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 Post subject: Sprint Idle
PostPosted: Wed Nov 06, 2019 9:43 pm 
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Joined: Tue Oct 15, 2019 8:00 pm
Posts: 16
Just fitted alloy carb mounts and tweaked the carbs and sorted the choke cable properly to get a decent fast idle and easy starting from cold. All is good BUT (there is always a but!) I have got the idle set at around 800 rpm (fast idle 1500 rpm) but at 800 rpm the engine is not as smooth as I would like. It is OK at 1500rpm. Happy with the carb balance and mixture (CO around 3.5% and not much reaction when lifting the pistons). The engine will "happily" idle down to 500 rpm But anything less than 1000 rpm and it gets a bit "lumpy" not excessive but not smooth. Timing is spot on and both mechanical and vacuum advance are working OK. Carbs are recent recons from Robsport.

Not having had experience of the Sprint engine before the question is does the engine naturally idle a bit lumpy or is there something amiss somewhere? Other than the lumpy idle it seems to run great, instant pick up, revs freely and pulls like a train - loads better than before.

Any comments gratefully received.

Roger

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1975 Sprint Man O/D in Honeysuckle Yellow

Too many cars, too little time!


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 Post subject: Re: Sprint Idle
PostPosted: Thu Nov 07, 2019 11:02 pm 
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Joined: Mon Dec 03, 2012 6:04 pm
Posts: 1017
I had a similar problem, in my case it turned out to be duff cheap rotor arm ( I use the large cap and the 1300/1500 rotor arm), ignition timing slightly out and vacuum leak (cracked breather).

Now you say the mixture is fine and you obviously have a gas analyser and the timing is fine. Have you checked for a misfire caused by one of the plugs not firing properly at low revs?

Another suggestion, is I don't know how accurate the rev counter in a Dolly is. It could be out a bit, though others will be able to clarify that.


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 Post subject: Re: Sprint Idle
PostPosted: Fri Nov 08, 2019 10:51 pm 
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Joined: Sun Aug 21, 2011 4:12 pm
Posts: 5196
Location: Highley, Shropshire
In my general experience, the idle on a Sprint is never silky smooth, you wouldn't be standing a threepenny bit upright on the cambox, even if it WAS flat! I think it's something to do with the limits the single cam imposes on this engine!

If the idle is constant and unwavering, i'd call that good enough!

Steve

PS I've just fitted a set of alloy mounts to a Sprint myself - and had to strip and rebuild the linkage cos I couldn't then get the hot idle below 1800RPM! All is well now though, it's crisp throughout the range and idles happily at 800rpm in neutral and 600rpm in drive (It's an auto so a good idle is pretty vital) This one too is a tad lumpy but it's no big deal and it doesn't cut or hunt.

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2 door '73 Toledo with Vauxhall Carlton 2.0 8v engine OWF 797M (The Carledo)
'78 Sprint Auto with Vauxhall Omega 2.2 16v engine EGP 247T (The Dolomega)
'91 Cavalier 2ltr 8v auto
'95 Cavalier 2ltr 16v auto
Spectrum Auto Services, Servicing, Repairs, Electrical, Recomissioning, MOT prep, Trackerjack brake fitting service.
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 Post subject: Re: Sprint Idle
PostPosted: Sat Nov 09, 2019 2:31 pm 
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Joined: Tue Oct 15, 2019 8:00 pm
Posts: 16
Thanks, I have checked (actually replaced) the breather pipes and also checked timing and mechanical/vacuum advance - so they are all OK. There is no misfire, the exhaust note is pretty stable as is the idle it is just "lumpy" i.e. the engine moves about quite a bit (yes the mounts are a bit soft - on the todo list - and I don't think the front stabiliser bar thingy helps as it tends to make the engine skew rather than rock which accentuates the problem)

Steve, I think that answers the question - "idle on a Sprint is never silky smooth" - and is kind of what I assumed. My son has a 106 Rallye with the 8 valve 1.3 engine and the idle on that is similar - i.e not smooth, so as you say could be related to the more aggressive cam timing.

Roger

_________________
1975 Sprint Man O/D in Honeysuckle Yellow

Too many cars, too little time!


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 Post subject: Re: Sprint Idle
PostPosted: Sun Nov 10, 2019 9:59 am 
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Joined: Mon Dec 03, 2012 6:04 pm
Posts: 1017
Quote:
Thanks, I have checked (actually replaced) the breather pipes and also checked timing and mechanical/vacuum advance - so they are all OK. There is no misfire, the exhaust note is pretty stable as is the idle it is just "lumpy" i.e. the engine moves about quite a bit (yes the mounts are a bit soft - on the todo list - and I don't think the front stabiliser bar thingy helps as it tends to make the engine skew rather than rock which accentuates the problem)

Steve, I think that answers the question - "idle on a Sprint is never silky smooth" - and is kind of what I assumed. My son has a 106 Rallye with the 8 valve 1.3 engine and the idle on that is similar - i.e not smooth, so as you say could be related to the more aggressive cam timing.

Roger
I don't think it should moving around excessively, once I had mine setup properly it wasn't doing that. Saying that i have replaced all but one of the engine mounts, including using the alternative gearbox mount recommended here, plus a new steady bar. So that could be the reason for the difference.


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