Volumex high rev high speed miss

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Fingers
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Volumex high rev high speed miss

Post by Fingers » June 4th, 2007, 4:14 am

I'm having a high speed miss with my Volumex Coupe. It only happens in fourth and fifth gear above 4000-4500 rpm. It will rev quite happily to 6500rpm in all other gears but will miss a little and hesitate in 4th and 5th, this is at full throttle, it seems to get a little better if I ease off.

The engine is modified as follows:
45DCOE
smaller supercharger pulley
Guy Croft IIIA inlet cam.
Large K&N inlet filter

Anybody have any ideas? It's only just started doing it, and is very frustrating.

Guy Croft
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Post by Guy Croft » June 4th, 2007, 5:32 am

Does the tacho needle flicker when it misfires?

GC

SteveNZ

Post by SteveNZ » June 4th, 2007, 8:13 am

My first guess, and its only a guess, is fuel supply. The pump cannot supply enough.

sumplug
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Post by sumplug » June 4th, 2007, 11:37 am

We have to determine if this is electrical or fuel related.
My guess is the there is a fuel line leak or the pump is airating.

Andy.

Fingers
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I think it's fuel

Post by Fingers » June 4th, 2007, 8:03 pm

No the tacho needle doesn't flicker and it feels like it starving of fuel. I haven't had a chance to have a good look yet but I'll pull the carb to bits and check for any blockages or debris. And it's still the standard fuel pump so maybe it's not keeping up.

But what I thought is that say 6000rpm is 6000rpm, the engine still has the same fuel requirement at 6k regardless of gear selection, the only thing that changes is the ram effect through the grille.

Guy Croft
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Post by Guy Croft » June 5th, 2007, 1:03 pm

If the tacho is stable it's not the primary ignition circuit but it could be HT leads, distr cap or plugs.

If it is not carb icing (can occur at 12 deg C or lower) then my guess is also fuel starvation. I would fit a Facet Silver Top interrupter pump, boot mounted, no regulators etc, connect it direct to the carb.

Don't use a 'Red Top', pressure is too high.

GC

SteveNZ

Re: I think it's fuel

Post by SteveNZ » June 5th, 2007, 2:50 pm

Fingers wrote:
But what I thought is that say 6000rpm is 6000rpm, the engine still has the same fuel requirement at 6k regardless of gear selection, the only thing that changes is the ram effect through the grille.
Fuel starvation is more common in the higher gears due to the longer time you can labour the engine at full throttle. In 1st gear you simply cannot run the carb out of fuel as there is not enough time before you change to 2nd gear.

Fingers
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OK

Post by Fingers » June 5th, 2007, 7:27 pm

Excellent, thanks for the suggestions, I'll have a good look at the ignition fuel systems. There's nothing obvious so far. I'll also get rid of the original fuel press regulator and see if that helps at all. I'll stick an exhaust gas analyser on it aswell and see what happens when it's running rough.

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Post by Guy Croft » June 6th, 2007, 5:30 am

I don't know the pressure output of the OE pump on the Vx, so if you are going to remove the regulator be sure to check it. Much over 6psi gauge pressure could lift the float needle off its seat and cause flooding.

Has it always had this problem? Could it be ex back pressure? What is the silencer set on your car?

CO on the analyser should be 3.8-4.5% under load.

GC

Fingers
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Post by Fingers » June 6th, 2007, 10:38 pm

No it's just a new thing, that's why it's annoying, I like to have everything doing what it should. The exhaust is just three straight through resonators, with just small perforations in their inner walls so not much obstruction to flow at all.

Fingers
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Sorted

Post by Fingers » June 24th, 2007, 9:11 am

All sorted, it just wasn't getting the fuel it required. But also cleanes up the plugs and dist cap contacts.

Now the open port at the front of the DCOE is just to provide an ambient vent isn't it?! Does that need any kind of filter over it to stop foreign objects getting in, there was a lot of road grime over the front of the carb and I'm wondering if this had anything to do with the drop in performance.

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Post by Guy Croft » June 25th, 2007, 8:44 am

You must always use a good quality air filter, otherwise the rings and valves degrade quickly from abrasion of airborne quartz particles, and yes debris (stones, rubber, flies - going straight into the carb (incl entering thru the float bowl vent) could easily cause blockages in jets etc. My personal favourite filter is K&N - and always has been.

You need to have a winter-summer setup for that unit. In summer temperatures you should induct at ambient temperature and in the colder months (12 deg or lower) induct from the engine bay to prevent car icing. This is an extraordinary but very real phenomenon with those units - ice forms on the carb and in the float chamber and blocks the whole thing up. It can wreck your engine if it occurs under load, you feel the performance dropping off and tend to put your foot down which makes it even worse. Of course some thermal lagging and water heating in the carb intake region is quite a straighforward thing to rig up.

In the colder months of course the engine bay air temp itself will still be pretty high so make sure the engine runs relatively cool (75 deg C preferred) and maybe use higher octane fuel to ensure the engine doesn't detonate. I would not however retard the ignition, the Vx unit is pretty insensitive to it anyway.

GC

Fingers
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Joined: July 1st, 2006, 8:35 am
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Post by Fingers » June 26th, 2007, 5:32 am

Excellent, thanks, I have a remote K&N filter on it but it's only for the intake, I'll look at getting something to cover that vent. I even had trouble with it icing during the summer with no baffling around it, but that's fixed now. I'm glad I recognised it from a previous car as I didn't realise it could do so much damage.

Anyway I'm friends with the VX again now, and it got a good polish and wash on the weekend.

Thanks again

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