Engine hits wall at 5k only in 3rd gear

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71duster06

71duster06

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Strange issue came up this year. 340 is running great but when I get on it to 6k in first and second I then go to 3rd still at full throttle but at 5k rpm it falls flat. I noticed this once last fall but it went away. Anyways any thoughts?
69 340
4 speed w/ 3.23 gears
Lunati 60403 with Hughes springs at proper spec
273 rockers
LD340 w/ 850 thermoquad
Older Points distributor with the pertronix electric conversion kit.
Stock fuel line 5/16" with electric pump

Strange that this is only a 3rd gear issues. Lack of fuel? Ignition issues? Valve float?
 
It must have to do with engine load and RPMs both.
I'd take a guess at fuel delivery or the timing changing with different vacuum load.
Probably fuel.
 
Sounds like you're emptying the fuel bowls faster than the pump can fill them. Get a fuel pressure guage and see what the pressure is when this happens. Shouldn't drop below 6 - 7. Also since you have an electric fuel pump try this:
disconnect the fuel line AFTER the regulator or at the carb. Get a one gallon jug and stuff the fuel line in it. Turn on the pump and see how long it takes to fill the jug. Should be somewhere near 30 seconds or less. You might be at the ragged edge with that 5/16 line. Or the pump might be bad.
Then again it might be something else entirely. Good luck, post what you find.
 
sometimes on thermoquads the secondary don't catch. See How to hot rod smallblock mopar engines By Larry Shepperd or other thermoquad mods info to see how to fix the problem...just a possibility
 
K.O. SWINGER said:
sometimes on thermoquads the secondary don't catch. See How to hot rod smallblock mopar engines By Larry Shepperd or other thermoquad mods info to see how to fix the problem...just a possibility
Click to expand...

That is a very good possibility, especially if you have been doing some tweaking.
 
I would run it easy through first and second so as not to drain the fuel bowls, then wind it up in third and see what happens.
 
I agree with the others that it sounds like it's a fuel delivery issue. If it were valve float it'd do it in all gears. Same if it were an ignition issue. Your just running out of fuel when you get into high gear. Weak pump, restricted fuel filter, rubber fuel line partially collapsed inside, etc...
 
Where is your fuel pressure? You gotta be careful. TQs dont like much more than 5PSI. So if it is fuel, it's volume, not pressure.
 
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