My truck makes a loud thunk type noise and loses all power at a certain point of accelaration. Its happens when you start to accelarate very easy but if you give it just a little more gas then its fine. Its a hard thing to describe but if you need more info then Ill try to give it to you. Thanks

 

If you have hydraulic lifters and your oil pressure is a little low the lifters may be collapsing.

 

What year truck is it? What size engine do you have and which tranny do you have. It may or may not be a tranny problem. I had to replace an FMX tranny in a 79 that did the same thing you are describing.

 

Wow, I was thinking it was the carb. Oil pressure seems fine at 60 when first cranked goes down to 20 in idle when warm and about 40 will cruising. My truck is a 73 F100 with a 390 and a C6 trans. The noise Im decribing isnt like a loud metal rapping noise more like if you dropped a centerblock wrapped in a blanket in a big room. Great desciption huh. I just dont know how to explain it. And of course when you want someone else to hear it, itll never make a sound.

 

Could the sound be the carb backfiring? I'd check the advance and timing while goosing the throttle. Also, if it has electronic ignition, replace the module. You should have a spare stashed somewhere in the truck anyway.

If you have an automatic trans, it could need attention. Check the fluid level.

Did the problem show up all at once or gradually?

As mentioned above, you'll need to give us more spec's on your truck if you want more effective help. Otherwise we'll be grabbing straws.

Good luck.

-Jim

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Sounds to me that you have a first gear shift/loading problem going to 2nd gear...

Try changing the vacuum Modulator at the rear of the trany.... On the new one... Turning the small screw (inside the vacuum hose connection port) IN will raise the shift point I thing I have mine in 4-6 turns from Out of the box setting....

__JOHN__Õ¿ö
72 F-250

 

Thanks for the ideas, I didnt expect to get the exact reason for my problem,just ideas, from such a vague description. I appreciate any help. I guess it could be the carb backfiring but what causes that and how can I check it to see. It seems that it only does it when Im in gear but it does it in all the gears at the place on the accelaration pedal. I have points ignition. I dont think its the transmission because you can hear the engine die for a second or like it only misses at that point. Thanks again for any help and If i need to tell you any more specs just let me know.

 

If you think it is the throtle position try Disconnecting the linkage and working the throtle at rest by hand... Quickly work the throttle butterfly`s and see if the engine stumbles...
If the problem is there it is probally Timing/Vacuum/fuel etc.. related..
Hook up a vacuum Gauge and see what the needle is doing when working the throttle! Check for binds and Interferance with the linkage to wiring etc...

Remember when all else fails... Jack up Radiator cap and change Truck.... Ok I know!!

__JOHN__Õ¿ö
72 F-250

 

I hooked up the vacuum gauge to the full vacuum port on my 600cfm edelbrock (the line that normaly runs to the valve cover). I dont know if That is the correct place to check vac. but it read at 18 and when you floor the throttle it goes to 0 and then back to a steady 18 when you release it. Does that tell anyone anything or did I do it wrong?

 

Those numbers look very good. Steady vacuum is a good thing. If it jumps around a lot, you could have compression or valve train problems.

Now hook the gauge to the port on the carb that goes to the distributor. It should do somewhat the opposite. You should see very little vacuum at idle and lots when you work the throttle.

It's looking more and more like a distributor problem. Check it out with a timing light or if you don't have one, take it to a shop where they can put it on their scope and find the exact problem.

The timing should advance when you rev the motor. More with the hose connected to the distributor and less without. Pull the cap off and make sure that the plate that the points are on rotates toward the diaphram and springs back. Grab the rotor and give it a twist. It too should rotate and spring back. If it seems gummy then you may have it sticking in mechanical advance mode and messing up your timing.

If you find a problem with the distributor, I can walk you through rebuilding it. It's fairly easy. If you don't, then I'm stumped.

-Jim
'76 F-250 4X4
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