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"jerking" between low ecceleration and eccelerator released


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I think the cuase is "wall wetting" ocnsquent upon manifold pressure changes. When running at low speed some of the fuel flows as liquid along the manfold floor.This is normal. Then closing the throttle to decelerate will lower the manfold pressurre to near vacuum and will evaporate that liquid fuel form the manifold floor. Fine,the engine can cope with that. But when the butterly is  cracked open slightly to accelerate the manifold pressure rises from near  vacuum to ca 05atm, and some of  the fuel is  used to rewet the dried-out manifold floor. As a result the mixure admitted by the inlet valves becomes very weak indeed and the engine stumbles. It may even pink as the high octane components of the fuel wet thew walls and the low octane pass the iv.  As Z320 show the damper is not operational at these very low airflows. Lowering the jet until the stumble goes  away is the test. Thinning the top two needle stations ( 1/4 in) would be what I'd try, by a thou at a time until the stumbling is cured.

Peter

 

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After the carbs freshly adjusted (but there was not much to change) and some less viscous damper oil it's better now.

After a test drive I changed the repro dampers and fitted a pair of AUC 8114 from an old pair of HS6 from eBay. But did not drive it up to now.

My wife gave me the advice to shift earlier down....?

Ciao, Marco

 

 

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Hi,

the test drive yesterday was not perfect but much better, this way it makes more fun to drive the car.

Propably the main benefit was the minimum re-adjustment of the carbs, also the different damper oil.

I don't know what the different dampers did for the revs about 1,500,

the car came with AUC 8103, sadly on one of the dampers the thread is broken off the cap...

The "empty way" on AUC 8103 is only 1.5 mm!

Another problem is: idling revs are at abour 950-1.000 - and sometimes drop th 500!

Carb or distributor springs?

Ciao, Marco

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Posted (edited)

another experiment with AUC Z320....

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"empty way" reduced to 0.5 mm

What will be the result?

Edited by Z320
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Posted (edited)

I've not been pleased with the test drive and swoped back to AUC 8114, which are the correct ones for the car (due to Burlen).

Next test drive will show. Still I can't find the reason why the revs drop from time to time....

Edited by Z320
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  • 2 weeks later...

the issue is a little better (richer adjustment) but still not OK, so I opened the carbs today.

I did not remember the valves in the throttle discs. What are they for, please?

zZSccYPFyaYwlRk50XzKxWjlvQoeCIziz_xrnka7

 

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32 minutes ago, Z320 said:

the issue is a little better (richer adjustment) but still not OK, so I opened the carbs today.

I did not remember the valves in the throttle discs. What are they for, please?

zZSccYPFyaYwlRk50XzKxWjlvQoeCIziz_xrnka7

 

They are "over-run valves" that open when the engine is decellerating the car. In that condition a  large vacuum exists in the manifold and combustion is impaired, leading to hydrocarbon emissions. The valve allows enough mixture to sustain decent combustion ,at the expense of engine-braking, Brakes provide that.

The valves impede flow at large butterlfy openings so it is very common to solder a disc in place of the valve in pursuit of max performance.

Peter

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Posted (edited)

Hi Peter,

thank you for your explanation.

Can this valve cause my jerking problems?

I have another pair of HS6 in my workshop without this valves.

And the parts suppliers offer the with and without valves...

Ciao, Marco

Edited by Z320
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If it was leaking the tickover would be too fast to regulate. If the springs were too weak, maybe jerking would happen. Ive never read of a test but reckon a tubing sleeve fitted tight over the full length of the spring might prevent air flow.  Peter

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12 hours ago, Z320 said:

the issue is a little better (richer adjustment) but still not OK, so I opened the carbs today.

I did not remember the valves in the throttle discs. What are they for, please?

zZSccYPFyaYwlRk50XzKxWjlvQoeCIziz_xrnka7

 

My solution was to clip the valves out with side cutters and fill the holes in the butterfly with solder, then file smooth.

Works just fine and the engine has ‘braking’ when the throttle is closed.

Peter W

169C2CB1-1588-40F6-81FC-0CC4B6E597B3.jpeg

D500FB6F-EE2E-4B40-8580-310157A3656F.jpeg

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I have another problem since ages:

I can adjust the engine idling at 900 revs and everything is OK for some time,

until suddenly on the next stop the revs drop down to 400-500 and the engine dies, or rise zu 1,400-1,500.

With a slightly new adjustment everything is OK for some time, until the game beginns from new.

This are the moments I feel like Bill Murrey....

I swap them next week

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2 hours ago, Z320 said:

I can adjust the engine idling at 900 revs and everything is OK for some time,

until suddenly on the next stop the revs drop down to 400-500 and the engine dies, or rise zu 1,400-1,500.

Hello Marco,

I had exactly the same problem and, as John said, it was worn spindles. (I have Strombergs, but the same idea).

Rather than go to the time and expense of removing the spindles and fitting new ones I used two of these on each carb. and held them pushed up to the body with a bunch of O-rings, to act like a spring.

https://www.onlinebearings.co.uk/VS008-Nitrile-V-Ring-Seal.html

The OD of the O-rings is small enough to fit inside the throttle shaft return springs.

Permanent fix ? Probably not but only took about an hour to do and less than £10. (Most of which was postage…)

I’ve had a steady tick over ever since.

 

Charlie.

seal2.jpg

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Hi John,

you mean the worn spindles / "bearings" return in different positions?

Hi Dave,

you think about the same with the advance mechanism?

Hi Charlie,

you did this always only in one side of the spindle? This helps to reduce leak air and John's issue?

Ciao, Marco

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33 minutes ago, Z320 said:

you think about the same with the advance mechanism

Hey Marco,

Yes, your symptoms: Jerky acceleration, Inconsistent idle are exactly what you get when the distributor is worn & poorly adjusted.  Shaft play, weak springs, worn bob-weights etc etc all show-up worst at, or just-off, idle. The pertronix/accuspark is less sensitive to a worn shaft than points would be, but none of these adaptions mask mechanical wear in the advance mechanism.

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I used a Powerspark, because of some problems pionts again, now a Pertronix.

Yes, there is a difference, jerking is less, but with Powerspark / Pertronix, idle problems are the same.

Perhaps there are different reasons.....

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9 hours ago, Z320 said:

Hi Charlie,

you did this always only in one side of the spindle? This helps to reduce leak air and John's issue?

Hello Marco,

No, I did it on both sides of each carb. The clamp that joins the "Inter spindle" between the two carbs keeps the o-rings pushed up on the one side and the clamps on the opposite side of the spindle do the same there.

The only hassle is re-syncing the two carbs. But as I said, less than an hour to do the whole job.

The thing about the V-seals is that they are designed to do the job, you could try it as a tempory test just using o-rings.

With the v-seals they have a lip that, with the suction through the worn gap, tends to fill the gap.

(That is my theory anyway...)

Charlie.

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I think it’s weak throttle springs 

causing jerkiness with gentle foot use. 
I have the main throttle lever spring where the rod enters the engine bay and 2 on the spindle to the inlet manifold in addition to the rotary springs on the spindle themselves 

racing scrutinisers like throttle return springs it seems !!!!

C8135058-5FA8-4071-AEB7-231BE5C34B25.jpeg

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9 hours ago, Hamish said:

I think it’s weak throttle springs 

causing jerkiness with gentle foot use. 
I have the main throttle lever spring where the rod enters the engine bay and 2 on the spindle to the inlet manifold in addition to the rotary springs on the spindle themselves 

racing scrutinisers like throttle return springs it seems !!!!

C8135058-5FA8-4071-AEB7-231BE5C34B25.jpeg

Perhaps the scrutinisers are all called Terry?

https://www.gracesguide.co.uk/Herbert_Terry_and_Sons

image.thumb.jpeg.ada234de50e38c14e7aa98f3c8676868.jpeg

 


 

 

image.jpeg

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I recently posted about what I thought was a failed AC pump that had ceased to suck. I duly bought a "modern" replacement as a temporary fix and fitted it but my problems persisted.

The symptoms of my diagnosis were the spitting back under acceleration and generally jerky throttle response. Furthermore the car had become difficult to start. 

My car has done a very low mileage since a full rebuild and when building the fuel system I bought Viton tipped needle valves for the carbs and Ethanol proof wired braided  hose at great cost fitted rubber end caps and correct clips.

So in essence a brand new fuel system bearings, linkages etc also renewed or refurbished.

When the new pump did not fix the issue I decided the check thoughout the fuel system. I checked each of the sections of braided hose and worked progressively back towards the tank. (See piccy below)

I eventually had to put and airline and apply a serious blast down the line to the tank to get any fuel through at all. once clear the fuel flowed under gravity easily.

I then checked the the internal sections of the rubber hoses and found that some had gone soft and there was a flap of loose internal that restricted the flow intermittently.

So Ethanol proof hose is not always what it says.

Fitted replacement sections and the car starts without choke in this weather idles cleanly and I think I have my issues fixed.

Moral of my tale is do not discount the most simple of possibilities.

Got to be worth a look if all else seems OK and was working.

P5300106.JPG

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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

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