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MG Midget and Sprite Technical - K&N air filters - now gutless

Hi all, I've just fitted a pair of K&N filters (1500 with Hs4s). I did my research and richened both carbs by one and a half flats. It was fine for the weekend - took them off (to fit stub stacks) and now it's gutless - won't rev past 3k and only performs if I pull out the choke.

I suspected something else was wrong so all doom and gloom did a leakdown test and all cylinders read ok, all equal 15% leakage, no sign of any bubbling in the coolant. Valves all adjusted ok, timing ok. Plugs are tan.

Put the old air filters back on and it's fine - but it does run on a little now. So can anyone give me a clue as to where to start? I think it's running lean (by the running on and the need for choke) but plugs look fine. My MPGs are down a bit.

Any advice really appreciated, looks fuel related but not sure where to start - don't understand why it was ok when I initially fitted the new filters and having removed them now it's not. They're on the right way up.

S Watts

i think you will find that when using K N filters you need richer needles, i did. someone will be along to confirm I'm sure.
regards bob.
bob taylor

Depends what else you've done tuning wise but the MOSS website recommends 'AAQ' needles for K&N plus performance exhaust....
http://www.moss-europe.co.uk/shop-by-model/mg/midget/performance-tuning/fuel-system-induction/1500-needles-for-su-carbs.html
graeme jackson

Do your K & N filters have the extra by-pass holes on the back to equalise the pressure on the carb pistons? Or, the use issue, are the holed blocked by the gasket. This will stop the engine from revving.
Guy W

Guy, yes they have the holes, holes are clear, no blockages. Can't understand why it initially ran ok - it wasn't hesitant at all when I first fitted them.

Graeme/Bob thanks for that info - got a Maniflow 4-2-1 to fit so stock header at the moment. I'll get those needles then.
S Watts

Only thing I can think of at the moment is the possibility of an intake manifold leak which would be leaning it out. The engine does have a different note to it now - raspy rather than just sucky (if that's a good description!). But that wouldn't explain why it runs ok with the stock air filter though. Stumped at the moment.

I'd assume if it's a blown head gasket I'd have had a low reading on the leakdown test and bubbles in the coolant.
S Watts

Well I've checked the compression, 175 on all cylinders. And I've sprayed propane across all the intake joints and there's no manifold leak that I can find.

Anyone got any suggestions? I'll put new needles in and change the exhaust but I just can't think why it ran ok in the morning and then had problems later, it was a long (3 hour) drive with a few stops -
could this be something in the bottom end causing it?
S Watts

So it ran good with the old filter set up, and then it ran even better, with the new K/N filter set up, but then added stacks inside the K/N filter set up and now cant rev past 3000 rpm


Hmmm... yepp its got to be a head gasket issue, no doulbt about it...haha

If the by pass hole is clear as guy mentioned, then my question is how tall is the sub stack vs how deep is the K/N air filter assembly...my guess is the mouth of the sub stack is to close the the K/N front lid... causing to much constriction in the air flow...constriction is good as it makes the air move faster but like all good things in life...to much can be a bad thing... so in laymans terms, your not sucking in enough air esp.at the higher rpm range where you need more air...

Try a shorter sub stack perfably a donut type of stack ... you just need a rounded shape to give the air a radi to enter the carb instead of the 90 degree angle the air has to navigate into the hole with out the stack

Even if the stack is short enough to not over constrict the air flow... another problem area that can be a problem is the shape of the stack. If you dont have a good size radius ring on the stack that can also have an effect on how air enters the carb

Try a book by danial stapleton on modifying mg midgets, I think he has a good read on sub stacks and the direction to go

In the mean time, id loose the sub stack untill you can find what works and what wont

Prop
Prop and the Blackhole Midget

It may well be without the stubstacks the fuelling was rich enough to run, sharp edge no stubstack creates bigger pressure drop on main jet ...more fuel.... stub stack fitted less pressure drop weaker. Try AAM needles, stub stacks and jets set down 60-70 thou from bridge.

Peter
Peter Burgess Tuning

What happens if you put the old filters back on? If all (reasonably) OK with the old filters then you have eliminated everything except the filters/stub stacks as the cause of the problem.
Chris Hasluck

Thanks a lot folks, Chris, yes tried the old filters and reasonably ok.

K&Ns on with or without the stubstack gives me a serious lack of power. It feels like retarded timing (but isn't).

I'll run the stock air filter while I source replacement needles and have another go when I've got those in my hand, and scratch my head in the meantime. I think it's fuel related (not HG). Want to replace the exhaust manifold with a Maniflow 4-2-1 but disinclined to do that at the moment while this problem is unresolved. Hmmmm.

Out of curiosity - the compression test results of 175 across the board - is that good? Can't find any reference data on what it should be.
S Watts

175 is very respectable... esp having all 4,

It sounds like peter has the inside track on this one

Prop
Prop and the Blackhole Midget

Peter, thanks - interesting! I'm learning loads! So AAM needles with the Maniflow header? If I can just get it so it isn't so leaned out I'll arrange a rolling road session.

S Watts

I'd say you have something wrong somewhere.

I have k&n and stubs. I can take the filters off completely, and the engine will still run fine, albeit a bit weak, so should yours.
Lawrence Slater

Lawrence, agreed - just can't work out what it is. I'll go back over it again tomorrow.
S Watts

Is your throttle linkage correctly fitted and adjusted to give full WOT with your foot to the floor
Guy W

Id like to see a few pics of your set up ... the carberation side along with your filter and stacks set up... inside and out

Is that doabe ?

Prop
Prop and the Blackhole Midget

When you mention old air filters... do you mean the factory orginal double sauce pan filters and housing ? Also called trumpets



Prop
Prop and the Blackhole Midget

,
Prop and the Blackhole Midget

Aren't you 1275 Lawrence?

The standard needles in the 1500s are very weak, remove restriction from filters/casing and very very weak. I attach a pic using Minty Lamb SU charts. ABT is very weak, two richer lines AAA and AAM

If you pull your choke out increasing amounts you will soon know how weak the setup is.


Peter


Peter Burgess Tuning

I highly recommend you follow Guy's first post to the letter.

I simply cannot believe that by changing filters out - all other things being equal - there is such a huge difference.

Thoroughly check that the bypass holes on the carb(s) are not being blocked by any stub stacks, or gaskets attached to the new filters or by a bit of crud.
Mark O

Agreed. When you took the new filters off to put the sub stacks on you didn't fit them upside down by any chance? Very easily done, and albeit with a 1275 it gave me the same symptoms.
Matt1275

Right, I've been very methodical and thorough this morning, cleaned the carbs, replaced the flexible fuel lines, blown through the fuel line between the carbs, double checked timing and points, checked the valve springs and it's now drivable again with the K&Ns on. Still puzzled as to why it was ok with the stock air filter though? Wonder if there was a blockage somewhere?

Thanks for all the advice, all really appreciated. Got some new needles ordered today as suggested by Peter so looking forward to seeing what improvement that brings about!
S Watts

I think its a good practice to order both jets and needles of am I thinking of something else

Prop
Prop and the Blackhole Midget

Peter, I agree entirely that stubs-tacks will improve flow (assuming they are not too close the the filter case)by eliminating some turbulence but won't their slight addition to the reduction in volume inside the filter make pressure drop ever so slightly worse? BTW, I made some very short elliptical stacks and put them in a big case with a remote filter to get a big volume between filter and carbs to hopefully eliminate pressure drop as far as possible. I'm running AAL needles but, when the car is on the road I'll hopefully see if you can improve everything with a R.R. tune-up
:-) !
Nick Nakorn

Nick
Only if the filters are not deep enough.
With more depth than the diameter of the throat (on top of the sub stack) it should not pose a problem
Onno K

Onno,

So would that mean on an hs4, you want the filter lid to be at least 1.5 inches from the mouth of the stack??

I.always wondered if there is a rule of thumb, for that

Prop
Prop and the Blackhole Midget

Sorry ...

Meant to say ....no more then 1.5 inches from the mouth of the stack
Prop and the Blackhole Midget

Hi Nick

You are welcome to come up and see if we can make it run better or confirm it is already spot-on.

Prop and Nick, it seems one doesn't need a massive gap on road engines and still not lose power. We have been down to 3/8th"/10mm on sprint engines and as long as the rest of the filter is big enough, not lost power.

Peter
Peter Burgess Tuning

When I eventually get up to your rolling road Peter, I'll be interested to see if I've made any improvements over an open carb !
Nick Nakorn

Yup, I'm 1275. Whoops, I Didn't read the original post properly did I. ;).

Are 1500s(Triumphs) that much more sensitive to driving without air filters then?
Lawrence Slater

Its possible that the problem was a bit of debris inside the fuel tube connected to the jet. At tickover and low revs the needle is low in the jet and keeps the crud pushed down in the tube section where the fuel can easily flow around it. As revs increase and the needle rises it allows the crud to get carried up the tube into the jet itself where it plays havoc with the fuel delivery..

Its a problem that you clear, or it can clear itself, without you ever actually seeing the debris. So a good clean through of the pipes can restore proper running without it being apparent why.
Guy W

Hi Lawrence

Over the years the models produced by MG/BLMC/BL etc ran weaker fuel ratios. The MGAs cruised at 5%, early Bs 2-3% rubber bumper Bs 0.5% CO. The same seems to apply to Midgets. The rubber bumper 1500s were set weak and the effect of removing filter/casing restriction is very marked.

Peter
Peter Burgess Tuning

Ah thanks Peter. I withdraw my comment then. :).
Lawrence Slater

This thread was discussed between 07/04/2015 and 09/04/2015

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