Air leaks in carb. Choke down (open) - high revs.

Justinas

Active Member
Hey guys,
I'm stuck with a question with my CB250G5 (1976) with pods and straight exhaust. Somehow when choke is down (open position) the bike revs like crazy. When choke is up (closed position) it just idles. Not nice, but at least it idles. Idle screw is all out, fuel screws are 1,5 turns out.
My first thought was air leak somewhere, but I sprayed the carb with wd40, changed all the gaskets in the carb, changed the inlet rubbers - nothing gave any results.
I have changed jets to bigger jets to accommodate pod filters and straight exhaust and at this moment my only idea is that the mixture is so fat, that when it gets some more air with open choke it makes the mixture right and this make it rev so high. But I don't know how much logic is in this...

Can there be other cause for these huge revs with open choke? Would be grateful for any ideas and advices.
 
I had a similar problem with my GS450. Turned out to be a crack/leak in the rubber boot connecting the carbs to the engine, but it could be a leak anywhere in front of the carbs that cause a difference in pressure that causes the problem...or something else entirely! :)
 
Maybe you have the idle set too high or the throttle is stuck part way open? Pull the pods and verifiy the throttles are closed. Might see if it does it with them off as well.
 
Thank guys for your advices. Although spraying carbs with WD40 (checking for air leaks) showed no change in revs, I have changed inlet rubbers to new, so there is definitely no leaks there. Choke and throttle flaps aren't stuck, checked that too. They really work fine and idle screw is almost all the way out (just to have it start).

As I open the choke just a little bit, the bike starts reving really hard. After a while revs drop a little bit and I can open choke a little bit more.

The thing is, if I block the air intakes completely, it doesn't start - if air leaks would be present, I guess it should start even with air intake blocked.

I have noticed that there is some petrol in air shafts, when I remove pods, but don't know what could this mean? Maybe this is the problem?
 
Turns out fuel in shafts was because of needles being too high. lowered them and problem is solved.

But open choke (flaps are open) - very high revs, closed choke (flaps are closed) - low revs, problem remain. Any other guesses what might a problem be?
 
Any other ideas on these high revs? Would be very thankful for any suggestions. After countless carb rebuilds and experiments I'm completely out of ideas:/

Got the jetting back to default and fitted the original air filters. Sadly it still revs like crazy with choke open.
There are no air leaks (checked with wd40 and fitted new inlet rubbers), the choke closes and flaps open and close like they should, idle screw is almost all out.
 
do a check of you linkages and cable and make sure they are returning to the idle position and not holding the carb open

pop the carb off if you ave to to make sure the butterfly is closed
 
cxman said:
do a check of you linkages and cable and make sure they are returning to the idle position and not holding the carb open

pop the carb off if you ave to to make sure the butterfly is closed

Checked it. Butterfly is closing and opening really nice and smooth and is not holding the carb open :/
 
I'm not much of a mechanic, but I'd recommend having a whiff of the exhaust to see if it is rich or lean and see what happens when you block the air intake it should die pretty quick, but will hopefully highlight other problems.
 
Maybe check compression in the cylinders? Not sure it would help, but the more info you have and the more things you rule out the closer you should get to the problem (the solution is another matter entirely!).
 
Paservank said:
I'm not much of a mechanic, but I'd recommend having a whiff of the exhaust to see if it is rich or lean and see what happens when you block the air intake it should die pretty quick, but will hopefully highlight other problems.

Thanks for suggestions. With air intakes blocked it doesn't start. If i block the intakes while engine is running it dies pretty quick too. That's why I'm pretty sure it's not air leak anywhere :/
 
Justinas said:
Any other ideas on these high revs? Would be very thankful for any suggestions. After countless carb rebuilds and experiments I'm completely out of ideas:/

Got the jetting back to default and fitted the original air filters. Sadly it still revs like crazy with choke open.
There are no air leaks (checked with wd40 and fitted new inlet rubbers), the choke closes and flaps open and close like they should, idle screw is almost all out.

Throttle cable too tight or sticking?
Try disconnecting it at carb end
If your using dual (opening and closing) cables it's pretty easy to adjust them so they counteract each other
 
No no, throttle cable is definately working ok. I replaced one of the cables with a spring on the carb, but even if I remove both the throttle cable and the spring from the carb it still revs like crazy when I open the choke at least a little bit.
 
Screws between carb and mounting plates are tight?
Both butterflies fully closed? (carb sync issue?)
Quadrant shaft locknut tight (the pivot for 'wheel' the cables connect to)
Clip missing between cable wheel and shaft allowing it to float?
 
Yeah, screws are tight and new gaskets there too. Carbs haven't been synced, but don't know how this could be sync issue:/

Just to be clear, here is a video of what happens when I open the choke. Cable is removed, idle screw just a little bit out, to get it started at all. The sound doesn't represent the real revs, somehow it sounds better in video;] Revs go up until 6k and would keep on going. The engine is newly rebuilt so I don't want to screw it up.

http://youtu.be/Iofx7Tua_yE

The left carb began flooding too, but I guess it's a different story, because previously it didn't and bike acted the same.Will have to solve this too.
 
Carb sync could be an issue, one side can hold the other open, or, prevent butterfly closing (same thing, said different ;) )
Have you taken a look at butterflies from engine side of carbs?
Here's a pic of throttle full open to show the adjuster screw fitted wrong, the washers should be either side of arm

The idle adjuster is missing, threaded hole lower right
Link bar between cable pulley and operating arm, this one was bent (angled down to left)
The sync adjuster bottom is in the middle, you can see 'U' shaped link between carbs is opened up and washeris 'missing' (both are 'on top')
Idle adjuster cable/screw fits into threaded hole


You do have the throttle shaft plugs in place?
Here's a pic of carb with missing plug

Check the bypass port plugs are in place, (the white plastic plug)


I've done a lot of CB/CJ carbs, 250 and 360
These are pics from some of them showing various problems, last pic is 'fixed' you can see the sync adjuster washer is in correct place 8)
 
Hi, just wondering if you every remedied the problem?

I’m going through the exact same thing on my Honda CL450 twin.... all advice points towards an air leak but I seem to have tried everything and cant stop the revs from ghost riding... I’m starting to think it’s possessed!

Any help you can give me would be greatly appreciated, I’m assuming you got to the bottom of your issue?
 
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