warmup.....motor speed change....why?

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Mar 19, 2005
Just a question ......

I noticed this many years ago, and last night, it happened again and I though...why not ask?

Typical get ready for a ride. Roll bike out and get it started, with or without choke as required. Bike starts, choke off and lower the revs to allow the oil to get out of the sump. Motor is running at about 500 or so and I go into barn and start to get my leathers on. This takes about five minutes or so. At about this point, long after the sump is empty and the bike has gradually increased in idle rpms...the motor suddenly picks up about three or four hundred rpms and settles into a faster idle. Almost like something warmed up, opened, released, or lord knows...the bike just runs faster.

I have a Boyer in it, always did, normal 932's, same happens with or without oil cooler, no choke on....absolutely nothing that would cause this...that I know of.

This is where you come in. Anybody else noticed this? Any ideas why? No...I'm not dreaming..this I have noticed at least a thousand times, over at least 30 years, and it just annoys me that I can't figure out a reason why this happens. It causes no damage etc, but it makes me curious. Some electric thing? Is the boyer warming up and changing the timing? Check your machine out, and let it warm up for five minutes and listen for sudden increases in the rpms....and let me know what you think.....
Do you really mean 500 rpm ? I don't think that I have ever tried to get a steady tickover as low as that.

I suspect that whilst the Amals are called "Concentric", they really meant "Eccentric," at least in behaviour if not in float chamber location.

I have never noticed a consistent change in rpm but it does seem, especially if it is sitting on the mainstand that the whole thing is juddering quite a bit so I assume the fuel is getting a good frothing and the float needle is having to work quite hard to get back on it's seat. On the mainstand, the power train has an effectively rigid connection to the concrete or whatever

Might it just be that as the rpm increases slightly with temperature, it reaches a point where the low frequency shakes cause it to over-fuel ?

On a similar point, I have found that if I overtighten my Isolastics (easy to do with the Mk111), it won't rev past 5000. I can only assume that fuel frothing in the non-rubber mounted carbs is to blame as backing the adjusters off a fraction is an instant cure.

You could try letting the bike warm up standing on a rubber mat and see if it makes a difference. Getting someone to hold the bike is not an option because they wouldn't be able to resist blipping the throttle :lol:

One other thought, are you sure that a wet sump clears in five minutes at a low tickover ? The manual suggests 3-4 minutes before dipping the oil but I reckon that is a certain way to end up overfilling the oil tank. I always take mine for a fast blat before checking the oil, otherwise the readings are not reliable.
warmup speed change

I have noticed the speed change. I have always assumed the engine and carby's have warmed up and the fuel is now able to vapourise and mix with the air in the correct proportions to give an optimum burn and therefore more power more revs.
Sounds good to me ! but is it true?
hewhoistoolazytologin said:
Motor is running at about 500 or so and I go into barn and start to get my leathers on. This takes about five minutes or so. At about this point, long after the sump is empty

From the oil pump flow figures given by John Hudson I worked it out that to empty the sump of about 1 Litre/Liter (roughly 1 US Quart) of oil with the engine running at 500 RPM would take approximately = 6 minutes!
Dear He who,

I can say this phenomina is not just a Norton Commando thing, A Bevel Ducati S2 with 40mm Delorto's used to do a similar thing, it would idle at a reasonable rate from cold then after a period it would speed up and stabilise. Now that was a wet sump motor.

Note I'm not sure warming an engine up at low rpm's is all that good for it. Camshaft lobes etc typically get worn out form this pracitce.

You are probably better to get all you gear on, crank her up and hit the road albeit gently for the first 5 miles.

I bet this starts a good yarn, hope so!!

Cheers Richard
Fast idle

I have a 71 Norton Command 750 Highrider. It runs perfect, starts first kick and idles great for the first minute or so. Once it warms up and i pull into a stop sighn, the idle just wants to take off from 800 rpm to about 13-1500. Like I said, it runs so good, dont want to mess withi anything at present. I have been told that it could be a worn advance unit down in the points area?? semper fi devildog
Thanks...all of you. Will try to address all ideas in one shot...here goes!

Yes...about 500 rpms. Motor is cold and at a closed throttle, it sits at about this speed.

Machine almost never started on main stand, due to rear sets, the fold up foot peg will fall down and rip an ankle up if on center stand.

When I say sump is cleared, I mean that the oil has long since stopped being pumped up the breather into the tank, whether the pump has cleared the bit of oil left in the sump that didn't go up the breather, don't know but tank is full and oil is getting pumped in.

Frothing...think not. The speed change is sudden, consistent, always happens about at the same time.....not irradic or inconsistent.

Six minutes....lord knows, never really timed this thing...but it is consistent, and the same with a weeks, days, or months accumulation of sump oil. This seems to be a temp or time related thing, would the sump finally being free of oil reduce the work the motor has to preform so much, that the motor can suddenly run much faster/freer?....

Always have warmed a cold motor on a low idle, worked now for 30 years, and still haven't had to split the cases...whether the lobes are worn, think not, but hard to tell, still look good....not tempted to change my warmup after all these years...something must be good about it.

Idle goes also up to 1500 or so when it warms up, real tricky to try and carb adjust this down to 1000 or so....had it a couple of times over the years, but hard to do. Carbs are only a year old by the way, and this rpm change, happened with old carbs too..

Common sense tells me this rpm increase is caused by a sudden change in timing after an initial warmup period.....Boyer installed...is this a possibility? Do Boyers warm up to operating temperature? I know one of the gurus must know if this can happen....Dynodave....got an idea?......maybe he, or one of the other gurus has the low down.

In the long run...this isn't an important question, as it doesn't adversely affect the running of the machine.....but after hearing the motor do this thing for so many years...and I know curiousity killed that cat...I'd like to know anyway :?
increase idle


I just commented about this in another string. Consider this. Idle increasing as the engine warms up is typical of an Amal concentric worn slide. Air gets passed the expanded carb body/throttle slide interface.

My original combat engine 932/19 & /20 carbs were sleeved 8-9 years ago and have gone 16,000miles. I get just a bit increase in idle when the engine warms up, but not as much as you mention.
I have new bodies and chromed slides, and this phenomenom happens with the old worn out carbs too...the point that the maotor speed increases with temp...that is known by all...my question remains ...why the sudden large increase in revs...almost like a door has been opened...
idle increase

OK, not so obvious. Look for other air leaks. What about the balance tube between the intake manifolds, carb bowls come loose.

If you can't find anything, go back to basics - ignition timing then carb adjustment starting from factory settings.
I reckon you're going to have to get hold of one of those nifty hand held infra red thermometer thingies and check out the temperature theory. Videoed evidence would be helpful :)

Apparently modern-day mechanics use them to check which cylinder is miss-firing and that sort of thing. I don't know why, are they scared of grasping a hot down-pipe ? Can't they spit accurately ?

I've been thinking about the Boyer aspect. They are certainly very current sensitive. If the cabling or internal wiring got hot and increased in resistance...? No, it couldn't make sense could it ?
Are we sure this doesn't happen on all Boyers? Is this happening, and no one else notices or maybe is just simply smart enough not to care?

Remember, this thing has happened since ever and ever...not a new occurance, and this has happened through carb changes, head rebuilds, ring replacements...etc...etc. The only thing I can place as being consistant, is that it always happens, and there has always been a Boyer in place. This is not...repeat...not, an important thing here, as the machine runs great and this is only bothering me, because I can't figure why it happens...sort of like when a motor will pick up speed when you shut the headlite off...less current pull, more for the ignition, and the revs pick up. This is JUST curiousity and wondering whether I'm the only one that has this happen. So don't sweat it.....if you have an idea...maybe we can nail this down...if not...oh well.....so be it... :wink:
G'day Hewho,
My 850 doesn't wet sump because i have the spring loaded non return valve and i haven't noticed any appreciable difference in revs apart from the normal warm up period, so whether this could account for the phenomenon you have i don't know, the climate here is sub tropical also maybe this is another thing.
I've got it !

You've got mice nesting in your exhaust system. When their fur starts to singe they hop out and the motor can run properly again. :lol:

I must say that I've never really noticed the symptoms and I have had a Boyer for 20 years plus. Mind you, I never leave mine ticking over on that long spiders-leg of a sidestand.

Are you sure that it's not a fuel level thing ? Do the revs increase equally on one both cylinders ? If so, I suppose that it must be ignition.

I've got a set of new points and backing plates if you want to give them a go !
could it be anything to do with air temp. at the intakes as the motor warms,air temp affecting air density / mixture ratio, filling of the combustion chambers ??.Ride safely,James.
The mice have been told to leave the area.

Have to research, but I think I have it. Thought of it while waiting to pass into never-never land.

Like I said, it is like a door opening...sudden and very noticable. what if.....during the initial warmup, at a very low rpm, the motor is running mostly off the battery, the alternator isn't getting a sufficient speed up to produce a good current for use by the boyer. The Boyer runs off the battery for a good five minutes or so......until, the motor has warmed up, the motor speed has slowly increased, and the alternator finally reaches the point where it produces enough juice to throw the voltage regulator into the status where the electrical power is going to, and not from the battery. There is now enough currrent to charge rather than discharge.
At this time, the Boyer also gets enough currrent to run as it should have run the whole time.....says....thanks a lot , and advances the spark to where it should have been in the first place and the motor picks up suddenly speed.
All because the Boyer hadn't had enough current, until the motor warmed up and reached the magic speed where the alternator stated to do it's job.....

Sound kooky? Might be true though, if what I have now so often been told about Boyers and low voltage, is true.

PS...this should get a good one going!!!

Are you going to try a meter to see if there is a voltage change as the speed increases? Observation with a strobe at the critical moment may be interesting too.

I have always understood that a Boyer with too little voltage sits on "full advance" and needs the 12v for it's internal circuitry to retard the ignition. This is why the blasted things backfire and kick back so badly when the battery is low. You're kicking a motor on full advance.

This doesn't compromise your theory (I think) because a smoother and more even tickover would occur with the ignition retarded slightly.
In the Norton owners manual it says not to attempt to adjust carburetors until the engine is up to running temperature, this must imply that with added temperature, carbuertor conditions change, yes? Also, once the engine components begin to warm up the effeciency improves, I believe. You know stuff like the scaveging effect, pistons and barrels expanding to normal operating tolerances, reducing friction, in addition to the sump emptying reducing the amount of energy required to turn the crankshaft because it doesnt have to rotate through oil anymore If I am listing irrelevant stuff, sorry, just speaking my 14 year old mind.
Everybody is entitled to a shot on this...if I knew what caused it...I wouldn't be bothering the rest of you with it. I should indeed check for voltage changes when this happens, and I will try to set that up, as for carb warming...agree this will increase rpms, but I think it will happen gradually, and not suddenly.
The only thing I am definately sure about is that this really does happen, because I've used this as a signal to myself to hurry and get the bike out of the courtyard before one of the neighbours complains...for years. When it kicks in...it's time to go... :wink:

Thanks for the tips....!!!!
Could it be something to do with the scavenge system finally emptying the sump? As the scavenge would suddenly draw in air so the load on the pump and therefore the engine would be reduced causing the engine speed to increase?

As you say there is normally a fair amount of oil in the sump when you start up have you observed how long it does take the oil tank take to refill? Or does this speed increase happen regardless of whether the sump is partially full or completely empty on start-up?

Could it also be something to do with the Combat type breather unit (that I think your bike has?) finally clearing itself of (wet sumped) oil and 'breathing' normally?

Sorry I don't buy the 'voltage' thing.

I would expect to see a voltage increase as the engine speed increased but that would (I think?) be caused by the engine speed and not the other way around?
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