HIF 4 Carbs

[Alex H.]

I've just rebuilt my HIF 4 carbs AUD 550 for my 1973 MGB. The incentive for this project was really bad performance.
On the throttle butterfly valve there is a relief valve of some sort. On mine they were soldered closed!
Can anyone tell me what these relief valves are for and why anyone would solder them closed?
Since I put them back together I can't get the engine to idle at less that about 1200 RPM. I suspect that the relief valves are leaking and that may be why they had been soldered closed.
The back carb also seems to be drawing much stronger than the front carb. I intend to remove the butterfly valves and resolder the relief valves in hopes of plugging the air leak.
Any info/speculation would be appreciated.

 

[aerog]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Alex H.:
I've just rebuilt my HIF 4 carbs AUD 550 for my 1973 MGB. The incentive for this project was really bad performance.
On the throttle butterfly valve there is a relief valve of some sort. On mine they were soldered closed!.<hr></blockquote>

Do you mean a small hole in the top and/or bottom of the carb body surrounding the butterfly valve?? The hole is the exit for the air that passes over the piston in the carb and should, I think, always be open (kind of defeats the design of the carb otherwise).

The valve on the butterfly itself is an "overrun throttle valve" and is, in effect, an emissions control device. The valve itself is a small spring-loaded poppet-valve that opens when negative pressure is applied on the engine-side of the carb.

I think the idea is that if you let the engine overrun at high speeds with the throttle closed you will get a higher than normal amount of unburned fuel in the engine (IE: it runs rich and thus your emissions go up) and when the valve opens it lets a "correct" mixture into the cylinders so a more normal (leaner) mixture is being burned, thus producing less emissions.

Just my guess, but I'd say at idle that valve shouldn't be working and thus isn't relevant since the butterflies should both be "closed" - whether it be via a closed poppet-valve or a solder seal.

I'm not terrible experienced with these carbs, but I've read several things that immediately reference checking vacuum lines when you have an idle problem. Assuming resoldering/sealing the butterflies doesn't work, it might be worth just buying some and trying it before you start tinkering with the carbs much more.

[ 04-01-2002: Message edited by: aerog ]</p>

 

[Alex H.]

Thanks Scott.
I do indeed mean the over run throttle valve. I suspected this as most of the emission control stuff has been removed. I'll have to check for vacuum leaks. I also suspect that the butterflys are not making close contact with the throat. I may be ordering new butterflys and shafts.
Do you have any suggestions regarding reading material that would be helpful? I have ordered an SU overhaul book from Burlen in UK. I hope it will help with my SU education.
Thanks again.

[ 04-01-2002: Message edited by: Alex H. ]</p>

 

[Lurker]

I have a Haynes book on SU carbs that I like a lot. Currently loaned to a friend who has a couple of MGs.

What I remember about the spring poppets in the butterfly valves is that it was an unpopular emmisions device. Many people either soldered the poppets closed or replaced the butterflys with earier solid type. This was supposed to give you more control when adjusting your carbs; allowing you to set your idle lower. Of course, I would never tamper with any emmisions device

. This is all just stuff I have read.

If you are having trouble setting your idle below 1200, you might want to look at your throttle shaft bushings. They are notorious for wearing out in the HIF 4s. Spray them down with Carb (not brake) cleaner and see if the idle changes.

Good luck.

 

[aerog]

<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by Alex H.:
Thanks Scott.
I do indeed mean the over run throttle valve. I suspected this as most of the emission control stuff has been removed. I'll have to check for vacuum leaks. I also suspect that the butterflys are not making close contact with the throat. I may be ordering new butterflys and shafts.
Do you have any suggestions regarding reading material that would be helpful? I have ordered an SU overhaul book from Burlen in UK. I hope it will help with my SU education.
Thanks again.

[ 04-01-2002: Message edited by: Alex H. ]<hr></blockquote>

I've heard stories about vacuum lines causing terrible idling problems - just an idea.

The book I have is the "SU Carburetters Tuning Tips and Techniques" by Brooklands Books, available through Moss. It has a lot of info on the basic theory behind the carbs, assembly/disassembly, tuning, etc - also covers SU fuel pumps. It isn't a rebuild manual really, but it does show a lot of the procedures for taking carbs apart and reassmbly.

Good luck! -- Scott

 

[Alex H.]

I will look up the Moss Motors book. I also understand Moss has a video which is useful. I'll maybe do a book review of the Burlen book out of the UK once it arrives.
Thanks again.
Alex

 

[Miles]

Hi, I recently experienced the same problem. After rebuilding the HIF carbs. I could not get the engine to idle well under 1200 rpm. Nothing was addressed about the spring loaded butterfly valves in any manual. After another overhaul the same problem. I was supposed to know the answer after spending years with the British auto. It ended up at a very qualified British mech. He took the spring loaded valves out of the butterflies and soldered them shut explaining the the spring loaded feature was designed to "seal" the hole when not needed. After soldering shut the car idled nicely. For additional leaks around throttle shafts try flowing propane gas from an unlit hand held tank around the shaft housings when the car is idling. If the idle increases when the gas is present it shows leakage around the shafts. This will also work on cracked vacuum lines.