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Hesitation when accelerating
I have a factory 74 MGB V8 and I have been trying for the last few weeks to get it tuned corectly. For the most part the car performs well except when accelerating there is a slight hesitation before she picks up. The engine is standard except for Lumenition Ignition, K&N filters and BAF needles, I have 20w oil in the dashpots and the timing is set at 9 degrees BTDC and a new vacuum advance unit fitted. Any help or suggestions would be appreciated.
Alan McLean
07/06/2012 @  12:08
Under topic: V8
 
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Hesitation when accelerating

Assuming your ignition is good it must be mixture.
I set the mixture just slightly on the rich side at tick-over to give a good pick up.
Are your dashpots free moving?
You may not be getting a rich enough mixture under acceleration - try 30 oil in the dashpot.
Finally a weakness (pardon the pun) with the SF carbs is that the spring needle, although removing the problem of needle alignment, does mean that the needle and jet wear in the tick-over region so tick-over adjustment then doesn't give the expected correct profile for low revs. You could try new needles as the easiest component to change.
Bob Owen
07/06/2012 @  20:57
 
Hesitation when accelerating
Your spec is identical to mine with the exception of a BBW needle. Clive W recommended that when I fitted K&N filters several years ago. I used the standard BBU needle for a little while but the flat spot was awful. Doesn't affect economy, just richens mixture enough during accel to make the V8 do what it does best.
Tony Lake
07/06/2012 @  21:51
 
Hesitation when accelerating
Hi Bob, The needles are new as are the air filters, the pistons are moving freely as I checked this when I was replacing the needles. You suggested going from 20w to 30w for the dashpots would that make the car less responsive? the reason I ask is that I was going to try ATF fluid in the dashpots (which is obviously lighter) as this was one solution given to me.
Alan McLean
07/06/2012 @  22:37
 
Hesitation when accelerating
Al, if you don't feel like you are getting anywhere, why not book the car in to Peter Burgess for a rolling road session?

My other half purchased her '67 Triumph GT6 mk1 back in April - it ran like a pig when we picked it up. A little tinkering on the drive with a gas tester got things a lot better. However, she booked the car into Peters workshop for a RR session for a couple of weeks later.

Peter really knows his stuff. Hand on heart it's the best 60 quid we've ever spent on a car. We were there from 9am until about 1:30pm and did a little over 20 runs on the dyno. Peak power remained unchanged but the difference in the power curve from the bottom upward was quite stark. The car is now a lot more tractable - no need to rev the nuts off it to pull away any longer and the fuel consumption has improved, even with "lead foot" Nicola at the wheel... In fact, we were so impressed, we've booker our new '72 Midget in for a RR session with Peter later this month and we've not even collected the midget from the vendor yet!


If you do book in, be warned there will be a bit of a wait at this time of year - we're in the middle of race season after all. It's a real education spending time in Peters workshop. I certainly picked up a lot.
JC Moulds
08/06/2012 @  00:59
 
Hesitation when accelerating
Hi JC, Sounds just like what the old girl needs, unfortunately I'm in Southern Ireland but if you or anyone else out there knows of someone offering a similar service in Ireland I would be very interested.
Alan McLean
08/06/2012 @  01:07
 
Hesitation when accelerating

Regarding dashpot oil: This only affects dynamic situations (OK, it also stabilises the piston to prevent flutter under steady state conditions too). The damper piston itself has a one way valve action so allowing the main piston to fall quickly (deceleration) but slowing rise (acceleration) depending on the oil viscosity. Whilst it is true that a higher dashpot piston pulls out the needle to give a bigger orifice and so more fuel for a given air flow past the needle, hence maintaining the correct fuel air mixture at different speeds, under acceleration a richer mixture is required and this is achieved by the damper fluid holding the piston down. Although the needle orifice is smaller the mixture is richer because the smaller venturi results in higher air velocity past the orifice and a greater jet pump effect. So a thicker oil gives more fuel under acceleration. Of course, you can over-do it and get too rich a mixture, that's when you need a thinner oil. The other extreme - taking the damper out altogether is the equivalent of having zero viscosity oil hence no enrichment under acceleration. Pressing the pedal then gives plenty of intake roar but no action....
Bob Owen
08/06/2012 @  18:56
 
Hesitation when accelerating
Hi Bob, Excellent explanation on the piston vs. dashpot oil all makes sense now. I will check the mixture at tick-over and richen it slightly if required as you suggested earlier and then road test. As regards the timing do you think 9 degree BTDC is correct setup given the fuels we are using now, I have read a few articles were owners are setting the timing at anywhere between 12 and 15 degree BTDC.
Alan McLean
08/06/2012 @  19:43
 
Hesitation when accelerating

Alan,
I agree that further advance than the 9 degrees at tickover gives better performance. The fear is possible detonation at high rpm with maximum distributor advance. Others, especially the racing boys, will know much more about this than me (cue for more comments, please). Modern cars, with their knock detectors and electronic management, can sail much closer to the wind on this and re-adjust themselves for changes in fuel - but we don't have that luxury and have to stay well clear of the worst possible knock threshold.
Bob Owen
08/06/2012 @  20:32
 
Hesitation when accelerating
The rate of change of flow area for the BAF needle is quite different than the BBW which goes with my K&N filter set up. By reducing inlet restriction air flow improves so at the intake manifold depression corresponding to the factory filter setup there will be a higher airflow, ergo there needs to a corresponding increase in fuel metering to maintain air/fuel ratio. At idle, station 1, and small throttle openings the differences are negligible. At station 8 on the needle using BBU dias as the baseline BAF will flow 14.5% more and the BBW flows 20% more. At station 12, BAF flows 35% more and BBW 52% more. At station 16, wide open throttle, BAF flows 69% more and BBW flows 109 % more. Will be interesting to see how the 30W oil helps.
Tony Lake
08/06/2012 @  21:58
 
Hesitation when accelerating
Hi Tony, That's a significant difference between the BAF and BBW needle's particulary at the higher end of the scale. You said your spec is the same as mine, but do you have the factory exhaust manifold and exhaust system? as the BAF needle I'm using was speced for standard manifold and exhaust set-up which I'm still using.
Alan McLean
08/06/2012 @  22:35
 
Hesitation when accelerating

I have the standard exhaust manifolds and K&Ns and have BAF needles. These were recommended by Beech Hill garage. Clive W recommends BBWs.
I find that mine is sensitive to petrol type.If set it up on regular it hangs back if I fill up with one of the Super grade fuels. I have to set the carbs for the fuel grade. This may be down to the ethanol content and the different stoichometric ratios for petrol and alcohol.
There was a BB series on needles and K&Ns in 2009, initiated by a rolling road test by Alan Rennie and followed up by Gordon Hesketh-Jones and others. Info was published in Victor's V8 section in April 2009 Safety Fast. This includes a very useful graph comparison of six needles, BBU, BAQ, BDL, BAF, BBA, BBW and BAC (in order of increasing richness). I have this on my PC but, sadly, it doesn't seem to have produced a V8 Note.
Bob Owen
09/06/2012 @  02:01
 
Hesitation when accelerating
Alan, mine are tubular manifolds with a single s/s exhaust with 2 boxes. I have Lumenition, K&Ns, static ignition timing is in the same area as yours. I guess it is dangerous to be too definitive on these issues. It would be fun one day to get a bunch of factory cars together in all their states of tune and put them on a dyno along with a list of their features to see exactly how they all behave. The Midget Register now do this as an annual event.
Tony Lake
09/06/2012 @  17:36
 
Hesitation when accelerating
Bob, Tony and JC, Thanks for all the advice help and information. There seems to be so many variables involved needles, manifolds timing and fuel that there is no definitive answer as to how to how the car should be set-up. One of the problems I have is that I have nothing to compare it with i.e another V8, so I don't really know if the car is performing as it should, as Tony suggested it would be great to have a group of factory V8's together at a rolling road to compare HP and torque outputs. Thanks again for all your help.
Alan McLean
09/06/2012 @  20:53
 
Hesitation when accelerating
Following a report from David Waterton on his V8 Roadster on Novatech's rolling road at Slough, I arranged for Novatech to have their dyno at MGLive! at Silverstone in 2009. The dyno was set up near the V8 Register Marquee and Parking Area and many V8 members took the opportunity of having their V8 on the dyno. We had an advance booking system so members could book a definite time for their session which avoided waiting in a queue on a "casual turn up" basis and it also ensured a high use of the available slots with Novataech over the weekend. If any contributor to this thread would like to organise another dyno session at MGLive 2013 or earlier with Novatech then do let me know so it can be promoted and an online advance booking system arranged.

The note mentioned by Bob Owen is in fact V8NOTE400 available online via the V8 website homepage search box at:
http://www.v8register.net/subpages/V8NOTE400.htm
See alos V8NOTE328 by Barry Franklin at:
http://www.v8register.net/subpages/V8NOTE328.htm

Gordon Hesketh-Jones contributed a very useful 3 page article on "Needles for SU carburettors" for the V8 Newsletter in April 2009. There is some detailed information on various needles on the charts in that article. You can see that at:
http://www.v8register.net/FilesV8/V8APR09G.pdf

You can see further information on the dyno test with the V8 Roadster (formerly owned by David Waterton) at:
http://www.v8register.net/subpages/novatechdynoMay2009.htm
http://www.v8register.net/subpages/novatechdynoMGLive2009.htm

Details of the Novatech dyno at MGLive! 2009 are at:
http://www.v8register.net/subpages/novatechdynoMGLive2009details1.htm
http://www.v8register.net/FilesV8/NovatechDynoresultsMay2009.pdf
Victor Smith
10/06/2012 @  01:45
 
Hesitation when accelerating
In fact I suffered from this recently but the cause was not one mentioned so far in the various helpful postings. Basically the N/S carb was not rising on accelaration as much as the O/S carb. This needed some careful polishing of the inside of the N/S chamber, and adjustment on the balancing screws
Gordon Hesketh-Jones
12/06/2012 @  02:59
 
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