Power loss above 4000 rpm

[PHOENIX]

Got my truck on the road again.
When I get on it, above 4000 rpm, the power flattens out.
Not sure if the engine is not getting enough fuel, too much fuel or something else.
It happens right about at 4200-4300.
Is it normal for the fuel line going to the pump (mechanical) to move while the engine is running.
You can see if slightly shifting back and forth as the fuel is pumped through it.
If I hold the hose with my hand I can feel the fuel gulping through the hose.
The hose is new and is only like 8"-9" long (from steel line on frame to pump).

[86 GMG]

does it seem to be sluggish at idle or any other driving conditions? i would start with a new replacment fuel pump and maybe a clogged filter somewhere that is causing pulsing the fuel flow. i am just making suggestions and dont take me for granted

[PHOENIX]

Weak valve springs?
The engine is a gm crate, I estimate around 70-80k miles, but really have no idea.
Do the valve springs usually go weak within 100k?
Also for awhile now, it has been rough around 4000k, now it does this.
How can I test for valve float?

[hotrodhomi]

what is the total advance at?

[TurnNBurn]

I had just about the same prob on my 79 K20 , i would get into 4th, 4 speed manual, stand on it and it would fall on its face and slowly drop off. If i let off for a second then hit it again it felt like pullin it down a gear and jumpin on it. it would blast off. I stared replacing parts, I had a clear fuel filter
so i seen that, fuel pump wasnt keeping up, but I only could see it if i stop on the road and look. idling it was full. so i replaced the pump no good, so
took off the gas line and blew threw the hose and ha ha, it was just about pluged. I dropped the tank and changed the pick up screen in the tank. problem solved (the gas line could be cracked and sucking air too) worked for me , good Luck hope I could Help

[corn]

I doubt its the springs, stock springs will usually go AT LEAST 5 grand before they will float the valves. Id put my money on a fuel problem. Filter first, then just for giggles make sure your fill hose isnt flat. If it is, id try it with the cap off. It almost sounds like a venting problem. After all that & you still havent found anything Id try a pump. The pump feed hose shouldnt really be bouncing around. I bet its in that area...

[piecesparts]

If the valves were floating there would most undoubtedly be some backfiring or cutting out of the engine. I would go with the loss of fuel/air and maybe, just maybe the advance on the distributor is hanging up.

[ureckme]

What has been done sence you could go 4000rpm's before. Whats differant?

[MylilBowTie]

Could it be something with the carb rebuild that it just had?

[hobbz]

the higher rpms could be causing the mech fuel pump to have too much pressure thus causeing it to be way rich at high rpms. might need a fuel pressure regulator

[PHOENIX]

Well before when I could go above 4k it would do it roughly (vibrations).
Now the power just falls off quickly.

The carb was rebuilt, the fuel lines by the pass. header were wrapped, rubber lines to and from pump were replaced.
I have a rubber hose from pump to carb. (replaced it).
I installed a new electric fan, same as I had before, but newer motor and new wiring.
I haven't checked the vacuum or timing yet.

If I disconnect the fuel line at the switch valve and at the pump I can blow compress air through there?
For the line from the tank to the switch valve do I need to remove the tank?
I know for a fact that the fuel tanks on this truck have not been removed/opened in at least 11 years.

I forgot to mention it only does this under load (while driving).
Sitting in nuetral I can easily rev it up to 5k, the revving is smooth also.
Somethings when I rev it is like rum-rummmmmm. Rather than rummmmmm.

What do you guys think?

[ureckme]

Would have to take a look at it. I had a problem like this 2 months ago I changed the fuel pump, I though I double checked every thing, went to set the timeing and got shocked. My coil was loose the 4 bolts holding it down came loose and at 4000rpm's it fell on it's face. titened them up and every thing was back to revven to 6500rpm's. I'm not say'n that whats wrong but its got to be some thing simple like that. Matt