MGB hesitation and loss of power at 2800rpm

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jonahnaylor
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MGB hesitation and loss of power at 2800rpm

Post by jonahnaylor » Sun Nov 17, 2013 5:27 pm

Hi I hope someone can please help.

I had a fantastic day at the NEC Classic Motor Show and drove there and back in my 78 MGB GT.

The car I thought was running great after adjusting the mixture, idle speed and timing the other week.

Turns out the car is only running great under 3000rpm...

I found when on the motorways as soon as I hit nearly 3000 the car would hesitate, the tach would drop down and jump back up and the car would regain power then a couple of seconds later lose it and hesitate and the tach would jump down and up again. Remaining at these revs the car would continue to stagger in this way, but as soon as I eased off back to 2500rpm the car would be fine again.

This basically meant I had to make the 260 mile roundtrip drive keeping at a speed so I didn't go above 2800rpm.

As I was out in the car for a few hours each way though I did notice a few things and get to diagnose the problem a little on the fast roads.

1. It would happen with or without Overdrive engaged meaning that it would be at about 60mph when I'd hit the 2800rpm hesitation with overdrive OR it would be more like just over 50 without overdrive. So the speed it happened at didn't matter, it was always the exact revs when the problem started.

2. The car would knock quite a bit in 3rd gear under acceleration. This makes me wonder as my initial thought was I should advance the timing? But won't advancing it create more knocking/pinking??

3. The tacho or rev counter would always plunge to zero and then bounce back to 3000rpm then plunge back to zero. The hiccups/hesitation etc was a soft feeling, not a harsh backfire or pop, but would shake the car and make you stagger!

4. Performance up to the 3000ish revs mark would be very good, responsive and the car felt quick and accelerated well - albeit with the knocking/pinking thrown in occasionally.

5. A few times I purposely took the car upto 3000rpm and then added a little choke to see if this helped, it didn't seem to help at all... So does this mean it's not running lean?? I've only just used a colortune a few days back to get the mixture right so I don't think we're too lean...

Could anybody please help at all.

I think it could be timing but I don't know how my car should be set. I tried to set it to factory 10 degrees BTDC and had to advance this after running the car a bit as it felt sluggish and would shake the car.

But on advancing too much I'd get a lot of pinking. Currently I'm about 20 degrees BTDC with vacuum disconnected so I can't imagine I'd need to advance further.

Could Carbs being not properly balanced cause this? Is it worth getting a Carbalancer..?

Also I've set the valve clearances to the book, changed the rotor arm, cap, leads etc - everything looks in good condition. I've got the electronic ignition conversion and also changed the vacuum line...

Any help is greatly appreciated!
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adamliptrot
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Re: MGB hesitation and loss of power at 2800rpm

Post by adamliptrot » Sun Nov 17, 2013 9:52 pm

Hey Jonah,

Are you running with the Accuspark still?
Forget the book when it comes to timing - you probably aren't running the same dizzy as when you left the factory, and certainly not on the same fuel. I tend to back it off until it's about to stall (making sure it's nicely warmed up) and then bring it back so it's smooth.

Enjoy the NEC?

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Re: MGB hesitation and loss of power at 2800rpm

Post by adamliptrot » Sun Nov 17, 2013 10:14 pm

Oh, with the faltering, check your fuel filter for rubbish and make sure your carb dashpots have oil in.
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jonahnaylor
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Re: MGB hesitation and loss of power at 2800rpm

Post by jonahnaylor » Tue Nov 19, 2013 2:53 pm

Hi Adam,

Thanks for helping as usual!

I topped up the dashpots before the trip so I think they're ok.

I also started out on a full tank of fuel - but I don't have a fuel filter in the engine bay so hard to check for debris... I think it may all come back to the timing again though.

Yes I'm still running the Accuspark ignition too. Checked all the wires and it all looks clean and good to me.

Today I warmed the car right up and then checked the timing with the vac plugged up and it was way up high at 20ish BTDC. So I tried it at 14ish and went for a revvy run. It actually seemed a lot better but the hesitation kicked in again when in 3rd gear at higher revs of 3,200 or so.

So I came home and have it now set again at 10 degrees. The thing is though, this went against my gut feeling. As previously hesitations have occurred when my timing has been quite retarded like at 10. I increased from this before because the car struggled and even shook the gear knob quite a bit.

So if anything I felt I should advance!! But I've back it off to 10 or 11ish and took it for a quick test and it seems ok. The engine was nice and hot and I've a feeling when I'm starting from cold again it'll be spluttering and shaking. But fingers crossed. Maybe since last time my mixture is set better (and I changed the air filters recently too) so you never know.

I'll let you know how the car drives the next few days - but it does seem to be a timing issue. How are you ever supposed to know what your exact timing should be though?

I only have a cheap timing light and I can't dial it up or down so that rules out trying total advance timing of 34 degrees as many others suggest.


The NEC was awesome - did you attend the show? I was on the look out for an Inca Yellow MGB GT in the car park (and the main hall !) but didn't see any! :shock:
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adamliptrot
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Re: MGB hesitation and loss of power at 2800rpm

Post by adamliptrot » Fri Nov 22, 2013 8:54 pm

No, didn't make the NEC, but I was there a few years back. Did finally manage to get to the Goodwood Revival in September which was fanastic, so that was my show outing for the year :)
Even at the MG50 event there weren't a lot of Inca Yellows about :)

Where's your fuel filter? If you haven't got one you could have some bits in your carbs restricting flow.
Did you make sure you put your vacuum pipe back?
With the car running, stick your head in the engine bay and see if you can hear any air escaping - an air leak (knackered gasket on the carbs or manifold for example) will cause lean running at higher revs.
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jonahnaylor
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Re: MGB hesitation and loss of power at 2800rpm

Post by jonahnaylor » Thu Dec 05, 2013 5:00 pm

Hi Adam, I'm hoping to do the Goodwood Revival next year, really excited to see what it is like!

I've had some time-out from the MG while it was in the garage for the chrome bumper conversion. But I'm now back in action at last!!

I'm not sure how I'm fixed with the fuel filter, I just know I don't have one under the bonnet like most MGB's I've seen. I think I must have the one at the back of the car after the petrol tank, but I'm not sure why there isn't one under the bonnet...

The car seems to be running well, while it was at the garage I did ask if they could reproduce the hesitation but they said they didn't find anything - but then again they didn't take it on a Motorway to Birmingham... They did however fit a new heatshield and check/retune my carbs so the problem may well now be fixed. The car pulls like a train the mechanic even commented, and on my drive home it really did. Seems to drive amazingly now but I'll report back once I get out on a long motorway trip to see if the problem has disappeared. You may be right with the gaskets... With the new heatshield going on and new gaskets in I'm hoping all is now airtight.
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Cruisinforicecream
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Re: MGB hesitation and loss of power at 2800rpm

Post by Cruisinforicecream » Tue Oct 07, 2014 10:07 am

adamliptrot wrote:Oh, make sure your carb dashpots have oil in.

Don't know much about carbs, how would I go about doin' this ^. And with what oil ?

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jonahnaylor
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Re: MGB hesitation and loss of power at 2800rpm

Post by jonahnaylor » Tue Oct 07, 2014 3:46 pm

Don't know much about carbs, how would I go about doin' this ^. And with what oil ?
Hi, to top up the dashpot oil just unscrew the two black plastic caps right at the top and just add 1 squirt of standard 50w20 engine oil from your oil can. Also just check the small breather hole on each is clear too.
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mike morrell
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Re: MGB hesitation and loss of power at 2800rpm

Post by mike morrell » Thu Dec 11, 2014 12:03 am

Just a thought but as the tacho is indicating a loss of power at the time the engine looses power it would seem logical that the fault is not associated with the fuel system as this would not cause a glitch on the tacho. As it seems to be when the engine has been run for some time it may be associated with a break down in the coil which can occur as it warms up during use and as load increases
The coil and tacho circuit have a common feed so I would be looking at the wiring connection blocks couplings and leads also . I had a very odd fault with my luminition system due to a poor connection block that failed unpredictably . It appeared fine when checked at rest but kept failing during use when hot due to expansion.

p.s I have an add problem with my ignition timing that I cant figure out so good luck with sorting yours

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jonahnaylor
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Re: MGB hesitation and loss of power at 2800rpm

Post by jonahnaylor » Sat Dec 13, 2014 4:12 pm

mike morrell wrote:Just a thought but as the tacho is indicating a loss of power at the time the engine looses power it would seem logical that the fault is not associated with the fuel system as this would not cause a glitch on the tacho. As it seems to be when the engine has been run for some time it may be associated with a break down in the coil which can occur as it warms up during use and as load increases
The coil and tacho circuit have a common feed so I would be looking at the wiring connection blocks couplings and leads also . I had a very odd fault with my luminition system due to a poor connection block that failed unpredictably . It appeared fine when checked at rest but kept failing during use when hot due to expansion.

p.s I have an add problem with my ignition timing that I cant figure out so good luck with sorting yours
I thanks for your help. I've got a lot of the issues now fixed and the car has been running great ever since. But as the colder wet weather has now hit, I seem to have a new issue, I've opened a new forum topic/thread here:

viewtopic.php?f=2&t=113" onclick="window.open(this.href);return false;

It may be related to the issues I had before, but seems different. This time the car just wants to stall whenever I come to a stop or put the clutch in. It starts great, idles well and sets off no problem, but then I come to put the clutch in to make a turn or stop for a roundabout and the car dies on me. The only way I can keep it going is by keeping a little bit of choke out all the time!
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