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01-16-2017, 05:10 AM | #1 |
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Misfire at high revs
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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I'm part of the only ever generation with an analogue childhood and a digital adulthood. Last edited by Synchro909; 01-19-2017 at 05:34 AM. |
01-16-2017, 07:05 AM | #2 |
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Re: Misfire at high revs
Not much to go on really w/o a little more testing, but my first instinct in incorrect coil polarity.
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01-16-2017, 07:14 AM | #3 |
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Re: Misfire at high revs
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! |
01-16-2017, 08:07 AM | #4 |
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Re: Misfire at high revs
Low probability, but you might check the points spring. Maybe a little float problem?
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01-16-2017, 08:24 AM | #5 |
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Re: Misfire at high revs
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
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01-16-2017, 11:46 AM | #6 |
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Re: Misfire at high revs
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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 |
01-16-2017, 12:06 PM | #7 |
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Re: Misfire at high revs
Once the problem starts, if it stops about immediately if you slow down, then, I too would suspect fuel vent issue.
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01-16-2017, 12:12 PM | #8 |
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Re: Misfire at high revs
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!
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01-16-2017, 12:30 PM | #9 |
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Re: Misfire at high revs
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.
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01-16-2017, 12:33 PM | #10 |
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Re: Misfire at high revs
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?
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01-16-2017, 12:44 PM | #11 |
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Re: Misfire at high revs
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.
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01-16-2017, 01:38 PM | #12 |
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Re: Misfire at high revs
I 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 ... Last edited by Benson; 01-16-2017 at 02:00 PM. |
01-16-2017, 05:18 PM | #13 | |
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Re: Misfire at high revs
Quote:
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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I'm part of the only ever generation with an analogue childhood and a digital adulthood. Last edited by Synchro909; 01-16-2017 at 05:32 PM. |
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01-16-2017, 05:34 PM | #14 |
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Re: Misfire at high revs
Arnold
Aren't you usually helping others here on the forum with their drivability issues Am i missing something |
01-16-2017, 06:06 PM | #15 |
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Re: Misfire at high revs
x2 on the spark plugs!
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01-16-2017, 06:20 PM | #16 |
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Re: Misfire at high revs
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 |
01-16-2017, 06:45 PM | #17 |
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Re: Misfire at high revs
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
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01-16-2017, 07:37 PM | #18 |
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Re: Misfire at high revs
Electronic ignition??
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01-16-2017, 07:43 PM | #19 |
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Re: Misfire at high revs
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01-16-2017, 07:52 PM | #20 |
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Re: Misfire at high revs
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!! 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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