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Discussion Starter #1
Hello my name is William I live in New Zealand I have a 1972 rolls Royce silver Shadow only problem is engine shakes at 600 rpm to 1400 rpm after that the engine smooths out I've pulled apart the carburettors replaced some seals and gaskets change the spark plugs check the leads set the points check the coil check the weights in distributor also have set the timing just can't find out what's wrong with it thanks hopefully someone can help


Only problem I think it might be as camshaft is worn I bet and causing it to shake
 

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Did you check motor mounts? How did you check your leads and how old are they? There could be a vacuum leak causing issues. Check oil in the carbs.
 

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Discussion Starter #3
Hi yes I have check the motor mounts they seem fine I've done a vacuum leak test and there's no leaks I've checked the dash pot oil and they're all fine I tested the leads using my multimeter and I've even swapped out the leads with some I had but made no difference
 

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Check arcing in a dark location and look for grey areas along the wires. Testing with a volt meter is for continuity....you have to make sure the insulation is doing it's job. If they are old, replace. Non-Lucas dist caps have also shown to be a problem.
 

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Discussion Starter #5
Hey thanks for that have checked for arking can't see anything will look along the wires for grey areas and will check distributor cap thanks
 

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Worn camshafts, on any engine, will cause poor running across the board not just at specific rpm's.
Back firing being one of them.
A worn camshaft will not properly let the exhaust gasses out or the fuel/air in, or both.

There really isn't a reliable way to check an old coil either, other then substitution with a known working coil, or testing it in a good running vehicle.

And as Wraithman said check the wires, if they are original, they've been in there since 72.
Many old resistor wires have Kevlar in them, that stuff breaks down after a while and the wires stretch causing arcing and climbing resistance.
 

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Discussion Starter #7
Hello thanks for that I will have a recheck some time this week and will post back thanks to all
 

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Very first thing I would check is the vacuum line to the speed control bellows, a leak in it will lean out the cylinder near the port that provides vacuum to it. If the leak is substantial enough that cylinder will misfire at idle.
Then do a quick cylinder balance test, short out each plug one at a time and note change in idle. If little or no change on one cylinder that is the one to focus on.
Short out spark plugs one at a time rather than pulling the wire off the plug for balance check. Faster, and no chance of causing an arc across the rotor or cap that may damage them.
Did you use a smoke machine to check for vacuum leaks?
 

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Discussion Starter #9
Hey whereabouts is the vacuum line to the speed control bellows I've tested the spark plugs by pulling off lead by lead but couldn't really notice anything and yes I did a vacuum test with a smoke machine I made from an old party house smoke machine 😂
 

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Any fault with a cap and rotor would be more apparent with load on the engine. If they were faulty and causing a miss at idle, it would have been worse with acceleration.
You are looking for a cylinder that isn't firing at idle because the mixture or compression is off. Could be low compression from broken or worn rings, burnt valve, or mixture too lean caused by an air leak.
If you can't tell by ear which cylinder is at fault get a dwell tach and hook it up to the coil at a higher scale reading. Then short out the plug wires one by one with a grounded sharp probe. You should see a noticeable drop on each good cylinder. No or very little drop you have pinpointed the cylinder. Then make sure any vacuum lines near that cylinder or the intake gasket are not leaking using a flammable brake cleaner. After that compression test on that cylinder.
Don't pull off the plug leads to test, use a grounded jumper wire onto the end of a test light to directly ground out each plug. We do it at the cap as it is quicker and easier than at the plugs. Pull up the dust boots on the leads so you can reach the connector. If they're tight spray some silicone spray on the wire and twist them and pull them up. You should be able to pinpoint the missing cylinder within a couple minutes.
You could of course have a bad cam lobe as you said in your OP not allowing the intake to open enough to get a full dose of fuel and air, you'll have to remove the valve cover on the pinpointed cylinder bank to see if one valve is not moving down as much as the others.
 

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Discussion Starter #14
ok thank you for that will give it a go this weekend I have done a compression test a couple weeks ago all cylinders were at 180 psi I'm hoping it's just a fuel mixture I tried to balance the carburettors with a air metre but my passenger side carburettor wasn't even registering on it and also when I try to adjust a fuel mixture the driver's side carburettor has to be fully turned in for the car to start idling all right
 

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Ok then if your compression is even like that start over then with the WSM to balance the carbs. It sounds like one carb is shutting off at idle and they are way out of balance. There could still be a vacuum leak but if you have ruled that out with brake clean, visual inspection etc then check balance and mixture back and forth until it idles smoothly. Start with setting the carb butterflies in sync, there is an adjuster to do that on the linkage at the front of the carbs, reference the WSM for instructions. Once they are set, you want to balance the carbs using the air volume screws. Quick easy method to check balance is to bend up some wire into a 'P' with a very long top part that will then fit down into the piston dashpot when the dampers are removed. Set them pointing at each other as close as you can get them. It will look like someone holding two divining rods looking for underground water. If you google "using an SU tool kit" you will find instructions on using the SU factory tool but you can just use some wire to do the same thing. The SU tool kit has aluminum tubes that the P wires fit into, but you can just bend a long wire with a U on the end to fit snugly down into the dashpot with the legs pointing towards each other. Get them so they point exactly at each other at stop, then start the engine and idle, if the carbs are balanced they will point exactly at each other when at idle. I suspect they will be way off, so then adjust air volume screws and see if you can get them to point at each other at idle. After adjusting the volume screws to get them to point at each other, you will have to check mixture on both and adjust if necessary, then balance again and adjust mixture again until the rods point exactly to each other at stop and idle. You check mixture by lifting the dashpot piston a tiny bit using the spring loaded pin on one side of the carb that pushes up to lift the piston from the bottom. Let it idle 15 seconds, lift the pin until you feel it touch the piston (you will see the rod start to rise) then lift a further 1/32nd of an inch. If the engine idle speed goes up and stays up, you turn the mixture screw anti clockwise a bit. Rev engine a little, let idle another 15 seconds and try again. If the idle speed drops and stays down when you lift the piston, you turn the mixture screw clockwise a bit. Turn the mixture screws no more that 1/8 or less at a time. You are trying to get to where the rods point to each other at all rpm's, and the idle speed will go up just a tiny bit then immediately drop back to what it was when you lift the pin.
Also pull the float bowl vent hoses off so the weakener system is disabled while you are adjusting just in case it is malfunctioning and interfering with the running. Doubtful from what you have described, but it's possible.
 

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Discussion Starter #16
Ok mate thank you for that will give that a go this weekend cheers thanks for everyone's help much appreciated
 

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Discussion Starter #17
Hi Jim Walters just one question the u bend on the steel rod how far does that have to go down the dash pot
cheers
 

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Far enough to stick into the inner bore of the piston and not touch any part of the housing. You want them to lift up and down with air flow past the piston when the engine is running. Four inches should be enough.
 

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Hello my name is William I live in New Zealand I have a 1972 rolls Royce silver Shadow only problem is engine shakes at 600 rpm to 1400 rpm after that the engine smooths out I've pulled apart the carburettors replaced some seals and gaskets change the spark plugs check the leads set the points check the coil check the weights in distributor also have set the timing just can't find out what's wrong with it thanks hopefully someone can help


Only problem I think it might be as camshaft is worn I bet and causing it to shake
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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