Series 3, Petrol RPM

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J

Jacksprat

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86
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london
Hi all, I recently added a new carb. To set up I reduced the idle until it sounded about right and then screwed in the mixture screw until the idle speed dropped and started to run rough then backed it a 1/4 turn until it settled again. All good, drives fine, battery light goes off and battery charging. I recently bought a cheap rpm tester (for another engine) and thought I would see what RPM it was idling at. The result was 400 rpm. Now Haynes says it should be 750 to 800 rpm and Glencoyne website suggests 600 rpm.

My question - is there any reason to increase the RPM? Or do I leave alone?

Thanks
 
Not entirely sure I agree with that. Most rpm meters aren't *that* inaccurate. But with a lazy/heavy engine like the Landy, idle speed isn't really that much of an issue, anyway. The only reason to have a higher idle speed is to ensure the engine doesn't stall. If it's running smoothly enough at a lower speed, then be grateful you're using less fuel. But if it struggles to pick up when you accelerate or just gets difficulty to drive without stalling it, then trim it up a bit. 800rpm is a pretty average idle speed for most petrol car engines. Engines with very light flywheels and internals (like motorbikes) need to idle somewhat faster (1200rpm-ish), otherwise they just die if the wind changes direction.
 
IMO, low tickover can cause low oil flow from the oil pump, which could starve the top of the engine of oil - the engine may run fine at 400 RPM, and have enough oil pressure to extinguish the light ....

Two checks:- expose the valve gear and make sure there is oil flow - get a proper tachometer to confirm RPM

Old Perkins were notorious for this in cold weather - leave a tractor ticking over long enough, and the top end would be worn out in short order ( relatively ).
 
If you set the rpm too low it causes things to shudder, at low tickover my handbrake lever rattles to buggery. With these engines, a couple of hundred revs extra at idle won't make any noticeable difference to mpg. They also behave a bit better running from cold. What carb did you fit?

Col
 
Fair comments, but my point still stands that just because your engine is idling nice and smoothly, it doesn't necessarily mean the rev counter/meter is inaccurate. They can idle quite well at very low speeds. And if it's really too low, it may well resonate with the vehicle and make things rattle, but since @Jacksprat said everything was good, I assume that's not the case.

And no, I wasn't seriously suggesting that a low idle speed was going to make any difference to the mpg! ;)
 
I don't know why but I've never been able to keep a constant tickover speed. For some reason, after a few seconds of ticking over sweetly the revs suddenly drop and the engine nearly stalls? I imagine there is a problem with the zenith carb. It is because of this I have the tick over set a bit high.

Col
 
The 2.25 petrol can idle at a low level with hardly any noise other than the sound of air/fuel going through the carb. Just as long oil is flowing along the rockers should be fine.
 
750 sounds too high to me, turn it down so it sounds nice and even and then blip the throttle. If it hesitates out bogs a bit, then turn the revs up a little
 
Colthebrummie said:
Even with my engine at its lowest revs I get about 40-45 psi of oil pressure. At least I do on the days the oil guage is working.

Col
Click to expand...
Cool - 40 psi is good.

The potential problem is that this is the pressure and not the flow, which might not be enough to make it to the rocker shaft in sufficient quantity....
 
Disco1BFG said:
Cool - 40 psi is good.

The potential problem is that this is the pressure and not the flow, which might not be enough to make it to the rocker shaft in sufficient quantity....
Click to expand...
This is true I spose, it only proves I've got pressure as far as the pressure switch thingymabob.

Col
 
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