Symptoms of a Hot Carb?

[Happyhenry]

Hello WiseOnes,
I’m having an intermittent running issue with my 1949 Longstroke.
When it occurs after I’ve been riding for a while and with the throttle about half open. If I open it just a tad wider to accelerate or pull up a hill the engine instantly dies. If I close the throttle a tad it picks up immediately. I’m convinced that it is a fuelling issue because the throttle acts like an on/off switch at that point.
I went for a 60-mile ride with friends today (hot day) and it started to act up after we’d crawled out of town.
After a short coffee break it then ran like a train on open roads for the rest of the day.
This has occurred previously.
First thought is blocked jet, but I suspect it may be a hot carb. I have no phenolic spacer on this bike, just a paper gasket either side of the drip guard.
Any ideas?
Cheers H

[KiwiGF]

Does the carb ever feel hot? Generally I have found that they stay cool even without a spacer, probably due to action of fuel “evaporating” in them, constantly. It would be a good idea to fit a spacer anyway.

My guess is that your bike is running slightly lean.

[Swarfcut]

  Worth a try...take off the fuel cap. Could be a blocked cap/ tank air vent. Other than that raising the needle to richen the mix as Kiwi's diagnosis is also worth a go.
 Swarfy.

[Ted_Flash]

I had similar symptoms, turned out to be the gauze filter on the petrol taps inside the tank blocked up. Running OK (fuel supply just on the edge of adequate) then ask for a little more and it can't supply.

[Greybeard]

My bike is generally very good but if I have to crawl through town traffic on a hot day the bike starts to feel unhappy. I put this down to an overheating carburretor.

[Happyhenry]

  Worth a try...take off the fuel cap. Could be a blocked cap/ tank air vent. Other than that raising the needle to richen the mix as Kiwi's diagnosis is also worth a go.
 Swarfy.

I did think of that.
However, when I had put the bike in the garage I could hear a hissing sound and worked out that it was coming from the filler cap. First thought was that it was a partial vacuum in the tank that was slowly 'catching up' - could this be the answer to my problem? I released the cap and did it back up again. The hiss continued.
By putting my cheek close to it (and then employing a bit of spit), I found that it was actually the petrol tank venting pressure. Hot engine below the tank, stationary bike in a cool garage.
So the petrol cap vent is certainly clear...
H

[berger]

is it clear enough? i had a problem with mine cutting out, same problem and it was the vent, lots of old crud about.

[Happyhenry]

is it clear enough? i had a problem with mine cutting out, same problem and it was the vent, lots of old crud about.

I've no idea, but I'll give it a good soak and a blow out anyway.

I did undo the cap while riding when the bike was misbehaving and it made no difference. I do have small bore copper fuel pipes hard-plumbing the carb in, so they may contribute although I also opened both fuel taps and that didn't help either. The issue is inconsistent so if it is fuel starvation it is intermittent like crud blocking a jet.

I'm using NGK B6HS plugs and they are a dry sooty black around the rim and dark coffee on the tip and centre. If anything I'm running a tad rich and wonder whether I should drop the needle or try a pair of hotter B5HS plugs (probably not).

I have a 3mm Tufnol spacer on the way, so I will probably perform my usual trick of implementing three potential solutions at the same time. If it's fixed I won't know which one did it...