"jerking" between low acceleration and accelerator released

[JohnC]

Hi Marco,

I'm with Stuart on this one. Look to your throttle linkage. There needs to be play, but only in the right places! I had this problem (kangaroo petrol at v light throttle openings). The root cause was a *combination* of no play in the throttle linkage *and* a missing bush in the throttle pedal cross-shaft. So the pedal was bouncing around and transmitting every movement to the throttle. Smooth roads - smooth engine; bumpy roads - bumpy engine.

I replaced the bush and slightly slackened off the linkage. Delight

JC

[Andy303]

4 hours ago, Z320 said:

Thank you for this idea, but I've got petrol enough,

currently testet at the Austrian mountains and on the Autobahn.

BTW. I love your valve cover

Marco:

You can get the Offenhauser valve cover here: https://www.whiteowlspeed.com/product/offenhauser-3132-triumph-finned-aluminum-valve-cover/

I have one and they are quite nice, however they use a breather cap and will not work with the Smith's PCV valve unless you drill and tap for a vent pipe in the side.

[Z320]

Hi, my last experiment was this:

I pressed 2 pieces of clear PVC hose on both studs SU bodies where the spindles

come out the carbs and filled the gap with grease to avoid any leak air going in there.

Result: no result.

 

So I lifted the bells and pistons to check the deep of the jets in the body: left 1.6 mm, right 0.5 mm, engine idling OK!

Why this? Different fuel level!

This is why I opened the float chambers expecting a higher fuel level in the right floar chamber. The oposite is the case!!

Left side, the jet 1.6 mm deep, the fuel level is 27 mm deep.

Right side, the jet 0.5 mm deep, the fuel level is 32 mm deep, 5 mm deeper!

Reason: I found the right floater bended.

Both floaters are sealed, no fuel in there.

I'm perplexed...

 

 

 

[Peter Cobbold]

Fuel level in jet is not critical for normal running as the constant depression is enough to lift fuel 8 inches out of jet. The fuel level in jet should not allow overflowing on slopes or during hard braking, otherwise it is not critical. 8inches makes a few mm difference in float chamber level of no importance.  Peter

[Z320]

Hi Peter,

thank you for your explanation.

Can you also explain why me carbs need a such different adjusted jet?

Needles and jets are "new", don't look suspicious. Is it the (maybe) different worn spindles?

Ciao, Marco

[boxofbits]

One of the problems caused by ‘ heavy’ return springs are excessive throttle spindle wear, and if using a cable, snapped cables.

Morris Cooper S’s for instance have two additional return springs hooked to the air filter casing which always caused cable breakages. As long as the spring has enough tension to close a freely rotating butterfly to the closed position without chatter and give ‘feel’ to the pedal , that’s all that should be needed.

Kevin

[Peter Cobbold]

5 hours ago, Z320 said:

Hi Peter,

thank you for your explanation.

Can you also explain why me carbs need a such different adjusted jet?

Needles and jets are "new", don't look suspicious. Is it the (maybe) different worn spindles?

Ciao, Marco

Marco,  Are the needles inserted in the pistons to the same depth?  Are the needles identical ? How was the tickover mixture set?  Peter

[Z320]

Needles are "TW", both the same depth, the offset plane to the lower piston surface.

Sadly I own no exhaust gauge, so I can't tell yu the CO concentration, the rear jet works well 1.6 mm deep, the front one I have to spin higher and higher.

[Peter Cobbold]

16 hours ago, Z320 said:

Needles are "TW", both the same depth, the offset plane to the lower piston surface.

Sadly I own no exhaust gauge, so I can't tell yu the CO concentration, the rear jet works well 1.6 mm deep, the front one I have to spin higher and higher.

Is the front psiton free to drop fully when damper rod is reomoved ? Lift it with a finger and let it drop- it should make  a click.  Peter

[Z320]

Yes, a clear "tock" with both,

both the same noise,

both pistons fall the same speed, both jets are 1/10" and look good.

next test drive will be tomorrow 

[BlueTR3A-5EKT]

3 hours ago, Z320 said:

Yes, a clear "tock" with both,

both the same noise,

both pistons fall the same speed, both jets are 1/10" and look good.

next test drive will be tomorrow 

Do both the pistons drop at same speed when there is no spring in them? 
Mine did not as I had muddled them up some years ago.
See here.  

 

[Z320]

21 hours ago, BlueTR3A-5EKT said:

Do both the pistons drop at same speed when there is no spring in them? 
Mine did not as I had muddled them up some years ago.
See here.  

 

Already checked, yes they do.

I found the gremlin, first test drive was auspicious, another drive tomorrow hopefully will confirm.

 

Edited July 10, 2021 by Z320

[Ian Vincent]

For what it’s worth, after I had my car set up on a rolling road a couple of years ago, I removed the piston and suction chambers and measured the depth of the jets. The front one was 0.89mm below the bridge and the rear one 0.6mm.

Since then, whenever I reset the carbs that is my starting point and I don’t have to move much from there. 

I’ve not worked out why the two measurements are different. 

Rgds Ian

Edited July 11, 2021 by Ian Vincent

[Z320]

Hi Ian,

you are absolutely right, me idea yesterday was to stop to find out the reason for the difference of 1 mm on my carbs.

The jet is adjustable and this is why there can be a difference. I`m convinced this difference is not the reason for jerking.

Ciao, Marco

[Z320]

Hi,

I worked step by step, simple work first, easy was to look for the floaters first.

From the set of HS6 I have in stock I found both of them in better condition than the ones on the car - after cleaning them.

The result was a minium less jerking - but this perhaps was only the result of my hopes and the jerking is anyway still absolutely annoying.

But I can't be wrong to have the same fuel level on both float chambers?

 

A minimum more effort was to work on the distributor. Does it not return correctly from full advance?

Mine has 2 identical advance springs, I own a second 25D and found the 2 identical advance springs in there (16.7 mm x 8 threads).

I've been told they must be 2 different ones and from a collection of advance springs from a box of bits and pieces I fitted one slightly shorter and stronger.

One with clearance (this is the case), one under load.

Result was a very "tired" car with snail accelleration (not getting on full advance?) - jerking still the same.

Perhaps members from this forum have some information for me about the springs?

Next was to fit the trottles without valve from my second pair of HS6, bought in a bundle with other parts for small money.

To bolt the carbs off and on the manifold is not that much effort with a plumbers installation bar fitted.

Here the trottles are.

And swaped

And the jerking has gone for 99%, what a pleasure again to drive....

Sadly the running on issue increased, but for that I have my push button under my dashboard and my petrol anti run on valves.

"Finally" I found this is another issue again of the missing idle jet on SUs, common on all "real" cabs (sorry folks) I know.

I give some thought on carbs from another producer, but first I will fit my other pair of HS6.

They look real used and rusty / dusty, but after a closer look the spindles have brass / bronze bushes and no play at all.

They look like to be in much better condition than the pair I drive on my TR4A.

Ciao, Marco

Edited July 12, 2021 by Z320

[Ian Vincent]

Hi Marco,

As you have discovered, having the correct mechanical advance is important for optimum performance.  I would have thought that someone with your ingenuity (and workshop) would have come up with a test rig to check the advance curve against the workshop manual.

Rgds Ian

[Z320]

Hi Ian,

I tested this only because I wanted to go down faster to less advance on lower revs.

In my opinion the question about the ideal advance curve for TR 4 cylinder engines is very easy to answer without a test rig.

When you drive, you always drive on full advance and not on a curve, because you can't drive the engine below 1,200 or 1,500 revs.

That's simply 22-24 deg when you drive (+ some deg. when speeding up on high revs) and 4 deg when you start the engine.

Btw this is how I also drive my pre war Nortons with manual advance.

Just MIN to start, than adjust manual to MAX, clutch, gear, speed-up the revs, engage the clutch, drive and be happy.

Ciao, Marco

Edited July 13, 2021 by Z320

[Z320]

Just a short update,

I cleaned and use now the bodies of the other pair of HS6 with the trottles without spring loaded valves.

They work much better than the pair of HS6 that came with the car when I bought it about 12 years ago.

Specially the mixture is better and very clearly to adjust.

But the car still suffers from not constant tickover revs. First the tick over revs  are OK, than revs fall slowly until the engine dies.

My idea is - I have not enough advance, that causes a spiral down: less advance - lower revs- less advance- less revs- less advance... death.

I'l check this next.

Ciao, Marco

Edited July 28, 2021 by Z320

[BlueTR3A-5EKT]

I too have still got the 'kangaroo petrol' problem.  I am following your tasks Marco and imitating some of them.  My next trick will be a carb swap to my spare HS6. 

I have not touched the distributor as it was restored by Distributor Doctor quite recently and the issue was there before the distributor was changed.

Peter W

[Lebro]

The Kangaroo effect is usually just positive feed back between the pedal, & your foot. As the car accelerates the foot (& leg, body etc) tend to react rearwards, which moves the foot off the pedal, this slows the car down, so your body parts move forward again, thus pressing on the pedal & so on. One way to stop it is to press your pedal foot sideways onto the side body panel to limit how much it reacts.

Bob.

[Z320]

The "Kangaroo effect" on my car was (absolutely annoying) when the car was rolling downhill under low braking load with low revs (high gear),

also when I change from "rolling without gas / low braking load" to minimum gas to keep the car gently rolling, gently accelerating.

I'm conviced this is because of the valves in the trottles and is now mainly gone.

 

I can only guess the valves open definitely by a high vacuum (high braking load downhill, high revs) without any problem, the mix is weak.

But open-close-open-close by low vaccuum and cause this by themself! The moment they close again (causes rich mix) the vacuum raises AT ONCE,

but because of that they open AT ONCE again (weak mix), brake the vacuum, and close again AT ONCE. Yes-No-Yes-No-Yes-No, very fast 

And the right one does this not syncron with the left one, influencing each other by the connection via the inlet manifold.

 

 

Edited July 28, 2021 by Z320

[stuart]

7 minutes ago, Z320 said:

The "Kangaroo effect" on my car was (absolutely annoying) when the car was rolling downhill under low load with low revs (high gear),

also when I change from "rolling without gas and low braking load" to minimum gas to keep the car gently rolling.

I'm conviced this is because of the valves in the trottles and is now mainly gone.

 

I can only guess the valves open definitely by a high vacuum (high braking load downhill, high revs) without any problem, the mix is weak.

But open-close-open-close by low vaccuum and cause this by themself! The moment they close again (causes rich mix) the vacuum raises AT ONCE,

but because of that they open AT ONCE again (weak mix), brake the vacuum, and close again AT ONCE. Yes-No-Yes-No-Yes-No, very fast 

And the right one does this not syncron with the left one, connected by the tube cross the inlet manifold.

 

 

Those little valves in the butterflies were originally fitted to TR7 as part of the drive to lower emissions in the late 70`s but they went with a different type of distributor too (AC Delco)

Stuart.

[harlequin]

My TR6 which has Stromberg carbs will misbehave (as sort of stuttering) at low revs, for example 30mph in 4th on a slight downhill. In my case it's caused by a lean mixture and is immediately cured by slightly richer mixture.

I find that by turning the fuel up just enough to stop the stutter I get an excellent plug colour and now I use the stutttter to set the mixture

George 

[Z320]

Short warm up today for checking the ignition advance at tick over revs with my ignition lamp.

Advance was very low at tick over revs, both dropped constatantly until the engine died.

I adjusted more advance until tick over revs and advance have been constant at about 15° advance at 1,000 revs.

Due to the manual max advance (24°) is alreday at 1,200 revs, this is what I told above.

I will see what happens next weekend.

[BlueTR3A-5EKT]

Still have the issue on my car.   I am lengthening the lever at the carb to reduce its movement compared to the accelerator pedal movement.  Will see if it works.   I have cable operated throttle not rod.