30 replies to this topic

[Spider]

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.

 

 

Ah, no theory actually.

 

SU weren't the only Carb manufacturer to do this. Many did, in fact the only one that springs to mind who didn't, was Weber.

 

Here's what goes on;-

 

Hoses connected on a HS4 set up. The one with the Red Arrow is for the Vacuum Advance, the Blue is straight Manifold Vacuum

 

 

and they are connected to two Vac Gauges here, with the engine at Idle

 

 

Note the 'Blue' one (Manifold Vac) is showing around 17" and the 'Red' one (Vac Advance) is showing about 0.5".

 

 

and again, at ~2000 RPM

 

 

Manifold Vac is around 19" and the Vac Advance is showing about 7"

 

 

This is looking into a HS4 with the butterfly at Idle (this is looking from the Manifold side)

 

 

and opened a little

 

 

and a HIF44 at Idle

 

 

and this is a HIF44 that I've modified (normally, the barbed port would be capped, but I removed it for this photo)

 

 

The straight tube is ported to line up with the butterfly so there's no vacuum at idle

Edited by Moke Spider, 22 July 2016 - 07:39 AM.

[ACDodd]

Just to add, running vacuum advance at idle significantly increases unburned hydrocarbons. On all carbs I rebuild, I use the ported vacuum to reduce emissions.

Ac

[Ethel]

Well that's confusing, are you saying the HIF is worse for emissions?

[ACDodd]

Any carb not running ported vacuum is usually worse for emmissions.

Ac

[Mini_Problems_]

I'm just looking for some clarification on this. I'm looking into buying a hif44 for a 1275 fast road build and would be happy carrying out this mod for the improved emmsions at idle. From what u can see at standard the hif44 has the vacuum advance on the manifold side of the butterfly as shown by mokespider Post. Your modified hiff has the take of closer to the air filter side of the butterfly to reduce vacuum to zero at idle? Is this correct?

[Spider]

I'm just looking for some clarification on this. I'm looking into buying a hif44 for a 1275 fast road build and would be happy carrying out this mod for the improved emmsions at idle. From what u can see at standard the hif44 has the vacuum advance on the manifold side of the butterfly as shown by mokespider Post. Your modified hiff has the take of closer to the air filter side of the butterfly to reduce vacuum to zero at idle? Is this correct?

 

Yes.

 

The modified one has the port aligned such that it come in on the edge of the butterfly when it's in the idle position.

[Mini_Problems_]

When you say the edge of the butterfly do you mean that the vacuum tube would be exactly on the edge of the butterfly so half would be on manifold side, half on filter side, or should the vacuum be set as close to the butterfly but still completly on airfilter side?

[Ethel]

You can get a good idea of what's required from Moke's HS4 photos. I think I'd use the choke to put the throttle in the fast idle position, mark the throttle disc position on the engine, and aim to be just a tad upstream of that. There's no flow to speak of, so drill a smaller pilot right through & counter drill just enough to accept your tube insert - you'll then have a bit of wriggle room by increasing the through hole size.

 

You could make up a jig to fit inside the carb's throat & butt against the mounting flange - then clamp it to your drill to check it'll break through in the right place before fixing the carb on it and going for it. I bet a 40mm solvent weld waste-pipe socket will be pretty close to 44mm OD.

Edited by Ethel, 23 June 2020 - 08:33 AM.
Actually the pipe is closer to 44 than the fitting, couldn't resist crawling under my kitchen sink with my verniers.

[Spider]

The hole size coming through only needs to be around 1/16" or 1.5 mm. The thickness of the butterfly, off hand is around 2 to 2.5 mm, so just align the hole, as best you can, with the edge of the butterfly at the idle position, don't loose any sleep if you end up a little off, but the closer, the better.

 

Then, as Ethal suggests, counterbore this hole do a little way so you can fit some 1/8" or 3 mm brass tubing in there, to make a connection point for a rubber hose.

[Mini_Problems_]

Thanks guys, appreciate the info. I plan on bizarding the carb a little, not the whole hog though as it's a road car at the end of the day. Although I can't see a reason why, would this effevt the desired position of the vacuum?

[Spider]

Thanks guys, appreciate the info. I plan on bizarding the carb a little, not the whole hog though as it's a road car at the end of the day. Although I can't see a reason why, would this effevt the desired position of the vacuum?

 

By Bizarding, do you mean 'Vizarding'?

 

If so, by all means work on the butterfly and spindle, but don't touch the bridge or other parts of the carb. While they may improve air flow, modifications to the bridge and / or piston upset the natural balance of the carb and you may never get the fueling correct for both acceleration and cruising.

[Ethel]

Mmm, there's another thought, which is the best profile for knife edging a throttle disc? My instinct is to feather the trailing edge where the disc tilts in to the airflow (Top engine side) and the leading edge where it tilts with the air (Bottom carb side).

[Mini_Problems_]

Oops typo, yes certainly not bizarding. I'll stick with working the butterfly and spindle for now. Also got a metro intake on the way.

[Spider]

Oops typo, yes certainly not bizarding. I'll stick with working the butterfly and spindle for now. Also got a metro intake on the way.

 

I'll also just add here that for me, rather than consider Blizzarding    the Carb, I've just gone up a Carb size if it's been warranted.

 

It is usually done for certain classes of racing where the standard carb size had to be used, but modifications to it were open.

[Mini_Problems_]

Thought I'd attach my Vizardification attempt, rather than working my 1293 I thought I'd try it our on my current 998. Results were quicker throttle response and a a small but noticeable increase in pull during wide open throttle was easy and worthwhile.