MG problems

[MG CHRIS]

After having our MGB GT back after it being at a friends house while i was preping and painting his mini the car backfires badly when being started up but as soon as it gets warn it runs sweat as a nut and i doesnt do it when it has be parked up for a show for a while neither just when it starts up in the morning sounds like WW2 when going down the street. It never done it when it was on the road last year. We have replaced the ignition leads and spark plugs but it still does it timing is spot on so cant be that as it would run bad all the time any suggesstions please

[saxxeblt]

If it's only on start-up and presumably when you use the choke initially would indicate to me it's a fueling issue when cold. Does the choke operate on both carbs?
What colour are the plugs? White, black, oily? As two of the plugs are fuelled by one carb each, if one pair of plugs are a different colour to the other pair would be a pointer to a problem with one carb.
If it's over-fueling with the choke on that may be causing the problem, overfueling may just be down to the float level in the carb and flooding it.
HTH

[alfa pint]

Carbs need tuned - they've gone off the boil. fuel mix issue not a timing one.

[MG CHRIS]

We had them tuned last year to get it through the mot as one carb werent even working probley and it ran perfectly and only grumbled a bit in the morning but thats all so that was my main idea to tune the carbs again will take it to my local place intune which did it last year and see what that does

[danger mouse]

Sounds like one of the SU pistons not moving freely when cold causing it to run lean on two cylinders until it warms up. Sounds quite cool, but not good really.

Firstly and most simply, there is oil in the dash pots: the same amount and grade in both right?

If yes, if it was me (this is what I did!) I'd take off the the dashpots, carefully take out the pistons (being careful not to bend the needles!) and sauce the lot in carb cleaner give them a blow out, a nice wipe down and put them back together with a wipe of dash pot oil on the inside of the dashpot and the outside of the piston. There is proper SU stuff but I've 10w40, rather than the 20w50 the engine needs, is good for this purpose. The tricky bit is making sure the dashpot is aligned properly so the piston doesn't snag-up inside. Tighten down the three screws on each gradually constantly checking the piston can move up and down freely.

If it's the MG it'll be easy as the throats are straight and you can see the front of the piston from the side of the car with the air boxes off. If it's the mini it's a right PITA because the car is in the way and the throats are angled if they're still standard. Might be easer to whip them off if you don't have skinny fingers. This job is also easier with the damper caps still off.

Fill the dashpots (I use 10w40, but that will probably start and argument...) and screw the caps down.

Doing this solved the same problem with mine, hope it helps you too!

DM

Edited by danger mouse on Friday 30th July 11:02

[perdu]

Might be no more annoying than dampness trapped inside the dissy cap

try whipping the cap off, spraying inside with WD40 and WIPING it out afterwards to remove the wetness with a cloth

There may also be a slight trace across the plastic inside there that shows a tracking leak

easily removed by painting over with Tippex

Misfiring when cold and improving as the car warms up is a really strong indicator of "dampness issues"

I hope it's no more than this, it's an easy fix

[MG CHRIS]

Found out problem took the dash pot off and cleaned out all the dirt and rubbish that was in the pot made sure the needlle for the carbs arent bent and then noticed a split in the fuel pipe between the two carbs replaced the pipe put it all back together and fired her up and ran smothly brill. Start her up in the morning as we are going to a show and see if it has worked when in the morning where the problem would start.

[danger mouse]

Glad to hear you sorted it. I just replaced all those fuel and breather pipes too, It all helps.

I was going to say check the vacuum line to the servo if you have one as well as a leak in that will make half the engine run weak too.

[MG CHRIS]

danger mouse said:

Glad to hear you sorted it. I just replaced all those fuel and breather pipes too, It all helps.

I was going to say check the vacuum line to the servo if you have one as well as a leak in that will make half the engine run weak too.

The vacume line was almost out some put it back in and redone the timing runs perfect now thanks for the advices really appreceated and the neighbours when it doesnt backfires done the street early on the weekend and wake everyone up

[SB - Nigel]

keep your eye on new petrol pipes

on my last car I had three diffent lots from three different suppliers and all three leaked

ETA: vacuum hose has to be the correct stuff and stamped as such too

Edited by SB - Nigel on Wednesday 4th August 22:21

[Kentish]

SB - Nigel said:

keep your eye on new petrol pipes

on my last car I had three diffent lots from three different suppliers and all three leaked

ETA: vacuum hose has to be the correct stuff and stamped as such too

Edited by SB - Nigel on Wednesday 4th August 22:21

I've never had fuel lines leak through but I know what you mean about vacuum lines.

It wasn't uncommon for the DIY'er to just bung on a length of coolant hose the same size and not realise it was collapsing under the vaccum

[MG CHRIS]

Problem still accuring was fine for 1 week or so then went back to what it was before so we think it could be a sticking valve or could be where it has been running rich there is loads of unburnt fuel in the head so we have bought a head gasket set and we are taking the head of tomorrow hope we will find something if not we havent got a clue what is causing the problem as the ignition system is pretty much new cleaned the carbs and balanced new fuel pipe cant think of anything else it can be. Both me and my father are mechanics so taking the head of will be no problem just hope this cures the problem

[MG CHRIS]

Fired the car up yeasterday and running like a dream more power, more compresson aswel as we wanted to see what differences it had before and after, needed doing mind as the valves were covered in unburnt fuel and the valve oil seals well 2 were different than the other 6 and 3 were damages aswel so really needed to be done thank good so we can enjoy the rest of the tax left on tthe car before it gets put of the road for the winter.