Toodling along on my 1965 B40F, I twist the throttle to accelerate, and the bike responds. But sometimes, it starts to accelerate, then loses impetus for a second, and then recovers. It's always just for a second, and it happens quite often. Any advice, please? Certainly get my money's worth from this forum, don't I? Phil
When you open the throttle quickly, it will allow more air in but it takes a bit more time for the fuel to flow. If the mixture is a bit weak to start with, the action of opening the throttle this way could result in a loss of power or hesitation. Starting point is probably to give the carb a good clean out and to then check the carburation is spot on - for example, via the colour of the plug.
Another possibility is a momentary misfire.
Check all leads, points are clean, condenser connections etc.
Quote from: AWJDThumper on 16 Jul, 2020, 17:52
When you open the throttle quickly, it will allow more air in but it takes a bit more time for the fuel to flow. If the mixture is a bit weak to start with, the action of opening the throttle this way could result in a loss of power or hesitation. Starting point is probably to give the carb a good clean out and to then check the carburation is spot on - for example, via the colour of the plug.
I don't think it's particularly when I open the throttle quickly. Nor is it when I first ask it to accelerate. It starts to accelerate and then a few seconds later pauses just for a second before recovering.
I guess I might soon be learning about carbs, but first I'll check connections and look at the plug colour.
Phi
Seems like some restriction in the fuel supply, float level first ok, then too low, then recovers.
Happens numerous times throughout a journey, always part way through an acceleration, always revovers after a second. Phil
Carbs are quite complicated things and it doesn't always help to try to analyse in detail what's going on in my experience. It's best to start by checking the basics first: that it's set up correctly for the for bike (jets, slide cutout, needle position, slide not too worn) and for whatever air filter is fitted (especially for the dodgy air filter fitted to a C15) and that the mixture appears to be correct from the plug colour. The hesitation could be due to a weak spark or to a weak mixture at the point it is hesitating.
had this happen to me a few years back filter in carb wanted cleaning out hope this helps , Roy
advance unit sticking ? maybe only one spring etc . might be worth a look
Quote from: royblackburn1@btinternet. on 17 Jul, 2020, 00:42
had this happen to me a few years back filter in carb wanted cleaning out hope this helps , Roy
You say "filter in carb". Is there a filter inside the carb? (mine's a Concentric.)
Ah, at the inlet to the float chamber.
That's clean.
Phil
I'm not sure you said that this has always happened? If not presumably not the carb set-up, but taking it apart to check for any evident grime and clean for this or non-evident grime worthwhile.
Also see if the slide will wiggle side to side. Mine does when it circa half open......but not full open or full closed. I presume that means too much wear and part of my issues. But I had assumed before it wasn't worn as I hadn't tried wiggling it at the half throttle position
If you do dismantle the carb make a note of the jet sizes/needle position. For your/our future ref.
Basically do all can in one strip rather than the 10 I've done so far without being sure where my issues arise!
Thanks Hoogerbooger. Because I haven't been riding it very often, I'm not sure, but I think it's maybe happened before but not to this extent. I'm annoyed that I don't know this.
The slide info is interesting, and seems to me to fit with the symptoms. Just thinking, if that were the problem then it should also happen when the bike is stationary and I increase the revs? I'll see if it does.
Anyway, meanwhile, here's a photo of the plug. Is it running a bit rich, or is it okay?
Thanks everyone for your help so far. Phil
That is definitely rich. If you've only done a short journey and have a choke, that couls cause that.
Raise the tickover to quite fast on the throttle stop screw then wind the mixture screw in and out to achieve the fastest revs then slow it down to normal with the throttle stop screw. AN easy place to start.
If it doesn't make any difference, drop the needle one groove.
I set the throttle stop to a quite high tickover. Wound in the mixture screw until ran unevenly, then out again searching for highest revs. Kept winding out, and revs kept increasing. Having wound about two full turns I thought is this right? How many turns would be too many?
Aaargh. I've lost track of where I was with the screws. Could someone give me a starting point for the mixture screw please? Phil
1 1/2 turns out usually a good starting point.
Further to my previous post, my carb is a Concentric 626, and the bike a 1965 B40F. I think a starting point for the mixture screw is found by winding it fully in, and then out by a certain amount? Is it 1.5? Don't know what makes me think that. Phil
Quote from: JulianS on 17 Jul, 2020, 16:01
1 1/2 turns out usually a good starting point.
Just seen this. Thanks Julian. Phil
By the way, craigc11g, just to say that I haven't ignored your suggestion of the advance mechanism. It's on my list of possibilities. Thanks. Phil
If all else fails, check to see you haven't got a damaged wire that moves under acceleration. It might have an internal break which may be difficult to spot, or damaged insulation and it's earthing out intermittently. In the second case, it won't blow the fuse if it's the wire from the coil to the points, but either fault could cause the plug to spark at the wrong time.
Phil,
Before you go too far, drop the needle one groove and try the mixture screw scenario again. You should be able to achieve a greater degree of response, ie not winding it out and needing more.
Well I've tweaked the screws again using Julian's recommended starting point and it sounds okay when stationary. I'll take it for a short run later if the weather improves s bit (with empty pc to try to find the leak.) Fingers crossed! Phil
Mixture, connections, fuel, advance...all good ideas, but you weren't to know how stupid I can be where motorcycle maintenance is concerned. I noticed when I was out on the bike this morning that when the little pause happened there was a slightly odd feeling at that moment in the twistgrip. Remembering that this afternoon, I checked the cable route, and look what I found! I've now altered that, and the issue has gone away!
Thanks for your help anyway, and sorry to have wasted your time!
I'm learning. Honest!
Phil
Well done. You haven't wasted our time.
Well I'm a bit disappointed Phil....I was hoping to find out the settings on your 626... .But I guess it'll be more useful to know if it runs ok on them ;)
Haha, yes, well if and when I take it apart - which I confess I'm tempted to out of interest - I'll let you know.
Great news Phil and really pleased you got to the bottom of it.