Stiff Throttle

Mitch Krause

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#1
I guess the throttle is on the engine, so this is the right place for this post I hope. The first picture is taken from the top of the engine and shows the two golden colored arms, one connected to the webers and the other connected to the throttle cable (9 foot Lokar). If I remove the connection from the throttle cable and then use a loop of wire in the same holes that the throttle cable connects to and pull at the same angle as the throttle cable pulls (using a fish scale), the pull peaks at about 8 pounds (right at the start) and is a pretty steady 5 pounds throughout the pull to wide open throttle.

If I have the throttle cable disconnected and laying on top of the engine and move the pedal to wide open throttle, then use the fish scale, it takes a pretty steady 2 to 3 pounds to pull the pedal back to the closed throttle position.

If I have everything hooked up and put my fish scale at the top of the pedal, it takes about 14 pounds peak force to get it moving and about 12 pounds pretty steady pressure throughout to move to wide open throttle.

I guess maybe this makes some sense for the pull since 8 pounds (weber alone peak) and 2-3 pounds (cable alone) is pretty close to that (14 pounds versus 10 or 11), maybe some additional force with things all connected and slightly different aligns.

However the pedal does NOT return to the closed position once it is at WOT. If you hook the fish scale around the arm where the throttle cable connects, it takes about 4 pounds to "help" pull it back to closed throttle. When the carbs at at WOT, the backwards pull is about 5 pounds as you let it run back to closed throttle.

Regardless, the 14 pounds at the pedal, make it very stiff. Of course both the pedal and the little arm where the throttle cable connects actually travel in a slightly circular arc as they go through their range of motion, which means you can never have perfect alignment to the opening of the cable in the throttle sheath. Perhaps I could have slightly better to eliminate a couple of the pounds.

Picture Two shows the routing of the cable from the engine to the left hand side of the drivers seat. The bends are pretty gradual, at least I think they are. Red lines underline the throttle cable where it is hard to see behind the driver and running through the bulkhead to start towards the front.

Picture three shows the cable from the side of the driver into the pedal, again I think the bends are as gradual as they can be.

I did have a "test" spring I put on the carb end to help pull things back. That made the pedal push peak at about 30 pounds, but it would (rather slowly) return to closed throttle.

So now the questions. Where to start to take some of the hard pull away? Is the 8 pounds on the webers normal? Are they stiffer than they should be (no gas in them, maybe not lubricated), will they ease in real use? What should the expected pedal force be in pounds (anyone ever measured that)?
 

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#2
Mitch;

Every bend in the throttle cable generates friction and hysteresis. The first bend near the pedal looks like a pretty tight radius. Can you re-route the cable?
 

Randy V

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#3
In my experience - Those cables generally need to be lubricated before they pull cleanly. I used a Teflon grease on another car and it worked wonders. Like other cables, the jackets need to be properly restrained. Any movement of the cable jacket (flexing etc.) is lost motion at the other end..
 

Bill Kearley

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#4
A cable lube tool will help get the lube where it has to go. Another thing I would look at is the bellcrank. If you have enough pedal travel change the bellcrank to give more leverage and reduce pedal effort. I have the same issue with Borla 8 stack.
 
#5
I used a push-pull throttle cable in my car but you probably don't need a similar cable. At Bonneville the SCTA (Southern California Timing Association) rule book requires that the throttle be able to be pulled closed by a toe loop at the top of the pedal. This is to prevent a runaway if the throttle does not return by its own springs.
 

Randy V

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#6
I used a push-pull throttle cable in my car but you probably don't need a similar cable. At Bonneville the SCTA (Southern California Timing Association) rule book requires that the throttle be able to be pulled closed by a toe loop at the top of the pedal. This is to prevent a runaway if the throttle does not return by its own springs.
Key reason that Motorcycles have two throttle cables (giving you the effect of) one push, one pull - they can be positioned with relatively tight radius’s without binding (much). The single push-pull cables you’re talking about typically need very wide radius’s.. Unless there’s something new on the market I’m not aware of..
 
#7
They do require a wide radius and to get smooth low-friction operation, so do all throttle cables, but a push-pull is more sensitive to radii.
 

Mike Pass

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#9
I would start by lubricating not just the cable but everything that moves and working it back and forth to get everything as free as possible.There are a lot of pivots and ball joints etc. on a weber setup that are very stiff when new.
The bend in the cable at the pedal end looks a bit too sharp a turn.
There is a common issue with using straight levers and a fixed rigid cable end position in that the angle of the cable at the exit with changes as the lever moves and causes the cable to rub against the exit of the cable. The greater the angle of the cable at this exit the more friction it will cause at the sharp corner. A pivoted arrangement for the outer cable end will allow it to keep in line with the inner cable.
Using a quadrant or snail cam will help to keep the inner cable run in line with the exit. This will keep the effective lever length constant and will eliminate the sharp cable angle at the exit. Straight levers suffer from a a change in effective leverage as the lever moves to different angular positions.
It's difficult to see exactly the arrangement at the pedal box but the same principle of thorough lubrication and straight inner cable line from cable exit should help.
Check that the movements at the Webers and the pedal end are the same.If they are different adjust the ratio of the effective leverages.
Hope this helps.
Cheers
Mike
 
#10
Mitch, your 90 degree bell crank (throttle cable to weber throttle arm) doesn't look ideal. Your initial pull is at quite an aggressive angle.

I would consider using a throttle cable wheel rather than just a bell crank. This tends to lessen the initial pressure needed and provide for a smoother and more linear actuation motion. It also tends to be a bit more adjustable as the wheel typically has a range of holes drilled for locating the throttle arm.
 

Bill Kearley

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#11
Wheels are nice as well, Try and get a diameter that gives the most leverage that the gas pedal will permit. I have also seen wheels that have a graduating radius.
 

Mitch Krause

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#12
Cliff and others,

Thanks for the advice. I am trying first to relocate to get a better angle on the bell crank, but would be interested in looking into a throttle cable wheel of some sorts. Any advice on where to source these, tried Pegasus, Summit and can't seem to find anything with throttle cable wheel as the search term (or snail cam or quadrant cam). Any advice on what to search for or where to look?
 

Mike Pass

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GT40s Supporter
#13
I make them by sandwiching together three pieces of alloy sheet. Two thin sheets on the outside and a thicker one in the middle. Roughen the areas of contact and use strong epoxy and rivets to fasten the pieces together. Easy to make and you can vary the shape of the curve. Either a constant radius or a changing radius (snail cam).
As they are quick and easy to make you can make a few to home in on the best shape for your setup.
Cheers
Mike
 

Howard Jones

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GT40s Supporter
#16
If you buy a new cable at some point, call these guys. They make custom cables that I believe are the best available bar none. Ask for their best low friction throttle cable. I have them on both my cars for the throttle and they made my custom shifter cables for my SLC.
I called them and they suggested that I could save some money if I sent them my cables and had them shortened when I told them that I need the same ones that I have but shorter by a few inches each.

Really good guys, extremely high quality, and cost effective. I was referred to them by a sprint car guy I know in California.

http://push-pull.com/
 
#17
Mitch
If the throttle comes back slow from full throttle you would look at that first.
If it does it with the cable off its at the carby end with it on its at the pedal / cable.You need to isolate why it does not snap back on its own.
I use a teflon lined cable /no lube required.
I would agree with the other guys about the wheel and make it larger in OD than the bell crank as it will change the ratio .
If you then change the ratio on the pedal it can help but must work with the ratio at the carby wheel or you can run out of pedal.
On my set up 8 TB I use a bell crank with a wheel on one end so it rolls up the back of the throttle pedal as you depress, changing the bell crank arm lengths can help I set mine up so I have to move more throttle pedal to TB movement as 8 TB are sensitive and if to sensitive it makes it hard to drive in traffic as it can be sensitive .
The larger the bell cranks and wheels at both end the easier it will become.
I make all my stuff so I cant give sources.

Jim
 

Mitch Krause

Member
GT40s Supporter
#18
All, thanks for all the help. I am done. I got about 5 pound out by straightening the cable pull at the throttle end by making a new bracket (that is the picture). A couple of pound gained by using some spacers that gave me a little angle at the pedal end and helped straighten that out a bit. Gained about 6 pounds by using some DuPont non stick dry film lubricant with Teflon that I squirted in the cable. Another pound or so by running the cable along the spine and getting a couple of curves out of it. At that point I had about 5 or 6 pounds of pull to move the pedal and it would usually spring back (sometimes stick at full throttle). Last thing I did was add a very loose helper spring at the throttle end. End result was about 7.5 pounds of pull at the pedal with everything mounted and the cable springs back to idle quickly every time. So I am calling it done. It is very easy to push with your foot now and even if it stiffens up a little now, I have the brackets made and can just add a little stiffer helper spring and it would still be fine.
 

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Terry Oxandale

Skinny Man
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#20
One more thing I might suggest. When I built my wheel (similar design to what Mike suggested), I also added a torsion spring. They come in quite a large variety (see McMaster-Carr), and can easily be put onto your bellcrank. Using one that has an I.D. just slightly larger than the mounting post you're using, you could bend the spring's straight ends to fit against the bellcrank's arm, and easily affix the other straight end to the post. Even though you've got things moving more freely now, with age, debris, and wear, it may not last.

Lastly, if you get some free time to reconsider the wheel, I would say take it up. I went to a lot of trouble when I made my wheel to allow almost no initial movement of the throttle plates just off idle, to a rapid opening speed at WOT. This is what I account for the phenomenal throttle response I have. Your bellcrank presently being used has a higher rate of opening off idle, and I assume at WOT, but a slower rate at half throttle. It's these little things that make the driving feel more enjoyable.

Example of what I'm talking about. This, and having the throttle plate arms perpendicular to the operating link when looking from the front/rear.




 
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