MG MGB and MGB GT Tech Talk
I've just bought some new carbs off ebay for a really good price (couldn't resist),but they are HIF's and on my GT its got the older HF's.
My old set are a mess and might take quite a lot of cleaning up and poss new needles etc but the HIF's are like new internally,so my question is should I go along with the new set or is there any advantage to spending more money and time on the old set
FWIW, I like HS4's much more as I think they are much easier to work on but that is just my opinion.
Try them, repair your old ones and keep as a spare. Replace HIFs when your done or sell one set. Whichever you dont like.
RS
I have HIF's on my 1973 B. They work and perform very nice.
Slap the HIF's on there, while you do the rebuild on the HS's.
Always a bonus to have a set of carbs in your hip pocket. No downtime for the car.
Fieldbuilder Wrote:
My experience is that when sold, they usually need a rebuild. Likely as not you now have 2 sets needing rebuilding.
I like the HS4's better. Jet/float issues are easy to deal with. I have them on my Spit as well.
The rebuild kit is $150+ a reamer either way.
Larry
In the US, the 72 model had HIF carbs from the factory. So you may be closer to original with the HIFs. I am not sure when the switch was made over seas.
Hurst
Hurst89 Wrote:
Derek up North Wrote:
Apparently, the throttle bushings on HIFs last longer. In theory, putting the fuel bowl closer to the jet decreases any lean out effects from hard cornering or braking. Also, HIFs get rid of that leak-prone hose between the bowl and the jet. But most people (including me) prefer to work on HS4s.
If you have a good set of HIFs I would use those.
I like HIF's but the main difference will be the spring throttle plate on the HIF's as opposed to the regular throttle plate on the HS4.
ourmg Wrote:
The HIFs don't allow for as much adjustibility of the jet height as do the older type H or HS carbs. That being said, it probably doesn't matter, because if you are trying for a good efficient tune, you won't be going there. Look up in Moss catalog the alternate rich and lean needles you can get for the HIFs then you'll be set.
NOTE!!!!!
On the HIF type the lean / rich adjustment is backwards from the other SUs. For HIFs you screw the adjusted in for richer and out for leaner adjustments and be sure you have a little shorty screw driver for the back carb as it's close clearance. These adjuster screws are on the side not the bottom of the unit as with older ones.
Les Brown Wrote:
Les nothing to convert- the throttle butterflys on HIF's have a spring plate in them to close slower as a part of emissions control. The earlier carbs don't.
I was under the impression some compromises were made in the design of the HIF carbs for emission standards. From reading the previous posts, I get the feeling there is not any real performance advantage between the HS and HIFs.
David
ourmg Wrote:
Les,
although the HS carbs look fine and more old (MG) style, the HIF have many advantages allready quoted. They perform better than the HS type and are not prone to leaking, as long as the float gaskets have been renewed at intervals.
At the butterfly, they are fitted with 'pop up valves' that do not realy help for performance but the butterflies can be canged to those of the eraly HS 4 type carbs and it is worth doing that!
The choke system is far superior to taht used on the HS 4, as long as the rubber sealing rings are o.k. or have been changed after they dryed out in storage.
The jet adjustment is fittet with a bi-metall element, that adjusts the jets depending upon the engine temperature. When tuning, the synchronisation procedure is the same as used for the late HS 4 carbs but be careful with the mixture adjusting screw. If turend to the right, it will give you a ritcher mixture, just oposite to the function of this screws on Weber or Solex carbs! For weakening it must be turned to the left to set the yet to it's highest position.
Comparing flow charasteristics between the HS and the HIF, the last one is capable for ~10% more than the HS is able to flow with the butterflies changed to plane ones!
Good deal, just service them and put them to your engine.
Ralph
Ralph 7h Wrote:
X2 on the concentric springs minimizing throttle bushing wear. Nice air cleaners too Les...here's mine.
Les Brown Wrote:
Oh dear,I can see myself going over the top here.I got myself a tube of autosol and a little bit got on the linkages,well I had to rub it off didn't I!
And now I find theres a super fine Autosol,this could be a long job,pictures to follow in a couple of weeks when my fingers get their skin back :)
HIFs flow ~10% more than HS-4s? I would REALLY like to see the data for that statement.
As for "performance" of the choke circuit, who cares? The choke circuit is only engaged for maybe 10 seconds.
ClayJ Wrote:
Derek,
Thanks for the numbers.
Who flow test carbs? I assume somewhere there's test-bed for that purpose.
It's not everyone who has a flow bench. For all his knowledge and skill with SUs, I'm not even sure if Jimmy Hilton does. If I had to bet, I'd put my money on Sean Brown.
this topic is turning into a massive wealth of information thats not found in the library or anywere else on the net for that matter,But! this me be a silly question HIF stands for horizontal integral float,thats easy ,but what does HS stand for?.And 4? whats that for,I've got an idea its MK 4.
I've an idea that a ton of bricks will land on my head in a sec when I find out,but remember I'm still a newbe round these parts :)
H is horizontal. I'm afraid I can't remember what the S is for.
The 4 is the sizing. The HS1 is 1 1/8", HS2 is 1 2/8", HS4 is 1 4/8", the HS 6 is 1 6/8" and HS8 is 1 8/8" (i.e. 2"). :)
It's the same sizing system for the HIF's (4 & 6) plus metric sizes (HIF38, HIF44; which are approximately 1 1/2" and 1 3/4")
Edit. OOPS. No HIF8!
@ Les
HS is for Horzontal Sidedraught (HS carbs can be altered in their setting of the horizontal position by the use of different rubber mounts for the float chamber, thats the main aspect why the comparable carbs from a Triumph will not work on the B without a modification)
@ Clay
Tests to the flow capacity were done for A-Series engines and are published in the A-Series tuning book
Ralph
This discussion got me thinking...
Q: Are there any downdraft SU carburetors?
Just curious.
D
BTW, I like my HIF's a lot.
Hmmm. First time I ever heard mention of a HIF7!! :)
http://www.su-stromberg.de/HTML/content/pages/identifizierung_su.php
Interesting. I like how the "benefit" of the bi-mettalic thingy in the HIF is also a "drawback" under the same conditions (Derek's quote first vs second paragraphs).
I have at least as much trouble adjusting the jet with air cleaners on with the HIF as with the HS... just a different type of access trouble.
Turns out Burlen Fuel Systems makes all the necessary cranks, rods and backing plates to do the conversion easily. They ain't cheap, however. You have to email or telephone because their website isn't terribly user-friendly.
I completely disagree about adjusting the jet on an HS with the filters on. You most certainly can and I do and without the stupid wrench (that I always dropped anyway).
The jet does not leak if you have everything properly installed, the float adjusted and the jet assembly (including sealing o-ring at the bowl) is in good order. There is no seal at the carb body (just a very close tolerance between the jet and the bush it rides in).
The HIF enrichment circuit is notorious for being a massive leaker when the o-ring therein fails.
I do very much like the throttle spindle arrangement on the HIF, however.
I can go from Death Valley to Pikes Peak with my HS by just moving the jet a bit... fat chance with the HIF.
Upshot is that proper maintenence and adjustment of whatever you use will yield superior results.
I wonder if there is some confusion between the HIF4 and the HIF44, the latter of which is more equivalent to an HS6.
Michael,
the HIF has it's point when it comes to flow, peak power and if you think about excelleration. When cornering fast, it does not have the hesitation that can happen with the HS carbs.
On one of the roadsters, I have a pair of HIF 6 carbs installed to the Stage 6 specs engine, after the HS 6 were not adequate and it was mutch more difficult to make a propper heatshield for the bowls and the jets than with the HIF's. Mine are fitted with ball bearing dampers and pulling the choke during hard excelleration, it outperforms a buddies B fitted with a similar engine and Weber 45DCOE. The different choke layout compared with that of the HS type is a great design that gives immediate response and I think about modifing it in the near future to work also during excelleration too.
Ralph
Derek up North Wrote:
Space requirements, I'd expect.
I've heard of the HS being described as a "semi-downdraught" design... because some are inclined at a slight angle, I suppose.
Ralph - Oh, my thesis was that for all practical purposes it matters not one whit since they both have good points and not so good points. If you are trying to squeeze more out of the carb, then all kinds of tricks can be employed. ;)
Say the 'D' stands for 'Diagonal' if you like. :)
Both will get the job done, here's a interesting fact, though, you don't see many folks messing with HIFs in racing, where they can run either you choose, so there ya go. I prefer the HS series, for no other reason than I can replace a needle and seat valve withy out disassbling and removing the carbs form the car, that along is worth it for me. To defend the HIFs, they are easier to rebuild. as for as any aboove issue related to HS series carbs like leaky float tubes, it's not a problem, if the work is done correctly and the proper parts are used. In your case since you have a set of each, I'd vote for using the HS4s, and if it's a later set, rebuild them with solid throttle plates, you can use solid throttle plate on HIF, but you may have to do mods to get it work, depending on the model of HIFs you have.
I have HIF4s on my GT and HS4s on my Spitfire. I have messed around with the HIFs on my GT twice: once when I bought the car and ten years later when I rebuilt the carbs. You can replace throttle shaft bushings yourself on HIFs. It's a pain to change the float needle, but how often do you have to?
I have been "friging" with the HS4s for years from getting them rebuilt to stopping the gas leaking where the jet tub attaches to the float bowl. I'll take the HISs over the HS4s anyday.
