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MG MGB Technical - HIF4 carb tuning

Gents,
I have HIF4 carbs on my 1977 MGB. I cannot get the rpm down below about 800. The rear carb seems to have a problem, because I am able to slow down the engine speed using the Idle Speed Screw on the front carb, but not the back one.
The back carb's Throttle Lever doesn't fully return on it's own, like the front one does. The spring is in place where it should be, but it doesn't fully return unless I push down on it with my finger.
I've checked for vacuum leaks, and I think all the hoses are tightly sealed. I found one that was leaking at the intake manifold, and tightening the clamp made a difference.
The air on the two carbs is now balanced using a Uni-Syn.
The mixture screws are both the same, screwed down two full turns from where the jets are flush with the bridge.
The distributor was replaced long ago with an electronic ignition, and it's fine, I believe.
The emissions system has been removed.
It just seems that the back carb is getting too much air somehow, and I think it might be related somehow to the way the throttle lever doesn't return on it's own.
Before I disassemble the carbs for a rebuild I want to know if there are any other ways to check for the cause of this problem. Any ideas?
Thanks,
Randy
Ithaca, NY
1977 MGB, Tahiti Blue/Tan with twin HIF4 carbs
Randall Olson

Hello Randall,

I imagine it is a carb swap from earlier MGB. My '73 B behave like that and I remember it was a choke level that I fond problem with those O'rings and washer when worn fool mixture.

Check there first, it is easy tofix.

Cheers,

Jean
J. G. Catford

Something is allowing the fuel/air mixture to get past at least one of the butterflies. Could be something is holding one or other of the butterflies open, or a butterfly isn't fully sealing the throat, or a poppet valve in a butterfly isn't fully closing, or possibly the spindles and/or bushes are worn allowing mixture to get past the fully closed butterfly.

From what you say something is holding the rear carb butterfly open, although whether it is something in the interconnecting linkage, or the spindle/bush or butterfly
itself, you won't know until you slacken the spindle clamping screws so that the two carbs are then independant.

When you find and fix it you should do a full set-up for air-flow balance and mixture from scratch, and the most important part of the former is that the air-flow should be balanced just off idle i.e. held slightly open on the pedal, as well as at idle. There must be a little play in the cable so that it is the idle screws that control the idle mixture, once setup correctly you should be able to stall it by backing off both idle screws by the same amount.
Paul Hunt

The carbs are now on the bench, and I can see the butterfly valves clearly. It appears the carb that is the problem (the rear one) has the butterfly disc closing tightly EXCEPT it appears to be closing directly over a brass tube on the bottom of the throttle body, shown in the attached photo. If so, air might be bypassing the butterfly valve. The front carb seems to allow the butterfly to close so the hole is on the air cleaner side.
What is the small brass tube on the throttle body? What it for? Can I solder it closed? Can it be adjusted so the butterfly disc can close completely?
Thanks,
Randy

Randall Olson

That brass tube is a "part throttle by-pass", intended to get better atomisation of the mixture at part throttle. DON
't seal it, but it should be flush with the aluminium body, which may be the cause of failure of the butterfly to properly close to idle. Also check that the disc is centred by slackening the 2 screws and allowing the disc to centre, then re-tighten, with throttle stops screwed clear. You might have to close the bifurcations to do this. If they break either replace them or soft solder.
Allan Reeling

The throttle bypass tube is flush with the aluminum carb throat, as it should be. I tried to get the two screws out of the spindle in order to center the disc, but only one screw came out. The other seems locked in place, and the head is stripped. I've tried liquid wrench to no avail.
With one screw out, the butterfly disc can move if I briskly tap it, but I couldn't get it into a position where it closed off the bypass tube.
Not sure what my next step will be, but I'm open to suggestions.
Thanks,
Randy
Randall Olson

Randall,

One other thing to look at is that the butterflies have not been put in backwards. The edge of the disc is not square cut, but is beveled so that the edges will meet the throat of the carb while the butterfly is not at 90 degrees to the throat.

Charley
C R Huff

Yes, the butterfly disc is installed correctly, and except for the location over the bypass, it seats well with the throttle body.
After trying to get the disc located perfectly without success, I was able to move it slightly to decrease the opening into the bypass. In doing so I was able to decrease the idle speed slightly.
Now, the carbs are installed, and the air is balanced. Idle is about 600-700 RPM...better, but not perfect.
I may adjust the mixture, but first I want to test drive it. Tomorrow.
Thanks for all the input!
Randy
Randall Olson

700 to 800rpm is the norm, but as Paul said unscrewing the throttle stops "should" stall the engine. HIF's have seals on each end of the spindles. These go incredibly brittle with time and should be replaced. If the spindles are worn, as well, air can be drawn in through the spindle bearing.
Allan Reeling

I agree. It's probably time to rebuild the carbs. I got them used from a junk yard in England, and bolted them on my car. That was about 12 years ago. But, with the perfect weather we are enjoying right now, it may have to wait. I'll test drive it today to confirm it's able to perform as it is. If it's not, I'll order the kits.
Thanks again,
Randy
Randall Olson

Oddly, the car's idle is OK now. I noticed that when I pushed down on the throttle's bracket on the rear carb only, the idle was slowed. This was even when the linkage between the carbs was connected. So, I installed another spring to pull it down, and voila!
Still plan on rebuilding them when time allows.
Thanks for all the help,
Randy
Randall Olson

Hi Randy,
I've the concern on my HIF 6 carb's, can you please a Pic?

Thanks a lot

BR
Frank
FW Frank

The attached photo shows the butterfly valve half over the brass tube on the bottom of the throttle body. I think that is where air is bypassing the valve and not allowing the RPMs to be lowered.
Randy

Randall Olson

Good picture. That will definitely allow a fuel/air mixture to bypass a fully closed throttle. The question is why? Can you compare dimensions on the carbs to see if it is the port too far forward or the butterfly too far back? If it's the port then I don't see what you could do about it except block it off, and if doing that I'd do both. It is only an emissions reduction measure, HSs didn't have them, although I suppose there is a small risk that it might have unwanted effects.
Paul Hunt

Randy,

Did you remove one of your butterfly retaining screws, or is it missing in action. If missing, perhaps the butterfly is not quite in its proper location?

Charley
C R Huff

Yes, I removed the butterfly retaining screw, then reinstalled it after trying to relocate the butterfly valve so it closed on the throttle body instead of over that bypass. I was only able to remove one of the screws...other one was too tight/stripped to get out, so I was only marginally able to move the butterfly valve.
Randy
Randall Olson

Randy, I had the same problem with the idle speed on my 76 roadster. It was fixed by changing the throttle spindles, butterflies, seals and butterfly screws. Items 13,14,15 and 17 on this web page.

http://www.moss-europe.co.uk/shop-by-model/mg/mgb-c-v8/fuel-system-induction-controls/carburettors/carburettors-su-hif4-from-1972.html

Also some good stuff here from Burlen on dismantling and reassembling the HIF.

http://sucarb.co.uk/technical-hif-type-carburetter-dismantling

Andy

Andy Robinson

Randy,
Quite often, it the bifurcations have broken when PO HAS BEEN "FIDDLING", the screws are put back in with loctite. This is also a dodge used by tuners, but since you still have poppet valves the latter looks unlikely. Don't know a good way to loosen Loctited threads except brute force, maybe someone will enlighten us!!
Allan Reeling

It occurs to me that you *might* be able to build up the back of the butterfly over that hole when shut, shaped such that it doesn't bind when opening. You wouldn't want it to become detached though.

Or if the top and bottom of the butterfly were machined slightly it might close just that bit more to cover the hole. But the real question is why it doesn't cover it now. I take it the butterfly is the correct one!
Paul Hunt

Randy-
It's not surprising that you're having problems. If you look closely at the diagrams that other posters on this thread have linked to, you'll notice that the appropriate throttle discs (Burlen Fuel Systems Part # WZX 1329, BMC Part # AUD 9808) for an SU HIF carburetor has a pilot notch at its bottom edge. This pilot notch is necessary because the SU HIF design has a small passage that runs from next to the fuel jet to under the throttle disc. The idle fuel / air mixture comes from the fuel jet through this passage, and is channeled into the incoming airflow through the brass fitting. It provides a better fuel / air mix, thus resulting in more efficient combustion at engine idle speeds, as well as lower emissions. Of course, the notch in the throttle disc has no effect when the throttle is open. What you have is from the post-1968 1 1/2” SU HS4 Series.
Stephen Strange

The pilot notch that Stephen refers to, where shown in Andy's links, looks like a cut-out. If that were the case then it would make the situation even worse, as the butterfly wouldn't even close off the throat if there were no bypass passage. Logically that 'cut-out' must be a section that projects forwards to cover the hole when the butterfly is closed. Which raises the possibility that it was necessary because they got the bypass port in fractionally the wrong place, and it was easier to modify the butterfly than change the body mould!

Whatever, you do seem to have the wrong butterfly on that carb at least.
Paul Hunt

Then again this http://www.mgexp.com/phile/1/110293/IMG_5751.JPG does appear to show a notch, but covering a raised exit to the passage. Your exit seems to be recessed. Also if you Google 'HIF carb buttefly' there seem to be several with no notch, like this one https://en.wikipedia.org/wiki/SU_Carburettor#/media/File:Sucarb2.jpg. However that does seem to cover the opening to the port.
Paul Hunt

I did not know my HIF carbs had the wrong butterfly valves, and that they should have a notch in the bottom. After looking at many photos of HIF butterfly valves it seems that some have the notch, while others do not. The ones with the notch may also have a raised bypass inlet, whereas mine is flush with the throttle body.
The car seems to run well at speed, which would underscore the fact that the butterfly would be open, removing the notch from the equation.
When the car is idling slowly, and the butterfly valves are closed, it might improve the seal if the butterfly valve was built up with solder to thicken it at the bypass, as someone suggested. This would allow the butterfly to seal tighter, and not allow air to enter through the bypass.
I cannot see how a butterfly with the notch would be able to seal off the air as tightly as the ones I have without the notch.
Thanks for all of the input. I think I know how to fix this, but I am reluctant to make a change too quickly. As I said, the car runs fairly well the way it is, and I don't want to mess it up.
Randy
Randy Olson

Gents,

Heat deteriorates loc-tite. With the carb on the bench and the throttle plates partly open, apply a soldering pencil for about five minutes. A little tinning on the tip promotes heat transfer. A slow but sure way to break loose loc-tited screws.

Regards,

Jim haskins
J. M. Haskins

Randy-
I can see that you don't understand the necessity of the notch in the throttle disc of the particular version of the SU HIF4 carburetors that you have on your engine. Permit me to go into detail-

Fuel for the fuel / air mixture is metered by means of a rotary valve that has a groove machined into its shaft that gradually increases in depth, ensuring that the degree of enrichment is progressive. The fuel then passes through a separate passage within the body of the carburetor that terminates in the high vacuum area that is close to the fuel jet aperture. There are two passages in the floor of the bore of the HIF carburetor; both are on the engine side from the fuel jet. One is a vent passage from the float bowl, and the other is an idle bypass passage that serves the idle discharge port. This bypass passage starts at the bottom of a recess that is located immediately behind the jet bridge and is aligned with the fuel jet so that fuel droplets will be drawn into it. The idle fuel / air mixture comes from the fuel jet through this passage. This is, in effect, actually an emulsion bypass passage that allows fuel droplets to be drawn along at small throttle openings by the high vacuum at the pilot notch in the edge of the throttle disc when it is closed, and atomized more completely with the faster moving air, thus creating an emulsion. At the junction of this bypass passage and the throttle disc there is a small brass discharge fitting that is press-fitted into the port. In order to convey this air, the discharge fitting is bored perpendicular to its length approximately in the middle and, of course, is also bored from the its top down. This allows the fuel/air charge to make the 900 turn upward to the throttle disc. The high level of vacuum at the juncture of the discharge fitting and the notch in the throttle plate draws the fuel/air mixture into the body of the carburetor, which is why the notch in the throttle plate is required.
Stephen Strange

"I can see that you don't understand the necessity of the notch in the throttle disc ..."

Insulting and largely irrelevant.

As Randy and I have both seen there are several types of arrangement of throttle disc and the port under it when it is closed. Unless the throttle disc can close off the throat of the carb, or very nearly so such that the engine idle can be reduced to stalling, you may never be able to get the correct idle. Whether that involves a raised port and a notch, or a flush port and no notch as Randy's carb has, doesn't really matter, other than to say if Randy had a notched butterfly it would make things even worse!
Paul Hunt

Paul-
OK, I'll bite, Can you explain the mechanics of why "if Randy had a notched butterfly it would make things even worse"?
Stephen Strange

According to the Moss catalog, the HIF-4 carb came with two types of throttle disc. The 1972 AUD493 carb had the throttle disc with notch. The 1973 to 1974 AUD550 and AUD630 carbs had a throttle disc without the notch.
So, it's likely I have the later carbs, and the throttle disc without the notch is correct.
While this doesn't solve the original problem of the high idle, it does shed some light on the issue.
Randy

Randy Olson

Randy-
If you look closely at the original image that you posted, your throttle disc appears to be blocking the fitting in the bore of the carburetor. That's why I believe that you have the version of the SU HIF4 carburetor that is supposed to used the throttle disc with a notch.
Stephen Strange

Stephen - look at Randy's second photo.

The butterfly is clearly almost directly above the port, which is recessed into the body of the throat.

If the port was proud of the body of the throat, as those with notched butterflies are, then an un-notched butterfly would hit the side of the raised port and would not be able to close as far as it has.

If the butterfly were notched with this recessed port then there would be an even larger passage for mixture to pass through the 'closed' butterfly, that's why I say it would make things worse.

It's just possible that the port isn't quite fully recessed for whatever reason, so the bottom edge of the butterfly is just catching on the top of the port and so being prevented from fully closing. But again given Randy's second photo there seems to be no gap between the sides of the butterfly and the throat. The real problem may be, as mentioned way way back, that because of the broken or missing fixing screw the butterfly is fractionally closer to the port side of the throat than it should be, and stops there, leaving a small gap at the top.

Some time back I did ask how it compared with the other carb, and I still think that is a valid question which has never been answered.
Paul Hunt

I would be concerned with the missing butterfly screw combined with second screw with a messed up head. This would indicate to me that someone attempted to tighten the last screw enough to ensure that the butterfly doesn't slip.

Anyone here who has removed a butterfly from the shaft knows that you must make sure that the butterfly is closing completely and in the correct location before you tighten the screws. This is probably one of the reasons that the screws are split on the ends to keep them from backing out.

Did you check and confirm that the butterfly is truly tight? If it shifts around, even very slightly, when opening and closing, the idle speed will move around. A very small leak here can cause pretty extreme variations in idle.
Richard Smith 1

This thread was discussed between 21/07/2015 and 30/08/2015

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