Throttle response

[flatdeck]

Hi guys, This question is not about how quickly the engine responds to more throttle but about what one should expect when closing the throttle. My A7 is taking about 3-4 secs to drop in revs to idle from about 1/3rd throttle when just letting go of the grip. I would expect it to be almost instantaneous normally(?) but then I don't really know what normal behaviour should be. Can someone enlighten me? I'm concerned that the ignition might be a bit retarded or something causing this ... or is it a trait of a longstroke with a heavy crank or what ? Cheers, Dave

[dpaddock]

Sounds like the throttle cable is pinched or bent. Remove the petrol tank and check the cable routing. If all is OK, lubricate the cable core. Next, stretch the throttle spring in the carb. We assume the slide isn't binding.

[Brian]

As dpaddock says check your cables and slides are free moving. When you twist the throttle and let go it should snap back. Also , do you have a manual or auto ignition control. If you have manual make sure you retard the ignition as you pull up. If you have a auto unit it could be worn, they are notorious for wearing out and if so when you pull up they take a few seconds to retard the timing which will cause the symptons you describe. There are a couple of other things that can cause your problem but I would start with these.  Good luck.    Brian.

[groily]

Agree with all that's been said. Carb slide and cable are easy to check. Sometimes a slide will snap back nicely when the engine isn't running and then mysteriously fail to do so when it is - for any of the reasons given or even for having a bent needle. Worth backing off the throttle stop screw while you check things out to make absolutely sure the slide can get to base every time. Like Brian, I'd suspect the springs on the ATD if you've got one. Failure to slow right down for lack of auto-retard is all too common, especially if the slow running is set a bit fast. The springs weaken to the point that they let the thing advance at too low an engine speed, and it flutters about not knowing what to do with itself. If it's really worn out, the weights can get out of line and get stuck, regardless of the state of the springs.  ButI if your mag's manual, forget all that. If none of the above make any difference and it's still erratic, I'd check the pilot jet if it's a Monobloc on there. Should be screwed out from fully-in about a turn and a quarter if memory serves. If things are in decent shape, small adjustments make quite a lot of difference . . . Good luck

[octane]

.. I'd suspect the springs on the ATD if you've got one. Failure to slow right down for lack of auto-retard is all too common...

I don't quite understand the reasoning.
Why would it stay longer in higher revs (slow return to idle)
because of that?

...as I see it; it would just run worse, when closing the throtle (as the timing is not-ideal for slow runing)
and probably loose rev's even faster, than if the ATD was working properly.

A ATD stuck in 'advanced' position will give poor low-rev/idle running.

[Brian]

Octane, I'm not quite sure how to go about explaining all this but I will have a go. To get a good steady tick over [idle] the timing needs to be retarded to just before top dead centre [tdC]. The best way to demonstrate this is if you have a friend who has a single cylinder bike with manual advance/retard. Start the bike and let it idle with the ignition retarded, now slowly advance the ignition and you will hear the revs increase without touching the throttle, then retard the ignition and the revs will drop to a steady idle. If your ATD unit is worn or the springs weak then it takes a lot longer for the timing to retard to the position where you get a steady idle. When you rev the engine the timing will advance easily but not retard as the revs drop. All this causes the problem Dave has and it is a common problem. Even though the ATD unit runs in oil all the time they seem to wear out. I have 4 A's and they all had worn out ATD units when I got them. If this is the problem with Daves bike then I am sure someone on the forum will know somewhere in the UK you can get a ATD unit overhauled [SRM ?]. If someone knows a better way to explain all this please do so. I dont know if this has helped or just made it all more confusing.     Brian.

[octane]

Hi Brian
I'm not putting you down or anything, but I have been messing with bikes
for more than 30 years and I perfectly well understand
the workings of the ignition system and 'time' of ignition
in relation to the position of the piston, and the ATD (and how they can go bad/get stuck)
and all that jazz,
.......and when you say

.now slowly advance the ignition and you will hear the revs increase without touching the throttle,.

..sorry;  it just ain't so.
What you're saying, is that a faulty ignition timing will make it run better.


An advanced ignition (or 'early' ignition, as I prefer to call it)
will make it stumble (at low rev's).
It's all very logical.
That's the whole idea why the ignition is set up
to make it 'earlier'/'advanced' as the rev's go up.
The flame-front of the combustion has to
be at the exact right position/time so the flame-front 'hits' the piston
at the exact moment the piston passes TDC.

The higher the RPM's: the faster the piston is moving and as a consequence
 the earlier (more distance before TDC) the combustion has to start (ignite) to hit that moment.


Try starting your bike with the ignition 'fully advanced'/'late'.

.

Start the bike and let it idle with the ignition retarded, now slowly advance the ignition and you will hear the revs increase without touching the throttle

Why ? What would explain that to happen?

The only possible explanation would be if your ignition timing was out of wack,
to a point where, 'retarded' is in fact WAY-TOO-retarded and
when you advance the ignition, it will in fact move the ignition timing to
the-right-amount-of-retarded.

[flatdeck]

Hi, I've been reading with great interest. The conclusion then is that the throttle should snap back and consequently the engine should drop in revs immediately back to idle (not taking 3-4 secs) - is that a fair comment?
I will check the cable(s) and slides (we have been trying to balance the carbs recently). The ATD was recently and professionally rebuilt with new springs and fibre drive gear so I am hoping that is not an issue. The timing has been done but I will, of course, be willing to go through that process again mainly to see if anything has slipped/moved/broken/bent etc etc :-)
Thanks/Cheers,
Dave

[groily]

OOPs.  All a question of degree if you'll pardon the pun. But the plain fact is that advancing ignition on an engine with the timing set correctly while leaving the throttle at tickover WILL increase the revs. Some early and very primitive engines used the ign advance retard as the sole means of controlling revs. Not good at all, but then early carburation was worse.
It's not a question of a fault making things better, because it isn't better - the spark is occurring too early and the motor, if it had a voice, would say it wasn't liking it. Take any engine where the spark advance is controllable manually . . .  in my case I have 4 such old vehicles in my own shed; 3 bikes with manual magnetos and and one pre-war 4 cylinder car with a distributor and coil. In all cases clock-like tickover requires a bit of retard; in all cases, a dose of advance speeds them up slightly. It's no big deal and in normal running one often leaves the levers at full advance. But it illustrates the point Brian made and I made - if an ATD has weak springs etc etc, it won't retard as per spec. The engine revs will not die down as smoothly as they should nor to the level they should unless and until the springs can retract the weights. And if the ATD flutters, the tickover will be erratic. The timing will also advance too early, although that will usually be undetectable unless it pinks under heavy throttle load, when a tad of retard is desirable. Hence the invention of the vacuum retard off the inlet manifold for the motor car to allow load as well as revs to enter the ignition advance curve equation - and similar more sophisticated things for modern bikes. My Yamaha has something of the sort, probably with some electronics up it. I have no intention of investigating!
Flatdeck's prob is more likely carb or cable related in view of his latest comments, but the finer points of carb and ign set-up are so inter-related it would be wrong not to consider both aspects when pondering a fault. There is never much point in playing with carburettors unless one is 100% confident that the sparks department is right and proper first.

[flatdeck]

Thanks Groily. We started with spark and timing after some early good advice from this forum. The magneto has been rewound, the ATD rebuilt and the timing checked and checked and checked. The points cleaned, leads and plugs replaced. The carbs have been apart and cleaned. Haven't actually replaced any parts therein though. I guess my original question was "how fast should the engine return to idle when the throttle is closed from about 1/3rd open?" As I said it is taking 3-4 secs at the moment which seems a little long when I am listening to it. I am expecting the revs to drop fast , say in 1/2 a sec ???
Dave

[RichardL]

In reading all this, let's call it "healthy debate amongst friends," I can't help but believe that Brian's and Groily's empirical observations, regardng revs increasing at idle when timing is advanced, are probably correct.  This belief of mine is based on what is becoming  a history of  hearing knowledgeable advice and comment from both. At the same time, Lars' point regarding having the flame front (or, maybe, "the fully developed energy of combustion") occur at just the right time when the piston is after TDC is also, as we all know,  quite true. I'm in no position to comment on this topic experiencially, but it seems to me that both circumstances, stumbling with ignition over-advanced at idle, or increasing revs on over-advance, may be possible (assuming perfect timing in both cases). Perhaps differences can occur between bikes based on type of bike, carburetion, fuel type, or other factors (my way of saying I'm really not sure). The thing that is missing here, for me, is the explanation, based on good science, as to why either condition takes place. Perhaps, with such an explanation we will come to agreement as to which, if either, is the  proper and expected behavior.

Richard

[Brian]

Octane, I was not implying that you did not understand ignition. If it came across that way I apologise. I was trying [obviously not very successfully] to explain why a worn ATD unit would cause the symptons talked about here. I wont say any more on the subject other than try what I said, find someone with a manual advance/retard on their bike and try it.    Brian.

[octane]

Octane, I was not implying that you did not understand ignition. If it came across that way I apologise.

Brian, I'm the one who apologize !
I'm sorry but I'm writing in a for-me-foreign language and sometimes
my choice of words are maybe not the best.
Please accept my apology if I somehow came of condescending, pompous or un-friendly.
That was certainly not my intention!

find someone with a manual advance/retard on their bike and try it.

You don't need a bike with manual advance/retard to do that.
Really you can do it with a bike with ATD.
When it's idling, the ATD has not yet come into play (bob-weights at 'rest'.)
and the timing is correctly set somewhere around 4-5 degrees BTDC. (yes it's BEFORE TDC,  at idle)
so the situation is the same as if the ATD wasn't there or you had a manual timing-thingy.
Now 'mis'-adjust the timing to an earlier/more-advanced position
and see what happens.



My guess is that (I may very well have gotten it wrong) some misunderstand stems
from the fact that using a manual set-up , you can set the timing at, lets say,
at TDC (zero degrees before TDC), or even a few degrees after TDC,
to make starting easier.
If that's the case; then after start, 'advancing' the timing will of cause make it run better
as you in fact adjust it into proper timing , (let's say 5 degrees before TDC) for idling/slow running.

regardng revs increasing at idle when timing is advanced, are probably correct.

Yep; that's my point about the possible misunderstanding above:
It really depends on what is meant by 'advanced':
 in relation to what ?
Of cause one can say it runs better with timing 'advanced' if it's in relation to, let's say 1 degree BTDC.
Then one couls say, the engine runs with 'advanced' all the time, including at idle.(around 5 deg. BTDC)
...but increasing 'advance' beyond the proper timing, will not make it run faster at idle /or slower return to idle.


That's the problem with using the terms 'advanced' and 'retarded':
some people  (I'm not refering to any of you) seriously think
that 'retarded' means anything after TDC and 'advanced' means anything before TDC.

I prefere to use the terms 'early' and 'late'
and at all times that's in relation to the piston travel UP TO TDC.

[groily]

Well, if I could express myself half as well in a second language as you do Octane, boy would I be happy. As I would if I could keep my workshop half as tidy as yours! I try in French because it's where I live, but can't do in that language what you can do in English! And I certainly don't play on French web forums where there's loads of give and take and even some leg-pulling. Good luck and keep the flow coming!