• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Carb balance/adjustment frustration [long]

Share this page

wkilleffer

wkilleffer

Jedi Knight
Country flag
Offline
Hi everyone,
I am trying to adjust the carbs on my 1974 MGB, with a 1972 engine and HS carbs. Just about the time I think I've got it right, it seems like I've done nothing right. The part that's really throwing me for a loop is the part where you're supposed to push up on the piston lifting pins to determine whether or not the mixture is correct for each carb. This will be my third try, it's getting late and I need to be able to drive the car in the morning.

The vacuum advance for the distributor has been connected the whole time.

I started out by disconnecting all interconnections and adjusting the throttle stop screws 11/2 turns in, raised the jet even with the bridge and backed it down 12 flats. I started the car and it got to NOT quickly. Then, I backed the idle speed down to just under 1000.

After that, I used a carb balancer to check the airflow into each carb. The rear always seemed to be higher than the front, so I'd tweak the idle screws to bring both carbs to the same place on the balancer.

Then, I started turning the jets flat by flat until I got the fastest running speed with an even fire and exhaust sound. This necessitated adjusting the idle speed and re-checking the airflow with the balancing tool. The airflow was never the same with either carb at that point.

At that point, the directions say to push up on the piston lifting pin first on the front carb and then on the second. If the speed rises, the carb is too rich, if the engine tries to stall, it's too lean, and if it briefly goes up and comes back down, it's just right.

What if I get none of the above? Well, sometimes it's obvious that it's trying to stall. But otherwise, I might get the slightest quick blip upwards, and then an attempt to stall. A few times have brought no change at all.

All the books seem to give a different amount as to how far up one should push the pin. Trying to push it up 1/32" and keep it steady is almost impossible. It's really difficult to push it anywhere but all the way up with any consistency.

Part of me just wants to stop before I get to that point and hope for the best. But that just doesn't seem right. I'm trying to use less gas while still having fun and retaining reliability. But it's no fun if unsuccessful attempts to adjust the carbs use 1/4 of a tank.

The carbs have been reshafted and rebushed in the last three years, and don't show any signs of leakage around the shafts.

There probably haven't been too many times that I've had the carbs adjusted all the way right, so I'm trying to do it this time. Any suggestions would be appreciated.

Thank you,
 
To which carb, front or rear, does the gas line go first? Is that the carb you've been adjusting first?
 
The gas line goes to the front first. That's the one I've been adjusting first.
 
Did you reset or check the float level for each carb? The proper fuel level in each carb jet is essential to getting them adjusted correctly.
 
[ QUOTE ]
Did you reset or check the float level for each carb? The proper fuel level in each carb jet is essential to getting them adjusted correctly.

[/ QUOTE ]

No, I didn't. I'm not even sure how to do it. The floats and needle valves were replaced with the rebushing, but that was then and this is now.
 
You only lift the pistons a bit, not more than a quarter of an inch, maybe 3/16 and just blip em, if no change all is ok. Lift em with a small screwdriver. You've prob had em dead on several times it sounds like.
 
I checked the floats. They're the plastic kind that don't have any way to adjust them. The edge of the float opposite where it attaches to the lid for the bowl will just barely skim a 1/8" drill bit.
 
[ QUOTE ]
They're the plastic kind that don't have any way to adjust them.[/quot]

Ah, but they do have adjustment...rather crude but it entails bending the metal 'arm' to which the float is attached.
 
[ QUOTE ]
[ QUOTE ]
They're the plastic kind that don't have any way to adjust them.[/quot]

Ah, but they do have adjustment...rather crude but it entails bending the metal 'arm' to which the float is attached.

[/ QUOTE ]

Not sure if I can swing that tonight. Are mine not set properly?
 
Don't know without putting a feeler guage under the little arm...do you have a shop manual?
 
There's a section in it on adjusting the float.
 
[ QUOTE ]
There's a section in it on adjusting the float.

[/ QUOTE ]

Yes, but it doesn't make much sense to me considering the fact that the float in the book has a metal adjuster and mine don't. I'm afraid to start bending the metal bars. I don't have spares and can't afford to break them. The B's my only car.
 
[ QUOTE ]

All the books seem to give a different amount as to how far up one should push the pin. Trying to push it up 1/32" and keep it steady is almost impossible. It's really difficult to push it anywhere but all the way up with any consistency.
Thank you,

[/ QUOTE ]
What the book doesn't say is that it's the piston lift that is important, not how much you push the pin up. There is some free play in the pin before it actually lifts the piston. The pin has a light spring & you can feel the pin lift get harder when the pin actually starts to raise the piston against it's own spring.

If you had the air filters off you could observe the lift directly. Lot of work though.

If you are in doubt about the lift & don't want to remove the air filters, remove a cap & damper, push something like a popsickle stick down into the damper, trim the sides of the stick until it is a light push fit if necessary.

You can now observe actual piston lift as you push up on the pin. Once you see & feel the movement a few times, the stick is usually not needed. The 1/32" is not terribly critical, 1/16" would still work.
D
 
I *think* that it's done now. A few good drives will tell alot. An SU tool kit and smaller air filters might help with this.

The piston lifts made a slight blip in the throttle, then things settled down, but were bumpier than before.
 
Unless your carbs are new new there will be a bit of a change in tuneing every time the piston comes back down until all shakes into it's worn spot. Now keep in mind this wearing starts as soon as the car is fired the first time. I would suggest to ignore it unless the engine stumbles badly. Drive it a bit and see how it goes.
 
William, I've had more success with getting the carbs close, then driving them a few days and then checking the plug colors. Then I adjust the mixture accordingly. It sometimes takes me a week to get it right, but eventually I get it dialed in correctly.
 
Remember that the reason you are adjusting the float is to get the level of fuel in the jet to come up to between 1/8 & 3/16 inch from the top of the jet.

Just setting the float does not guarantee this.

Vacuum out the jet & hit the fuel pump until the fuel stops rising.

Adjust the float as necessary, paying no attention to where the other float is.
(work on one at a time)
 
Well, thanks to everyone's advice, I think I've got the carbs properly adjusted and balanced. The car seems to run better and have more power, and I didn't notice any stumbles on accelleration. I've got the idle speed set two gauge needles down from 1000 RPM.

You might have read my other post on valve clearances. From the sound, I might be one or two thou's off from correct, and one rocker might be a hair looser than the rest. Practice makes perfect, so I'll give it another try this afternoon, and all should be well.
Thank you,
 
Hello William,

the HS floats are not designed to be adjustable, but the thickness of the washer under the float needle body will affect the setting.

Alec
 
Back
Top