350 Howell TBI question

[aroising]

To all the TBI Guru's here, maybe someone can shed some light on my situation.

82 FJ60 350 TBI NV4500 Howell harness 1 tons.

The problem I've been having is the engine will run flawlessly for about 2 minutes, and then develop a random thump that you can feel through body. I've read nearly every post associated with a 'thump' but have not come across anything solid. The ALDL is not throwing any codes and I've checked the timing gears. Inline fuel holds at 11psi. Map sensor is at 5.1v. New plugs, rotor, cap, coil, EGR, Vacuum lines:confused. The engine has a mild cam on a fresh rebuild. Engine revs great, but develops the thump when brought back down to idle. Again, no codes. Thanks in advance

 

[brokenparts]

does the motor stop running?

seems like the psi is on the light side with a mild cam but not too far off.

 

[aroising]

No , engine continues to run just continues to thump.

 

[LCOwner]

Is the thump rythmic? Like it is tied to the rotating assembly? Or random?

 

[aroising]

its not rythmic. It seems to appear only after warmup and sometimes 2 thumps then nothing for 20 seconds and then a thump. I'm curious if its associated with the cam causing problems with the computer after rotating into closed loop. I'm thinking of just swapping to the stock cam and seeing if anything changes. Thanks for your input

 

[peesalot]

Is the belt o.k. ? have someone watch the crank pulley when ou start it, does it walk in out out about an 1/8" ?

 

[brokenparts]

Is it like a Knock (detonating) or like shoes in a dryer?

How did you go about setting the timing?

 

[trona]

Check to see if it happens in open loop

Try jumping A and B pin and putting the motor into open loop and see if it happens.
Could be plug wires look at the motor in the dark see if you see any sparking
Dave

 

['74 UA FJ]

Does it do it only at idle, and not revved up?

 

[aroising]

Well I replaced the cam and the truck engine is running great. I read that TBI's hate cams and apparently mine is one of them. Thanks for everyones input.