Just a thought . If the bike originally had CDI ignition , there may have been an advance curve built in . With your fixed ignition timing now , your mid range may require carb adjustments
from stock settings . Try your needle clip a notch either way . If you're onto something , it'll show up quick .

I will be watching this thread with interest! I posted the same problem a couple years ago with my bike, err...on a different forum and never figured out the issue. Granted I haven't really had time to work on it since then so I have no answers.

Interesting that you and I have this issue on bikes with "timing changes". I installed a power dynamo 12v on mine. It works correctly, charges correctly, etc but I have this bad surging issue too. Idles fine, lots of power if I get on it ad fast but...if I try to level off the speed I have very annoying surging. I took it to a local guy and he changed the timing. He said the surging was better but there was a performance decrease. I didn't really notice a decrease in either.

I haven't rode the bike much or far with this issue. I don't believe mine surged just by turning the throttle up. I believe it ony surged driving down the road.

This image is hidden for guests.
Please log in or register to see it.

What do you think? Same thing?

vic

Oh, I don't know what the bike ran like before this because it didn't run when I bought it. Perhaps it was a problem then too.

vic

I'll have to try needle settings. I think leaner in the midrange may actually help, but I'll use my colortune tool on a warm engine to determine what the burn color is in the combustion chamber first. If I have a lack of advance from the DT250 timing setup, maybe leaner is the answer to that.

I'm going to buy a TrailTech cylinder head temp gauge. I like knowing what my cylinder head temperature is at all times since I've seen people hole pistons when they were just cruising at 35mph not knowing their needle jet settings were going to kill the bike.

Is the front exhaust pipe a candidate for plugging up? When I had it off I blew through it to make sure it wasn't obviously plugged up with a mouse nest or something. I didn't feel any restriction, but I didn't run oven cleaner through it to clean out carbon. I did clean out the muffler and remove the baffle insert to see what was going on. There was not any baffling material on the insert, so I installed some steel wool around that. That actually quieted the bike down nicely and may help to make it run slightly better.

DT125a: From what I've read the surging is a pretty common issue with these bikes, but I've also read that people have tuned it out of the bike. So that's my goal. I'd like to minimize it as much as possible since I'm kind of a mechanical zen nut.

The Temp gauge is a great idea. My RT1 is a tiny bit crisper with the needle up a notch but it runs a lot hotter because of it.

Normal cruisin' around is in the 350 range but it'll get over 400 when climbing. With the leaner needle setting I was having to pull over and let it cool .....

Welcome to the forums, Luke! You and I talked a bit on my YouTube video page of my DT250 timing. I think you should do a leak-down test on the bike. If you have any one of your seals leaking, then ALL of your carb tuning is going to be in vain. I was able to MOSTLY build a leakdown rig from parts I had lying around... a vacuum gauge from Harbor freight, a bike pump, and some tubing. Then I just had to get a tapered exhaust pipe fitting and rubber expansion freeze plugs from my local auto parts store. The picture below shows my setup:

This image is hidden for guests.
Please log in or register to see it.

The engine doesn't have to be out to do this. Take off your exhaust and plug it with a rubber expansion plug (and I put a slathering of grease on mine to help it seal).
You'll see that in my carb boot, I have an exhaust fitting from Autozone... one end is perfectly sized for the carb boot, and then I put a rubber freeze plug in the other end to seal it off. Next, I drilled a hole in the exhaust tube and JB Welded a brass barbed fitting in place. After my JB Weld set up, You can see that I attached a piece of blue hose to it. You'll also see that I then use a Y connector to branch off that hose... one branch goes to the gauge, and the other branch goes to my bike pump.

Put in about 5 psi and see if it hold solid there for an hour or so. If not, start spraying down areas with soapy water to find the leak. Don't go much more than 5 psi! You don't want to blow out your seals!

On my 250, I had a nasty air leak that I ultimately tracked down to the point of oil injection! There was a bad basket where the line hooked up to the cylinder, and it was leaking badly. After I replaced that, it was air tight! The leakdown test really is a critical step in ensuring these 2T motors run correctly.

NOW we're getting technical Is the leakdown test done with the piston up top or down low or does it matter really?

Also did you have a one way valve between your bike pump and the device you made? I either have or can source the exhaust fittings and rubber freeze plugs locally.

One thing I haven't done yet is a hot compression test. My bike is brutal to kick start. About every other time it bucks back and throws my leg up into the air (or back into the passenger peg mount.) It seems like the compression release feature may not be working right.

on the leak down test, the piston should be down, this allows more air to enter the crankcase.

on the hard to kick over problem, remove the head and decompression valve assembly and there is a small hole that goes inside the cylinder.

these can be blocked shut with carbon.

it is just a matter of cleaning out the carbon blocking this hole.

What sort of safe operating range of temperature do these engines like? Tonight I got my TTO Trail Tech cylinder head temperature gauge in the mail. I hooked it up and went for a rip that lasted about 30 minutes. I noticed that the temperature kept climbing as I was riding. It would fall off slowly if you let the bike idle. I got up to 385F as my maximum temperature and the cylinder and exhaust were starting to make some not so awesome sounds. Almost like piston slap type sounds. It might be predetonation. I'm curious with the bore job and new piston what my hot compression numbers are. I'm going to pull those today. I'm running the B9ES plug (which is pretty cool) at the factory plug gap.

I'm going to try to build a leak down tester tomorrow and see if I can find anything new. But I wanted to check and see what sort of cylinder head temperatures are safe to hit without worrying about seizing or holing pistons.

The TTO head temp was a wise investment.... 385f is within normal operating range on flat level highway.
If you were to ride a long uphill grade at highway speeds it is normal to see your head temp to approach 430 to 450f max !

I consider anything above 450 to be the danger zone. When trail riding you will see temps from 320 to 360 as normal.

Also learning to use your choke on long uphill grades can really help cool the motor too.... I use it especially when I

want to run my RT hard down the highway at 70 to 80 mph !