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Similar to the post below "292 stumbles on acceleration" but different.
Our 66 does not accelerate at all. It struggles to get up to 50MPH and tops out there. It almost feels like the brakes are on but I can glide with the clutch in just fine.
The only thing we have done to it lately is to put a choke tube on it to see if that would help the idle drop down faster.
You didn't mention which engine/transmission/rear axle ratio you're talking about. From your picture the engine appears to be one of the I-6s. Did it ever run right and now has gone wrong? If, so, what changed? Your problem sounds to me like it's timing, spark advance, or vacuum leak related. You're axle ratio could also be playing a part in having a lower top speed than you'd like. Hope this helps.
Sounds like you may have answered your own question. If the ONLY thing you have done to it lately was working on the carb - Check your exhaust to see if its black - choke is not opening up like it should. BUT with out know what motor or carb you have, we could be talking about a Datsun. There are two most likely areas to check. Since you changed something on the carb - check that first. Another thing that can keep the motor from getting up to speed/power, is the timing being off. points slipping or the distributor slipping. Both will throw off the timing. Start with those. And please post what motor it is along with what - carb and anything else you have done since it started causing your problem. No matter how little it might be. Bad gas letting garbage plug up the fuel supply to the carb. Not enough fuel, NO power.
Good luck, someone here will solve your problem
Sorry its a 240 I6 engine. Carb its a 1 barrel mystery carb. The truck ran fine 2 weeks ago. I will take the tube off to see if that makes a difference. It did make a difference in how long the idle was high (warm up).
Axle ratio did not change and it ran/drove fine on the highway on the trip home a few months ago. I have only been local since.
Update:
I took the tube off and adjusted the mixture and it is a little better. I will test it out with quick drive tonight.
Update 2: Nope no change. I can still floor it and it will still take forever to get up to 20MPH.
So try checking the timing, spark advance, and look for vacuum leaks. As Masked Ride said, look at the distributor points and condensor and look for slop in the shaft. Also look to see that your harmonic balancer isn't deteriorated. Good luck!
Agree with comments regarding symptoms of timing, and/or tune up issues. Been quite some time since I have had to tune and time a 6 cylinder engine. Seem to recall, if spark advance operates off primary vacuum then I had to disconnect the vacuum line and plug when setting timing, found golf tee's work great. Suggest estableish TDC at #1 cylinder and compare rotor location to timing marks. Rather than pull the valve cover, I am basically lazy and try old school method of removing #1 spark plug and with ignition off rotate engine and immediately stop when compression forces, I use my finger, out of the spark plug opening. For me, this is just a quick check to determine if rotor and timing marks are within limits. Once the engine is running should be able to fine tune keeping in mind when adjusting carb idle mixture then need to recheck timing??
Ok I figured it out with the help of my 11 year old this time. It turned out to be the acceleration cable. I had it pulled too tight so there was less room for the cable to move.
Hopefully that makes sense. It makes sense to me now that I saw it.
Thanks for all the input and replies. Sometimes it is the simple things.
Check your exhaust to see if its black - choke is not opening up like it should. BUT with out know what motor or carb you have, we could be talking about a Datsun. There are two most likely areas to check. Since you changed something on the carb - check that first. Another thing that can keep the motor from getting up to speed/power, is the timing being off. points slipping or the distributor slipping. Both will throw off the timing. Start with those. And please post what motor it is along with what - carb and anything else you have done since it started causing your problem. No matter how little it might be. Bad gas letting garbage plug up the fuel supply to the carb. Not enough fuel, NO power. 
