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MG MGB Technical - Rolling road success!

As some of you maybe aware I have been rebuilding the engine on my MGA coupe over the last few months. Yesterday had the car to the local Weber specialist to check and tune carbies. All seemed well until we strapped the car down and an intermittent bad misfire occurred - distributor, coil and leads had been tested earlier and were deemed okay. Turned out the bolt on the steering universal joint clamp was hitting the silicone HT lead where it exited the distributor cap to No 3 depending where the steering wheel was positioned- I was aware of this problem from having the engine out in the past but as it hadn't caused any problems previously I thought I'd get away with it - bad move. (Problem really resulted from shoe horning a 18GB engine and 4 sync overdrive box into an MGA - it's tight in there!). The solution to the problem was to find a smaller height distributor with side exits to replace the Scorcher distributor. Luck would have it my tuner sourced a 123 distributor with USB tuneable curve to which you can fit a Bosch side exit cap
http://www.123ignition.nl/product.phtml?id=189
Don't ask the price! These are beautifully built units that are tuneable through out the rev range via a laptop. Copied another high performance MGBs settings and made a couple of alterations and we were away (it also comes with an adjustable built in rev limiter).
Carby float levels reset and synchronised, dyno sensors attached (knock sensor, air fuel ratio, inlet air temp, exhaust temp, fuel pressure, tacho) and we were away.
Attached are the results - I am interested in Peter B and other's comments.Tuner is reluctant to pull any more fuel out as likes to keep A/F below 12.5 on B series engines. Drop off at 5,800 appears to be the HT leads breaking down or a faulty spark plug- used the leads off the old Scorcher system. (rev limiter set at 6,500) - will change leads and reckon it with be okay to 65.
Mike



Mike Ellsmore

Mike
Thanks for sharing that

Usually when you have an electrical missfire the A/F ratio will show as lean as a result of the unburnt charge in the exhaust
As your mixture is still plenty strong when the power drops off i doubt that an electrical missfire is the cause of your dropoff
Can you actually hear it missfiring or does it just go flat---to me it has the symptoms of a restricted exhaust

just my two bobs worth

willy
William Revit

Hi Willie,
Pretty sure it is not a restricted exhaust - car has big bore side exhaust with Genie straight through stainless steel muffler (see photos). There was a very audible miss at about 5,9. Am fitting new leads and will see if that fixes it.
Mike



Mike Ellsmore

Sweet! Congrats. I'm sure you'll get all the bits sorted.
Max71

Hi Mkie
I reckon he is spot on with fuelling, a B does like to run rich. If the dyno operator says it is a power drop off and not sampling or smoothing of the data causing the perceived drop then up the rev limit to 7500 for dynoing as the 123T tends to soft cut around 500 rpm lower than the rev limit when they really hard cut. We have seen this with all 123Ts set at lowish limits. Set the limit to 7000 after dynoing, unless you are racing.
Peter
Peter Burgess Tuning

That screwed my theory then-----------
Peter
With the 123T, --does the rev limiter cut individual cylinders during the transition to full rev limiting or retard ign timing during this period---just thought it would show a spike in the mixture if there was missfiring happening
willy
William Revit

Hi Willy, I am not sure how the cut works, I will try and take note next time I have one in and see what effects there are on the AFR. It does annoy me it comes in so early though!

Peter
Peter Burgess Tuning

gd/day Peter, does this always apply to the 123Tune?
Last time we dynoed my supercharged car we decided 6000 was high enough for a street engine so I set the rev limit at that. It cut at about 5900 so reset it to 6200 then ran it to 6000. Seemed OK and made good power. Maybe I should have set it a bit higher as the power was still going the right direction. My 123T is the USB type that I got from you a fair while ago. Been great and so easy to use.

Denis
Denis4

Hi Denis, I'll set a 123T low for limit next time I have one in and see the effect and see if we can show it graphically.
Peter
Peter Burgess Tuning

Guys, thanks for your comments. Interestingly the dyno guy who tuned my car does many competition/racing MGs and other classic cars many of which are equiped with 123 distributors and he never mentioned to me about the rev limiter set point not being accurate. I'll give him a call tomorrow to see if he has encountered this problem and I'll report back.

The rev limiter spec for the 123 distributor I have fitted
http://www.123ignition.nl/product.phtml?id=189
states
"Rev limiter: Programmable soft rev limiter ( max 7900 rpm ), 60% of the sparks will cut off"
If it turns out that it activates 500 rpm lower than the set point then it needs raising with the manufacturer (from my curve if this is the problem, it appears to be about 700 rpm below the set point!)

I now have the new HT leads so will give it a try on the road also to see if this fixes the problem. Does running it up to the rev limit in the lower gears replicate the dyno load in top gear - I suspect not.
Mike
Mike Ellsmore


Hi Denis
The reason I asked about the limiter is that I can remember having a problem with the Elan once going fluffy at around 5500rpm- buzzed it up and when it went off it showed a jump lean,checked fuel pressure under load and all was well and it turned out to be the spring on the rotor button (6250 limiter)had given up and was scattering spark everywhere
and resulting in the false lean reading
Fitted a new button, and all returned to normal
It just interests me as to how the 123 does it's thing compared to the slidey Lucas rotor when Mike's dyno readout shows no sign of the mixture spiking up (lean) at the point where he is experiencing the missfire

willy
William Revit

Willy My last dyno session was near on a couple of years ago when I set it up with a Mikuni HSR48 slide carb. Still have the printouts and no sign lean anywhere. Almost flat Air/fuel 11.8-12:1 right across which is about right with the SC. On the first pull when it hit the limiter it just went flat and unfortunately didn't do a readout as we new what the problem was. After the reset it just wanted to rev so I recon its OK but certainly something to watch. Finished painting the V8 GT, body guards, and doors together, getting there slowly. Denis
Denis4

I'll steal this thread for a sec.-Mike won't mind I hope
So Denis did you end up painting the GT yourself - colour ??
willy
William Revit

Rev limiters will "cut" all cylinders equally, unless it is some strange thing. Why have any rev limiter at all on a dyno, except to check its function after the engine has been dyno'd. Turn it off and see what happens.
b fisk

Knowing what I know now I would agree but at the time I was being overprotective of the new engine. The engine previous to the rebuild had over the years been on the dyno quite a few times and never made any more power after 5500. The larger carburetor and Peters SC head changed the top end more than expected.

Yes Willy painted it myself the old tried and true dark BRG and came up pretty well, not concourse but wont be ashamed to park it with ones that are. I live in the sticks with 10-12km dirt road, so have to have cars that are "drivers". Denis
Denis4

Just an update - fitted new HT leads and plugs. (Opted for the warmer NGK BP6ES over the 7s set at 0.032" on the recommendation of my dyno tuner rather than PB's 0.035"). Car runs nicely but rev limiter came in at 6,000 rather than set point of 6,500.
I spoke to the dyno tuner about this and Peter's similar experiences and he says he hasn't seen it before - his own race car (Fiat) is also fitted with a 123 distributor and he says rev limiter comes in on the set point - he will keep an eye out for this problem with future B series installation. When I get a few more kms on the engine I will increase it by 500 revs and report back.
Mike
Mike Ellsmore

Mike, glad you feel you have got to the root of the problem. Albert explained the gradual soft cut to me when we first used the 123T and we saw the effect. It is a gentle reduction of sparks I think and if you don't take the hint from engine performance the hard cut comes in at the set point and let's you know in no uncertain terms! We have seen the effect on all 123Ts not just B installations.
Careful with the 6s, it can sometimes be the difference between detonation damage and not. Remember your dyno guy set the engine to the 7s not 6s so you are moving away from what he has done for you. We tend to only use 6s in mild tuned Bs used in towns only, any spirited use and we use 7s. You will also find the heat range has good overlap with the 6s so should cause no problems.
Peter
Peter Burgess Tuning

This thread was discussed between 31/03/2017 and 07/04/2017

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