High Speed Misfiring

[Hegg]

I've got a Federal +2 (Strombergs) that I've been having a bit of an annoyance with lately.

When the engine is at a higher speed -- above 3500ish I'd say -- it seems to occasionally misfire. I can feel the car just studder/shake a little occasionally. It's not consistently misfiring, but does it whether under hard accelleration or just maintaining highway speed.

I've tried a few things such as replacing the coil, messing with points gap, and playing with the timing, but it's intermittent and I can't get it to happen other than driving at 3500+ rpms, so it's hard to troubleshoot while in my garage. The coil, timing, and points gap didn't seem to change the issue.

I just read the post on spark plugs and I think I'll stop by on the way home and try a different set. I'm not sure what I'm running now, but I think I've got the BP7ES in there if I remember right.

What else might I check? Most ignition components are new -- plugs, wires, cap, rotor, points, condensor. The carbs have been rebuilt as well. Like I mentioned, I swapped coils and it didn't make a difference.

How can I diagnose effectively if it's a fuel delivery problem or an ignition problem? If it was timing, it seems to me that the miss would occur with every RPM, not just intermittently. I've opened my choke up to see if enriching the mixture (albeit only on the front carburetor) would help.

I appreciate any feedback! Happy motoring!

[Hamish Coutts]

Hegg,

Read my recent active post on plugs. Suggest you try Champion N9Y if is a standard engine. I found a hell of a difference when I changed from BP7ESs and BP6ESs.

Hamish.

[Hegg]

It just occurred to me also that my carbs might not be balanced. I'll try that as well.

Thanks for the info on the plugs. I swapped them out today and it didn't make the problem go away (although it seemed to run better in general).

[twincamman]

perhaps a dodgy cap or old plug wires ? ed

[Hegg]

Wires and cap are new, but I could try swapping the old ones back in and see if it changes.

[type26owner]

If you have a problem with the ignition misfiring then it's usually possible to observe it with a timing light. It's easy to spot when the strobe is intermittently not flashing. Just ramp up the rpms while watching for this happening in turn on each plug wire. You can complete the diagostic check of the ignition system in under five minutes in this way. While you're at it look for any timing scatter. The vertical shaft play gap of the dizzy must be maintained from zero to .002" to keep the scatter to about one degree. If the scatter is really bad at ten degrees or more then it can cause a misfire like you're experiencing. The twincam is EXTREMELY timing sensitive and precise timing is required to stay smack dab in the narrow power band right below the threshhold of pinging. Make sure the ignition system remains in perfect order at all times.

Randomly swapping parts is a poor way to find most problems. It just leads to frustration. All these problems have been solved in slick ways and the internet connects you to vast resource of knowledge. Use it.

[type26owner]

For some unknown reason the outside of the spark plug ceramic insulators get dirty quite quickly on the twincam and this can cause a misfire. Keep those sparkling clean. Also make sure the wire's boot only overlaps onto the insulator by no more than 1/4".

I've had a run of bad spark plugs lately; especially the resistor type. Don't keep buying them from the same vendor if you have the same problem because the counterfeits are everywhere.

[Hegg]

I'm not sure if it is an ignition problem or not. I'd suspect ignition but it could be carburetor related if it's not pumping enough gas through.

I'll go through the timing and adjust the carbs this weekend (hopefully) and see where I get.

I appreciate everyone's feedback!

[1964 S1]

My 1969 Federalized Stromberg Elan +2 has severe high speed misfire if the battery is low... there was another thread a few months ago on this subject.... mine loads up, runs rich and sputters along. I attribute it to a lack of spark and running rich, I charge the battery after nursing it home, and then all is fine... how's your charging system???

[Hegg]

My battery could be a little low, and I'm not sure about my charging system. When the car is running with no accessories, my ammeter stays about in the middle (at 0). When using things it jumps around, so I know it's showing something.

Thanks for the comment.

[Hegg]

Just an update:

Put in a new batter, no change.
Changed plugs, problem still there, but car ran slightly better overall.
Balanced carbs, problem still there, but a huge difference in car performance overall.

My next step(s) are to check for misfiring using the timing light as previously mentioned. Then I'll play with timing and see where I get.

Thanks again!

[Hegg]

Another quick update -- haven't had much time to play!

I glanced at a post about adjusting the timing chain the other day and thought I'd try it this weekend. The chain was in a bit too tight and I adjusted it until it sounded a little better. Took it for a test drive and the problem is 95% gone now!

However, I'm experiencing some power fade past about 4k rpms. I'd assume this can be attributed to timing, but anything else I should check? Car seems to take off pretty decent up to 4k or so now.

Thanks for everyone's help and suggestions.