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MG Midget and Sprite Technical - fuel?

1275 IOW Frog recently replaced head gasket, head skimmed, all work done by experienced classic car garage. been running smooth ever since. Almost spluttered to a stop the other day and I realised fuel was very low. Topped up fuel but running rough and BIG backfire through exhaust, accompanied by kangaroo hopping!
I've cleaned out the carbs, jets and needles were new a couple of months ago, still kangaroo hopping.
No loss of coolant or mayo in the oil and no smoke (white or blue) from exhaust.
Electronic ignition was fitted several years ago and never had any problems with it. Unfortunately tacho is not reading because its an RVI.
Any ideas where to start?
aj robinson

Id say fuel is a good starting point


if it was running great then the fuel got really low.. and now it runs lime trash... id say drain a d replace the fuel and see what happens if still crap id look for some debri inside the carbs somewhere ... the float needles or the jet / needle... fuel filter area

so based on what you said... it think the fuel is ither defective or you drained the tank so low you picked up trash from inside your fuel tank and its...

now trapped somewhere between the little filter inside the fuel tank and the intake valves inside your engine

good luck.... it can be a fun job....granted it could be something else but i always go back to what changed that caused the problem


prop
Prop the cow killer

But id try replacing the fuel 1st ... before anything as thats the easist to isolate as just being defective fuel.. try to get rid as much as possiable of rhe old fuel before adding the new fuel


prop
Prop the cow killer

Ha ha....Prop beat me to it. He's got some good advice.

Low octane fuel or any little bit of contaminate will cause the car to run as you describe.( I mean water)

I know its a pain to dump 10 or 12 gallons of fuel...but at least you can rule it out when your finished.

Sounds like that might be it by the way you've got your engine tooled up.

I recently went through this with my cousins MGB.

He swore up and down it wasn't the fuel because he just topped off. I ran 2 bottles of Octane boost and
3 bottles of isopropyl alcohol threw 1 tank of gas and spent the night barely able to drive off the whole tank.

I should have drained off the contents and put it in my Toyota Camry.

The car straightened out after a fill of known good high octane gas!
S

okay, thanks guys. Replaced fuel and a slight improvement in the idle; little bit smoother.
Then noticed that the rear carb is lifting much more than the front; could that cause the kangarooing and backfire through the exhaust?
aj robinson

Yes....You will have to investigate that. Good catch by the way.

Check that both butterfly throttle plates are opening at the same time... {Synchonized} and for vacuum leaks on the intake gasket.

You'll have to find out what's causing the imbalance.

Sounds like your on you way to fixing it
S

This is a scary thought... look for loose or missing screws on the carbs

high 5 steven

Prop
Prop the cow killer

im wondering about a vacuum leak because the car recently had the engine and gearbox out and the head gasket replaced, head skimmed.
Is there a 'tried and tested' method to look for vacuum leaks?
aj robinson

Spray carb cleaner around suspect area's.

It's non combustable and will slow the engine where there is a leak .

Gavin
Gavin Rowlesx

thanks Gavin
aj robinson

I would read any warning label on the carb cleaner about flammability as the stuff I've seen is highly flammable.
David Billington

Electronic ignition can definitely do this. I had one that was intermittently temperamental according to what you describe until it quit altogether. I had another module on a different car and switching them out located the fault.
Glenn Mallory

thanks guys. Interesting point about the electronic ignition
aj robinson

Gavin...

you might have your chems confused or be using an exotic carb cleaner

( Most ) carb cleaners are slightly combustable and the engine will actually speed up momentaraly untill the carb spray thats sucked into the open gaps is burnt off

prop
Prop and the Blackhole Midget

Gday all
FWIW, whatever's in the spray can may be flammable but the propellant most likely will be. Take care spraying anything near a source of ignition.
Rod
R W Bowers

any other (safe!) way to check?
aj robinson

If you had a compression tester...just to check the cylinders...I would do a compression test just to know there's no issues with the head gasket.

I had an issue with an engine years ago where the back carb piston kept fluttering up and down and ran poorly..Im pretty sure it was a head gasket blown between 3 and 4.

What do the spark plugs look like?

If you had a vacuum gage and put connect it to the intake manifold that a safe way to check for a leak and will tell a lot as to condition of your engine.
S

Steven

thats a. Interesting thought....

i bet if removed the rockers so the valves all closed tigjt and made a piece of sheet metal with some gasket paper glued to it and bolted that over the front of the carbs vwnture using the air filter bolt

and the put a vacume plus a vac gauge to it... that would tell you if you got a vac leak somewhere or not ....

in fact you could use a marboro cig and run it around the joints ... any leaks the smoke would be sucked into the gap...provided you can keep applying enough vacume if it was a leak


prop
Prop and the Blackhole Midget

If you don't have a gauge, a simple way of checking gasket integrity between cylinders is to remove on plug at a time, cranking the engine while plug is removed.

Any significant leak in a cylinder adjacent to the one with the remvoed plug can be clearly heard via the plug hole. Gasket leaks will be heard on both of the cylinders either side of the leak.

If a leak is heard on one cylinder only in no's 2 or 3, then it indicates a leaking exhaust valve courtesy the siamesed port and the fact that when 2 is on compression 3 is on exhuast so any leak through no 2 exhuast will be heard, and vive-versa. Similarly, any leak at no 2 or no 3 inlet valve will be heard via no's 1 and 4 resp.
Paul Walbran

Vacuum gage testing...

http://www.youtube.com/watch?v=SdlNwm8OHco

I've got one and its helped me out trying to tell the condition of unknown engines to quickly figure things out.

Definitely a great tool to keep on hand.

Its an old tech way that's still works today.
S

Im with you steven... i love vac gauges

they can tell you alot...and the smartphone with internet to understand the readings is just indispensable

prop


Prop and the Blackhole Midget

If you have been running very low on fuel, chances are you may have picked up some residue which normally lies in the bottom of the tank.

Therefore

Thoroughly clean out all in line fuel filters - forget about the strainer in the tank for the moment.

Test car

If performing as previous, physically remove both jets and thoroughly clean them out with either a soft piece of copper wire - I know I know...and/or blow them through with compressed air. Either way, the jets' orifice MUST be perfectly round. Check by holding them up to a bright light. Carb cleaner is no good at all for a hard entrapped particle of residue which may be rust based.

Check the needles - wipe them clean etc whilst at it. Perhaps check to see if they are properly centralized.

Test car

If still no joy, then check out the rear carb more thoroughly - spring, damper oil, etc and yes, check thoroughly for any leaks in hoses - splits usually - and at flanges. After that, check fuel pump delivery and possibly float height setting and the floats themselves.

Oggers

"If you have been running very low on fuel, chances are you may have picked up some residue which normally lies in the bottom of the tank."

I've heard/read this on many occasions, but I've never quite understood why that should be.

The fuel pick-up is at the lowest point in the tank, so it's always drawing fuel from the bottom.
Dave O'Neill 2

Dave... i have experianced this

i thjnk you can just get to low maybe a couple of inches where all the nasty lives

or maybe like in my case... there is no pick up tube/ pipe as in missing

prop
Prop and the Blackhole Midget

****If performing as previous, physically remove both jets and thoroughly clean them out with either a soft piece of copper wire - I know I know...****


Ogger...

you say that like its a bad thing..... im sure we ALL have a soft pieace of copper wire hidden under our door step of the tool shed just for that purpose ... hahaha

guilty as charged


prop
Prop and the Blackhole Midget

Dave

Quite right - nothing to do with where the pick up is, but....if a pump is run below NPSH - which could be possible if the static head is diminished by running on low levels of fuel, and the tank strainer on the suction side is clogged - it could start to cavitate. This produces spikes in the discharge pressure which could damage filters further downstream and allow particles held within it to pass through.

In addition, lower fuel levels in the tank can cause a greater degree of sloshing about and increase the risk of picking up the crud than otherwise.
Oggers

thanks everyone, I've been ill for a couple of days so will get onto this soon.
Methinks a thorough clean as per Oggers advice is a good start.
aj robinson

and i'll get a compression tester!
aj robinson

If I had to choose between the compression tester or the vacuum tester ( I have both).

Id get the vacuum tester...it tells you more.
S

Finally sorted by my local tuning genius!
Turned out there were a multitude of problems, some of which are hard to explain. Anyway, the car is now running better than ever!!!
Ignition timing was out quite a way and the electronic ignition module wires were slightly loose in the dizzy. Coupled with a loose needle valve in the rear carb, a couple of valve clearances wrong and carbs not accurately synchronised, I'm surprised the damn thing ran as well as it did.
So all is fine and dandy, EXCEPT he has now aroused my curiosity.....
Following an approximate valve timing check, I'm told the camshaft is 20 degrees retarded. So, it it a straightforward job to adjust? I understand that 1 tooth of the crankshaft cog = 20 degrees? or is that the camshaft cog?
I realise that most people will now remind me that 'curiosity killed the cat!'
aj robinson

The degrees will be the same either way. 1 tooth on the cam gear is only half the degrees of the crank gear, as the cam gear is twice as big. But it is also turning at half the speed, so in crankshaft degrees it will be the same.
Paul Walbran

Is there a 'tried and tested' (i.e. 'easy ') way to do this?
aj robinson

Read up on keith culver and david vizard

its widly belived that these cams are ground to the INcorrect specs and many times can be out as much as 10 degree maybe even more

if your really going to do this (if its running fine and not causing any issues ..then i would not unless you want to tinker for learning)

i would get whats known as a venier camshaft sprocket... it allows you to adjust your cam spec settings on the fly...or else you will need an offset flyweel key so buy sevalral in varing off set specs

btw... this wont be an easy project nor will it be a sunday noon lazy time project. The entire front end of the engine and car has to come off and you will need a good supply of tools as well

and possiably im not certian but pull the head to measure the cam lobes with a DTI

i was fornate i was rebuilding my engine at the time and had it bolted to a stand ... but it was educational and i learned alot and i screwed it up several times before i got it correct

once you get it apart the work isnt physically hard it becomes a mental challange at that point

my best advice... dont work tired ..think about it 1 hour for each screw or nut...measure 6 times before setting in concreate and take lots of breaks a d never belive you have it set correctly because you may have.. but there is always something to throw it off a tad..like accedently moveing the timming chain in reverse by an 1/8 inch...haha

if your into this kind of thing its alot of fun but if your under any kind of dead line ... your screwed

obviously a good knowledgeable friend to drink your beer for you and watch is the best idea

but keep us posted on what you do

prop





Prop and the Blackhole Midget

Thanks Prop, I kind of guessed I could be making a rod for my own back, but you know how things are when curiosity strikes!
Think I will leave well alone..........for now!!
aj robinson

If it is in fact 20* off, and if one tooth is in fact 20*, then you don't need an adjustable cam wheel, and you don't need an offset key. All you would have to do is move it one tooth. That is pretty easy. Just make sure that you only move either the crank or the cam while the other stays in the same place.

Paul is right that it doesn't matter which one. I once wasted hours and did a lot of head scratching making that discovery. It is kind of counter intuitive.

If the crank wheel has 18 teeth, there would be 20 crank degrees per tooth (360/18 = 20).

Charley

C R Huff

I was thinking of completely slacking the crank cog so its free to move, then just turning the camshaft cog 1 tooth, then securing the crank cog. Sounds almost too simple.....
Any advice?
aj robinson

Think I'll start a new thread about adjusting the cam timing.
Thanks to everyone so far,
aj robinson

This thread was discussed between 27/07/2016 and 15/08/2016

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