bigbravedave
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| posted on 31/12/11 at 11:46 AM |
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Help me diagnose this poorly mgb engine, very down on power
ok from the start, my father was a bmc b series engined car that had a full engine rebuild 10 years ago, was used for a couple of years then left
standing for 8 until he recently brought it as a non-runner.
Once home we removed a lumenition rev limiter we got a spark.
It now starts and runs but its down on power, I suspect its not running on all 4. you can disconnect plugs 1 and 4 and it makes little difference to
the engine note! To drive it will do 40mph flat out regardless of gear.
Things we have done so far include:
Replace points, set up dwel.
compression test, good accross all 4.
new fuel pump and checked fuel supply to carbs
new ht leads and checked spark at all 4 with colour tune plugs.
removed exhaust to check nothings made a nest in it!
checked the ht leads are on the correct plugs!
The car has passed an mot! although being 1960 something, emmisions or running on 2 didn't matter.
we have removed the twin su carb float bowls and checked they are not stuck.
when you remove the plugs 2 are white ish brown (plugs 2 and 3), two are black and wet (1 and 4)
Im left scatching my head with this car.
the next step is clean/rebuild the carbs.
All sugestions are welcome. I know its going to be something really simple, I just want to get this car running for the old man!
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rusty nuts
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| posted on 31/12/11 at 11:58 AM |
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Start with the basics, check the valve clearances, too tight and you will have problems , It may be suffering from valve seat recession which would
close up the clearances. Check the ignition timing is correct as well as checking the auto /vacuum advance is working as it should .Some of the MGs
left the factory with an advance curve that wasn't suited to the engine Use the distributor number to get the correct timing setting. Siamese
ports tend to make the outer cylinders run richer than the inner cylinders . Setting up the carbs should be the last thing you do when tuning an
engine. Don't forget to check your getting full throttle!!
[Edited on 31/12/11 by rusty nuts]
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andrew.carwithen
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| posted on 31/12/11 at 12:12 PM |
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Have you renewed the spark plugs and checked the gap?
You could try swapping the plugs over between cylinders 1 & 2 and 3 & 4 to see if the fault moves with them?
And you definitely have checked the firing order is correct?
What's the condition of the dizzy cap? - any sign of tracking? worn contacts etc?
Do the points use a condensor - have you replaced that?
Maybe the coil itself is on the way out?
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bigbravedave
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| posted on 31/12/11 at 12:34 PM |
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cheers guys,
great replies,
we have new pugs correctly gapped, and have swapped them about.
we have fitted a new coil, a ballested one and tried another new coil just incase!
it definately gets full throttle sweep at the carbs.
the points do use a condeser coil, good idea to change
the vaccume advance is working as far as I can tell
Im going to order a new distributor cap, It has got a side exit cap that the ht leads clip onto, I will check its the correct type for the
distributor.
Im just wondering if part of it was the faulty lumenition rev limiter module I removed, I couldn't find any info on it, I just want to ensure
that it didn't require any wiring changes to install it that It's missing now that its removed.
as for valve clearances the engine does lack that tappet rattle Im used to. but if the valves wern't seating would it still compression test
ok?
Im going to bullet point all your ideas and go through everything one by one, Its such a simple engine which makes it so frustrating that I
haven't sorted it yet!
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digiman
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| posted on 31/12/11 at 12:34 PM |
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I had a similar problem with my old MGB, compression test was fine, but my uncle (an ex-jaguar mechanic) immediately said it was an exhaust
valve....and he was right. Take the head off and have a look.
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Wadders
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| posted on 31/12/11 at 12:47 PM |
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You'll have to bear with me here, cos its 30 years since i owned an MGB, but IIRC each carb serves two cylinders, could it be the choke is stuck
on one carb, or the float is stuck causing it to flood a pair of cylinders?
You could do to identify which pair of cylinders is not running, if the dead pair is fed from one carb, i'd start looking there.
Might also be worth pulling the rocker cover off and checking none of the valves are stuck open.
Al.
[Edited on 31/12/11 by Wadders]
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r1_pete
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| posted on 31/12/11 at 01:44 PM |
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After 8 years the carb slides (pistons) will almost certainlt not slide smoothly in the dashpots, if they are not stuck all together.
Take the dashpots off, clean insde etc, then, what I do to get smooth operation is to coat the outer edge of each piston in solvol autosol, and slide
it up and down the dashpot intill it is nice and smooth running, then clean off all traces of solvol, re assemble, and top up the dampers with engine
oil.
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daviep
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| posted on 31/12/11 at 02:02 PM |
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Have you tried swapping the pug leads of cylinder 1 & 4? I don't mean replacing them with new leads I mean removing the lead from plug 1 and
connecting it to plug 4 and vice versa?
Sounds to me like they are the wrong way round it would fit perfectly with all symptoms. The correct way to do it would be to take off the rocker
cover and check the timing and firing order properly but if 2&3 are firing then I would try swapping 1&4
Davie
“A truly great library contains something in it to offend everyone.”
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jacko
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| posted on 31/12/11 at 02:48 PM |
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May sound daft but is there oil in the carbs ?
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Gear Monkey
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| posted on 31/12/11 at 02:55 PM |
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I've had similar problems with a bike that had been sitting for 5 years. Modern fuel is so full of additives which when left for a number of
years gum everything up. I stripped and cleaned the carbs two or three times, soaked then over night and blasted them with air but despite making
minor improvements in the end I just bought new carbs and it ran perfectly.
GM
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owelly
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| posted on 31/12/11 at 03:19 PM |
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This reminds me of an old mechanic friend of mine who spent hours and a small fortune with a similar fault on his MGBGT.
"of course I've checked the fecking firing order, you twat" he kindly replied in frustration. But he's thought the rotor arm
span the other way....
Keep it simple...
http://www.ppcmag.co.uk
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bigbravedave
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| posted on 31/12/11 at 04:20 PM |
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more great replies, cheers guys,
right, its cylinders 1 and 4 that aren't firing, swapping leads has no effect, good strong spark at 1 and 4 though.
the slides in the carbs move freely with the correct damper oil in.
I will strip and clean the carbs over the next few days.
Im suspecting sticking valves although can't get my head round an engine compression testing fine yet having stuck valves. that will be next on
the list.
If valves are stuck whats the best method of unsticking, it has got a high compression head and a fruity cam fitted.
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bill132hotrod
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| posted on 31/12/11 at 04:29 PM |
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Hi there
A few years back I bought a Austin A40.
It had very similar problems and I traced it to a Mice nest blockin the Exhaust have you tried disconnecting the downpipe at the manifold, If it is
that she will rev and breath a whole lot better.
Give it a go.
Regards Bill.
PERSEVERANCE is the word of the week. Stick with it the results are SWEET.
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Peteff
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| posted on 31/12/11 at 04:43 PM |
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quote:
Originally posted by bigbravedaveIf valves are stuck whats the best method of unsticking, it has got a high compression head and a
fruity cam fitted.
Take the rocker cover of and hit the rocker over the valve with a plastic mallet.
yours, Pete
I went into the RSPCA office the other day. It was so small you could hardly swing a cat in there.
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Wadders
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| posted on 31/12/11 at 04:51 PM |
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Ha Ha, i was going to post the same, but thought better of it in case i got derided for being a rough arse 
figured there would be a 'proper' way to do it that i didn't know.
Al
quote:
Originally posted by Peteff
Take the rocker cover of and hit the rocker over the valve with a plastic mallet.
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plentywahalla
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| posted on 31/12/11 at 06:30 PM |
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IIRC the exhaust manifold siameses the cylinders 2&3 and 1&4. so try looking there. Its the only bit where cylinders 1&4 share a common
path and any blockage would give you the symptoms you have.
Rules are for the guidance of wise men ... and the obedience of fools. (anon)
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britishtrident
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| posted on 31/12/11 at 07:47 PM |
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Idling on the inner 2 cylinders is common on these cars it is caused by the the siamesed layout it occurs when the carb are not set up properly
(idle speed too low and mixture out). It is unlikely to have anything to do with the car running out of steem at 40mph.
Did you check the pistons in the carbs are rising and falling freely ? Also try it with the damper caps on the carbs removed --- over filling
the damper dashpots will cause this problem as will using too thick an oil in the dashpots; 5w/30 engine oil or ATF works a treat.
The Lucas distributor is also prone to the centrifugal advance sticking due lack of lubrication.
If none of the above check the fuel flow rate from the SU pump --- important to check this at the actual float chamber as it is not uncommon for SU
float valves to be choked.
[Edited on 31/12/11 by britishtrident]
[Edited on 31/12/11 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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Lightning
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| posted on 31/12/11 at 08:35 PM |
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If the dizzy has been out . Could it be 180 degrees out .
Steve
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rusty nuts
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| posted on 31/12/11 at 10:05 PM |
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quote:
Originally posted by Lightning
If the dizzy has been out . Could it be 180 degrees out .
In that case the engine would not run at all!
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Lightning
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| posted on 31/12/11 at 11:37 PM |
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True. Still think leads are wrong though
Steve
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britishtrident
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| posted on 1/1/12 at 01:21 AM |
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MGB 101
On the B series on the MGB the ports are siamesed
Cylinder 2 shares a port and carb with cylinder 1
Cylinder 3 shares a port and carb with cylinder 4
There is balance pipe between the front and rear carb but minimal cross over between the two.
Also Cylinders 2 & 3 share an exhaust port and exhaust manifod branch.
This really has a major effect on mixture distribution at idle.
Firing order is 1,3,4,2 or to put it another way 1.3.4,2,1,3,4,2,1,3,4,2,1,3,4,2
Consider the firing interval spacing between cylinder
Cylinder 2 lead cylinder 1 by 180 crank degrees but after cylinder one fires the crank then has to turn a full 540 before we get back to cylinder
2.
The same applies to the rear pair of cylinders.
Clearly because of the different inlet pulse spacing the outer cylinders (1 & 2)will always get a different intake charge from the outer
cylinders (1 & 4)
On the B if the idle speed and mixture is correct this isn't a major problen but if the setup isn't quite right the engine will idle
quite happily on the centre two cylinders.
[Edited on 1/1/12 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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bigbravedave
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| posted on 27/1/12 at 09:25 PM |
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Finally got to the bottom of it, one carb had very wrong jets and needles wedged in it, then someone had tried to balance the other carb to it two
complete rebuild kits are going into them tomorrow.
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britishtrident
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| posted on 27/1/12 at 09:45 PM |
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Ya think so DiNozo ?
"Jammed in" wrong jets sounds highly unlikely.
I would hazard a guess its simply a stuck piston in one carb. As I said back at the start "Did you check the pistons in the carbs are
rising and falling freely ?" --- this is always the first thing to check with SUs and Strombergs , particularly if they have been lying
up or neglected..
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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paulf
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| posted on 27/1/12 at 10:18 PM |
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I had a very similar problem on an MGB that I owned years ago.I changed everything including the cylinder head , points, coil carbs,exhaust and
eventually checked the fuel pressure and found that although the pump always ran it had next to no fuel pressure and the carbs were running dry above
about 40mph.I replaced the pump and it ran perfectly as it would with everything else replaced.
Paul
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rusty nuts
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| posted on 28/1/12 at 11:17 AM |
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quote:
Originally posted by britishtrident
Ya think so DiNozo ?
"Jammed in" wrong jets sounds highly unlikely.
I would hazard a guess its simply a stuck piston in one carb. As I said back at the start "Did you check the pistons in the carbs are
rising and falling freely ?" --- this is always the first thing to check with SUs and Strombergs , particularly if they have been lying
up or neglected..
Perhaps not as unlikely as you might think, Many years ago I worked for a garage in Cambridge that lent a customer an Austin 1800s that had just come
in in part exchange. They had the car for a few days when it broke down in London, the AA or the RAC was called and they found one of the needles had
dropped out of the piston which was promptly "Jammed " into the piston as hard as it would go, the customer then drove a few blocks before
it broke down again and was abandoned in The Inn In the Park. I had to travel to London with a few tools to sort out the problem so I could drive the
car back to Cambridge
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