Triumph Repair: Triumph GT6 - Stromberg Carbs, stromburg, triumph gt6

Question
QUESTION: I have a '71 GT6 with Stromburg CD 150 carbs.  I had the carbs rebuilt. My question is are the carbs supposed to work together - when tested the back carb puts out pressure and the front carb puts out nothing until the engine is revved.  Wondering if I have a vacuum issue or??? Is there supposed to be a vacuum hose connected to the front carb?  It's my understanding from what I've read that the carbs should be synchronized to work together at the same time. Any help or thoughts appreciated.

ANSWER: Hi Wendy,
Yes, they need to be synchronized. There are several ways to do it. The best is with a Unison gauge or it can be done by listening to the hiss noise that each carb makes. This is done with a length of hose. They also need to have the jets (needles) adjusted for mixture. If you had an experience shop do the rebuild and installation, they didn't finish the job as a rebuild should have included this unless you just removed them and had them rebuilt off the car.
Howard

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QUESTION: Hi Howard.  They were removed for the rebuild. The guage is what is telling me that the front carb is not working properly.  We have tried adjusting but nothing is working we can't get anything to come out of the front carb - the guage does not move without revving the engine. I know this isn't right based on the manuals I have.  Guess I will have to have someone experienced in strombergs and british cars look at it. Thank you for your response - it verifies that I am correct in that something is not right somehow.
Wendy

ANSWER: Hi Wendy, What gauge? Are you using a Unison gauge? If so, and you see no movement of the ball when you open the throttle, then the front butterfly is not opening because no matter what settings were done or even if who ever rebuilt them didn't and you have a torn diaphragm and the piston don't lift at all, there will still be air going in when you open the throttle and thus you have to get a reading on the gauge.
Yes, I agree that something is not right here. Do you have any mechanical background and hand tools? if so, I can walk you through the testing. Let me know.
Howard

---------- FOLLOW-UP ----------

QUESTION: Hey Howard. Yes a Unison guage. No movement whatsoever unless as I said the engine is revved. Where is the diaphram? Also I'm thinking that there should be a vacuum connection somehow from that front carb to .. is it the manifold? same as the back carb.  I have limited mechanical knowledge -my husband is pretty good but no knowledge of Strombergs - we have a repair manual we've been using but this has us stumped. When my husband is handy I will write another follow up and better describe to you what we've done in attempt to figure this out. I have trouble remembering the names of all the parts other than calling them thingys (LOL).  I will get back you to soon to tell you what we've done - see there is an emission control valve that seems to have something to do with the problem. I have ordered a new diaphram for that but haven't replaced it yet.  Shouldn't there be some sort of vacuum advance hose from the front carb like there is from the back carb? I'm looking at the photos in the manual and in a British Victoria catalog and something doesn't look the same.  I'm sorry I'm not too good at explaining - like I said when the hub is around I will have him help me explain it to you.
Thank you Howard for your patience.  
Wendy

Answer
Hi Wendy,
Understanding how the Stromberg works will help you figure out how to repair and adjust it.
First, if the Unison shows no movement at idle, it means there is no air entering that carb. So the two throttle shafts and thus throttle butterflies are not in sync. and the one with no reading on the Unison is all the way closed.
Remove the four Philips screws from the top of the carb and lift the top off and remove the long spring. Then lift out the piston with the large diaphragm. Be careful not to damage the long needle on the bottom of the piston. Inspect the diaphragm for tears or holes and note that there is a "Tooth" in it's outer rim that must fit into a groove in the carb. Also note that the two holes in the bottom of the piston must face the engine when the tooth is aligned correctly. There should be oil in the center tube. Reassemble it in the order you took it apart being careful to get the needle into the center jet and the tooth into it's notch. After you put the top back on remove the center cap and lift it up and push it back down, if you feel hydraulic resistance it is ok, if not put some engine oil in the center tube. It is a shock for the piston, necessary for a quick throttle opening and to smooth out the pulses at idle.
This is a constant velocity carb, meaning the air going across the jet is held at the same speed from isle to full throttle. Only the volume of air changes. The air going across the bridge (where the jet is) forms a low pressure area at the two holes in the piston which is transmitted up above the piston lifting the piston by the diaphragm.
This also lifts the tapered needle out of the jet which increases the fuel supply. All of this confirms that the carb is ok. You still need to sync the two throttle shafts and set the idle stop screws with the Unison gauge.
I hope this helps,
Howard