DCOE throttle return

[foster461]

This winter I did a few small projects on the TR6 including new linkage for the Webers and converting from the bell crank to cable operation. It all went really well until the test drive today. The problem is with the throttle return. If I use a spring strong enough to fully close the butterflies it makes the throttle very binary, on or off with a lot of resistance when I first press the accelerator pedal. If I use a weaker spring the pedal feel is better but the butterflies are slow to close.

 

I'm using the standard LHD pedal which has a metal rod that crosses the bulkhead with a short lever for what was the Stromberg bell crank. I have a cable attached to that lever which loops around to a bracket on the center carb. You can see that cable and bracket in the picture below.

 

I'm thinking that I am screwed and that with this arrangement I dont have enough leverage to make this work properly but I am open to suggestions if there is anything that could do to improve it. Plan B is to go back to the bell crank config under the carbs using a solid lever.

 

Stan

 

[graeme]

Hi Stan,

 

Could you use a longer lever on the carb end of the cable? Might give more control.

 

BTW, why did you change from rod to cable?

 

Cheers

Graeme

[Waldi]

Stan,

Tou have a nice engine bay.

One comment: I note you have connected the crank case breather hose with the brake servo hose, this will reduce the vacuum on the brakes, hence it's efficiency.

The line may (partially) plug over time as well due to condensed vapors, further reducing brake efficiency.

Regards,

Waldi

[Guest]

Perhaps not an answer, but a suggestion to have a look at: I used to have a Spitfire 1500 with two SUs (the European set-up). The throttle pedal on that car has a different layout: A pivoted throttle is adjusted to a cable that runs through the bulkhead on the LH side, over the top of the engine to the other side where the carbs are.On the other side a spring for closing the butterflies.

 

The longer cable gives some 'slack' and when the spring pulls down and the butterflies close, it's not as if there's an on/off switch under your feet.

 

From the Moss Europe site - not drawn to scale, I may add.

 

[Guest ntc]

Hi Stan

 

You have got the angle for the dangle wrong!

 

Where your cable attaches on the rod, it is on the wrong side it should be towards the rocker cover. So you will need to shorten the outer cable. That way a smaller spring will be required.

[foster461]

Graeme, cant use a longer lever as I already had to shorten the original lever a smidge to clear the rear carb.

 

Waldi, there is a PCV valve in the breather hose and the servo operation is not affected

 

Menno, I wonder if a longer cable would help especially if I do as Neil suggests and bias the cable lever towards the engine and use a shorter spring.

 

Thanks all for the suggestions. I'll play with it some more later today.

 

Graeme, I switched from the bell crank to the cable because I thought the cable would work better, would not be buried under the carbs next to the exhaust and would remove the congestion around the center carb where the carb throttle rod and the long rod from the bell crank to the linkage were competing for the same space.

 

Stan

[boxofbits]

Hi Stan

 

Looking at the photo your throttle cable is not in a straight line and appears to run uphill to the lever.

 

Also, the crank doesn't seem to be at a workable angle. It looks as though the crank needs to be set higher so that it can rotate in an arc towards the cable. As it is, it looks at the bottom of its stroke. I think I would set the crank back 20 degrees or so and adjust the angle of the cable so that it pulls in as straight a line as possible. With the cable disconnected check the butterflies close with the lightest Spring setting and then go from there.

 

Kevin

[Tom Fremont]

Stan,

 

Too bad the solid linkage didn't work out for you. I've had no issues with them on my cars apart from bell crank drag which was easily rectified. My driver has a ball bearing in its bell crank but the other one has a bronze bush.

 

There are stronger throttle return springs available from WEBER. ( I used these a while back when I put in longer pump rods which compress the pump springs more; a condition predicted in the literature; now they're in the bits collection. )

 

If that helps...

 

Cheers,

Tom

[Guest ntc]

Tom

Sorry to say it is crap with solid linkage.

Edited April 11, 2017 by ntc

[Tom Fremont]

Tom

Sorry to say it is crap with solid linkage.

Hmmm...might it have to do with the fitter or the method ?

 

In the furthest recesses of my memory I have a faint recollection of learning how to drive it with solid linkage; I'm guessing it took a couple of minutes, tops. Haven't put any attention on it in 20+ years / 120K+ miles. Satisfaction with this simple method which employs almost all of the original TR250 linkage is right up there with the WEBERs themselves - most reliable parts on the entire vehicle .

 

In this hobby I've seen that complexity trumps simplicity, the latter typically frowned upon and rejected in favour of the former . Call me the last term of an engineer's credo ( KISS ) if you like .

 

Cheers,

Tom

[Don H.]

Tom

Sorry to say it is crap with solid linkage.

 

Have you ridden in Tom's car, Neil? (smile). It is most definitely not crap in that installation...

 

As an idea (from one running a generation or two earlier than a TR6), have you considered alternative Bowden cables, Stan? In the bicycle world, they've gotten quite sophisticated in their low-compliance, low-friction attributes. Modern shifting systems have demanded it. The design of a modern bike cable with self-lubricating liners is nothing like the ones of old. Are they using those kinds in your linkage?

[Guest ntc]

Don

It is and will always be so.

[Tom Fremont]

Thanks, Don .

 

I guess cables have more damping ( friction ) inherent in them . I reckon this reduces kangarooing, a special trait beloved by youth especially if the tyres chirp obediently .

 

Cheers,

Tom

[foster461]

 

Have you ridden in Tom's car, Neil? (smile). It is most definitely not crap in that installation...

 

As an idea (from one running a generation or two earlier than a TR6), have you considered alternative Bowden cables, Stan? In the bicycle world, they've gotten quite sophisticated in their low-compliance, low-friction attributes. Modern shifting systems have demanded it. The design of a modern bike cable with self-lubricating liners is nothing like the ones of old. Are they using those kinds in your linkage?

 

The cable is a Lokar brand Don (http://www.lokar.com/product-pgs/throttlecables-kickdowns/tc-kd-pgs/throttle-cables.html) and is designed for this purpose. I think my issues are geometry/leverage. With the bell crank there is a lot of room under the manifolds so the lever on the bell crank can be as long as you need it to be. With the cable I am restricted by the rear carb body and the bonnet which is making both of my available levers quite short. The linkage moves freely and there is nothing binding.

 

We were busy today running errands in the freakish 90 degree weather but tomorrow I will adjust the angle of the lever on the linkage bar and see if that makes a difference.

 

Stan

[pfenlon]

Don

It is and will always be so.

Neil you should go to evensong more often, and pray that others will aspire to your heights of knowledge.

 

but you have borrowed Menno's spade sadly in this instance.

 

So its penance at morning Mass for you sunshine. or at least try and accept that others can also get things right.

 

Chapters 6 verse 4. night night.

[BlueTR3A-5EKT]

This winter I did a few small projects on the TR6 including new linkage for the Webers and converting from the bell crank to cable operation. It all went really well until the test drive today. The problem is with the throttle return. If I use a spring strong enough to fully close the butterflies it makes the throttle very binary, on or off with a lot of resistance when I first press the accelerator pedal. If I use a weaker spring the pedal feel is better but the butterflies are slow to close.

 

I'm using the standard LHD pedal which has a metal rod that crosses the bulkhead with a short lever for what was the Stromberg bell crank. I have a cable attached to that lever which loops around to a bracket on the center carb. You can see that cable and bracket in the picture below.

 

I'm thinking that I am screwed and that with this arrangement I dont have enough leverage to make this work properly but I am open to suggestions if there is anything that could do to improve it. Plan B is to go back to the bell crank config under the carbs using a solid lever.

 

Stan

 

Hi Stan,

 

Admission - I went to cable op throttle on my TR2 in 1971 as I got so annoyed with the worn out linkage that jumped off/stuck/jammed & was going to cost me a weeks wages to replace with new.

 

A couple of notes on the cable installation you have.

 

Disconnect the cable from the lever - is there enough pull on the throttle pedal return spring to pull the pedal back up from fully depressed? Are the throttle pedal support bushes too tight? The orig TR6 throttle return spring is usually enough to make it work.

 

With the cable disconnected do all three relay levers to the carbs from the connecting top rod snap closed from full throttle opening? If not investigate and rectify. Heavier Weber throttle springs are mentioned above.

 

Does FULL throttle opening at the carbs equal full accelerator pedal depression? Had that issue on a friends 'race tuned' MG once. Huge Weber that opened only 2/3rds on full accel pedal depression - New lever length at the carb made the car go like stink. Full pedal movement should equal full carb opening.

- Set the mechanical pedal stop when full carb opening is achieved so you do not wreck the throttle cable inner wire when stamping on the loud pedal.

 

Most importantly - As NTC has commented, the angle of the lever on the shaft that the cable attaches to should be rotated back toward the rocker cover - Then you will have equal quadrant setting. - Best would be to have the input lever the cable is attached to at right angles to the relay lever(s) that link to the carbs.

 

Go for a weaker cable return spring - For neatness - I fitted what looked like a long ball point pen spring on the exterior of my throttle cable inner wire between the cable stop and the lever. Thus when the throttle pedal is pushed and the inner cable shortens, the spring compresses. When released the spring pulls the cable back through its outer casing. PLUS - You can wind them on/off the inner wire without removing anything. Just do not go coil bound on full throttle.

link to type << http://www.ebay.co.uk/itm/Compression-Springs-Spring-3-16-inch-1-2-inch-Various-Sizes-Choose-Quantity-/262667618990 >>

 

Hope my notes are of use.

Peter W

[roy53]

 

 

Most importantly - As NTC has commented, the angle of the lever on the shaft that the cable attaches to should be rotated back toward the rocker cover - Then you will have equal quadrant setting. - Best would be to have the input lever the cable is attached to at right angles to the relay lever(s) that link the carbs.

 

Hope my notes are of use.

Peter W

There you are Stan spot on advice from the above two .

All you will need to do is solve your issue with the cable length. a new bracket on the weber or a longer cable is required.

 

ROY