gawd's sake, your below the Equator, opposite to us guys so you have anti-gravity, right ?...Did someone remove the anti-gravity bungs from your carbs ?? Are the floats set measured from the bottom of the float bowl, like they should be ? How bikes run at all way down under is a mystery to me .....

[Hush]

We have to fit a mechanical force feeder pump to the carbs just to get gas to flow!

[j_oly1999]

Hush,  i've got an '81 and have just went through all the carbs, and here's what "I" think is the deal, mind you i'm no certified mechanic, but follow the logic for me.

#2 float is cracked or set too high, whatever it is, letting in too much fuel.  Which, in turn if you look between the carbs, there is a small rubber T fitting i believe, that vents the flaot bowls on #1 and #2 carb.  If #2 is flooding, it would/could back flow into carb #1, right?  #1 would pulse prob. from excess fuel, but not quite enough to bog or foul it out.

[Hush]

Yes same conclusion I came to, will pull the bowl on #2 today and fit replacement float.

[Hush]

OK I've had a #$%* of a day getting intimate with my old girls hard to reach bits.
I dropped the carb bowls on #1 and #2 a grand total of 7 times, would they seal? would they Hell as like.
I swapped out bowls and floats and float valves and pivot pins...and still petrol everywhere!!!
The #1 carb still pulses (through the isolator) when the bike is running, but the bike runs great at 8000 rpm?
#2 carb leaks like a sieve, gas everywhere, I parked the bike up before I took to it with an axe!
Anyhow now I'm wondering if the #2 carb leaks because it's getting massive amounts of gas from #1 carb and the pulsing isolator is the huge amount of gas the bike is trying to swallow because #1 has a sunk float???
It would explain the reason the bike runs fine at high revs and burbles at idle.
Damned if the bike doesn't run perfectly on #3 and #4 alone! and those carbs don't leak at all.
Anyhow I've run out of patience and daylight and need a few hours ZZZZZ before night shift so I've put it all way for the night, any bright ideas from forum members I will definately entertain.

[scunny]

connect some clear tubing to your carb drains and see what the levels are. might point you in the ballpark

[manjisann]

any bright ideas from forum members I will definately entertain.

There is a sheep in your airbox 

Could there be a minor deformity in the #2 valve seat that allows petrol to flow past it even when the float needle is all the way up?

Could the posts where the float pivot pin sits be worn weird causing the float, any float, to hang up and not float as it were?

I'm not sure if the float pivots on the pin, or the pin pivots in the shaft or a combo of the two, but could there be any burs that would inhibit this?

Sorry can't remember if you have, but check float height. Maybe it's been off for a while and somehow was not noticed.

Just a few brain storms I came up with, hope they help.

Brandon

[Hush]

Thanks Scunny as per usual the voice of calm reason.

Brandon, I replaced just about everything including the #2 bowl so I doubt it is that.

Off to the shed to do battle again, man the weather is blue sky perfect, I had a run with the harley club arranged today.......AAARRGHHHHHHHHHHHHHH.....OK I'm better now.

[BobbyR]

The #1 carb still pulses (through the isolator) when the bike is running

When I replaced my isolators with new they pulse. $ years later they still pulse. I do not have a 650, but it sounds like you are flooding and I would check the level in all carbs with a clear plastic hose.

[Hush]

Dear diary: I better write this down while I can still remember the sad tale.
1) Petrol tap had minor leak
2) removed tank to tighten leak and stripped tap thread.
3)Recieved replacement tap from Scunny
4) Decided to replace the old gas hose while I was at it. 
5) Made the hose end larger with a screw driver (someone shoot me now) 
6)Now the #1 and #2 carbs leak, strange "pulsing" in isolators?
7)Removed bowls 9 times (new SOHC4 record I would think) and swapped parts with old carb set.
Two days of frustraition later and take Scunny's advice to use a clear tube to check float hieght.
   (but I knew this, I knew to use this to check, brain in neutral)
9) Find small bit of rubber fuel hose on the tip of float valve.
10) Re-set floats and the leaks stopped.
11) Got to ride 40 miles today, still a wee leak when i finally stopped but that seems to have cured itself now.

[Hush]

All I can think is that I was too close to the problem and needed to take some time away and get some sensible advice.
I need to remember that the Japanese did not build these carbs to fail, they were built to be precise, economical and not needing a lot of servicing.
Anyhow just getting out and doing some road time must have dropped my blood pressure a few notches.
I must have got the float heights pretty close too, on the motorway I was maintaining 70 mile per hour and there was plenty more but a nasty cold cross wind convinced me not to push too hard.
Glad I didn't have to muck around with my carb sync, hate to waste $130.

[manjisann]

I dunno Hush, road time to cure a problem, sounds suspicious  Anyhow, glad to hear you have worked it out and can ride again, stop off and buy yourself a chocolate fish for figuring out the answer to your perplexing problem!

Brandon

[Hush]

You know what a chocolate fish it?