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MG Midget Forum

difficulties with adjusting the mixture

Moss Motors
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Ballerup, Copenhagen, Denmark   DNK
The engine was build for the US market, but that I have removed all the emission control parts and plugged the pipes on the manifold and plugged the holes at the exhaustports. I only have left the charcoal cannister and the hose from the valve cover to it. And I have fitted the 9:1 pistons, or are they 9,3:1.....I dont remember. Apart from this surgeries it should be stock........well with only one muffler.....
I previoulsey made other topics about the ignition and mixture, here is the situation right now:
All the parts in the ignition system is new and the CSI ignition should be adjusted 10 degrees btdc dynamic at low idle as explained in the instructions.
New gaskets all the way down the inlets are fitted, so should be without air leaks.
The carburetors are with a AUD549F label.
New plain throttle valves in stead of the "poppet" valve types fitted.
When cold the idle is very fine and smooth, rising when warm. When adjusting the mixture for the best smooth idle when warm, the idle is still as high as approx 1400-1500 rpms even with the idle screws fully out. When enrichening the mixture a few flats, I can get the idle a little down, but at the cost of the very smooth idle, off course. When I drove a longer trip yesterday, I had the spark plug wrench with me, so that I once in a while could check the colours on the plugs. They are, both at idle and at speed very clean, allmos with no coloration and I'm very afraid of driving too lean with the effects that could cause. When driving up to approx. 40 mls/h steady, the vehicle bahave very gentle and smooth, but when cruising above that speed, at approx 45 -55 mls/h steady, the vehicle begins to behave uneven, not much, just a little, when not under significant load. That would tell me that it runs somewhat lean, and pulling the choke handle a little confirm it, I think, I can get the vehicle to behave and have a very smooth running at the higher steady speeds mentioned.
It is as if there is a disproportion between the mixture setting at idle and when at speed.
The needle tubes are right now 2mm or 0,079" below the bridge.
The needle in the pistons now are ABC.
The other day I ordered at Moss Europe new AN needles and new "blue" springs, only to find out the the present needles are of the spring loaded types, and the present springs are much longer than the new blue ones, well I'm learning all the time........
I can not find any instructions about fitting the spring loaded needles correctly.....
Finally, I don't see the posibility to adjust the float level, since the float is nylon.....
Inputs very welcome smiling smiley



Mightymidget



Edited 2 time(s). Last edit at 2017-06-19 06:46 AM by Jan Kruber.

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Kerr Avatar
Kerr Platinum Member Norm Kerr
Ann Arbor, MI, USA   USA
"the idle is still as high as approx 1400-1500 rpms even with the idle screws fully out."

This is key: from your description of things it is very likely that there is a vacuum leak somewhere.

You described all new gaskets fitted, but did you test that by spraying an aerosol all around each gasket joint, including all of the intake manifold, up to the cylinder head to make sure? To do this test you listen for a change in the engine running sound while spraying. If there is any change that identifies a leak at that point which needs correction.


If there is no air leak anywhere in the intake system, but it is still acting lean as you describe, the next place to check is the vacuum advance to the distributor (look for a leak), or the crank case ventilation has a leak.


N

Kerr Avatar
Kerr Platinum Member Norm Kerr
Ann Arbor, MI, USA   USA
Regarding the crank case ventilation system:

- does your Danish Midget have a charcoal canister for emissions (collection of hydrocarbons)? If so, then your oil filler cap should be sealed, and there should be a metering nipple on the top of your valve cover with a small "breather" hole in it. If, say, a vented oil filler cap were to be fitted (or the old, sealed one is leaking), it will allow too much air into the intake (via the hose from the engine front cover to the carbs). For more detail, see page 320 in the Bentley Manual, section Z3, Engine Emission Control. A quick test of your sealed oil cap condition, is to apply some cellophane film (like Saran Wrap) over the oil filler neck, re-install the oil cap, re-start the engine and see if that eliminates the lean condition - if so, buy a new cap (the oil filler cap was meant to be replaced annually to maintain a perfect seal).

- does it have an air pump on the exhaust system? If so, then check the vacuum sensing tube that goes between the intake manifold and the gulp valve for a leak. For more detail, see page 322, Exhaust Emission Control.




Norm

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Ballerup, Copenhagen, Denmark   DNK
In reply to # 3535322 by Kerr Regarding the crank case ventilation system:

- does your Danish Midget have a charcoal canister for emissions (collection of hydrocarbons)? If so, then your oil filler cap should be sealed, and there should be a metering nipple on the top of your valve cover with a small "breather" hole in it. If, say, a vented oil filler cap were to be fitted (or the old, sealed one is leaking), it will allow too much air into the intake (via the hose from the engine front cover to the carbs). For more detail, see page 320 in the Bentley Manual, section Z3, Engine Emission Control. A quick test of your sealed oil cap condition, is to apply some cellophane film (like Saran Wrap) over the oil filler neck, re-install the oil cap, re-start the engine and see if that eliminates the lean condition - if so, buy a new cap (the oil filler cap was meant to be replaced annually to maintain a perfect seal).

- does it have an air pump on the exhaust system? If so, then check the vacuum sensing tube that goes between the intake manifold and the gulp valve for a leak. For more detail, see page 322, Exhaust Emission Control.




Norm
Oh, sorry, Norm, I forgot to not, that the engine was build for the US market, but that I have removed all the emission control parts and plugged the pipes on the manifold and plugged the holes at the exhaustports. I only have left the charcoal cannister and the hose from the valve cover to it. And that I have fitted the 9:1 pistons, or are they 9,3:1.....I dont remember. Apart from this surgeries it should be stock........well with only one muffler.....



Mightymidget

Ballerup, Copenhagen, Denmark   DNK
In reply to # 3535317 by Kerr "the idle is still as high as approx 1400-1500 rpms even with the idle screws fully out."

This is key: from your description of things it is very likely that there is a vacuum leak somewhere.

You described all new gaskets fitted, but did you test that by spraying an aerosol all around each gasket joint, including all of the intake manifold, up to the cylinder head to make sure? To do this test you listen for a change in the engine running sound while spraying. If there is any change that identifies a leak at that point which needs correction.


If there is no air leak anywhere in the intake system, but it is still acting lean as you describe, the next place to check is the vacuum advance to the distributor (look for a leak), or the crank case ventilation has a leak.


N
I'm a little reluctant in spraying around the hot parts, especially I'm afraid to hit the exhaust manifold, thinking about the risk of fire, but I can feel, that there's no way out, your statement make sense.



Mightymidget

Ballerup, Copenhagen, Denmark   DNK
In reply to # 3535322 by Kerr Regarding the crank case ventilation system:

- does your Danish Midget have a charcoal canister for emissions (collection of hydrocarbons)? If so, then your oil filler cap should be sealed, and there should be a metering nipple on the top of your valve cover with a small "breather" hole in it. If, say, a vented oil filler cap were to be fitted (or the old, sealed one is leaking), it will allow too much air into the intake (via the hose from the engine front cover to the carbs). For more detail, see page 320 in the Bentley Manual, section Z3, Engine Emission Control. A quick test of your sealed oil cap condition, is to apply some cellophane film (like Saran Wrap) over the oil filler neck, re-install the oil cap, re-start the engine and see if that eliminates the lean condition - if so, buy a new cap (the oil filler cap was meant to be replaced annually to maintain a perfect seal).

- does it have an air pump on the exhaust system? If so, then check the vacuum sensing tube that goes between the intake manifold and the gulp valve for a leak. For more detail, see page 322, Exhaust Emission Control.




Norm
Norm, I have eited the preliminary topic, about the changes I made, removing the exh. emission control parts and so on. I bought and fitted a new filler cap during rebuild, but it's not a good fit compared to the old one, allthough they look excactly the same, I'll check here. I have the metering nipple with the breather hole in the valve cover.
Jan



Mightymidget

Ballerup, Copenhagen, Denmark   DNK

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ice Avatar
ice Gold Member Larry Ice
Lawrenceville, GA, USA   USA
Jan, just start the engine cold and spray starter fluid or some flammable fluid around the intake manifold. Like Norm says you should get a surge of rpm if you find a vacuum leak.



Iceman

Atlanta GA

60 AH MK1
62 AH MKII
67 Midget
71 Midget

Kerr Avatar
Kerr Platinum Member Norm Kerr
Ann Arbor, MI, USA   USA
the manual you want is specifically for '67~'74 Midgets (not something general for all British cars, which is what the one you linked looks like)

here is a link to an online version (not as good as paper beside you in the garage, but a useful reference):

http://www.spritespot.com/referencedesk.htm



To answer one of your questions from the beginning of this thread, the pistons are unique between spring loaded and fixed needles. There is an adapter that can be fitted, but I am not sure who makes it (maybe Joe Curto?), the easiest answer is: if you have spring loaded needles use spring loaded needles (they all have 3 letters). By the way, I doubt that changing your needles will be at all helpful to solve your issue as described.


Also, if you can post some photos of your engine that can help us to troubleshoot.


thx,
Norm

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Midgies Dad Avatar
Midgies Dad Ben M
York, Maine, USA   USA
This is the Bentley manual -- your 1275 bible.

https://prodimage.images-bn.com/pimages/9780837617480_p0_v1_s192x300.jpg



Edited 2 time(s). Last edit at 2017-06-19 02:59 PM by Midgies Dad.

flat350 Avatar
flat350 Steve Alleman
Schaumburg, Illinois, USA   USA
Make sure that the new throttle plates that you installed to replace the ones with the poppet valves are installed on the shaft's correctly,just a little off and they will hold open.



1971 MG Midget MKIII

Ballerup, Copenhagen, Denmark   DNK
In reply to # 3535908 by flat350 Make sure that the new throttle plates that you installed to replace the ones with the poppet valves are installed on the shaft's correctly,just a little off and they will hold open.
Steve, I believe I have installed the throttle valves correct, I looked through the bores against a light and centralised the valves before tightening the screws and no significant air gap was present. With the old throttle valves still in I could see air gap severeral places between the bore and the valves, especially in one of the carbs.
Jan



Mightymidget

Ballerup, Copenhagen, Denmark   DNK
In reply to # 3535469 by ice Jan, just start the engine cold and spray starter fluid or some flammable fluid around the intake manifold. Like Norm says you should get a surge of rpm if you find a vacuum leak.

Larry and Norm, yesterday after the drive home from work I leaved the car for a while so that the exhaust manifold was not that hot. I was concerned about the risk of fire. I then started the engine and after a very short time of idling, I sprayed plenty ether based "start spray" on all the connections in the inlet system. There were no signs of rising engine rev's ...........
Jan



Mightymidget

Ballerup, Copenhagen, Denmark   DNK
In reply to # 3535322 by Kerr Regarding the crank case ventilation system:

- does your Danish Midget have a charcoal canister for emissions (collection of hydrocarbons)? If so, then your oil filler cap should be sealed, and there should be a metering nipple on the top of your valve cover with a small "breather" hole in it. If, say, a vented oil filler cap were to be fitted (or the old, sealed one is leaking), it will allow too much air into the intake (via the hose from the engine front cover to the carbs). For more detail, see page 320 in the Bentley Manual, section Z3, Engine Emission Control. A quick test of your sealed oil cap condition, is to apply some cellophane film (like Saran Wrap) over the oil filler neck, re-install the oil cap, re-start the engine and see if that eliminates the lean condition - if so, buy a new cap (the oil filler cap was meant to be replaced annually to maintain a perfect seal).

- does it have an air pump on the exhaust system? If so, then check the vacuum sensing tube that goes between the intake manifold and the gulp valve for a leak. For more detail, see page 322, Exhaust Emission Control.




Norm
Norm, yesterday I fitted a little plastic bag over the oil filler cap and fastened it on the valve cover just below the cap with some rubber band. I disconnected the timing cover breather hose from the carburettors and put it into a can. I plugged the two pipes on the carbs. I started the engine, disn't notice any change, took it for a spin and didn't notice any change compared to the previous setup. I noticed too, that the plastic bag continued to stay "neutral" - it was not pumped up with air and there were not accumulated any vacum, must show, that the oil filler cap is tight, right?
Jan



Mightymidget

Kerr Avatar
Kerr Platinum Member Norm Kerr
Ann Arbor, MI, USA   USA
It does sound like we can rule out the throttle plates and the oil filler cap.

Next step: the timing advance vacuum line

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