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I just received this great Carburetor Synchronizing Kit from British Tool Works (very nice quality) – thank you, Kelly!

My 76 Shadow runs pretty strong but the idle has always been just a bit off, especially on hotter days here in Los Angeles. I have never been able to fully smooth this out. Over the past 5 years, I rebuilt the carburetors, upgraded to a PerTronix ignition, new plugs, and added high-quality wires and it made a big difference. I also replaced all my vacuum hoses with fresh snug-fitting silicone tubing. The car always passes our strict California smog test every two years. After all this, the idle is still slightly off. So when I learned about these gauges I had to try them.

As expected, my carbs are not balanced. The part that has me puzzled is one of the carb pistons has a bad case of the jitters. See attached a 5-second video showing the gauge readings. The reading on the left carb is rock solid. The one on right is wildly jumping. I know the gauges are good because I swapped their positions and they both read the same for each carb. I'm thrilled to have these gauges because they are revealing something that has been hard for me to pin down, but I not sure how to adjust for this fluttering.

Any guesses on what could be causing this?

 

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IMO the jitters are due to a mechanical issue in the engine.

I would find someone that can perform a cylinder power balance test.

This test will help pinpoint the cylinder(s) that are causing the spikes in the intake airflow.
 

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Those carb piston fluctuations look pretty bad. It's possible that you have a perished seal on the carb butterfly spindle which is allowing air to be sucked in and interfere with the operation of the dashpot piston. As a temporary measure - without having to open anything - I would push a greased string into the seal area of the carb butterfly spindle and see if the dial test gauge readings improve. If the dial gauge needle settles somewhat then replace the butterfly spindle seals. Hopefully it's something as simple as that. Also, when replacing the vacuum hoses did you possibly overlook the hose for the cruise control which is just above no.1 cylinder on the A-bank side of the engine? A vacuum leak in the cruise control can cause idle fluctuations.
 

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Discussion Starter #4 (Edited)
Good news. I think I have it straight now (see video).

When I did the test yesterday the car was not warmed up. Today I warmed that car up to full operating temperature. Then pulled the air intake snorkel off and noticed a slightly loose vacuum hose underneath (it's been a few years since l looked under there). I put the gauges back on and started the car in neutral. The flutter had dropped significantly, though there was still a little jump on the needle. The gauge needle reading on one of the carbs was almost a "full turn" past the other carb reading -- way off. I reset the volume adjustment screws and was able to get the readings on both gauges aligned.

 

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Well done, simple fixes are the best. Do you have HIF7 or HD8 carbs? I think the leaking vacuum hose you found is the one for the cruise control, you may find that your cruise control works better now also.
 

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Just ordering mine! I chose the pro kit, as being in the UK, you never know when I might be able to help someone out with the different versions of SU carbs that the pro kit will fit and only a few dollars more.

Thanks for posting this, another toy/tool for the toolbox!

Jon
 

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Just ordering mine! I chose the pro kit, as being in the UK, you never know when I might be able to help someone out with the different versions of SU carbs that the pro kit will fit and only a few dollars more.

Thanks for posting this, another toy/tool for the toolbox!

Jon
Kelly at Britishtoolworks also makes the dial gauge adapters for Zenith/Stromberg carbs just in case any of your friends drive Triumphs etc.
 

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I just received this great Carburetor Synchronizing Kit from British Tool Works (very nice quality) – thank you, Kelly!

My 76 Shadow runs pretty strong but the idle has always been just a bit off, especially on hotter days here in Los Angeles. I have never been able to fully smooth this out. Over the past 5 years, I rebuilt the carburetors, upgraded to a PerTronix ignition, new plugs, and added high-quality wires and it made a big difference. I also replaced all my vacuum hoses with fresh snug-fitting silicone tubing. The car always passes our strict California smog test every two years. After all this, the idle is still slightly off. So when I learned about these gauges I had to try them.

As expected, my carbs are not balanced. The part that has me puzzled is one of the carb pistons has a bad case of the jitters. See attached a 5-second video showing the gauge readings. The reading on the left carb is rock solid. The one on right is wildly jumping. I know the gauges are good because I swapped their positions and they both read the same for each carb. I'm thrilled to have these gauges because they are revealing something that has been hard for me to pin down, but I not sure how to adjust for this fluttering.

Any guesses on what could be causing this?

Below is a response to William of NZ I gave a few minutes ago for a related (?) problem. Maybe a clue??
I had a similar problem with my 1978 Rolls-Royce Corniche DRH32489. I did all the same things as you. My problem may have been a little more difficult since I have a 4 throat Solex carburettor rather than SUs. In the end, the problem was leaking inlet manifold gaskets. To check whether this is a problem, squirt carburettor cleaner around the inlet manifold gasket areas and you should temporarily find the revs go up and the engine smooths out. This is due to the fuel mix becoming closer to ideal as the carby cleaner fluid replaces the excess air leaking in. Temporarily I cleaned the area up and sealed with silastic. It ran perfectly until I replaced the inlet manifold gaskets. Large parts of the gaskets (42 years old) had been sucked into the engine and presumably gobbled up. You might be able to see where the missing gasket material is located after cleaning the area. Good Luck.


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