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 Post subject: ROUGH IDLE WHEN HOT
PostPosted: Mon Oct 09, 2017 5:31 pm 
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Joined: Wed Dec 18, 2013 5:05 pm
Posts: 214
I expect this issue has been done before so I went back through a few pages but couldn't find anything.

My engine idles at 800 rpm as it warms up and when at operating temperature. After a longish drive when the engine is really warmed up, but with the coolant temperature still at 180-185 degrees, it will idle normally as you come to a stop and for maybe 15-30 seconds but then the idle revs drop and the idle gets rough. Sometimes it recovers enough to keep idling and sometimes the engine stops.

I drove down to Bareco today and had no problems with engine idle on the way down, although I didn't idle much after the M5 run. The problem started in traffic after the run back up the M5.

I had slight pinging on the way to Bareco so retarded the timing slightly while at Bareco. The engine didn't ping on the way home, even when I tried to get it to, and has good power up to 4800 rpm.

The tappetts were set a month or so ago and needed little adjustment, plugs are all at 32 thou, points are near new and set at 15 thou. I checked the carbie balance a few weeks ago and they were fine without adjustment. I didn't take too much notice of the mixtures as they haven't needed resettting for awhile.

I have had the idle problem before and resetting the mixtures usually sorted it. So off with the air filters and checked the mixture by lifting the piston. Rear carbie was fairly rich, front carbie a bit lean so adjusted until I had a slight increase in rpm with the piston lift on both carbies. I did notice the rear carbie jet adjusting nut is much higher than the front carbie jet nut, maybe a full turn or more difference. Not sure that is relevant.

I then went for a drive and again it idled ok for 15 or so seconds as I came to a stop and then dropped and went rough.

I left the car standing in the garage for about 30-40 minutes. I just went out and started it and it idled beautifully at 800rpm for about 5 minutes. However, I know when I go on a longer run the rough idle will return.

I would like it to idle better when the engine gets hot. Ideas please.


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 Post subject: Re: ROUGH IDLE WHEN HOT
PostPosted: Tue Oct 10, 2017 8:09 am 
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Joined: Tue Aug 02, 2016 8:57 pm
Posts: 51
Location: Nowra NSW
Possibly might be getting blowby past the throttle shaft or shafts. Then moving to a tighter spot then getting rich with less air. ????


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 Post subject: Re: ROUGH IDLE WHEN HOT
PostPosted: Tue Oct 10, 2017 10:18 am 
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Joined: Wed Dec 18, 2013 5:05 pm
Posts: 214
I hope not Ian. The carbies are only 5 or so years old from new. But a possibility.
It only happens when the engine is really heat soaked after a lengthy drive.
I seem to recall a discussion with Aladdin many many years ago that the various tolerances on these engines can go out of whack when the engine gets hot and a rough idle can be the result.


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 Post subject: Re: ROUGH IDLE WHEN HOT
PostPosted: Wed Oct 11, 2017 11:17 am 
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Joined: Thu Dec 19, 2013 3:40 pm
Posts: 106
Hi
On the info available I would lean towards a needle valve problem on the rear carb. If it has a slow leak then you would not notice it when driving but as it idled the level would slowly build until it started to affected the idle. It is not the float level set but the level would slowly build.
Hope this helps.


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 Post subject: Re: ROUGH IDLE WHEN HOT
PostPosted: Wed Oct 11, 2017 11:10 pm 
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Joined: Thu Dec 19, 2013 11:22 pm
Posts: 42
Have you checked that the needle in the slide valve/piston is set in the right place and the screw is tight?

Graham


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 Post subject: Re: ROUGH IDLE WHEN HOT
PostPosted: Thu Oct 12, 2017 1:35 pm 
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Joined: Wed Dec 18, 2013 5:05 pm
Posts: 214
Thanks gents.

I spoke with Aladdin yesterday evening before I saw these posts. Having followed his advice I have confirmed it is the rear carbie which is flooding No 3 and 4 cylinders when the engine is at idle for more than 30-60 seconds.

The rear carbie jet was a bit off centre so I reset it last night. I drove up to Pittwater this morning and the problem is still there but the car is running well otherwise.

I spoke with Midel and they recommended draining the auto transmission fluid out of the dampers and using engine oil, which I have done. It hasn't solved the problem (didn't expect it to but as they advised it I felt I should take their advice).

I am heading to Midel to get new needles and seats and will change them this afternoon.


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 Post subject: Re: ROUGH IDLE WHEN HOT
PostPosted: Thu Oct 12, 2017 1:48 pm 
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Joined: Thu Dec 19, 2013 9:35 am
Posts: 60
When the idle gets rough try oiling the shafts either side of each carb, if the idle is affected it would indicate air induction past the shaft.

Check the spindle stops actually prevent the butterflies from moving past the right idle spot and also the various joints in linkage chain which might affect the cold Vs Hot throttle position.
How secure are the manifold gaskets or the vacuum advance connections?

After that I agree that a needle check might reveal a loose one, or maybe even a loose butterfly.

Rob


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 Post subject: Re: ROUGH IDLE WHEN HOT
PostPosted: Thu Oct 12, 2017 4:57 pm 
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Joined: Wed Dec 18, 2013 5:05 pm
Posts: 214
As it turned out, the correct answer was none of the above.
I drove up to Pittwater and the idle problem was still there after centering the jet and changing the damper oil.
I drove over to Midel and bought new needles and seats, and some SU oil just to see what it was like.
Changed the needle and seat and the problem was exactly the same.
Went back to basics and pulled the plugs, all OK. Shut the garage and turned off the lights and inspected the leads with the engine running to see if there was any sparking/leakage. Nope.
Pulled the dizzy cap off and checked for cracks etc, nope.
Thought the points gap looked too tight. Got out the feelers and they were only about half the .015".
Reset the points.
Idle was a bit lumpy but the problem had disappeared. Retuned the carbies which were now showing as too lean and the idle seems to be as it should be.
Why didn't I check the pojnts earlier, well I had done them a couple of weeks ago in a full tune up and because the symptoms didn't indicate a points problem. The engine ran OK, the idle was fine before it suddenly went off and the engine had good power, although the drive to Pittwater did reveal power was down a bit on the hills at Roseville Bridge and up Wakehurst Parkway.
In hindsight, I suspect because the plugs weren't getting a good spark after a short period at idle they would carbon up to the point the idle would suddenly drop off. The rear plugs carboned up more than the front, probably because the rear carbie was a bit richer than the front, leading to misidentification of the rear carbie being the cause.
Anyway, a process of elimination sorted it out.
I would have taken a fair bit longer to fix it without the ability to bounce ideas with other owners. Thanks all.


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 Post subject: Re: ROUGH IDLE WHEN HOT
PostPosted: Thu Oct 12, 2017 6:35 pm 
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Joined: Tue Aug 02, 2016 8:57 pm
Posts: 51
Location: Nowra NSW
Glad you got it sorted. So now you can enjoy the trip to Tassie without that bothersome problem. Well done.


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 Post subject: Re: ROUGH IDLE WHEN HOT
PostPosted: Thu Oct 12, 2017 8:01 pm 
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Joined: Wed Dec 18, 2013 5:05 pm
Posts: 214
One problem sorted, another waiting in the wings to take over.
Here's to a trouble free Tassie trip for everybody.


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