Mission Impossible : Idle speed adjustment

[cooter131]

Hello friends,

As a great fan of the carbs and especially the twin IDFs I decided to spend some time trying to fine adjust them.
The entire project began when I noticed that when the engine (and oil) warms up the idle speed drops significantly.
(Not to mention the case where the radiator fan starts up!)
I learned to live with that as I had no choke installed.

The setup includes two hoses coming from the oil-vapour separator using a 'Y' divider. These hoses drive the vapour down to the carbs inlet.

It would normally work fine as long as oil temp remains low. After the oil temp reaches 70'C or more (according to the gauge) things change. What I noticed is that the vapour contains much more oil spray and thus the gas mixture becomes less inflamable. Result: idle sppeed drops.

Oppositely to water temp that tends to remain stable, oil temp might increase if driving hard. (no oil radiator installed, and weather in Greece can become really hot).
This makes oil more capable of running easily through the hoses and enter the carbs.

After many efforts I realised that the only way to solve this is to remove the reurning hoses from the carbs.

Have you faced the same problems, or is it maybe that my engine needs a rebuilt to get rid of this very heavy oil & vapour mixture coming out of the hoses?


Thanks,

George

[baltobernie]

Hello George,

Your post is interesting. Have you detached the vapor hoses temporarily from the carburetors to test your theory that the hot vapors are responsible for the RPM drop?

[cooter131]

Yes, I did that and I immediately had a RPM increase.

According to the gauge the idle speed increased from 850 to nearly 1100.
Then with the hoses back on I got again 850 rpm.

[TulsaSpider]

850 RPM sounds fine to me... are there any other problems? Have you done a compression check?

[mdrburchette]

I'm running the oil separator to the turtleback air cleaner on one of my cars and not running the hose to my carbs on my other car. Do you have a picture of where you're running the hoses?

[sptcoupe]

Also, be sure the idle speed is not dropping when the fan comes on - which is normal.

IDFs will also idle higher right after you open the throttle, as the gas is higher on the emulsion tubes until it runs a bit, then the idle will drop. Your idle speed mixture will also effect your idle speed if it is either too rich or too lean. Idle will be inconsistent if one or more ofthe bores is either too rich or lean at idle - sometimes running smooth, then for no apparent reason, running rough (usually lower).

Too high (or low) gas pressure will do the same thing. IDFs need to at 2.5 or 3.0 psi, even on highly modified engines. Your stock fuel pump will deliver too much, and a pump from a FI car will is much worse.

Also, poor balancing will cause a a whole bunch of idle and other issues. Be sure you eliminate this source of problems.

Start by letting a warm engine run a bit after starting, without moving the throttle plates. with the car runnig around 8-- - 1000 roms, carefully balance the bores of each carb using the air by-pass screws, adjusting to the highest drawing bore on each carb. Then carefully synchronize the carbs by using the inter-carb adjustment linkage. All the bores should be drawing the same, and you may have to go back and re-adjust the bore with the air by-pass screws several times. Balancing and synchronizing the carbs is a critical part of the set up process, so take the time to get them as close to the same as possible, as you will forever be chasing seemingly unrelated problems that are really caused by small tuning errors.

Then adjust the idle mixtures and idle speed so that each bore is running at its strongest mixture, and the idle is about 800-900. Recheck that the carbs are still synch'd, especially if your throttle plate bearings are worn a bit.

Not as complicated as it sounds, and is basically a two beer process. Unless it is Denise, who got silly on the gas fumes when we were teaching her how to do this over the phone!

[mdrburchette]

Not as complicated as it sounds, and is basically a two beer process. Unless it is Denise, who got silly on the gas fumes when we were teaching her how to do this over the phone!

Yeah, make sure you point that crankcase hose away from your face while tuning carbs.

[cooter131]

Thank you all for the advice.

So far I have checked that the carbs are pretty much balanced.
It is true that after opening the throtlle a bit (or a bit more sometimes ) the rpm take a few moments until they finally settle down at the determined speed.

And yes, I have noticed that the radiator's fan operation causes some rpm drop as well.

Unfortunately the previous owner probably liked the view of the carbs as much as I do so he discarded the turtleback and thus the smelly crankcase hose is a problem. In fact the smell is the problem and not the hose itself.

Eitherway, best results are achieved when the vapour is driven away from the carbs inlet.

I will definitely try to fine tune the engine over the weekend, and also try to take some photos of the beast's heart!