The V8 Owners Forum -

Hi

Something that has always happened but I just drove around the problem.

In the GT40 RV8 3.9 JE102 cam balanced and a screamer! Hotwire ecu

So drive on road, lift off and use engine braking, once slow, put in the clutch and engine dies.

Same place, driving, dip clutch and brake to stop and the engine idles perfectly.

Only non standard thing that I can think of is that the normal take off for the advance on the dizzy comes straight up near the butterfly, but no height available under the deck lid means it was blocked off and a T taken from the fuel pressure regulator to plenum tube.

I presume that the vacuum reading is the same in bothtake off positions so this may be a red herring.

OR
does the standard advance take off sample the vacuum before the butterfly and hence will be at a different vacuum to the plenum at idle when the butterfly is closed?

Only reason I ask is my daughter drove the car for first time over the weekend and really battled not to engine brake and stall the engine each time she slowed.

Any thoughts guys?

Thanks
Ian

TVRs had a similar problem caused by a faulty idle stepper motor if I remember correctly.

The dizzy advance take off point has no vacuum at idle or overun. If you have connected the dizzy advance canister to full plenum vacuum the timing will be all over the place unless your dizzy has been set up for that situation like on many Americam V8's

DEVONMAN wrote:The dizzy advance take off point has no vacuum at idle or overun. If you have connected the dizzy advance canister to full plenum vacuum the timing will be all over the place unless your dizzy has been set up for that situation like on many Americam V8's

Many thanks for that so would it be better to run no vacuum?

Not many miles done each year so any loss in consumption not to big a deal!

Ian

Yes, Just clamp the tube and see if it makes any difference. It would be worth checking the idle timing with and without the canister connected. You may need to speed up the idle rpm if you disconnect the canister.

Thanks I will give it a try.

Just to clarify, if the vacuum take off is on the filter side of the butterfly, surely it only ever sees ambient air pressure? Or does the air rushing in cause a Venturi effect to generate the vacuum needed for the dizzy? Just I cannot see any Venturi near the butterfly / original take off position.

Cheers
Ian

Ian Anderson wrote:Thanks I will give it a try.

Just to clarify, if the vacuum take off is on the filter side of the butterfly, surely it only ever sees ambient air pressure? Or does the air rushing in cause a Venturi effect to generate the vacuum needed for the dizzy? Just I cannot see any Venturi near the butterfly / original take off position.

Cheers
Ian

The vacuum take of is set just next to the edge of the butterfly so that it gets vacuum as the air rushes past it when the butterfly is just cracked open. (Light throttle conditions). The small gap between the edge of the butterfly and the bore effectively becomes a venturi point and the air rushing past causes a vacuum in the take off and advances the ignition only on light throttle.

Ian, If you have had this problem long term then I suspect that you may have a problem with your road speed sensor. Check that it is working correctly, google it, it can be dismantled and is only a magnetic rotor and a reed switch in series with a 120R resistor.
Failing that check/set up your base idle speed, again plenty of info on the net.
Hope this is of help.
Andy.

If the speed sensor is faulty then on engine braking it may be telling the ecu that the car is moving faster than it really is, and therefore the injectors pulse is switch off too long on overrun. However this switching off feature should also be controlled by engine revs.

On a down hill approach to a fast road roundabout my engine often dies if I have to suddenly dip the clutch and brake due to quickly appearing traffic from the right.

Thanks guys

No road speed sensor not on. 1990 system CUX Hotwire not CU flapper

And thanks for the explanation re the Venturi and butterfly just cracked open, certainly never thought of that, just one side of butterfly or the other.

Next time the car is out I will try with no vacuum connected and see how that goes

Ian

DEVONMAN wrote:


On a down hill approach to a fast road roundabout my engine often dies if I have to suddenly dip the clutch and brake due to quickly appearing traffic from the right.

Exactly what happens on mine.

Ian

Ian, The Flapper injection system does not have a road speed sensor but the Hotwire system needs one.
If you PM me your email address I will send you a simple explanation of the operation of the speed sensor that describes the problems you have.

Many thanks for the snip of the article which I believe is fully found here.
http://www.gt40s.com/forum/site-questio ... nside.html

So I have never had a road speed sensor, also have no spare connectors that are unused, other than purge valve. So where in the loom would these normally connect?

With this in a GT40 replica it is not a standard Rover /TVR / Morgan gearbox so no connections to a speedometer. I can sort out a bolt counter on the CV or similar that get a road speed signal, but where to connect the other end?

Also as this thing does about 160 top end I certainly do not want it to be limited by the same system that limits other vehicles to 115mph.

Perhaps I should still go carb, none of this electronic stuff getting in the way of driving it

Ian

Road speed sensor is wired between ignition switch 12v and pin 6 on ecu .
Expects to see about 6000 pulse per mile.