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MG MGB Technical - HIF basic carb tuning

MGB nearly ready for the road. I warmed her up in the drive with an eye to tuning the carbs. They are clean, no fuel leaks, and the dash pots have oil in them. Couple of things, the rod you can push up on the side to lift the piston up only works on the front carb, although the pistons move freely on both carbs, does this mean anything? Also the screws! to adj the fast idle and choke are nuts and bolts, one having a screw head making it easier to adj. What is correct?
Engine idles when warm fine with a little vibration, what are the basic things i can do to make sure its running well without going to far ?
Thanks in advance
Gerry

Don't worry much about one lift pin not working; they don't do too much that's useful anyway. It would be nice to have all 4 speed adjustment screws to be slotted head. I believe that's what is correct. Other things you can do is make sure the throttle plates in both carbs begin to open at the same point when the linkage moves and also that airflow to both carbs is nearly equal (need a carb sync tool to do this accurately). Also, if you have a digital tachometer, you can set mixture fairly accurately according to the procedure described in the handbook (involves looking for slight changes in engine RPM relative to the mixture screw turns).
wiilieL

If you have both lifting pins present but only one lifts its piston then I would investigate carefully, there is definitely something wrong which could affect running. Remember the pin has to be moved a significant amount before it *starts* lifting the piston. You can see when this starts to happen by unscrewing the damper cap and peering inside, if the air cleaners are fitted and you can't see the body of the piston.

The lifting pins are *essential* for correct setting-up.

The fast idle and choke are adjusted with the same screw, the normal idle with another. They *should* be slotted as well as hex, but if any are hex without slots it should be a simple matter to hacksaw a slot in them, it certainly makes adjustment easier.

To set up the throttle and choke interconnecting shaft clamps should be slackened to make the two carbs completely independant.

The jets should be set to their starting position by screwing them up until they are flush with the top of the bridge (judging by eye is good enough as it is only a starting position) then screwing down two full turns.

When the engine is running and up to temperature set the idle screws so that both carbs are pulling the same amount of air as judged with a listening tube or balancing tool.

Adjust the mixture on each carb, one at a time, to achieve the highest idle speed to get the coarse mixture setting, then start using the lifting pins to get the fine mixture setting. What you are looking for is for the idle revs to rise momentarily then settle back down when you lift and hold the piston 1/32". If the revs drop it is weak so screw the mixture screw in a bit. If they rise and stay up while the piston is up it is too rich so unscrew it a bit. It can be quite tricky to judge the correct point on an HIF, bad enough on an HS. But if you turn the mixture one way far enough to just detect a weak mixture, and far enough the otherway to just detect a rich mixture, then the correct position is between the two. With practice you should be able to get these two points closer and closer together. You don't need a rev counter, the human ear is more sensitive to very small changes.

Because the carbs are linked by a balance pipe when you alter the mixture on one you will affect the other two cylinders, so after adjusting the 2nd carb go back and recheck the 1st. You will probably have to tweak this a bit, so then go back and recheck the 2nd carb again and so on. After a couple of iterations they should both be correct.

Then you need to tighten the throttle interconnecting spindle screws, such that there is slight play in the linkage and cable before movement of the cable starts to open the butterflies. Now recheck the air flow balance just *off* idle, it is more imprtant to get corerct balance off idle than it is at idle, for obvious reasons.

Then tighten the choke interconnecting spindle clamps so that when the cable is half pulled it is making a right-angle with the opertaing lever on the spindle. Too mcuh one way or the other can make the choke hard to pull. Again there should be slight play in the cable so the knob has to be pulled a little bit before it starts moving the cams on the carbs. Again set the fast idle screws such that in fast-idle both carbs draw the same air. On HIFs there is an arrow stamped on the cam, this should be under the screw when you set the fast idle speed to 1100/1200 rpm.
Paul Hunt

Thanks guys. I tried to buy some slotted screws yesterday but nobody has them. Luckly i found an old carb that had two so i'll use them. I'll take the dashpot off and see why the piston doesn't move up, the lifting pin seems to have full motion.
Gerry

The Haynes manuals and some on-line aricles and this Site have proven invaluable to my understanding the SU...I used to think there was a Voodoo associated with SU's...but after learning the theory, they are easy to tune and refurbish...and indeed a truly elegant design....I've updated my HIF's (bushings/seals...etc...)and they are great instruments...and I wonder why anyone would replace them....
pete

Quite. Setup properly, and not fiddled about with afterwards (which means you have to setup from first principles again) they are extremely reliable and mine keep their tune from one year to the next.
Paul Hunt


Just to add that John Twist does a video at

http://uk.youtube.com/watch?v=ASeMfXfjNpw


Regards,

John
John U

My HIFs had slotted screws with hex-shaped heads, and a nut on each that secured the screw in place. I found it cumbersome to have to loosen the nut then turn the screw and then tighten the nut again after tuning, so I put springs on the screws instead of the nuts to secure them. Then I was able to simply turn the screw whenever needed without having to tighten down the little nut. Made it much easier to adjust the initial idle and fast-idle settings, and they have not budged since setting them up. This was actually a tip from one of John Twists videos.

Erick
Erick Vesterback

In fact the tip is in the video link above, towards the end of the clip.
Erick Vesterback

Indeed, the springs are super!....Got the idea from the Twist video...so much easier than those damn lock nuts....
pete

Since we're all singing the joys of well adjusted carbs, besides Barney Gaylord's MGA site, have any of you wonderful guys seen the balancing rig for the SU's that uses very lightweight graded sticks or tubes placed vertically down the dashpots with a horizontal light weight tie between them. So, it looks like a capital "H" to start. The suck balance is better done this way as it measures the rise of each piston, whereas, as you know, the " In-Sync" balancer uses only the suck force in each carb at idle.
I have a memory of Moss offering it once years ago in their "SU Tuneup" kit. And I definitely recall, years ago when we covered it in this forum, a few guys had testimonials on making their own setups. Vem
vem myers

That sounds really ingenious. At the minimum an inverted 'U' of wire sounds like it would do the job. One problem could be that HIFs or some versions of them have a spring-clip on the damper piston that fits inside its cylinder meaning you can't completely remove the damper caps (presumably so you can't mix them up). Still get roundable by balancing them on the edges of the damper cylinder, or anything else that moves up with the piston. You would have to check the 'U' was square with the engine off, then at idle, and finally just off-idle, but it would save having to adjust the likes of Carbalancer between the idle test and the off-idle test.
Paul Hunt

This thread was discussed between 27/05/2008 and 08/06/2008

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