Misfire at high revs

Synchro909

Synchro909

Well-known member
My ever faithful outback touring car has developed a miss at high(ish) revs. It won't rev past about 45 in top but pulls as well as ever at low revs. I figure that being when cylinder pressures are the greatest, the plugs should be OK. I have recently changed the head gasket but everything went back as it was. The motor is not stock so here are a few details but I bet the first response asks for something I missed!!
High comp head (cold compression test gave 96-99 psi on each cylinder)
FS auto advance distributor.
Down draft Australian Stromberg carby.
Motorcarft TT10 plugs with few miles on them.
Coil recently replaced and mounted terminals up.
Distributor body and rotor button recently replaced
Nothing else that has been done could affect the running of the engine (O/D, synchro gearbox for example) so I am left scratching my head as to what could be causing the misfire. Engine idles nicely and up to about 40+ mph, runs just as it always has.
What ideas form the forum gurus?
 
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Model A basic road side troubleshooting, high speed miss check for fuel flow, water in gas.
My highhspeed miss was my gas cap wasn't venting! Removed cap gasket and left it loose!
 
I've had what Jack mentioned about points spring. The points opened/closed normal during setting while hand cranking. So no problem seen until I was dead on the side of the road. At this moment it was discovered the points were seized in the open position. It was fine at lower speeds then ran like crap as the rpm 's rose. Hope this helps. Jeff
 
Syncrho,
Many suspected carb problems are "electrical". I too suspect that you have an electrical issue. At the faster speed and load to the system at higher speed ( probably still present at low speed) but it is more pronounced and noticeable.
I like Sun's old slogan.... "Test, Don't guess" .... simply stated but make good sense.

I would start checking the system and go from the distributor back... as was suggested above check the distributor connections, wiring, grounds, use a digital volt/ohm meter to make sure you don't have excessive resitance anywhere which would indicate a problem. The pigtail where it goes into the distributor, ignition switch, terminal box, and all associated connections and connectors are suspect. I've seen a lot of crimp style connections that had issue.. so pay attention to those.
I believe the FS is electronic, correct? If so grounds are very sensative to the module and ability to function fully.

I know sometimes these are a little tough to solve but am confident you will find it. You're digital volt ohm meter is your best friend. You can also use it to look for low voltage which is an indicator to a poor connection nearby.

Keep us posted!
Larry Shepard
 
Once the problem starts, if it stops about immediately if you slow down, then, I too would suspect fuel vent issue.
 
Patrick L. said:
Once the problem starts, if it stops about immediately if you slow down, then, I too would suspect fuel vent issue.
Click to expand...

I agree with Patrick make sure you have a good fuel flow.. that hopefully has already been confirmed. If you're running an electric pump make sure you have sufficient flow and pressure. Keep us posted!
 
Potential causes:
Coil primary connections backwards;
Coil secondary breakdown;
Insufficient oil in coil / leakage;
Coil wire loose and or poor terminal connections;
Arcing between primary & secondary terminals of coil;
Condenser breakdown;
Defective rotor;
High voltage leakage in distributor body /cap;
Fuel tank vent closed;
High speed jet in carburetor too small or partially clogged;
Carburetor float setting too low;
Closed GAV;
Too much ignition advance;
Loose connections at ammeter;
Defective ammeter.
 
This is like the old prob where the fuel line went too far into the carb and hit the screen, but he does not have that set up. Could same thing happen on downdraft carb?
 
Unfortunately, this is one of those times when having an electronic ignition system makes diagnosis more difficult. That being said always eliminate the easy stuff first. I'd start with the fuel delivery. Make sure the gas is good.
 
:DI would change plugs from the cold TT10s to Champion W18. Especially true if the car has been driven on multiple short trips or idling for extended periods.

TT10 and even C16C / W16Y plugs are too cold for short trips.

These cold plugs worked fine when I was driving 25 miles one way 7 days a week.

They foul out with short trips of 5 to 10 miles in town.

Since it is summer I suspect that you are on long camping trips maybe ... but just in case there are no long trips involved ...
 
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Bob Bidonde said:
Potential causes:
Coil primary connections backwards;polarity is correct
Coil secondary breakdown;possible and something I was "umming" about. I'll try a known to be good coil
Insufficient oil in coil / leakage;As above
Coil wire loose and or poor terminal connections;coil wire would be checked as part of changing it. Other terminals will be checked from switch to plugs
Arcing between primary & secondary terminals of coil;Slight possibility - I'll pay attention to this when I put the other coil in
Condenser breakdown;No condenser - electronic distributor
Defective rotor;I have a new one, I'll try it even though the current one is also new
High voltage leakage in distributor body /cap;Exactly as above
Fuel tank vent closed;Checked and clear
High speed jet in carburetor too small or partially clogged;The jet is the correct size. I've been running this carby for about 30,000 miles. I'll give it a clean out to be sure. I'll verify again that the fuel flow is adequate too.
Carburetor float setting too low;Unless it has partially sunk, it shouldn't have changed. It's been good for many miles. I'll pay attention to it when I clean out the carby (which I expect to be clean anyway.)
Closed GAV;Down draught carby - no GAV
Too much ignition advance;It was a smidge advanced but I've brought it back and the misfire is still present.
Loose connections at ammeter;I'll check that
Defective ammeter.Same here.
Click to expand...

I really appreciate the suggestions left by all those who have done so. I've quoted Bob's list because it covers just about everything others have said. My responses in red.
It's going to be too hot here today to spend much time in the workshop but I'll do what I can and get back when I find the gremlin (and fix it). I would have expected some of the above suggestions relating to HT leakage etc to be more likely to give trouble when the motor is lugging and cylinder pressures and voltages are at their highest. Weird stuff happens so I'll look at it all just the same.
 
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Arnold, make sure you get that thing sorted,we don't want to be being driven around in a miss firing car.
Remember a certain A that played up for years,was a blocked fuel line.
Check the flow from the carb drain bung,measure it,check from the fuel bowl,OH you have a fuel pump.
My $$ on fuel.
repacking wheel bearings on the 33 and van today,leaving friday ,as my brother is visiting.
Lawrie
 
I agree with Jackson lll . check the points spring. After flexing 10 thousand quzillion times the spring gets tired and refuses to spring back like it did when it was young.. ken
 
BRENT in 10-uh-C said:
Electronic ignition??
Click to expand...

Yup. So no points spring but that is a good place to go if there were.
OK, so a few reckon the plugs might be a bit cold. I have only been driving it around town lately so that could be right. On Sunday, the problem was still present after doing about 15 miles, most of it at as fast as it would go with the misfire on the freeway.
What plugs does the consensus suggest? Benson suggested W18's. I promise, I'm not going to ask about oil!!:D:D
It's getting hot in the workshop but I've changed coil, distributor body and rotor button. Checked fuel flow, tank cap dash terminals and ammeter. Too hot to test drive now and I'm busy tomorrow. I'll get back later.
 
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