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MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Moss and a T Blower

Just read that Moss has a blower coming for the T series engines. Don't know when it'll be on their list. Interesting. PJ
P. Jennings

That looks like the Eaton style that Tom Lange is selling.

Jim
James Neel

I read more, it is the Eaton style.

Jim
James Neel

Fellows - I have not yet had a chance to look their blower over, but will do so at my first opportunity. I see they include a new but very odd 1-3/8" SU. I remember that Marshall tried a 1-3/8, and it was a disaster...

Their price is $4,195 including the carb; mine is $4,000 plus the carb. We have sold roughly 50 of these blowers, theirs are untried. We'll see... Their blurb does sound like they have read mine!

More when I know more.

Tom Lange
MGRrepair.net
t lange

We've had a Magnacharger for years on our '51 TD. It has, what appears to be, the same carb as on the Moss website for the new blower package (float bowl on side towards engine though). Always SEEMED to run respectable... except is always low on oil, checking every 6 months. I may convert it to drip oiler like the Marshall.

After transplanting it onto the '50, I added an O2 sensor and air/fuel ratio meter. I don't like what I see, at all. At around 55mph, it runs out of gas and registers leaner and leaner. I transplanted the 1-1/4" SU from our Marshall blower (now on the '51), and it too, craps out at around 55 mph. The car will do 80mph, but most of the time the gauge is completely off scale lean, just popping up on the radar for a split second at a time. Got two fuel pumps, new fuel lines, coated gas tank with 2 different feeds, no rust in strainers or bowl, even a Joe Curto steel needle/seat. Pumps about a pint of gas in 1/2 a minute, either pump. Hogged out bowl-carb banjo bolt. Pulling enrichener has no influence.

I picked up a Bendix/Zenith carb and a Keihin carb for Harleys, both with accelerator pumps & chokes and the Zenith with adjustable jets. Both dirt cheap, and also got a nice air cleaner and an intake manifold that is getting machined for new manifold. I'm going for the Zenith for now, as the needle & seat dwarf the SUs. Keihin looks like it is engineered for better midrange control, but the Zenith doesn't have plastic parts- looks vintage.

Tom, what carb/needle combos do you provide, or is that proprietary?

Someday, I've got to strap on our S.Co.T and Shorrock blowers for comparison, in my spare time (HA!).




Jim Northrup

Measure the throat on our so called 1 1/2" carbs Tom. You will find that they are indeed 1 3/8". I keep the float bowl on the out side Jim to keep it away from the exhaust heat sink. Never have a problem with vapor lock. I did just move the choke ( enrichment) bracket to the inside as it was vibrating against the float bowl and wearing a groove in it. Also added a fast idle for the choke. Making 9 lbs of boost now with the smaller pulley and she will do a whole lot better than 80. Hell it did a whole lot better than 80 with the 6 lb pulley. Comes on much quicker now. Went from 32 MPG in stock form to 21 with the 6 lb pulley. Wonder what she will do now. Hard to keep your foot out of it cause its so much fun. Threw out about a quart of oil every 1000 miles on my long drive last year. So I made my own drip catcher and worked the alloy side cover over. I hate to jinx it but no oil leaks now. I can see the drip cotter pin and it's bone dry. All the damn oil slick was coming out the tappet cover vents.

I would like to see the Moss version up close for comparison. Looks like the intake manifold might be a little w*nky. Also pisses me off they didn't set it up for the TF as well.
LaVerne

As Laverne says, the entry to the carbs is smaller than the size stated.
SU carbs are designated by the throttle bore, not the entry. The number is the bore size in eighths over 1".
Hence H1 - 1 1/8", H2 = 1 1/4, H4 = 1 1/2, H6 = 1 3/4, H8 would be 2". There is no H3 = 1 3/8"

HS and HIF series use the same designations, but the entry is closer to the bore size, so they flow a little better. (late HIF have metric designations but the sizes are the same HIF38 = HIF4, HIF44 = HIF 6)

The lean at speed is due to a total lack of understanding about how the SU operates.
It has nothing to do with gross fuel flow capacity. (it might after you correct for the following)

The basic tuning parameter is that the air piston weight plus spring only allows the piston to reach the top at maximum airflow, which is the max power output point. If you increase airflow in any way, you have to change the weight/spring to suit. Putting a small carb on a big engine, or using a blower, causes the piston to top out way too soon. Porting the engine, changing the cam, increasing rpm limits, increasing the blower drive ratio, will all cause more airflow and may cause the piston to top at high rpm. Even exhaust system or air filter changes can push it past topping if it was close before.
It is best if airflow restriction is minimal, so stock carbs are set to top only a bit above the max power point.
If you start out with a carb that is reasonably well calibrated for 40bhp, and it tops at 45, then expecting it to function at 75bhp is insane; it will go off once you hit the airflow of the 45bhp mark.

Since the fuel is regulated by the tapered needle and the piston movement, once the piston tops out you have no more fuel feed adjustment by the needle. It turns out that while fuel flow will increase with increasing airflow through the now constant throat size, it does not increase as fast as the airflow does, and the mixture goes lean. The relationship is referred to as the discharge coefficient of the jet.

You can go to a bigger carb, but the same parameter applies; the numbers will be different. Required airflow is about the same for a given power output, so if you can find a carb in your size that came on a engine that produced the power you expect, you can use those settings as a starting point. Not likely for a blown engine with a small carb.
The sole justification for using a small carb on a blower motor is that you already had it when you bought the aftermarket blower - not the case when you are buying a modern kit. Just because the blower CAN suck air through a small carb is no reason to force it to do so, and it gives other tuning problems, especially extreme and unnecessary touchiness of needle profiles. It would be best to use a carb that was used on an engine of similar power output. For a blown T at 75bhp, this would be H/HS/HIF6. A -4 would work OK on mild engines, and a -2 is too small.

Once you get the piston/spring in range, you then work with needle profiles. Should you get the engine to the point where the piston again tops out, you must increase the spring and start all over with the needle.

FRM
FR Millmore

Moss used to carry a T-Type supercharger kit that looked similar to this one. The older blower unit is probably NLA so this may be the newly designed version to accommodate the new model of blower. Same thing happened with their MGB kits - the blower became NLA so they had to redesign the entire system to fit a new model. It would make sense if all Eaton T-Type blowers on the market used the same general guts, and therefore had a similar outward appearance.

FR, excellent post with some very interesting theories. Good brain food, that.
Steve S

I don't want to hijack the thread, but whilst digesting FR's 'brain food', a thought burped up.

I can imagine that a single H2 is much too small for a blown engine, but;
If the very plausible theory is correct how does that explain why my YA (or any Wolseley 14/40) with one H2 on basically the same XPAG as the TC/TD with two H2's doesn't top out continually (assuming that it doesn't)?

The heavy brass piston doesn't explain it because that is fitted to the TC too? Am I correct in thinking that no H2's have springs?

Maybe the twin H2's have a larger HP margin on the top end?
Willem vd Veer

Unless I'm reading this wrong, the Moss blower is for the TC/TD 1250 engines. No provisions, as of yet, is made to install this on a 1500 engine, correct? Seeing how the A and B blowers mount on the other side, what is the alternative for a 1500 blower? PJ
P. Jennings

PJ - with an H4 and proper jetting provided by Joe Curto, my blower works fine on a 1500 XPEG engine.

Tom Lange
mgtrepair.net
t lange

t lange, Maine, USA, tlange@acadia.net

PJ - with an H4 and proper jetting provided by Joe Curto, my blower works fine on a 1500 XPEG engine.


Thanks for that info Tom. When in that price range, I always, when possible, go for a proven setup. I'm not ready to go that route yet, I can't even get to my car to get going on it again for another month or two.
Also, will one of my stock 1500 carbs, re-jetted, work on your blower? PJ
P. Jennings

The carb body will work fine with some mods, but you will need to replace the float bowl with a straight one, and various choke linkage bits. Because of the pieces needed, I generally recommend getting a dedicated carb for the blower set-up.

We have only a few weeks' lead time for supplying a blower now.

Tom Lange
mgtrepair.net
t lange

I ran a Marshall on my TD for a few years. Its seemed to always starve at anything over 4000 rpm. Once, while in New York Cuty, I mentioned this to a chap who suggested I go and see Charles E. Stitch, right in downtown NYC who maintained vintage cars for clients living in NYC.

When I met this man, he must have been in his late 70s then (1953). He quickly sized up the situation and dug into a pile of scrap carburettors and came out with a bigger one (later, I discovered it was an H6). He cleaned it up, ground out the inlet, made an adapter plate, all in the space of an hour and a half, and sent me on my way - no charge!!

But the bottom line, was that the H6 made a significant difference across the entire rpm range. And it could pass anything except a gas station!

Gord Clark
Rockburn, Qué.
Gordon A Clark

I'm certainly not a flow expert but I'd say the 1 1/2" gets the job done. I do intend to get a little dyno testing done just to see what the A/F mix is across the board.

http://www.youtube.com/watch?v=5t6_LseoaHo&feature=related
LaVerne

When you say H-4 carbs, do you mean HS-4s? I think I have 4 or 5 of them from early 1800 engines buried somewhere. When needed, it's easy to send one out for a rebuild. But, do they need a float bowl change to work? PJ
P. Jennings


PJ -
H carbs have the brass levers that lower the jet for cold start, and the float bowls mounting arms are angled to suit that application. Fuel feed is through the carb body to the jet. These come as -1 through -6, and possibly as -8 for rare old competition applications.

HS carbs have the plastic jet fed by a flexible tube from the float bowl. Fuel feed is through the tube. All the float bowls are the same, and the angle is changed by the grommets where the bowl mounts to the body; they come in various angles and materials. Angles go from zero = horizontal to about 30 degrees. HS2 have vertical pattern flange to the manifold as H2 & H4 series do, HS4 are diagonal, and HS6 & HS8 are square 4 hole pattern. (as are H6) The HS4 diagonal pattern is half of the square of the larger sizes; and, they are left and right, or front and back, mirror images. A rare form of HS6 was used on Volvo B18, very short body and unique triangular air filter mounting flange.

And there were various Japanese forms under license, used on Datsun and such. Not real familiar with these
Hitachi, but I think they were basically HS type.

Another form is the HD, which uses a different jet/cold start arrangement, usually only seen as -6 or -8, though there are -4. Mostly seen on Jag/Rover/Healey. The HD4 were on MKIII & MKIV Magnettes & probably the similar Riley.

Early Jaguar used a predecessor/variant of the HD called the Thermo, It had no cold start, but used a separate cold start carb. This was retained on some later cars which used the HD6.

FRM
FR Millmore

The TD engine I bought to replace my broken one had a MOSS supercharger on it. This was back about 7 years ago. As I remember it had a very big SU carb with it.

Not comfortable with a supercharger, so sold it prior to seeing how it performed.
Bruce Cunha

Willem-
"I can imagine that a single H2 is much too small for a blown engine, but;"
>>>It is one of the beauties of the SU design that it can be made to work over a very wide range of flow, but you need to tune accordingly. The piston flow characteristics are the first part of this, since everything else, eg needle profiles, depends on it. Using a carb too far out of range makes it difficult to get the needles correct, and significant pumping losses are experienced with too small a carb. No point in shooting yourself in the foot!

"If the very plausible theory is correct"
>>>This is the principle of operation, not a theory. The piston position is determined by airflow, and the fuel feed is controlled to match by a suitable needle.
Once the piston position becomes constant by topping out, there is no longer adjustment of the fuel.

" how does that explain why my YA (or any Wolseley 14/40) with one H2 on basically the same XPAG as the TC/TD with two H2's doesn't top out continually (assuming that it doesn't)?"
>>>The common error of thought is that two carbs have twice the flow of one. This might be true on a common manifold, but is not on divided manifolds. All else being equal, the engine breathes through each carb half the time, alternately. The total flow at any given time is always through one carb. The major advantage of two carbs is better manifold design, ie less restriction and better mixture distribution, worth a couple of BHP generally. A good single carb manifold can often match or exceed this. Experience shows that twin carbs & manifold are worth about 3-5hp on most engines in this power range, with no other changes.

"basically the same XPAG"
>>>"Basically" doesn't count here. We are looking at specific requirements. ANYTHING that changes airflow will change the carb setup. I don't know much about the differences in these engines, but if the single carb ones have lower compression, milder cams, more restrictive (quieter) air & exhaust silencers, then engine airflow goes down and so does required carb flow.

"The heavy brass piston doesn't explain it because that is fitted to the TC too?"
>>> also first TD. If the pistons were in fact the same weight, then the twin carb ones could possibly have been lightened up a bit, to small benefit. On the other hand, if the twin carb engines did have different cams etc, then they may have been able to utilize the added capability of the carbs. Remember that factories don't like making special parts, unless the sales benefits exceed the cost.

"Am I correct in thinking that no H2's have springs?"
>>>No, they changed from brass to spring loaded aluminum mid TD production.

"Maybe the twin H2's have a larger HP margin on the top end?"
>>>Possibly.

Bottom line is that single carbs are cheaper, and may give better drivability and economy. Twins may make a couple more bhp but they sell "high performance" cars (which tend to be more profitable)

FRM.
FR Millmore

FRM, Thank you for the education on SU carbs. It was very well received and your time to do this is very much appreciated! PJ
P. Jennings

Thanks, FRM for a really enlightening exposition. It all makes perfect sense to me.

Now I can take off the air cleaner manifold and watch the piston position at various steady revs, and also the range of movement when the throtle is flipped, to see when it tops out. The TDII engine has been breathed on and this may explain some of the slightly lumpy performance characteristics.

IanB
Ian Bowers

I don't agree with Fletcher's statement

""Am I correct in thinking that no H2's have springs?"
>>>No, they changed from brass to spring loaded aluminum mid TD production"

All the later SU H2 carburetors I've run across are either brass pistons or aluminum pistons with a steel weight (donut) around the outer circumfrence. None of them are supposed to have springs. Attached is a picture of one such example.

Everything else is pretty spot on.

If however, you are running a low boost (6 to 6.5 psi) Judson unit, the single H4 with an RA needle works fine.

warmly,
dave


Dave Braun

Dave-
I do not have references to TD or 1250TF, except memory. Memory says the later ones did have springs, but of course they could have been changed long ago, or perhaps that was on H4 cars; Memory goes back to 1965 or so, (but it has a lot of British, so it leaks!). Moss show no spring for TD w/H2, but springs for H4 TD & TF. There were certainly lots of H2 with springs in other applications, and likely all service parts after some point were later type. Keeping as close to the time as possible by my guaranteed accurate SU master book, "all BMC January 1954 on", ZA Magnettes had springs in H2, changed to H4 around the ZB change (but not exactly). The 4-44 Wolseley is shown as weighted until it became the 15-50, which has spring, both H2 (singles).

Bottom line is if the piston is heavy, either brass or weighted, then it does not use springs.

Note that there is no functional difference between piston weight and spring force, but springs are cheaper and easier to change (for the factory), especially on larger carbs. For modifiers, weighting pistons in addition to the springs you have (or adding springs to heavy pistons) can be a useful avenue for experimentation.

Ian-
Watching the pistons at steady revs is not likely to be of much use, unless you are strapped to the wing at speed!
The engine must be loaded, since the airflow utilized is only that required to produce the power to keep the engine at the speed of interest. The engine should be able to develop that plus enough to move the car at less than wide open throttle. So if you hold it wide open at no load to get full airflow, it should still produce enough power to overspeed and blow itself up, before it ever gets to full-load airflow. Certainly if the pistons top under no load at high RPM it is severely out of range.

The topping out problem will have no effect until the airflow gets to that point - any problem before this is down to other causes. Any change to piston weight/spring will require reprofiled needles over the full range. It is possible that a small increase in weight/spring could solve minor top-out, but still be adjustable by the usual jet twiddling.

Anyone who experiences a definite shutoff point on a modified/blown engine, as Jim N. and Gordon report, should investigate this first. Gordon's fix is the best one.
(From Vizard dyno info:) For comparison and with no other changes, on a modified A series 1430cc engine, an HS4 gave 108bhp starting to fall at 6500rpm, but an HS6 was 118 and still rising. Even better, the HS6 gave 5 bhp more at 3750 with the advantage difference increasing as speed increased. This is definitely the far reaches for a -4 carb; the difference was about 8bhp at the usually accepted useful limit of 60/70bhp for a -4. If the -4 had been setup for a 70bhp engine, the restriction to flow would have been less at lower speeds, which would decrease the advantage of the -6 at lower speed/flow.

FRM
FR Millmore

Again thanks for the illuminating answers.

I guess this means that using such kit as ColorTune on a static vehicle is not much use beyond the tick-over mix.

Back to the old trick of hareing down the road, shutting down and whipping out the No3 plug to see if it is buff coloured.

Or rolling road testing.

IanB
Ian Bowers

Ian,
I am very familiar with the shutting down, and whipping out a plug to check the mixture,,,, But,, I must ask,,, why the N0 3 plug????? wouldn't #2 work just the same?????

SPW
STEVE WINCZE

This thread was discussed between 06/03/2012 and 09/03/2012

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.