MGB struggles on hills

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atemgb
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Joined: Fri Nov 27, 2015 7:49 pm

MGB struggles on hills

#1 Post by atemgb » Fri Nov 27, 2015 7:57 pm

Hi there,

I'm looking for some advice on my MGB GT 1978.

I've had the car a long time but it's only been on the road for a couple of years. The recent MOT failed on emissions being to high (although it had been fine previously), so the garage reset my timing, adjusted carbs and cleaned the brake servo until the emissions passed.

But on the drive home it couldn't climb hills, I had to drop it down to 2nd gear and creep up at 20 mph or the car stalled. It was fine on the drive to the garage. The fuel tank is low but not empty, could this contribute ?

I'm due to drive it to a wedding next weekend, any advice would be much appreciated !

Thanks
Andrew

kevin
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Joined: Sun May 12, 2013 7:49 am

Re: MGB struggles on hills

#2 Post by kevin » Sat Nov 28, 2015 8:24 am

Check the breather to the fuel tank is free. Does it run any better with the petrol cap off?
I'm wondering if there is a blockage or sediment in the carb? How fresh is the fuel? Old stale fuel doesn't burn well at all so brim the tank and see if any improvement occurs.

Kev

suffolkpete
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Re: MGB struggles on hills

#3 Post by suffolkpete » Sat Nov 28, 2015 8:59 am

Since it was fine before, it must have been something the garage did. I can't understand how "cleaning the servo" affects the emissions. It sounds like timing or points badly out of adjustment or weak mixture. I would systematically work through these and also check the vacuum pipe to the servo for leaks and that the carb pistons are free.
1974 Rover 2200 SC
1982 Matra Murena 1.6

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JPB
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Re: MGB struggles on hills

#4 Post by JPB » Sat Nov 28, 2015 9:58 am

Has this garage ever tested a car with SUs before? B series engines - like many others on SUs - can be touch & go, in cases where the HCs are too high at normal hot idle, the car should be tested at around 2,000rpm and if it passes the HC test at that crankspeed, it will pass on emissions. Garages with experience in older cars will be aware of this, some younger testers may not understand. Many folk will set these far too lean to the detriment of efficiency and performance.

I'm not accusing anyone of ignorance but nor do I believe in coincidence and you did mention that it was OK on the way to the test.. ;)


It could be that the low fuel level is affecting the car's ability to climb hills but unless there's paint or other debris blocking the pickup I'd expect this to be an issue only in the seconds before the tank runs dry entirely since the pickup would have to be pretty well out of the fuel by that time.
J
"Home is where you park it", so the saying goes. That may yet come true.. :oops:

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JPB
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Re: MGB struggles on hills

#5 Post by JPB » Sat Nov 28, 2015 10:08 am

Additionally, I don't know why they would have been anywhere near the servo when doing the test. Manifold vacuum evacuates the servo, that's how they work, so how it would have become dirty internally is a mystery unless someone sprayed something into the filter to stop a rattle from a broken or weak spring but even then, the servo would be back to its usual state within minutes and unless they'd left off the hose, there'd be no lasting effect on the emissions but you'd find the brakes very hard work!
Last edited by JPB on Sat Nov 28, 2015 1:16 pm, edited 1 time in total.
J
"Home is where you park it", so the saying goes. That may yet come true.. :oops:

kevin
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Re: MGB struggles on hills

#6 Post by kevin » Sat Nov 28, 2015 11:23 am

If this was my classic and I wasn't handy enough I'd be looking for a classic friendly glad age to redo the set up, sounds like they have made a right balls up of things especially so if it drove fine before hand.

Kev

atemgb
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Re: MGB struggles on hills

#7 Post by atemgb » Sat Nov 28, 2015 5:55 pm

Thanks for the input.

Sorry my poor description on the servo, they cleaned the hose between servo and manifold.

I pulled the choke out on the hill climb which made a small improvement so maybe I'll try tweaking the mixture.

Years ago I reset the carbs according to the Hayne manual, and it went perfectly. But yesterday I was reluctant to mess around with them too much after the garage has the emissions just right.

Do you think I should turn the mixture screws in 1/4 turn on both carbs and try the hill again ? Or does it need to be a bit more scientific than that ?

I will fill up the fuel and fit new filter first to rule that out.



Thank again

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JPB
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Re: MGB struggles on hills

#8 Post by JPB » Sat Nov 28, 2015 6:07 pm

atemgb wrote:they cleaned the hose between servo and manifold
..and that makes me think they could have caused it to leak since this would make the car far too lean. I'd be tempted to take it back and ask the garage to do the mixture correctly since you'd have no comeback if you were to fiddle with it yourself in between times. Did they perform the emissions test at idle though? If so, ask them to test the car at around 2000rpm, this is acceptable practice for cars with SUs and it might have passed without having them weaken it too much had they done that at the time of the test.
J
"Home is where you park it", so the saying goes. That may yet come true.. :oops:

History
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Re: MGB struggles on hills

#9 Post by History » Sun Nov 29, 2015 8:03 pm

Warm engine.

Drive car in top gear at 30 mph. Then give it full throttle. If it doesn't pink advance the timing a bit. Try again. If it still doesnt pink advance more. When it pinks retard the engine until it doesn't.

If when going full throttle from 30mph in top the car hiccups and stalls.

Warm engine.

Choke fully off.

Check damper oil.

Using the lift pin lift the piston in any carb. If the revs rise and stay smooth the mixture is to rich. Screw main jet nut up. If the revs don't rise and falter. Screw main jet nut down. If revs rise briefly and falter the mixture is about right.

Power balance.
Using a rev counter note idle rpm. Then disconnect each cylinder spark plug in turn. And note each rev drop. They should all be within 10% of each other.
If say no1 is 200 rpm drop and no3 is 150 rpm rev drop then no1 carb and cylinders is doing more work. So no2 carb needs more air and no1 carb less.
This is all done with air filters fitted.

Because the carbs are front carb 1 and 2 cylinders and rear carb 3 and 4 cylinders then if there is excessive rev drop between a given pair say 1 and 2. Then a compression check etc will be required.

Idle speed. There is little advantage in slow idle rpm where the clutch doesn't drag. I would say between 700 and 800 rpm max. So if you get a nice smooth idle which is a littke faster than the book don't worry.

Damper oil. Many different opinions. My opinion is any mineral oil that is not too thin or thick. Engine oil or AFT fit the bill so what evers handy will do. I can tell if a damper has no oil but cannot tell what oil is in the damper.
I reckon the garage has weakened the mixture and advanced the timing. Twin SUs need to run at least 3% co. Any less and lack lustre performance and also less mpg. Too rich or too weak means less mpg. Once twin SUs are dailed in properly they sparkle and stay set for many miles

My dad used to constantly tinker with his Marina SU the settings ended up the same as what he started with.
Bob.

mr rusty
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Re: MGB struggles on hills

#10 Post by mr rusty » Tue Dec 01, 2015 10:15 pm

You said the garage 'reset the timing' why did they do that? If it was running OK before, but a bit rich, the timing would be the last thing I'd be fiddling with because there's simply no reason to do so if it hasn't been disturbed since the last mot.

I'd start there, either check it with a strobe, see what they've done, or adjust it manually a bit at a time until the car runs right.

Or do they actually mean they fiddled with the points? Is it still on points? I'd be checking there, make sure the gap is correct and they're not burnt.
1968 Triumph Vitesse Mk1 2 litre convertible, Junior Miss rusty has a 1989 998cc Mk2 Metro, Mrs Rusty has a modern common rail diesel thing.

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