Engine wont go back to idle unless I blip the throttle

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SecombeS
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#1 Engine wont go back to idle unless I blip the throttle

Post by SecombeS » Thu Jul 26, 2018 9:17 am

I have just put the engine back in the car having changed the clutch and spigot bearing. On starting the engine it seems to run OK when cold and on choke. Once the engine has warmed up it does not drop back to idle when gently taking my foot off the accelerator. The engine will stay running at up to 1500 rpm. However, if I blip the accelerator it quickly goes down to an idle speed of 650rpm. I have checked the linkage and it is not stiff. All springs are present and no more than 3 years old. My only suspicion is whether the carburettors are not operating smoothly. Prior to the clutch change the engine had not been started for 9 months. Any ideas?
Steve
1968 Series 2 UK FHC 1R20020

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malcolm
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#2 Re: Engine wont go back to idle unless I blip the throttle

Post by malcolm » Thu Jul 26, 2018 9:23 am

Sounds to me like the linkages are simply not returning to idle efficiently. the "Blip" simply gives the springs more momentum. Although the springs are new, perhaps check that all is clean so nothing is making it hard for the linkage to return properly. Have a look under the bonnet with the engine running. Raise the revs manually by hand on the linkages. Let them go back slowly so your revs stick at the 1500. Then push the linkage further closed. This will show you what's sticking.
Malcolm
I only fit in a 2+2, so got one!
1969 Series 2 2+2
2009 Jaguar XF-S

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SecombeS
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#3 Re: Engine wont go back to idle unless I blip the throttle

Post by SecombeS » Thu Jul 26, 2018 9:49 am

Thanks. I also suspected the linkages (which are stainless steel) but they seem OK, if a little loose. I guess I could try putting stronger return springs on the carburettors.
Steve
1968 Series 2 UK FHC 1R20020

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christopher storey
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#4 Re: Engine wont go back to idle unless I blip the throttle

Post by christopher storey » Thu Jul 26, 2018 9:50 am

There are so many joints in he linkage, and it only needs one to be slightly misaligned, or overtightened ( on the ones where there is a screw in end cap ) for this to happen. Also, if the bulkhead bush on which the shaft rotates is worn, this can cause similar troubles . I should first disconnect the cross rod from the triangular bulkhead swivel, and see if this helps. If not, then work forwards towards the carbs trying each joint in turn

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mgcjag
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#5 Re: Engine wont go back to idle unless I blip the throttle

Post by mgcjag » Thu Jul 26, 2018 9:52 am

You could try something like this....
Image
Steve
1969 S2 2+2 & Building a C type replica

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Heuer
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#6 Re: Engine wont go back to idle unless I blip the throttle

Post by Heuer » Thu Jul 26, 2018 10:36 am

Is the spring in the accelerator housing (4.2 cars) in good condition? As others have said disconnect the linkage at the throttle at the slave shaft and then manually slowly accelerate the engine up to 3,500 rpm and slowly back it off. If the problem is still there it is with the carbs, if not it is with the linkage. There is adjustment at the bush assembly to ensure the slave shaft is parallel and does not bind. Be wary of replacing the bush assembly (3.8 cars) with new because in my experience they don't fit!

it may also be worthwhile disassembling the the linkage and cleaning all the springs, screws and ball joints. They should not be greased, just lightly oiled. And finally can you please put the series of car you have in your signature as the advice depends on that.
David Jones
S1 OTS OSB; S1 FHC ODB

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politeperson
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#7 Re: Engine wont go back to idle unless I blip the throttle

Post by politeperson » Thu Jul 26, 2018 4:51 pm

When I had SUs and rods, I had this problem until I had the idea of setting the rods with the pedal jammed open to full throttle.

In my case it allowed full movement of the mechanism and snapped the butterfly's completely shut.

I also prevented me from stretching the rods a little bit every time I buried the throttle pedal with my foot, as the pedal would hit the stop before the rods got stretched.
James

L.J.K. Setright was right.
"You just cant beat a good E-type"

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SecombeS
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#8 Re: Engine wont go back to idle unless I blip the throttle

Post by SecombeS » Thu Jul 26, 2018 7:44 pm

Thanks for the comments. I have read them all with interest and am trying to following the suggestions where I can. I have adjusted the bulkhead bracket holding the rod to the carburettors. Although the engine still doesn't go back to idle quickly this has definitely helped to reduce the over-revving. Will continue the tweaks tomorrow!
Steve
1968 Series 2 UK FHC 1R20020

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ALAN COCHRANE
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#9 Re: Engine wont go back to idle unless I blip the throttle

Post by ALAN COCHRANE » Thu Jul 26, 2018 8:48 pm

Steve

That could be a picture from my engine bay. I ended up attaching a spring here as well.
My problem was that the throttle would jam wide open, fortunately I discovered this in the garage not the open road. No amount of adjustment would stop this from happening intermittently.
As Christopher has stated the 3.8 throttle linkage is prone to jamming for any number of reasons. I suspect in my case the bulkhead throttle bush was the culprit. I'd replaced the original and no amount of realignment would fully cure the problem.
Coincidently my problems also started after reinstalling the engine. Just shows how little tolerence there is with the 3.8 setup.

Cheers

Alan
1962 S1 OTS-850389,1968 Triumph TR250, 1971 Triumph GT6 Mk3, 2008 Porsche Boxster RS60 Spyder

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MarkRado
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#10 Re: Engine wont go back to idle unless I blip the throttle

Post by MarkRado » Fri Jul 27, 2018 5:42 am

Hi Steve, I had the same problem, albeit on my 3.8;
I changed suspect parts in the linkage and re- adjusted it spot-on, only to find that one of the butterflies needed a bit more force to close completely (I assisted each of the return springs one after another while engine running).
Checking the carbs individually it turned out that one of the throttle shafts was a little stiff - I put a drop of thin machine oil down the shaft on each side of the gland cork seals. Worth a try, but no success in my case.
I removed the carb and checked/ readjusted the butterfly, but could not find a satisfactory alignment (check for gaps against bright sunlight). Seems that the reaming for the bushes and gland seals was not done perfectly. Carbs will go to David Lonsdale this winter; the shortterm remedy was to increase spring tension by adjusting the spring lever. But accelerator control is very aggressive now, makes for an embarrassing driving style from standstill :oops:
Mark
1963 OTS 880436 - since 1988

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SecombeS
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#11 Re: Engine wont go back to idle unless I blip the throttle

Post by SecombeS » Mon Jul 30, 2018 4:01 pm

I am beginning to suspect stiff carb spindle. The engine seems to enjoy sitting at 1500 rpm unless I blip the throttle. I have tried all sorts of adjustment of the linkage but without success. If I disconnect the linkage and just rev using the final link to the control arm it still sticks. I have noticed that one the carbs seems to be sticking a little and only shuts down completely if I nudge it to idle with a screwdriver blade. I will try some SU oil on it to see if that frees it a little.
Steve
1968 Series 2 UK FHC 1R20020

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Heuer
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#12 Re: Engine wont go back to idle unless I blip the throttle

Post by Heuer » Mon Jul 30, 2018 4:06 pm

Have you checked the tension on the three throttle return springs? The hook onto a thin metal frame which bend easily if manhandled.
David Jones
S1 OTS OSB; S1 FHC ODB

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#13 Re: Engine wont go back to idle unless I blip the throttle

Post by mgcjag » Mon Jul 30, 2018 4:17 pm

Hi Steve...after checking the springs as posted above you need to seperate the 3 carbs at the throttle spindles...then you can look at them individually.......also check the pistons to ensure they are not sticking in the dashpots...in the service manual there is a very detailed section on the carbs.... Steve
Steve
1969 S2 2+2 & Building a C type replica

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SecombeS
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#14 Re: Engine wont go back to idle unless I blip the throttle

Post by SecombeS » Mon Jul 30, 2018 4:57 pm

So.………….

I had earlier already replaced the return spring on the suspect carb but no difference. I have just retrieved the original spring that had one quite a long straight end. I cut this in half and then created a new hook using needle nose pliers. I replaced this on the carb and the idling problem was cured immediately. I can now rev to 2000 rpm and gently take my foot off the throttle and the revs drop back immediately. What would I do without this forum. Thanks to everyone!
Steve
1968 Series 2 UK FHC 1R20020

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