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Discussion Starter · #1 ·
I completed the carb rebuild on my 1976, and I passed the California emission test last week!

As luck would have it, the following day I went for a drive and my car was idling really rough. I checked and replaced a few vacuum lines. I checked the plug wires. I tested the Pertronix ignition and the coil. Everything was fine, yet the rough idle was still there. It was behaving like a vacuum leak.

This evening I started systemically pinching all the vacuum lines. When I pinched the vacuum line running through the Anti “Run-on” Solenoid the idle smoothed. I removed the solenoid and tested it by attaching two lead wires connected to a battery. It was working. When the power was applied the solenoid would close and block the airflow. I used a multimeter to measure the voltage coming from the wiring harness. I did not get a reading. It appears the solenoid is not getting power.

Before going into the wiring harness and looking for the problem, are there any switches in the emission system that controls power to the solenoid that may be faulty? If any of you have experience with this, please let me know your suspicions.

Many thanks.
 

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Always check your fuses first. Since you have an SY1, and I'm not intimately familiar with the fuseboard for the one series, someone else is going to have to reveal which fuse is most likely to be the faulty one in this instance. The wiring diagram in the workshop manual should also trace back to to the correct fuse to look at.
 

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Discussion Starter · #3 ·
Thank you, Brian. Turns out the main power wire going to the harness was disconnected. The solenoid is now working.

However, I found a new problem. When I start the car it idles nicely at 650 RPM. If I open the throttle to about 3000 RPM and then let it drop back to idle position, the engine drops to 1200 RPM and goes no lower. If I turn the engine off and restart, it goes back to 650 RPM at idle. Every time the throttle is opened it never returns to the 650 idle speed and just hangs at 1200 RPM. I'm not sure where to investigate next.
 

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You mentioned the rebuild of the carbs so check the choke fast idle cam is doing what it should and not sticking or wrongly assembled. Bit odd that it starts of at the correct rpm though, but it sounds like the problem is in this area.
I assume all this is happening with the engine at normal operating temperature.
With the engine hot, start the car so it is idling at the correct speed then while watching the fast idle choke cam, increase the revs to see if it this cam moves and the fast idle adjusting screw lodging on the cam.

Jake.
 

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Or sticky pistons in the carbs....
Quite right, after jeyjeys comment it made me wonder if during the carb overhaul the suction chambers went back on the carbs they came from. If you are sure all the parts stayed with there respective carburettors then fine, but if not, and the choke mechanism is operating correctly, worth swapping the suction chambers over to see what happens with the idling speed.

Jake.
 

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Discussion Starter · #7 ·
Thank you JeyJey and Jake. To be honest I did not keep track of the suction chambers. It's very possible I mixed them up. This might also explain why I'm having a tough time balancing the carbs. I'm using dashpot dials, and the left carb appears to have a low piston lift. It does not respond much to the air adjustment screw. If I back the screw almost all the way out, the piston lift does not increase. I work on it tomorrow and report back. Many thanks.
 

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Discussion Starter · #8 ·
I did the dashpot/piston drop test and the pistons were mismatched. I switched them and balanced the carbs using the dashpot dials. The idle is smoother but remains inconsistent. There is also a little flutter on one of the suction chamber pistons which you can see in this 19-second video.

The car is still not holding idle between 600-700 RPM. When fully warmed up the car still idles at 1000+ RPM about 75% of the time. I suspect the throttle spindle might be worn out? It's the only thing I did not replace during the rebuild, though I did replace the throttle spindle seals. When pressing down in the throttle lever attached to the throttle spindle, the idle drops to 600 RPM. When I say press down, I mean I have to push hard, and very little moves on the linkage, but the idle drops. I read online the throttle spindle has a small notch on the shaft that allows the throttle lever to grab on. Maybe this is slightly worn allowing for a little play and is preventing the spindle from fully closing?
 

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There is a high probability of vacuum leaks on cars from 75-80 due to the amount of tubing. The cruise control bellows is often overlooked.
 

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There is a high probability of vacuum leaks on cars from 75-80 due to the amount of tubing. The cruise control bellows is often overlooked.
Also, unlikely but possible, inlet manifold gasket. A vacuum leak can cause the carb piston to fluctuate as your dials show.
If this idle speed problem has only occurred since the carbs were rebuilt, just perhaps something has not gone back from where it came from?

Jake.
 

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Discussion Starter · #11 ·
I having a great day!

Interestingly enough though, the idle problem has vanished completely and mysteriously! After fighting this problem for many weekends I decided to take my car to one of the best and busiest Rolls Royce mechanics in the Los Angels area, Barry Griffin. He is the owner of Independent RR Sevice located in North Hollywood. It took me about 90 min to drive there in our extremely congested LA traffic. During the commute, my car was running perfectly, and when I arrived the car was still running perfectly. I had taken the morning off work to make this appointment and felt like a fool showing up with a perfectly running car. Barry was great. He inspected everything and hooked it to a sniffer/computer to measure the air/fuel mix and emission, etc. He explained the complex emission systems in the 1976's are notorious for causing problems but that mine is running tip-top and this was reflected in the measurements.

While I was there, I pointed out some other frustrating issues I was having like the popping sound I hear when turning the front wheels. I have replaced most of the parts in my front suspension and I was stumped by this noise. He found the problem in two minutes and then proceeded to put the car on the rack and fixed it right in front of me. The sub-frame rear mounts were loose! This never occurred to me. He tightened them. He then inspected the entire front end and found the camber bolts were loose as well as a few others. After all this Barry did not charge me thing. I asked to pay him but he refused. His shop is very busy and I had taken up about two hours of his time. And the guys in his shop had to stop their work to move other cars around to get mine on one of the racks. Barry had gone out of his way to be helpful. This kind of service is rare and contrary to my experience with other shops. Independent RR Sevice is now my go-to shop when problems become too big for me to manage at home. Thanks Barry!

The idle problem may return at some point, but I'm going to enjoy my noise-free and smooth-running magic carpet for now.
 

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Discussion Starter · #12 ·
There is a high probability of vacuum leaks on cars from 75-80 due to the amount of tubing. The cruise control bellows is often overlooked.
Thanks. Given my bellows looks original and they are prone to leaking, I just ordered a new one.
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Discussion Starter · #13 ·
Here is a picture of the throttle disk from my HD8 carburetor. It has a spring-loaded poppet valve mounted on the other side, similar to the one shown in the third image below.

Looking through the rebuild parts list for RR HD8 carbs on the SU site, I do not find any throttle disk with this spring-loaded poppet valve.Parts for AUD 648 for Rolls Royce & Bentley Silver Shadow 6750 cc - SU Carburetters - SU Carburetters

For those of you who are familiar with the 1976 model, does this look right to you? Do the spring-load poppet valves wear out? If so, what might the symptoms be? I'm wondering if this could cause the idle to hunt.

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Not present on my 74 SS1. Must have been a later emissions addition.

Here's what I read on a classic sports car site:

Carbs built after about 1968 feature throttle disks with a spring-loaded poppet valve that improves emissions, but the valve also impedes air flow. Fortunately, earlier flat disks can be fitted.
 
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