RazMan
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| posted on 22/4/11 at 10:32 PM |
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Throttle Body Whiplash Behaviour
Now that I have my throttle bodies all nicely set up and running beautifully I have just one niggle which spoils an otherwise perfect setup.
When closing the throttles at mid - high revs and slowing down (say for a corner) I find it quite tricky to step on the gas again gently. It feels
like the the throttles are sticking in the closed position and then 'let go' causing a rather jerky pickup. Everything works fine at low
revs so I assume it must be the high vacuum caused at higher revs on the over-run.
I have made a progressive throttle cam to give the pedal more movement in the lower part of the range but it has only made a partial improvement.
Is there any way of getting this jerkiness out of the throttle? Any suggestions would be appreciated.
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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Mr Whippy
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| posted on 22/4/11 at 11:05 PM |
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weird, throttle valves are hinged in the center so the air flow is balanced on each side of the butterfly valve so there should be no force on them
regardless of engine speed. Only chokes valves pivots are off center.
Have you tried moving them by hand rather than the pedal or cable to see if they still stick? I suspect the throttle cam is going over center making
it stiff to get going
Fame is when your old car is plastered all over the internet
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RazMan
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| posted on 22/4/11 at 11:22 PM |
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Everything works totally smoothly when the engine isn't running and the butterflies open without any tight spots. I have checked the TPS and
made sure it matches the fuel map too. The ironic thing is that I didn't have this niggle when the butterflies weren't sealing very well -
its almost like they seal TOO well now!
I wonder if a little more fuel in the 0% TPS map positions might help - I initially had them too lean and had lots of popping on the over-run. Maybe
richening the over-run would smooth things out a bit?
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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bi22le
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| posted on 22/4/11 at 11:24 PM |
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Is this not just the quick response given by TB?
I just thought this was the way my engine was. Hard to drive at times due to a very light car, respectable power (150 BHP), supplied by a responsive
engine (4age 20V).
Not being funny (infact probably just showing my inexperience!) but do you not just need to get some more throttle control?!?
Biz
Track days ARE the best thing since sliced bread, until I get a supercharger that is!
Please read my ring story:
http://www.locostbuilders.co.uk/forum/13/viewthread.php?tid=139152&page=1
Me doing a sub 56sec lap around Brands Indy. I need a geo set up! http://www.youtube.com/watch?v=EHksfvIGB3I
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RazMan
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| posted on 22/4/11 at 11:43 PM |
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quote:
Originally posted by bi22le
Not being funny (infact probably just showing my inexperience!) but do you not just need to get some more throttle control?!?
Biz
That's exactly what my first thoughts were, but even after making the progressive cam, adjusting the pedal ratio AND teaching my right foot to
be verrry delicate I still have the problem, but only at higher revs. It's almost like a switch at times - gentle pressure has no effect until
WHAM it opens up again - it makes driving in the wet interesting to say the least!
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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jollygreengiant
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| posted on 23/4/11 at 06:23 AM |
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quote:
Originally posted by RazMan
The ironic thing is that I didn't have this niggle when the butterflies weren't sealing very well - its almost like they seal TOO well
now!
Some times we say the answer and don't realise it. Perhaps this is the case and you you need to adjust the throttle stops so that the butterfly
don't completetly shut. It always used to be best practice on carbs that the butterflys never completly shut, very close yes, but not totally
sealed.
Beware of the Goldfish in the tulip mines. The ONLY defence against them is smoking peanut butter sandwiches.
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Krismc
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| posted on 23/4/11 at 06:24 AM |
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Mine is like that, on lift off at high revs/high speed the car lurches a little and the exhaust bracket bangs- but at no other time does it do
this!
What TBs are you using, ive got R1's they have 2 return springs and ive put a extra one on pedal to stiffen pedal up a bit.
Built, Ivaed, Drove and now Sold - 2011 MNR VORTX RT+ 2000cc Zetec on R1 Throttle boddies.
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rusty nuts
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| posted on 23/4/11 at 07:17 AM |
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I agree with JGG , it sounds like the butterflies are jamming in the TB when fully closed.Have had loads of cars over the years where the butterfly
hasn't been correctly centralised in the bore. It may be possible to put a light chamfer on the edges of the butterflies that close onto the
bore to physically stop it jamming but still control the speed at idle
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big-vee-twin
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| posted on 23/4/11 at 07:42 AM |
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When building my throttle body set up, I did notice that they did stick momentarily at the fully closed position.
I have adjusted them so they very nearly close but not fully they are GSXR750 one's
Duratec Engine is fitted, MS2 Extra V3 is assembled and tested, engine running, car now built. IVA passed 26/02/2016
http://www.triangleltd.com
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RazMan
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| posted on 23/4/11 at 08:13 AM |
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Thanks for the input guys 
I can appreciate that if the throttle stop is raised a bit then it might improve the 'sticky' feel but unfortunately that would also raise
my tickover too - unless I play around with the idle timing again to bring it back down that way.
I sent DAYS on getting all 12 butterflies to seal properly. I machined a chamfer on each of them to ensure a good seal, but eventually had to apply
some Molycote (oem's use this stuff for the same reason) to take up the very small gaps that were left - the result was a perfect idle speed but
now it seems the seals are too good! Aaarrgghhh !!
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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big_wasa
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| posted on 23/4/11 at 08:20 AM |
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A small bleed hole maybe ? I guess thats why bikes use the second set of butterflys.
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RazMan
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| posted on 23/4/11 at 08:37 AM |
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A bleed hole would have the same effect as raising the throttle stops but I see how you are thinking. I do have a vacuum takeoff from each bore so
maybe I could get the ecu to switch a solenoid to let a small amount of air past at high revs & 0% TPS ....... but that could get complicated
Methinks I will raise the throttle stops a tad and then tweak the idle timing to bring it down again without letting things get too hot. I will give a
progress report later
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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GeoffT
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| posted on 23/4/11 at 09:09 AM |
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I get a similar effect to this with my setup, more noticable for me when you try to pull gently away. The vacuum seems to 'stick' the
throttles onto the stops slightly making for a jerky takeaway, it's definitely caused by the vacuum as the stick only happens with the engine
running.
I came to the conclusion that I need more throttle leverage at the low end, maybe even a 'logarithmic' type of linkage. I'll be
interested to hear how your idle timing tweek works out though.....
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RazMan
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| posted on 23/4/11 at 11:36 AM |
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I've just spent a very frustrating hour tweaking this and that. It appears that I can have a low stable idle with the jerky pickup OR a high
idle speed and a smooth pickup ..... just not the combination that I want 
I was quite surprised how the idle mixture was affected after opening the throttle stops a bit - I had to bung an extra 5% of fuel in just to get the
AFR within realistic levels. I suppose it is all about compromise so I will continue tweaking ( I keep my netbook in the car now) and see how it goes.
I'll have a good chance to get things closer during the Stoneleigh run on Monday.
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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RazMan
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| posted on 23/4/11 at 11:40 AM |
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quote:
Originally posted by GeoffT
I came to the conclusion that I need more throttle leverage at the low end, maybe even a 'logarithmic' type of linkage. I'll be
interested to hear how your idle timing tweek works out though.....
 
Description
I did exactly that and made a cam from a Mini handbrake quadrant - it gives about 30% more pedal movement in the lower throttle range and helped quite
a lot in reducing this problem. I've still got a little more to do it seems
[Edited on 23-4-11 by RazMan]
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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GeoffT
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| posted on 23/4/11 at 03:54 PM |
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quote:
I was quite surprised how the idle mixture was affected after opening the throttle stops a bit - I had to bung an extra 5% of fuel in just to get the
AFR within realistic levels
Yes, a minute change in TPS at idle makes a huge change in the fuelling requirement - mine varies from 20-50% across the rpm range for a TPS change of
just 1. For this reason I found that you have to ensure that there's absolutely no slack in the TPS linkage to prevent lean hiccups when coming
off idle.
The mini quadrant looks interesting - my setup has just the normal eccentric cam cable pickup on the t/b's, probably gives about 50% extra
leverage at the tickover end, but I could do with much more......
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rusty nuts
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| posted on 23/4/11 at 05:20 PM |
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Not just any Mini handbrake quadrant though!
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corpi
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| posted on 24/4/11 at 08:26 AM |
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could it be that there is some part of the throttle linkage "flexing" in any way, a bit before the initial movement of the throttles,
which then, once the throttles start moving springs back, giving you more opening than you want?
Like a sort of stick-slip due to the vacuum holding the throttles closed, and once they are cracked open there is less resistance.
I hope this makes any sense...
I know I have similar symptoms, and my throttle cable goes over a pulley, the mount flexes a tiny bit.
1% more opening from closed, obviously gives more effect than at half throttle!
just my 2P worth
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RazMan
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| posted on 24/4/11 at 08:39 AM |
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quote:
Originally posted by corpi
could it be that there is some part of the throttle linkage "flexing" in any way, a bit before the initial movement of the throttles,
which then, once the throttles start moving springs back, giving you more opening than you want?
Like a sort of stick-slip due to the vacuum holding the throttles closed, and once they are cracked open there is less resistance.
I hope this makes any sense...
I know I have similar symptoms, and my throttle cable goes over a pulley, the mount flexes a tiny bit.
1% more opening from closed, obviously gives more effect than at half throttle!
just my 2P worth
You might have something there - the cam arrangement (above) does have a tiny bit of flex which is probably exaggerated when pushing against the high
vacuum 'stiction' effect. I might try a temporary brace and see if that makes any difference - it certainly can't hurt
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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britishtrident
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| posted on 24/4/11 at 08:47 AM |
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I would take a careful look at the design of the throttle linkage, on production cars the amount the throttle opens in response to throttle movement
is designed to be progressive, ie the first 12mm or so of pedal travel only opens the throttle a small amount.
By careful design of the actuating levers and throttle pedal you can get a rising rate of movement of the throttle in response to the pedal's
movement to give smoother control.
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RazMan
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| posted on 24/4/11 at 09:01 AM |
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That's why I used the aftermarket Mini handbrake quadrant - it was almost custom made to do the job. Not only did it have lots of holes to mount
everything  but it has a progressive action too. All I had to do was sort out the cable nipple fixing and job done.
The flexing idea has got me thinking now - I'm on a mission 
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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MikeRJ
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| posted on 24/4/11 at 10:04 AM |
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One thing whic is often overlooked is that the brackets that hold the outer cable at either end must be strong and absolutely rigid. Any flex here
will give the exact symptoms the OP is seeing as the brackets bend first as pressure is applied to the pedal and then the throttles will jump open.
The same thing applies to the throttle pedal itself, and an old, sticky cable will also cause problems.
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littlefeller
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| posted on 29/4/11 at 08:59 PM |
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just richen the mixture
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sebastiaan
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| posted on 29/4/11 at 09:08 PM |
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You probably need a throttle dashpot to dampen the lasf few degrees of the throttle closing. You might be able to adapt something from an older
production car. Changing the pedal to throttle action helps too, as you found out.
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RazMan
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| posted on 29/4/11 at 10:39 PM |
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Just to update a little more .....
Things have settled down quite well now and with a combination of tweaking the throttle stops, pedal ratio, retard at idle and richening the mixture a
tad, the car is very driveable around town using the lower end of the revs. The throttles still have a tendency to stick a little when using more revs
on the over run but it is not half as bad as before. Maybe I can get the ecu to let some air past the butterflies (using a solonoid valve) under
those conditions to help improve it a bit more.
All things considered I am more than happy with my homebrew throttlebodies and even if they don't give me as much power as I expect they will,
they are worth it just for the noise! I've got a V6 that really sings at WOT now
[Edited on 30-4-11 by RazMan]
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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