Cuts out at WOT

[bob58o]

420cc engine, 32mm flatside carb, stock gas tank...

I gave my buggy to my nephews over the weekend. It was one of the few times I had a chance to ride it since putting the 420 on it. After holding WOT for some time (maybe 30 seconds or so) the engine would cut out for a second or two. Then it would start firing again. It would run fine for a little while again, then would temporarily cut out but would always start firing again after a very short time.

I first thought it might be fuel delivery. It is the stock gas tank with no extra vent. But now I am thinking it might be an ignition issue.

If the bowl is running out of fuel, I would try adding an additional vent to the tank, replace the fuel filter (it is a paper type now, which may not be right for a gravity fed system), and/or adjust the float height in the carb.

Does it sound like a fuel issue or a spark issue???

 

[itsid]

try again, and once it cuts out immediately check the spark plug...

if it's crusty white you know you got a fuel delivery issue..
(to lean or indeed emptied bowl)
if it is glossy and dark chances are the airfilter is clogged and/or
too much fuel is spoiling the plug (too rich or airfilter).

when in doubt, you can also run the engine at WOT for a while with the tank cap removed..
that way you can at least rule out a venting issue,
and can jump directly to checking fuel lines, and filters

'sid

 

[65ShelbyClone]

What if it's driven around at low throttle for a while after the WOT problem? Will it start the cycle again? If so, that sounds like inadequate fuel flow.

Something I would try at least initially would be to disconnect the fuel line at the carb and see how it flows with the filter and fuel cap tightened. If it flows well and then slows down, it's probably a venting issue. If it's just always slow, then the filter is at least part of the problem.

 

[bob58o]

What if it's driven around at low throttle for a while after the WOT problem? Will it start the cycle again? If so, that sounds like inadequate fuel flow.

Something I would try at least initially would be to disconnect the fuel line at the carb and see how it flows with the filter and fuel cap tightened. If it flows well and then slows down, it's probably a venting issue. If it's just always slow, then the filter is at least part of the problem.

I don’t even have to let off the throttle to get it to cycle again. I can keep it WOT the whole time. It will run fine at WOT for a while, cut out for a few seconds, and even if I keep it floored when it cuts out, it will begin to fire again. Then it will run for a while and repeat.

The spark plug looked pretty dark when I looked at it, but it wasn’t after a WOT run. I had a lot of idle time on the plug when I looked at it. I needed the choke when I cold started it, but it had been sitting for a while. I’ll do a thorough carb cleaning before I do anything. IIRC, I have the smallest idle jet available installed.

I never really considered that too fat of a main jet would cause this type of problem, but I’ll look into that too. The only tuning I did prior to this was on jack stands. I held the band brake floored to simulate a load while I gave it gas. I did this at 1/2-3/4 throttle to adjust the needle clip and repeated at WOT to tune the main. I burned up the brake during the process so I ordered a couple new bands and will install one soon. After that, I’ll see if I can get this carb and fuel delivery dialed in.

Added: I guess I misinterpreted “the cycle”. I was thinking the combustion cycle. It seems like it will happen every time I hold WOT for an extended time. If just cruising at part throttle, it doesn’t happen. If only holding WOT for 10-15 seconds, it doesn’t happen. But when I try to go a few blocks at WOT it will cut out periodically. It seems to me like the bowl is running out of fuel. Like the delivery cannot keep up with the consumption at WOT. So to answer your question, I feel like after it cuts out if I cruise at part throttle the bowl fills again... but if I then floor it for 20-30 seconds, it runs out again.

But if the bowl is really emptying, why does it only cut out for a few seconds??? If I hold WOT while the bowl is empty, shouldn’t it die completely instead of just cutting out for a second or two??

 

[itsid]

As per your description the engine needs quite a while at WOT to cut out..
so if you are having a fuel delivery issue, it's in the realms of a drop or two per second..
Once it cuts out the rpm drops and with it the fuel demand...
which maybe gives the bowl enough slack to refill again
that does make sense, doesn't it?

It could also be an electrical issue (coil or spark plug wire) opening up a tiny crack and disconnect when hot enough.
(maybe testable with a hot air gun)

but my first guess is indeed fuel ..
So I'd test that first

'sid