My SU's are on the way back almost good as new, I hope. There is one thing I was told was that the piston springs were streched out almost double the normal length. I was told to give the carbs a try and see how they worked but he was at a loss to explain why someone would strech the springs.

Any ideas? If I have problems he told me to get new springs.

By stretching the piston springs it seems that it would increase the air flow required to lift the piston and needle.

Hi!

Maybe some info on this page may be of some help in your endeavour:

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

Good luck

The pistons are raised by the vaccum created by the engine, not the airflow - the greater the vac relative to the airflow quantity, the higher they're raised hence the weaker the mixture. Putting stronger springs in or stretching the original ones will keep the pistons lower thus increasing the air speed over the jet, increasing the depression and making a richer running mixture. It has little or no effect on acceleration enrichment. Spring stretching has been a quick fix for lean running with Stromberg CD series carbs for a long time because nobody dares move the metering needle in the piston for some strange reason. Not a good fix but cheap and quick. (many Volvo dealer mechanics used to do it!)

Jonathan;

Your assertion about airflow vs. springforce is absolutely correct...while configuring a single SU HS8 (2", from Jag.) for fuel metering duties in a turbocharged 122 a while back, at first(before installing a heavier force spring), the dashpot was too easily raised up by the (higher) airflow of the B18 through the single carb. After installing the new spring, the dashpot operated in its normal range (as opposed to wide open, where it doesn't have fine control), and the carb worked great. Buy new springs! BTW. I have some used clean for sale...cheap.

Mitten-The propensity for people to experiment with things never ceases to amaze me either, (hell, I might be one of those doing the experimenting - although I can honestly say I've NEVER stretched carb springs)...but then, if they didn't experiment, Wright bros would have never flown either!

Regards,

My strombergs (similar design) had streched springs and they only helped to use more fuel, not to mention that the previous owner's mechanic had installed the carbs out of 2 single carb engine onto my B20 without changing the needles which made it worse again!!!!!!

I'ld get the standard springs since any mods to the springs would only useful if the engine was modified to a point of requiring it

Why? Never underestimate the tendency of your fellow man to misunderstand, modify, and alter anything. The more mysterious, the more common the modification. On a scale of 1-10, SUs rate a strong 9.5 for mystery. I once bought a 544 that had had the linkage modified so that they worked as primary and secondary.

Perhaps the PO had a problem with stumble on accelleration. Rather than use heavier oil, or use oil to begin with, he stretched the springs?

Have faith in the creativity of our fellows.