Flat spot on a new engine

Competition engines and ancillaries - general discussion
Grundo
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Flat spot on a new engine

Post by Grundo »

Sorry for another topic on a Volumex - can't help it...

I have just re-installed a rebuilt Volumex engine into my beta and successfully started the engine - getting it running basically. The car idles beautifully with no rattles or anything mechanical but won't rev beyond 3000rpm and when put in gear there is no power whatsoever - the car can move on engine idle but stalls as soon as the revs are engaged with the car in gear. Before I start tearing the carb apart etc I thought I would ask the learned folk here as to what could be obvious.

Some facts about the car:

I bought the car off another member 18 months ago after he removed the engine in 2008. Since removing from the car it has had the following modifications:
1) lightened flywheel
2) new con-rods
3) GC forged pistons with associated re-bore
4) new bearings etc
5) re-conditioned head (no porting or polishing at this stage)
6) new clutch and LSD (although this won't have anything to do with the engine)

What is the same:
1) DCNF at the jetting last used by the previous owner after a high degree of fettling
2) Same supercharger and same drive ratio
3) same fuel delivery system although he ran the car on race fuel and I'm using 98
4) same exhaust with a 4-2-1 straight through with a short silencer exiting in front of the rear wheel
5) same airfilter box and filter

I have installed the same DCNF that the previous owner used, and all other ancillary equipment so haven't changed the inlet. Timing has also been checked - but I wouldn't assume that it is perfect.

The car has a lambda sensor installed which doesn't show the mixture leaning out at all (running very rich but then its a narrow band sensor), whether its accurate or not I don't know. If I was a betting man its just not getting air although the local garage guys thought it was leaning out (but then they also got the clutch adjustment wrong..)

I would love some advice please on a methodical approach to working through the problem please. I'm also slightly paranoid about running the engine lean so any information I can get would be extremely valuable.
Grundo
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Re: Flat spot on a new engine

Post by Grundo »

Sorry also forgot to add that the car has a boost gauge that hasn't moved off zero - not hugely surprised given there wouldn't be much boost at those revs and with the engine under no load. It could also of course have a blockage.
Guy Croft
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Re: Flat spot on a new engine

Post by Guy Croft »

No boost bothers me, a VX should go to boost when you 'blip' the throttle. I'd first check the backfire valve on the inlet manifold - it may be stuck open. Unscrew from the inboard inlet manifold and dismantle it and give it a good clean and new O ring.

G
WhizzMan
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Re: Flat spot on a new engine

Post by WhizzMan »

Exactly. The compressor is powered by engine revs, not by exhaust gasses. More revs should mean more pressure.

I would check pressure as Guy advises. Also, check ignition timing (not just static, also advance) and third, take the carb off and write down anything about jetting, settings etc. People here will be able to tell you what a good base setup would be like and how much you are off.
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Grundo Farb
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Re: Flat spot on a new engine

Post by Grundo Farb »

I have checked the relief valve but will again. I don't want 'cloud' the discussion on jetting just yet, remember the previous owner tuned it to suit the air/fuel flows and its not too dissimilar to what I have jetted up on a 45 DCOE I had on another car.

I'd rather understand how one circuit in the carb can effect another if there is dirt/wrong levels etc and exhaust those areas - if that indeed is what it is before mucking around with jetting.
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Guy Croft
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Re: Flat spot on a new engine

Post by Guy Croft »

Here are the other principal probs that non-standard VX engines suffer from:

1. Excessive exhaust system back-pressure
2. Carb chokes too big
3. Distributor bob-weights stuck
4. Flooding carb
5. Blocked jets or defective pump jet (the main jets are too big ever to block really, if the idle jets were blocked it wouldn't idle but if the pump jets are defective the engine will never pick up)
6. Worn out rings & bores (but even then it would likely develop more power than a standard atmo 2 liter!)

None of which explain why:

A. Your previous user ran perfectly well on current carb/jetting
B. You have no boost.

Long-distance fault finding will be utterly impossible here if one asserts that A & B are true but that there is nothing untoward in depts 1-6.

Your post is good, but you need to confirm jetting, investigate the pump jet fuction (should spray into barrels when the throttle is opened - test with engine turned off) ie: give me a bit more to go on.

GC
Grundo
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Re: Flat spot on a new engine

Post by Grundo »

Hello, checked the timing and cam settings - all fine there. Took the airfilter off and checked the petrol delivery, gives a good squirt when the throttle applied - in fact when running it seems apparent that the mixture is too rich as it runs fine until when the throttle is opened the main jets open and it goes rich. The chokes are 30mm so hopefully not too large there.

When the throttle is blipped the pressure gauge does register boost so that seems to be working fine and I don't think its the over pressure valve.

I haven't checked the distributor weights as I don't know how - I have another one off another car which I can use to see if that makes a difference but couldn't see how or where to check them on that - can anyone help with that?

I haven't taken the carb apart yet to check the jetting - that will have to be later sorry.

The exhaust is running pretty freely so doesn't appear to be much back pressure there.
Guy Croft
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Re: Flat spot on a new engine

Post by Guy Croft »

No - don't just swap parts like that! Do it properly.

Turn the engine to TDC with cyl 1 firing and mark the distributor body relative to its mount so you can put it back in the same orientation then unbolt and remove it. Get it on the bench and examine the state of the bob weights and springs, ie: that they are not seized or broken and that they have a bit of oil or grease lubrication.

If you are not familiar with the mode of operation of an (old fashioned) centrifugal distributor this article explains:'

http://www.mgb-stuff.org.uk/ignitiontext.htm

If you click on the 2 photos top left they will expand and show you the various internal parts.

The vacuum advance pipe on the VX distr is ordinarily connected to the inlet manifold, I would blank off at the manifold but leave the vacuum device in place on the distributor. It is a part-throttle economy device only and really not needed - even assuming it works at all.


G
James Bowen
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Re: Flat spot on a new engine

Post by James Bowen »

Are we sure its fuel related?

The symptoms are similar to a defective coil / coil pack. Might be worth trying a spare before unbolting other stuff?

Regards, James
Guy Croft
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Re: Flat spot on a new engine

Post by Guy Croft »

Could be James.

Above all - the main thing is check and test one thing at a time with fault-finding and correction.

G
Grundo
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Re: Flat spot on a new engine

Post by Grundo »

Hello,

right, engine to TDC, took the distributer off, and moved the weights, seemed to move freely, oiled the mechanism and put back on. Checked the jetting of the carburettor;
chokes 32
Main jet 145
Air corrector 120
Emulsion tube F24
Needle vale 300

Checked the flow of fuel to the carb, by taking the connection to the carburettor off prior to the carb, flows well and no blockage there.

Spark plugs white and top of cylinders quite white. I guess that shows lean running?
nabihelosta
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Re: Flat spot on a new engine

Post by nabihelosta »

Hello..

Sorry I don't know much about DCNFs, as I've always worked with IDFs and DCOEs, but a couple of remarks could me made here..
1st: are you sure about the jet sizes?
2nd: if yes, doesn't a 120 air corrector size seems a bit weird?? I mean the main jet is listed 145. During all my work on Webers, I can predict that a 145 main jet, may cope with air correctors going from 175 to 210, regarding the engines modifications. Or maybe the application on the VX is different?

Good luck running your VX brother! These engines are awesome torque beasts!


Nabih
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WhizzMan
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Re: Flat spot on a new engine

Post by WhizzMan »

Could you please also give the following:

- idle jet
- float level

Agreed, the air corrector sounds like off. With that amount of main, I'd expect something closer to 200 as well.
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Grundo
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Re: Flat spot on a new engine

Post by Grundo »

Took the carburettor off, emptied bowl, blew the connection between the bowl and the main jets/emulsion tubes etc out with compressed air, tested the operation of the float which works very well. Checked the static timing which again was fine. Disconnected the vacuum advance.

Started the car again and fuel now flows through one of the auxilliary venturies and the engine now revs higher indicating that the problem was fuel flow, fuel not flowing through the other auxilliary venturi so I will take the carb off again and blow through.

As for the jetting, I will test the carb with both venturis operating and then use a wide band O2 sensor to test the response but I feel the flat spot and lean running has been solved.

I take your point on the air corrector, the information I have suggests a 170 for race day but will see how the engine operates under load first.
Guy Croft
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Re: Flat spot on a new engine

Post by Guy Croft »

chokes 32
Main jet 145
Air corrector 120
Emulsion tube F24
Needle valve 300

OK but the complete list must include:
What size DCNF?
What size idle jet?
What size pump jet?

The big float needle is not needed but it will no do any harm. The fuel level recovery rate would be perfectly adequate at 200 size or even 175.
The F24 tube is OK
The main jet is probably OK the air corrector is miles out. It's job is to tailor the top-end mixture and a 120 - being tiny - would bleed an worthlessly small amount of air into the emulsion tube. Such a small a/c is invariably a sign that the engine is struggling for gasoline - leading to the conclusion that either a) the main jet is too small (which it isn't) or b) the chokes are actually too big.

That said - you may not notice the influence of the choke/air corrector except at higher rpm ie: 5000 plus on that motor (higher on tuned atmo units) may even then erroneously assume that all is well. But in actuality the power and torque will be abysmal and, well, rightly so - on that choke/a/c setup.

You should read James Bowen's post on jetting-up his SOHC where I wrote extensively on the subject. There is no difference betw jetting a VX and an atmo engine except that with the supercharged suck-thru engine the more highly tuned the motor the smaller the chokes have to be for given carb size. This may seem weird but it's true.

GC
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