Adjusting Fuel Pressure for Better Running Engine, 1.7 Engine Performance Options

[ChrisFoley]

You should be tuning by adjusting the MPS not the FPR.

[brant]

You should be tuning by adjusting the MPS not the FPR.

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[Rand]

You should be tuning by adjusting the MPS not the FPR.

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I was waiting for this. I don't buy the idea that you can just turn up the fuel pressure above what it's supposed to be and get magic power. If that actually works, something else is out of whack. It needs to be tuned properly.

[Minerva's 914]

You should be tuning by adjusting the MPS not the FPR.

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I was waiting for this. I don't buy the idea that you can just turn up the fuel pressure above what it's supposed to be and get magic power. If that actually works, something else is out of whack. It needs to be tuned properly.

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I'm in total agreement here. My interest in increasing FP is not to compensate for something being wrong nor to increase power with this adjustment alone but rather something being out of balance. It's possibly out of balance because of the increase in displacement in my rebuild. I don't expect there will be very much of an increase needed and I'm sure it will only be evident at higher RPMs, which I won't see for the next 1200 miles or so after the break-in period however, the factory set the FI for the original engine design and I am bumping up the displacement. So, I will at least play with it and will of course relay any improvement or lack thereof back to the board. I'm not looking for magical increase in power, I'm looking to insure that if the injectors need more fuel at higher RPMs that it's original programing dictated then the FP adjustment will deliver. I want to adjust the feed to match the need.

... then again I could be wrong, it's happened before, though not often (hehehe) unless you ask my wife, she seems to think I'm wrong way more often than I do. Go figure (IMG:style_emoticons/default/confused24.gif)

[914_teener]

You said it was a stock motor. (IMG:style_emoticons/default/stromberg.gif)

You didn.t mention anything about increased displacement or that you had rebuilt it.

[r_towle]

If you increased the displacement you need to tune the MPS
If you have never cleaned advanced plates, it really needs to be done or you will chase your tail.
If you opened the case it is possible the camgear is not set properly
You can add a variable resistor (1000 ohm volume knob) inline with the CHT and keep turning and driving till it respond how you want.
This is a bit of a hack, when you like how it runs, check the resistance through the volume knob, then go buy a resistor of that size and put it inline forever. This will increase the fuel across the whole range, so idle will suffer

[Minerva's 914]

You said it was a stock motor. (IMG:style_emoticons/default/stromberg.gif)

You didn.t mention anything about increased displacement or that you had rebuilt it.

I am not the original poster and you are correct that he did not say anything about an increase but I was chiming in with my own separate interest in playing with the FP

Sorry to confuse

[R Shaff]

Thank you, all. I appreciate the input.

My reactions, for what they're worth:

First, we don't know that the MPS needs adjusting. It might. Probably does. It's only 45 years old. I wish I could do it easily. From what I've read, I doubt I could do it thoroughly or well, even with a lot of effort. (Taking it apart sounds like a daunting task.) If someone has a good way to adjust it and test it properly, without running the risk of screwing it up, I'd love to hear it. In short, if I can get the car running well enough without risking the MPS, I'll probably do that.

Second, even if the MPS could use some adjustment, I don't agree that I shouldn't be adjusting the fuel pressure. We don't know how accurate my cheap pressure gauge is and at this point I don't think it matters. The pressure that worked best on a new car in the factory in 1972 might not be the pressure that works best on this particular car, with this mixture of old and new components, some original and some after-market. I see the 28 psi as a starting point for the tune, not necessarily the right end point.

Third, I will check the advance mechanism (weights and vacuum). Easy enough and worth doing. I'm skeptical of the benefits it will show, however, because of the way the timing is set for this engine (at 3,000 rpm). Clearly the centrifugal advance is working well enough for that, so the timing with the hoses connected shouldn't be far off, if it's off at all.

Again, thanks all. I'll report back with more results when I have them.

[R Shaff]

If you increased the displacement you need to tune the MPS
If you have never cleaned advanced plates, it really needs to be done or you will chase your tail.
If you opened the case it is possible the camgear is not set properly
You can add a variable resistor (1000 ohm volume knob) inline with the CHT and keep turning and driving till it respond how you want.
This is a bit of a hack, when you like how it runs, check the resistance through the volume knob, then go buy a resistor of that size and put it inline forever. This will increase the fuel across the whole range, so idle will suffer

Clever hack about the volume knob. I get it. Might give it a whirl. I could even put the volume knob in the cabin and adjust it on the fly. Try different settings before and after the car warms up, to find the right balance. Thx.

[r_towle]

The weights stick when the old grease gets hot, then say after you wind it up at a stop sign and the centrifugal force overcomes the sticky grease the weight get stuck at full or partial advance and when you shift up, the timing is now terrible for the lower rpm and the car bucks, sometimes quite a bit.

It will keep you chasing your tail for a long time, cost hundred of dollars in parts. Or you could do it and spend 0.05 on grease. I just use normal bearing grease. And I have found it on every car I have touched....quite a few.

The CHT
In 73 the factory used a resistor inline on the 2.0 liter models to enrich that specific year. I used a volume knob because I could drive and tune....takes maybe 30 minutes to simulate all conditions...load, no load, accel, coast on highway . I planned a route to radio shack, measured the switch when I got there and replace it with the resistor....same route home to verify results. Good for years (that car, that elevation...)

I too changed fuel pressure (it was the distributor weights (IMG:style_emoticons/default/smile.gif) ) and that screws up idle and makes a hard start sometimes. 28 is the right number old or new motor, leaky or not. The mps and CHT control the duration of the injector opening....those are the keys.
If the mps holds a vacuum properly I would look elsewhere, but not fuel pressure.

Lastly, I removed my cold start injector from the loop, wiring and fuel..... those drip quite a bit and can make tuning hard.

[r_towle]

Also, the ONLY vacuum line required to the plenum is the mps. I get rubber pipe caps at the hardware store and remove and block off every vacuum line at the plenum. Drive it. Then add one single vacuum line back, drive again.

You have to leave the mps hooked up, but you test it and make sure it's not leaking.
The test for me is i suck on the hose and if it sticks to my tongue it holds a vacuum....move on.

[TheCabinetmaker]

The cap'n would have thoroughly chewed ass here. If your guessing at the pressure it might be 35 lbs or even 40. That will quickly destroy parts of your system. Injectors will be the first to suffer. They will start leaking internally flooding the combustion chamber, and possibly washing the cylinders and damaging the rings. Then they can leak externally and possibly burn your car to the ground. Many very knowledgeable people here have told you it's a bad thing, but you continue to ignore them. Good luck with car.

[R Shaff]

Also, the ONLY vacuum line required to the plenum is the mps. I get rubber pipe caps at the hardware store and remove and block off every vacuum line at the plenum. Drive it. Then add one single vacuum line back, drive again.

You have to leave the mps hooked up, but you test it and make sure it's not leaking.
The test for me is i suck on the hose and if it sticks to my tongue it holds a vacuum....move on.

Thanks!

[R Shaff]

The cap'n would have thoroughly chewed ass here. If your guessing at the pressure it might be 35 lbs or even 40. That will quickly destroy parts of your system. Injectors will be the first to suffer. They will start leaking internally flooding the combustion chamber, and possibly washing the cylinders and damaging the rings. Then they can leak externally and possibly burn your car to the ground. Many very knowledgeable people here have told you it's a bad thing, but you continue to ignore them. Good luck with car.

Sorry Captain, but you won't be hearing any fire engines racing to this house.

(I'm not an idiot. I used a gauge when I first adjusted the pressure, so I saw how sensitive the adjustments are. It was at 32 when I started. I turned it down to 28, and the engine wouldn't pull at upper rpms. I turned it back up a bit, probably in the 30-32 psi range, and it ran great. No fires. And clearly the ignition advance is working or it would not run well at higher rpm.)

That's all. I said the car was running well. Still is. I was just putting my experience out there in case it helps someone else.

For those of you who still think I should adjust the MPS, I'm eager to hear how you would do that. Mine holds a vacuum fine, and it passes the static electronic tests. And again the car is running pretty well. I'm reluctant to tear into the complex and fragile MPS when things are working, but I'm open to suggestions (like the variable resistor idea on the CHT).

Thx.

[TheCabinetmaker]

Buy the mps tuning kit from Chris Foley. I believe it comes with instructions.