MGB Roadster 1970 - Mixture set-up?

[Jack Valiant]

Guys, I have the original twin SU set-up on the Roadster. Fitted new float valves and seats with new needles and gaskets etc. Car starts and runs ok albeit a bit rich. Assuming that all the mixture setting are wrong what would be the best way to set it up so she runs reasonably before I can get it to a emmission tester? (Timing and points settings all setup as per the manual)

Thanks

Chris

[MG Mark]

Jack Valiant said:

Guys, I have the original twin SU set-up on the Roadster. Fitted new float valves and seats with new needles and gaskets etc. Car starts and runs ok albeit a bit rich. Assuming that all the mixture setting are wrong what would be the best way to set it up so she runs reasonably before I can get it to a emmission tester? (Timing and points settings all setup as per the manual)

Thanks

Chris

If memory serves correctly (and the manual should confirm it, but mine is at home and I'm at work....) set the jets (the tube that the needle slides in and out of, which I presume you have also replaced) so that the top of the jet tubes are level with the platform of the housing where they sit in the throat of their respective carburettors. Then adjust the jets downwards from that level by turning the adjusting nut by 12 flats. That should give you a starting setting good enough to work with until they are set up properly on an emission tester. Do that for both carbs, then start the engine, slacken the clamp(s) on the connecting bar that connects the throttles and set the idle and balance the carbs individually then tighten the clamp(s), recheck the idle and balance with them connected together and the job's done. If you haven't got a balancing flow meter to hand, then a simple way to do it is to unscrew the dashpot plungers and leave them resting loose in the dashpots - when you gently rev the engine up, the plungers should rise by the same amount.

MG Mark

[Jack Valiant]

Thanks Mark, did that last night and its still running like a dog! I can see fuel coming out the jet on the left hand carb but nothing on the right, so I think the engine is trying to run on one carb? Had the right hand needle jet out and all seems well also the tube and seat. Fuel in the float chamber and coming down the tube into the jet chamber. I will change the carbs if I have to, but seems like something simple. The fuel pump is working and ticks at ignition on ok, could lack of pressure be a problem May try a squirt of cold start into the right hand carb inlet to confirm my diagnosis ! Any suggestions would be appreciated!

Chris

[MG Mark]

The SU setup is a simple system, and they will run reasonably well even when most of the components are, shall we say, tired. So if you've adjusted the jets equally, and you have fuel getting to one carb alright, then it should be getting to the other. In that case, it shouldn't be the fuel pump, as that only needs to create sufficent pressure to suck/push fuel from the tank to fill the float chambers - the induction system is not pressure fed and works with fuel simply being sucked from the float chamber, through the tube, and up the jet by the low pressure created across the bridge of the jet when the throttle is opened. Check to see whether it is something really simple like the new float needle in the right hand float chamber not operating freely, not seating correctly or sticking closed - they can be quite sensitive to adjustment in terms of the distance between the fork and the top of the float when full/empty, the seating of the valve, and if the fork does not operate smoothly.

Mark

[hotrod59]

Hi'Ive had be's for ten years and Marks spot on follow the fule to the carb and filler on the side as i had a prob,with one carb, playing up too. On your change of battery,I did this but at ths same time fitted a kill switch inside behind the drivers seat on the panle infront of the Batt, as I had a direct feed fire on the starter and the ignition key will not stop it. Well 2fie ext, and 5 galls of water later when it went out.
Rod
Ps Dry powder do more damage than a fire. I use CO2.

[Jack Valiant]

Thanks guys for all the advice. Carbs now sorted due to a combination of worn needle and seat, mixture setup and sticky float. All balanced up and running like a well oiled machine so well pleased. MoT tomorrow so fingers crossed we will be ready for the summer roof down

Cheers

Chris

[MG Mark]

Jack Valiant said:

Thanks guys for all the advice. Carbs now sorted due to a combination of worn needle and seat, mixture setup and sticky float. All balanced up and running like a well oiled machine so well pleased. MoT tomorrow so fingers crossed we will be ready for the summer roof down

Cheers

Chris

Marvellous - glad to have been of help and to have saved you both grief and some money! Now enjoy!

Mark