MGB Register Forum

The throttle action on my BGT is very aggressive at the bottom end of the range (ie idling and up a little).

Unless you are really * footed it is all too easy to have the throttle snap open (or shut) like a switch causing very loud clonks from the poor old back axle.

Now this is where I am likely to be quite wrong in my assumptions and my attempts to correct the problem.

First let me say that everything is very well lubricated and there is no trace of snagging with the cable which operates very smoothly, as does the foot pedal.

On looking at the linkage at the carburettor end I have noticed that the lever to which the cable is attached has what would appear to be a "stop" underneath which rests on the choke control rod at the closed throttle position. In this position the angle between the cable and the lever is extremely acute - not conducive to good operation I would think as the leverage is wrong.

To correct this and bring the cable much closer to an included angle of 90 degrees with the lever I have slackened off the cross shaft pinch bolts and raised the lever, adjusting the cable to take up the slack. This means that the "stop" underneath now no longer rests on the choke control rod.

Both throttle butterflies still return to their stops as they should and the arms on the lever connected to the cable both engage in the forks which operate the butterflies but I have taken care to ensure that there is slight clearance both above and below in the forks and that there is free end float for the lever. Both throttle butterflies would appear to open together.

I have tried this set up on a short road run but have not noticed any significant difference- probably because I am so used to * footing the throttle that I cannot tell if there is an improvement.

Thing is, can anyone tell me whether I have done the right thing and if not tell me what I ought to do?

There is a human side effect - I have aggravated the painful backache, gained from the recent battery installation, as a result of leaning into the engine bay - even though I put plenty of blankets over the radiator area to lean on. Motorcycles on a work bench are SO much more easy to work on!

As I am hoping to attend the rolling Road tests this coming Sunday, maybe I shall find the answer to my throttle linkage "problem" there by asking the experts what they do. Rog.

You might find that with the linkage now not touching the choke mechanism, that when you cold start with the choke out, that the choke now will not open the throttle slightly to increase revs.

Thanks for that observation re the choke, Anthony. In fact what I have done seems to have made no difference to the choke or cold starting which as always is pretty instantaneous once the pump has primed the carbs, and the fast idle speed does not appear to have been affected.

I am still not sure I have made a great deal of an improvement in reducing the abruptness of the initial throttle action though. Most of the time I manage to live with it by being very gentle with the pedal but the occasional jolt from a bit of rough road does upset things somewhat.

Hi Roger,

I get that * as well, have to be so careful with the right foot at times or it would carry on like a kangaroo down the road, and I get big laughs from my passenger. Anyway, I've asked questions on this before, and the answers I got back were to lubricate throttle cable, but my cable is new and of high quality and I made sure it moves very freely in the outer cable cover when fitted. I questioned if it could be the weight of the oil in the dashpots, is it fluttering? maybe needing a heavier weight oil, but I have 20/50 engine oil in these as recommended in the hand book, so should be good enough.I know some put in a light weight oil such as 3 in 1, but I can't see that this would help, and would only make it worse and disappear quicker and keep needing to be topped. So I'm still trying to work out what the cause is, if you put your finger on what it is please let me know..

Have you allowed a bit of slack in the cable?

You should be able to pull the cable, very slightly, out of the guide in the heatshield to prove it's not under tension.

Don't forget you can fine tune the throttle adjustment if you include the stop bolt above the pedal too.

Hi Adrian,

I do have a small amount of slack in the cable but adjusting the stop at the pedal end is something I had not considered for the simple reason that I have not attempted to peer into the dark cavern of the foot well - it's bad enough having to change a battery at my age. Foot well things are for the young and agile!

Whatever the difficulties with the throttle it performed pretty well on the Rolling Road test last Sunday except for a slight fuel supply problem at 5000+ revs! - but who uses that end of the rev range except on track?

Cheers

Roger

Looking back to Dave Linkson's comments in November I can report that after opening out the angle between the cable and the actuating arm attached to the throttle spindle I can detect a slight improvement in the throttle action - it is slightly less abrupt though my wife still has trouble occasionally.

I am pretty sure that Dave's kangaroos will not be triggered by the movement of the pistons in the dashpots, whatever oil is used though I believe there is a relationship between the viscosity of the oil and the way the pistons control the mixture - Haynes says use Castolite or similar SAE 20 oil. I top up my dashpots regularly with 20/50 engine oil and have not experienced any problems. If there is a problem with Dave's pistons it may be that they are sticking and the dashpots should be removed, cleaned and checked over to make sure all is still in accordance with spec.

Roger, The pistons have been cleaned and checked, and rise and fall nicely, I have done all the checking & cleaning nessacary, so appears to be a patronising comment in your reply here, and would be courteous to reply properly to me and not refer about me, or don't reply at all! as it really doesnt help! Thanks..