[MidnightAuto]

Looking for advice on a weird sound my fuel regulator makes. This video does not do it justice, but have a listen. I put the phone besides the fuel regulator (when video is dark) and you can hear a fast paced pulse/knock. It’s coming directly from a robbmc fuel regulator. You can feel it strongly when you put your hand on it. I have a 1/2 line going in and 5/16 return line. Any ideas on why?

Also when motor shuts off, you can hear some shreaking coming from the regulator for a couple seconds.

https://youtube.com/shorts/R2_jxBZA9...XxUvUp09yxl0Sx

[ID67goat]

Normal for a mechanical pump. Is that what you are running?

[steve25]

What do you have for a pump, mechanical?

[MidnightAuto]

Robbmc 1100 mech pump

[steve25]

Does this regulator slapping take place as soon as you start the motor when it’s cold or only once the motor is warmed up and using as much idle fuel.

Also did you install the regulator at the same time as the pump?

I don’t know what size return line that manufacturer recommends., but 5/16” seems on the small side to me .

I don’t know but with that regulator it may just be a normal sound as it responds to the pumps fuel flow pulses at times of low fuel volume needs of the motor at low engine speeds.

Also where is the regulator mounted in regards to the carbs needle and seat height wise?

If the regulators inlet and main outlet sits higher then the outlet of the carbs needle and seat then that could be a issue for todays alky cut fuel when the engine compartment is hot.

[78w72]

Originally Posted by MidnightAuto

Robbmc 1100 mech pump

have you contacted Robbmc to ask about it, might be a bad or weak spring inside the regulator?

I have the same 1100 pump on my 400/467, or at least the older version that can be adjusted for PSI, I run a small mallory regulator right now that doesnt make any noise but have also ran the pump without a reg adjusting it all the way down to 7psi that works OK on a later 70's Q-jet.

I would message Robbmc for their advice, they might send a replacement for the internal parts.

[MidnightAuto]

I need to contact robbmc, but figured I would ask here. Regulator is below carbs so ok there. Installed on inner fender well. Pump and reg and lines all installed at same time.

I’ve been reading about a fuel damper. But those seem to be for efi. With that said, this is definitely a “fuel hammer” issue. I just don’t know why it’s happening to remedy

[Dragncar]

The return line needs to be at least the same size as the inlet. If its a size bigger that is not a bad thing.

[MidnightAuto]

I sent the vid to Robb. They are checking it out. As part of this set up, they had me install a reducer fitting in the return line to work with my parameters.

[78w72]

Can mech pumps run a full return line like electric pumps? I recall Robbmc or another source tell me not to run a normal return set up on a mech pump when I inquired about it, said they arent designed for full returns like elect pumps. The "return" some mech pumps have is just a vapor return, Robbmc offers that on their pumps but its a fitting with a very small port for vapor return. Doing a full return with the same size line as the feed can reduce the output quite a bit is what I recall them saying.

Ive been running the robbmc 1100 with and without a regulator and never get any noise like that. Hope they/you figure it out.

[HWYSTR455]

The regulators regulate using an orifice, and what's similar to a needle, like a fuel bowl. (Needle & seat).

A mech pump operates using an arm & a diaphragm, and the arm moves on an arch. So the psi does vary.

The knock is the 'needle' slamming into the seat, or, the arm lifting off and slamming the metal contact point on the diaphragm.

With a rotor vane pump, since the psi is more steady, as it builds up to the setting, it can actually sound like a whine or whistle as the diaphragm is closing the gap to the seat.

That's the best way I can explain it, it's generic explanation, but, it's normal behavior.

In general, it's best to use a deadhead regulator with a mechanical pump. The orifice gap remains constant.



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[MidnightAuto]

the vapor return set up is exactly what i have too, and they told me the same thing when i was setting up - it needs to be restricted or it will rob pressure. so I have my terminology right - I think you are saying to kill the return line entirely (or in this case, its a vapor return line and just let the fuel terminate into the regulator?

Should I not worry about back pressure? I do have this set at 5psi for the tripower.

certainly willing to disconnect the return and see if it makes a difference but wanted to make sure i understood any repercussions.

[HWYSTR455]

Not sure who that was directed to, but I would think trying it without the vapor line is a great test.



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[HWYSTR455]

Many mech pumps don't have a vapor return line, or a regulator for that matter, and work fine.

You needing to drop/regulate psi pretty much dictates a regulator, and a dead head one would be the most appropriate IMO.


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[MidnightAuto]

Thanks hwy. You were on the money. Here is the feedback from Robb.

At idle, the pump is supplying pulses of fuel to the regulator at a pretty slow rate. As each pulse reaches the regulator, the pressure in the regulator body goes up until it reaches the pressure you set using the adjustment screw. At that point, the regulator plunger seats against the regulator body, stopping flow from the pump. When the plunger seats, it makes the knock / pulse noise. Then, the pressure pulse from the pump drops off as the eccentric on the engine’s cam pushes the pump lever down to draw more fuel into the pump.

While pressure from the pump has dropped off, the vapor return fitting on the regulator body is returning fuel back to the tank, causing pressure in the regulator body to drop. This causes the regulator plunger to unseat from the regulator body until another pressure pulse comes from the pump. When that pressure pulse happens, the plunger reseats against regulator body, causing another knock / pulse noise. And so on. Once the engine gets much above idle speed, the pulses from the pump come so fast that the plunger doesn’t move near as much, so there’s little or no knocking sound.

You can check this theory by pinching the return line closed while the engine is running. If the noise goes away, or is reduced quite a bit, that confirms what I’ve explained above.
The good news is, this noise doesn’t hurt anything. You can ignore it.