First experience with "dieseling"...scary!

jbswear · Post by **jbswear** » Sun Jun 20, 2010 5:30 pm

Okay, so I'm playing with the GT380's carbs, trying to figure it out and get it running.

It'll run, but I'm having a hard time setting the idle--more than likely my timing is off, so I'm gonna wait until I get a dial indicator to continue.

The right cylinder is burning the strongest--the pipe is scalding hot to the touch, while the left is pretty hot, but not quite so, and the center pipe is just hot enough to burn skin. Does that make any sense?

I was having overflow in the left cylinder, but I adjusted the float tab and now that isn't a problem.

I have tiny sparks across all points when they're open and fat, blue sparks at the plugs; it seems that my battery is strong.

The bike kicks and starts fairly easily, but it's not running smoothly.

I have the air screws turned out 1.25 turns and have the stock jets in the carbs. I'm running pod filters (screen, not foam).

It's been so long since I've have the carbs completely apart so I don't remember where I have the clips on the needles. Where should I start with them?

Anyway, as I'm fiddling with the idle, I revved it to keep it up as I'm adjusting the knob. Then the engine jumps to 9k rpm. I killed it. Still running. Pull the plug leads. Still running. Shut off the gas. It died after a minute or so. This happened twice.

Before I break or further damage anything, what would you guys recommend I do?

Post by **Suzsmokeyallan** » Sun Jun 20, 2010 9:52 pm

Timing will make or break any engine, you need to time it properly first.

Frank · Post by **Frank** » Mon Jun 21, 2010 6:36 pm

Have you ever pulled the heads? I'm wondering if there's not a lot of carbon built up which (when hot) can lead to what happened. I was riding with a friend once (he had an old CZ250 motocross bike) when he fell in sand, pinning the throttle open. We went through the same thing you did until it finally ran out of gas.

jbswear · Post by **jbswear** » Mon Jun 21, 2010 8:00 pm

Frank wrote:Have you ever pulled the heads? I'm wondering if there's not a lot of carbon built up which (when hot) can lead to what happened. I was riding with a friend once (he had an old CZ250 motocross bike) when he fell in sand, pinning the throttle open. We went through the same thing you did until it finally ran out of gas.

I think I'll pull the heads and check.

Arne · Post by **Arne** » Sat Jul 03, 2010 11:48 pm

I had an RD that dieseled at uncontrollable rpms like that, it went something like:

kick over motor to see if it fires
sh*t
hit kill switch
f****
turn key off
holy mother of *** its still running
rip plug wires out of coils using end of wrench fearing that I would get a massive shock
sh*t now what
start taking the carbs off

Then it ran out of gas.

Of course that bike had chambers on it that made it sound like someone was running two logging chainsaws with no mufflers so my ears rang for a few days.

It ended up being a stuck carb slide that I hadn't noticed on reassembly.

Its pretty exciting in a bad way.

From what I know, if you run a two stroke to high rpms with no load they can heat up and cause 'dieseling' (whether or not its actually exploding from compression or from something hot enough in the cylinder to ignite the mix I don't really know).

I'd go through, check all your assemblies to spec paying particular note to the carb slides, any open ports on the intake, maybe stick a suction hose under the pistons and see if the crank is full of fuel, then give it another try.

If you kill it before the rpms go over 5,000 I would highly doubt it would 'diesel' again. (keep your eye on the tac and your thumb on the kill switch when you start it the first time). Of course it doesn't hurt to clean the carbon off your top end.

If a bike was 'dieseling' on me again, I would cut off the air supply to the engine (seal off the carb inlets if they didn't have an air box on yet)

Good luck.