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MG MGB Technical - HIF carb setting

Hello,

Normally I'm on the "T" side of this forum. I'm here because a friend took his HIF carbs apart and "rebuilt" them. He has "69 B. Now he wants me to install them and set them up to run properly. What a friend! Anyway, what is the correct number of turns that the main jet screw should be turned out for a reasonable starting point? He doesn't have a clue and the last "B" that I had was 30 years ago. I think that I can go from there with the air flow gage and piston movements which I assume are similar to the T's. The workshop manuals that I have don't seem to give much info on the adjustment of the units. Any little secrets out there that I should know about?
My friend claims that the car was "missing" once in a while so he did the rebuild. This should be fun. Probably electrical, as is usually the case, but I have to get the car running in any event to be able to proceed.

Thanks in advance,

Mark
Mark Strang

Mark,
screw the adjuster until the jets are level with the bridge and then two full turns down (clockwise I think!). That should get you started then use the usual methods you mentioned.

I hope your "friend" got everything back in the right place or this could turn out to be a marathon!

You might want to recheck the usual suspects eg float location and height, cold running mixture enrichment, which is a bit different from HS and earlier units with separate float bowls.

Best to use an exhaust gas analyzer for mixture setting if you can get your hands on one.

Good luck (I think you might need it!)

MGmike
M McAndrew

MGmike,

Thanks for the info. Hope I'm not getting into more than I want.
I'll let you now how it turns out.

Mark
Mark Strang

Hi Mark,

I generally start by setting the jets 0.060" below the bridge, however many turns that is....

Make sure you check /everything/ as this car has been fiddled with. A '69 would have HS4s, not HIFs.
Rob Edwards

I think you will find that the HIF carbs use a screw adjustment in the side to adjust jet depth. The HS carbs had the nut underneath. Have a look here
http://www.v8register.net/FilesV8WN/SUbookletsAUC9939AHIFtuningandservicing.pdf

Changing from dual HSs to a single HIF44 is a favourite amongst the midget fraternity.
Steve Church

Steve, I know HIF's have a screw adjustment, I've just finished the HIF6's on the V8 (see rant in general;o). But guess what, SU either designed in some consistency for the set-up or had a fluke!

The refurb guide supplied with the full service kit states two full turns of the mixture screw as an initial start point. I didn't measure it but I would guess this is close to the .060 Rob also suggests.

MGmike
M McAndrew

Well, I got the carbs on and the linkages all adjusted. The car actually started but ran extremely rich with the jet settings as pointed out above. To make a long story short, I raised the jets as far up as they would go and it still ran rich but better. I made sure that the units were drawing the same amout of air flow using the old hose trick and the gages. Out if desperation I dropped the needle down to the bottom of the piston and it runs better but still bad. Obviously I am missing something here. It is simply getting too much fuel. At higher rpm's it runs reasonably well but not smooth. I think that I'm close but still in left field. Any ideas?

Mark
Mark Strang

Mark- 12 flats ends up around 60k. I like to use a squared off 60k sparkplug gage wire: it fits in the jet depression and you can use a second feeler ribbon to get the depth set exact for both carbs...when the top of the 60k wire is flush to the carb deck. At sea level with a #5 or #6 or equivalent needle, 60k turns out to be very close to what works wonderful. What color springs were reimplanted? What weight oil are you using? Did you in fact get the right pistons in the right air chambers? What does "rebuilt" mean???? Unless a new bush and throttle spindle were fitted you probably are still sucking air and will never attain a steady idle. The HIF are more prone to this than the HS it seems. Now days some rebuilders are using a silicone seal at the new spindle/bush ends. Finally, were the jet needles set flush to the piston bottoms? You can cheat a bit and lower the needles about a 1/16th to 3/32 nds if you still have the jets up flush to the carb body deck as I think you say. What you need is an Innovate lC-1, exhaust sniffer and wideband gauge to set at 14.7 or there abouts. Remember Lawrie Alexander's immortal observation: 95% of all SU carb problems are electrical..... Cheers, and let us know, Vic
vem myers

Mark-
From my earlier posts:
"HIF have tiny O rings in the cold start mechanism; when they die, the "choke" is effectively ON, and you get all the symptoms described. Since the choke effect diminishes with RPM, leaning out will help a bit at/near idle but kill performance and eventually the engine at higher throttle openings.
Rebuild the carbs, with special attention to the cold start.

Further to my above:
There are two holes in the floor of the HIF carb bore, just engine side from the jet. One is a vent to the float bowl, the other is the cold start discharge port. Start the engine and run it up to 3-4000 rpm, while looking down the throat with a good light. If fuel can be seen issuing from the port, then the O rings are perished or omitted; or the cold start is ON or wrongly assembled.

I first discovered this on a car that unaccountably had little screws installed in the discharge ports, couldn't be started cold. When I took them out I got just what all of you with rich HIFs are reporting.

The other possibility is that the floats have gas in them, causing high fuel levels in the bowls - common on HIF."

Or, floats are set too high.

FRM
FR Millmore

FRM,

I think that I'll start with the float settings since the car ran "reasonably well" prior to his "rebuild". I don't know where they are set so that is a good atarting point. I'll let you know in a couple of days when I get some time to check it out.

Mark
Mark Strang

Mark-
Makes much more sense to do the visual check on the cold start before removing the carbs. You have to remove them to set the floats or dismantle the cold start. If the tiny O rings were bad and the CS were dismantled, the pieces may have fallen out, or they might be installed incorrectly, or if carb soak was used they might have just dissolved.

FRM
FR Millmore

Mark
I'm with FRM on this. Best to do the checks on the cold start first. I did suggest this might be an area of concern in an earlier post.... It appears your "friend" might have left you a test!
If you do take the carb's off to check the float, you might as well take the cold start unit out to check the o-ring is in place and its assembled correctly. It sounds like you'll end up doing it anyway!
The link Steve put up gives a complete picture of the components and the setting procedure. Follow it methodically and you won't go far wrong.

Hopefully this will only be a 10k run and not a marathon!

good luck

MGmike
M McAndrew

"One is a vent to the float bowl"

??

According to my Haynes the one that isn't the enrichment port goes to under the butterfly and is described as "An emulsion bypass passage (that) allows unevaporated fuel drops to be drawn along at small throttle openings and mixed more completely with the faster moving air". It's the lack of this passage on HSs that is the reason for having to run the engine at 2500rpm for one minute every three minutes during the tuning process, to 'clear its throat'.
PaulH Solihull

Paul-
"One is a vent to the float bowl"
On the car (74)I found the blocking screws in, the second hole was simply drilled straight through the roof of the float chamber. It was the first time I really studied an HIF, and I was a bit surprised at this seemingly redundant "vent", but it is on all the other HIF I've looked at. My only idea was that it is to prevent filling the charcoal canister in the event of float valve leakage. On a 73, when a float sunk, the excess fuel did in fact come out this vent, flooding the engine (but not the canister). This "extra vent" would seem to cause some mixture disturbance, since air would be drawn through the normal vent, across the float bowl, and into the carb.

I'm not really up on all variants of the HIF, since we did not get many cars with them, just 72-74 MGB are common, and everything kept changing on US cars of the time. Your description from Haynes sounds like it may be referring to the ones with the notched throttle plates, and I've not studied those, though I've seen them (72 only). Alternatively, the Haynes description is murky enough that it may simply be referring to the notched plate, rather than a drilled passage. How would such fuel droplets get into the passage, since it is on the side of the bridge, not the bottom of the bore? (Not certain of this as I do not have one to look at)

"According to my Haynes the one that isn't the enrichment port goes to under the butterfly and is described as "An emulsion bypass passage (that) allows unevaporated fuel drops to be drawn along at small throttle openings and mixed more completely with the faster moving air". It's the lack of this passage on HSs that is the reason for having to run the engine at 2500rpm for one minute every three minutes during the tuning process, to 'clear its throat'."

FRM
FR Millmore

The port starts immediately on the butterfly side of and in line with the jet, at the bottom of a recess, so droplets will fall into it. The enrichmnt port is to one side of that. The notched butterfly, and a slot in the bottom of the piston, are there because the port is there, image attached. A hole between float chamber and carb throat would disable the American anti-runon system, as it would simply suck air down from the throat, rather than raising the level of fuel in the float chamber thereby sucking it out of the jet, and American cars used HIFs and anti-runon in 73 and 74. They did use HIFs in 72 i.e. the year before anti-runon, but if the intention of the drilling was to stop fuel from a leaking float valve going into the charcoal canister the carbs wouldn't have need a vent port at all, connected to the canister or not.


PaulH Solihull

Paul=
Yes, well, your pic is as I recall the 72 carb, but not the 73-4.
As stated, I was pretty surprised to see the extra hole, symmetrically opposite the enrich port. On the face of it, I would at first agree with all points but...
While it is always said that the anti-run-on "sucks the fuel out of the jet", it is perhaps not true. It is only required that the system eliminate any pressure differential across the jet, which this would do, and the resulting pressure would be less than atmospheric. Exact figures depending on the various orifice sizes through the system.

Would be nice if Mr Strang or somebody would post a pic of the throat of an HIF without notched throttle plates. I am interested, but not quite enough to go dismantle a customer's car to see, since it is a few miles away etc. But I will remember this next I see one.

FRM
FR Millmore

This thread was discussed between 29/11/2011 and 13/12/2011

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