Hi guys, after doing some reading I believe I have the dreaded "4 stroking" issue on my 76 DT400 that some of us seem to have. Here's what I've done; rebuilt carb, removed and cleaned the reed valve cage with new gaskets, removed the compression release, cleaned that up and put a new O ring on it, so I'm fairly confident those are not leaking air, inspected around the head gasket and the base gasket of the cylinder, neither appear to be leaking at they look brand new (I have probably less than 50 miles on it since I rebuilt the top end) . As some of you know I just converted it 12 volt, when I did I looked at the seal on that side there was no oil leaking from it but I'm sure that's still not a indication of it leaking air. Checking the spark plug there is oil on it so I made sure the oil injection pump was properly adjusted. Below is a video of it running as well as a picture of my oily spark plug. Thanks for looking and as always appreciate the help! Tony

Video

Here's the video embedded... just need to paste the link here and site does it



Sounds like a bad CDI ignition system to me.

Thanks Mark, the CDI is brand new with the 12 volt conversion. It was doing the same thing before I converted it...

Hmmm. I don't know. Maybe you could describe what you're doing with the throttle and how it responds?

I don't think the 76 400 still had the dreaded o-ring on the jet setter...

How is the starter plunger? (choke)

Good morning Mark, when you hold the throttle steady is when it does that but if you just rev it then it's fine. As for the starter plunger, and I'm glad you clarified it as choke, it's the only thing I didn't change/rebuild in the carb but I did remove it for cleaning. I'm going to go over my the instructions for wiring the 12 volt system again to see if I missed anything like a ground.

Here's a bit of new information; I have noticed it was smoking quite a bit and after leaving it all night there would be a small puddle of oil on the floor beneath the spark arrester so I pulled it out to find it covered in oil after I had just cleaned it in my ultrasonic cleaner, as you can see a puddle of oil came out with it. I have no where near the knowledge you have with these bikes but I'm thinking that oil has to be coming from somewhere, all of that certainly isn't coming from the oil injection tank - I did look to see how much was missing from the tank, there wasn't anywhere near enough gone to suspect that... I suspect the exhaust pipe is full of oil too!

What is 'the dreaded 4 stroking'?

I've never heard of it.

The other source of oil is the transmission/clutch reservoir. A leaking crankshaft seal or missing/defective o-ring where the primary drive gear meets the crankshaft bearing inner race are potential problem areas.

Performing a leakdown pressure/vacuum test is a good way to find leaks.

You may want to go ahead and remove the primary drive/clutch cover and inspect the crankshaft seal. These can be popped out of place from high pressure in the crankcase.

Four stroking basically describes the condition where the engine breaks up and does not run cleanly at a steady throttle position due to an excessively rich condition. You can replicate it if you pull the choke on a properly jetted bike with the engine warm.

The video in the original post appears to show a hanging, surging idle from an air leak.

In the video it wasn't idling, I was giving it gas to show that it breaks up at RPM, it idles perfect at 1500 RPM.