Idle Adjustment - 350I Flapper

[ROOODBOY]

Hi folks.

I'll post details in the "What's Going On In Your Garage.." thread later, but basically I've sorted a problem with my engine breathing and now the idle is around 1,300rpm.

So, I found the idle adjustment screw and it bottomed out with the idle still around 1,200rpm.

According to 'The Bible' this screw should be two and a half turns out, which puts my idle around 1,400 rpm.

Anyone have any idea what I can do now...?

Thanks,

Martin.

[adam quantrill]

Maybe there is air getting in through a leak, raising the RPM. Check for leaks.

[honestjohntoo]

ROOODBOY said:

I found the idle adjustment screw and it bottomed out with the idle still around 1,200rpm.

According to 'The Bible' this screw should be two and a half turns out, which puts my idle around 1,400 rpm.

The "2.5 turns out from fully home" recommendation is merely a starting point for the idle CO adjust screw on the AFM - not the idle speed control screw on the plenum chamber - which would typically be expected to be something like 4 to 6 turns out from fully home, for a normally functioning system.

With the idle screw fully home and the idle speed is running at 1200 rpm it does seem that the idle screw is having a modicum of control as you can vary the idle speed with it, but it is being swamped by metered air bypassing the throttle plate in one of two places.

Either:

• Around the throttle plate itself due to a condition known as "Idle Speed Hang-up", the principles of which are discussed here

http://www.vintagemodelairplane.com/pages/Download...

Or

• The EAV is still partially open when the engine is fully warm because the connector at the rear of the unit has become disconnected or the wiring is damaged.

Either way the metered air is asking for extra fuel from the ECU and the idle speed is simply too high.

Yes, it might also be a rogue air leak into the plenum chamber, but when that happens the idle performance is very poor and unstable - not reported thus - and the system does nor normally behave correctly when driving either.

Finally, as this issue manifested itself after fixing a breather problem, you may wish to check the integrity of the whole crankcase ventilation system again.

CRANKCASE VENTILATION

The crankcase ventilation system is an integral part of the air supply system to the engine, but it is often overlooked when diagnosing problems. An air leak or a blocked pipe in the ventilation system will noticeably affect engine performance.



OS Oil separator
F Filter

The system works as follows:

Air is drawn out of the crankcase by depression felt at the pipe connected to the plenum chamber in the butterfly housing. This pipe connects to the front of the right rocker cover via an oil separator (OS) which is fitted to ensure that lubricating oil is not drawn into the engine inlet. As the impure air is being drawn out to be burnt in the combustion chambers, it is replaced by fresh air drawn in through the filter (F) located on the rear of the left rocker cover. The small hole below the filter should be about 0.040" diameter, max.

The volume of air taken into the engine in this way bypasses the air flow meter, and therefore must remain a 'constant' amount to maintain the programmed air/fuel ratio. Any faults that occur within the crankcase ventilation system will affect the running of the engine. These include:

• Air restriction due to blocked filter, oil separator, external pipe etc.
• Excess air due to leaking rocker/inlet manifold gaskets etc.
• Excess piston bypass problem due to worn cylinders/rings etc.
• Incorrect size hole into the LH rocker cover.

Just trying to cover all the bases!

[ROOODBOY]

I did wonder about an air leak, but couldn't find one. In fact I'd just cured one that I didn't realise I had!

Thanks guys, some good info and suggestions there for me to go on.

Cheers,

Martin.

[350zwelgje]

Had this issue (wasn't able to adjust idle on the plenum) and first thing to do was to clean the CO-adjuster on the air flow meter. Cleaned it and put it back. This enabled to adjust idle as I liked!

But as mentioned before, air leaks cause it as well. For example cleaned the plenum and throttle plate a week ago and idle went up as more air could pass. Adjusted idle back down to 950 rpm. I prefer high idle rpm as it keeps the oil pressure a bit up with a warm engine.

Rob