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Old 04-08-2018, 10:46 PM
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harry harry is offline
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Default Inlet port obstruction

I've been in the motor trade all my adult life and played with engines way before I was an adult.

But you live and learn every day.

I've been fettling the AJS for a year or so while riding it in the fine weather and doing major jobs in the winter.

So I'm talking amal carbs here.

I've struggled to get a nice reliable tick over and get rid of a flat spot just as you open the throttle, so that it makes the bike fall into corners as it loses power when it should be powering out of the corner.

I've done all the usual stuff changing jets moving needles changing throttle slides etc etc It changed things but never made it better.

Another annoying thing was the insulating spacer between the head and carb was too thick making the carb too close to the oil tank thus making it hard to get the bell mouth on. So I read all the parts lists and eventually sourced the correct one. But the diameter of the hole in the middle is 2mm more than the one that was on there. But amazingly the tick over and flat spot are cured.

Lots of research on the internet and eventually I've found the explanation.

Inlet port obstruction. Many carbs squirt the fuel into the middle of the airflow but amals don't the fuel especially at idle enters the airflow through a drilling at the bottom of the venturi. so the fuel droplets tend t run along the bottom of the air passage. If they meet a tiny obstruction like a spacer 2mm too small the fuel tends to go back into larger drops which don't combust well.

So yes that 1mm step in the inlet tract was enough to cause all those running problems. Yet in all my years I've never heard of it before.
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Old 05-08-2018, 05:33 AM
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I wonder how many people ovr the years have come across a similar problem and have sold the bike or chucked it away in frustration on not being able to cure it?
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Old 05-08-2018, 05:35 AM
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Or the bike is still festering in the back of the garage shed and has done so for many years....
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Old 11-08-2018, 07:55 PM
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Quote:
Originally Posted by Friar Tuck View Post
Or the bike is still festering in the back of the garage shed and has done so for many years....
More than likely.
Carbs are a difficult even to experienced hands. A mate has had a huge problems with carb related running issues on his ZZR1100.
Sorted now, but it took a long time.
It doesn't hurt to pop a dose of carb cleaner in the petrol every now and then.
Carb expertise is a dying skill.

Last edited by HOS; 11-08-2018 at 08:00 PM.
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Old 11-08-2018, 08:10 PM
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Through carefully looking and considering the new individual carb filters on my UJM I noticed that the rubber sleeves that fit over the carb bell mouths are slightly thicker than the standard rubber connectors from the old airbox.
But the bell mouths have two slots at different positions around the outside of the bell mouths which the new filter rubbers would have partially blocked.
A 1mm overlap on a 2mm slot basically.
So before fitting them I took the Dremmel with a router bit and very carefully eased out the rubbers on the inside to prevent the overlap.

Here you can see the slot arrangement.
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Old 12-08-2018, 07:08 AM
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I've noticed recently tht the rubber between the carbs and engine have cracked just thruogh age really. So when I get around to recommissioning the bike I am not looking forward to removing the carbs to replace the rubber tubes, as this means dismantling half the bike just to get access! sheesh. Me just being lazy really....
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Old 12-08-2018, 08:33 PM
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Quote:
Originally Posted by Friar Tuck View Post
I've noticed recently tht the rubber between the carbs and engine have cracked just thruogh age really. So when I get around to recommissioning the bike I am not looking forward to removing the carbs to replace the rubber tubes, as this means dismantling half the bike just to get access! sheesh. Me just being lazy really....
The carbs would probably need to be stripped, cleaned and checked.
If you look at the whole job it looks like a lot of work. So I split the job up in my head into bite size pieces.
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