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Discussion Starter #1
I have been having an issue with my 75 silver shadow. The carbs were recently rebuilt. I took the car on a drive of about 30 mins the other day down A1A here. its a 35 mph constant road. Did not stop the car at any traffic light for about 20 mins. I came to the first traffic light and noticed that the car started to run rough when stopped. Almost like it was stumbling/rumbling feeling. When I hit the gas it would bog down or stumble like you would think that it would stall but did not do so as I applied more throttle and got it going. If I let off the gas it would not stall but it had that feeling. Every traffic light after that did the same thing. I have been able to replicate the problem on every drive since. Starts up and runs fine then once it is warmed up it runs rougher. Any ideas what I should check? Carbs were rebuilt, new wires, distributor, fuel pump and filters were also replaced. Thanks in advance.
 

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Definitely check the timing. Play with it...loosen the clamp and turn a very small amount either way. Remember combustion relies on 3 parameters...air, fuel and ignition so anything not up to par will yield poor performance. After ignition I would check fuel delivery (you have 3 filters). If this problem recently started ..walk it backward to review what you touched. If there is a catalytic converter on your car they can be suspect due to clogging, robbing your acceleration and overhaul power but this problem gets worse over time.
 

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I wonder if you have stale fuel in the tank following a long lay up, it might run OK with stale fuel while the choke is on but once it warms up and the choke opens it might run rougher with stale fuel. Your symptoms also could be as a result of electronic ignition module failure ( if you have EI) or condenser failure if you have ignition points. Did you change the spark plugs when you replaced the wires? Is your air filter clean? Check all vacuum pipes around the carbs for leaks.
 

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To add to the list of things to check, make sure the automatic choke is fully off when the car warms up. Also check the choke solenoid is switching off. I'm thinking this may be a possibility as the engine runs well when cold and then becomes lumpier as it warms up.
 

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Runs fine cold fuel not the issue. Coil most likely, ignition rotor possible. Distributor replaced with ? What type of ignition module, original Lucas OPUS or Pertronix or something else? Not timing. Exhaust will be black if choke not coming off.
 

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Discussion Starter #6
I really appreciate all the suggestions. I am going to check all this and see what I can find. Plugs were also changed along with wires. I don't think it is stale fuel because it has gone through a couple tanks since it was redone. I may replace the ignition coil just for the heck of it as some of you suggested. I am expecting a perfectly smooth running engine. Am I expecting too much from this age of a car? After all rolls Royce says all you should hear is the clock. I hear the clock and feel the engine...
 

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Discussion Starter #8
Fuel pump was replaced with after market. I replaced it again just for the heck of it. Would a Silver Shadow I (1975) also have 3 filters one main and two by the carbs? My research for some reason turned up that it was only SSII's that had the two additional filters by carbs...am I mistaken? Thank you
 

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My 1974 Shadow (SRX18501) has 3 filters. I have replaced all 3. One in each carb where the fuel inlet pipe is routed and a large one just forward of the fuel tank, which is accessed from beneath the car.
 

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You should have a perfectly smooth running engine when it's sorted. When my 1979 Silver Shadow is idling I can hear the clock ticking.
 

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Discussion Starter #11
Can someone point me in the direction of a diagram showing the location of the two fuel pumps by the carbs? I want to double check and make sure they were changed when I had carbs rebuilt.

Shadow 11, you have confirmed that I still have some work to do.
 

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The carbs have inlet filters. Follow the fuel lines to the carbs and use a line wrench The main fuel filter is underneath after the fuel pump. Also..refer to the applicable TSD for your car.
 

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I'm assuming you meant the 2 fuel filters by the carbs. The pic below shows the right hand side carb fuel filter. The two red arrows point to the two setscrews you will need to undo to remove the filter. With the setscrews out, just wiggle and pull the hexagonal shaped insert out.

You will need to remove the carb air duct to get access and then disconnect the fuel feed pipe.

The filters are small - about 3/4" dia and 1" long (very approximately). There is a 13/16" x 3/32" O-ring inside the housing that might need replacing. Don't forget to wear protective glasses.

My car is a 74 "series 1", which will be the same as your 75 model.

Geoff
 

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Think about it. If it runs fine when cold, ie with the choke on, it will not be a fuel filter issue. If the float bowls are receiving enough fuel when the choke is on, when the air/fuel ratio is richer, then by default they will receive more than enough when it is warm, choke off, less fuel demand.
Look for weak coil or failing rotor, running fine when cold but worse when warm is a classic symptom of each failing. Substitute coil to test, it does not have to be bolted down and grounded to try, just lay the substitute coil on top of the engine and plug the wires into it to see if it makes any difference. To test the rotor, when warm and it is stumbling, shut off engine. Remove coil high tension lead from distributor cap, remove cap, hold end of coil high tension lead within an eighth of an inch of the centre of the rotor. Have someone crank over the engine, if a spark jumps to the rotor the rotor is dead shorting through the dielectric to the distributor shaft. You can also get a good idea if the coil is weak by noting the colour of the spark from the coil lead to ground, if yellow it is weak, a good coil will show a bright blue spark.
Also, are you sure the mixture is not set excessively lean on the carbs? Is the weakener system working correctly? Remove the float bowl vent hoses and leave them open to the atmosphere to eliminate the weakener system as a possible culprit.
 

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Discussion Starter #17
Jim is there any particular ignition coil you recommend for replacement? is aftermarket ok? Distributor cap and rotor already replaced with Lucas unit. Thanks
 

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Discussion Starter #18 (Edited)
I can get a Bosch ignition coil on amazon quickly. Looks like it should work?

Also, I wanted to report on some further testing I did this morning. While car was idling parked, I gave it some gas. It also stumbled the further I pushed the gas (basically revving the car from idle to almost full throttle). Wouldn't this indicate that in fact maybe the mixture it too lean? Thanks again. I am learning as I go but I want to get to know this car inside and out so your help is greatly appreciated.
 

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Missing when engine is under load is the classic symptom of ignition faults. Coil or rotor or ignition module. If the coil fitted is the original from 1975 it is very, very likely faulty.
You need a coil that is suited for the ignition system fitted. If it is Lucas OPUS you need a coil with 1Ω resistance. If Pertronix you need a coil with .32Ω to 1.5Ω depending on which version of Pertronix used. You haven't said what type of ignition module you have, Is it the stock Lucas OPUS or Pertronix or some other? Regardless of what rotor is in the distributor now, I would get a new #425620 rotor from the Distributor Doctor. http://www.distributordoctor.com/
 

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Discussion Starter #20
I apologize its all Lucas in there. So I am going to go ahead and replace the ignition coil as that seems to be the culprit. It will be here in a couple days. Its cheap part and easy to replace and lets see what happens and I will report back if symptoms have improved. Thanks again.
 
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