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MGB HS4 carb question- 18GK engine, 1974 MGB [long]

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wkilleffer

wkilleffer

Jedi Knight
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Hello everyone,

Now that the valves are adjusted and the ignition is working properly on my MGB, I decided to have another crack at balancing the carbs. The engine's an 18GK and the carbs are HS4.

I used a new to me tool that involves putting metal rods in place of the dashpot pistons to gauge airflow, and making adjustments based on carb behavior when pushing down or lifting up on the rods. Pushing down on the front carb seemed quite springy. The back carb was less springy and didn't go down so far, even though the indicators said they were breathing in the same amount of air. Also, I think the back carb was no less than three flats richer than the front.

Carb adjustment is a major source of frustration for me. I'm just coming to the realization that it doesn't always work according to what the books or instructions say. But is something wrong with my carbs? They look old, but I had them rebushed and shafted about four years ago, and the rebuilder put new floats and gaskets on them as well.

Unless someone here comes up with a better solution, I'm going to take them off the manifold and have a look inside of everything to make sure nothing's impeding the airflow.

Also, anyone know where I could get a good sporty-looking K&N setup for this car? One of the supporting vendors of this site makes something, but they're kind of rich for my blood right now.

Thank you,
 
Try the air flow type balancers..They can be found on Ebay if you search for SU carb. I have the same tool you talked of and I could never figure it out.

You could be suppling all your air though one carb.

How do the plugs look?
 
[ QUOTE ]
Try the air flow type balancers..They can be found on Ebay if you search for SU carb. I have the same tool you talked of and I could never figure it out.

You could be suppling all your air though one carb.

How do the plugs look?

[/ QUOTE ]

A Unisyn?
 
Using your kit, when the carbs are balanced, the wires will be aligned and the dashpots will be equally springy -- they have to be. If they don't have the same "springiness," then they won't align with the engine running. Make sure that you align the wires to each other perfectly before you start the engine. What you're using is ten times better than a Unisyn (IMO).....

HTH!
 
[ QUOTE ]
Using your kit, when the carbs are balanced, the wires will be aligned and the dashpots will be equally springy -- they have to be. If they don't have the same "springiness," then they won't align with the engine running. Make sure that you align the wires to each other perfectly before you start the engine. What you're using is ten times better than a Unisyn (IMO).....

HTH!

[/ QUOTE ]

The problem that could be interfering is that I had to use a pair of pliers to make one of the rods able to hold the wire, and I had to use a screwdriver to open the slot on the other rod up a bit to get it to grip the inside of the piston. Hopefully, I haven't ruined the set doing that. The possibility of ruining tools is also a source of frustration.

I have a air meter tool, but bought this to avoid removing the air cleaners. That's why I asked about a smaller, sportier air cleaner assembly. The stock filter assemblies for a 74 MGB might be effective, but they're bulky and hard to work around.

Thank you,
 
Re: HS4 carb question- 18GK engine, 1974 MGB [long

you didn't ruin them. there's nothing magic about them. They're just an extension of the top of the dashpot so that you can see that they are rising and falling equally. Unless you've got them bent so far off to the side that they are rubbing on the opening on the top of the dashpot then you are OK. The fact that you can see different rise/fall rates is why the rods are a good tool to compliment or replace the unisyn. Unisyn will give you a near-perfect balance of air between the carbs but that might be counter-productive if the dashpots are moving at different rates. They don't have to be perfect but they should be pretty dang close.

if one isn't as springy as the other then I'm betting that the dashpot itself is binding. With the filler caps off they should both drop quickly from 'full-up' with a resounding clunk when they hit bottom. HS4's don't have the needle centering problem that H6's do they? I don't think so. That means the dashpot itself is probably the problem.

There really isn't much in the carb throat itself so I kinda doubt you have an air restriction. I'm betting it is a physical mechanical problem.

Pull the pots and make sure that they are smoothly sliding up and down and are very very clean. I'd think you'd feel a noticable difference between the two. One of the pots might be a little egg-shaped and binding, crud could be binding, burrs, whatever... Egg-shape can be gently improved with a rubber mallet carefully applied - the metal is a little more pliable than you'd think. Just don't go crazy sanding ANYTHING in there. It is a very tight tolerance.

Oh... They are a machined set too so don't mix em up. But you might switch-em around and see if they move better in case the DPO mixed them up in the past.

Oh... part 2... The tube that the oil goes in could be bent a little off and binding too. They too bend pretty easily and can be bent back gently by hand.

Dig in, play with them, figure out in your head how they are supposed to move, play with them some more, and it will probably become clear to you where the problem is.

<edit> Oh... and don't worry about pulling the carbs off completely as of yet. Just pull off the dashpots and the pistons. No need to make more work for yourself. You're only really talking about 4 parts... pot, piston, spring, cap. OK 5 if you count the needle... but you shouldn't need to pull it off either.

do roll the pistons on a flat surface and watch the tip of the needle very closely. It should be pretty dang straight... if it's bent you could be binding too.
</edit>
 
Re: HS4 carb question- 18GK engine, 1974 MGB [long

Thank you for the advice. I did some work on it this evening, but it's almost too hot down here. The car seems to run a bit hot, making it hard to do this sort of thing accurately.

Something I noticed is that under the piston, there is a number stamped into the metal of the carb body. One is stamped AUC 1300, and the other is AUC 1384. Is there any significance to that?

Thank you,
-Bill
 
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