Rough running 1990 RR 3.9 EFi

Evening all

The saga of the Green RR continues. I have abandoned solving the rumbling/whining noises from the front end in favour of this (new) problem:

Idles OK-ish at about 750-800. As the engine speed increases it's apparent that the engine is running less smoothly than usual, a sort of 'roughness about the edges'. As the speed increases the roughness becomes more pronounced and can be felt through various attached items e.g. transfer box selector. It reaches a peak at around 2,000 RPM, when it could almost be described as a vibration, and then gradually decreases.

It feels a bit like a gentle - as in quiet - misfire.

I wondered if the ignition advance had died but Mr Gunson's Tachostrobe indicates that there is some advance in the ignition timing as I rev the engine. I'm not convinced that the vacuum advance is working cos I can suck on the vacuum tube without effort. I was tempted to remove the transparent cover from under the rotor arm to look at the vacuum advance. Is it worth it?

Incidentally what should the timing be set to?

Also can I, if necessary, use the vacuum advance from a 1988 3.5 EFi RR?

TIA

Richard

Reply to
Richard
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Vacuum capsule is shot. Remove dizzy cap and try turning rotor arm by hand - if it doesn't return ALL THE WAY under its own spring tension, your dizzy needs to be overhauled. Vacuum capsule can be changed in-situ, awkward but possible. Some are torx screws, earlier ones are phillips, knack is locating the capsule over it's drive peg......

6 degrees before TDC, no vacuum.

Yes, no issues there. have a play with that one first to see what I mean re. locating the capsule's operating lever over the peg on the baseplate. Change the capsule before setting the timing.

Badger

Reply to
Badger

I would suspect a worn cam lobe or two. A compression test is probably in order.

Reply to
EMB

Mm! What I neglected to mention in my haste to post the question was that these symptoms appeared on the way to work about 2 weeks ago. Having had a hopeful stare under the bonnet I noticed that the hose to the fresh air flap vacuum reservoir had parted company at the reservoir and the roughness did not continue when I left that evening.

If it is anything more than a very simple fix, this will hasten it's disposal.

Any suggestions for a replacement? Must be capable of passing below a

1.87m height restriction.

Richard

Reply to
Richard

Thanks

I know that the vacuum capsule on the 3.5 is Torx screws 'cos I Loctited them in last time they came loose ;-)

I'll have a tinker over the weekend.

As you will see in my reply to EMB I was slightly economical regarding the story. Apologies etc, but I don't think it would influence your thoughts - I hope.

Richard

Reply to
Richard

First thing I'd check is the plug leads. And also check their routing - they can crossfire.

Reply to
Dave Plowman (News)

My RRC did this. Thought it was the loose dizzy, tighten it up and all was fine until i used the windscreen washers. This annoying mis fire/rumble/call it what you want came back. Found the cause in the end....the pipe from washer bottle to Y junction was split and each time i washed the windscreen it also washed the right hand side plugs as well!!!!.

Can't you make the height barriers taller Richard??!!

Dom

Reply to
Dom J

Morning Dom,

Dizzy tight and now fitted with working vacuum advance ex the blue RRC. Interesting selection of plugs fitted, of which #6 was a balck as sin and having no effect on the engine (possible Hurrah!). Top of pot 6 also black and perhaps oily, possibly fewer hurrahs. All other plugs look fine, swopped #1 and #6 plugs and roughness has gone. Will peer into pot 6 to see if the black has gone. If not, what do you suggest? Redex or ignore it??

TTFN

Richard

Reply to
Richard

Check the lead resistance from inside the dizzy cap to the plug cap, carbon filament HT leads are the devils gonads IMO just another thing to check and discount as being the cause. I did check Peggy's ( EFi in the heady days when I could afford

Reply to
Derek

These 'carbon string' leads have a resistance proportional to the length. IIRC, about 10k ohms per foot. The total resistance including carbon brush and king lead should be of the order of 30-50k.

Reply to
Dave Plowman (News)

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