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MG MGB Technical - Pondering carb intake mani venturi effect

I had an amazingly frustrating situation with a new rebuild and nothing I tried fixed it until finally someone with a A/F meter showed how the carbs were being straved in the higher RPMs. (that was the problem where the car choked at higher RPMs and nothing fixed it). Played with all kinds of needles, etc.

Took off the K&Ns and put back the stock filter cans and the A/F showed a better graph and cured the problem.

In recent reading I was interested in reading the supposition that too much porting and polishing of the intake manifold hurts the fuel intake more than it corrects anything. The theory was that the rough casting interior-wise helped the Venturi effect.

I originally had Huffaker PP my intake and it ran great (with stock cans). When that gave up the ghost I had a replacement PP'd.

I'm wondering if there's more of an art to how much and where the metal is being removed in the intake.

Experts here have significant experience and beneficial opinions which I hope to elicit.
Max71

Max
You have answered your own question
If the problem was fixed by fitting the original air filters, it means you need some richer needles to go with the K&Ns---
As far as polishing the manifold goes,Lots of theories on this, but yes it can be a problem if the manifold has longish or curved runners where the fuel charge can become devapoured by settling on the surface---- a rougher surface can create enough turbulance to stop the charge settling on the walls of the tunnels
This isn't a problem with MGB manifolds, being short and straightish so polish away.
The main thing to watch for with your pp'd manifold is to make sure it hasn't been opened up larger than the port entrance in the head---this is a no no and is worse than having a piddly little manifold
The manifold needs to be either matched to the head or the start of the inlet ports in the head matched up to the larger manifoldf to remove any obstruction for the incoming charge

willy
William Revit

No - don't polish, a matt smooth surface has the best effect on airflow.

Opened up ports will slow air speed and affect low down torque but will increase flow at higher rpm.

Yes - K&Ns need richer needles - usualy AAM or AAA.

However the K&Ns need to be the deep ones - the shallow type restrict airflow into the carb.

Also fit stub stacks inside the K&Ns to smooth the entry.
Chris at Octarine Services

Oh I did try 4 different needle profiles and even a rare one from a head builder that were richer at the higher RPMs. Nothing worked. None had any positive effect.

These were the deep K&N. Builders were all surprised since other cars just needed a little richer needle.

While fixed, I have wondered why this happened. When I read that article I was curious if the intake could've had anything to do with it.

Huffaker did the head and it was re-worked (new valves) by Sean at Top Speed in Oregon who was the top head guy until he stopped doing it.

When I originally had it on a dyno the dyno guy claimed it was the wrong cam or something. All turned out not to be the case.

Even had the venturi's fitted to the plate going into the intake.
Max71

There shouldn't be a problem with moving mixture faster, with fewer obstructions. Putting the original paper filters in will only slow down the flow and weaken the mixture. But there may be a problem shifting the exhaust out. You can't get more in unless the combustion chamber has been properly scavenged. The cam, or the cam timing could be the problem or the extraction capabilities of the exhaust system.
Allan Reeling

Cam is fast road cam. 10:5-1 compression. PP head. Stock carbs. Stock exhaust.

For the longest time I thought it was cam timing. Who knows maybe that is a culprit. The only thing I cheaped out on was getting a vernier grear. So its a special key
Max71

Interesting that no needles made any difference
Impossible really
The aircleaners wern't blocking off the dashpot vents by any chance-------
What was the gas analyser reading at revs when you had the problem
It doesn't sound like a cam timing problem to me if it goes ok with the std filters
willy
William Revit

Allan,

At any given rpm the engine is pulling the same volume of air - depending on the restriction in the intake system the weight of air will vary. Porting, K&Ns,etc results in a heavier and thus denser charge of air.

The amount of fuel added to the air by the carb will vary depending solely on the level of vacuum in the manifold and the needle profile.

Fitting paper filters will increase the level of vacuum and thus lift the needle higher and supply more fuel while at the same time reducing the weight of the air in the charge - this is why a richer needle is needed with K&Ns - more weight of air needs more fuel, less vacuum in the system means lower needle position.

Because there is a greater cross section with ported inlets and the charge flows easier with less of a pressure drop, it slows down, not speeds up.

Max,

I am intrigued by an air flow meter used with the filters on - what sort was it?
Chris at Octarine Services

Chris, Thanks for that, the bit about the filter restriction increasing venturi vacuum makes absolute sense. Also get the air speed thing, hence twin port V8 manifolds etc... What's your take on Max's problem? Could it be that the exhaust is too restrictive and can only handle waste from a "thinner " charge? My own modified 1800 was ported by me, 3 angle seats, bulleted bronze guides, 60thou bore and balanced + a slightly hotter cam, with K & N's, and ABD needles. It did have a really good exhaust system and went like a train. 131 ft/ilbs and 105bhp at the flywheel. Power started dropping at 5000rpm.
Allan Reeling

Chris,

Thanks for the very good explanation on basic engine operation. I for one sometimes forget the basics, and tend to read advertising claims to increase performance, etc. and spend money needlessly for very little gain. Funny how that works as we get older ;-)

We have a stock '74 B/GT with the factory HIF SU's, and full factory smog gear in place. I installed K&N filters (from Moss USA) that fit inside the factory air cleaner assemblies about 25 years ago (approx. 40K miles), and kept the standard needles. Originally thinking (believed) I would get better gas mileage, and a boost in performance. Once I had the SU's adjusted I found little to no difference in either, and in the end probably a waste of money given how we use the car. Mostly club events, and touring.

Regards,

Larry C.
Lawrence Cordeiro

Allan,

ABDs are much the same as the standard AAU needles - I would normally fit AAM or AAA needles with K&Ns.

Not enough info on Max's issue to offer a cause or solution, I doubt it is the exhaust as the standard one is pretty good anyway. I doubt it is the inlet manifold either.

Chris at Octarine Services

Something that has crossed my mind (short trip) is that maybe the fuel supply or fuel filter or whatever is at it's limit or slightly blocked and with the extra demand from the richer needles with the K&Ns it can't cope A pressure/flow test would tell the story here
willy
William Revit

Or indeed the fuel pump. My SU pump had given up last year, and on switching to the back-up Facet, the difference in high end performance was really noticeable.
Allan Reeling

"maybe the fuel supply or fuel filter or whatever is at it's limit or slightly blocked"

Assuming electric easy to check. Direct a fuel hose into a container (it may spurt when disconnecting if the ignition has been on recently), turn on the ignition and it should deliver a minimum of one Imperial pint per minute - and in practice more than double that - in a continuous series of pulses with minimal bubbling. If it can do that, it's not pump or delivery to the point you are testing at.
paulh4

Sorry for the delay. I'll do my best to answer.

Because it was a maddening problem that lasted years I was resigned to thinking the lump would have to be pulled and everything checked.

I don't remember the brand of A/F this gentleman had but it went into the exhaust pipe. We went for a run and looked at the graph which showed the issue. I put in multiple needles with the same issue.

Another person recommended putting on the stock cannisters which I was deeply suspicious but thought why not. Did a run and could bury the tach deep into readline without issue. At that point I didn't want to test fate anymore and left it like that with needles that had a richer profile at the top end. I could probably get away with less of a profile but it was fixed and I was delighted.

I can assure you there were no restrictions in the intake manifold nor the PP'd head with big valves. Vacuum is stable and took readings with a vacuum gauge which I don't have at my fingertips but readings were normal.

Looking into everything I even looked into running differnt pressure fuel systems beyond the stock. I tested the stock and it was fine pressure. Honestly, I gave up after trying everything people suggested.

Then when I read about leaving intake rough to improve the venturi effect I became curious in case it could help someone else.

If this isn't enough info please let me know.
Max71

All good--as long as you're happy now - that's all that matters really
willy
William Revit

We have had problems trying to fuel SUs when the inlet manifolds have been polished, they are best left rough to encourage eddying at the boundary layer helping fuel and air to stay mixed properly. Radiusing the inlet manifold on the top side also has a bad effect of fuel homogeneity.
The worst problems have occured, for us, when folk have smoothed the raised sharp edged section in the carb where the fuel jet is. It is sharp edged for a reason, to promote localised pressure drop to lift fuel at low gas speeds. Smooth it off and it will show more flow on a bench but not lift and mix fuel into the air stream properly .
Once into the ports on the head and surface finish seems to have less effect, however fuel economy is better with a rough surface.
Peter
Peter Burgess Tuning

That's it!! Thanks Peter.

Every intake I've had then was done incorrectly and I imagine that was part of the problem. I'll keep a copy of your reply whenever I can source a new intake.
Max71

The factory went to great lengths to 'tune' the manifold. We tidy up where core slippage/joins are, otherwise we leave well alone. That includes experimenting with the restrictor in the cast balance tube, the factory 'got' it right :)
Maniflow make a steel manifold for the B carbs. We had great problems selecting the correct needle profile, in the end the manifold was worth a horsepower or so which is great for a restricted race class but a little fussy to set up for road use.
http://www.maniflow.co.uk/index.php?view=product&from=162&product=1204

Peter
Peter Burgess Tuning

Peter
Yep all good but
Here we have Max going out to get another manifold when in his opinion his car is now going good with std. filters - but won't perform with the K&N filters
I just can't see how the manifold can cause this, I accept that polishing the runners isn't the ultimate but if it goes ok with one lot of filters and not the other how can it be the manifold causing it, there must be something else going on there---
willy
William Revit

The cannisters create a greater pressure drop on the jet and increase fuelling which may be enough to more or less overcome problem. It really needs to be on a rolling road to find the reason but I am guessing as I have seen this with polished manifolds and/or bridge work. Radiusing the manifold instead of leaving the OE design really does upset the fuelling. The factory did its best to fuel reasonably at all rpms and throttle openings.

Peter
Peter Burgess Tuning

Peter
Please don't get me wrong, I fully accept your take on this, I'm not questioning that at all and I know how easy it is to progress on backwards when modifying things, it's just interesting that the filters can't be compensated for with needles having the manifold as a constant part -- With the correct mixture for both sets of filters I wouldn't have thought there would be all that much difference in performance between the two really-- I'd love to know what was happening with the different needles that were tried in it- like did the mixture get richened up to suit the filters and then it still wouldn't go or didn't the needles used do the job to get the mixture up to the mark----who knows---not me
Cheers
willy
William Revit

I know it is a head scratcher Willy, would have been nice to see it misbehaving on our rolling road :)
Peter
Peter Burgess Tuning

Sorry I'm a ways from the UK. I bought several needles trying to resolve it and even that one profile which still didn't: MME

I tried the following:

#5, #6 #7 AAA AAM, CPM, MME (richer at top) needles.

Not racing out to get a new intake. Peter, do you think its worth it to re-fit the K&N's? Or leave well enough alone?
Max71

Leave well alone :)
Peter
Peter Burgess Tuning

:)
Max71

For what its worth my MGA roadster with 1800 3 brg engine, alloy std head, mild cam, extractors, with 1.5" SUs runs MME neddles with K&N tapered filters. Seems to go okay.
Mike
Mike Ellsmore

Max, have you had a look at the mintylamb website (odd name i know!) that has an SU Needle Compare-o-rama that shows the difference of between 5 needles at a time. see link below
http://www.mintylamb.co.uk/
Mike
Mike Ellsmore

Thanks Mike! I have checked SU needle sites. No sure if I looked at that one which I will now.
Max71

This thread was discussed between 23/03/2017 and 01/04/2017

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