Loss of power under load

[ngsd]

I am a little stumped and could use some help. My old engine ran great but had a rod bearing starting to go so I swapped in another running engine I had in the shop. I used all of the old parts form the old engine including intake, carb, dist, fuel pump etc and basically the only thing that is different is the long block. It started and runs fine at idle and revs up but when you put it under load it just loses power and dies. It will start back up and seems like it is starving for fuel but runs and idles ok. I checked for vacuum leaks, timing, fuel pressure and all the things that are common sense but cant seem to locate it. Would a weak timing chain cause this? Just strange that it is just under load when I try to move.

 

[oldiron]

Sounds a lot like a blown power valve. Worse when cold?
What carb?

 

[DC_Gearhead]

Sounds a lot like a blown power valve. Worse when cold?
What carb?

Agreed. Sounds like a carb issue.


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[jckkys]

I agree it sounds like a carb problem. What exactly that means depends on what kind of carburetor you have. It does seem to be in the power enrichment circuit. Ford and Holley carbs have power valves. Carter AFBs and quadrajets have metering rods. A lean mixture under load with a power valve means it's either plugged up or the internal spring is broken. A perforated (blown) diaphragm would run rich, particularly at idle. Metering rods are functioned by a piston. They sometimes stick in their bore. I have an AFB (that some people call an Edelbrock) that does that in hot weather.

 

[blubuckaroo]

Start with the basics.
How's your ignition system? Ignition wires in good shape ant plugged in tight? How are the ignition points? Is the dwell set right?
Make sure the distributor is advancing. I've seen the advance stuck.

 

[SHX669]

Some more info might help ; electric or mechanical pump , what carb , what tranny - manual or stick ? A few questions ; what motor ? IF mechanical pump is the fuel pump eccentric on the cam supposed to be the same for both motors? Is the fuel pump installed correctly in relation to the pump arm and fuel pump eccentric? Did you check for obstructions in the fuel line?

 

[Madgyver]

timing chain jumped a tooth?

 

[ngsd]

The only difference from the engine that ran good before was the long block. All of the parts just swapped over and had no issues. I agree it sounds like a carb issue but given that the day before it ran fine and only when I changed out the block did it have problems it makes me think I have a timing issue. It does advance and checked points, dwell wires are fine, plugs are good, and all of that area checks out as it did before. Runs fine at start, and revs like normal at and idles but nothing at all under load, cold or warm. It is a cheap Chinese knock off carb so I have little faith that something is not wrong but given the time frame it is unlikely. I may rebuild the original carb and try that. For the questions above, it is a stock 302 from another 71 Bronco. Two barrel carb and stock manifold with three speed trans. All new gaskets on carb, intake etc.

 

[blubuckaroo]

Sounds like the only thin left is the condition of the engine you put in.
I'd pull the intake to see if the intake gasket sealing the runners. I has one that split the gaskets and had a huge vacuum leak from the valley.

Also, there's a way to check the timing chain slop by turning the crank back and forth while watching the distributor rotor.

 

[EPB72]

Could check coil output weak spark could cause cutting out under load

 

[hucklburry rev2]

Are you sure of the firing order on the replacement engine? Swap the 4 wires and see how she runs?

 

[jckkys]

Carburetors don't like to sit after use. The gas turns to a slimy glop that blocks passages. So it's a good idea disassemble it blow carb cleaner through all the small passages and follow that with compressed air. I really don't trust Chicom parts so I would rebuild the original 2100 and run that. As I said, if the power valve diaphragm is perforated the idle would be super rich and you stated the idle is "fine". That only leaves a blockage in the power valve or the main circuits. A weak spark could also fail under load but that didn't happen with your old engine so why would it happen now?

 

[ngsd]

Could check coil output weak spark could cause cutting out under load

I was thinking about that as well. The one thing that has me thinking not is that after it stalls I need to keep my foot on the accelerator to get it started again so it feels like it is starving for fuel. I will swap it and try that though as it is one thing that makes sense.

 

[ngsd]

Are you sure of the firing order on the replacement engine? Swap the 4 wires and see how she runs?

They are both early bronco 302's so that would not be the issue i wouldn't think. How can you tell if it were? Wasn't the change for late model 302 or 351's?

 

[ngsd]

Carburetors don't like to sit after use. The gas turns to a slimy glop that blocks passages. So it's a good idea disassemble it blow carb cleaner through all the small passages and follow that with compressed air. I really don't trust Chicom parts so I would rebuild the original 2100 and run that. As I said, if the power valve diaphragm is perforated the idle would be super rich and you stated the idle is "fine". That only leaves a blockage in the power valve or the main circuits. A weak spark could also fail under load but that didn't happen with your old engine so why would it happen now?

The carb was a brand new one and only ran a few times prior to the swap. It did not sit longer than a few days since the last use so no issue there. It could be the carb and just coincidental that it happened after the swap. I did buy a rebuild kit for the original carb and at least I will have something to swap out and see after I get it done if it makes a difference so I can check that off the list. The intake gaskets may not have sealed but I would have thought the idle or revving it would not be good if that were the case though.

 

[broncogt]

Timing chain installed 1 tooth off?

 

[hucklburry rev2]

They are both early bronco 302's so that would not be the issue i wouldn't think. How can you tell if it were? Wasn't the change for late model 302 or 351's?

From experience, I have a 78 block someone must have swapped the cam, with the old firing order it started and idled, I had some backfire revving it up no load, and no power under load. Swapped 4 wires after 7 days of troubleshooting and its really a nice running engine.

Its similar issues to what you are saying, and with all the work you have done, you said you swapped dist. over? I'd look at plug wires and timing.

 

[DC_Gearhead]

From experience, I have a 78 block someone must have swapped the cam, with the old firing order it started and idled, I had some backfire revving it up no load, and no power under load. Swapped 4 wires after 7 days of troubleshooting and its really a nice running engine.

Its similar issues to what you are saying, and with all the work you have done, you said you swapped dist. over? I'd look at plug wires and timing.

Don’t ask me how I know i have a 302 cam in my 351w. Hahahah. Sounded fine with wrong firing order initially, but under load it acted funky with backfiring.


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[oldiron]

Did it ever backfire or "sneeze" through the carb when you were first trying to start it?

 

[ngsd]

Timing chain installed 1 tooth off?

That would be a thought as well if it was newly installed but it also ran fine before the swap.