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MG MGB Technical - 1974 MGB GT

It's been a while since I have been here, I haven't the time to work on the MG's as before. Now I am seeking advise on a number of issues,I am working on a 1974 MGB GT. I have been able to get it to start,and I am trying to get it to idle smoothly. The carbs are secondhand HIF4's, the jets have been turned down 2 turns, i have set the idle speed @ 1000rpm. Also I can hear the tappets rattling a bit, can anyone tell me how to go about adjusting them. Any info would be much appreciated, I now most of you guys have been through most of this before and would have good suggestion on how to solve these problems.
Thanks in advance.
Glenroy
Glenroy

Have a look here, especially at the very bottom of the page:-

http://www.bmcno.org/tech%20tips/techtip.htm#DL_LIBRARY

Derek Nicholson

2 turns down from flush with the bridge (not as far up as they will go) is the starting position. You then coarse tune each carb at a time by turning the mixture screw to get the highest idle speed. Then fine tune them by using the lifting pin to raise the piston about 1/32" (the pin itself goes up more than this before it contacts the bottom of the piston). If the idle just tends to die it is too weak, screw the mixture screw in a bit. If the revs rise and stay up while you hold the piston up it is too rich, so screw the mixture screw out a bit. When correct the revs should momentarily rise then settle back down. It *is* hard to judge, particularly on HIFs, and even more if you have never done it before. You can train your ear by making it obviously rich and obviously weak, and if you can find the ideal spot then find the point where you can just detect richness, and again just detect weakness, noting how far you have to turn the screw from one to the other, and turning it back again half-way. After making a adjustment to one carb you will have to check the other and probably make a small adjustment to that as they are interdependant. After a couple of iterations back and fore they should both be correct.

But the carbs should only be done when everything is correct with the ignition like HT lead, cap and rotor condition, plugs condition and gap, points condition and dwell/gap, timing, and correct operation of the centrifugal and vacuum advance mechanisms. Before starting note where each plug lead goes, especially No.1 (the plug nearest the front). Normally this goes to the distributor cap in about the 1 o'clock position when looking down on the cap from the side of the engine. There are various reasons why it might be other than this, but it is important to get it back where it was before if the engine was running before. The firing order is counted from there 1 3 4 2 *anti*-clockwise.

And *that* should be done after getting the valve clearances correct. The above URL comes up not found for me. The valve clearances should be .015" with the engine cold, I use .014" as a go and .016" as a no go as it is easier to judge. Just slacken the locknut with a ring-spanner and use a screwdriver to turn the screw, hold the screw there, then lock the nut, and recheck the clearance. You adjust each valve according to 'the rule of nine'. This means that you adjust valve No.1 when No.8 is fully down, No.2 when No.7 is fully down etc. i.e. turn the engine in its normal direction and whichever valve goes down subtract its number from 9 and adjust the result. They are also counted from the front. The B-series engine in the MGB *is* clattery, if you make it too quiet you probably don't have enough clearance and will burn valves and seats.
Paul Hunt 2

Glenroy,

You've already attracted some excellent advice. I can only add that if you are new to working on MGBs, you should get a Haynes Manual, and/or better yet, a Bentley Manual for the MGB. The Haynes is rather a condensed Bentley and is quite adequate about 85% of the time. There are many other excellent books and manuals about the MGB, but with one of these and a good parts catalog (Moss, etc), you can go far with this.

I'll add one small note to Paul's advice: when adjusting valves, I use a BIG screwdriver. Otherwise, it's really hard to keep the adjuster screw from turning with the jam nut. As Paul says, err on the loose side!

FWIW,
Allen
Allen Bachelder

Thanks a million, I am going to print these off so I can read them carefully to know where I am starting from. Derek thanks for the website, I bought a copy of that boot a long time ago, just in-case it came in handy, I think the time has come. Paul to get to the site you have to type www.bmcno.org and then go to techtip. Allen I appreciate any advise given. I will let you guys know how the works coming along. I am hoping that this weekend I will be able to tackle this job once and for all. I have another question I could do with some help on.
I have a 1969 Roadster and all the lights work, except the front right indicator. When it was working, it would not flash, I checked the bulb and that's working. when I turn on the indicator, if the sidelights are on, the sidelight goes out on the right side and if they are off, they turn on. What could be causing that?
Thanks again
Glenroy
Glenroy

Glenroy,

' Sounds like a faulty ground to me. With dual element bulbs, when the ground is faulty, one element will "seek" a ground backward up through the other filament and continue backward up the circuit until it finds one - unless the other filament is turned on.

With everything turned off, I clean these buld sockets with a male battery cable wire brush. While your at it, polish the (+) contacts in the socket and on the end of the bulb. Then, looking at the back of the socket assembly from inside the front wheel well, you'll see a spring clip holding a bullet connector for a black ground wire. Clean the clip and the bullet down to shiny metal. If that still doesn't do it, the clip is not grounding the socket housing assembly where it screws on. Unscrew the assembly and clean all contact areas between the clip and the housing. All contacts cleaned/polished down to shiny bare metal. This should solve the problem.

FWIW,
Allen
Allen Bachelder

Hi Guy's

Didn't get much time this weekend to work on the cars. I did however work on the 74 BGT, I checked the distributer, and number 1 plug is at 11:00 o'clock not 1:00 o'clock. I could not determine where TDC was as the manual is a bit confusing when it describes how to locate TDC. Can anyone tell me in terms of engine rotation which mark is TDC. As it is at present, I have to keep the choke pulled out a little for it to idle, and if I put my foot on the accelerator to try getting the revs up, the engine dies. It wont idle without the choke, or rev without the choke being used.This will cause a problem getting the car up to running temp to adjust the carbs. Any suggestions.
Glenroy

Glenroy,
to find TDC take out all the spark plugs and place a wooden spoon with a long handle into number 1 cylinder,then put a big socket onto the pully wheel nut and turn the engine until the spoon reaches its maxium height,then look at the timing marks on the pully wheel,there should be a arrow or grouve on the pully wheel next to the TDC timing mark.

The timing grouve might not be visiable on the pully wheel due to years of useage and wear and tear off the fan belt,so run your finger nail over the wheel to locate the mark,then either clean it out with a pointed object or mark it with tipex.

HTH phil.
phil mcneil

Thanks Phil,

What I can't figure is which mark on the timing chain cover goes with that mark on the pulley to establish TDC.
Glenroy

The engine turns clockwise, viewed from the front, so the most clockwise pointer is TDC, the others are BTDC, counting in 5s anticlockwise ie 0-5-10-15-20
Martin Layton

Glenroy,

on a 74bgt the timing marks are approximately at 10oclock when standing infront of the car,the pully wheel rotates clockwise so the first mark it comes to is 20degrees before TDC,then it is 15,then 10,then 5,then TDC is the last marking before the 12oclock position.

phil
phil mcneil

Thanks again,

From what I can see, the static timing is set to 15 degrees BTDC. It would kind of explain when I try starting the engine that it locks as though the piston is being forced back in the other direction. If I have to reset the timing,the firing order so that no: 1 plug is at 1:00 o'clock, where would I start?
Glenroy

The crank pulley grove *can* be out as it is part of a rubber sandwich which can delaminate. A simple check is to put a probe in No.1 plug hole (don't drop it!) and turn the engine till the piston is at its highest point then check the groove on the pulley is at the TDC mark. That is close enough for checking, but if it looks out there is another method for getting a more precise indication.

I'd set it at 9 degrees TDC static, i.e. the groove just past the 10 degree pointer, then with a test lamp or meter connected to the points turn the distributor anti-clocwise until the points are closed, then slowly clockwise until they just open. Should be good enough to get it started, then use a timing light and set it dynamically.
Paul Hunt 2

This thread was discussed between 28/01/2007 and 11/02/2007

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