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MG MGB GT V8 Factory Originals Technical - Transmission Snatch

OK, here's a question to try and resurrect this board a bit. My conversion has the 3.9 hot-wire fuel injected motor from a Range Rover and a R380 gearbox. It also has Hoyle IRS with a Ford Granada 3.14 diff. I have driven around 18,000 very enjoyable miles in this car since I built it and very good it is too.

But there is one wee problem - transmission snatch at low speed, making traffic crawling tricky. I have to feather the clutch all the time, which is a pain. The engine is fitted with a Clive Wheatley steady bar on the front of the left hand cylinder head and simply cannot rock from side to side. I have tried removing the bar temporarily but it made no difference. The engine mounts are in good condition. As far as I can tell the ignition and fuelling are good and the engine will idle happily at 800 rpm.

Any ideas anyone?


Mike Howlett

Hi Mike
We get a similar problem here with manual gearbox Falcon XR6's No problem with autos
The fix for the manuals is to drill a 1/16 hole in the throttle plate It's just enough to hold a little throttle on and take the clatter out of the driveline when you are just rolling along with no throttle on
Falcons are bad for it at about 60kph in 4th or5th gear on a very slight downhill coast just as you are lifting off the throttle If that is where your problem is it might be worth a try
willy
William Revit

"The fix for the manuals is to drill a 1/16 hole in the throttle plate "

How is that different to the throttle being open anyway, and doesn't it prevent the idle speed being adjusted right down?
Paul Hunt

Check the alignment of flanges on the propshaft.
1st Are the spiders aligned with each other, and 2nd Are the flange faces parallel while sitting on all wheels. There is also a weak bracket on the Hoyle rear end holding the dif in place on the cage. Check it isn't broken and check the torque of the long set screws which go through the aluminium spacer.
Allan Reeling

"Transmission snatch" usually doesn't come from the transmission, but from the differential mechanism inside of the rear axle. Set the handbrake so that the halfshafts (quartershafts) cannot move. That way your measurements will be as accurate as possible. Grip the differential flange that connects to the rear U-Joint (Universal Joint) of the driveshaft and rotate it to take up any freeplay, and then scribe a mark on its edge and a corresponding mark on the axle housing. Next, rotate the flange in the opposite direction and scribe another corresponding mark on the axle housing. If the marks are 4.5mm apart (6 degrees of rotation), you have a like-new differential. If the marks are 8mm apart (10 degrees of rotation), you have a usable differential. If the marks are 10mm apart (13 degrees of rotation), you have a worn differential. Note that this method of measurement can be misleading because holding the tire and rotating the pinion will show you two free-plays combined: half shaft spline wear / play, and pinion / crown wheel play. Consequently, measuring the play of a differential using a dial indicator on the bench is the most accurate technique.
Stephen Strange

Paul
Yeah I've jumped in feet first a bit here
The XR6 has fly by wire throttle and therefore has no idle speed adjuster
The throttle plate , from fully closed to fully open is controlled by a stepper motor directly from the PCM at all times


Just went 12.00
HAPPY NEW YEAR

continuing on
Why the small hole works on these is because it gives a little extra throttle on coast and just above idle to trick the PCM but when at actual idle the PCM reduces idle back to normal idle speed

All I can say is it works on these cars but I'm not sure if it will work for Mike's car
I only suggested it might be worth a try - no promises
It really depends on the throttle operation
Does you Rangie there have fly by wire throttle ???
Fly by wire - it will work
Cable throttle - don't know but MIGHT be worth a try
willy
William Revit

Mike - you have probably got the answer you need already - stephen's reply sets out the test for the most likely culpret - least likely in view of your choice of a relatively modern gearbox and limited mileage, is the clutch. FWIW.
Happy New Year
Roger W

Thank you everyone. I will be checking the drive train following the advice of Allan and Stephen. To answer Willy, the Hot Wire injection system does not have fly-by-wire operation, but a simple cable to open the butterfly and operate the TPS. The fuel system seems to be working well, with easy cold starting, good response and a steady 800 rpm idle.
Mike Howlett

Mike
Thanks for the reply - Probably won't work in your situation as it would be no different to bumping up the idle speed as Paul mentioned
One little thing I thought of that might help-
Don't know what sort of idle speed control you have but most efi cars have a set method of adjusting base idle speed.
Some set base idle with the air bypass disconnected
Some are completley pcm controlled with no adjustment
Some have the bypass diconnected by the pcm with the idle pin on the data connector earthed

There are several others as well
It is VERY important to have the base idle adjusted properly to the correct spec. otherwise if your car is fitted with an idle air speed controller and the base speed that it works off is out the controller ranges around looking for where it is supposed to be causing jerking at very light throttle and if it is a manual gearbox can cause snatch/rattle in the driveline if just trying to roll along at low revs

As an example I had a Mazda turbo for quite a while and it had a diagnostic pin that had to be earthed to set ign. timing and idle speed 700rpm
If you (me) set the idle to say 1000rpm as soon as the diag. was disconnected the pcm would pull the idle back to it's required 700 and it would idle at 700 all day but drive horrible The same applied if it was set at 400 the pcm would pull it back up to it's required 700 but it had no bump up cold start speed like this and still jerked all over the place at low speed
When it was actually set to spec (700) it was a different car - smooth as

Might be worth a look at

Nice looking GT by the way I'm a bit of a fan of blue

cheers willy
William Revit

That's useful information Willy, thanks. The Lucas system uses a stepper motor to control air bleed into the plenum and there is a procedure for setting the base idle. I'm not at home at the moment, but once there I will look it up and have a go. All the components are 20+ years old (from a 1991 Range Rover) so it could easily be out of optimum setting.

By the way, the car is Delft Blue, a Triumph colour. Sh, don't tell anyone!
Mike Howlett

Really nice colour - had it on another Pininfarina design - Peugeot 504 Coupe.
Roger W

This thread was discussed between 07/12/2013 and 02/01/2014

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