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Vacuum Advance Fitting


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#1 Albino_Hedgehog

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Posted 19 July 2016 - 04:03 PM

Hello

I am about to fit my new carburettor, but I have noticed that the vacuum advance nipple/fitting is missing and has been replaced with a bolt instead!


Anyone know where I can get one to replace the missing one?

Cheers

#2 Moke Spider

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Posted 19 July 2016 - 08:33 PM

They were fitted by SU at the time of manufacturer and while I'm not 100% sure, I think they may have even been fitted at the time of casting the body. They are not available as a spare part.

 

However, all is not lost. I reposition the same fitting on the HIF44 range, although I leave the original in place (the first one I tried to pull didn't end well).I use some 1/8" Brass Tube, drill the body of the carb and then press in a short length with Loctite.

 

The Brass Tube is available from most Hobby stores and in short lengths. Use a good quality drill bit that's not bent. Drill Bits when sharpened properly and of good quality will actually drill a hole a few thousands smaller than their nominated size, so that's also helpful here.

 

I'd suggest drilling it through in a position where it will line up with the edge of the butterfly when in the Idle position, this so that you'll get very little vacuum at idle (which can lead to over heating while Idling and pre-ignition).


Edited by Moke Spider, 19 July 2016 - 10:17 PM.


#3 AlexMozza

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Posted 19 July 2016 - 09:27 PM

They were fitted by SU at the time of manufacturer and while I'm not 100% sure, I think they may have even been fitted at the time of casting the body. They are not available as a spare part.
 
However, all is not lost. I reposition the same fitting on the HIF44 range, although I leave the original in place (the first one I tried to pull didn't end well).I use some 1/8" Brass Tube, drill the body of the carb and then press in a short length with Loctite.
 
The Brass Tube is available from most Hobby stores and in short lengths. Use a good quality drill bit that's not bent. Drill Bits when sharpened properly and of good quality will actually drill a hole a few thousands smaller than their nominated size, so that's also helpful here.
 
I'd suggest drilling it through in a position where it will line up with the edge of the butterfly when in the Idle position, this so that you'll get very little vacuum at idle (which can lead to over heating and pre-ignition).


Is there anything you don't know?!?!

#4 Dusky

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Posted 19 July 2016 - 09:55 PM

Cooking probably! :D 

So with the hif44 standard location you overadvance the timing at idle?


Edited by Dusky, 19 July 2016 - 09:55 PM.


#5 Moke Spider

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Posted 19 July 2016 - 10:07 PM

Is there anything you don't know?!?!

 

Lots, but the lady folk would easily top that list!

 

Come on mate, I'm learning just like the next guy.

 

What was it some named me the other night ???  (not on this forum) Ah, "Back yard Hack" yeah,,,,,,, :D



#6 Moke Spider

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Posted 19 July 2016 - 10:09 PM

Cooking probably! :D 

So with the hif44 standard location you overadvance the timing at idle?

 

Yes that too!

 

IMO, yes. The HS and Earlier series have the vacuum Port arranged such that there was no vacuum on that take off at Idle. I think the HIF Series it was there all the time to help with emissions, but on some models, they also suddenly had to do things like include Anti-run On valves so they wouldn't run on when switched off.



#7 Ethel

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Posted 20 July 2016 - 09:07 AM

Never made the connection with run on and vac advance, interesting. There's obviously still a fair bit of airflow to keep an engine idling and about half of it will go past the vac pipe. I doubt the butterfly will perform like a switch - it might even increase the vacuum by Bernoulli effect???

 

The crankcase vent is supposed to do similar.



#8 Albino_Hedgehog

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Posted 20 July 2016 - 09:20 AM

Ok cheers!

So I should be able to drill the bolt out, and put a small pipe in a braze it etc!

That'll sort it right?

#9 Moke Spider

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Posted 20 July 2016 - 10:06 AM

Ok cheers!

So I should be able to drill the bolt out, and put a small pipe in a braze it etc!

That'll sort it right?

 

I tend to doubt it.

 

The 'bolt' would have no doubt distorted the hole, so getting a good vacuum tight seal could very much be an issue.

 

I wouldn't try to Braze it and given the Carb body is an Aluminium Alloy, I don't think you'd be able to. Loctite is simple, effective and safe



#10 Moke Spider

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Posted 20 July 2016 - 10:33 AM

 I doubt the butterfly will perform like a switch - it might even increase the vacuum by Bernoulli effect???

 

 

"On many cars it is found desirable to use vacuum operated ignition advance to obtain optimum part throttle consumption figures. The take-off point for this vacuum is arranged slightly to the air intake side of the butterfly, and in such a position that opening the butterfly allows the throttle disc to pass over the vacuum take-off point so that it then communicates with the manifold depression. By this means the vacuum is small at the distributor during idling and full throttle conditions, and is large at part throttle, being at a maximum when the throttle is open a few degrees."

 

http://sucarb.co.uk/...su-carburetters

 

Scroll down a little less than about 1/4 way down the page.

 

If I remember tomorrow, I'll snap off some photos to show the difference between the HS and the HIF series in this regards.

 

<EDIT: Still not sure? Connect a vacuum gauge to the Vac Advance take off Port on a HS Carb, that'll show what's what. >


Edited by Moke Spider, 20 July 2016 - 10:46 AM.


#11 Albino_Hedgehog

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Posted 20 July 2016 - 11:15 AM


Ok cheers!

So I should be able to drill the bolt out, and put a small pipe in a braze it etc!

That'll sort it right?

 
I tend to doubt it.
 
The 'bolt' would have no doubt distorted the hole, so getting a good vacuum tight seal could very much be an issue.
 
I wouldn't try to Braze it and given the Carb body is an Aluminium Alloy, I don't think you'd be able to. Loctite is simple, effective and safe

I meant if I was to replace it with a brass bolt of the same thread.

And braze it to that?

#12 tiger99

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Posted 20 July 2016 - 04:24 PM

Why are you so insistent on brazing? Loctite, or failing that, epoxy adhesive, will not harm the carburettor, but brazing most probably will, as has been explained already.

#13 Albino_Hedgehog

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Posted 20 July 2016 - 04:29 PM

Why are you so insistent on brazing? Loctite, or failing that, epoxy adhesive, will not harm the carburettor, but brazing most probably will, as has been explained already.


I was just pointing out that I would be intending to braze the bolt and not the actual carburettor.

#14 Moke Spider

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Posted 20 July 2016 - 08:11 PM

 

Why are you so insistent on brazing? Loctite, or failing that, epoxy adhesive, will not harm the carburettor, but brazing most probably will, as has been explained already.


I was just pointing out that I would be intending to braze the bolt and not the actual carburettor.

 

 

Ah, OK.

 

Yes, I don't see why the tube couldn't be brazed to the 'bolt', though you'll only need very low heat.

 

Like tiger, I thought you were trying to Braze the Carb body, which would be a challenge.



#15 Ethel

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Posted 21 July 2016 - 01:20 AM

 

 I doubt the butterfly will perform like a switch - it might even increase the vacuum by Bernoulli effect???

 

 

"On many cars it is found desirable to use vacuum operated ignition advance to obtain optimum part throttle consumption figures. The take-off point for this vacuum is arranged slightly to the air intake side of the butterfly, and in such a position that opening the butterfly allows the throttle disc to pass over the vacuum take-off point so that it then communicates with the manifold depression. By this means the vacuum is small at the distributor during idling and full throttle conditions, and is large at part throttle, being at a maximum when the throttle is open a few degrees."

 

http://sucarb.co.uk/...su-carburetters

 

Scroll down a little less than about 1/4 way down the page.

 

If I remember tomorrow, I'll snap off some photos to show the difference between the HS and the HIF series in this regards.

 

<EDIT: Still not sure? Connect a vacuum gauge to the Vac Advance take off Port on a HS Carb, that'll show what's what. >

 

Ta, I have an idea of the theory and don't doubt SU made it work. I'm just curious about the actual application, if I had a running carb'd engine I'd be tempted to experiment as you suggest. I was wondering if the Hif was an improvement on the same theme or an attempt to do something a bit different.






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