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MG MGB Technical - Changing Carbs

In an effort to keep as many hairbrained ideas in motion at the same time, I am slighting changing topics from my posting on Timing & Mileage.

As was pointed out on my previous posting, my 72B should have HIF4 AUD 493 carbs, not the HS4 AUD 265 carbs it does have.

It does appear though that my 25D dizzy is appropriate for the 72B.


If I were to ever come across a reasonable price for a HIF4/493 carb setup, what would be the advantage of this change back to stock, if any? Assuming the HIF4 set were in good operational condition.

Also, what other components parts would be needed to make this swap? Heat shields, linkage, etc.?

Knowing that I have other repairs to do before even considering this carb change, I am curious as to what difference it would make, other than being more proper.

Thanks to all for the detailed responses to my other postings. I haven't switched over to a Miata yet!

R.W Anderson

I wouldnt expect any difference between the carbs if they are properly set up. Do you know what needles you have in the HS4s? I think your engine is tuned, usually the = to AAA is a good choice here, standard is no 5. Other possible sources of poor fuel usage are leaking/worn jets. They are easy to change.
Stan Best

Pros and cons. The HIF's have a different fuel enrichment device, which some say is better than the dropping jet. They also have an automatic fuel temperature compensator. AND a nasty, prone to failure, and obstructive "poppet" valve in the butterfly. Definitely something not to have! But easily got rid of.
I have to say I prefer the simplicity of the earlier carbs, but I doubt whether you would notice a performance difference. The main reason for their developement was probably to satisfy US emmission legislation.
Standard needles on your carbs would have been FX. Are they fixed or bias needles?
Allan Reeling

HSs are indeed simpler to work on a major factor being the easily accessible floats, valves and jets, they also have less bits and passages to go wrong/get blocked. Their only advantage is the temperature compensated jet, that and a couple of other features being emissions related more than anything else. If I was choosing it would always be HSs over HIFs, having two of each.
PaulH Solihull

Okay, thanks for that insight.

Now to consider parts to replace within the HS4 AUD 265.

According to VB Ltd catalog and above, the 265 has FX metering needle as standard.

I have no idea what fixed vs. bias is nor how to tell the difference. For some reason I keep thinking that on that tag on the carb, there is a F after the 265. I need stronger glasses and brighter lights to reconfirm this. Would that imply "fixed", or does the FX indicate "fixed"?

If I'm going to open up the carbs to check which needle is in it, I would just as soon replace things at the same time. But I am not clear on whether any of the metering needles are considered interchangeable.

The car had emissions equipment on it originally, but all was removed before I got car. I still use the canister and associated hoses to valve cover, gas tank, and the valve in the intake manifold, with a closed vent filler cap.

So, should I simply replace needles with FX? Should I automatically replace the Jets, as suggested above? And while I'm at anything else?

I will add that I know at least the front carb throttle shaft has a vacuum leak, not sure about the rear. I think one cannot simply replace a shaft to mend this air leak.

Am I at the point where $75+ in parts (2 FX needles, 2 Jets) may be worth the effort, or am I throwing good money away, not having everything (throttle shafts, etc.) rebuilt at the same time.

I'm used to rebuilding old GM & other Rochester and Carter carbs, so I have some experience at this. Not sure how temper mental HS4s are.

Your thoughts? And THANKS.
R.W Anderson

As Paul says, no great advantage to changing to HIF, and some disadvantage. One is that they are prone to developing leaky floats, and you have to remove the carbs to do anything to the floats/float valves. The thermostatic jet adjustment is useless or worse, and the cold start enrichment can be a bother. I'd stay with the HS4.

"Fixed needle" means the needle is rigidly attached to the piston via a setscrew; "biased needles" have the needle in a sprung holder, so the needle floats and also rubs on the jet wearing both jet and needle, requiring renewal every couple of years. The fixed ones are better as they can last forever, but require that the jet be centered as detailed in the WSM. AUD265 should have fixed needles, but it is possible they have been changed by substitution of pistons (and we hope the matched vacuum chambers)(or the tag has been switched!). Later HS4 and all HIF have biased needles.

The "F" means front of a pair, the rear one had an "R" suffix. Nothing to do with needle designation, which is stamped on the needle shank. Fixed and bias needles have different designations, even if they are the same profile; they are not interchangeable, though they can be adapted.

If the real world, most of the throttle shaft leakage can be fixed by simply replacing the shafts; the wear is usually about 90%+ on the shaft and 10%- on the bushes. I've replaced 20x the shafts as bushes in my 50 years of doing this. Frequently I find that replaced bushes are worse than the "worn" ones they replaced. Air leaks cause the mixture to go lean at idle. Since the mixture is set at idle, you richen it to compensate, which makes it too rich everywhere above idle. Kicks the snot out of fuel mileage!

Do you have the Smith's diaphragm PCV valve on the manifold"
Do your carbs have the PCV ports in the body" AUD265 should not, but parts do get switched. Later HS4 bodies AUD326/405/465 do have these and biased needles.

Finally, and in reference to your timing problem, besides engine basics like compression, valve timing, can condition, ALL of this information is critical to getting the correct distributor setup. You should have supplied Jeff with all of it, and the distributor must be setup to match. If you get a WSM and go through the tuning specs, you will see that there is a different distributor for each carb setup. "Different distributor" means the specification number (xxxxx), not the model, which is what 25D4 or 45D4 is. All distributors up to 75 are 25D4, but replacements for them are now 45D4. A distributor of either model can be setup to suit any engine.

When you get this straight, you will have the answer to the carb/manifold vacuum question, as it all depends on the carb/distributor combination. I think that you probably want a setup similar to the 68 engine, and that will be a very different distributor to the 72. I personally would set it up as a 62-67 low compression engine. The carbs will be as the AUD265, but the distributor will be different.

FR Millmore

As I prepare and further consider opening up the carbs to make some repairs, I see there is a standard replacement throttle shaft as well as an .010 oversize shaft for $30 vs. $17 for standard.

How good is the gamble that the standard would work, vs. going with oversize which I would suspect may require minor boring to fit properly.

The dollars are starting to ad up though.
$17 standard throttle shaft
$13 standard FX metering needle
$25 jet

$45 x 2 = $90.

Another consideration is needle valve & seat $21, if I think there is any chance there is leakage there too.

What is an average price for rebuilding both carbs by a pro.

Or again, shall I simply drive and live with the mileage?

Next weekend I may find time to open things up to look and see what is in there.

Thanks again for all the insight.
R.W Anderson

air filterhere has been a great deal of discussion of the relative merits and vices of the 1
Stephen Strange

I think my brain hurts right now! That is a lot of information, and I'm not sure 50% of it has sunk in yet.

But to get to one quick answer, yes when spraying carb cleaner at the point where the throttle shaft comes out of the carb body, idle does go up; confirming a vacuum leak. I will double check tonight to see if that was just the front carb, or if rear carb does the same.

I do not think I need to go to the point of replacing drive chain, as the last time the engine was out (last Fall) I recall the marks were in alignment.

If I can find a neighbor to assist, I may also document timing change over rpm range, but 10 BTDC dynamic (no vacuum) at idle does correlate with about 35 BTDC (with vacuum) at 3500-4000 rpm. I'll double check this too.

I think distributor is working perfectly, as the engine is runs so smoothly, compared to pre-rebuild.

I should add that I did change air filter elements before current tank of gas. Not sure if they are "cheap", but they are paper.

I should also clarify, that I have not changed driving habits. Still drive moderately, on same roads and have same mix of highway crusing to and from work. I presently do have timing dynamically set to 20 BTDC (no vacuum) at idle. I haven't checked this tank of gas yet for mpg.

Thanks again. And BOY! Some of you folk sure have studied up on this topic.
R.W Anderson

This thread was discussed between 10/06/2011 and 13/06/2011

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