Tuning Car won't go over 70 km/h

[W Bakker]

Hey all,

As stated in the title my car wont go over 70 km/h. As soon as I hit 70 it starts bucking as if it's running very lean. Is it possible this speed is where the carb switches jets? Possibly to a clogged? It's a 650cc with the weber 28imb carb. the car uns fine below 70.

 

[smart51]

The carb switches from the idle circuit to the main jet based on throttle position not road speed. I wonder if spark advance might be your problem.

When the throttle plate is closed, the piston tries to draw air in to the engine but only creates a partial vacuum instead. This draws in fuel from the idle circuit. As the throttle opens, the pressure drop is less, but the transition holes are uncovered to let in more fuel. With the throttle open, there is little pressure drop under the throttle plate so little fuel is drawn in. Instead, there is a pressure drop in the venturi caused by the speed of the incoming air. This draws fuel through the main jet.

I don't recommend doing this normally, but what happens if you drive your car in 3rd gear from, say 30 km/h to 70 km/h? Does it run alright, or do you get the engine problem, maybe at 45 km/h? Does the problem only happen in 4th gear, and only at 70 km/h?

 

[W Bakker]

The carb switches from the idle circuit to the main jet based on throttle position not road speed. I wonder if spark advance might be your problem.

When the throttle plate is closed, the piston tries to draw air in to the engine but only creates a partial vacuum instead. This draws in fuel from the idle circuit. As the throttle opens, the pressure drop is less, but the transition holes are uncovered to let in more fuel. With the throttle open, there is little pressure drop under the throttle plate so little fuel is drawn in. Instead, there is a pressure drop in the venturi caused by the speed of the incoming air. This draws fuel through the main jet.

I don't recommend doing this normally, but what happens if you drive your car in 3rd gear from, say 30 km/h to 70 km/h? Does it run alright, or do you get the engine problem, maybe at 45 km/h? Does the problem only happen in 4th gear, and only at 70 km/h?

Just went for a quick spin and it does indeed also do it in third at a lower speed. So it is related to engine speed. If this indeed sounds like spark advance. I honestly have no clue on timing etc, how whould I go about changing the spark advance?

 

[smart51]

Spark advance is done automatically. There are weights inside the distributor which move with centrifugal force. The faster the distributor spins, the further out the weights move and they turn the distributor shaft forwards as they go. This makes the spark happen sooner.

At the top of this picture you can see a spring. It connects to a black metal weight to the left, which is this picture has some white paint on it. This is one of the weights. There are two, one either side. You could check to see if they are moving freely. Mine seemed to move freely but I still had a problem.

In the centre, there is a piece of felt in a tube. Routine maintenance requires a little engine oil on there. It can't hurt to add some.

 

[W Bakker]

How should the weights move? The weights move in and out from each other, but they don't rotate if they are supposed to. I did add some engine oil to the little piece of felt. The gap between my contact points also seems a little wide. Could that be causing my problem? And if so, what should the gap be?

 

[smart51]

The weights are hinged on a pin at one end. The other end moves out towards the case. The movement is very small at 18° of arc.

The points gap should be between 0.47mm and 0.55mm. I have found it tricky to set, as tightening the screw makes the gap bigger. I set mine a with a feeler gauge that is a little under size, so that tightening the screw sets it to the correct range. I usually have to do it a couple of times to get it right.

 

[W Bakker]

Alright, the movement does seem to be correct then. I'll try changing the gap on the contact points. Thanks!

 

[dangfiat]

Do you have a timing light with advance? You could get the car warmed up, check timing at idle (10 deg BTDC). Then adjust the idle screw to a high (3-4k rpm) and check timing again (which I think is 27deg, but will let the experts here comment on)

Should tell you if your dizzy is performing as it should

 

[bugsymike]

Hey all,

As stated in the title my car wont go over 70 km/h. As soon as I hit 70 it starts bucking as if it's running very lean. Is it possible this speed is where the carb switches jets? Possibly to a clogged? It's a 650cc with the weber 28imb carb. the car uns fine below 70.

Is it possible you are not getting enough fuel delivery to the carb at those revs assuming jetting is correct and no blockage.
What is it like pulling hard up a hill in lower gear at similar revs?
Unless you have some fancy ignition with a built in rev limiter, I would think a timing issue would gradually run out of power and revs at the limit rather than the "bucking" you describe when you reach that point.
Unless some part of the ignition system like a coil is breaking down at that point, or the contact points spring is so weak it is causing the points to bounce, though not common.

 

[W Bakker]

I've just taken off the top of the carb. The fuel level in the float chamber seems low. Is it possible adjusting the float fixes my issue? If so, is there a guide on doing so somewhere on the forum? I have no clue how to go about checking/adjusting the float.

 

[fiat500]

Hey all,

As stated in the title my car wont go over 70 km/h. As soon as I hit 70 it starts bucking as if it's running very lean. Is it possible this speed is where the carb switches jets? Possibly to a clogged? It's a 650cc with the weber 28imb carb. the car uns fine below 70.

When I experience this issue it was because the fuel pump wasn't delivering enough fuel. It was very noticeable if I was going fast and went over a big undulation in the road. Everyone talks abbout that exact protrusion above its spacer and gasket of the fuel pump actuator-rod. I'm repeating myself from previous posts, but I have found that there are many variations in pump manufacturer and not all of them are exactly dimensioned as the original specification, so setting things up according to the Fiat manual may not be appropriate. In my case I fitted a different pump that seemed to have the operating lever sitting closer to the flange of the pump. After fitting that the problem was resolved.

Relating to this, also check that the pump is secure as being slightly loose could cause this problem.