Help needed, rough idle problem

MikeMelb

MikeMelb

Active Member
Help please, hoping the collective wisdom can resolve the problem.

Our P6B started to idle a little roughly a couple of months ago.

New leads and plugs had a slight positive effect.

Clamping the vacuum hose from the manifold to the brake power booster smoothed the idle significantly so a vacuum leak in the booster was suspected.

Through Mike Coombs a reconditioned booster (ex Dr Flynn) was obtained and fitted on Monday, again a slight improvement to idle whilst in Park or Neutral, but it becomes quite rough when in “D” or “R”. (It is possible that the recond. Booster may have a leaking diaphragm as well)

The vacuum pipe to the distributor is fine, there appears to be no leak around the inlet manifold.

Over the past two years the BW 35 has been rebuilt, engine and transmission mounts replaced and new exhaust, from manifolds back, fitted.

If I think about it carefully the roughness may have co-incided with the fitting of a new reaction valve to the brake booster and that valve (because it was new) was transferred to the reconditioned booster.

Off idle the car runs and performs extremely well, took it on a 170km run to Arthur’s Seat – holding it in “2” for a bit yesterday and it returned 10.4 litres/100km. It runs on the cool side of the temp gauge and does not lose any coolant.


Any suggestions would be greatly appreciated.

Thanks

Mike K
 
Remember a blocked breather can have a negative effect as well as a leak. Both will disrupt the mixture balance at idle.
 
sounds like a vacuum leak on intake side.I use a very bubbly water spray and wet around gaskets.pipes etc when its running. it will tend to 'suck' in the bubbles puts smooth idling if we hit a leak point. do check dizzy cap too. as potential for wear or tracking of contacts in cap and rotor arm internally. a strobe gun on timing marks can often see if we have worn dizzy etc as will wander around in line with surge /hunting. other possibility would be weak mixture.
 
Thanks guys, liquid was used round the manifold, dizzy cap and rotor look fine, though there is a slight unevenness on the edges of the Lumenition chopper.
Will be meeting with mechanic tomorrow and will wash out the breather lines.

In the old days would have trickled Redex in but it seems to be no longer available in Oz
 
Thanks for all the input

Have just been with the mechanic, seems that the “Flynn reconditioned” booster which still is sticking on and also losing vacuum is probably no better, and perhaps worse than the one we replaced (which was a changeover in 2000), so in the new year both boosters will be sent off to a brake specialist, in the hope that at least one of the master cylinders has already been resleeved, for reconditioning.

Have a safe and trouble free holiday period
 
Just had an email from Mike Coombs offering to refund what I’d paid him for the Flynn booster.
He’s obviously a true gentleman, but he acted in good faith and certainly did not misrepresent the item so I certainly can’t accept his very kind offer.

Mike K
 
Update

I owe Mike Coombs and Dr Flynn's brake man an apology!

If the car was driven daily I may have worked out the connection earlier...............

Chronology.. had the exhaust redone around mid 2016, soon after noticed the top of the Air valve/reaction valve on the booster had broken so tried to locate a replacement. Got onto some-one who told me there was a Jag unit that was common to the Rover, so I bought and installed it a Jag 420 Reaction Valve kit.

Subsequently thought the car was idling rough so had the carbs checked (ok) replaced the leads and plugs and still had the problem which was diagnosed as a vacuum leak in the booster. That's when I started to search for a replacement booster which could be sent for reconditioning.

Then Mike supplied the reconditioned unit from the Flynn collection. However as the air valve looked a little old and the housing was corroded we replaced them with my new one on fitting the unit. The idle became worse and the vacuum leak was so bad that power assistance disappeared after a couple of applications of the brakes.
So the decision was taken to send both boosters to the specialist.
However on Monday I suggested that rather than leave the car brakeless at the mechanics workshop we send off just the original booster and leave the Flynn one on the car so I could drive it home, but we did swap back the air/reaction valve so the "new" one could accompany the booster to the specialist.

Lo and behold, with the Flynn valve back on the Flynn booster the booster behaved perfectly, there is no vacuum loss and the idle is back to normal.

Moral of the story: whilst the Jag 420 kit air valve/reaction valve cover and diaphragm are the same as the Rover, I assume the plastic plunger valve must be different and that is what caused the problem.

I guess I would have preferred not to have learned this lesson but pass it on to others for reference.
No doubt the parts supply in the UK is better than in OZ so you don't need to search out alternate replacements.

Mike
 
You must log in or register to reply here.
Back
Top