MGExp

MGB & GT Forum

Converting to ported vacuum

Moss Motors
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor

pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN
Making some progress...

. Hide banner ads & support this website by becoming a > Gold Supporting Member <

Attachments:
carbparts.jpg    44 KB
carbparts.jpg

Steve64B Avatar
Steve64B Steve Opitz
Phoenix, AZ, USA   USA
1966 MG MGB
I could be mistaken, but I do believe the difference between manifold and ported vacuum is based on which side of the butterfly the signal is coming from.

Its not a ported signal unless it's coming from the air cleaner side of a closed butterfly. Top or bottom makes little difference.

pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN
In reply to # 2164206 by Steve64B I could be mistaken, but I do believe the difference between manifold and ported vacuum is based on which side of the butterfly the signal is coming from.

Its not a ported signal unless it's coming from the air cleaner side of a closed butterfly. Top or bottom makes little difference.

OK... enough of this speculating...

It dawned on me that I have a set of HS4s in a box so I dug them out and looked.(I forgot I had them... oldtimers)

Checking the HS4s I have on hand confirms that I drilled the hole in the correct position. The hole is supposed to go exactly where I put it and is supposed to be covered by the edge of the throttle disc at idle. The hole is positioned at the leading edge of the throttle disc and in order to make it the same as the HS4 I had to drill the bottom of the carb to account for the reversed rotation of the HIF carbs throttle spindles.

Drilling the hole anywhere aft of the throttle disc will result in manifold vacuum which will not allow the ported vacuum distributor to function as intended.



Edited 1 time(s). Last edit at 2012-09-07 09:38 PM by pinkyponk.

. Hide banner ads & support this website by becoming a > Gold Supporting Member <
chris Avatar
chris Platinum Member Chris Roop
Pendleton, OR, USA   USA
Well ok then! I thought that anything from the bridge pipe of the intake forward toward the air cleaner would have been good enough, and manifold would be anyingth on the bridge pipe.
Glad you're correct!


Member Services:
Used English car parts.
pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN
In reply to # 2164463 by chris Well ok then! I thought that anything from the bridge pipe of the intake forward toward the air cleaner would have been good enough, and manifold would be anyingth on the bridge pipe.
Glad you're correct!

It could be fine anywhere ahead of the butterfly for all I know... but I'm sticking with where Abingdon decided they would put it... just in case. I only know that it can't be aft of the throttle plate for ported vacuum. I'll post a picture of the HS4 port tomorrow for posterity's sake, just in case anyone needs to drill their carb.

pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN
OK... got it all done. I just got back from the test drive.

Totally worth the effort! The throttle response is now excellent! No more giant bog until 1500rpm. My clutch will last much longer now. It goes through the gears much quicker. I drove it like I stole it.

The vacuum port works perfectly... there is no vacuum at idle but just touch the throttle and there it is. The timing mark progresses quite linearly when I rev the engine while watching the marks under the strobe. No pinking anywhere so it's on the safe side. I'll get to mapping the curve some other time.

The carbs are also now rebuilt and and it drops back to idle very quickly and it's the same idle!! No more idle of the "particular moment in time that we're at." I set it to 850 and it just sits there at 850.... boggles the mind! (This is using the tach on the timing light... the dash one reads a tiny bit higher.)

If you can't tell... I'm totally chuffed! Can't wait to recurve the distributor and get all the power I built into this new motor.



Edited 2 time(s). Last edit at 2012-09-10 02:03 PM by pinkyponk.


Attachments:
carbies.jpg    43.1 KB
carbies.jpg

gooser Avatar
gooser Drake Myers
Danville Va, USA   USA
moral of the story:
















no pinking from pinkyponk.

pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN
Ha!


(no pinking yet...)

. Hide banner ads & support this website by becoming a > Gold Supporting Member <
gooser Avatar
gooser Drake Myers
Danville Va, USA   USA
congrats brother.

slice Avatar
slice Andrew McCue
Chelsea, QC, Canada   CAN
1979 MG MGB
Comgrats on the fine work Adrian. I have been wanting to do this a for awhile now just to gain a little more control of the idle speed. The manifold vacuum pushes both the timing and idle speed up to 1000 rpm and I have no more idle adjustment (no vacuum leaks, new shafts, solid throttle plates). Of course the the temperature compensator in these HIF carbs does not help any. My distributor is a 123 and works interchangeably with manifold or ported vacuum. So how did you drill the hole so precisely. Any hints or advice would help, as I do not want to ruin the carb.

. Hide banner ads & support this website by becoming a > Gold Supporting Member <
pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN
In reply to # 2166471 by slice Comgrats on the fine work Adrian. I have been wanting to do this a for awhile now just to gain a little more control of the idle speed. The manifold vacuum pushes both the timing and idle speed up to 1000 rpm and I have no more idle adjustment (no vacuum leaks, new shafts, solid throttle plates). Of course the the temperature compensator in these HIF carbs does not help any. My distributor is a 123 and works interchangeably with manifold or ported vacuum. So how did you drill the hole so precisely. Any hints or advice would help, as I do not want to ruin the carb.

Thanks Andrew. It made my car run a lot better... I'd recommend it. I can't say the improved idle is just down to the ported vacuum because I rebuilt the carbs and it's also a totally different distributor. It will idle lower than 850 but that was the spec I found for a 68 MGB. (I used a 68 distributor)

I almost hate to tell you how I drilled the hole. I noticed the brass fitting for the idle bypass(?) was bang on centre in the carb right where the butterfly rested so I shut one eye and drilled right next to it. I used a drill press though. I drilled it out at 3/16" until I was almost through (for the fitting) and then switched to the .047" drill. The tricky part is the small bit is too short to drill the hole with a drill that has a chuck bigger than about 3/4". (the carb body gets in the way) I have a tiny electric drill for model work like a Dremel and it was just barely able to fit. I had to flex the tiny drill bit a little to get clearance. A pin vice would hold the drill bit and would have been safer but I like to rush in... and I couldn't find my pin vice.

If you get this all done on yours how about sharing the best advance curve for this setup in your 123 distributor? Might be useful to others.(like me)

. Hide banner ads & support this website by becoming a > Gold Supporting Member <

To add your reply, or post your own questions




Registration is FREE and takes less than a minute!


Having trouble posting or changing forum settings?
Read the Forum Help (FAQ) or contact the webmaster