Hi!

I see lot of red stars here, but no simple solution. I humbly want to tell you what I used to do with my 1800cc MGB with double SUs, assuming the ones in your Volvo are similar. Everyone told me it (MGB w SUs) was supposed to be a nightmare. Was not. Found an abbreviated workshop manual. Said essentially this:

Take off air filter(s). Adjust linkages so throttle valves open simultaneously. Set idle at 800-1000 rpm. Stop engine. Screw hex nut ("needle" adjust) at bottom of each carb in to full stop. Then, counting exactly, screw out 2 full turns for both carbs. Start engine. Put modest size screwdriver into one intake and lift a few millimeters on carb "cylinder". Engine rpm should increase slightly for a few secs and then drop off. That carb setting is then approx right. Do same with other carb. If, lifting the same amount and with same speed, you get similar response at both carbs, they are set right.

An increase in rpm, when you lift, that stays up indicates too rich fuel. Screw hex nut up 1/6 of a turn, and try again.

A decrease in rpm, when you lift, indicates too lean fuel setting. Adjust hex screw down 1/6 of a turn.

Srewing up and down, and listening, will teach you the trick. Fiddle around with it for an hour or so. Once content, put back air filter(s). Adjust idle rpm screws to desired idle speed. You can, with filters on, adjust richness of mixture by turning screws exactly same amount on both carbs, but don´t be surprised if you get lost and have to take off filters again and have to repeat procedure as above.

Once you learn this you can do it in 5-10 mins, most of the time taken by getting filter(s) off. Later, you will learn to do this every 3 mo or so, to get it just right. Why: Provided you have done all the checkups of ign etc, you should now hear the MGB/Volvo 1800 SU song, if you accelerate at 3500-4500 rpm, with window down.

More to read and with figs at:

http://www.mgb.bc.ca/service/su-tune.html

Points 8 and 9 on that page are essentially same as above and are the ones to start with first. Do the other stuff if those don´t help. The page also contain´s stuff on variants of choke system.

In my experience, these carbs required sparks exactly according to standard specs, for regular driving. So called sports specs spark plugs did not work well. Lots of uneven engine performance.

Good luck!

Harry;

In addition to the other good advice given (especially timing advance/repeatability checks), you should check to be certain your linkages return and are not sticking somehow. You did not mention the SU type installed, but the later HIF type (OE in 69?) had a thermal choke, and did not have the external fast idle linkages which on HS6 type should be checked that they are not engaged when choke is dissengaged.

Carb sync alone cannot be totally responsible for idle inconsistancy you described...it would cause engine to idle rough but not cause low to high idle RPM condition you described. This does suggest an ignition inconsistancy.

Also, synching or out-of-synch condition of SU's is overused as the reason for hating them...at idle, (presuming linkages have been synched, and jet position is similar) the amount of rise of the dashpot shows the amount of induction per carb...adjusting the idle screws in small increments until rise (and induction noise heard by holding head close to and between carbs*) is about equal, should give smoothest idle. What's so tough about that?

* this is no joke, and takes away the variable factor introduced by the tube method - which is highly dependent on tube placement!

Good Hunting,

(BTW, I recently posted a choke tech. article on swem site which might interest you.)

In addition to all the advice already given, most people recomend setting the valves correctly BEFORE trying to tune the carbs.

Also I have heard good things about Crane electronics to do away with the points completely. Somewhat difficult to install but they claim no more variation in idle.

It sounds as though you need to get your carbs in synch. Start by backing your idle adjustment screws out until neither is touching the throttle linkages. Next you need to adjust your linkages so that the throttle plates in the carbs are opening simultaneously. If you look at where the throttle linkages contact the ends of the throttle shafts you will see U-shaped links with a pin from the throttle shaft inside the U. The U should be just touching the top of the pin. If they are not loosen the lock nuts on the linkage adjustment rods and lengthen(or shorten) the rods until they just touch the pins. After you are sure that both throttle plates are opening in synch, turn the idle adjustment screws in until they just touch the linkage then give them another two turns (you may have to experiment with this until you get them set so that the engine will idle, just make sure that whatever number of turns you give one, you give the other). This should allow to get the engine running consistently enough to allow you to set the mixture and then fine tune the idle. If the throttle plates are not in synch on these carbs, it allows one carb to draw more air through it than the other. And since the amount of air being drawn through the carb actually controls the height the pistons are rising in that carb, the mixture in the carb which has more air passing through it will be running richer than the other.

Also if the distibutor han't been overhauled for a while, check the centrifugal advance springs for sloppiness due to stretch which can cause the idle to vary wildly. Point a timing light at the timing marks on the front of the engine - marks should stay still and not "hunt". Then gently increase idle speed watching marks - timing shouldn't alter until about 1500 rpm. Performance-type repairers usually have distributor graphing machines to restore original spec timing curves.

Could be binding linkage. Air leaks are still a possibility but you've taken care of the most likely cause with new throttle shafts. These carbs are a real pain in the backside but once you get them right they tend to stay that way. Good luck!