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Distributor Vacuum Connection To Hs4


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#1 Zami

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Posted 17 May 2020 - 03:35 PM

Two questions. There is a large brass fitting, normally capped off, that I planned to use when I switch to a distributor with a vacuum adv. the standard connector/hose from MS is much smaller any suggestions on how to make this connection? Secondly will the larger diameter brass fitting cause any problems with the operation of the vacuum advance on the distributor? I suspect that the connection was designed to use the same small diameter fitting as is on the distributor advance.

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#2 Chris1275gt

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Posted 17 May 2020 - 04:13 PM

The large brass fitting is the connection for the engine breather system on my 75, 1275gt. the vac for the advance as you look at the carb points upwards (hence the rubber bend) at the front in the middle as per photo.

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#3 Zami

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Posted 17 May 2020 - 06:46 PM

Thanks Chris, I must have an older set of HS4s as neither carb has the port on top. I suppose I could get a fitting and drill a hole to have a vac. Source.

#4 Chris1275gt

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Posted 17 May 2020 - 07:54 PM

Thanks Chris, I must have an older set of HS4s as neither carb has the port on top. I suppose I could get a fitting and drill a hole to have a vac. Source.


I'd check where the hole is on a HS4 carb before drilling it as it generally has to be in a specific place to benefit from the vacuum effect within the carb.

#5 imack

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Posted 17 May 2020 - 07:57 PM

Use a spacer with vacuum take off.

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#6 DamoMini

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Posted 17 May 2020 - 09:21 PM

If you use the spacer won’t that be manifold vacuum instead of ported, so at idle you’ll see high vacuum and therefore high vacuum advance?

#7 Zami

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Posted 18 May 2020 - 12:29 AM

Found some previous postings from 2017/18 on this subject. Moke Spider had some photos showing the location of the port vacuum. The hole appears to have a specific location relative to the butterfly as noted by Chris1275GT. I am a little reluctant to start drilling away unless I can get some specific details from someone who has performed this operation. Thanks for everyone’s input.



#8 timmy850

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Posted 18 May 2020 - 05:23 AM

I added ported vacuum to my car as a trial. It ended up being better to drive off from the line due to having a little more advance off idle

My vac advance can only gave a maximum of 5 degrees more under full vac load so it wasn’t going to cause any pre ignition issues - as soon as any load is on the engine the vac advance is then zero

#9 Moke Spider

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Posted 18 May 2020 - 06:14 AM

To get the best from the Vacuum Advance and not have it operate when you don't want it, the position of the Port is indeed important as DamoMini mentioned.

 

I've not had to drill a HS4, but I have drilled loads of HIF44s.  If drilling, you want the Port to come on top of the edge of the butterfly when it's closed.  When I've done the HIF44's I've fitted Brass Tube in to the body of the Carb after drilling. The Brass Tube I get from the Hobby story. Off hand, I think it's 1/8" OD. I fit it in with some Bearing Retainer grade Loctite for insurance. I cut the Tube to length before pressing in to get a neat end on it and to help press it in I have a short parallel pin punch that fits inside the tube. I fit the tube over this and then tap the tube in. I Find it's an easy way to hold such a short piece of tube.






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