1948 8n can't get full throttle

[jchatterton]

Just rebuilt a 48 8N engine with a front mount distributor. Converted to Pertronix (1247) electronic ignition. The engine starts great, and idles smoothly, but will not get past about 700-1000 rpms if I had to guess. Good spark to each plug. These are new suppression plug wires as Pertronix recommends. I initially bought an aftermarket new carb. I have since rebuilt the original carb and installed it, but am still having the same issue with the throttle response. I disconnected the linkage between the governor and the carb, but I still get the same result. I didn't do anything to the governor, but with the linkage disconnected I was assuming that the carb would produce full rpms, but it doesn't. Any advice would be appreciated.

 

[Kondoge]

Mine wouldn't get full throttle and it turned out there was a plug in the carb up underneath where it bolts to the manifold on the carb, that plug was gone. I hammered a random bolt in, which I myself don't recommend. if that hole is open and has threads find one that fits. otherwise distributor issues, or wiring issues, maybe spark plugs?

 

[wore out]

Just rebuilt a 48 8N engine with a front mount distributor. Converted to Pertronix (1247) electronic ignition. The engine starts great, and idles smoothly, but will not get past about 700-1000 rpms if I had to guess. Good spark to each plug. These are new suppression plug wires as Pertronix recommends. I initially bought an aftermarket new carb. I have since rebuilt the original carb and installed it, but am still having the same issue with the throttle response. I disconnected the linkage between the governor and the carb, but I still get the same result. I didn't do anything to the governor, but with the linkage disconnected I was assuming that the carb would produce full rpms, but it doesn't. Any advice would be appreciated.

Restricted exhaust?

 

[jchatterton]

Mine wouldn't get full throttle and it turned out there was a plug in the carb up underneath where it bolts to the manifold on the carb, that plug was gone. I hammered a random bolt in, which I myself don't recommend. if that hole is open and has threads find one that fits. otherwise distributor issues, or wiring issues, maybe spark plugs?

I'll look and see, but I've put on two different carbs, so I'm kinda leaning towards something else. Thank you

 

[jchatterton]

Restricted exhaust?

New exhaust pipe.

 

[Lynn Patrick]

What plugs are you using? Just remember - new parts doesn't mean GOOD parts!

 

[Non-farmer]

Just a shot in the dark here - is this something that can happen when the governor balls get worn down with flat spots? What does that end up doing, symptom-wise?

 

[soaked]

Hi jchatterton, welcome aboard!
If your fuel delivery is plugged up, the carb bowl cant fill, cause the engine is burning the small trickle as it comes in.
Engine sounds labored if ask for more throttle, as it is leaning out. Suggest JMOR's flow test --take the carb drain plug off, and see if flow is a pint in about 2 minutes.
1247 is 12 v neg ground, and needs 3.0 ohms on the primary curcuit. Are you running a 2.5 ohm coil, and the factory resistor, or other?

 

[wore out]

New exhaust pipe.

Does it have a hew manifold? There have been issues with restricted ports or internal "leaks" with the one on the market now.

 

[jchatterton]

What plugs are you using? Just remember - new parts doesn't mean GOOD parts!

I went back with Champion H10C plugs, gapped to .027

 

[jchatterton]

Does it have a hew manifold? There have been issues with restricted ports or internal "leaks" with the one on the market now.

I used the old manifold, it still looked really good

 

[jchatterton]

What plugs are you using? Just remember - new parts doesn't mean GOOD parts!

Is there a better/preferred plug to use?

 

[jchatterton]

Hi jchatterton, welcome aboard!
If your fuel delivery is plugged up, the carb bowl cant fill, cause the engine is burning the small trickle as it comes in.
Engine sounds labored if ask for more throttle, as it is leaning out. Suggest JMOR's flow test --take the carb drain plug off, and see if flow is a pint in about 2 minutes.
1247 is 12 v neg ground, and needs 3.0 ohms on the primary curcuit. Are you running a 2.5 ohm coil, and the factory resistor, or other?

I bought the 12v conversion kit from J&D Productions. It's a 2.5 ohm coil with a .5 ohm resistor

 

[jchatterton]

Hi jchatterton, welcome aboard!
If your fuel delivery is plugged up, the carb bowl cant fill, cause the engine is burning the small trickle as it comes in.
Engine sounds labored if ask for more throttle, as it is leaning out. Suggest JMOR's flow test --take the carb drain plug off, and see if flow is a pint in about 2 minutes.
1247 is 12 v neg ground, and needs 3.0 ohms on the primary curcuit. Are you running a 2.5 ohm coil, and the factory resistor, or other?

I will try the fuel test next, thanks.

 

[Jim L WA]

Another thought, could the centrifugal advance be stuck in the distributor?

 

[jchatterton]

Hi jchatterton, welcome aboard!
If your fuel delivery is plugged up, the carb bowl cant fill, cause the engine is burning the small trickle as it comes in.
Engine sounds labored if ask for more throttle, as it is leaning out. Suggest JMOR's flow test --take the carb drain plug off, and see if flow is a pint in about 2 minutes.
1247 is 12 v neg ground, and needs 3.0 ohms on the primary curcuit. Are you running a 2.5 ohm coil, and the factory resistor, or other?

I got good fuel delivery. 1 pint in just under 2 minutes

 

[WER-FL]

I had a similar issue and disconnected the Throttle to Gov linkage at the below the battery box and very carefully controlled speed. My linkage was too short. The linkages are adjustable, I had to welded on an additional 3/4 inch due to rust.

 

[jchatterton]

Another thought, could the centrifugal advance be stuck in the distributor?

I think you might be onto something there. I pulled the distributor back off and this is what I saw. The farthest right arrow is where I found the retaining clip. The thin washer was oblonged and worn on the inside diameter. The retaining clip had walked out of its groove and slid up the shaft. I've ordered new parts. Hopefully thats what it is. Thank you for the heads up on this!

 

[Lynn Patrick]

Is there a better/preferred plug to use?

I use nothing but NGK 3112s after having several other brands fail early, some in a couple months & some in a couple hours. I have heard from the gurus here that H10 was sufficient for the gas in the 1930's-40's but today's gas needs a hotter spark.

 

[Mad Farmer]

I use nothing but NGK 3112s after having several other brands fail early, some in a couple months & some in a couple hours. I have heard from the gurus here that H10 was sufficient for the gas in the 1930's-40's but today's gas needs a hotter spark.

Those work good.

I used to use ACs but they went hell