David Jenkins
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| posted on 21/5/07 at 10:33 AM |
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Hesitation on acceleration
If I'm driving along at around 2000 rpm and floor the accelerator. the engine bogs down for a second before picking up and pulling hard. If I
ease the pedal down it doesn't hesitate and just pulls.
The accelerator pump works OK - it does squirt when I move the accel linkage quickly.
Any suggestions?
BTW: It's a 1660cc X-flow with a Weber DGV 32/36 downdraught carb and MegaJolt ignition.
Thinking of MJ - it couldn't be that, could it? Not enough advance at high MAP reading, for example?
cheers,
David
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cadebytiger
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| posted on 21/5/07 at 10:35 AM |
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accel jet undersized?
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JoelP
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| posted on 21/5/07 at 10:36 AM |
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bogging down when you stamp on the throttle is i think just caused by a rush of fuel without enough air and engine speed to use it. Always better to
wind it in slower, but getting the carb(s) tuned on a rolling road will help somewhat. Mine used to do the same on twin webbers, boggs brothers made
it a lot better.
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David Jenkins
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| posted on 21/5/07 at 11:02 AM |
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quote:
Originally posted by JoelP
bogging down when you stamp on the throttle is i think just caused by a rush of fuel without enough air and engine speed to use it.
Ah - I was thinking that there wasn't enough fuel - maybe it's too much in one go. Sort-of makes sense, as it doesn't happen when
the engine's running faster (4 or 5K rpm) - the engine just pulls harder immediately. I'd guess that at those revs the huge amount of air
being sucked in allows the engine to cope with the sudden petrol boost.
Just have to go lighter with my right foot then!  
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flak monkey
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| posted on 21/5/07 at 11:05 AM |
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If its too much fuel you will be getting an excessive amount of black smoke when you blip the throttle (i say excessive as most cars which run with an
accelerator pump smoke a bit when you increase the revs sharply from idle), or at least in theory you should.
David
Sera
http://www.motosera.com
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David Jenkins
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| posted on 21/5/07 at 11:22 AM |
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When I floor the throttle I'm in no position to look at the exhaust!
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UncleFista
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| posted on 21/5/07 at 12:08 PM |
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I have the same set up as you, and had the same symptoms.
I just set all my jetting back to standard (was a right mix and match), and raised the main jet sizes until the hesitation had gone.
Probably different problem, but you never know 
[Edited on 21/5/07 by UncleFista]
Tony Bond / UncleFista
Love is like a snowmobile, speeding across the frozen tundra.
Which suddenly flips, pinning you underneath.
At night the ice-weasels come...
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ayoungman
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| posted on 21/5/07 at 12:34 PM |
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If your used to driving modern cars with ECUs on board, you are probably not used to the old fashioned driving technique required. Carb fed engines
need the power fed in much more slowly. Remember, there isn't an onboard brain to adjust fuel, timing,advance air flow etc.
HTH
"just like that !"
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David Jenkins
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| posted on 21/5/07 at 02:33 PM |
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Good point! It's the only basic carburettor engine I've driven for many years. Just have to take more care!
I'm reluctant to mess around with the jets, as in every other respect it's working well.
My next upgrade, when I can afford it, is to go to bike carbs. We'll see how it behaves then.
cheers,
David
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britishtrident
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| posted on 21/5/07 at 03:35 PM |
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If you still have the original jets it is because you have change or done away with the original air filter.
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David Jenkins
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| posted on 21/5/07 at 03:50 PM |
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I do have a free-flow foam filter instead of the standard one.
So would I have to go up a size or two, or down?
It has been on a rolling road in the past, but I wasn't over-impressed - I would have thought they'd have sorted this problem, but I ended
up with the same jets I went in with!
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MkIndy7
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| posted on 21/5/07 at 04:35 PM |
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Cam timing possibly if the blocks been skimmed or its got a faster cam.
Our fast road cam was hopeless after a skim until we got a vernier pulley
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rusty nuts
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| posted on 21/5/07 at 05:04 PM |
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Not forgetting the go faster cam which could affect fuel requirements
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David Jenkins
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| posted on 21/5/07 at 06:03 PM |
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I think I need a session with a different rolling road - can't afford that just now, though!
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rusty nuts
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| posted on 21/5/07 at 08:26 PM |
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I used Peter Baldwin at Wilshers garage at Wimpole when I had the car set up . He has a very good reputation and didn't cost too much . Tractor
Boy has also used his services and was pleased with results. May be a bit too far for you though. ( just outside Cambridge)
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NS Dev
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| posted on 21/5/07 at 09:16 PM |
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Ignore the comments about old engines vs new etc, the carburettor is more than capable of a very sharp blip from idle.
Plenty of accel fuelling and a decent amount of advance is what is required. The numbers for both of those statements I'm afraid I can't
help with, but our old pinto engined, 38dgas fuelled sierra (ahh brings back amusing memories) would take full throttle from idle, and just barked
like a dumper truck and spun the wheels up!!
Retro RWD is the way forward...........automotive fabrication, car restoration, sheetmetal work, engine conversion
retro car restoration and tuning
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paulf
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| posted on 21/5/07 at 09:33 PM |
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I used to have a downdraught carb on my crossflow when first on the road and found it to have a similiar problem, but manged to get it to respond well
by adjusting the mapping when i fitted the megajolt, its worth trying some more advance at the bottom end and intermediate throttle settings.Your
unlikely to do any damage to the engine as if it is to much you will hear it pinking and your not likely to be driving at low revs and full throttle
for extended periods.
Paul.
quote:
Originally posted by NS Dev
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Plenty of accel fuelling and a decent amount of advance is what is required. The numbers for both of those statements I'm afraid I can't
help with, but our old pinto engined, 38dgas fuelled sierra (ahh brings back amusing memories) would take full throttle from idle, and just barked
like a dumper truck and spun the wheels up!!
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