Dual Carbs Vacuum Problem

[3zeR]

Hi and hello.
This is my first post here, but I've hope that it wouldn't be the last one.

As I wrote to Mr Croft I've built Lada Samara engine for the user "kowal".
It is 1500 cm3, 8v, dual carbs engine for amatuer rallies. Everything works great but there is a problem with vacuum in one of the barrels. There is too much underpressure after throttle. All other barrels are nearly on the same level, but not this one. We've checked the carbs, but everything is ok. What should be the problem? You can see it clearly on the photo.
best regards
Konrad

[Guy Croft]

Konrad, hi

start by reading this thread that I was working on last week and see if there are any checks you can do like the ones I descrined, let me know how you get on:

http://guy-croft.com/viewtopic.php?f=13&t=1664

G

[3zeR]

Hi again,
I've read this topic, but everything is rather correct. Even when you open the throttles, the pressures are rather similar. The problem occurs on the idle run. There are a little regulating screws just next to the vacuum test pipes, but screwing in or out doesn't do enything to the vacuum after the throttle. I've regenerated this carbs few months ago and everything should be ok. The engine runs well, only this reading on the vacuumeter is weird.

[Guy Croft]

What does a compression test tell you - on the cylinders?

G

[3zeR]

Yes, this one we've didn't checked :/ We must repair our measuring instrument and then we'll do this. But why there would be so big difference?

[Guy Croft]

First a bit of background.

At idle speed (ie: very low speed) the throttle plate is not set fully closed - because the engine would stall (and anyway there is fractional leakage around the spindle and edges of the plate). So if you imagine the first full 2 rotations of any cylinder, at commencement of the very first intake stroke the region between the throttle plate and valve - the port, in effect - is actually at atmospheric pressure. As the inlet valve opens and the piston accelerates down the bore the seal created by the throttle plate causes a pressure drop there. When the inlet valve closes the pressure in the port starts rising again - the rate determined by the sealing of the outer region.

This cycle is repeated every two revolutions (because of course a 4 stroke engine only inducts every second revolution). Thus, in the port, depending on the 'whole' cylinder condition and valve sealing you actually have a cyclical variation in depression that that doesn't really show because of the viscosity and inertia of the mercury in the manometer. I say whole cylinder condition because it's not just a function of bores and rings, it's the whole combustion event. The more advantageous the the cylinder condition and sealing the higher the vacuum and vice-versa and thus the vacuum reading is indicative of both. It is not relevant in terms of fault finding, but still worth noting, that vacuum is very much affected by ignition timing, the more advance the higher the reading, which goes to show how much vacuum is affected by the firing event.


The sealing is affected by the following:

1) the quality of the throttle plate sealing
2) the relative throttle plate position barrel to barrel on each carb
3) relative carb to carb throttle position (ie: the balance carb to carb)
4) the setting of the air bleed (bypass) screws if used on those carbs
5) presence of any air leaks where the inlet manifold joins the head or where the carbs are bolted to the manifold
6) servo or any other tappings in to the manifold. A servo (booster) must always be fitted with a non-retunr valve.

Naturally, if the bores of the inlet manifold are cross-linked downstream of the throttle plate the vacuum readings will be completely meaningless and it's not a design practice I agree with. Bores should always be kept sealed from each other.

The 'whole' cylinder condition is affected by the folowing:

1) the sealing condition of valves, rings/bores. This is pretty obvious.
2) pressure wave effects in the ex pipes on valve overlap, given that the ex valve is still open some amount after TDC on the intake stroke. It is common to see real odd firing behaviour due to, say, variations in primary or secondary header pipe lengths or pipes way too short or even completely incorrect linking of the primaries. Scavening of the cylinder after the firing stroke is heavily dependent on the header configuration. Contaminants in the cylinder (exhaust residuals) dilute the new charge and slow down the combustion and reduce power on the next stroke. (This is what EGR - exhaust gas recirculation does, oddly enough actually forcing the driver to increase the throttle to get more power and take the engine out of a range where combustion is least efficient)
3) quality of combustion - spark intensity, fuel/air mixture. Suffice to say 'dodgy' spark plug or lead or a fuel leak into the manifold on one carb due to say, flooding or a dripping pump jet, can really upset the firing event.

Note: With aggressive cams with a lot of overlap one expects the effect of residuals to be troublesome at lower rpm (and of course it is one reason such cams can cause pretty dismal performance 'off cam' as it's called) and it can be really compounded by high static pressure of exhaust gas (back pressure) in the pipes between cylinder and muffler. High back pressure is a very different thing from pressure wave effects and it affects all cylinders equally and is typically a result of 'over baffling' - unlikely to be a cause for concern here.

I will assume you know perfectly well how to check the sealing issues above. Now, how to check the 'whole' cylinder condition.

First do a hot compression test and do it wet and dry to check ring and valve sealing. Make a careful note of the results.
Then get a Colortune device (see links) and put it in each cylinder in turn and examine the firing even thru the window in the Colortune plug. You may find interesting differences in the stability and quality of the burn that might just tie in with the difference in vacuum reading. My guess is, that if you have fulfilled all the sealing checks this may actually pin the problem on a firing event. The other simple way to see how well each cylinder is behaving is to hook up a an accurate rpm gauge and note the steady idle speed. The pull off one plug lead at a time and let the engine stabilise for a few seconds and note the rpm drop. On a good engine they will all be the same! This is called a 'power balance' test.

I'd like some photographs of your setup please. In particular I want to know all the dimesions of the header and the type of layout (4-1, 4-2-1). Remember the less detail you share with me the harder I have to think to derive the right answer.

I hope this is helpful at this time.

GC

[kowal]

Guy,

Since Konrad in the brain of the whole operation and I am 'only' the owner of the car you are discussing about I won't help much but the you will find photos of the engine in Readers' Cars section: Lada Samara 1.5 8V 2xSolex C40.

The exhaust manifold is 4-2-1 (starting with 38 mm diameter to end with 50 mm).

Anyway thanks for your help, I guess it'll keep Konrad busy for a while:) When he gets home he'll probably give as much information and as detailed as possible. I hope that between you experts you will come up with some solution.

Regards,
Piotr

[Guy Croft]

What are the lengths of the primary and secondary pipes Piotr?

G

[kowal]

This only Konrad can tell, by measuring it (I'm in Warsaw, Konrad and the car are in Cracow) since I don't have such info, I'm affraid.

Regards,
Piotr

[3zeR]

Hi Guy,
I know exactly what are you talking about, but this doesn't solve the problem that the pressure in the manifold is too high not too low. But the engine is running well. We will see how it would be working after breaking in.
regards
Konrad

[Guy Croft]

I don't have the engine here but what you may be seeing is the high vac one correct and the others too low. How high the highest is depends on the throttle plate setting. The best firing cylinder will develop the highest vacuum at the lowest throttle setting the engine can take before it stalls.

You need to go thru the check regime now don't wait till after break in. If you don't a) you might damage the engine and b) I have completely wasted my time writing that reply..

G

[3zeR]

ok I'll check compression first. that should maybe tell us something. If not I'll try to check the rest.
thank you
Konrad