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MG Midget and Sprite Technical - Question for Deb Evans(re 1500s)

Or anyone else who can help.
I've been doing some work on a freind's 1500 midget and one of the jobs he needed help with was setting the carb mixture.
He has SU HS4s and has done away with the waxstat jets and put standard jets in. When I came to try and adjust them I found that the jets were wound right out about 35 flats, so I wound them right back up again to start from scratch. In the fully up position the jets were standing well proud of the deck so I wound them down till they were flush with the deck then a further 12 flats as a starting point to start to balance the carbs. Although it is now better than when it came to me, I'm still not happy with the idle mixture as it is rich and the engine runs lumpy.
It has new HT leads, dissy cap and rotor arm and I checked them timing with a strobe. It was a little retarded but is now spot on. So where do I go from here? The carbs have ABT needles which were I beleive the ones that were in when it had waxstat jets. Should I be fitting ADN needles or just try to play with it a bit more? Oh BTW, I fitted, at his request, all new gaskets.ie manifold, both sides of the new heat shield and carbs, in case there had been an air leak. Also new air filters and gaskets.
I've not done alot on 1500s as my car is an A series 1098cc., so any help would be gratefully received.
Thanks in anticipation.

Bernie.
b higginson

I spoke to my freind last night and he told me that the car had not run for two years and the petrol was also that old. could that be the problem, does unleaded fuel deteriorate over time to the extent that it will not combust properly, or is that an old wives tale? I've heard stories of people transforming the running of a car just by throwing out old fuel and filling with new. On the other hand i've also heard of cars being started after years of standing and running fine.

Bernie.
b higginson

hmmmm - well, as regards the fuel - I don't know if it is or isn't an old wives tale, but, my 1500 can be a tad awkward with fuel, so, I try not to leave it in too long ...! It does pick up every bit of dirt that it can find in the fuel tank (I changed mine, and try not to allow it lower than 1/4 tank full and that does wonders!), I also add redex every 4th tank ful, which seems to keep the carbs semi-happy. So, yes, in answer to your question, if it was mine, I'd change it.

I would keep trying for a bit longer before changing the needles - although, you didn't say how long you've been trying, and it could be years, in which case ...! The other thing to consider is that, if it's not been checked for months, then it does seem to take alot longer to set up - then needs re-doing quicker (after a couple of weeks rather than a month or so). I'm currently suffering the dire consequences of this after winter and idleness!

You don't say what it's idling at currently?

Sorry this isn't much help, but, no-one else had answered and at least I could say that who knows about old wives tales - sometimes they do seem to have something to them ... something scary about women's intuition :)
rachmacb

hi Bernie.iv got a 1500 the same as rach and mine is also very picky with old petrol.for the sake of 10 mins draining the tank id just drain it and start again with shiny new petrol.use the old stuff for your lawn mower then its not wasted. the stale petrol thing may be an old wifes tale,however i have heard a few 1500 owners say the same thing so who knows.if that is what it is then its an easy fix. Roy
roy j

I have a Honda engined lawn mower. There is a sticker on the tank that says, it big red letters, "Drain and refill with fresh fuel" and about 100 words in small print about petrol degradation during winter months. I also heard one of the Shell experts talking about the "shelf life" of fuel they use in their F1 cars the other day saying they are trying to extend it to be at its optimum while stored in the cars tanks over night under parc ferme conditions. I know that's a bit extreme but it does go off over time. You can buy off the shelf additives that extend the shelf life but Im thinking it would be easier to pour in some fresh to eliminate a potential problem
Nick Sayle

Old fuel certainly does 'go off' which will cause issues with smooth running but that would not be the cause of the engine running rich.

WRT to needles for the HS4 carbs fitted to the Midget 1500:

The ABT was specified for cars running the AUD665 carburetor. This was the pre 1976 model with fixed jets. It is identifiable by the fact that the dashpot damper cap is flat.

The ADN was specified for cars running the FZX1327 carburetor. This was the post 1976 model that replaced the sliding bush jet bearing assembly with a ball bearing design. These carbs were also set up for lower emissions in that they had Wax Stat jets. They are recogniseable by the fact that the dashpot damper cap is domed.

NOTE: You may have FZX1258 carbs fitted by a previous owner (I've seen this on a number of cars). These are a Wax Stat type with a ball bearing jet assembly (originally fitted to the Spitfire 1500 between 1976-1977) but with a flat dashpot damper cap and fitted with ABT needles.



The ABT needle certainly runs richer across the range than does the ADN, albeit this is more noticeable at wider throttle openings; the ADN being fitted for emissions reasons (see picture). However this would NOT account for the car running rich the way you describe (especially at low rpm), and indeed, the ABT is a better set up for the engine in stock form.


Reading your post you've basically started setting up the carbs but there are a few tweaks still to come.

1.) Winding the jets all the way up will ALWAYS put them above the bridge of the carb, so you are correct in winding then down level with the bridge as a starting point.

2.) Winding the jets down 2 full turns (12 flats) is merely a baseline to get you into the ballpark so that you can start the engine, let it warm, and then accurately set the timing - 10 degrees BTDC at 680rpm, vacuum line disconnected.

Make sure you have already accurately set the Dwell Angle on the distributor: 51 +/- 5 degrees which equivalates to a contact breaker gap (on new contacts) of 0.014" - 0.016".

Setting the jets to this position will ALWAYS be on the rich side because it is a position designed to allow you to easily start the motor.


3.) Once the motor has warmed up to normal running temperature by running the engine at a fast idle, throttle up for about 30 seconds to clear the inlet manifold of any excess fuel (do this every few minutes throughout the procedure). Now check the idle speed, if it needs adjusting switch off and slacken the clamp bolts for the throttle interconnection (ie at each throttle fork).

4.) Restart the engine and balance the carbs. Now set the idle speed using the throttle adjusting screws by adjusting each in equal amounts. Now re-balance the carbs. Keep doing this until you get the correct idle speed and carb balance.

5.) Assuming you don't have an exhaust gas analyser, the way to set the mixture is as follows:

With the engine running at idle, wind up both mixture jets one flat at a time until you get the fastest idle with the engine running smoothly. Keep going to the point where the engine speed JUST starts to drop then back off by one flat.

Now re-set the idle and carb balance as before. A good check that you have the correct mixture is to press on each carb piston lifting pin - the idle should increase momentarily before dropping off again.

Generally speaking I find the jet positions on 1500s to be around 8 flats wound down from the bridge for the correct mixture setting. Ie considerably leaner than the ballpark 12 flat setting for initial start up.


6.) Now set up the throttle interconnection clamps. There should be 0-012" clearance between the throttle forks and the lever. Ie there is 12 thou of movement before the throttle starts to open. make sure the opening is equal on both throttles.

7.) Now set up the choke opening cams - there should be 1/16" (1.5mm) free play before the choke cable operates the cams. You set this by adjusting the fast idle screws on each carb. Make sure choke opening is equalised.

If you are still having problems check that, when the choke cable is pulled and released, the jets seat fully home. They can get sticky with linkage wear or if the jet springs weaken with age or if the jet bearings are worn. This shouldn't be a problem with new jets if they have been assembled correctly, HOWEVER most fixed jet conversions do NOT come with the relevant jet bearing kit so any wear in the bearing will cause stickiness. I always advise renewing the bearing when ever changing jets.

Hope this helps.
Deborah Evans

Graph of ABT vs ADN:


Deborah Evans

Exploded diagram of HS4 Carburettor.

Note Main Jet Bearing assembly at centre of picture (these vary between the carb types):


Deborah Evans

Thanks Deborah for the detailed setup procedure and everyone else for their input. I'll go through it again tomorrow. BTW New petrol didn't make any noticeable difference, so it must be the setup or the carbs themselves. It has electronic ignition, not sure what make.
Thanks again for your time Deb and everyone.

Bernie.
b higginson

"Thanks Deborah for the detailed setup procedure"

You are most welcome Bernie. I hope you get it sorted out. :)
Deborah Evans

Eureka! ( I think) As I said above, the waxstat jets had been changed for standard items. so that type of carb according to the Moss catalogue is the type with over run valves in the butterflies and I think that the front carb's valve has developed a fault, which although OK on part or full throttle is weakening the mixture on idle to such an extent that it starts to misfire until I richen it with the jet adjuster nut. I hate those bloody valves anyway as they interfere with the air flow through the venturi. So I think I'll tell him to get a pair of standard butterflies and spindles and see if that makes it better. I notice that even on your diagram Deb, of the carb, it doesn't show the over run valves, nor does the Moss one , it just lists a butterfly kit with valves for the waxstat carbs only.
So. Onwards and upwards.

Bernie.
b higginson

This thread was discussed between 01/06/2010 and 03/06/2010

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