Carb Trivia

Formatted for the SOL Morgan web page by John T. Blair (WA4OHZ)

Last updated: 30 May 2009

©By: Bob Nogueira Last update: 08/28/1999

If I may provide a little Stromberg history here. The actual design of the Stromberg CD (ED. CD stands for Constant Depression) carb was started by Triumph who was paying BMC SU division a very high price for SU carbs. They were having a problem developing the carb and not infringing on SU patients. With a change of management at Triumph the CD project was sold to Stromberg to complete with a promise of a market if they got it to work.

After Stromberg managed to get a workable CD carb, Triumph lived up to its end and started using Strombergs. This great plan was somewhat derailed when BMC and Triumph merged and SU became a sister company of Triumph. At that point Triumph tried to make harmony in the family by using both Stromberg and SU on engines.

I've had Strombergs and SU and each has advantages and disadvantages with little or no difference in performance . Morgan used what ever came in on the truck that week.

Strombergs vs SUs

©By: Greg Solow Last update: 08/28/1999

I believe that TR-4 engines for 63, 64 were fitted with Zenith-Stromberg carbs. My 1964 model certainly was. TR-4As were all fitted with HS-6 carbs. The factory received engines form Standard Triumph complete with carburators and manifolds and installed them as received. The Zenith carbs were used for a short time after they became available because SU was owned by British Motors Corp.,(manufacturer of the MG) and Triumph did not want to buy carburators from their competition if it could be avoided. After Triumph and BMC merged to become "British Leyland" Triumph went back to using SUs.

Greg Solow

SU's - H6 vs HS6

©By: Jerry Murphy Last update: 08/28/1999

It's kind of easy to remember the difference between H6 and HS6, because the HS6 has a short body (presumably, S for Short). It goes (stock) on an intake with longer runners than the earlier version, with an overall length from the engine to the outside face of the carb about the same for both setups.

My '67 +4 came with HS6's and a long intake manifold. Now it has HS6's and a short intake manifold and K&N filters all fitting nicely under the stock hood as described more fully in "The Morgan Bedside Reader."

One of these days I'm going to mount the twin Webers sitting in my garage...

Jerry Murphy

Carbs is carbs
©By: Fred Sisson Last update: 08/28/1999

My opinion - the ONLY way to set a carb. is with the air/fuel ratio meter. Then you KNOW what's happening. For the Weber - jets are available & it's simply a matter of correcting the mixture at various loads/rpm as determined by driving & watching the meter.

The engineering in Webers is quite complex- emulsion tubes, air jets, fuel jets, neat little enrichment holes, holes in the butterflys, etc. If you want to go nuts & have fun - get all the books on theory & tuning of Webers you can find (I think I have eight..) and try to figure them out. I did pretty well with the books, lots of thinking and lots of (pricey...) little brass parts. A real "zen" process for me - probably a simple correction for an expert. It is fun but - the books were written before the availability of modern air/fuel ratio meters. The car ran "great" but without a meter- I really had no idea if I had maxed out the mixtures.

In the real world- I'll bet that with an air/fuel meter and a fuel jet change or two, you can get it running kick-ass. Or.. if you want more- read the books AND use the meter.

I have yet to check an engine that I couldn't improve the performance of with a simple tweek of a jet/needle. The meter tells you.

Don't get nuts about mixture at all speeds- it ain't gonna happen with carbs. Just check hard acceleration at wide open throttle- say pulling third gear. You want around 12-13:1 mixture.

Check mid-range cruise- say a nice 50 mph on flat, high gear, steady throttle. Strive for around 17:1.

Check idle- somewhere on the meter- 17:1 would be great but not that important- just so it runs nice and is not way off the meter.

That's it.

An engine will run on a godawful mixture. We have all have kept engines running by simply pouring gasoline down the carburetor now & then or spraying starting fluid in the hole.... That's an extreme, but it works. Just about any carb will "run", but how good? A car can be very drivable even though it is very rich/lean in places. That's where tuning comes into play. The closer you get to proper mixture at various loads/rpm - the better it runs though. Forget anyone's recommendations about jets, needles (SU), settings, etc. They are all educated guesses but YOUR engine is different - an individual.

Get the meter and KNOW what the mixture is - then go from there.

Fred Sisson

Return to the Index of Tech articles