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Discussion Starter · #1 ·
Carburator is "spitting" fuel out the back (out the air intake) which kills engine

I got a 1972 Honda CB125 about two weeks ago.
She ran fine but i was out for a drive when she backfired a few times and stopped dead, now i have issues.
She starts on the first kick no problem.
Idling is not very smooth but i think that is due to all the messing around that has happened since the issue.
My main problem is that once in a while (random amount of time), i get a spit, for want of a better word, of what seems to be fuel, out of the air intake.
it makes a little "ppfftt" noise and it makes the engine cut out.

I dont really know what this is.
My only thought is that maybe a small particle of something (maybe from the tank) is flowing through the carb and blocking a passage for a brief moment, which is forcing air/fuel mixture out the wrong passage.
I am trying to figure out why i broke down in the first place and if these symptoms show that is was a carb issue all along.

* Sparkplug is good and giving a good spark
* carburetor has been cleaned, no blocked passages (as i can tell anyway)
* I had a little run round the block at low speed and no issues while running except for cutting out when slowing down to a dead stop at a junction

Any questions, thoughts or ideas on this would be most welcome
I am really new to this but keen to learn so apologies if some of this does not make sense.
Thank you :)
 

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Discussion Starter · #3 ·
To be honest i am not 100% sure what the inside of the tank looks like.
This seems to be the next job for me then as soon as the weekend comes around.

there is currently no fuel filter in the system and i have read that some in line filters can be restrictive, any thoughts on this?
 

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The float Bowl constantly maintains fuel inside and the velocity of fuel through the carburetor is controlled by the availability of fuel from the float Bowl so while some fuel filters can be restrictive as long as the float Bowl remains full or at least has enough gas for normal operation then however restrictive as the fuel filter is in the fuel line it will not affect the way fuel gets into the carburetor.

I'm using talk to text but hopefully that makes sense

...in the end if you were to go WOT for an extended period of time in a race you probably would deplete the float Bowl if you had a restricted fuel filter but because the way the system is set up in any sort of normal driving operation it will not make a difference... because fuel can still replenish the float bowl while you are backing off throttle Etc

Sent from my SM-G930V using Tapatalk
 

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What you're witnessing is backfiring- the intake charge is being kicked back out.

A carburetor's job is to mix fuel into the air that passes through it. And a carburetor is dumb- it doesn't care which direction that air is flowing.
Ever hear the "lumpy" idle of a hot rod with a radical camshaft, one that delivers a "blub-blub-blub" idle but makes insane power in the upper rev range? That's because there's so much overlap (that works to your advantage at high revs when intake velocities are so high that they cram air and fuel into the cylinders even after bottom dead center) that at low revs the air-fuel mixture gets pushed back out the intake manifold, back through the carburetor (where fuel gets added to the mix again), then sucked back in (fuel added yet again) so that the mixture is so rich that it doesn't burn properly. All that to say that something that interrupts or mis-times the spark delivery can result in the intake mixture being spat back out of the carburetor.

The fact that this happens occasionally suggests that you've got some sort of intermittent interruption to your ignition. This could be from the aforementioned poor cam timing, or it might be from dirty or pitted points, or maybe weak spark from a failing coil or a corroded connection in a spark plug cap or failing resistor in the cap or a dirty spark plug or something else that's causing mistimed or failed delivery of the spark.

Or I could be completely wrong.
 

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Discussion Starter · #7 ·
If there is air and gas blowing out the rear of the carb, it means that you have a problem with the intake valve, or the cam timing has been retarded, like the chain jumped a tooth.
TOOLS
These two issues sound slightly out of my current range of being able to fix.
I have got the service manual but i have never attempted adjusting either of these.
suppose there is a first thing for everything though :)
 

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Discussion Starter · #8 ·
What you're witnessing is backfiring- the intake charge is being kicked back out.

A carburetor's job is to mix fuel into the air that passes through it. And a carburetor is dumb- it doesn't care which direction that air is flowing.
Ever hear the "lumpy" idle of a hot rod with a radical camshaft, one that delivers a "blub-blub-blub" idle but makes insane power in the upper rev range? That's because there's so much overlap (that works to your advantage at high revs when intake velocities are so high that they cram air and fuel into the cylinders even after bottom dead center) that at low revs the air-fuel mixture gets pushed back out the intake manifold, back through the carburetor (where fuel gets added to the mix again), then sucked back in (fuel added yet again) so that the mixture is so rich that it doesn't burn properly. All that to say that something that interrupts or mis-times the spark delivery can result in the intake mixture being spat back out of the carburetor.

The fact that this happens occasionally suggests that you've got some sort of intermittent interruption to your ignition. This could be from the aforementioned poor cam timing, or it might be from dirty or pitted points, or maybe weak spark from a failing coil or a corroded connection in a spark plug cap or failing resistor in the cap or a dirty spark plug or something else that's causing mistimed or failed delivery of the spark.

Or I could be completely wrong.

The sparkplug cap is nice and clean on the inside (cant comment on the resistor), the sparkplug is a bit dirty (looks like from running rich) so i have got a new on on the way, as for the other items i will have to check these.
I feel like i am more confident with the electrial side than the mechanical so i will start in my comfort zone and build from there.
I really appreciate the help :)
 

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Get a proper Factory Service Manual, they can be downloaded if you look and ask enough.
You'll need to reset cam chain tensioner correctly, verify timing and set valve clearances

It seems complicated now, but with some reading and a few YouTube videos (different bike model videos are OK as long as the presenter explains things) you'll eventually get the sense of how these simple machines work. Understanding theory of operation always makes the difference.
 

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Discussion Starter · #11 ·
Thank you all for the help!! To keep a long story short. Today I ended up taking the flywheel off after some attempts to set timings and other adjustments that did not work. The woodruff key was sheared completely level with the crankshaft. This is then what has kicked the timing completely out. Please correct me if I’m wrong. I’m also adding a photo of my improvised flywheel puller lol. Maybe it will help someone
 

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Discussion Starter · #12 · (Edited)
Unfortunatly in my naivety and eagerness to get this bike i did not check enough and have now found that this is not a CB125, the engine is a CG125E (Nov 1984- April 1985) but the bike is registered in 1972 which was before the CG......
 

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Discussion Starter · #14 ·
Feel like a bit of an imposter now though. It’s not the classic I thought it was. I’m not even sure what this one actually is lol
 

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I saw this with mower engines. A blade strike and rough running, another, and no run. Not sure how a bike would stress an engine that way. Perhaps a popped clutch stalling the engine.
 

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Looks like it was run for a while, loose on the taper, until it sheared the key. You were lucky, the crank usually looks worse than that afterwards.
 

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Still a classic even if fitted with a modern EV-motor...
Feel like a bit of an imposter now though. It’s not the classic I thought it was. I’m not even sure what this one actually is lol
 
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