small increments 78 3,3 engine??

[jeos]

Hi there,
My 930 78 engine which is 100% original has been measured entirely, and all the pressure perfectly, ignition is perfect, the fuel system is perfect, there are no leaks on vacumsiden, but when I accelerate slightly there will be some very small increments from the engine, both when cold and warm. what is it
has experienced it?
, Maybe it's just me who is hysterical

[JFairman]

When you say increments during slow acceleration I'm guessing you mean it mildly surges as it accelerates slowly in first and maybe second gear. Thats is usually caused by a slightly lean fuel mixture.

I have no idea what country you're in and what type of gas you're using so if all your fuel pressures are "perect" and by the book and you're using current day E10 gasoline the old CIS book specs are now wrong.

They are wrong because E10 gas has 10% ethanol in it and the ethanol molecule contains oxygen and adding that to the gas causes a leaner mixture than pure gasoline. It also absorbs moisture out of humind air and that causes other problems with the different metal parts used in mechanical fuel injection.
There are more problems with E10 garbage gas in humid climates like phase seperation in the gas tank and lower differential chambers in the fuel head but I won't get into it more than that right now.

Anotherwords.. if ignition and system fuel pressure is thought to be correct and there are no air leaks anywhere and you're using E10 oxygenated gasoline you need to lower control pressure a little across the board to get rid of the lean surging during slow small throttle accelerations in low gears if that's what you are experiencing.
Lower the control pressure so it runs just a little richer on s__t gas and it will drive better.

Bosch k-jetronic or CIS is an old mechanical system and it was designed and tuned to pass exhaust emissions 30 years ago using real gasoline that is no longer available in the USA.

If it has a 25 year old lambda system on the fuel head and you need it on there working to pass emission testing and or random checks then get used to it because you'll have to live with it running a little lean during idle, slow acceleration, and steady cruise... the way it is.

[RarlyL8]

I just went through this on a client's 930. We had to lower the idle AFR to 13.2 before the surge would subside. In the winter we will have to adjust again as both the air desity and fuel formula change. As Jim says, it is what it is ...

[jsveb]

Excellent question and answers

I have the same engine, same year, and the same symptoms.

Now I know.

A way of testing this: could we pure in some high octane race gas ( off road of course ) and then the bucking should go away, or at least the condition improve?

[JFairman]

The mild surging during slow first gear accelerations I think the origonal poster is referring to as increments has nothing to do with fuel octane. It is caused by a slighty lean air fuel ratio while accelerating slowly like during normal street driving.

Bucking is much more dramatic than mild surging and thats something very different in my opinion but I didn't write the origonal post so I'm not even sure what he means by increments.

If you have an adjustable control pressure regulator the fix for mild surging during slow acceleration is turning the 4mm allen head on the bottom in the middle for overall and steady cruise control pressure adjustment.
When adjusting it I turn it a quarter turn at a time with an L shaped allen wrench so it's easy to see the change on an AFR gauge while test driving afterwards. Then it's easy to pull over and park the car and turn it back a quarter or 1/8 turn or whatever if I want to.

[T77911S]

Originally Posted by RarlyL8

I just went through this on a client's 930. We had to lower the idle AFR to 13.2 before the surge would subside. In the winter we will have to adjust again as both the air desity and fuel formula change. As Jim says, it is what it is ...

what control pressure did you go with?

[jsveb]

I confused myself there.

While "E-type" gas will lean out AFRs in a car tuned for gasoline, it will actually raise the octane rating of the gas.

Sorry for the confusion.

[jsveb]

Originally Posted by T77911S

what control pressure did you go with?

As you can see in my post above, I am clearly not an expert

But, I wouldn't try and copy that. I have initially set my wcp to fact. specs -I believe 3.65b. It still needed fine tuning. For this an AFR gauge is a must.

[JFairman]

While "E-type" gas will lean out AFRs in a car tuned for gasoline, it will actually raise the octane rating of the gas.

True, but if you are in a humid climate and the tank is not completely full all the time then the ethanol in the E10 will absorb water right out of the air and and the ethanol water mixture will sink to the bottom of the tank and stay there rolling around on the bottom rusting the bottom of the steel gas tank because water weighs more than gasoline.
That is called phase seperation and it is a possible reality.

Then it gets worse... the acumulated water and ethanol mixture rolling around on the bottom of the fuel tank will continue to absorb ethanol out of the E10 gasoline and that lowers the octane of the gasoline considerably and then you may have something like 89 octane gasoline when you origonally filled the tank with 91 or 93 octane E10.

Phase seperation can also occur in the lower differential chambers of the CIS fuel head or anywhere else E10 gas sits if you are in a humid climate.
If you have a cast iron CIS fuel head you may see rust in the bottoms of the lower differential chambers when taking it apart and that rust was caused by the water and ethanol mixture collecting there during phase seperation.

You don't have to believe me though so google phase seperation in E10 gasoline and read/learn up on it yourself. It is not BS, it is a reality if the car is in a humid climate using todays E10 pump gasoline.
This is the worst thing about E10 gasline and the corrosion it can cause on metal parts in fuel systems that were designed and put in use many years ago before E10 gas became a reality forced on us by the government.

If you have converted to modern EFI or have a car that came with EFI then there is only the gas tank for phase seperation to possibly occur in because the fuel rails and fuel pressure regulator are continuously flushed out with gas returning to the tank while the fuel pump is running.

With a modern car using EFI and a corrosion resistant gas tank then E10 gas phase seperation pretty much becomes a non issue. As we all know the government doesn't have any interest or care in classic cars and car enthusiasts.

[jsveb]

I believe you.

That's why in aviation you always, if possible, refuel soon after landing to avoid leaving a partly empty tank overnight. This becomes even more important because of overnight cooling causing even more condensation along with this.
Also if you refuel at daytime. The fuel tends to be warmer than it will be at let's say 2am. This means at 2am the fuel has contracted hence become more dense. This causes the tank to not be completely full, once again leaving room for condensation. The bigger the tank, the bigger this problem is.

There is more to it , but this is about to be OT, so ill stop here:

Three options:
Drive your car all the time
Always keep the tank full
Drain on a regular basis.

[RarlyL8]

Option 1 works best.

We set the warm control pressure at 53psi but understand that is one point in a range. The fine tune is set with the fuel mixture screw.
This really is no big deal once you've done it. 30 seconds twice a year and you're good.

[JFairman]

I agree driving the car all the time would be best if possible and you don't mind driving it in the summer rains.

and, for a little control pressure data comparison trivia on another boring rainy day in S. Floriduh...

With the CIS flowtech 20% added flow modified 1987 lambda fuel head, 964 cams, and B&B headers with a gutted B&B muffler my warm motor control pressure is a little higher to compensate for the high flow fuel head.

It is: 59 to 61 psi with AFR readings of 13.5 at idle, and around 14.8-15.1 during steady cruise depending on what kind of mood it's in or maybe it's planetary alighnment...

Cold start control pressure and AFR depends on ambient temperature but it's usually somewhere around 30psi or a little lower. It starts and runs smooth and perfect all the time and I can drive it off immediately after a stone cold start anytime without a hiccup or any kind of hesitation.

CIS with it's mechaical air flow meter that has to work against fuel control pressure to move and do anything is slow responding but when it works, it works nice.

[jeos]

my warm control pressure is exactly 3.65 bar, what will believe it to be made down to?
the manual is + - 0.2 bar so I can set it to 3.45 ??

And thank you for your many answers
super with this great forum.

[RarlyL8]

Yes the range for a Euro 930 CIS is 3.45-3.65 bar. I shoot for the middle then fine adjust as needed. The sweet spot is not the same for every engine as control pressure varies with system pressure and idle fuel mixture settings. Fuel formula and air density affect fine tune. It is helful to set pressure to spec with a fuel pressure gage and fine tune with an AFR meter.