I put a Holly AFPR on my 350 TPI over the weekend. Yesterday I get the thing back together and borrowed a FP gauge from my buddy to set the FP and the needle is shaking all over the place. At idle with the vacuum DISCONNECTED the needle will shake back a forth 7 to 10 PSI. If I rev it to 2500 RPM the needle will stables to about a 2 PSI shake and I set the AFPR to about 48 PSI to start. Now, with the vacuum CONNECTED the shake is the same at idle but if I rev the motor the needle shake from 40 PSI to 60 PSI so dammed fast you can’t see the needle, it will just be a big blur. I didn’t check the FP with the stock FPR so I don’t know if there was a problem before, if it was as bad, or if was something I did wrong (more then likely). After work I’ll put a vacuum gauge inline with the AFPR it see what the vacuum is doing. What the “H” is going on? I don’t see how the FP could go higher than the set level without a positive PSI on the intake.

 

I just installed the Holley FPR a couple weeks ago and I had no troubles (other than clearance with the plenum). I had check my FP before with the stock FPR and there wasn't any fluctuations. Now I have the Holley FPR and using the online Accel FP gauge I have no problems either...right now the PSI is set at 42.

I'm only guessing here, so don't panic, but I would suspect some faulty injectors are throwing off the pressure. Or worst case scenario would be the fuel pump. Did you make sure that the diaphram was installed the correct way? Otherwise the installation is pretty simple...as long as it's not leaking fuel, the installation should be ok.

 

Would bad injectors do this? I have had a hard re-start the past couple of months. I was in a hurry last night so I didn’t pay attention to leek down, I’ll check tonight. Any one have more ideas of things to check?

 

Usually when the needle starts to jump around, it is a sign of a fuel pump getting weak.
Why do you have it set so high at 48? In my experience when you go to higher psi settings (with stock pump), the needle will jump more because it puts more strain on the pump to maintain the higher pressure.

 

48PSI is high but not excessive. It will put a little more strain on the fuel pump but if it's bouncing around like that already, it may need to be replaced. I hope not for your sake as it's not an easy or cheap job to do.

I only thought the injectors...is they were in a sense leaking I thought they may throw off the pulse...but like I said, just a guess. A leak down test may answer that.

Try putting the PSI down to 42 with the vacuum off and see if it gets any better. If so, then maybe the FP is getting weak.

 

Well, It’s looking like the injectors are defiantly bad. Less then a second after the pump shuts off the pressure drops to 0. I hope that that is the only problem. I’ve helped a friend change the pump in his Z28, PITA!!!!! This will just be my excuse for getting the #24 SVO injectors. By the way the 48 was just a starting point to correct a lean condition. I KNOW, I KNOW, DIY PROM. I’m getting there this is just temporary. Hopefully by the spring I’ll be doing some custom PROM work.

 

If you can change the FP on a Z, then you should find the injectors too bad. It was actually much easier that I thought.

 

Dont be too quick to assume its the injectors. My fuel pressure drops to zero immediately on stopping the engine. Before and after changing the injectors!
Put some rubber hose on the jaws of a set of locking vice grips and pinch the return line.
Turn on the ignition to let the pump build pressure, then turn it off and see if the pressure still drops rapidly.
Unfortunately in my case, the pressure fluctuates also, and I will tell you that shortly I will be replacing the fuel pump (Oh Joy).

Sorry but my 0.02 worth

Cheers

Richard