3500 RPM runs flat

-
A

Arrow

Member
Joined
Oct 2, 2015
Messages
13
Reaction score
0
Location
lambertville
At about 3500 my 340 ( in a 66 GT Dart ) seems to flaten out, have a Carter manual fuel pump, PSI at 6 drops to about 3-4 under power. Could it be weak valve springs or broken one. Have mix bag on electronic ignition, dist, coil and box not matched. any ideas ? Thanks
 
How does it run at idle? how does it start? is it responsive and crisp under normal driving conditions?
Could be running out of fuel
Could be a the exhaust manifold heat riser flap is frozen shut
Could be a few wiped out lobes on the cam
So we need more details.
 
Hi 318 ,Fires right up, rough idle, like it has a mild cam in it. Don't know history of motor it is a 1969 340 . runs ok underload till about 3000/3500 starts to break up like it has a limiter on it. has new 600 carb not smoke. 4 speed. soft springs ? maybe try a electric full pump ? Thanks
 
So does it break up at WOT and clean up if you backoff/level out... possible air fuel ratio
 
fuel break up is different than electrical break up. If it's snapping and popping like electrical, well it probably is electrical. If its more of a cough than a snap, probably fuel.
I would doubt valve springs.....
 
Doesn't sound like a fuel to carb issue if its 3-4 psi while driving. If it goes to 0 at wide open throttle (or any throttle for that matter) you'll have a few seconds before the carb runs out of fuel. Been there, done that. :)
6-7 psi is the 'cutoff' pressure, in other words the mechanical regulation which only comes into play when the carb's inlet is closed or nearly closed. 3-4 psi while driving and accelerating is typical on a stock type pump.

I think instead of guessing with you, we should try to help you diagnose it.
First, help us understand the conditions this occurs. Does it happen revving the engine with gear box in neutral? How about cruising in 3rd on highway? Or is it just at full throttle acceleration?

Next, take a look over each of the systems to check condition. In other words, diagnosis will be isolating the problem to one of three things needed to fire a cylinder; spark, fuel+air, compression. I'm assuming for the moment this only happens under load, so unless you have a shop with a load dyno and oscilliscope, the best way to start is look for clues.

So look at these items:
Remove and check spark plugs (take picures if you need).
Check the ignition wires and inside cap and rotor. Look especially for arcing or carbon traces or moisture.
If the coil is labelled, check the resistance to specification from manufacturer

If you have a timing light and tach, check the timing curve. (remove the vac advance hose and plug with golf T or similar. Then measure the timing from idle to 3500 rpm. Write it down! Record every degree or every 200 or 250 rpm or whatever is convenient. Re attach vac line and note how much the timing increases at idle vac)

Holley carb:
check fuel level (sight plugs) on primary and secondary.
Check choke is fully opening when engine is warm
That's all for now. After you answer the questions on when it happens, we can decide if it is worth looking at the primary main circuit to see if its clogged up or way off or something.

Engine: If you want, check compression dry and with oil. Or if you have the inclination, a full leak down check. And yes shure you could check valve springs but I think its best to eliminate possible spark or fuel issues first.
 
And make sure the coil wire isn't burning the center core down inside the wire.
 
Mattax said:
Doesn't sound like a fuel to carb issue if its 3-4 psi while driving. If it goes to 0 at wide open throttle (or any throttle for that matter) you'll have a few seconds before the carb runs out of fuel. Been there, done that. :)
6-7 psi is the 'cutoff' pressure, in other words the mechanical regulation which only comes into play when the carb's inlet is closed or nearly closed. 3-4 psi while driving and accelerating is typical on a stock type pump.

I think instead of guessing with you, we should try to help you diagnose it.
First, help us understand the conditions this occurs. Does it happen revving the engine with gear box in neutral? How about cruising in 3rd on highway? Or is it just at full throttle acceleration?

Next, take a look over each of the systems to check condition. In other words, diagnosis will be isolating the problem to one of three things needed to fire a cylinder; spark, fuel+air, compression. I'm assuming for the moment this only happens under load, so unless you have a shop with a load dyno and oscilliscope, the best way to start is look for clues.

So look at these items:
Remove and check spark plugs (take picures if you need).
Check the ignition wires and inside cap and rotor. Look especially for arcing or carbon traces or moisture.
If the coil is labelled, check the resistance to specification from manufacturer

If you have a timing light and tach, check the timing curve. (remove the vac advance hose and plug with golf T or similar. Then measure the timing from idle to 3500 rpm. Write it down! Record every degree or every 200 or 250 rpm or whatever is convenient. Re attach vac line and note how much the timing increases at idle vac)

Holley carb:
check fuel level (sight plugs) on primary and secondary.
Check choke is fully opening when engine is warm
That's all for now. After you answer the questions on when it happens, we can decide if it is worth looking at the primary main circuit to see if its clogged up or way off or something.

Engine: If you want, check compression dry and with oil. Or if you have the inclination, a full leak down check. And yes shure you could check valve springs but I think its best to eliminate possible spark or fuel issues first.
Click to expand...




This.

Make sure the spark plugs are not used up. Or you don't have a bad cap or rotor.
 
Thanks to everyone for the ideas in solving this. Will take some time to run by the many ideas. Not a easy one Thanks again for the help. Will advise when done. Thanks
 
You must log in or register to reply here.
-

Back
Top