High rpm/speed miss

[prb51]

I have a high speed miss that doesn't come on until the high 90's/100 mph range around 4k in 5th. The ambient temp is around 100 in Az today.
Everything up to that point is very smooth and I don't hear any pinging.
What would be the most likely culprit?

 

[prb51]

Oh yea, 56 TR3

 

[donbmw]

Check plugs, wires, cap or points.

Don

 

[Geo Hahn]

100 mph in 100 degree heat in a 53 year-old car... geez Pat what do you expect?

Seriously, high speed miss always makes me think first of plugs & wires.

Oh yeah, I drove about 70 miles today and it was 100 by the time I was home. Oh well, driving season is just beginning in some parts of the world.

 

[prb51]

You know, I've had those points awhile..I'll start there.

 

[Brosky]

Does it do the same thing while accelerating through any of the gears at the same RPM, or does it only happen in 4th gear?

 

[prb51]

Good question...I only noted this in 5th gear and I was accelerating hard in all gears

 

[DougF]

Sounds like a good time to try Pertronix?

 

[Brosky]

PRB,

When this occurs at any rpm in high gear, (that is actually under a load condition and more obvious if going up a hill) you should check the following:

1. Ignition timing at that rpm.

2. Each individual ignition wire for cracks or arcing, by spraying a mist of water from a spray bottle on the wires and see if it starts breaking up at idle.

3. Play in the distributor shaft bushings.

4. Pull the plugs and make sure that you are not running too lean.

5. I'm sure that others will chime in with more good suggestions, but these are quick and easy and cover a lot of bases.

 

[bgbassplyr]

Wound up in high gear and loaded puts the greatest load on the ignition system. Any problems would appear at this point. Probably wires, plugs, point float, or coil.

Jim

 

[prb51]

Since I've relatively new wires and coil (for the MSD6 unit) I'll start with the points then the plugs.
I've had the points on for probably 6k and the point faces don't wear due to the spark generated by the MSD so probably the spring is tired.

 

[Brosky]

As the old X-Files sign in Mulder's office used to say "Trust No One".

Spray the wires down and see if starts to skip at idle and up to 4,000 rpms. Eliminate the simple stuff first. You'd be amazed how many new wires that I found were bad over the years. It only takes one to do what you're describing.

Also, before I forget, talking about simple, make sure that every plug wire is on tight, as in very tight. Another cause for a miss or skip under load.

 

[bgbassplyr]

And, unless you are using solid core wire, that carbon string junk can and will separate if pulled on. If string, check connection on both ends for solid contact.

Jim

 

[Roger]

Whenever I've had that kind of problem - a high load misfire - it's usually plugs.
The rest of the ignition system either works at such-and-such RPM or not, most of the time. But when loads come on, plugs fail first - assuming carburation is right, of course.
Have you tried a plug cut? While misfiring, clutch out and cut engine, remove plugs and study. You might see just one with misfire signs, or you might see, for example, high speed / load lean mixture signs.

 

[philman]

Where is your power source for the MSD box? It was recommended to me to run separate power lines directly to the battery, not to the starter or alternator. My MSD box also had a miss until I replaced the lucas alternator with a delco alternator (I was told that Lucas alternators could generate noise in the DC circuits). At the same time I replaced my Bosch plugs with NGK plugs because the Bosch plugs were causing noise in my FM reception at high rpms.

 

[LastDeadLast]

I had a very similar issue with my TR6 back when I used to use the OEM dizzy. It turned out to be play in the distributor bushings.

-s

 

[TR3driver]

Not too long (maybe 6 months or a year) after I installed a MSD 6 on my 59 TR3A, it started running really poor on the way home from work. The center contact in the stock distributor cap is a resistor, and the MSD had burned it up! The spark had actually been jumping around the resistor for some time, until it burned away the center tower enough that the path from the coil center contact to a side contact was shorter. Anyway, you might have a look at your resistor.

Also worth noting that "carbon core" wires will quickly burn where the original cap pierces them; while MSD specifically says NOT to use solid core wires with the MSD 6. I used a wire set that had tinned copper wire at the dizzy, but a discrete suppression resistor inside the plug boot; which seemed to work very well for me. Unfortunately, I don't know of a current source for them ... if you find one, please let me know! They were made by Whitaker as I recall, and I got them from JCW around 1986 or so.

 

[prb51]

I'll mist the wires and change out the plugs (due anyway I think).
I've the carbon wires with the MSD6 unit, might try shortening the wires and reattaching and check for the burns you mentioned.
Will also check the rotor cap (get one from Advanced) and replace the points.
Great feedback guys.

 

[billspohn]

I did a double take on seeing 'high RPM' and 4000 RPM - until I remembered I was reading the Triumph section :thumbsup:

Just giving you a hard time. The other guys have it right - it will be either electric - something breaking down, a high resistance joint, a weak coil, some sooty plugs, or a fuel problem - check the filters in the carbs, the output of the pump etc.

Good luck tracking it down - these things can be most annoying. I went through this not long ago on an MGA - turned out to be a brand new electronic fuel pump that wasn't putting out enough volume!

 

[RonMacPherson]

Good points, however. Usually electric load breakdown can be induced on an uphill grade, or by putting a load on the vehicle(used to run a Clayton chassis dyno) in one of the lower gears. So you should be able to duplicate the symptom without the speed.

If it ONLY occurs at speed, I would also be curious as to fuel flow. Kinked rubber hose, filter getting contaminated, fuel tank vent fully open, fuel pump pressure starting to deteriorate, low float level... Those are items I would be checking also, if you do not repeat the symptom in 2nd or 3rd gear under full throttle load..