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MG Midget and Sprite Technical - One eyed frogeye!


As long as I drive my frogeye (10 years now) it hesitates at a speed > 60 mph WITH WARM WHETHER. Before hesitating I hear a different double sound from the exhaust. After I slow down to 55 for 5 minutes I can drive her to 85 miles for 3 minutes or so. If I stop her direct after the hesitation the valves , spark plugs and pistoms are to white. A rolling road session showed a fuel/air mix 1/12-13 (and no hesitation while the water temperature was > 190 oF. )
The problem comes at a higher speed if I open the heater or the choke.

What did I try to solve this problem?
Cool thermostat (water temperature seldom comes above 150 oF now)
New Peter Burgess valve guides
More valve clearance.

New ignition (3x), now 123 with Bosch coil lovely!
Tried NGK BP ES 5, 6, and 7, champion the English ones.

Electrical fuel pump 0,5 psi (2x)
Re routing and isolation of the fuel lines

More distance pieces between carb and manifold.
Insulating the H1 carbs.
Insulating the heat shield
Rich EB instead of CC needles

Adding Tetra ethyl lead to the BP ultimate. How lovely smooth and silent she goes with that!
Removing one headlamp (photo), so the under bonnet temperature went down (she drove faster than ever: 89 mph wich is fast with al those speed cameras here around Amsterdam. No wind)
This all and still hesitating

What else can I do?


Flip Brühl 948 frog 59

remove the other head lamp and go even faster!

have you ever rebuilt the carbs? and what kind of intake system is on there (filters trumpets)
Onno Könemann

BCCP did rebuild the carburettors, K & N Filters (I forgot to mention checked the air hole in the fuel tank.)

Flip Brühl 948 frog 59

I would say, coil, resistor or contactbreaker.

Typical stuff that would react different when coping with high revs and/or high temp.

Bas Timmermans

My neighbour had an old fuel/ vacuum tester. I started to test the fuel pressure when hesitating: it stayed at 4- ??? I guess 0.4 atm. as the Pierburg pump should give this pressure. So no problems in the fuel supply till the fuel reaches the carburettors. Tonight wit less traffic I will try the vacuum on the inlet manifold. On the gauche is printed leaky valves. I wonder what will see. Any suggestions? I do not have any experience with a vacuum meter.


Flip Brühl 948 frog 59


I have tried 3 different coils, it has no contact breaker and as far as I know I don’t use a resistor. The vacuum meter showed no changes with and without the hesitation. With full open throttle on top speed it reads 10. Does that exclude problems with the valve train? What else can give those hesitations and diminishing of power if I maintain top speed for more than some minutes?

Flip Brühl 948 frog 59

Seems like you've done everything!

What about the level of fuel in the floats? Are you sure that the amount of fuel in the floats doesn't diminish with sustained high usage? My idea if the fuel pressure is fine, but maybe the flow isn't enough so the floats empty with use.

You could check this by switching off ign when misfire comes in, and check the floats when stationary. Check the current level with the normal level with ignition on. (Maybe not enough flow past the needle valve(s)?)

BTW if you valve start to play up... really means stick open, then vacuum in inlet would drop, since (if inlet) pistons now pulling in air and pushing it back out in compression stroke and if ex then the exhaust pressure from working cyls would fill cylinder and enter inlet. The engine would sound a little different...


Anthony Cutler


I still miss something, I don’t think the float needles are a problem. I now use delrin? tipped needles it did not cure the hesitation and the "double sound". But it is not difficult to stop the engine when it happens and look for the float level. Any further suggestions what to try next are welcome!


Flip Brühl 948 frog 59

The H1 carbs use a pair of grommets where the float chamber connects to the body of the carb. Are you sure these are in good nick and not obstructing flow? I have seen these give trouble before, but that was when the poor bloke tried to use Sta-Bil in his petrol and it ate the rubber.
David "W.A.G." Lieb
David Lieb

So now we are at the "rare" section and I can mention that a vacuum in the fueltank can cause some simmilar symptoms. Easy to check. Take off the cap and do a testrun.

Bas Timmermans

Didn't Flip cover that in his second post?
David "sounds like a lack of fuel... perhaps an electric fuel pump to each carb?" Lieb
David Lieb

Maybe the screen on the fuel pickup in the tank is clogged.
OTOH, 0,5psi sounds very low, I thought we should be getting around 2.0psi? At least, with an electric fuel pump there is no difficulty in testing it. How many liters/hour is it able to put out if you measure at the carb input?
I have no manuals (or books by Daniel Stapleton) here at work, so someone tell me what is considered normal.
David "grasping at straws" Lieb
David Lieb

i do hev Daniel's book at hand and found out the lightes pump does 7 gallons per hour

most are in the 12-15 galon/hour region
Onno Könemann

So that would be just over half a litre per minute. Borrow your wife's favorite measuring cup and timer and check it ;-) If you are still alive tomorrow, let us know what you found out. Might be worthwhile to blow out the fuel line (probably want to bypass pump and filters).
David "not going to touch that line" Lieb
David Lieb

@ David, " (I forgot to mention checked the air hole in the fuel tank.)"

If that means Flip checked the hole in the fuel filler cap, then yes!

I once had a crack in the cap seal. When tightening the cap down the seal would expand and cover the hole. Took a while to find out:-(

The seal wasnt 100% so the problem only occured when driving fast or spirited:-) Thats the moment it went out of fuel. It happend on a bridge and at that very moment the signals were on and telling me the bridge was about to be going upwards. I can assure you that wasnt a pleasent moment.
Bas Timmermans

I use a new pair of grommets. Float level is nice. My wive is in Colmar France for work. So I could borrow "our” favorite measuring cup. Long ago I gave her a rust free metal one. Plastic ones where not good enough for the fluids that I measured before. With the help of my yongest daugther I measure more than one litre a minute. The problem is past the point where I measured flow and pressure .5 atm. =7 psi. And not .5 psi. The pressure in the inlet manifold did not increase at the moment the strange sound starts and de power drops. Will that exclude the inlet valves to be the problem?

Flip Brühl 948 frog 59

I think you are running into too much advance under those sustained high load conditions. It takes considerable time (like the times you quote) for the engines insides (combustion chambers) to get to stable temps under load. I have read that it is common for rolling road operators to set too much advance, which gives good power readings for a short time, but results in detonation under sustained load. That would account for both the power loss and the noise.

All the conditions you report as helping are consistent with this theory. Headlamp = lower underbonnet and air intake temps; heater = lower coolant/chamber temps; tetra ethyl = higher octane; choke = richer (cooler) mixture; slowdown = cooler chambers.

I would back off the distributor a few degrees and try the top end. If that works, you may then need to recurve to get the low & mid range back. Since you have a 123, that should be easy. If there is not a suitable curve included, then contact Marcel Chichak, who invented the 123. He is a Mini guy, and an inveterate tinkerer, and seems accessible. The great advantage of an electronic distributor is that you can put "bumps & squiggles" into the curve, which you cannot do with mechanical systems.

I suggest building a dedicated cold air intake, since you have already demonstrated that it is beneficial. If the timing changes do not fix it, then I'd look at other factors affecting chamber temperature. Water pump might be a good start; I think the 948 pump can be upgraded; Vizard mentions using a Cooper S pump with a shortened impeller on early engines. You could be getting pump cavitation at high rpm, which means you want to slow the pump down with a bigger pulley. That might mean problems at high load/low engine speed, which the bigger pump would cure! It's possible that upgrading the pump or similar could let you go back to the previous advance or more - it's all about chamber temps.

In looking up info re advance curves lately, I found possibly the best resource ever for people who really want to know what's going on in an engine, and how to find out on the cheap.
This will take you to a discussion of advance timing, and part 2 gives a most splendid cheap diagnostic tool, useful for lots of other stuff too. I am off to buy the components (all of about $10) this week! Links from this page will give you hundreds of similarly useful articles, tools, and info. That includes a bunch of stuff on the effects of charge temperature too.

FR Millmore

Further to my previous:
Note that all else being equal, detonation is caused by high combustion chamber temps, and backing off the advance is an expedient "fix", not necessarily "the cure".
One easily checked cause of high chamber temps is restrictive exhaust. I've had a couple of dramatic cases. A customer kept burning exhaust valves, on a Morris or Austin saloon. He was a large animal vet and drove into fields to access his patients. Every time it burnt a valve, we also replaced the front exhaust pipe because it was flattened from hitting rocks & stumps. He used high revs in low gears to get through the fields, and was always late, so flat out on the road in top too. Also, I had a Jaguar XK120M, on which the DPO had installed a muffler that fed both 1 7/8 pipes into a single 1 1/2" pipe. This resulted in the temperature gauge acting like a speedo over 60mph. Killed top end too!

Your are reading water temp in the rad I think, so it is not a good indicator of temp in the head.

Drill a small hole in the exhaust just past the manifold, weld in a fitting, attach a couple of feet of 1/8 or 3/16" copper or steel tube (to dissipate heat), then rubber hose up to a sensitive pressure gauge, and run it. Your fuel pressure gauge will do for gross first measurements. Pressures should ideally be below about 2 psi, preferably 1. Really good systems will be around 0.25psi, and awful ones could be 7 or up.

Note that backpressure is exponential with flow through the engine, so it can rise drastically after starting out not too bad. Any improvement in inlet flow, cam, etc will also cause great increases. If you wind up changing the exhaust to help this, you will need to reset fueling on the top end, and you can maybe get your advance back and more.

FR Millmore

Yes; high combustion chamber temperatures. That could well be the case. The white combustion camber interior proved it. I saw it with a Heine scope. My neighbour has one. Why no problems on the rolling road with very high water temp and why not when it is freezing cold?
I retarded the ignition; it helps a little bit but does not cure. Yes I hit some speedbumps, see photo! I use the original small diameter exhaust pipe. I will order a new one. It is stainless (gives higher temps…). The hesitation was there before I hit the bumps.
Yes; cavitations are another candidate. I use an aluminium BLMC mini pump with big blades. The blades just touched the motor casing when new. The original pump was steel and had a much smaller impeller. Where to buy a bigger pulley or smaller impeller pump?
Another possibility is the place where the carburettor overflow pipes end. As I bought the frogeye as a basket cage. There were some plastic overflow pipes when I saw my baby for the first time. I changed them for copper ones. (Photo) I don’t know where they should end. If there is overpressure there at high speed, the float level can be lower than designed. I did control if the were clogged, the where not. I can use the vacumeter to see if there is overpressure at high speed.

Flip Brühl 948 frog 59


Flip Brühl 948 frog 59

Conditions on RR are "different", fans, airflow, etc. And the full throttle run must be held for at least as long as it takes the problem to show up on the road, like 5 minutes.
Float vent tubes should be fine.
Water pump you have is what you want, but bigger pulley would be good. Must be lots of those about, check Mini people.
I don't know what, if anything, is changed on your engine. Greatly affects exhaust requirements. You are in the range where some decent head work could make big difference, if that's not been done. I'd be calling Peter Burgess or similar, and probably starting with a 998 Cooper/MG1100 head.
Are you using stock 948 manifold? I'd be using a manifold and pipe off a bigger engine, but I no longer recall which ones are better. Haven't seen anything but 1275 flange type for years. Stainless pipe will not cause a problem, that dent is probably not the major problem either. Bigger pipe might help, depending on what mods if any, but silencers are the usual main bad thing. Silencers are very hard to figure - you cannot tell by looking at them, even ones that look just like "good" ones can be horrid. Pressure measurement is the only way to tell; measuring at front and back will show if the pipe is contributing a little, but the usual place to measure is just before the first silencer.

FR Millmore

Flip do check the smal pieces of hose that conect the overflow of the float to the copper pipe.

Recently had a GT in the shop witch has simular troubles and it all came down to those small pieces of hose being degaraded and clogged
first time we blew them clean with compressed air but that only helped for 150KM
after that the clogged again
changeing them solved all problems

you waterpump and pully should not be a problem at all
bigger pully's are only needed for sustained periods over 6500rpm (racers)

silencers are good if you can look through the pipe!
annything other than a perforated pipe with dampening around that is rubish

and if you want the best small bore head the 12G202 is the way to go.
in std form it is no stormer but it has potential to flow way beter than the 12G295 it can come close to a wel ported 12G940 (1275 head)

But one thing i noticed you have not mentioned the specs of your engine anny where?
what is it (including in and exhaust)
Onno Könemann

Thank you FRM and Onno,

It is an almost standard 948 cc. I never rev it over 6500 rpm:

Not standard: Cylinders first overbore, K & N filters, H1 1 1/8 carb with EB needles, new float needles (photo). Burgess valve sleeves, lightened flywheel. 3.9 diff. 123 ignition curve B; max 33 deg advance @ 4500 rpm.

Standard; not changed but renewed:
Valves, single valve springs, low 8.3 compression. Exhaust hot spot manifold, I can look trough the pipe of the muffler. Spark plugs NKG BP 6 ES, Positive earth.

I will cheque the overflow pipes again


Flip Brühl 948 frog 59

change that to 32deg max advance verry quickly
in my experiance they get the best power than and if the curve is correct have the bast torque curve

but nothing strange to explane the troubles

I do not know the hot spot manifold but it sounds bad.
Thought about a nice tubliar yet?
buy a proper one there is a lot of crap on the maket.

In the folowing months i will be a bit busy (finish midget paintjob and renovate cousins house)but i would love to drop by sometime and help
2 sets of eyes see more than one!
Onno Könemann

Flip, this looks like a high speed lean out where a combination of the engine mods are drawing more air through (relatively) small carbs and the pistons are maxing out earlier than max revs. In a situation like this the car runs lean as the pistons are drawn to the top of the bellchamber and if sustained the engine will splutter and go no faster. Backing off the throttle will allow the pistons to drop and the calibration to normalise, and then more welly will give the car a period of rich mixture, allowing the car a short lived higher speed till the mixture runs lean again as the pistons max out. So it could be the SU calibration that's wrong here. The calibration is set by the needle and damper spring. Although the 2 are inter related I'd suggest that you look at trying a heavier damper spring, and if it shows an improvement, book the rolling road again to finalise the spring and needle combination. As an aside look at John Twist's videos for more info on the SU and other stuff.
f pollock

I tried to retard the new 123 ignition again; now by 10o! so I put it on TDC. Because it was at 10o before TDC (The dyno engineer put in on that). I made a test run on TDC: No top speed, a lot of spluttering in low revs when cold but less hesitation at top speed. The hesitation stopped in 30 seconds when I pulled open the choke. Sparkplugs snow white. I forgot to take a camera with me so I made a photo after slowly driving home. At home the spark plugs are more or les light/normal (photo)

Second test run 5o before TDC no hesitation at all!!! Spark plugs normal!!! Top speed ok. The motor did not pick up quick in low revs, in high revs no problems. Coming home I saw I forgot to connect the vacuum hose…
It is only now that I read the installation instructions of the 123 ignition. 12o advance at 12 inch Hg. I will choose another vacuum advance 10o degrees at 4500 rpm. As the (centrifugal) advance is 33 o at 4500rpm. It will be too much together with the vacuum advance. I will let you know as soon as the traffic jam is over.


Flip Brühl 948 frog 59

3rd test run: 5o before tdc vacuum connected (max advance 33 max vacuum 12o). The double tone started but no hestitation.

f pollock,
I don’t think it is the carburettor. Fuel/ air ratio is always better than 1/12 See dyno graph. It realy helps to pull the choke but that wil cool and retard the combustion.

I will keep you informed as soon as I can drive with full throttle (FOT)


Flip Brühl 948 frog 59


Flip Brühl 948 frog 59

flip i have checked a 123 manual online
i presume you have the mini/aseries one

try curve 8,9,A or B
i would start with A this should give you back a lot of that low down torque
if that works out without pinking try curve B and see if it is anny beter

if you heve anny other 123
they all are arranged in the same way

you have 4 max advance numbers and for those you have 4 curves in al 16 options

however max A-series power usualy comes near 32 deg max advance.
so you are left with 4 curves of witch you want the steepest (most advance at the least posible rpm) at witch the car show no signs of pinking!

Onno Könemann

4th test run: stroboscopic at 800 rpm: 8 degrees before tdc, vacuum connected (max advance 30 max vacuum 10degrees). The double tone started at 90 km/h and hesitation at 115-125 after 3 minutes.

5th run idem 3 degrees before tdc. No double tone no hesitation top speed 130 km/h (with more advance it can be 142 km/h)

FRM wrote: I have read that it is common for rolling road operators to set too much advance, which gives good power readings for a short time, but results in detonation under sustained load. That would account for both the power loss and the noise.

So that’s it I suppose. He gave me 10 degrees static and more with vacuum and rpm.

Tank you Onno. As you see I tried curve 7 before I received your mail. I am trying curves 8, 9 etc till the hesitation and double tones start again.
I kept the motor warm as you can see. (Photo)


Flip Brühl 948 frog 59

Very high tech solution on keeping the engine warm Flip!
Better your wife doesnt find out tho! :)

Good to hear you got it solved.
Arie de Best

It is my car, its my live, I love this car, and sometimes (now) I hate her.
Yes I can drive on top speed for longer then ever. Say 5 minutes than the hesitation starts and at 50 miles se starts giving the double tone. I did control the ignition. Plotted a white dot on the pulley at 32 degrees and yes with 4500rpm, as seen with a flash light, it is on the mark. While doing so I smelt fuel. It poured out of the H1 carburettor overflow. The engine splutters and stopped. The telephone rang. I had to go back to work. I am puzzled there is only an overflow above 4000 rpm. Any advice is appreciated.


Flip Brühl 948 frog 59

sou you have the only spridget that gets more economical over 4000rpm?!??

Why do all the pully ploting with an 123 you have got the flashing led don't you?
Onno Könemann


Does your frog have a mechanical pump? The faster the engine, the more fuel pumped. Seems you have have a leaky flo-jet on one of your carbs?

Anthony Cutler

The pump is electrical (Pierburg) I controlled the pressure it is constant 0,4 atm at any speed. I guess the overflow can be caused by vibrations.(overflow comes from both overflow pipes. It is there at 4000-4500 rpm and over when reved higher or lower. I controlled the bowls; the float needles, the float-hinged lever and both overflow pipes. A friend of mine has 1.1/4 SU2 carburettors. We are going to try these. Has anybody had this overflow with H1????

Why try a flashlight?
I would like to know if and how much the vacuumadvance adds at >4500 rpm.(yes it adds!) By doing so I found out it is the carburettor an not only the ignition...And yes the 123 does what it says! It gives the advantage that correspondent with the selected curve. That is not always with the things I buy for my frogeye.


Flip Brühl 948 frog 59

I though it was float level related, gas is pouring out the overspill pipes at 4500 rpm. The old needles have a spring inside. The SU engineers did so with a reason. So I did replaced the new (9 year old) viton float needle by the 50 years old example. I did a test run and ran on for 10 miles. No hesitation. I think it is over.

Thank you for your help


Flip Brühl 948 frog 59

This thread was discussed between 09/10/2009 and 01/11/2009

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