Itinerant Air-Cooled

Hi all,
I have a 79 bay with the FI engine. I changed all the fuel lines and got it going again after 2 or 3 years. It starts up well, idles high when cold and settles down when warm as it should. I had the CO tested and it is at 0.18%, so I checked the plug colours and the insulators were pure white. Is the any way that I can enrichen the mixture as I am scared of overheating.
I checked the mixture screw and it is only 1/3 turn from the bottom so I think something else is wrong... It drives well without hesitation.
Has anyone any suggestions?
Thanks..

Hello there, and welcome to IAC!

Plug readings are best done after a warm and long run through typical driving conditions. I take mine for 10-20 miles on the freeway, then pull over and shut it down immediately. I let it cool off for a few hours, and then pull the plugs. This way, there's no excessive idling to give you false readings.

Did they read the air-to-fuel ratio when you had the CO tested? Or the hydrocarbon count?

Does your '79 bus have the "California style" oxygen sensor plugged in to the left side of the exhaust system? Or is it a non-sensor model, sometimes called "federal style?

A well-driving bus means you're definitely close, but we can't make any assumptions without knowing more; good luck!
Robbie

Hi Robbie,
Thanks for the warm welcome to IAC.. Glad to be part of the forum, it is a great source of information..

The plug readings were taken after a fairly good run of around 15 miles of mixed traffic, but I share your opinion of shutting down immediately and not having it idling for long to modify the signs on the plugs.

The bus was originally from California but hasn't got an 02 sensor. The test revealed the CO reading of 0.18% along with HC of 79PPM. I don't read an awful lot into the readings but trust plug colour more, but I do take the CO readings as a second opinion.

Previously, the bus was running well, nice light tan plug colour and CO readings of around 2.5% and no issues.

I am just finding it somewhat strange that the mixture screw is nearly against its stop and the mixture is still so weak..

Thanks,

Stephen.

Hello and welcome.

That mixture screw you are referring to is just for idle adjustments, so I wouldn't worry about it right now since you describe your cold start warmup as running fine already.

Before you go adjusting mixtures, a Cylinder Head Temp gauge is a must. Specifically the one made by Dakota Digital and installed under the #3 spark plug ring, so you can compare numbers with everyone else as needed. Colin has an excellent writeup on adjusting it here on the IAC forums.

http://www.itinerant-air-cooled.com/vie ... php?t=7761

Mark where you start so you can go back if you need to, esp if you don't have a gauge when you go about it.

Hi Stephen,

I think you've got a good brain on your shoulders, so I'll ask YOU the questions and you can find out the answers yourself…… :)

As Chris noted, the mixture screw on your AFM is for adjusting idle mixture only, it has little to no effect off idle. It is, as a matter of fact, an "air bypass" screw, meaning it's just a slight air leak to combine with the fuel that is being injected. So if your screw is all the way in, is it at the richest or leanest setting?

What do you mean your mixture is "weak?" That's not a compatible diagnostic term. Rich? Lean? How do you KNOW, without knowing the air-fuel ratio?

And the kicker here:
Why do you think your idle mixture screw bottomed out? I have a theory, as it plagues more than half the fuel-injected buses I look at.

Robbie

1) all 4 plugs should be even color between themselves.
2) white insulators can also be an indication that the engine is inhaling a lot of water. Does this engine have water vapor injection on it?
3) The best way to check mixture is with an air to fuel meter across several different RPM ranges.
4) The CO test is often done at idle. If the test was done at RPM PRE- CATALYTIC CONVERTER, a 1.8% CO test result at RPM under load would imply an air to fuel ratio of about 13.82 to 1. That is not lean although it is a bit lean for a bus at sustained freeway speeds. The California 1979 bus with an O2 sensor would run about 14.7 to 1 at freeway speeds, which is leaner still.

You cannot tell what the proper setting is without the correct meters. Stop trying to read plugs to fine tune. That is not possible with today's gasoline blends. It can only be used for a ballpark and not a final setting. There is a lot more to reading plugs too than just the overall color.

Hi all,
I understand that the mixture screw is only for idle mixtures and that doesn't affect the mixtures overly at freeway rpms... I also appreciate that a vacuum leak certainly would affect the mixtures.. I will be going through all of the hoses in the morning and plug them off one by one to check for improvements.
I am sort of used to checking plug colour and have done so for many years, and whilst it may not be fine tuning, it would show the difference between an engine that is running safely and one that is about to drop its valve seats out..
Every day is a school day and through discussing with you guys and also reading the writeup about AFMs, I have certainly learnt a lot, and I thank you all for that..
I'll keep you all posted as to the outcome..
Ta ta for now..
Stephen.

not tryin to question your experience. So much has changed since plug reading was in vogue. Example - what is the effect of alcohol on the color of a plug? What about alcohol that is carrying an abundant amount of moisture it has absorbed? How does that compare to the older fuels of the 1970's and early 1980's before the change of additives that gave color and texture? What effect does the addition of inert EGR gasses have on mixture color?

tinkerer wrote: ↑

Thu Apr 27, 2017 5:14 pm

I understand that the mixture screw is only for idle mixtures and that doesn't affect the mixtures overly at freeway rpms... I also appreciate that a vacuum leak certainly would affect the mixtures..
Ta ta for now..
Stephen.

Based on your report, leave that engine alone! It is fine.

White insulators means clean valves and little carbon build-up. You are happy and you don't yet realize it.

The danger sign is white powder on the outside shell.
Colin