Carb problems

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spookefoote1956
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Re: Carb problems

Post by spookefoote1956 »

1608 wrote:If I may piggy-back this post, my new carb is running too rich. Lowering the needle makes it too weak. What to do.
Quarter turn out on the air screw, What carb?
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Drunclej
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Re: Carb problems

Post by Drunclej »

What throttle setting is the carb running rich?
Have a look at the link I quoted.. some great info on amal carb troubleshooting.
MikeM.
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Re: Carb problems

Post by MikeM. »

I had carb problems last year on my 59 Model 31, for no apparent reasons it started to run extra rich and lumpy especially on pilot jet, I finished up reducing the jet to a 20 from a 30 and the main jetting from 400 to 390. The bike runs much better now and though could possibly do with a slightly larger cutaway on the slide ( 3 to 3 1/4, increased with a file!). I use Esso Supreme and in a supposingly ethanol free distribution area but am sure the fuel companies have done something with the fuel.
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1608
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Re: Carb problems

Post by 1608 »

If only it were as simple as tweaking the slow running / air screw. I replaced the old carb because the body was distorted and the throttle stuck open, engine has rattled since ( Ariel 650). I 'll have to strip the new carb down and investigate. I'm a little wary of these new carbs, and the quality control. The last new one I bought ( for G12 ) came with the the wrong jet block fitted ( too smaller bore) I just fitted the old one as that wasn't worn.
Apolgies to Jean-Noel for hijacking your post, bad form :oops:
JEAN-NOEL
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Re: Carb problems

Post by JEAN-NOEL »

No problem, John ! Your posts are very interesting regarding carb issues.
Finaly, the the jet block gasket was in place, and the block was perfectly aligned with the carburetor body. It was was very difficult to dismantle the jet block, wich was hardly stuck in the body. I had cleaned the carb last year, but it was full of white stuff, like wheat flour. I have used a Dremel and many Tools to clean and polish all the parts. May be the tiny hole above the air screw was blocked ? i Will reassemble carefully the carb today, and take a road test without dealing with the needle, to see if there is an improvement. I hope to post better news tomorrow !
JEAN-NOEL
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Re: Carb problems

Post by JEAN-NOEL »

Yesterday I ran the bike on the bench, but no road test because it was raining a lot. No improvement : the air screw has remained almost unresponsive...i got again a steady tick over @ 950 rpm, but with the air screw completely closed, no change ! Also, previously, the tick over became slightly faster before stalling when the fuel tap was closed. Now, It stalls without change.
This morning, back in the shed, I re checked the pilot jet, release a bit the throttle cable, to have more gap on the twist grip...and the bike started on one cylinder ! Left plug was warm, the right one stone cold...Tried the left one on both leads, and there was blue sparks...On the right one, no spark ! Both are brand new Champion N4C from AMOC...and are cleaned with brake cleaner and a soft rag.
What on earth could be happened between yesterday 7 p.m. and this morning 9 a.m ?
I think I'm going crazy, so I closed the garage to go in the cellar and have a beer.
56G80S
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Re: Carb problems

Post by 56G80S »

Jean-Noel, that was probably the best thing to do! I tend to try and stick at that kind of stuff until I go round in circles and end up seeing causes and relationships where there are none.

Good luck with it.

Johnny B
Drunclej
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Re: Carb problems

Post by Drunclej »

Yep, it can get confusing can't it?
Can you clarify at what throttle opening the fault occurs,?
I found it helpful to regard the carburetion as three succesdive phases...idle (on idle jet), off tick over to say, 3/4 throttle (needle) and full throttle (main jet). I find this approach stops me fiddling in desperation with anything ( ie needle position) , which can be adjusted, but by using logic, should not be affecting the bikes running at that particular throttle opening. Its easy to introduce new problems by changing various jets and settings, until you confuse yourself. That is the situation I put myself in a couple of weeks ago 🥴
To clarify....was the bike running well prior to this episode, and if so, can the carb be put back as it was, and start the process again?
I'd also echo Mick d's post.
Also, presuming the bike was running better before it was refurbed.....what has changed?
JEAN-NOEL
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Re: Carb problems

Post by JEAN-NOEL »

Back from the shed ! Today I have fitted Champion N3C, as one of the N4C was defective. Bike starts easily, cold or warm. Now the air screw is a bit more responsive, but I can screw it fully closed without stalling. And there is always a flat spot towards 1/3 throttle aperture, and its misfires. Seems to be be too rich, even at tickover. Re checked the float level : 2 mm above the "pimp". Very boring...May I have to bite the bullet and order a brand new carb ? This one appears to be in good condition but...
peter ranson
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Re: Carb problems

Post by peter ranson »

Is the carb an original Amal monobloc ? if so has it had a fibre washer fitted under the float needle seating as shown in the Burlens parts diagram ?. If it has this is the reason the fuel level is too high , this washer was not fitted on the original carb. I have seen this a few times and it produces the same symptoms you describe.
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