Drive Train Wind Up

[AlanG]

Hi,
I've just about finished recommissioning my 1972 Sprint and a first road test has shown up a problem. Under light to medium throttle openings it is very jerky. Almost like the throttle pedal is moving under my foot. Full throttle opening stops it. Could this be wind up in the drive train? I have fitted new doughnuts. Could there be a problem with these?
Any ideas or thoughts would be appreciated.
Thanks
Alan.

[JonB]

Sounds like the throttle cable needs to be lubricated, or it’s routed incorrectly.

With the engine off, try to press the pedal with your hand. Is it moving smoothly, both directions?

[sprintsoft]

Hi Alan,

I have a Sprint with new rotoflex couplings and can confirm they will not surge without some amount of torque reversal through the drivetrain. It is more likely your carbs are not too well setup and are ‘hunting’ giving you this effect.

In general terms minimising flex in the drivetrain also involves new engine and gearbox mounts, and the 4 diff mounts.

Regards
Iain

[sprintsoft]

Jon makes a very good point, the quality of the throttle cables stocked nowadays is pretty basic and can lead to jerky progress! On mine I made up a custom cable from Teflon inside coated bike brake cable, it’s a big improvement over the standard item,

[vxah]

This issue can sometimes be caused by poor throttle cable routing, engine movement under load moves the cable reducing throttle opening, reducing torque, reducing engine movement, increasing throttle opening etc.. Movement at higher throttle openings don't change it enough to cause noticeable issue?
Or maybe lean running? Blocked jet?

[Elan45]

Alan,

You mentioned replacing the doughnuts. Are you sure the replacements are the Sprint type w/ the extra steel pieces between the mounts where the bolts go through. The old original ones were very torque sensitive and I believe I've seen some vendors selling the old style, primarily for use in vintage Formula Ford.

Roger (been playing w/ Elans long enough that I remember what the old doughnuts were like)

[Grizzly]

I know some one who had a very similar issue which turned out to be one of the ears broken off the diff casing.

[alan.barker]

When i bought my Sprint the dpo had fitted cheap Hillman Imp doughnuts
Alan

[AlanG]

Thanks all for the replies.
JonB
The throttle cable is lubricated and moves smoothly.
Ian
I will recheck the carbs setup and report. The engine and gearbox mounts are new.
vxah
Interestingly the H/L vacuum non-return valve was stuck open so this would cause lean running. I've just fixed this but not had the opportunity to road test it again yet. Soon as the roads dry out I'll let you know.
Roger
Pretty sure the doughnuts are correct. got them from Sue Miller.
Grizzly
Diff is good.

Alan.

[sprintsoft]

Hi Alan,

you said:
"Under light to medium throttle openings it is very jerky...."

Is this whilst holding a steady throttle and speed, or, is this happening as you open the throttle and increase speed?

If you open the throttle quickly (pump jets working) does it clean up the response?

Iain

[john.p.clegg]

Just something to consider....

Had the same effect , it was a wire which was on its way out , from the coil to the distributor , as the engine moved , it would break contact , as the engine died it would make contact again...had me fooled for a couple of days...

John