carburation standoff problems..causes?

[SirYun]

Hi Guy

i have an 4 cylinder a series engine that is giving me a headache.

It is a BL a series engine 1310cc ( midget/sprite/mini)
head is just a rather normal 35/29 ported .
RC40 exhaust, maniflow freeflow/cooper header
cam is a 276 *0.290 lift symmetrical profile

Checking height: 0.016" @ lobe

Timing: (Intake): 34.5/61.5 (Exhaust): 63.5/32.5

Nominal lobe lift: (Intake): 0.290" (Exhaust): 0.290"

Duration (Intake): 276 (Exhaust): 276

Lobe centre angle (Intake): 103.5 (Exhaust): 105.5

Lobe separation angle: 104.5

Lift on overlap: 0.085"



carb is an SU HIF44 (constant vacuum)




the combination of parts is nothing special but it just does not want to play ball.

on the first chassis dyno session it was uncontrolably rich down low and uncontrolably lean up top.

Suspecting only fuel delivery problems I spend ages fiddling and fitted pump/reg and such and had it running decently for just driving around.

However , on return to the rollers it turns out the problem is still there and still huge.



AFR goes to beyond 7 down low to the point it bogs down and leans out up top.

nothing will cure this as it is impossible to get a needle to do this

after taking the air filter of it showed a large amount of standoff/spitback

removing the bellmouth (exponential one) it went from bad to horrific with clouds of fuel drfiting from under the bonnet. (blair was right about the exponential bellmouth). the huge fuel saturated cloud suspended in fron of the carb mounth causes it to go wildly rich...i can get that but how can it affect top end fueling?

i know what i am seeing.
I sort of know what is going on
I have no clue what can cause it
i have even less of a clue on what to do about it.


how far would the cam timing have to be out to cause this much grief ?

induction noise is very LOUD ( as in not legal for track use .. let alone road) and as i suspected there lot of energy there messing up the fueling.

i had a manifold fitted with larger and slightly longer ports and that seems to work a bit better but stupid me replaced it with the one we used before.

the exhaust is not ideal for this cam but could it be such a major factor?

any thoughts very very appreciated

[Guy Croft]

The most likely culprit in the first instance is the camshaft.

I have seen this effect on radically reprofiled cams where the extent (number of degrees) of the overlap event - inlet and ex open around tdc) is too great and the accelerative rate on the cam flanks is too low and chronic reverse flow occurs. This gives rise the noise (roaring) and outgoing pressure waves that cause the standoff, and yes, the fuelling is all over the place and the power is hopeless. Cams like that are very insensitive and finding any optimum FL position is extremely difficult, a problem compounded in your case with a single cam engine, by the fact that you cannot move ex and inlet lobes relative to each other.

You do not state the origin of the cam or the rocker ratio (or valve lift rather than lobe lift), I've guessed at 1.38 rr but of course that doesn't give me the actual lift profile. I strongly suggest you get an A series cam from Kent Cams - these will have known characteristics. All their billet A series profiles are well proven. Copying a spec from someone who's done is always easier than 'going it alone'.

Then, if the results are not measurably better, go down to a 40 carb as I have suggested before. I do not see that you can do much else. I am mindful of the fact that the SU is an extremely difficult carb to tune, it has a needle, a jet and a spring - any of which can mess up the calibration if unsuited to the application. I'm not going to offer advice on that because I can't..

GC

[SirYun]

the cam is ground by Piper cams and the profile (Calver/ Russsel engineering) is actualy quite proven (is has been in use in australia for ages and with good effect and normally produces around 110-120 bhp at the in house engine dyno at Russell).

the rocker ratio is 1.5 but these things are nearer to 1.56 and lift is 0.450 isch.



I have since emailed with Keith Calver and according to him there are several very very similar setups that work without problems.

So far everybody points into the direction of the cam as the culprit.

i can only come the the conclusion that the cam timing is off even though i have rechecked it two times .. .

a Weber will be tried

[Guy Croft]

You may find the cam is just too radical to run with a single carb... all those conflicting pressure waves. And with that kind of rocker ratio the header could add to the problem if not near-perfect for the setup.

You ought to ask how they set them up in Australia with SU carb - if they say it works. Isn't there some guidance in the David Vizard A Series bible on all this?



GC

[Guy Croft]

I was talking to the notable Paul Ivey (Owner of Race Engine Components and contributor to DV's A Series book) the other day and I asked him about running the single 1 3/4" SU on an engine of your kind of type. Paul has built, developed and tested a huge number of race engines over the years and is fantastically helpful to me every time I speak to him.

He sais that when he was working on the units for the Midas kitcar they did use that carb on the 1275cc unit, along with Janspeed LCB (long centre branch) header, Abingdon 544 cam (I found a rather nice website with info see:
http://www.teglerizer.com/midgetstuff/s ... mspecs.htm )
Also 5,6,7 needle, std jet and spring and they were getting 100bhp.

Hope that is some help,

G

[SirYun]

thanks a lot of inquiring Guy! Mr Ivey is quite an A series luminary ..and a 544 is quite enough cam

i have picked many brains , compared how the australians are running them (mostly single hs6 or weber 40's) i have come to the ''conclusion''..

which has to be tested. the dynoguy suggested i do the spannering myself (as he is very busy..as allways) and i will be adressing to following points.

differential diagnosis:

A:the cam timing is off- i quite shure it is not, but not 110%.. it might need some retard.. IVO seems early.

B:the exhaust manifold is an unhappy marriage for this setup and basic calculations show that the LCB (i have one on the shelf) is pretty much ideal. while the freeflow should work ( according to many.. but i have so far found no people running this manifold on a Bigbore engine ) i might not and i have dyno data from a 1000cc with a RE13 that compares various manifolds..this one was the worst of the lot and that was on a much smaller engine.

C: total pipe lenght might need slight tweaking

D: intake is too short/runners too small ( all the aussies run lots more manifold volume and this engine seemed to run better with the larger volume MG metro manifold.. but i changed it back)

E: the combination off all factors including a generous rocker ratio makes for bad stuff happening.

I will get to the bottom of this ! and will post back

[SirYun]

I checked the global cam timing by checking lift on overlap. It seems that the chain has stretched quite a a bit and now the cam is an estimated 3-4 degrees retarded compared to what I set it at. so a bit off. Alas I have to deconstruct half the car to be able to move the cam timing forward a bit