smoke on over run :-(

Post by **Coops** » Sat Dec 05, 2009 10:36 pm

well thought things were going to good,
engine has done over 500 miles now,
and decided to open it up a bit,
thing is, if you you rev her up through the rev range, then shut the throttle down and let the engine do the braking you get a lot of smoke out the back, and i mean a lot,
while driving and accelerating no smoke,
only on the overun shut down,
any ideas?

theres no smoke on start up from cold or hot,

CastleMGBV8 · Post by **CastleMGBV8** » Sat Dec 05, 2009 10:51 pm

Tony,

Sounds like you are sucking oil either into the the induction system or down the inlet valves under high vacuum, have you got an efficient PCV system installed?

If only happenning on a closed throttle it's got to be one or both of these.

Also not overfilled with oil is it as occurs if you have the wrong length dipstick?

kevin

Post by **Coops** » Sat Dec 05, 2009 10:58 pm

same as per the 3.9 set up i had mate (sump dipstick intake etc)

was quite scary seeing the smoke behind,

wonder why its doing it on this eninge, i know its a higher comp engine,
i wonder if i open the throttle idle screw a bit more and turn the stepper down a bit so its a bit less of a vacuum,?

i have the small round foam filter on the passengers rocker cover,
the large round wire mesh filled breather on the drivers rocker that goes to the breather pipe stub on the twin plenum,

BUT the twin p has been modified to use a stepper instead of the over run valve, could this be causing a prob? if so how can we over come this~/

Post by **Coops** » Sat Dec 05, 2009 11:04 pm

just to let you know,
this setup with the stepper and not overrun valve was used on the 3.9 also

CastleMGBV8 · Post by **CastleMGBV8** » Sat Dec 05, 2009 11:30 pm

Tony,

The little foam filters are a bit on the small size and i would put a PCV Valve in the hose between the flame trap and the plenum which is what it should have anyway.

You are running standard rockers aren't you and not roller rockers?

If you get excess oiling to the valve chest areas it can get sucked down the inlets valve stems at high vacuum.

kevin.

ian.stewart · Post by **ian.stewart** » Sat Dec 05, 2009 11:33 pm

Could be overfilled,
what oil are you using, syn can cuse ring bedding in problems,

Post by **Coops** » Sat Dec 05, 2009 11:36 pm

not over filled mate,
oil is comma 15W40 nothing fancy just normal mineral oil.

ramon alban · Post by **ramon alban** » Sun Dec 06, 2009 12:13 am

thev8kid wrote:BUT the twin p has been modified to use a stepper instead of the over run valve, could this be causing a prob? if so how can we over come this~/

Hello Tony, YES!

Irrespective of the source of your burning oil on overrun, the original overrun valve nullified any grossly excess vacuum.

This extract from my full scientific analysis of various overrun phenomenons, explains how/why that overrun valve came about.

Rover and Lucas' Solution

Rover and Lucas' solution to this phenomenon (sic - poor combustion on overrun) was to introduce a valve that responds to the high vacuum generated in the inlet manifold to allow in a more generous quantity of metered air from the air rail which in turn operates the AFM flap resulting in the signal to the ECU to inject an appropriate amount of fuel. A secondary effect of the opening valve is to reduce the pressure differential across the main injectors nullifying the excess sucking forces on them.

My full scientific analysis is here:

http://www.vintagemodelairplane.com/pag ... run02.html

As to finding a fix:

What about reintroducing another excess vacuum release valve but of lesser capacity than the original.

OR

Revert to the original Rover/Lucas solution.

JSF55 · Post by **JSF55** » Sun Dec 06, 2009 12:16 am

It might be the straight hose from the rocker to the plenum? is to large and your sucking too much

try and fit a restrictor or the correct PCV. Had the same problem on the pop when i was running efi and left the baffle of the top of the rockers, you have got them on ?

JP. · Post by **JP.** » Sun Dec 06, 2009 8:38 am

Are the baffle plates still in place in the rockercovers ???

Do you use an oil catchcan between the mesh filled breather on the drivers rocker & breather pipe stub on the twin plenum.

mgbv8 · Post by **mgbv8** » Sun Dec 06, 2009 9:14 am

Are you sure it isnt just a valve stem oil seal popped off, or that you have too much oil flow to the rockers??

Post by **Coops** » Sun Dec 06, 2009 10:08 am

on the hot wire set up there is a t piece in the breather,
it goes from the mesh filled breather to the trumpet base via a small hose,
and the other side goes to the plenum breather stub just before the throttle plates,
this was not used on the 3.9 whne we had the tp fitted as there is no pipe for this on the tp trumpet base.

Would this help if i fitted a small bore pipe and reintroduced the t piece?

spend · Post by **spend** » Sun Dec 06, 2009 11:24 am

Have you investigated the internal state of the flame trap, pipes and trumpet base. If the crankcase ventilation is open they get fouled up pretty quickly.

Connecting a vacuum gauge to the plenum and to the oil filler tube will give you an idea of differential pressures (crankcase vs manifold) whilst you simulate over-run in the garage. You can also pull plugs immediately to see if it is even across pots.?

Ian Anderson · Post by **Ian Anderson** » Sun Dec 06, 2009 6:16 pm

Have you checked your brake fluid level?

I remember you working aload on the vac booster

If the seals don't work it is possible to get brake fluid drawn past the seals and then sucked into the plenum and burnt - of sourse this would happen at the highest vacuum - ie massive overrun

Ian

CastleMGBV8 · Post by **CastleMGBV8** » Sun Dec 06, 2009 6:31 pm

Ian,

Good point and easily overlooked until you have no brakes!

Kevin.