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Conference terri::cars_uk

Title:Cars in the UK
Notice:Please read new conference charter 1.70
Moderator:COMICS::SHELLEYELD
Created:Sun Mar 06 1994
Last Modified:Fri Jun 06 1997
Last Successful Update:Fri Jun 06 1997
Number of topics:2584
Total number of notes:63384

1168.0. "slight misfire when going downhill" by NSDC::SIMPSON (File Under 'Common Knowledge') Thu Jul 26 1990 18:01

I have the dubious distinction of having BOTH my cars showing the same
irritating habit.

When I coast down a moderate hill in gear (i.e. no accelerator, couple of
thousand r.p.m.), the engine starts to pop n' crackle - a slight misfire.

Anyone any theories on what could be going on - and whether I should bother to
investigate further!

Cheers

Steve
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1168.1Heard that before!!IOSG::REESThu Jul 26 1990 19:404
    Don't know anything about it, but my dads Granada does the same
    thing...!!
    
    
1168.2ANNECY::MATTHEWSM+M Enterprises. Thats the CATCHThu Jul 26 1990 19:435
Don't all cars do that ??? All mine do. As to the reason, it is
probably related to the engine sucking in petrol/air due to the
engine turing over, rather than you pumping in the petrol/air
when you have the accelerator open ... this could be complete BS
by the way.
1168.3Trabi's don't!HOO78C::DUINHOVENDutch treatThu Jul 26 1990 20:039
    Al cars I've driven do the same.
    
    The only ones, which don't are two-stroke ones like Trabi's.
    They don't have much "brake" performance downhill anyway.
    That will improve, if ignition is switched off downhill.
    You'll have some fun, when switched on again at the end.....
    
    Hans
    
1168.4I don't think that its BS!!NSDC::SIMPSONFile Under 'Common Knowledge'Thu Jul 26 1990 20:163
You could all be right - I've only noticed it because I've got the window
open in this hot weather. Here in Switzerland there a lot of hills to pop
up and down, so I guess that I notice it rather more than I would in England.
1168.5Carburettors onlyIOSG::MARSHALLHarry PalmerThu Jul 26 1990 20:3412
Petrol isn't pumped into the engine (unless you have injection, but assuming
it's a carburettor...) the throttle controls the size of orifice through which
the petrol / air mixture is sucked, hence the quantity of mixture reaching the
engine.  If you close the throttle but (due to going downhill) the engine revs
stay high, you're bound to get the wrong quantity and mixture, hence mis-firing.

If it's noticeably bad, you may find the idle speed adjustment (ie the small
amount the throttle stays open when you take your foot off the pedal) or
mixture adjustment (ie ratio of air to petrol) need adjusting.  There may also
be a problem with the bypass-idle circuit, if your carb has one.

Scott
1168.6But...ANNECY::MATTHEWSM+M Enterprises. Thats the CATCHThu Jul 26 1990 20:497
re: .5

 Could the suction created with the revs high and zero throttle opening
 cause the throttle to open, and gulp in some petrol/air ??? Suppose it
 depends on how strong the throttle return spring is ...

Mark
1168.7The cars in question....NSDC::SIMPSONFile Under 'Common Knowledge'Thu Jul 26 1990 21:107
Thanks for the info. so far. 

.5 shows me that I ought to say what sort of fuel feed I have - 'cos one the
cars does have injection.

I've got a VW Transporter - transitorised ignition; still has a distributor
	   VW Golf GTI.
1168.8race tuned exhaust?SHAPES::BUCKLEYCBareback on the SharkThu Jul 26 1990 21:114
    Having a small hole in the exhaust, near the engine can cause popping
    
    Chris
    
1168.9GVA01::STIFFPaul Stiff, EHQIM-OIS DTN:821 4167Fri Jul 27 1990 20:264
    My modified 1275GT Mini does that, but I retarded the ignition a
    few degrees, and that and the pinking stopped.
    
    Paul
1168.11'Cool, man'!CSSE::WAITEFri Jul 27 1990 23:167
Back in my high school days, a properly sounding 'poping off' sound
was thought to be a sign of a well tuned/timed engine and was 'cool'!

Nothing quite like a slightly warmed over Ford V8 flathead to produce
the desired effect.

Dick_who_is_showing_his_age