Uneven idle

[DarwinT]

I did some work on my '71 Midget. It had a lack of power at lower RPMs and vibration while accelerating in lower RPMS.

I put new points on it and timed it.

It has a Weber carb, and the vacuum advance from the distributor was hooked to the intake manifold, and the vacuum advance port on the carb was not plugged.

I moved the vacuum line to the carb and put a vacuum gauge on the line from the manifold.

I also put a Harley muffler on it.

It idled smoothly before, but now the idle speed varies quite a bit. It isn't a regular variation, it is erratic.

I drove it and it runs very well, except when I take my foot off the pedal and push in the clutch, sometimes the engine would die.

When I got back from my drive I tried adjusting the mixture screw on the carb, but couldn't hear clear differences of idle speed, mainly because the idle speed varied so much.

Here's a video that will let you hear what's going on. The true idle speed is twice what the meter shows.

https://s2.photobucket.com/albums/y2/darwin-t/?action=view&current=idle.flv

I used a propane torch (unlit of course) to look for a vacuum leak and did not find any.

 

[Midget78]

I also have a Weber and one of the issues I had was the throttle linkage had a tendancy to bind (The choke also). Sometimes the choke would be shut off yet the cable wouldnt close it all the way. Needed a little lubrication and adjustment. Id look at it and it would be fine then every now and then it would stick. I ended up re-routing the throttle & choke cables and adjusting the throttle lever nut. There is a fine line between too tight and not enough at that point. Make sure you dont have any vacuum leaks around the carb or through the linkage (being worn). The carb spray trick while its running works great to find the smallest issue. Im not to sure about the torch.
Do you know how far your mixture screw is turned out (rotations)? With the engine at normal operating temp where do you have your rpm's set? 800 or so?

 

[jhorton3]

Take a can of WD40 or carb cleaner and spray around the carb and intake rather than the torch to check for air leaks. Check that the intake is secure on the block. Have you tried putting the the distributor advance tube back on the intake vacuum port and plugged the carb vacuum port? I watched your video and noticed the diameter of the exhaust exit looks really small. Could it be that there's not enough of an opening and you've got too much back pressure building up?

 

[DarwinT]

I double checked the linkage and cables - all seemed to be fine. It has a manual choke, so that makes things simpler.

I'll check the mixture screw - I forgot when I was out there and it'll probably be tomorrow morning before I get a chance to do it.

I did the WD40, and if there is a vacuum leak, I can't find it.

I don't want to switch the vacuum hose back to the manifold. When it's hooked there, the timing is advanced at idle, and then not when accelerating. On the carb port, there is no vacuum UNTIL you accelerate - I checked it with the vacuum gauge to make sure. The former owner's placement of it was part of my problem.

As for the exhaust opening, it's a Harley muffler. The standard exhaust on my Midget is 1 1/2". The intake on the muffler is 1 3/4" and the opening in the back of the muffler looks like it's 1 1/2" or pretty close to it.

I timed it again - static timed it this time and tried adjusting the carb again - it seems to be somewhat better, but not there yet.

When I had my neighbor's timing light before, the mark was steady - it didn't wonder around.

I had put a new electric fuel pump on it,too. It pumps enough fuel because I was driving it at pretty high revs today. It is rated for 2 - 3.5 lbs pressure, which shouldn't be too much.

I'm thinking maybe some dirt got into the carb.

 

[DarwinT]

Do you know how far your mixture screw is turned out (rotations)? With the engine at normal operating temp where do you have your rpm's set? 800 or so?

1 3/4 turns.

I don't know what the idle was set at when I made the video, but will set it to around 800 when I get it going.

 

[Jim_Gruber]

12 Flats is the normal / standard to get you close I do believe.