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MG Midget and Sprite Technical - Ascertaining mix condition

Gents

Quick Q. I have CV bike carbs on the Zetec powered midget. I have no gunson thing. Spark plug condition apart, how do I quickly ascertain if the mix is reasonably OK?
Oggers

Oggers, I have no experience of bike carbs on a car, but I see you have had no replies as yet so I will dive in.
The usual sequence would be to set a medium tickover and then adjust fuel mixture strength up or down until you get the fastest tickover. Then reset tickover down, and repeat on the mixture strength. Do this a couple of times and the mixture should be near. At that point you would then richen it slightly to cope with snap acceleration (or do these carbs have an accelerator pump?) but also to safeguard against lean running at high revs which could result in piston damage.

Now you need some input from someone who knows what they are talking about!
GuyW

Guy

That is all sound advice, but these are slide CV carbs and mix adjustment is not easy. At present it is a case of how do I know rather than how do I rectify. Usual stuff applies of course - black smoke, stinking of fuel, plug colour etc, but I'd like to gain a reasonable idea of the mix condition before I take it for a run as the clam shell bonnet is a PIA and access is limited when it is back on.
Oggers

I guess you are looking for mix through out the rev range and load range? In which case a wide band lambda is by far the best way to "dynamically" check the mix.

A garage can use their gas sniffer to look at the idle, but that is limited use.

Seat of the pants is as good as any of the other ways of guessing. You can tell if it is grossly wrong just because it won't pull. But it's not accurate enough for proper adjustment.

Malc.
Malcolm

Malc

Yes - My normal method is somewhat seat of the pants. Drive enthusiastically up the local steep hill and see what happens! - and then check plugs. But as I say, the bonnet is a pain to replace/remove and engine/carb access is not good with it in situ. As an aside, I think initally the carbs were canted upwards slightly - just to get them to fit into the engine bay - causing the petrol level in the float bowls to be off the horizontal, in turn causing the floats not to rise as they should, not shutting off the fuel correctly, thus causing flooding, thus causing rich mix.

Since then I have ahem "adjusted" inner wing - cut a small hole in it - which allows the carbs to be pushed downwards so the float bowls are now pretty level. Seems to have cured it, but want to make sure before I replace the bonnet.
Oggers

<<bonnet is a pain to replace/remove and engine/carb access is not good with it in situ.>>

There is an obvious answer there!
It would not have deterred Prop!
GuyW

Guy

Bonnet is off, and I have driven it around with it off for 5 minutes when no-one is looking...Wonder if that is illegal?
Oggers

Doesn't appear to be illegal to drive around with an uncovered engine on a motorbike.

Or a trike.


Or a 3 wheeled Morgan. 😁
GuyW

Guy

Good point, but it may be that if was originally built to take a bonnet, it should have one in place on the road. Lord knows how those new Morgan trikes pass the pedestrian impact tests....
Oggers

I think the essence of the problem is that the carbs are too big for the engine. I have tried several things since to lean it out, but nothing is working. It is still far too rich - although it does go like a rocket! I think I'll just have to reduce the carbs from 40s to 37s and take it from there.
Oggers

As an experiment, could you fit a restrictor plate between carb and manifold, to narrow them down a bit? I guess that won't weaken the mixture but it might be interesting to see what effect it has.

For a short term test you could probably cut them out of cardboard! Or baked bean can lids would cut with scissors.
GuyW

Guy

Good idea - I'll hold on to that thought. Just agreed with supplier to swap them for 37s though. Will fit them up and revert when re-assembled.
Oggers

Have these carbs been checked and adjusted for use. Plus if going for billy-big-bollocks components and parts they require to match and balancing in with the other bits (extra go-faster stripes and extra characters to the boot badge at least) or plugs, ignition system, wotevar.

Isn't there usually some messing around with jets and stuff.
Nigel Atkins

'ere we go.

“Zetec
This depends on the spec of the engine is there are a lot of race-spec heads about that need as big as 42 mm carbs. The average road-going or standard Zetec is happy on 36-38 mm carbs” (their typo not mine).

“It’s very important that bike carbs run no more than 3 psi of fuel pressure.”

“Once the carbs have been stripped and cleaned they need to be drilled out to the correct 1.6-1.7 mm, which Steve recommends as a good starting base”

"or use complete aftermarket ignition systems from MegaJolt or Weber Alpha. For an EFi engine, such as the Zetec, then the latter will be the only option," IIRC do you have the MegaJolt(?).

Nigel Atkins

Hi Oggers are the carbs specced for your engine?
Peter Burgess Tuning

As I am having a clearcut, I could sell you my Gunson exhaust gas thing. But it sounds like you really need an A/F sensor in the exhaust
Dominic Clancy

Gents

Bike fuel pump installed - no more than 2 psi and self regulating

150 main jets - ie 1.5mm. Main jets really make no difference until WOT

Running on a NoDiz ECU- fine with the previous Weber, should be equally fine with bike carbs.

TPS wired in to ECU and calibrated.

Peter B - Broadly speaking yes! Balanced as well. Reading around, others have used 40s to good effect o nthe standard EDDC Zetec. Not sure whether they are exactly the same carbs though. I "think" the issue is that my 40s are too big to adjust satisfactorily at normal road speeds and idle for my standard EDDC engine (Zectec 2.0 Blacktop). I adjusted the floats so they sit high and de-shimmed the needles by 0.5mm so it raises them in at attempt to lean things out. Although it does run leaner, there is no throttle response - suggesting it is too lean! The happy medium is very difficult to find.

Think I will have more luck with the 37s
Oggers

Sounds like you're on the right track, those other boys were running with dials that go to 11 and you've going for 9 and even 8 might be fine, And as you put your dials might have been different sizes anyway.

Which is a Zetec engine anyway, paint the top any colour you like, bright red is always the most powerful and fastest.

Good luck, if you get it right you should see reasonable mpg too subject to tread and weight of right boot.

Doesn't the T9 box slow you down though?
Nigel Atkins

You'd think it would suck 40's ok
I've never had anything to do with these particular carburettors but normally for initial setup on anything different, I'd concentrate on full throttle mixture first then fiddle with the inbetween bits---reason being
If you get all the progression stuff right then change main jet mixture it throws all your hard work out the window
What size is your Zetec- is he variable timing
A friend here has an Escort with a std 2.0 non VVT Zetec on 48mm throttle bodies and it's fine, you can give it full throttle at 1000rpm and it will pull hard and clean from there right through
William Revit

Of course with your carbs the idle balance and mixture has to be adjusted spot on first as the idle mixture on most bike carbs effects the mixture quite a way through the throttle range , so idle setup first, then get full hook mixture done, then you can fiddle and fine tune the middle bits if you need to
If they're anything like most of the Jap. bike carburettors we get here, they're built to meet emissions and have to be set up really lean at idle, otherwise they'll carry an overrich mixture through the progression/mid range
I'd imagine PB would know them inside out being a Harley man
William Revit

Hi Willie


You'd think - but it doesn't!

All CV bike carbs work pretty much the same. Ensure correct float height, make sure the valves don't stick. After that, then roughly first 1/4 throttle is idle jets and their air-mix. Mine were 1 5/8 out as per manual. Idle jets as stock - which should be fine - and all cleaned out. 1/4 to 3/4 is needles and needle jet/holder. Mine were stock - unadjustable but shimmed by 0.5mm. Then 3/4 to WOT is main jet. Mine were 150 which should be about OK. The carbs are Keihin CVRs from a ZX9R Kawasaki, and theorectically it should just run fine out of the box - albeit with a modest amount of tweaking required thereafter. Slides move up fine, butterflies are fine, it's just pulling too much fuel.

I have a feeling I am missing something obvious. The carbs may be damaged in some way that I cannot see, or the carbs are just simply over-sized. It either runs far too lean or far too rich - hence the request to reduce the size and increase the chances of an acceptable solution in the most used throttle ranges.
Oggers

I don't tune bike carbs on cars as I cannot stock enough parts and the conversion manifolds tend to be crap, also most of the carbs are old ones with plastic slides that have worn out. With the few I have done for mates we have had good results when folk change the needles to the ones with 5 grooves so you can tweak the mixtures. The main jet, as you say, is only for flat out power. Be aware that idle mix adjustment does show up throughout the rev range when you log all the afr readings and compare. This surprised me when I first saw this effect. Like with Webers, was told to set mid range foot flat main jet, tweak the flat out with air correctors then tweak the idle and part throttle. Not so as the various jets have an effect over the whole range!
Peter Burgess Tuning

Sorry Peter, I feel like I might have dumped that on you a bit there
My apologies
willy
William Revit

No probs Willy I have been following the post but didn't feel I had tons to add and didn't want to appear negative re bikecarbs on cars.
Take care and stay safe
Peter Burgess Tuning

Peter

My manifold is a pretty decent one - fabbed up by a guy who specialises in bike carbs for things like Westfields Locosts etc., and it does seem to be well made. You are quite right - 5 position adjustable needles would probably help me out here, but mine cannot be lowered - non adjustable, and I cannot find any of similar profile which are.

Agreed about the idle. Its the first place you start. However, it is screwed in pretty far in - lean, but yes, as it does affect all the rev range, it may need even more. When it ran lean they were 1 1/2 out and needles lowered as far as they could. It then bogged down - suggesting too lean. Perhaps don't touch the needles and screw the idle in even more? Anyways - sent back to exchange for 37s which should be easier to tune.

Thanks for all the help folks....some good pointers here
Oggers

First tinker with the 37mm carbs looks good. No smoke at idle, cannot see anything when opened up beyond that either. After a good rummage in Oggers' garage full of curiosities - or junk as Mrs. Oggers disparagingly puts it - I came across an old Colortune kit. Duly plugged it in, and lo, a light blue flame at idle and similar at a wider throttle.

Question therefore - does this mean it is running slightly rich or slightly lean? I glimpsed a much more saturated blue when it was coming down from about 3/4 throttle, and there was also a bit of popping back when opening the throttle - I am guessing slightly lean.....

Oggers

Good progress Oggers.
You would expect it to run a little lean on the overrun wouldnt you. But if its popping, maybe its a bit too much. How easy is it on those carbs to richen it a little? Do you have air filters fitted yet? If not, then simply fitting them may be enough to richen the mixture a little.
Assume you have no exhaust manifold air leaks!
GuyW

Guy

All good points - Fiddled about some more this afternoon and the latest state of play is as follows.

Enriched the idle some more - all cylinders with the Colortune are now showing blue at idle. A tad bluer perhaps - but difficult to discern. Needles also adjusted up - so they run a tad richer. Air filter fitted. Now, pretty much from idle to about 1/3 throttle is pale yellow, from 1/3 up is predominantly blue. The popping has gone on the overrun.

It seems about right, but not taken it out on a run, and not checked the plug conditon thereafter. 37mm carbs definitely easier to tune - although the needle clips are on the lowest groove - could shim them to take them up some more I guess. Personally I always feel running about in and plug condition gives a more definitive view than the Colortune, but OK for a first pass.

Look promising, sounds smoother, and seems to breathe far easier!
Oggers

Drive it! It's the only way if what you are after is a car that you enjoy! Personally I could never get a coloutune to be consistent enough to be conclusive.
Just drive it and have fun!
GuyW

Guy

Yes- tried Colortune before on a plethora of classic bikes and the previous mk3 Midget. It's OK as a guide, but that's about it. I think its main drawback is the average Joe can only really use it when the car is stationary and the engine is not pulling a load. Mix requirements at load may well be different.
Oggers

This thread was discussed between 25/11/2020 and 13/12/2020

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