Slow Idle

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Joe
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Slow Idle

#1 Post by Joe » Tue Dec 22, 2020 9:05 am

Hi,
A question about an intermittent idle problem, what happens is that car runs well normally(had carbs rebuilt and tuned0 it is a 77 TC auto HS6 SU's. However sometimes especially if it is hot( this is Australia hot not UK hot sorry!!!) It would be running well and sometimes when I pull up at traffic lights the idle drops right away to nearly stalling and the symptoms feel just as if I pulled up and pulled out the choke, it runs rich and smoke comes out the back. However when I pull away it does it smoothly and runs well afterwards and may not do it again for quite a while? When it is cool does not happen anywhere near as much. Have been asking all the carb experts I know with no real answers. When I had carbs rebuilt had jets changed from the waxstat to manual.
Cheers Joe

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valencia
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Re: Slow Idle

#2 Post by valencia » Tue Dec 22, 2020 7:01 pm

Hi Joe,
it does sound quite like fuel starvation due to fuel evaporating because as you say it's worse when hot. As soon as the car pulls up to idle- no air flow goes through the engine compartment and a much smaller amount of fresh fuel is pumped through. It adds up to lots of heat boiling a smaller quantity of fuel. Modern fuel, E5 and the like are particularly susceptible to this. I would attempt to reduce fuel temp by insulating fuel pipes and heat shields for the carbs, also re-routing pipework may help although I have only carried out the heat shielding and insulation myself. It seems to have worked.
I do hope that you can get it sorted out.
Michael.

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Re: Slow Idle

#3 Post by johnnydog » Tue Dec 22, 2020 8:24 pm

In addition to Michaels suggestions, a 77 car on HS6 carbs was originally fitted with heatshields mounted between the carb body and inlet manifold. Unfortunately, they were relatively brittle and easily break whilst working in that area of the engine bay. Often they were never replaced if damaged / broken. If yours doesn't have any fitted, then Chris Witor has had them remanufactured. This may be worth doing first assuming that everything was done correctly during their rebuild.
I would also just check that the piston body in each carb is free falling when lifted manually when the engine is up to normal temp. I have known them to stick in a slightly raised position on a hot engine, which has sometimes been cured by just slackening the top dash pot cover screws and ensuring the piston falls freely by continually lifting it with your finger whilst you retighten the three screws a 'smidge' at a time. Don't be tempted to tighten each screw fully before tightening the next one - you just need to tighten them a little at a time working round the three screws per carb until tight, ensuring the piston still lifts and falls freely whilst nipping them up.
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Joe
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Re: Slow Idle

#4 Post by Joe » Thu Dec 24, 2020 10:42 pm

Thanks for replies, gives me something to hunt down, mine had one heat shield missing when I bought it, so got a set from CW a while ago. Will certainly look at carb slides and fuel line routings.
Cheers Joe

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Re: Slow Idle

#5 Post by johnnydog » Thu Dec 24, 2020 11:04 pm

I didn't mention it, and I'm sure you are aware, but the checking the free movement of the piston should be done with the damper removed.
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1967 Mk1 2000 in Gunmetal Grey
1969 Mk1 2000 in Royal Blue
1970 Mk2 2000 in Valencia Blue
1972 Mk2 2.5 PI in Triumph White
1973 Mk2 2.5 PI in Sienna Brown
1976 Mk2 2500S in Carmine Red

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