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MG Midget and Sprite Technical - Twin HS2 Carbs

Hi All,
I'm sure this question has been asked a thousand times before, my 1973 1275 Midget is running far to rich, the plugs are black strong smell of petrol. when I balanced the carbs including warming the engine it used half a gallon of fuel. the car was adjusted as lean as you can get fully up with no more adjustment.
This is what Ive replaced so far
Jets
Jet Bearings
Jet needles (AN)
Dashpot Springs (Blue)
Fuel floats
Dashpot oil being used 20/50

nothing seems to be sticking jet seem to be centred ok. would the fuel pump cause any problem its a replacement electronic one though it was from Moss

Thanks
Rich
Rich65

Hi Rich,
Have you checked / adjusted the float levels in the float chambers? That should be a start point.
GuyW

Yes, I was wondering if the jets in the bowl could be faulty
Rich65

I wasn't meaning the fuel supply valves (jets), but the setting of the float levels. I see you have fitted new floats but you do have to set them up so that the fuel level is correct in the float chambers when the valves shut off. If its too high, then the fuel delivery jets in the throttle body of the carbs cannot be set right and you will get a rich mixture.

The Burlen Fuels site (SU carbs) has detailed downloadable instruction sheets for how to set up the carbs.
GuyW

Rich,
if the fuel pump is just a standard one but electronic then it should be fine.

You're right to question any new parts (and how they were installed).

Do you mean you've replaced the needle and seats to the fuel bowls(?), IF so these vids might help too with general info -

(the first part of this video up to 2:32) -
222 MG SU Carburetters: Setting the Float Height (needle & seat comparison) - http://www.youtube.com/watch?v=82YNx-RkGNI

(all of this vid)
232 MG Carburetter Needles and Seats (odd sizes) - http://www.youtube.com/watch?v=pVX_JClyeDQ

(all of this)
147 MG Carburetter Tuning - http://www.youtube.com/watch?v=4nvGLgO6pj0

(all of this)
41 Matching SU Air Pistons - http://www.youtube.com/watch?v=IfU47Oqq9wA

(all of this)
44 Secrets of the SU Carburettor Part 1 of 2 - http://www.youtube.com/watch?v=7GRAcqDySog

(and this)
43 Secrets of the SU Part 2 of 2 - http://www.youtube.com/watch?v=60Bj_2cZQnc
Nigel Atkins

FWIW Haynes do an SU carbs manual which I have found informative. My copy covers instruments up to 1985.
Haynes is not always totally accurate, but the carburettor book seems pretty good so far.
Personally I find a book a bit more accessible. Just sayin'....
Greybeard

I much prefer a real manual or book to a video. With a video, the critical stage might be passed in seconds whereas the manual stays open at the page by your bench to re-read. Also the manual doesn't have a battery to go flat whereas a tablet or laptop runs out of battery at a critical.......... ;-)
W Bretherton

As I find written instructions (or my reading of them) not always to be clear I like a mixture of written and verbal/shown instruction such as videos but when under the bonnet what's on paper can be best (as Bill's put, no worry about batteries, internet connection, screen contrast etc., etc.).

For setting up the carbs there's written instructions in the official (reprint) of the (factory) Driver's Handbook (not a workshop manual as such) - for '73 model see Ref:0057 here - http://www.mgocshop.co.uk/catalog/Online_Catalogue_Handbooks_5.html

Who knew one little booklet could contain such much useful info (?).

Nigel Atkins

Amen Bill. You can think of a book as "non-volatile memory" lol!
Greybeard

A paper manual is also much harder to accidentally delete. And flipping between the front and middle is much easier.
Martin

I downloaded the information sheets and setting up sheets from that Burlen site, and then printed them off as paper instructions. They are similar to the stuff in Haynes but a bit clearer to follow and a bit more detailed.
GuyW

I'm using the tuning su carbs Manuel which Nigel sent me, the floats from Moss are slightly different, black instead of clear plastic resistance to modern fuels and there was no adjustment on the old ones but the new ones can be adjusted by bending the metal . I set them using the small jet adjusting spanner which is supposed to be the right thickness. I didn't replace the float needle or seating here's a couple of pics of the new and old floats

Rich65

New float

Rich65

Rich,
I think the problems of E fuel is often vastly overstated but as you have the new floats you might as well use them.

If the car was running well-(ish) before you could compare all the old parts with the new parts to see if there are differences (that's not to say the previous parts were necessarily correct).

I saw this, I don't know if it still applies -
"On the H series, some after market kits include jet bearing washers which are too thick, preventing the jet from reaching its full height. The jet bearing gland nut may not be tightened sufficiently, again preventing the jet from reaching full height.
The shoulder of the needle should be even with the face of the piston – place too deep into the piston will cause rich running."

I also wonder if you have the damper springs fitted ok (I think there's a right way up but not sure it'd make that much difference).

If you've mixed up the damper pistons and dashpots that could also make a difference, not necessarily the answer here but worth checking any way, I meant to put this up last post - 41 Matching SU Air Pistons - http://www.youtube.com/watch?v=IfU47Oqq9wA
Nigel Atkins

Rich,
in this Moss video it also has float needle and seat problems, as the most common, I know you not changed them but could they have gotten damage or muck/crud in them or dislodged(?) - http://www.youtube.com/watch?v=wLRFnTJ28oQ&list=PLUeqA1PltU0_eXDI_bLUw_EkdZ9clVzQ8&index=2

http://www.youtube.com/watch?v=NVLhUC-fKc8&list=PLUeqA1PltU0_eXDI_bLUw_EkdZ9clVzQ8&index=3

note the bit about fibre washers in this vid, I don't think John Twist fits any, you could see if you have any fitted and if they make a difference - http://www.youtube.com/watch?v=Z6Le_HkrkKQ&list=PLUeqA1PltU0_eXDI_bLUw_EkdZ9clVzQ8&index=4

PS, sorry I'd already posted JT vid for matching piston/dashpot.
Nigel Atkins

ETA: I meant to say you should first look for signs of over-fueling as noted in the first Moss vid I put up before going to too much effort with float/needle and seats.
Nigel Atkins

Thanks Nigel I suppose because because all I know about the carb is that it has done 66,000 miles, my thinking is that if I change the parts I know where I stand with the carb. Obviously the parts have to be fitted correctly.
I will change the float needle and seat and I'm going to check the surgestions on the videos also recheck the work I've already done
Rich65

Hi Rich,
Make sure you have not pushed the needles too far into the piston. The shoulder of the needle should be level with the bottom of the piston.
Also, and I am not sure if you have a choice with the hs2, have you got jets with the correct diameter hole in them? Some one will clarify, can you get jets in different sizes for the HS2?

Good Luck

Dave
Dave Brown

Rich,
don't change the needles and seats just for the sake of it, I meant to check and see if they're OK (with a magnifying glass if you can) and check for muck/grit/crud/debris in the needles, seats and bowl tops.

It's possible to change perfectly good existing parts for new parts that are not as good, possibly including needles and seats.

I also wonder about the float height adjustment, the difference between the existing and new type and if washers(s) (if fitted) might make a difference. I can't see your photo of the new black float in enough detail, and I could well be wrong, but it looks to me as if the float arm to needle *might* be high(?) (others would know much better than I).
Nigel Atkins

Of sounds like your jet tube it to high at the bridge... as it relates to the needle valve of the float

Sometimes the float gasket can be to thick raising the highth of the needle valve

Try 1/8inch flat washer under the jet tube at the bottom of the carb
1 Paper

Lowering the jet tube will make it run richer - or flood.

I would suggest that simply replacing parts because they may be worn is not the way to go. It costs more and you may well be throwing out better quality original items and replacing them with poor quality replacement parts and also introducing a degree of uncertainty.

You need to diagnose the source of the rich running. As a start - if you turn on the ignition, but don't attempt to start, do you hear the fuel pump click? It should click rapidly for a few seconds and then stop or slow to a very infrequent or occasional pulse. If it continues to click fairly rapidly then it is very likely that your carbs are flooding. This will either be leaking fuel needle valves, or floats set too high. Both will allow fuel to overflow the carb jet tube and give poor running.
GuyW

Float height setting seem to be ok, I did blow in the fuel inlets and they did seem to be sealing.
You can see from the picture the front looks a bit corroded, I thought maybe should have been a washer with the seat but the su website hasn't one with their kit.

Rich65

just a thought but are the dashpots fully clean? had someone recently with a rough runner found quite a bit of "gunk" and sludge at the bottom of the dashpot cylinder and after cleaning and refilling with oil the car was running sweetly again.
a reid

Hmm, overfilling of the carb float bowls was only one suggestion of possible causes of engine running rich. There may be other things you could check.
How about the choke mechanism? Are the choke levers fully returning and with clearance when you push the choke in at the dash knob?
Another fault which occasionally arises is inadvertently putting the gasket between the carb and the air filter in upside-down. It is an easy mistake as it fits ok, but then obstructs the air balance tube which in turn causes rich and uneven running.
Is the rich running the same on all 4 cylinders?
Do you have the correct grade of plugs fitted?
GuyW

Rich,
this might seem a bit bonkers but it's an easy check and might just be the cause (was for my 1300).
Even though the float hight has been set and the needle valve checked by external means, it still doesn't prove it's functioning properly. Remove the dash pots and pistons so you have a clear view of the carb bridge and the jet. Switch the ignition on but don't start the engine (it won't anyway!). The fuel pump should fill the bowls and stop clicking. If you continue to hear a click every few seconds one or both of the needle valves aren't doing their job properly. Normally you would expect the bowl to flood and fuel to pour from the overflow but.... Have a look at the top of the jet to see if fuel is being pumped out of the jet and running down in to the manifold. Mine was. My working theory is that the valve closes sufficiently to restrict the flow but not fully. The jet height is below the overflow on the bowl and, as such, the first route out is into the carb. Only when the flooding is significant will it appear from the bowl overflow.

This is somewhat supported by the amount of fuel used in setting up the carbs as mentioned in your first post (sorry it's taken me a while to respond, might have been able to save you some heartache ;-)

Best of...
MGmike

M McAndrew

Did I hear an echo just then? ;-)
GuyW

Is it my bad eyes and/or the photo but are the needles and seats all a matching set and pairings?

Front *look* to me with my bad eyes to be bigger (unless it's a split photo and the two sets weren't both photo'd sitting psychically side by side) and they seem rougher in appearance.

Rear seat seems to have one flatter side(?).

The photo is too low-res for me to see too much and i might be talking out of my ar*e (again, not for the first time, before Eddie says so :), again). :)
Nigel Atkins

hee hee Guy, I should have read all the posts but got lost in the manuals and flicked to the end ;-)
Could say great minds think a like but mines well short of that unlike you :-)

best of....
MGmike
M McAndrew

Same suggestion Mike, just yours was more detailed and better explained!
GuyW

This thread was discussed between 15/05/2017 and 19/05/2017

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