Stuttery engine on throttle blip

[mikeinatruck]

I have a flapper efi in my landy which is only really an off road toy, it runs fine seems to have a good bit of power but for ages it has always had a stutter when you blip the throttle when at idle.

I have checked and rechecked everything, metered out the loom etc but I cannot seem to to find the cause of the stutter.

It has a rising rate fuel pressure reg which gives a steady 336psi at idle.
The dizzy and amp were when they were fitted some time ago.

TPS is correct and I believe the flapper is ok.

Anyone got any ideas?

[DaveEFI]

It's the rate of movement of the TPS which tells the ECU to richen the mixture - are you certain it is giving a clean signal when moved? They are mostly pretty worn given their age. Other thing is dry joints in the ECU - again due to age.

I hope your fuel pressure isn't 336 psi at idle the correct figure is 36, vacuum disconnected. Which will be about 28 at idle with the vacuum connected and a standard reg. On my SD1 EFI, when I still had the standard injection and tried a rising rate reg, I found setting it at 38 psi vacuum disconnected worked best. But I don't like them.

[ChrisJC]

TPS. Check it at the ECU connector. Or swap it out with another one.

Or, switch the ignition on but don't start the engine. If you blip the throttle, the injectors should pulse (which you can hear if you stick your head under the bonnet).

Chris.

[mikeinatruck]

LOL.. 336 psi, dam keyboard! I will double check the vacuum disconnetced reading never new that! You learn something every day.

I will also recheck the TPS voltage, I might just replace it, I thought I already had but maybe its playing up...

Thanks for the advice.

[DaveEFI]

One tip for checking the TPS is to remove it and wire it up as a volume control between say a CD player etc and amp. If it is noisy - ie loosing the signal when turned - you'll hear it crackle. The posh way is to use a 'scope.

[ramon alban]

Anyone got any ideas?

Hello Mike, I have just re-read your January post

http://www.v8forum.co.uk/forum/viewtopi ... ght=#66672

and its reminded me that you have made a few changes to the original Efi system that we are mostly familiar with:

Three come to the fore: eg, KN filter, Rising Rate FPR, AFM mounted on its side,

Each of the above might affect behaviour in an un-predictable way.

# The KN filter takes in air at a different temperature and density than the original arrangement and may also provide a different resistance to rapid airflow as the throttle is blipped.

# The RRFPR probably responds differently to rapid changes in throttle position.

# The AFM was designed to operate in a horizontal plane, and its (two) flaps now both affected by gravity, may be respond very differently when asked to move rapidly as the throttle is blipped.

If any of the above are responsible it may be possible to make changes. If all three are involved and interacting adversely, it may be easier to live with the stutter!

[mikeinatruck]

Hi Ramon,

A few changes since that post, the AFM is now normally mounted and only 6 inches from the butterfyl, the rising rate few pressure regulator is ok, 36 psi with no vacuum and 26 psi idling.

Checked everything again today
Injector Ohms as per book as was thermal switch, AFM, TPS, timing where it should be. at idle vacuum is 20 inches Hg. Mixture is good set at around 2%.

Only thing different than a 'normal' v8 is fuel pressure drops of quick when testing with engine off.

Starting to think I have a a possible valve train issue.

Once off idle it seems to be ok....

[ramon alban]

OK Mike, just been thinking a bit laterally and ask you to check that the vacuum advance system on your distributor is working OK, and not just pulling under vacuum but also it is not sluggish due to lack of lube, or otherwise binding.

Reason as follows.

Vacuum Advance is not conventionally added to Total Timing, it being generally referred to as a separate process. It senses manifold depression only when the throttle plate moves away from the idle position and advances the timing to capture maximum efficiency from the combustion process at low engine speed before handing over ignition advance duties to the mechanical system. By improving launch efficiency it fulfills its primary function of optimizing economy. It becomes relatively unimportant at higher loads and high end performance with little or no vacuum available during part/wide open throttle, therefore no vacuum advance anyway! Some vacuum is available at low load, low engine speed, cruising situations which the vacuum advance system will sense it and feed in some small amounts of advance accordingly.

That paragraph taken from a fuller description of vaccy advance, itself just a small part of my full treaty on the electronic ignition commonly fitted to RV8's of the era.

http://www.vintagemodelairplane.com/pag ... ion04.html

There is also a sister essay on Components & Testing the whole system.

[DaveEFI]

But surely at idle the vacuum advance has little to no action? And blipping the throttle causes minimum vacuum so again the vacuum advance would be doing nothing.?

I'd also expect it to be a major ignition timing issue to make it stumble when blipped at idle. The far more likely reason is the mixture isn't richening as it should under sudden throttle movement.

[ramon alban]

But surely at idle the vacuum advance has little to no action?

I think you might need to read the heart of the paragraph again, Dave?

It senses manifold depression only when the throttle plate moves away from the idle position and advances the timing to capture maximum efficiency from the combustion process at low engine speed before handing over ignition advance duties to the mechanical system.

And blipping the throttle causes minimum vacuum so again the vacuum advance would be doing nothing.?

If no instantaneous vacuum existed when the throttle plate moves away from the Vacuum Advance port, why has the system been installed over all these past years during our cars' era?

[mikeinatruck]

Hmmmm.. I was wondering about the dizzy but its not that old so kind of disregarded it, as you say on idle with 20' in hg of depression the vacuum should advance it up quite a lot the throttle blip collapses that vacuum but that aside if the vac was faulty it would already be close to static so would be more of a problem when the vacuum increases again?

I also had the plugs out and they were a bit sooty but not coked up and the gap was good.

One thing I am considering is the valve train, I am lucky to have over 25psi oil pressure at hot idle, nope its true! I have a capillary gauge to prove it so oil pressure can get quite high, i was wondering if I might be pumpin the lifters up a bit too much? A couple of them tap quite a bit from time to time..

I am thinking of throwing in some HLC to see what happens...

[ChrisJC]

Or, switch the ignition on but don't start the engine. If you blip the throttle, the injectors should pulse (which you can hear if you stick your head under the bonnet).
Chris.

Have you done this? It will cost nothing but time.

Chris.

[DEVONMAN]

Hmmmm.. I was wondering about the dizzy but its not that old so kind of disregarded it, as you say on idle with 20' in hg of depression the vacuum should advance it up quite a lot the throttle blip collapses that vacuum but that aside if the vac was faulty it would already be close to static so would be more of a problem when the vacuum increases again?

...

Going by the statement above, you seem to be under the impression that the 20 in HG idle vacuum is advancing the ignition at idle. On a RV8 there is no vacuum advance at idle and as Ramon has said it only occurs as the butterfly just moves away from the idle position. This would be at light cruise condition only. The vacuum advance then reduces again as the butterfly moves to the fully open position.

Not much help with your problem I know.

If it was a carb engine the strutter would usually be due to lacking in the accellerator pump area so my guess is it's a fuel supply/mixture problem.

Denis

[mikeinatruck]

Hmmmm.. I was wondering about the dizzy but its not that old so kind of disregarded it, as you say on idle with 20' in hg of depression the vacuum should advance it up quite a lot the throttle blip collapses that vacuum but that aside if the vac was faulty it would already be close to static so would be more of a problem when the vacuum increases again?

...

Going by the statement above, you seem to be under the impression that the 20 in HG idle vacuum is advancing the ignition at idle. On a RV8 there is no vacuum advance at idle and as Ramon has said it only occurs as the butterfly just moves away from the idle position. This would be at light cruise condition only. The vacuum advance then reduces again as the butterfly moves to the fully open position.

Not much help with your problem I know.

If it was a carb engine the strutter would usually be due to lacking in the accellerator pump area so my guess is it's a fuel supply/mixture problem.

Denis

So the point at which the pipe comes off to feed the vac advance on the dizzy is before the butterfly? so at idle it only has atmospheric pressure, but when the throttle opens there is a short pressure drop advancing the timing?

Well the dizzy is okay as i can suck on the hose and hear a change in engine speed, what about if the small hole is blocked so I had no advancement?

Chris yes have tried the throttle blip, all injectors are pulsing.

[ramon alban]

as Ramon has said it (the vacuum sensed at the distributor) only occurs as the butterfly just moves away from the idle position. This would be at light cruise condition only. The vacuum advance then reduces again as the butterfly moves to the fully open position.

Not much help with your problem I know.

Dennis, What I asked Mike to check (in my earlier post) was

OK Mike, just been thinking a bit laterally and ask you to check that the vacuum advance system on your distributor is working OK, and not just pulling under vacuum but also it is not sluggish due to lack of lube, or otherwise binding.

The point being that even though the engine may have a usable vacuum, if the Vacuum Advance mechanism is laggardly or binding then blipping the throttle would not invoke a quick enough response to capture the energy normally provided by a good Vacuum Advance mechanism.

Sure, it's a suggestion right out of Left Field, but it will remain untested until Mike confirms the mechanism is AOK or crap, instead of debating it's merit!