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New engine running like an old one....

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M

MoHealey

Senior Member
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All,
Could really use your collective experience.
After sorting the machine shop snafu's on my '69 GT project, I have the car up an running...sort of.
Virtually everything is new; motor rebuild, fuel tank, pump, dizzy (pertronics). I cleaned and reassembled the carbs, but did not rebuild (they were functioning great at shutdown).
The car starts and idles beautifully. Quiet, no miss..perfect.
At road speed, I am losing at least one cylinder anywhere north of about 2500 RPM. Sounds terrible. If I just allow the revs to drop to idle, I'm good again until I accelerate gradually under load up to 2500. Rowing through the gears (exceeding the 2500 RPM's), this never happens. Only at road speed.
I've gone over the entire ignition system 3 times and everything checks out. Have inspected the carb floats and needle valves, and found no sticking or crud. Adjusted valves - they are right on spec, and quiet as I mentioned.
Any ideas? This "feels" like it's fuel related, but I'm 8 hours into diagnosis and could use some ideas.
Many thanks
 
I'd re-check the distributer, it's cap and wires. A bad plug wire can do what your describing with a slight voltage drop. Just because the distributor is electronic, doesn't mean it can't act up. Also, crazy as it might sound, a wire connector might be slightly loose and the force of air from the fan at speed could possibly shake it and cause a defaulted connection. Keep it simple at first and let the more complicated stuff come later, if needed. More MG problems are <span style="text-decoration: underline">misdiagnosed</span> as a fuel problem when it's the ignition system that is the culprit. I always look there first. One thing I never use any more is carbon ignition wires. You probably don't have them, but I had a set where one wire went bad and the result was very much what you're describing. JMHO. PJ
 
That's what I originally thought (ignition), which is what prompted the installation of the new distributor(was going to do it anyway). But I did re-use whe wires.
I'll swap them out today and see what happens. Certainly can't hurt.
Thank you
 
Just another thought. I didn't ask as I thought they would be but, are the plugs new? I installed a set of Iridium plugs in my truck at $8.00 ea. and under a load, one of them would break down. I found which one it was and they replaced it for free. Some of this stuff can drive you crazy trying to figure it out. Over the years, I've heard Champion plugs were great for giving problems. Maybe their better today. Who knows.PJ
 
Good suggestions all, but still getting no results. Changed the plugs and wires today, and have the exact same symptoms.
This may be desperation setting in, but is it possible for a bent fan to cause something like this? I know one of the blades on mine has been "adjusted" by the PO, and it shows some "wobble".
Plan to replace, but did't seem that critical.
 
The fan should not cause your issue, but would cause a consistant vibration. Years ago my "65" B fan let loose and ate my radiator.....not pretty. If your's has a wobble, I'd address that quickly.
In the past, I've had some strange/random engine missing issues. Check out the lead from the coil to the dizzy. Mine was not fully seated and caused a miss as certain RPM due to vibration or whatever. Also had a loose ground issues with the tack and at higher RPM's the car would miss again due to vibration. IMHO, gotta believe you have an ignition/wiring issue. I'd also address the fan issue soon!!
 
How the plugs look after you run the engine a bit? What needle and jet height on the carbs?
 
Plugs look generally good after running. Pehaps a bit toward a lean indication.
I'm not sure which needles are installed. Have only done basic tune to ear procedures at this point.
 
Sweet success!!
Dismantled the carbs again, and this time I found a sliver of old rubber clinging to the front needle. Rinsed everything,reassembled...and the new motor is running like....a new motor!!
Thanks for your guidance.
 
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