engine won't fall back onto idle

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john.nash
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engine won't fall back onto idle

Postby john.nash » Wed Jul 06, 2011 4:26 pm

Just done an engine change on an old enfield.
Different carb, that came with it, but using the same mag etc etc.

The engine wont fall back onto idle very quickly, it seems to want to race for a short while and then settles back.
I would usually put this down to worn bore on the carb , but:

  1. it responds to the mixture screw (which in my experience worn bore carbs rarely do)
  2. strangely enough likes to start, from cold, without either a flood nor the choke closed
  3. Once it's at idle, it's fairly steady.

There are no carb leaks at the head (tried the wd40 trick), it's all been cleaned out, there is play in the throttle cable and the slide is hitting bottom without hanging up. engine pulls fine as well.
John "Josh" Nash
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2009 royal Enfield,1978 t140 bonneville, '67 CJ750, 196-ish Ural M62 outfit, 1960 k750, ''51 kH500, ''49 soon to be Ariel bobber, 47 VH twinport, '44 Ariel WNG, '43 Ariel WNG, '41 Ariel WNG and piles of rusty scrap ....

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Postby nevhunter » Thu Jul 07, 2011 3:55 am

Check how worn the slide is ( Carb) or if it is bottoming on the jet block and not fully closing. Sometimes people will remove a bit from the bottom of the slide to reduce the cut-away. There may be a overly large nipple on the end of the cable also stopping it from closing, then check inlet valve guide wear. Nev

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Dave.Barkshire
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Postby Dave.Barkshire » Thu Jul 07, 2011 8:31 am

Sounds like a mystery. If the carb is good as you say with no leaks or wear and the slide returns fully home then the only other thing that you might want to question is the float level. I have had some strange float level behaviour in the past where vibrations at certain revs cause the float to vibrate and free flow and I've also have floats which touch the sides and drag which can cause some funnies. These problems have been on vintage carbs though. Anyway, it's just a thought.

PS : You mentioned WD40... I remember that a while ago Wali sprayed some WD40 into his inlet and wrecked the engine due to the inlet valve drying up. There were some postings at the time on the old forum.
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Postby brenton.roy » Thu Jul 07, 2011 10:40 am

Hi John,
If the slide is bottoming and isn't worn, it could be an air leak somewhere else. Do the symptoms change as the engine warms up?
Does the slide drop immediately when you shut off or is there a slight lag? The return spring could be past it. Have you checked the throttle cable for frays/kinks/lube and is the path to the carb via gentle bends?
I have heard that air can get in via a worn valve guide and cause these symptoms, but I'm not sure if this is just myth.
regards, Brenton
'51,'56 Squares, '48 VH, '27 Model C, R67/2, Mk IV Le Mans, '06 Super Duke and Ariel projects.

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Postby john.nash » Thu Jul 07, 2011 4:10 pm

Do you know Brenton, I think it might get worse when nice and hot. I ran it down to the MOT, this morning, and it stalled twice on idle on the way back.
Where do I look for that ? Mixture ? I set the screws when warm.

The slide definately hits bottom on throttle release - HOWEVER who knows if it's been messed wth slightly.
The throttle stop screw has an effect, and a quarter turn either way will speed things up/slow things down.
The mixture screw does likewise.
I lapped in the float needle and it's sporting the float chamber and new float off the previous enfield engine (the body off that was a clip fit and different size unfortunately)

The carb is a pre-monobloc 276, with a longer body to get it over the magdyno.
I'll order a new slide (£25) just to see if there is an improvement, but I am thinking the body is worn as usual and the mixture screw working or better/worse when warm is just one of those things. I cannot get just a new body, they will only sell me a complete carburettor at a mind numbing £277. A concentric won't fit.


.. and I will be careful wth WD; I only use it to check for manifold leaks, at the joints.
John "Josh" Nash
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2009 royal Enfield,1978 t140 bonneville, '67 CJ750, 196-ish Ural M62 outfit, 1960 k750, ''51 kH500, ''49 soon to be Ariel bobber, 47 VH twinport, '44 Ariel WNG, '43 Ariel WNG, '41 Ariel WNG and piles of rusty scrap ....

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Postby Vincent.vanGinneke » Thu Jul 07, 2011 10:56 pm

Did you actually looked into the carb inlet when it went revving?
To cancel out a 'misbehaviour' of the slide I would look there first.
Just a thought.................

regards, Vincent

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Postby john.mitchell » Thu Jul 07, 2011 11:10 pm

Have a look at ebay 260661784991, John. A New Enfield India carb for £42 inc postage.
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Postby brenton.roy » Fri Jul 08, 2011 7:23 am

Hi John, the most likely things are what you are checking for. So long as the body isn't really worn, a new slide and a stronger return spring will most likely bring a cure.
The bodies tend to bell out where most of the movement is - around the bottom of the slide and the slide can rattle around a bit, causing symptoms like yours.
You can occasionally get an oversized slide - and have the body cleaned up /reamed /bored to fit it. This is way cheaper than re-sleeving.
I have cleaned up a worn carby body to suit a slightly thicker slide by making a "reaming tool" with some wet rub sandpaper and steel tube. Not exactly good engineering practice, but it worked!
I asked about whether heat changed the symptoms because if there is a crack or leak somewhere, this will either be better or worse when things expand (usually worse!).
Is it running lean when it has warmed up? A "plug chop" is the best way to check this. http://www.barkshire.co.uk/AOMCC/Knowle ... spx?id=735
If there is an air leak, it probably is running leaner than you would like - and warmer than you expect.
Brenton
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Re: engine won't fall back onto idle

Postby adrie.degraaff » Fri Jul 08, 2011 8:26 am

If you are using a gasket that's to soft you bent the carb. housing giving similar problems.
Last edited by adrie.degraaff on Sat Jul 09, 2011 10:45 am, edited 1 time in total.

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john.nash
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Postby john.nash » Fri Jul 08, 2011 10:57 am

john.mitchell wrote:Have a look at ebay 260661784991, John. A New Enfield India carb for £42 inc postage.
RegardsJohn M


John, it's a 1940 enfield. Has an amal 276 with a long intake to get it over the magdyno. Otherwise, I'd have done the usual and fitted a new concentric; just not enough clearance.

Thanks for the other advice lads.
I am acting on it.

Found that Autocycle engineering (birmingham) want £50 (pluspost etc) to resleeve.
A little while to do it though, so I'll fiddle and live with it, until the winter.
Missed two months summer riding on an ariel last year 'cos of a mag.
John "Josh" Nash
webmaster@arielownersmcc.co.uk
AOMCC No.4119
2009 royal Enfield,1978 t140 bonneville, '67 CJ750, 196-ish Ural M62 outfit, 1960 k750, ''51 kH500, ''49 soon to be Ariel bobber, 47 VH twinport, '44 Ariel WNG, '43 Ariel WNG, '41 Ariel WNG and piles of rusty scrap ....


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