Engine Missing at only Highway Speeds...

[Creatrixx]

My engine seems to be missing only at highway speeds. It was doing this before we swapped the head, it started on our way back from the Pittsburgh vintage grand prix, so it had been doing a lot of highway travel just before it started. Any ideas what could be causing this?

Seriously, I just want to be able to drive the [censored] car for a week without issue.

 

[dklawson]

Seriously, I just want to be able to drive the car for a week without issue.

Surely not! Whenever someone looks at my LBC and starts asking questions like they want to buy one I hit them with "Oh, do you like working on your own cars? You know there are only a few shops around who work on these."

What symptoms go along with the high-speed miss? Does it happen immediately or does the car have to get hot for this to happen? How bad is it? Have you noticed anything else that happens before the miss starts? When it does start missing are the conditions always the same?

 

[aeronca65t]

Missing at high speed is often related to fuel starvation or sometimes, weak ignition.

Always look at the cheap and easy things first:

See if your gas cap is properly vented (modern cars have non-vented caps)....if you've got the wrong cap, the fuel pump will have to work against a vacuum after running for a while. Loosen the cap and run the car. If it runs better when loose, you've solved the problem.

Change your fuel filter(s). You may have two...one near the tank and one near the carb.

Remove the top of the carb float chamber (if you have an SU HS-type carb) and clean it out. While you're at it, remove the piston chamber and use low pressure air to blow back through the main jet discharge (this is the hole the tapered piston needle goes into). Do this while the float chamber is apart (this allows any junk to be blown out).

Start the car in a very dark place at night and let it idle. Get a plant mister or spray bottle filled with plain water and mist it all over the engine. This won't hurt anything but it will reveal weak ignition insulation. If you see sparks jumping, you've located an area where the igniton isulation is weak (maybe the plug wire, cap or coil).

There are many more things that could cause your problem, but the stuff above is cheap or free and fairly easy to do.

 

[Creatrixx]

Doug;

It has only happened twice and it was both of the last times that I drove it. Both times it had been on the highway for a little bit before it started, but it was hot before we got onto the highway. We didn't notice anything change before it started. It does seem to be missing bad enough to cause concern. We had really only tried richening up the mixture previously. That didn't help.

aeronca65t;

Thanks for all of that! I can't loosen my gas cap, the last owners put one of the locking ones on, but since it's a new problem, I don't think that's the issue anyway. The rest of it sounds great, though. Hopefully my boyfriend is up for another weekend of helping me on my car (since I am definitely not touching the carburetor myself).

~Melissa

 

[dklawson]

I was trying to narrow things down a bit before making any suggestions. Nial has given you a very complete list of things to look through.

Go completely through the ignition system before tinkering with the carbs. Dress the points(or fit new ones) and carefully set the gap. Check your ignition timing and make sure it's correct. Check the color of the spark at the plugs. If it's orange, make sure you haven't put resistor plugs on the end of resistor wires (only one should have a lot of resistance). If the spark is orange and the correct plugs and wires are installed, replace the condenser. Make sure the cap, rotor, and wires are all in good shape. Check and change things one at a time as you find them. Do not shotgun this and change everything at once. You'll never learn the source of the problem if you do everything at one time.

Ignition coils can cause misfires when they are hot and failing. However, they are often blamed for problems they don't cause and replaced when they are OK. Didn't you say your boyfriend just bought (or was buying) a Spridget? I was thinking that with the temperature problems your car has had... IF the problem only showed up after things were hot you might want to try a different ignition coil. Borrowing the one off your boyfriend's Spridget would probably work but make sure that you know before swapping if this is a ballast or non-ballast ignition system. They use different coils. Write back if you don't know, it's easy to determine with a multimeter.

Once you're comfortable with the ignition system, then move onto the fuel system and/or carb adjustments. All the ignition adjustments and checks should be right before you move onto adjusting carbs.

 

[Creatrixx]

Thanks! We've got lots to work with now. I'll keep posting as things move forward...

 

[Creatrixx]

Yesterday we did notice that the engine misses while warming up as well, but it seems that keeping the choke pulled for a short while fixes the issue (just while it's warming up).

 

[ThomP]

Fuel starvation is often the cause of this. Crud in the fuel system will do this by blocking the fuel out of the gas tank. I had to rebuild the tank on both my Mini and my A30 to repair the engine miss problems.

Additionally, Mini gas tanks have a vent line, the gas cap is not vented. The vent line is located at the top of the tank and exits through the bottom of the boot. If a critter decided to build a home in your vent line, it would be plugged and prevent fuel from leaving the tank.

As a word of encouragement these problems only happen once every 20 years or so, so, once you have fixed them, you'll eventually have a reliable car!

 

[Creatrixx]

As a word of encouragement these problems only happen once every 20 years or so, so, once you have fixed them, you'll eventually have a reliable car!

Dear lord I hope so. I am thinking that I'm going to sell the mini in early 2008 and buy a Smart, though. My current "other car" (which I'm trying to sell) is a Mazda3 and it just feels huge to me; the mini actually feels like I'm not toting around a ton of extra car. But a Smart ForTwo... that would be tiny *and* not an old LBC!

We'll see!

~Melissa