MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB GT V8 Factory Originals Technical - Topping up the SU carburetters

I have a 1973 factory GT V8 that I bought in May this year, it was rebuilt in 1999. I recently aquired the handbook and I noticed the section about topping up the Carburetters with oil, I also recall my dad telling me when I got the car that I must keep the carburetters topped up with oil otherwise it will likely 'break'. I am still VERY new to this car and the Rover V8 and SU carburetters are completely new. I have had a quick look at the diagram in the handbook but I am confused. Where am I supposed to put the oil? Once the cap is unscrewed and the piston is lifted up (either one) it comes out, there is then some slight pressure (suction) when returning it, where would I put oil? The handbook says:
'Unscrew the cap (11) of each carburetter suction chamber and GENTLY lift the piston up to the top of their travel. Fill the retaining recess (12) with oil and push the damper down until the cap contacts the top of the suction chamber. Repeat this filling procedure until the oil level is just visible at the bottom of the retainer recess. Screw the cap firmly into the suction chamber.
Failure to lubricate the piston dampers will cause piston flutter and affect acceleration'

This has left me confused and a bit worried about taking a more lengthy journey.

Any help appreciated.

Andy Butterworth
A Butterworth

Well you have found the right place for the oil - most people just cover the cylinder inside (at the bottom of the 'neck' of the carb. The manufacturer used to specify SAE30 but most of us used to use engine oil.
Roger

Roger

Andy,
The oil acts as a damper to preven "flutter".
The car and the carb will run without the oil but not just as well as it is designed to.

If youy want to get really fussy there is an oil retailed by SU specifically for this purpose but as Roger said engine oil will do the same job and is usually more likely to be on hand.

I used engine oil in mine for about twelve years with no problems.

Whichever oil type you decide to use make suer both carms have the same type and amount.

Cheers, Pete.
Peter Thomas

Here's my two cents... I ran cars with both HIF4 and zenith carbs. I've always heard that you should never run engine oil in these...the viscosity is too great. I did a few times (quick fix) and the carb piston(s) always seen to get stuck a bit (only noticeable at idle). Basically the car would idle high. I found that machine oil was the closest to the manufactures recommended and if not available the brake fluid (in a pinch). I might be wrong here, but check the lubrication page on the manual for a SU auto and I believe you'll find it days to use 3 DOT oil.

Safety Fast!!!
Hal

Hal

FWIW, both my MGB and MGC factory manuals specify engine oil. I've been using Kendall 20w20 to good effect....
Rob Edwards

Automatic transmission fluid also works well, as do the lighter weight shock oils. But in any event avoid the use of gear oil, as throttle response will slow significantly.

Actually they function fine even without oil but a lubricant is a good idea.

Jim
Jim Blackwood

Thanks for the replys guys but you haven't really answered my question. The handbook says use engine oil and I am happy with that, what I can't quite understand is where to put it...... As I said if I unscrew the top of the Carb 'dash-pots' the piston lifts out, the handbook says it 'will travel as far as the top', not actually come out??. Anyway the handbook also has a diagram of where to put the oil but it is a very small area and I would need something in a dispenser like WD40 to get to where the handbook states to put the oil, hence why I am stuck..

Any more help please.

Andy
A Butterworth

Andy,
The black knob on top of each dashpot unscrews and removes. You can then lift the piston with your finger, if the air collector is off, until it reaches full height in the carb. I find that 1/2 teaspoon of ATF is just right on HS4 carbs, but since the dashpot is shorter on the BGT V8 HIF6 carbs and hence the chamber that holds the dampening oil, perhaps just a little less will suffice. Hope this helps.
Jake
Jake

What a divvy surely everybody knows that
unknown

To "unknown"

This BBS is a learning tool...please do not "dis" those who may not know, yet are NOT afraid to ask!

Methinks that you, too, were once a novice.

rick
rick ingram

I think if you put too much oil in, it doesn't really matter as far as how the engine performs, because any excess will be pushed out the vent hole in the plastic cap.

OK, it may make a little mess that can easily be cleaned up, but nothing else I don't think.

However, I stand to be corrected??
M G

Agree with Rick "unknown" is divvy.

Paul
Paul

Firstly you will not break anything if the oil gets low, just have hesitation on acceleration. Not all damper pistons lift right out, HIFs have a retaining clip which lifts the main piston and eventually will pull the clip out, which is of no concern. If you unscrew the plastic cap lift it up, push it gently back down, and you feel the resistance of the damper in oil before the plastic cap reaches the main piston cover you have enough oil. The gap between the cap and the cover indicates how much of a 'reserve' you have. If you need to add some just pour a little engine oil where the damper cap screws. If you add too much it doesn't matter, excess will be drawn into the engine and disposed of very quickly. Note that some drawings of supposedly correct oil level are incorrect in that it shows the oil level as being *above* the top of the damper cylinder. If you try and maintain it at this level you will be topping up after every journey. You shouldn't need to top up more than once in a thousand miles or less, but whereas HSs have a blind damper cylinder and so cannot leak the HIFs are drilled right through and plugged. If the plug becomes displaced you will lose the damper oil very quickly if not immediately.
Paul Hunt

Zap - "unknown" is now just carbon. I have tried different oils in the pots in both a 1971 B and a 1973 v8. The oil seems to disappear within an few weeks in both cars. Can anyone suggest why?
Iain Hallifax

The SU manual indicates that the oil level should be just over the top of the inner piston tube - WITH THE DAMPER IN PLACE.

So just lift the damper cap and push it to one side, use an oil can to squirt in oil till it just gets over the top of the inner tube.

Any more than this will leak down inside the carb or get ejected through the cap.
Chris at Octarine Services

Iain - my 75 V8 has HIFs and as I said these *can* leak if the plug at the bottom of the damper cylinder is missing or displaced. However a 71 roadster should have HSs which have a blind hole and should not lose oil.

Chris - that is precisely the error I'm refering to. Any oil *above* the top of the damper cylinder will be lost very quickly . As long as the oil level reaches the *bottom* of the damper piston with the damper in position and the piston at rest there will be enough, but only just. That still leaves the space above the top of the damper piston but below the top of the damper cylinder as a 'reserve'.
Paul Hunt

Thanks, Paul & Chris.

Your input clarifies the situation very precisely.

cheers



M G

This thread was discussed between 08/09/2003 and 20/09/2003

MG MGB GT V8 Factory Originals Technical index

This thread is from the archives. Join the live MG MGB GT V8 Factory Originals Technical BBS now