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Potential vacum leaks on HD6 carbs on my 1968 1800S

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Old Nov 17th, 2019, 19:40   #1
fishyboy
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Default Potential vacum leaks on HD6 carbs on my 1968 1800S

Hi all
For a while I've had an issue with the fluctuating idle on my 1968 1800S's HD6 carbs (https://www.volvoforums.org.uk/showt...ing+carburetor). I set up the carbs and get a nice steady idle at 800rpm, then blip the throttle and the idle settles at about 900 rpm.Flip the throttle again and it may drop down to around 800rpm (or lower).

I think these appear to be the symptoms of a vacuum leak. I have tried spraying car cleaner or Easy start yet (maybe tomorrow), but there is a small but perceptible movement on the throttle shaft.

Were HD6's originally bushed? If not should I try replacing the shafts (i have a pair of new ones) or will I need to get the carbs re-bushed?

Secondly how to I remove the throttle stops from the end of the shafts?

Thanks in anticipation.

Phil

Last edited by fishyboy; Nov 17th, 2019 at 19:46. Reason: spelling
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Old Nov 17th, 2019, 23:49   #2
blueosprey90
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See here from 5:50.

https://youtu.be/f7PW00fLExc

He also has another video replacing the bushings on an HD8 for what it is worth.

To me, your symptoms suggest that your carbs are not balanced and that you are running on only one carb at idle.

With a worn throttle shaft, first, if you sprayed the shafts with carb cleaner or ether, the engine would speed up. A symptom that you don't seem to be getting. Also, with worn throttle shafts, you would have rough idle and poor performance, symptom that you don't mention.

I think your throttle stops are probably pinned.

Last edited by blueosprey90; Nov 17th, 2019 at 23:52.
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Old Nov 18th, 2019, 16:11   #3
fishyboy
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Hi blueosprey90,

Thanks for the response.

I am pretty sure the two carbs are well balanced. I used both a Carburetor Air Flow Meter (photo below) and the SU Syn Tool with the two wires (as used by Thomas Bryant - https://thosbryant.wordpress.com/201...uretor-tuning/). Both of these independent tools shows the carbs were balanced at idle. I have also set the mixture getting good colored plugs with the jets set within 4 thou of each other.

When I get time this weak try searching for the leak with the carb cleaner/ether. My understanding from the 'net is that the carb cleaner/ether will cause the idle to drop if there is a leak (enriching the mixture, drooping the idle - http://www.sucarbs.com/?page_id=18). The air leak in the absence of the carb/ether will cause the idle to increase (leaner mixture increased idle - https://itstillruns.com/symptoms-car...k-6902940.html and https://www.ahexp.com/forum/the-3000...tuning.192276/)

Phil
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Old Nov 18th, 2019, 18:53   #4
blueosprey90
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Try disconnecting the carbs from the throttle linkage to see if that makes any difference.

Also check your throttle return springs.

It sounds as if you have a fairly subtle problem at idle. Just 100 rpm at idle. I'm not sure it translates to worn throttle shafts. Do you feel any play in the shafts?
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Old Nov 19th, 2019, 14:16   #5
Derek UK
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Disconnect the joining clamp between the two shafts before checking for play. Obviously you can check them better with the carbs on the bench. There should be NO play but realistically there will be just the tiniest movement. Any more than this then you will get unpredictable idle. If you can live with that that's fine, raising the idle to about 900/950 rpm helps. If you have a dynamo, setting the idle to have the charge light just flickering or just out is sensible. Bottom line, if the shafts have noticeable movement get them rebushed + new shafts. Shafts do wear and can be replaced. You might get lucky, but look on it as a short term fix.
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Old Dec 1st, 2019, 14:59   #6
fishyboy
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Default Engine rpm not retruning to "idle" when throttle opened up

Hi all,
Sorry for not responding earlier.
Went out in the car today and put 50 miles on the clock. Got back to the garage to find the car was ticking over at 1050 rpm (connected up my tacho-strobe as rev counter reads approx twice actual rpm). Turned engine off then on and engine idles at a steady 750rm. Rev up to around 14-1500 rpm and idle returns to 750 rpm. However, take the revs above 1500 rpm and the idle returns to 1000 to 1100 rpm. Tried turning the engine on and off several times and the car always idles at 750 rpm after start up. Open up the throttle above 1500 rpm and the idle allways settles back to around 1000-1100rpm. Very repeatable.

Sprayed car cleaner around throttle shaft points and no change in rpm.

Tried waggling the shafts and disconnecting clamps and no perceptible movement.

Checked the carbs and both appear to be well balanced.

Checked the return springs and they seem to be returning everything correctly.

All very odd.

Any further ideas?

Phil
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Old Dec 1st, 2019, 15:40   #7
Derek UK
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If you are sure there is no play around the carb shafts make sure that the linkage allows the shafts and butterflies always settle to tick over each time. There should be a faction of play in all of the connections that allows this. The centre "W" clamp can hold one side up if the adjustment screws have been done with it tight. Yes we all do it but it does mean that if you adjust the idle by adjusting one screw more than the other the difference in the butterfly position reverts to the setting set by the clamp as soon as you open the throttle. Also make sure that the choke cable is adjusted correctly. It should only adjust the idle speed for the first 1/8" before starting to pull the jets down.
This minor tweaking is a bit of a black art but mostly down to common sense if you know what does what. It's not helped by the various instructions that are available being different persons adaptation of the same thing. Haynes is fine but read it through a few times before you get your tools out.
Of course you could have small air leak which is temperature dependant. Extra air will increase the revs. Spray around the carb and manifold joints, when the revs are high, with WD-40 to check. Don't use carb cleaner on a hot engine.
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Old Dec 1st, 2019, 19:36   #8
fishyboy
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Thanks Derek
When I get some time I'll take a look through the whole system.
I checked the throttle stops (at the outside of each carb) were going fully back and not sticking and they looked OK.
What I don't understand is that I must of have turned the engine off six times while it was on high tickover and each time the engine started with an idle of 750rpm. Very strange.

Phil
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Old Dec 1st, 2019, 23:27   #9
Derek UK
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When the engine is running at idle again after a spell of normal running it may be pulling more vacuum in the intake system and it may have a small air leak. When you start again and go straight to idle there will be less vacuum until you drive it a bit. This may indicate an air leak as mentioned. Yes, clutching at straws a bit so check all of the intake manifold nuts, especially if you have the alloy inlet manifold, and then the carb to manifold nuts. Don't go crazy, if the gaskets are good you shouldn't have to torque them up hard. New gaskets? Now there's a thought. Not a big job and not expensive.
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Old Dec 16th, 2019, 15:32   #10
fishyboy
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Default I may have found the vacuum leak

Hi all,
Finlay got around to trying to find the possible vacuum leak on my 1968 1800S.
Having found no obvious leak around the throttle shaft on the carbs I sprayed carb cleaner around the input manifold and observed no changed in RPM. I then checked the function of the brake servo which appeared to be working fine.

As a last effort I went down to my local motor factors and bought some replacement servo hose had decided to replace the hole system from manifold to servo and also from the T-piece to the PCV. The last piece I replaced out of the PCV had a 1cm tear (see photo) right next to the Jubilee clip. Replaced the torn section. I've now run the car up to temp three or four times and found a steady idle. Still to do a road test, but hopefully the problem has been solved.

Thanks for all your inputs.

Phil
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