Smooth idle, a reality or a fantasy

[pauljdav]

Hello, My car, recently aquired, runs great, except the idle is not smooth. It is currently set around 850 and it will idle at lower settings. The idle mixture screw does not seem to make much difference.

Last night I swapped the intake manifold (The PO had s dual planed manifold with the 32 ADFA) so I put in a single planed. I also replace the gaskets in the carb, cleaned it up and used compressed air to blow out any orifices.

Anyhow, is it a dream to have a smooth Idle, should I possibly look to a new carb?

All spark plugs looked very good with normal brown coloring.

Let me know if I seek the impossible on a Fiat Spider!

Paul

[So Cal Mark]

when everything is right, your engine should idle perfectly smooth

[bradartigue]

If you have the stock cams then the idle will be nice and smooth. Bigger cams are a bit more lumpy. Anyway, what size motor do you run? 32ADFA is a strong carb for the smaller engines and a fuel efficient carb on the larger ones (meaning its not all that powerful). Since the car is new to you and it sounds like it is running fairly well then I'd leave the carburetor alone until you get to know the car a little better.

[pauljdav]

I have the 1608 on the car. The car is a 1978 and I found out I had a different motor when the water pump I ordered for the 1800 did not fit.

The carb installed is a 32 ADFA and the manifold was the dual plan from the 1608.

Yesterday I put a single plane manifold from an 1800 on and replaced the gaskets in the carb.

The difference I notice today is the power transition between the main and secondary barrel is smoother. The Idle is exactly the same. It does not die, but it is missing at idle speed. I doubt heavily that the cam is modified since it still has the original 4-1 exhaust.

If I were to replace the carb, which direction should I go?
Paul

[So Cal Mark]

why replace the carb? The 32/36 is plenty adequate for a stock 1608. the first thing you need to do is evaluate the engine. Take a compression test to measure cylinder strength, inspect the cams for flat lobes

[mdrburchette]

Have you checked your points?

[bradartigue]

It is unlikely that the problem is your carburetor, unless it is just tuned poorly. It is plenty big for that motor.

You should tune to "spec" then start looking for problems. You may have done this, I don't know. I have the factory+weber procedures laboriously defined in http://www.artigue.com/fiat/Pubs/Artigu ... 124_MM.pdf

Now "to spec" isn't always perfect, but it is a great starting point.

If it idles like crud afterwards then let's start talking about air leaks, bad compression, and other nasty things.

[pauljdav]

I was attempting tuning again, and I turned the idle mix screw all the way in and the car still idled (although even rougher).

I currently have it out 3 turns and around 1000 rpms.

Paul

[bradartigue]

Setting the idle mixture is not just a function of the idle mixture screw; you have to adjust the idle speed set screw and probably the ignition timing as well to get it all right.

Set the ignition to 10 BTDC. If it is an electronic ignition then block off the distributor vacuum at the carburetor while setting the mix.

You turn the speed set screw to get the car around 900 RPM
Turn the mixture screw to the point where the motor runs fastest and smoothese
Turn the speed screw again to get 900 RPM
Do the mixture screw again
Repeat until further refinement is not improving things.

If you are still running quite rough then find some carb cleaner and, with the engine running, spray around the carburetor base and manifold. If the engine speed changes then you have an air leak. If that doesn't do it then do a compression check.

[spkx714]

That's great advice about the carb tuning. Mapp gas, acetelyne, and the regular burns o matic works well to help you find an intake leak, As stated, it the idle goes up you have a leak. This maybe too late but document all of your base settings and rule out issues in the correct order, if you do have any kind of a vacuum leak it would be almost impossible to tune the carb. Good luck with this challenge and please keep us posted.

Steve

[pauljdav]

Well, the problem is not my carb. I had a backup and it was the same. I did get it fairly smooth. I have an electronic ignition that I plan to install, then I will do the compression check. Hopefully the ignition is my issue.

Paul

[So Cal Mark]

unless it misfires on acceleration, it's probably not the ignition