Engine surges and a/f ratio increases during cruising rpm, not in boost.

This is a 4.6 dohc ford mod motor in a jet boat. I am not running a bypass valve. When manifold vacuum shows close to zero, fuel pressure is already climbing. I've figured out that my fuel pressure regulator is seeing boost signal from the carb hat before engine sees boost and is overpowering the floats.

2 part question.
1. What brand of, or model open vent bypass valve should I run so I can vent charge tube to be equal to manifold until manifold sees boost?
2. Where should I mount bypass valve, carb hat/charge tube? Or pre intercooler?

Charge tube and intercooler have 3" i.d. Lots of volume.

Thanks in advance.

 

The problem I see is that at near zero vacuum the bypass will already be closed. The proper fix would probably be tailoring the boost to come in later, but wonder if it would be worth a try to reference your fpr below the throttle plates, and keep the base fuel pressure as high as feasible, or maybe maybe run it as is, but with a lower base fuel pressure.

 

Thank you for the reply. I have tried lowering the base fuel pressure as low as possible and it does no good. And tried referencing manifold to regulator and setting pressure high. To get it high enough to compensate for carb hat pressure it would flood also. Are the Procharger race bypass valves able to be adjusted so they are open until zero vacuum and then slam shut?

 

So I understand, when you say AFR increases when it surges, is it going up numerically, or down. What #s are you seeing.I dont know if the procharger race valves close at zero. I know the smaller procharger valves do.

Im assuming its going rich.

Ive never had issues with fuel pressure causing issues at cruise with no bypass valve. I have had problems with fuel regulator over powering the needle and seat when dumping throttle and the air stack up in the hat for the split second spikes the regulator.



IF you have a good hat on it, technically the bowls should be pressurized equal to the boosters and hat pressure, which follow reference to the fuel pressure regulator . So just the pressure shouldnt make it move anymore fuel. But if the bowls dont have the same pressure, which is common, especially with short and/or crappy hats, fuel pressure could over come the NS. Vent tube extensions could help this situation. This will guarantee the bowls are getting equal to or more than hat pressure, though it very well could richen up your current tune.

All that being said, I would definitely put a bypass on it first.

 

Tbird, yes, AFR is richening. I run under part throttle, cruise 3000-3200 rpm, 13:1 AFR. It will surge uo in rpm and afr goes into the 12's. I run a CSU 750 carb and a CSU carb hat. I have base fuel pressure set at 6 psi. As i get down to 3-4" vacuum at about 3k rpm it begins to push fuel pessure up. I have seen fuel pressure as high as 11-12 psi before manifold pressure is at zero. CSU also said this was due to no bypass valve. I have an extremely efficient exhaust system. This is a boat, so at 3k rpm it is like at truck pulling a 6% grade. There is a constant load on the engine. I have already swapped out fuel pumps and regulator. I believe CSU and the reasoning makes sense. I am asking what recommendation on bypass valve and location.

 

Optimal location is as close to the hat as possible. The farther it is from the hat the less effect is has.
And if you have any turns right before the hat , like say a 90 deg turn, put it on the long/outside of the turn.

The only bypass I have ever used is the procharger proflow surge valve on street/drag cars. With blower cars at cruise it would push some pressure but never has effect on driveability.
Turbo cars had no effect on fuel pressure at cruise.
Im not positive it will be big enough in your situation. Might want err on a bit larger. Honestly though, any bypass is a huge help. Just make sure its a BYPASS and not a BLOWOFF. Or that is stays open under vacuum.

 

keep us updated

 

FWIW, I had ZERO luck with the fpr referenced to the intake, at least with a junk hat. Apparently there was too much pressure differential between the hat and the intake to keep the bowls full. It would pop and sneeze about 400' out at the track.

 

Yeah, it's not the way to do it, but thought it may be worth a shot, especially since it's on a boat, and they're loaded different than we're used to with cars, and gear changes.

 

Tbird, thank you. I'll have to wait until it's warmer to get the boat back in the water. But I'll update once I find out

 

I missed the comment about at or near zero vacuum if a bypass is closed you might still have the issue. Agreed that it might be a problem.
Could you do a cable operated bypass. I dont know all the load variances you have. Not a boat person here. You could do a slotted cable hooked to your throttle lever so the bypass stays open in your high loaded cruise range and as you open the throttle where you want to make boost it will pull it closed.


What about your power valve. Do you have a vaccum ref , or boost ref.

 

Didn't even think of the PV, but that could easily be it.

 

I have no idea about the pv. It is a CSU carb that I ordered specifically for this build. The guy at CSU is the one who recommended the bypass valve.