Momentary loss of acceleration

[Phil C]

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

[AWJDThumper]

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.

[AdrianS]

Another possibility is a momentary misfire.
Check all leads, points are clean, condenser connections etc.

[Phil C]

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

[LightningAndStarfire]

Seems like some restriction in the fuel supply, float level first ok, then too low, then recovers.

[Phil C]

Happens numerous times throughout a journey, always part way through an acceleration, always revovers after a second. Phil

[AWJDThumper]

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.

[royblackburn1@btinternet.]

had this happen to me a few years back filter in carb wanted cleaning out hope this helps , Roy

[craigc11g]

advance unit sticking ? maybe only one spring etc . might be worth a look

[Phil C]

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.)

[Phil C]

Ah, at the inlet to the float chamber.

[Phil C]

That's clean.

[hoogerbooger]

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!

[Phil C]

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

[Blackthou]

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.