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"jerking" between low acceleration and accelerator released


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Hi there,

my TR has some "jerking" when driving between low ecceleration and the eccelerator released.

My impression is it's getting worse from year to year. Can a poor shimed diff be the issue? I know it has some play.

Ciao, Marco

Edited by Z320
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I’ve had a similar issue on my TR3 for some years and failed to find the culprit. My TR4 doesn’t have the issue even though the diff has far more free play so I don’t the that the diff can be the problem. 

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  • Z320 changed the title to "jerking" between low ecceleration and eccelerator released

Sometimes an over adjusted throttle control can do this, it’s good to have a little free play on the linkage. 
Stuart 

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Good morning Stuart,

the TR4A linkage is special but installed and adjusted correctly.

But indeed it does not give the "forks" of the carbs free (no gap) anymore,

I can force it gently and hope WD40 does the job until I'm back from office this evening.

I can't believe this is the reason, but if it is I will be very pleased.

Thank you, Marco

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8 hours ago, Drewmotty said:

I’ve had a similar issue on my TR3 for some years and failed to find the culprit. My TR4 doesn’t have the issue even though the diff has far more free play so I don’t the that the diff can be the problem. 

Hi Drewmotty,

thank you at least for that comparison.

Ciao, Marco

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 I get similar 'kangaroo petrol' issues at slow speed in first gear with light throttle pressure.  - almost as if I am learning to drive again!

My car has cable throttle to the SU carbs so I am not able to do proper comparison to yours with rod linkage.

Peter W

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Its very obvious when travelling the lane into my workshop as its not very smooth and as the throttle pedal on the TR is a pendulum type your balancing on the heel and pressing with the front part of your foot so its not that easy to be subtle with pressure applied. If theres no free play in your linkage then you will get jerky application. I found it particularly obvious when I first fitted the Dellortos to mine as I had to make up a new mechanism from scratch and rather than going down the cable route I opted for a version of the usual rod links, the first version was far too direct and so I had to add some slack into the system and now its very smooth.

HS6 have forked linkages for the self same reason to allow some free play.

Stuart.

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Hi Stuart,

it seems to be less jerking now - but maybe it is only my wish....?

Not only the linkage has s return spring, both forks on the SUs two...and they always worked.

Hi Dave,

I use an original distributor, last year with points, this year with a Pertronix Ignitor I,

this seems to work better - but maybe this is only what I wish...?

Damper oil is OK, mixture is rich, spark plugs are lovely "dark"....

 

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Hi,

this is my measurement from yesterday evening

P1210303-b.thumb.JPG.a629f0ec771450702cdf5422779ae70e.JPG

I can turn the diff flange about 5 mm from stop to stop, this is about 7° play on the 79.50 mm circle.

The pinion has 10 tooht x 36°? And I have 7° play?

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I’ve finally spent half a day converting my TR3 to a cable throttle and it’s made a huge improvement to the kangarooing on bumpy lanes at low throttle. The car now behaves at least as well as my TR4 which has always been fine with the standard setup. 
That said I still haven’t been able to identify a root cause to the problem. 

E51E6209-DE7D-4576-89E8-76DF431DB273.jpeg

926BDA4A-6F37-436F-8C21-3DF2C9E0F338.jpeg

3023B8CF-BA39-4582-9532-96323C75F8A3.jpeg

F6F6857F-45E9-46E1-BE46-1D7E61650C98.jpeg

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7 hours ago, Z320 said:

Hi,

this is my measurement from yesterday evening

P1210303-b.thumb.JPG.a629f0ec771450702cdf5422779ae70e.JPG

I can turn the diff flange about 5 mm from stop to stop, this is about 7° play on the 79.50 mm circle.

The pinion has 10 tooht x 36°? And I have 7° play?

I’ve got more than that on my TR4 which doesn’t suffer with jerky response. 

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On 6/14/2021 at 8:50 AM, BlueTR3A-5EKT said:

 I get similar 'kangaroo petrol' issues at slow speed in first gear with light throttle pressure.  - almost as if I am learning to drive again!

My car has cable throttle to the SU carbs so I am not able to do proper comparison to yours with rod linkage.

Peter W

Peter, Might be weak mixture. Weak mix burns slow> rpm falls> mixture richens > burns better > rpm rise >mixture weakens................

Test is to try a little choke. Or lower the jets.

Tick over mixture is normally very rich but a manifold air leak can weaken it.

Peter

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43 minutes ago, Drewmotty said:

I’ve finally spent half a day converting my TR3 to a cable throttle and it’s made a huge improvement to the kangarooing on bumpy lanes at low throttle. The car now behaves at least as well as my TR4 which has always been fine with the standard setup. 
That said I still haven’t been able to identify a root cause to the problem. 

 

926BDA4A-6F37-436F-8C21-3DF2C9E0F338.jpeg

 

 

Have you set the clearance on the throttle forks?

Stuart.

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They were already set and remain unchanged. 
Chilliman has come up with an interesting theory:

weak return springs.

Sounds plausible especially when the mass of the operating rod is considered as it moves directly backward and forwards. 
I’m not in a position now to double up on springs to test the theory but it wouldn’t take long to test. 
Maybe give it a try Marco. 

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3 hours ago, Peter Cobbold said:

Peter, Might be weak mixture. Weak mix burns slow> rpm falls> mixture richens > burns better > rpm rise >mixture weakens................

Test is to try a little choke. Or lower the jets.

Tick over mixture is normally very rich but a manifold air leak can weaken it.

Peter

Thank you Peter, I will drop the jets a flat each and see if it changes things.

I had also wondered about piston flutter with the different piston dampers that are available.  I run the AUC 8114 not AUC 8103 type.  The 8114 having more clearance for the piston to move before the damper comes into full effect.

Perhaps I need to swap the dampers to test…..
Cheers

Peter W

 

93C37736-64E9-4541-A62C-4C201273C7A8.jpeg

Edited by BlueTR3A-5EKT
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4 hours ago, Drewmotty said:

I’ve finally spent half a day converting my TR3 to a cable throttle and it’s made a huge improvement to the kangarooing on bumpy lanes at low throttle. The car now behaves at least as well as my TR4 which has always been fine with the standard setup. 
That said I still haven’t been able to identify a root cause to the problem. 

E51E6209-DE7D-4576-89E8-76DF431DB273.jpeg

926BDA4A-6F37-436F-8C21-3DF2C9E0F338.jpeg

3023B8CF-BA39-4582-9532-96323C75F8A3.jpeg

F6F6857F-45E9-46E1-BE46-1D7E61650C98.jpeg

Might I suggest you add a couple of cable terminal covers to your battery to solenoid cables.  That choke cable would make a lovely conductor to earth if it touched the solenoid cables.

Peter W

https://www.autoelectricsupplies.co.uk/product/159/category/32

C7F7FE0C-1D89-4BC6-A132-AFF90F073D26.jpeg
 

PS   Be sure to carry a spare throttle cable and method of fitting.   I tuck my spare behind the accelerator cable bracket.   

 

 

2C7C9E42-E0B3-4754-B468-8D0D517BB4ED.jpeg

BF9FD711-364D-4BDF-8A2C-405783829FFD.jpeg

Edited by BlueTR3A-5EKT
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9 hours ago, BlueTR3A-5EKT said:

Thank you Peter, I will drop the jets a flat each and see if it changes things.

I had also wondered about piston flutter with the different piston dampers that are available.  I run the AUC 8114 not AUC 8103 type.  The 8114 having more clearance for the piston to move before the damper comes into full effect.

Perhaps I need to swap the dampers to test…..
Cheers

Peter W

 

93C37736-64E9-4541-A62C-4C201273C7A8.jpeg

Peter,

Peter, Could it be a leaking over-run valve in one of the butterflies ? - though these were later fitments I think, not used on TRs until the 7.  Peter

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9 hours ago, BlueTR3A-5EKT said:

Thank you Peter, I will drop the jets a flat each and see if it changes things.

I had also wondered about piston flutter with the different piston dampers that are available.  I run the AUC 8114 not AUC 8103 type.  The 8114 having more clearance for the piston to move before the damper comes into full effect.

Perhaps I need to swap the dampers to test…..
Cheers

Peter W

 

93C37736-64E9-4541-A62C-4C201273C7A8.jpeg

Peter,  In first gear crawl there should not be any pulsing of air pressure at the piston or jet as the butterflies are barely cracked open. At less than 30deg opening the flow past the butterfly is sonic and iv pressure pulses cannot pass. So I would not suspect dampers to be operating. Peter

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2 hours ago, Peter Cobbold said:

Peter,

Peter, Could it be a leaking over-run valve in one of the butterflies ? - though these were later fitments I think, not used on TRs until the 7.  Peter

Overrun valves were fitted to the carbs that are on my car.   I snipped them out and filled the holes in the butterflies with solder many years ago so that is one item off the list.
The possibility of an air leak could be worn spindles.   The carbs were new in 1975.   Will do a spray test with the engine running.


Thanks.

Peter W

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Today I put the air filters off,

the dampers do not work on the first 3-4 mm (I guess, not measured),

I realize this is "the empty way" of the damper and it is out of function while idling.

Is this correct?

I can lift the pistion without much force only against their weight and the spring inside.

Should I use other springs?

Also I pressed the pin up to lift the pistons while ideling: revs increase, than fall lower than before.

Means: the mixture is too week?

But the spark plugs are black. I should try hoter ones?

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10 hours ago, Z320 said:

Today I put the air filters off,

the dampers do not work on the first 3-4 mm (I guess, not measured),

I realize this is "the empty way" of the damper and it is out of function while idling.

Is this correct?

I can lift the pistion without much force only against their weight and the spring inside.

Should I use other springs?

Also I pressed the pin up to lift the pistons while ideling: revs increase, than fall lower than before.

Means: the mixture is too week?

But the spark plugs are black. I should try hoter ones?

I know this says mg but they are SU carbs and this guy knows his classics and shares his knowledge

 

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  • Z320 changed the title to "jerking" between low acceleration and accelerator released

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