SU HIF Adjustments.

Group -

I am quite the newbie when it comes to working on cars and my 77 MGB. My issue is that after setting the floats, attempting to balance the airflow (by ear and piston lift method) "MaggieB" still idled near 1100. Any attempt to get lower would cause the engine to rock like it was going to stall. I believe my floats are out of adjustment, but the haynes manual and some other internet posts all speak to a "mark" on the float itself that should reside .04in or 1mm below the base of the carb when inverted. I cant find this mark and therfore I believe I am putting too much fuel into the each carb. Can someone point out the mark via picture or drawing and a easy way to identify the .04in or 1mm position? Thanks in advance your time and help.

If you have a 77 and there are two units hanging on the side of the lump the OEM single Stromberg has been replaced with an earlier set up. Most any earlier twin set will fit but we need to know which carbs you actually have in order to offer advice. Read the little triangular tabs held on to the float chambers by a screw through the lid if you have HS models or just say that you have HIF units if there are no float bowls hanging off the sides. A picture or two of your engine from the front and from the driver's side might help too.

Jack

Chris,
On a different note, I just had Jeff rebuild my distributer and I have never had the car (a 77B also) idle so well at lower rpms until now. Less choke needed when starting and it actually idles at around 7 to 800 RPMs smoothly. Maybe your carbs are right?
Dean R

Chris,

You would probably benefit from getting a synchrometer. Doing it by ear is tricky at best.

Make sure you've got the throttle linkage loose while doing the this process.

Use the synchrometer to get the air flow the same through the carbs. Once you got the flow to the carbs synced, work on the mixture. Turn the screws anti-clockwise until the rpms drop, then back clockwise until you get smooth running and maximum idle speed.

Now go back to your idle speed screws and adjust the idle downwards and recheck your sync. If all is well you should be able to get down to 850rpm without trouble. Now go back through the mixture adjustment process and you're done. This is important because the mix will be different at different throttle openings. You're probably running into this issue when you are trying to get the idle speed down.

Tighten up the throttle link and check the off idle sync by putting a feeler gauge between the throttle lever and the choke/enrichment link. This will cause the throttle to be actuated by the linkage which will tell you if the linkages are indeed moving the butterflies at the same time. You may have to fiddle with the clamps a bit to get this right.

Guys -

thats for the points, I'll double check those this evening. Even though the idle was at 1100 it was real smooth, no sputter or backfire or "spitting" from the carbs. I took it for a test spin and everything held pretty good, idle held and everthing seemed fine. I went out Sunday for a quick spin and out of the gate it was spitting, backfire or popping from the exhaust and a little sluggish at accelerating and the idle kept wanting to drop down...I'm not sure put is it possible that the idle screw nuts came loose and allowed the idle set screw to move? Or is it possible a float has become stuck flooding one of the carbs?

This baby starts right away, no need to choke or anything, it fires right up, but then starts into the whole backfire popping mess. I'll get some pictures and post as well on the carbs to aid in the discussion.

Thanks again!

First, check out joecurto inc (google)..

he has decent prices on tools of manipulating SUs.

I have used a Uisyn for decades and decades. The hose is cheap, but IMHO not useful in the hands of a novice.

Second, the tune up procedure is this:

A) adjust valves (important--especially the first time)

B) adjust points to 60 degrees dwell (dwell meter at about $50 is much more accurate then using a feeler gage on the points, especially old points). Otherwise, use a matchbook cover (about 0.015"). It's crude, but so are Lucas points. Later you will want a Pertronix and you can forget this frequently repeated step.

C) adjust ignition timing to roughly 10 degrees BTDC. it will be changed as you work along

D) set mixture. This is the almost-impossible thing for a newcomer. The instructions in the manuals (you have one, don't you?) are not really helpful. Jack Austin may peel in with his prescription for setting SUs (Jack, you ought to put it in the library and make it a reference url<BG>). He is right. But it is still almost impossible without someone there showing you. Iowa is a bit of a stretch for anyone I know<G>

E. Set carb idle speed/synchronization. Then repeat C), D) and E) because E will upset C & D.


If you know what you are doing and there are no problems, this is an hour or so work.

Chris,

Your problem doesn't sound carb related to me (from reading the second post), it sounds ignition related. If the car is running correctly and then starts to go south the hotter it gets, I would suggest taking a closer look at your points and condensor. You may have improper points gap or bad condensor.

The old adage is that 90% of carb problems are ignition.

Bill

AS promised here is a pick of the enigne...I'll investigate the points a bit later...but can someone tell me is there an easy method for setting the float level?

They only way I know is by removing the carbs and turning them upside down. This is where the HS series carbs have an upper hand, as the can be left in when adjusting the float levels.

Pics of the inside of the carbs on the way...

Eric and group -

here the float removed from the carb...I read in a latter post that the "U" should be 1mm below the casting line of the bottom of the carb...I think I found the "U" as it was staring at me the whole time...please see the pic and the arrow...let me know...also ever seen an O-ring on the bottom of the HIF expand to the point it wont go back in? Its actually a size to big now...I am letting it dry, but I dont know if thats the right thing to do or not...

I measured my 1mm from the lowest end of the "U" when the carb is inverted, that is the end that is up in the photo above.

I also had my gasket expand when I took the bowl cover off. I actually used and exacto knife and cut a section out to make it fit again, probably not the best thing to do, but it doesn't leak...

Great, so my measurements are correct and I'll try the exacto knife trick tonight!