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MG MGB Technical - Drilling a HIF carb for vac advance

This comes up in the archives a lot but I thought I would add my experience in case anyone else finds it useful.

I should say though I am 1). not sure this will work and 2). haven't tested it yet. My engine is still being built!

My car is a 78B with a desmogged US motor. The motor is a bitizer it seems. It has a 25D distributor (which I want to keep) and has twin SU HIF4 carbs. When I got the car the vac advance line was taken from the manifold. Actually the vac advance was missing and there was a matchstick in the manifold!

Reading up on all this (in books and in the archive here) I find that the 25D should really have vacuum taken from the carb, not the manifold. Peter Burgess mentions this in his book and explains why.

So I set about adding the port to my HIF. Luckily I have a great mechanic who lent me an old HIF with the port in it so I could measure where I had to drill. Unlike the earlier SUs the HIF4 has it's port under the carb just under the throttle disc. If you look at a HIF from underneath you will see a small brass plug lined up under the centre of the dics. This ISN'T the vac advance line as far as I can tell. This is part of a bypass port. On the earlier example carb this comes up through to the edge of the throttle disc and the disc itself had a small cutout for it. On my later carb (which had poppet valves in its disc BTW) there is no cut out in the disc. This port is flush. Looking at my example carb there is a tiny hole off to the side of this port that goes straight through the the outside of the body to a steel tube with an olive on the end. The hole is on the side of the carb with all the linkages. This IS the vac advance port.

If I read the archive correctly some carbs come with the port already there but blocked by a brass plug? Maybe it is possible to drill through this or remove it and there is your vac advance line already. Mine doesn't have this so I had to drill. By measuring carefully and with lots of peering inside the carb I found the hole is 1.2mm diameter (found by trying various tiny drills in the hole by hand till one just went though). It comes up right on the edge of the closed throttle disc but is not totally covered by it. The hole seems half uncovered on the air filter side of the disc. I think the important thing is it is upstream of the disc.

I used a mill to drill the holes and held the carb itself upside down in a rotary head. I got everything level and drilled a 3mm holed into the body to about 10mm depth. This hole is about 7mm from the centre brass plug towards the linkage side of the carbs and offset about .7mm towards the centre of the carb. The offset ensures the hole is on the upstream side of the throttle disc. These measurements aren't exact!

I then drilled a 1.2 mm hole through the bottom of the 3mm hole to make the port. This was tricky. A standard chuck and drill can't be used since the body of the carb will get in the way. I had to use a pin vice to hold the tiny drill then put that in the chuck of the mill. With the mill up as high as it could go I just had enough room to do this!

On my carb the port hole ended up slightly off position. I would say about .5mm too far upstream. So the hole I drilled is not half covered by the disc when shut but it is right next to it. I am not sure this will matter or not? I suspect not. A short tube will be epoxied into the 3mm hole to attach the vac line too.

As I said I haven't tried this yet as I am still rebuiding my engine. But I think this should give a much better result than using the 25D distributor with the manifold vacuum port. Drilling the carb body itself is kind of scary but if it doesn't work I don't see why the hole can't be blocked off easily.

When the engine is done I will post a follow up and let people know if it worked or not!

Simon


Simon Jansen

Simon,
Your port position should be OK, though there is a bit of difference in the transfer from vac to no vac as the throttle opens. I'd have put it on top where it is on the HS carbs, since you are in effect trying to duplicate the older setup. More important, the distributor and carbs used don't make any difference, as long as they have the correct advance and mixture curve characteristics; they are very different between the manifold and carb takeoff systems. All this had to do with emissions. For ballpark, you probably want to duplicate some earlier setup, based on compression and cam. Use carb needles, vac and mechanical advance that was stock on an engine with the same compression and cam. Work from there.
FRM
http://www.usachoice.net/gofanu
FR Millmore

Yep, certainly trying to unamericanise this engine. I am fitting the proper compression pistons, the earlier distributor and having the correct vac advance. As far as I know the cam was the same the entire time the MGB was made? I am just using a (new) standard cam.

Simon
Simon Jansen

I believe that there was a different cam, used on late cars, probably after the single ZS and its ridiculous manifolding and compression, 75-on. I have seen references to it, but no specs, I could be wrong. And aftermarket suppliers are lax with the substitutions. Moss lists them all the same, Roadster Factory shows different for twin carb - 88G303 and single carb - CAM1156, Do I have to tell you who I believe? Just get one for a pre 74, or better, go up a notch with a mild performance one.
Pistons change about 73, from 8.7 to 8.0, even the 8.7 could stand a bit more.
The best Bs were the 65-67 ones, so try to duplicate that. You will need to figure out what needles to use, since the HIF, while otherwise superior to the HS, uses a spring-loaded needle. As you seem to be an enterprising lad, you could figure out how to modify the HIF to fixed needles. I've been meaning to do this for a while, but haven't gotten around to it yet. Email me and we can collaborate.
FRM
http://www.usachoice.net/gofanu
FR Millmore

According to The Original MGB by Clausager, North American engines from Dec 74 had a new camshaft, Cam 1156. Clausager doesn't provide any specifications.
The Haynes MGB manual shows Engine Nos 18V883AE(,CC, 18V884AE(CC,O), 18V890AE(CC,5), 18V891AE(CC,O), 18V892AE(S) and 18V893AE(O) to have the following valve timing; Inlet opens @ 8 degrees BTDC, closes @ 42 degrees ABDC. Exhaust opens @ 54 degrees BBDC, closes @ 18 ATDC. Don't know if this is correct information but Haynes does agree with the MGB shop manual on early cam opening and closing numbers.
Clifton
Clifton Gordon

That cam tming certainly explains why the thing was such a pig, relative to early ones! Thanks, Clifton.
FRM
http://www.usachoice.net/gofanu
FR Millmore

Most of the manuals say the cam stayed the same but the cam advance changed with the RB. Its a bit hard as in different parts of the world there different for the same year. Denis
DENIS

"Most of the manuals" ?? Whose? Simon's motor is likely one of the #s Clifton gives, 883 or 884. There is a lot of bad info, I tend not to trust any that differs from factory, and occasionally the factory is suspect too! I don't have a factory book for post 74. Haynes is pretty trustworthy though, so for comparison here is the spec for early cams from a factory book.
These are checked with clearance set at .021, not at running clearance.
Inlet: Open 16BTDC - Close 56ATDC = 252 total
Exhaust: Open 51BBDC - Close 21ATDC = 252 total

Compare these to the specs Clifton quoted of 230 total inlet and 252 exhaust. This is the intake lobe from very early B (and A) series engines, suitable for low horsepower, low speed farm vehicles and suchlike. Really bad cylinder filling, followed by no compression; it would likely run on paraffin with only a small power loss; then again, it hasn't got much to lose!

BIG question - which cam is "Big M" selling you under their one number scheme?? Do they change it at will while hiding behind their arbitrary number system? By my reading of US and California law, it is illegal (not to mention unscrupulous) to sell a non-spec part for use in any emission controlled car for road use. Since the change would be more likely to show up on a post 74 car in testing, it is likely that they are selling the late cam for all. For anyone (like me) who has driven a correctly set up early car, getting a post 74 cam for a "real" MGB would be sufficiently maddening as to cause me to do things which would make newspaper headlines!

FRM
http://www.usachoice.net/gofanu
FR Millmore

FMR I know I quote Peter Burgess a fair bit but he is an expert that is recognised the world over. In his book in the intro he states that all Bs used the same camshaft but the cam was advanced 4* to help the low end power for the heaver RB. I do know though that there were things changed in certain countries at times, like in AUS a 69b has no emission controls and a different distributor (29d) and I suppose this could be the same with the cam in later cars. After many years in the motor trade I tend to think there are many mistakes in most W/S manuals, sometimes misprints but also because of these variations. Denis
DENIS

Denis,
In no way would I argue with Burgess, but I would guess that he is speaking of UK and possibly most of the world. The 883 & 884 engines are certainly US spec, and from the other things I stated above, appear to have the CAM1156. I don't know what the other numbers Clifton gave are, possibly German or some other tough emission place. These numbers (18V***)are explained in the Bentley manual for pre75s, but I don't have one for the later cars. While I did not have the CAM1156 specs before, I have always felt that these cars ran (or didn't!)like they had such a cam.
I would like to thank everybody for putting me on to Burgess - there are so many books and so little cash, and I hate buying books that turn out to discuss trim and upholstery cleaning with little useful info!
FRM
http://www.usachoice.net/gofanu
FR Millmore

FR I'm wondering if the different part numbers for the cams could be because of a keyway position change or something as simple. I have never driven a RB car so I cant pretend to know. But an advanced cam will take away your ability to rev. Denis
DENIS

If Clifton's data from Haynes is correct, the CAM1156 intake lobe is different, having 22 deg less duration; and the exhaust is advanced 3 degrees while being the same profile as the early cam. Way more than a keyway/timing change.
If the cam is advanced 4 deg, as you quote Burgess, there would likely be a useful low-end boost, with a small top end drop which most people would never notice. Most cars never get up to the top end anyway.
FRM
http://www.usachoice.net/gofanu
FR Millmore

I haven't created the vacuum port on my HIF yet. I've gottem some good information from Bill of http://www.sumidel.com. His comment follow:

The vac take off port should be to the right of the idle passage [looking from manifold flange side] and should be in a position so as the butterfly edge covers the hole and is not seen when butterfly is in the fully closed position, Half of hole should be uncovered on back side of disc .
Hope this helps ,number 57 is correct drill.

Bill also sent me a photo of the carb with the takeoff. If anyone would like to see it, please e-mail.

Steve
Steve Lipofsky

This thread was discussed between 08/06/2004 and 18/06/2004

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