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MG MGB Technical - Rich carb tuning using vacuum blocking

Sorry, couldn't come up with a better title for this non-mainstream technique. While Googling for info on flow and AFR in the HS carb, I ran across the fellow, somewhere in the Midwest, USA, young, mechanical engineer, who'd done some flow work on the HS and HIFF carbs. He was leaning out dangerously-22-35 AFR-- and tried both heavier oil and stronger springs to slow the piston rise under WOT. They worked, but not enough, so he tried blocking off one of the 2 vacuum port holes in the chamber piston and plugging down the orifice diameter of the other hole. Claims it was the ticket and he was able to dial in the AFR thereafter.
I've never heard this one before. Have any of you of the LBC ilk tried this as a tune technique?
Further, he devised a measure stick to track AFR at each of the 14 stations. He drilled the pot damper top screw for a 1/8 inch sleeve and inserted a < 1/8 inch rod to extend down to the top of the piston shelf, Then he could measure the rise of that piston in the air chamber: every 1/8th inch reflecting a station rise at the needle. He claims he used a digividcam mounted on the fender to record the rise under road conditions, speaking the rpms and AFR readouts at incremental RPMs into the mike that fed into the vid cam. So at 3500 rpms, 4th gear freeway, speed 65 MPH, the AFR showed 15-16, and he could see on the vidclip where the needle was at that point. Later when he played the clip, he saw both carbs and could also tell if piston rise was balanced by comparing each needle height. He claims a max-high resolution allowed zoom in that very accurately showed the needle position on the scale. I don't recall if he was using the LM-1 here though.
Incredibly clever I thought and a first as far as I know. Anyone here used this or similar for station
mapping? Cheers, Vic

vem myers

Holly Cow! Methinks that the young engineer has entirely too much time on his hands. I would like to invite him to my place where he could expend some of his excess time on digging French drains, mowing lawns, repairing a leaky TD fuel tank...the list just goes on and on.

This approach reminds me of the Air Force move after the C47 was something like 40 years old and stillbeing used by them. They finally came out with directive that no more engineering "improvements" would be installed in the C47 and that further problems should just be fixed.

I find it difficult to imagine that as long as the SU carburetors have been around ans as simple a device that they are, an engineering solution like our young engineer came up with was really necessary to fix something that is in all likelyhood a simple vacuum leak due to soem worn parts.

I do think that this young engineer should be given a A+ for interest and effort and hired by some company who is looking for a real go getter with inovative ideas, then keeping him long enough to temper his excessive enthusiasm for engineering fixes with a few years of experience. Some company could wind up with a real winner. Cheers - Dave
David DuBois

Des Hammill describes in his book (How to Build & Power Tune SU Carburetors) how to make calibration sticks for checking the SU needle operating stations at different piston lifts. His reason for doing this was to change the profile of the needle by correlating the the lift of the piston to the appropriate positions on the needle. A fixture was made to chuck the needle in a drill and re-profile the needles with a strip of sandpaper for best performance. I built some sticks from two pencils and tried profiling some needles, but I didn't get anything that worked better than a pair of AAA needles. I did that about five years ago. I think the big problem with the Des Hammill procedure is you are not able to make checks under power. I'm attaching a photo I scanned from the book, it's not very sharp, I didn't want to ruin my book.
Dave, I can't help you, I'm not an engineer.


Clifton Gordon

Clifton- Looks like straws with 1/8 inch increments. Yes, this is the idea as shown in your book. My interest is to map afr at 14 stations, or a representativ number to "estimate" a curve, then plug it into the "Build a Needle" program under the program.
vem myers

Sorry to bomb your list, I am a TR guy that strays. In my quest to find the perfect needle, I have often thought it would be neat to be able to measure the piston lift under load, using either a camera or midget under the hood. When I read the first posting of this thread, the bit about trying heavier springs to cure a lean mixture really jumped out at me. According to Des Hammill's book,the stronger the spring, the lower the piston will ride and the weaker the mixture for a given throttle opening
Berry Price
BTP Price

Berry A heavy spring gives a richer mixture.

To me, Hammill's discussion about how the spring weight changes mixture is a little vague.
Here is a link about the theory and operation operation of SU carburetor's. This article has a better description of the springs effect on the fuel mixture.
The quote below is from that article and supports and explains what Denis said.


The spring within the suction chamber loads the piston in a downward (closed) position. The tension of the spring is selected such that the piston reaches its fully open position when the engine reaches its maximum air demand, that is maximum brake horse power output, not maximum RPM, at any given engine speed up to maximum BHP:

§ If the spring is too weak, the piston will be elevated to a level higher than its optimum position causing the engine to run too lean.
§ If the spring is too strong, the piston will not rise to its optimum position causing the engine to run too rich.

Why is this so? Surely, if the piston is higher than it should be, the engine will run richer, and if the piston is lower than it should be, the engine will run leaner?

Wrong! Remember the volume of air the engine draws is proportional to its speed.
So… If the piston rises too high the throat area will be larger than optimum, the vacuum between the piston and the bridge will be low. That is the air velocity across the jet will be low, drawing less fuel, thus causing the mixture to run lean.
The opposite occurs if the piston does not reach optimum height the throat area will be smaller than optimum… The air velocity will be high between the piston and bridge causing a larger volume of fuel to be drawn through the jet assembly causing the mixture to run rich "

Clifton Gordon

OK, okay, so the stronger the spring, and I see 14 CHOICES, BLACK COLOR AT 5 1/4 OZ, UP TO LT BLUE- RED, AT 18 OZ., the slower the rise, and the richer the mixture doth maketh.
In the Haynes, SU Carb Tune, overhaul...." the MGB is shown to have stock springs from red ( 4.5oz) to blue-black (4.5oz) for "competition. Go figure. Jet choice runs from 90-100-125k. Seems you could try tuning with different jet sizes as well.
Am I right thinking that on a 125k jet, the needle rise, incrementally, will allow more fuel to pass for the same rise in say the 90k jet?
Anywho, to get back to the tuning by suction chamber hole blockage, has anyone save the bright guy above have hands on for this technique?
Finally, can someone recommend a needle rise kit for station measure, or is there none out there? Cliff, does Hamill credit the straw stick measure kit shown ion the photo? Cheers, Vic
vem myers

Clifton's info is correct.

The " somewhere in the Midwest, USA, young, mechanical engineer," needs to do some more thinking.

The port holes conduct vacuum to the area above the piston, but there is very little actual flow - just leakage around the piston OD. Restricting the holes will only slow down response of the piston to vacuum changes - exactly as the damper does. It will have NO effect on any steady state operation - UNLESS the restricted flow rate is less than the leakage rate. So, if this finely reported story is true, he's using a worn-out carb.

The effect of this "modification" is simply to do exactly what a stronger spring or heavier piston in conjunction with higher viscosity oil will do: the piston will be lower and the mixture will be richer, and the damping rate will be higher.

An additional confounding issue is that removing the damper or drilling holes in it can have an effect on the damping characteristics (additional to the obvious one). The area above the oil in the damper is a small pneumatic cylinder, and the size of the vent hole will alter the damping. Again, this is a transitional function, not a steady-state one. Putting a non-vented damper cap on a carb that calls for a vented one could really screw up the works, turning the damper into an air-spring.
Some carbs - those referred to as "dustproof", have the dampers vented internally through a drilled passage into the vacuum chamber. On these, a non-vented damper cap is fitted. Removing or drilling or using a vented cap both changes the damping and greatly increases the leakage rate.

If the lean-out condition is happening at wide throttle openings, then it is likely that the piston has topped out - too small carbs or too light springs. When this happens, the mixture goes way lean. (while this seems contrary to the general rule of lower piston=richer, it's because of the "fuel discharge coefficient" changing at different airflow rates. FDC is the rate of fuel feed relative to airflow, and is dependent on the geometry of the carb, so it's an experimentally derived function. this top-end lean-out is very common on modified engines that need more air than the carbs are calibrated for. The cure is stronger springs or bigger carbs.
Lean out under less than WOT is commonly a result of air filter removal or alteration, calls for needle changes, sometimes drastic.

The visual representation of piston position has been done many times in many ways. Never had a mini vid camera to try that method, but it was one of the first things I thought of when these cameras came out.
FR Millmore

This brings up an interesting thought.

I have this issue where my '74 with HIF's will want to die when I pull up to a stop. If I give it a little gas for a few seconds, it settles back into a good idle. If I enrich the mixture, the problem decreases, but then from the smell of the exhaust, it's obviously too rich.

What this thread has made me wonder is if the piston is taking too long to drop, causing lean running for a few moments before it settles into a good position for idle.

I replaced the existing springs for new (red) ones a few years ago. I wonder if the solution to this long-standing problem is simply a stronger spring.

Any thoughts?

Matt Kulka

Vern, The Hammill book I have was printed in 2000. I said there are no station measuring tools availiale, you have to make them. I used two pencils. I made the calibration scales on paper and taped them around the pencils. I removed the oil from the dashpots and cut the pencil length so the station 1 is at the top of the damper with the scale inside the dashpots. I also made calibration scales on some fine wet/dry sand paper to determine where to re-profile the needles. I'm loading a photo of the sticks I made. Matt, your pistons may be dropping too rapidly, many older cars had a vacuum pull throttle off(I forget what it was called)to delay returning the engine to idle speed when stopping. That's just a guess


Clifton Gordon

A close up of the sticks. I wraped the ends with masking tape for a close fit in the dashpot cylinder.


Clifton Gordon

A stronger spring will give a weaker mixture. It holds the down, and whilst it leaves the needle lower in the jet which would tend to weaken mixture, because the piston is lower there is a higher vacuum on the engine side of the piston which sucks more fuel out of the jet anyway. For confirmation of this just consider the momentary richening of mixture that happens on opening the throttle - the damper restrains the piston for a few seconds to give the richer mixture.

But going back to the original post, if someone found the need to do all that then either they have a non-standard engine, or there is something wrong with it.
Paul Hunt 2

I have a few thoughts:
Cliff- Sorry, didn't catch your statement that the measure sticks were not available to buy and had to be made. I did hear you say you made your own though. Seems I saw a balancing tool once in the SU Rebuild and Tune cassette with Lawrie Alexander, but the memory is fuzzy. I saw a balancing rig, using a vertical stick in each carb's damper hole with a horizontal cross tie. Then under throttle, you could see if the horizontal tie stayed level. This setup,similar to yours, and available somewhere retail, could be adapted to measure needle rise. If you're offing the piston damper, the piston will rise faster, and give the correct needle height at whatever throttle I assume. It also seems the pencil weight is negligible, yes, and will have no effect on final rise height. It'd be ezier to make the rig then flop around trying to find one I think.
MFR- Naw, the guy from the midwest is way ahead in his thinking and we're just jealous. I agree, and we all believe:
"The effect of this "modification" is simply to do exactly what a stronger spring or heavier piston in conjunction with higher viscosity oil will do: the piston will be lower and the mixture will be richer, and the damping rate will be higher"
You'll recall, I hope, in my report above that he was experimenting with a tune technique for the lean out at WOT. Perhaps you missed it:
"He was leaning out dangerously-22-35 AFR-- and tried both heavier oil and stronger springs to slow the piston rise under WOT.They worked, but not enough, so he tried blocking off one of the 2 vacuum port holes in the chamber piston and plugging down the orifice diameter of the other hole. Claims it was the ticket and he was able to dial in the AFR thereafter."

I'll try to find his URL and you can enjoy his work at your leisure.
Cheers, Vic
vem myers, .
vem myers

Vic, The link doesn't work for me. No problem, it's not something I want to try. I used the Hammill sticks to attempt re-profiling some lean needles but found it's very difficult to make two identical needles plus it's very time consuming. It was easier to buy a pair of AAA needles, they are very close to what I needed with my engine.


Clifton Gordon

Paul Hunt,
Your last thread started 'A stronger spring will give a weaker mixture'. That seems to be the opposite of what you go on to explain further on, and what was posted previously.

I was nearly starting to understand things up to that point :)

Charles Goozee

Interesting discussion. Hammill does seem to contradict himmself. My earlier posting about stronger springs producing a leaner mixture was taken from the first chapter, page 16, Later, in the book (page 49), is a chapter on rejetting, which idicates that the stronger the spring the leaner the needle can be to produce the best fuel economy at a cost of slower acceleration. Conversly, the weaker the spring the richer the needle will have to be and engine acceleration will be improved at a slight cost to fuel economy. Su carbs provide endless entertainment,with as many combinations as a Rubic,s cube. And Vic wants to increase the possibilities by trying different size jets.
Berry Price
BTP Price

Well, bottom line is as Cliff states: ezier to buy the nearly correct needle, and for me, that is after using the program and the "select a needle" option. The stronger the spring, the more the dampening of the upward piston movement, and the richer she gets.
I've never seen nor heard of anyone using the 125k jet on a 1 1/2" SU, or a 90k on a 1 3/4. I was just wondering if it could benefit AFR tuning in a stubborn, 3rd standard deviation case.
Sorry Cliff- the URL is indeed dead. I got the balance kit info from Barney Gaylord's site. If anyone's interested, go goggle him. Cheer Vic
vem myers

Vic The other Jet sizes are for different needle series or types and not to use with our "100" needles.
There are plenty of needle sizes available off the shelf for the normal naturally aspirated SU, but to get the supercharged SU spot on takes a bit of work. We are lucky that there's only one carb.

Charlie - it's my age. Yes, a stronger spring gives a richer mixture for the reasons I mention later.

Berry - a stronger spring *can* result in use of a weaker needle, because the mixture *is* richer. A weaker mixture would need a richer needle. I'm not sure a weaker than ideal mixture gives better economy, any more than a richer than ideal gives better performance.
Paul Hunt 2

This thread was discussed between 27/07/2007 and 02/08/2007

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