My throttle body is too big!

OK. When I was "designing" the fuel injection system for my mini I made a booboo.

I simply thought "Well, if this was carb'ed I would go with a 1.75" SU so something in the 44mm range ought to be fine". A bit of ebay browsing landed me on half of an arctic cat 1000cc snowmobile intake as the 55mm throat seemed "close enough". So now I have a butterfly that wouldn't be a restriction on a 2 liter on a wee 1330cc mill.  

As a result the low rpm range is impossible to control and difficult to tune. I fear I am going to tear up the motor mounts in pretty short order like this.

I do not want to start over again so I am thinking of making an eccentric pulley. 

So I ask the hive.

Is simply moving the pivot off center the magical ingredient here or does the shape of the "ramp" contribute more? In the past I have worked with sport bike ITB setups that used the egg shaped pulleys. 

Can anyone think of a pulley I could repurpose?

Watching. I fear my ITB's at 48mm each are going to prove challenging to tune on a 2.0. 

You can get creative with the shape of the eccentric, but the basics are going to get you most of what you need. I have a similar part on the throttle body for my 1UZ powered spitfire, it helps with the low rpm/light throttle modulation.

Trent said:

OK. When I was "designing" the fuel injection system for my mini I made a booboo.

Kinda curious . . . how much too big do you figure you went?

I did something for the throttle linkage for my 48mm Dell'Orto DHLA.

It was replacing a very small progressive 4bbl carb.  What I did was make a secondary shaft that the original carb's quadrant attached to.  Then I mounted a linkage to the carb and to the shaft, with the carb linkage forming an acute angle.  This way the motion ratio works out that it is less than 1:1 at the beginning of stroke, when the throttle angle-throttle area ratio is greatest, and it is more than 1:1 near WOT, when throttle angle-throttle area ratio is lowest.

Project page with all the words

In reply to Pete. (l33t FS) :

Cool, thanks for sharing that. The 50mm's on the 2.5 in my e30 want some sort of progressive solution as well.

L5wolvesf said:

Trent said:

OK. When I was "designing" the fuel injection system for my mini I made a booboo.

Kinda curious . . . how much too big do you figure you went?

I feel like the only reason the HIF44 works on the little motors is because it is a "constant depression" type of carb with a variable venturi.

I feel like I overshot the number by about 40%. 35-40mm is probably the right number for a 90hp motor. What would it really need? 150cfm? It doesn't take much to flow that.

Trent said:

L5wolvesf said:

Trent said:

OK. When I was "designing" the fuel injection system for my mini I made a booboo.

Kinda curious . . . how much too big do you figure you went?

I feel like the only reason the HIF44 works on the little motors is because it is a "constant depression" type of carb with a variable venturi.

I feel like I overshot the number by about 40%. 35-40mm is probably the right number for a 90hp motor. What would it really need? 150cfm? It doesn't take much to flow that.

Geez, 40%. I didn’t think it was that much. So, I think you know venturi is where the fix is. With my Fiat 850 air flow was and is an issue so . . .

Over 180 degrees of bends before the mixture gets into the head. That probably makes the flow even slower before it reaches the throttle body. Then the mixture makes 2 90 degree turns – slowly. I would guess the mixture may be puddling at low rpm. I know you don’t want to redo the TB (which is the actual problem) so maybe speed up the flow before the TB with smaller tubing.

No such thing as throttles too big, only cam too small :)

I'm kidding.

Progressive bellcranks should help you.  Don't be afraid to go bigger than you think you need.  There for a while, many factory cars came with offset pulleys like the one you pictured.

In reply to Curtis73 (Forum Supporter) :

The last of the small block Chevys had them, which was right at the end of the cable throttle era.  I thought this was kind of amusing since GM used to love to go the other direction and make throttle tip-in overly abrupt, to make the engines feel powerful.  Too bad there rarely was much extra once past that first 25% or so.

A lot of manufacturers in the 80s put the throttle mechanism on a progressive linkage.  I have heard it referred to as a square-root linkage.

In reply to Pete. (l33t FS) :

Indeed.  The TBI GMs were notorious for that.  So hard to tip in the throttle without jerking the vehicle.  The LT1s used a progressive bellcrank and people complained about the throttle response.

Mom had a 91 4.3L TBI blazer (160 hp) and she claimed it was so much more powerful than her 96 blazer MPFI (190hp) because of that jumpy throttle.

Dad used to have an 88 TBI truck, and I always put the right part of my foot on the trans tunnel and rocked the left part of my foot on the accelerator to allow gentle acceleration.  If I just put my foot on the pedal and pushed, there was no way to gently accelerate.

In reply to L5wolvesf :

My 843cc Fiat 850 used a yamaha raptor 44mm throttle body.  It too was oversized but with the progressive pulley it was fine.

I wonder where I put that thing?

Progressive pulley will help. If it doesn't, GM's solution was to add a lip to one side of the throttle plate. This way, as you cracked the throttle, only one half of the plate was creating an opening for the first several degrees of travel, and eventually the lip would end and you'd get an opening on that side as well. May be slightly challenging to fab up, but it works well enough. The restriction it causes at WOT may not matter if the TB is already that far oversize.

I yanked the throttle pulley off the TB to take some measurements so I could machine something fancy but while I was looking at it the quick and dirty option occurred to me. 

I moved the pivot 5mm, as scribbled on the part above. 

Three cuts and a few zaps of the MIG and I figured it was good enough to try.

I have to say I am impressed. 15 miles on in town driving says definite improvement achieved. Crawling around the neighborhood at 15mph in 2nd gear is much easier. 

I felt it would be nice to have a successful end to a thread like this.