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 Any SU carb guru on here :)

Can't get the hillman imp running right .I've got rebuilt twin HS2 1-1/4 ,the imp is 875cc with 4-2-1 headers and Griffiths intake manifold

Current got new Red springs and MME needles a guy in the UK is using this and works well on his IMP but he has a sports head and sports cam so flows a little better but nearly the same setup has same manifolds as me too 

I can't seem to get it to increase the idle a little if I push the lift pin up abit inleast I wind up the fuel mixer alot. I either have good top end/bad low down (bogging)or good low down and average up top by playing with mixers 

I've set floats by using a 3mm drill bit between middle of plastic float and float lid and I've balanced both carbs reasonably close 

 

Any suggestions be good! maybe I'm using wrong needles and springs, I found a post saying to use Blue springs and E3 needles but that info was from like 40 years ago :doubt:

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I was under the impression, and it's how I tuned my Mini, that when lifting the lift pin on the carb the idle should stay the same if the mixture is OK. If the idle increases when the pin is lifted it means the mixtures is rich. 

I didn't get involved in changing needles and springs so can't help there. 

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It will increase briefly but return to normal. If it stays high your mix is out

 

Your mix should be at about half way of the nut, otherwise you need different needles. 

Should check it with a wideband, makes it much easier

 

 

Pretty sure there's M needles in my A series twin carb set up and bd needles in my sidevalve twincarb 

 

https://www.7ent.com/pages/articles-tech-tips/chart-carburetor-needle.html

 

 

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Sounds like youve got the incorrect needles. Your spring selection should be based on having the piston at full height at redline under load. TBH having a O2 sensor like the kits that 14.7 sell makes life much easier when setting carbs up on a midified engine. Also ensure that your mechanical and vacuum advance are working correctly. cant count the number of times that has screwed up tuning a vehicle.  

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Just a couple of little basic things, have you done the drop test on your pistons? You can have identical spec carbs tuned perfectly at idle that immediately go up the shit under load if one piston rises faster than the other or jams up 

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If the idle does not increase momentarily when you push the nipple up that indicates toward a vacuum leak no?

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2 hours ago, NickJ said:

If the idle does not increase momentarily when you push the nipple up that indicates toward a vacuum leak no?

I hadn't heard that one... interesting because my triumph is like that, I just assumed that it was in the  ballpark at that point... seems to idle down low enough 

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How high have you got the jet wound up ? It shouldn't be much above the bridge if at all. Have these been rebuilt ?  Jets centered ?  Will need a strip to do that. A book of needles would be good....but of course the ones you need won't be available. But with the book you can get something close.

 

 

IMG_20200725_175819.jpg

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On 24/07/2020 at 19:45, gibbon said:

Just a couple of little basic things, have you done the drop test on your pistons? You can have identical spec carbs tuned perfectly at idle that immediately go up the shit under load if one piston rises faster than the other or jams up 

I actually forgot to do it when I had the carbs off :doubt: I've done it in place seems about the same 

On 25/07/2020 at 10:36, NickJ said:

If the idle does not increase momentarily when you push the nipple up that indicates toward a vacuum leak no?

I'll check for vacuum leaks 

On 25/07/2020 at 19:12, Motu said:

How high have you got the jet wound up ? It shouldn't be much above the bridge if at all. Have these been rebuilt ?  Jets centered ?  Will need a strip to do that. A book of needles would be good....but of course the ones you need won't be available. But with the book you can get something close.

 

 

IMG_20200725_175819.jpg

Im actually not sure I got told to set it to about 9 flats down to get it running, so I adjusted jet from all the way up and adjusted it from there

That's what I was thinking of how you do it?

 

Yes fully rebuilt from someone who's rebuilt a few of them had new shafts etc

 

Fitted new blue springs and e3 needles I'll try retune it this weekend and see if it's any better and I'll let you guys know how I get on :-) 

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I whip the plungers out put a straight edge across the brass jet the needle lives in set it flush for your base setting it should run ok as a base to tune from. Vids pretty long but very detailed pour a scotch find a comfortable chair....

I put SUs on everything they look cool on anything and actually work very well when setup and tuned right.

Ignition issues will mess up your results big time make sure everything is perfect there.

 

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On 25/07/2020 at 10:36, NickJ said:

If the idle does not increase momentarily when you push the nipple up that indicates toward a vacuum leak no?

You sir are correct! Vacuum leak around manifold gasket will pull it off tomorrow and see if I can sand manifold flat..

 

5 hours ago, Muncie said:

I whip the plungers out put a straight edge across the brass jet the needle lives in set it flush for your base setting it should run ok as a base to tune from. Vids pretty long but very detailed pour a scotch find a comfortable chair....

I put SUs on everything they look cool on anything and actually work very well when setup and tuned right.

Ignition issues will mess up your results big time make sure everything is perfect there.

 

Did that yesterday I think,  fitted the new needles and use a ruler to make sure  the shank on the needles were flat with the plunger. I'll watch the video tomorrow just to make sure :)

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