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.

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

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?

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

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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

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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!

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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.

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

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

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 !!

A small bleed hole maybe ? I guess thats why bikes use the second set of butterflys.

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

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.....

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.

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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.....

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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

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Not just any Mini handbrake quadrant though!

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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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

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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.

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

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.

just richen the mixture

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.

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]

Raz,

I can't remember which ECU you are using but....

I had a similar problem to yours, which was (in my case) due to slightly sloppy linkages, and a TPS that didn't see the angles perfectly.

Having recently changed to Speed-Density (for the turbo) it seems that this is way less sensitive to these subtle throttle position things at idle (not great everywhere, but good at this tip-in range).

So, back to the ECU thing. If you are using megasquirt, then the ITB mode allows you the best of both worlds (Speed density at low power, and TPS at high power).

Just a thought,

Matt

p.s. I'll try to find you at Stoneleigh as I'd like to see your car in the flesh. I still need to sort the map on mine (new engine, different cams, turbo etc.), but the new lambda probe arrived yesterday and Stoneleigh isn't from here (only 45miles).

I'm using an MBE 970 which seems to have soooo many whistles and bells it can be a bit daunting to a novice like me. The density that you mention must be the equivalent to an index map - a monotonic curve that reflects the relationship between throttle angle and airflow but can be tweaked in so many ways. I would guess that some improvements might be possible in this area but as a complete novice I would not like to play around too much in case I do something silly - that's a job for the dyno guys.

ps I'll be glad to meet you at Stoneleigh. I think Sunday is my chosen day - just waiting to see if SWMBO can make up her mind

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Originally posted by RazMan
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.

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My throttle bodies are not really conventional and are contructed from the original secondary butterfly castings - and I suspect that is part of the reason for the strange behaviour.

Check out this thread for the full story
http://www.locostbuilders.co.uk/viewthread.php?tid=145114

Anyway, I can now happily report that I have got things working nicely now. It took a lot of tweaking, fettling, remapping, swearing etc but overall I'm really chuffed with the result