Dong Fang DF 250 RTB Bobber Information

**heist222** · 04-29-2015, 01:00 AM

So I believe I found part of the issue why my 250Rtb-B wont achieve speeds over 45 mph. I cannot shift into 5th gear when it is running. No problems getting to 4th, very smooth. But there is nothing there after 4th gear when riding down the road. I was wondering if anyone here had any thoughts on what could be wrong? And what possible fixes can be made

**barnone** · 04-29-2015, 02:53 AM

Originally Posted by heist222

So I believe I found part of the issue why my 250Rtb-B wont achieve speeds over 45 mph. I cannot shift into 5th gear when it is running. No problems getting to 4th, very smooth. But there is nothing there after 4th gear when riding down the road. I was wondering if anyone here had any thoughts on what could be wrong? And what possible fixes can be made

You mean all this time the highest gear you have been running is 4th gear?
Can you shift in 5th gear when the engine is not running? Probably not your problem but the first thing is to make sure the clutch cable is properly adjusted.

Do you have a friend that is knowledge about motorcycle? Ask him or her to ride you bike and count the number of shifts.
1st gear to 2nd gear is shift #1, 2nd gear to 3rd gear is shift #2, 3rd gear to 4th gear is shift #3, 4th gear to 5th gear is shift #4.
Neutral is between 1st and 2nd gear.

If the person can't get the bike into 5th gear and the clutch cable is properly adjusted then there is probably transmission damage.

How a transmission works at
https://www.youtube.com/watch?v=7EMI...ature=youtu.be

**beniamin82** · 04-29-2015, 08:10 PM

Heist222 the first gear is down from neutral, but i think this basic information you have already...

**axa** · 04-30-2015, 05:48 AM

Originally Posted by beniamin82

Heist222 the first gear is down from neutral, but i think this basic information you have already...

never know... on my first bike, I kept adding 2 stroke oil to the transmission until finally it stopped shifting.

**ben2go** · 05-01-2015, 02:29 AM

Originally Posted by axa

never know... on my first bike, I kept adding 2 stroke oil to the transmission until finally it stopped shifting.

**barnone** · 05-04-2015, 07:33 PM

heist222,
Any reply would be appreciated.

**bowtiewagon** · 05-09-2015, 11:16 AM

Wow I was worried for a moment, did not see the site, was getting all kinds of domain expired and sight no longer available screen when I clicked on it. Glad to see it is still here.This is the lifeblood to my bike.

If not for this place I would have been lost with a few repairs.
Anyway, hope someone else chimes in to keep the thread alive.

By the way, still looking for a replacement for my shift linkage. Have not been able to locate anything yet. May have to resort to making something myself.

**axa** · 05-10-2015, 04:41 AM

Originally Posted by axa

not sure I ever answered these questions.
with 10CR I still use 87 octane and don't see any sign of detonation problems... this leads to the conclusion the dynamic CR of this engine is way low.

Reuse the metal film gasket. use rtv silicone as assurance. if it leaks it's probably for some other reason.
or... as they say, if doing all this work why not start with a new gasket... depends on your philosophy and skill level...

turns out it would be a good idea to run a higher octane fuel.
i was having some issues at full throttle/under heavier loads i couldn't diagnose.

first there was engine surges.
then i noticed a high frequency clicking, first thought was valve lash, but that didnt make sense as it didn't kick in until said conditions.
but i dismissed all this as i thought it must be the sound of my keys and little shopping loyalty cards flapping in the wind and dismissed it.

maybe a few days later i stalled out during a full throttle incline, i then concluded that since i was low on gas and moving at a good clip up an incline, i must have been starved for gas.
sat a few seconds then started right back up. and again dismissed it...

got gas the next day but again on the way home stalled out agian, sat a few seconds started right back up and proceeded home cautiously. Now i knew there must be something wrong.

got home and couldn't start the bike again... thought it must be a blocked jet...
proceeded to drain and clean the carb but found no problems...
It wouldn't start but i noticed it was sure turning over easy.
pulled the plug and noticed very little compression... engine needed to come apart.

head cover off first check valves. sure enough, the exhaust was so tight it was keeping the chamber open, adjusted valves, reassembled, started right back up...
thought that would be the end of the story but that tight valve seemed to be the effect, rather than the root problem...

Now if you follow my story you might know i have steel rods, so with a lower expansion rate, it didn't really make sense that there would be such a large change in the valve train.
but with everything looking fine i took the bike to work. 2 miles in the head gasket blew...

now this was really odd as it blew on the cooler intake side no more than an inch from the intake itself. got it home and proceeded to take the engine apart.

first thing noticed was the head bolts were not at all tight...
This turns out to be my 1st rookie mistake.
after rebuild out re-torqued is required after the engine heats up one or more times.

i took the head off to check for damage, specifically for any sign of detonation/pre-ignition... lucky no damage but the gasket itself.
but this revealed my 2nd rookie mistake. as it turns out that high frequency clicking was in fact the signs of pre ignition or detonation. the anit knock sensor all new car have is actually just a tiny microphone that listens for a 6400hz ping, the tell take sign of pre ignition/detonation.

With the head off i inspected my work, revealed my 3rd rookie mistake. as the intake valve is the larger it comes closer to the edge of the combustion chamber. and as the head has a round cavity milled to clear the valves as they close, when i shaved the head down i ended up making a sharp edge at this valve clearance than it was previously. i missed reliving it when doing the head work. i knew nothing of hotspots. and sure enough this is exactly where the gasket failed.

so in summary i created a hot spot in the combustion chamber that was helping the low octane fuel ignite at the worst times...

i thought running 87 would be fine. but depending on which chart you look, a 10:1 engine requires at least 92 octane... and now i believe i know why.

But im sure looking forward to seeing even better performance when my new gasket arrives...

**SR.MAILMAN** · 05-10-2015, 04:09 PM

As you know(and others may not) higher octane fuel burns slower and with less heat. No power advantage unless timing and compression are increased.

I use 94 octane, no ethanol fuel only because of the low mileage I do and the 2 year shelf life.
On the plus side... My chrome single wall pipes do NOT blue and 84.5 miles per gallon.
My engine is 10:1 compression,but the mfg. recommends 87 octane.

Good luck with your repairs...

**barnone** · 05-10-2015, 11:56 PM

Made it to mountains of Western North Carolina where there are curves and elevation changes.

After several days of mowing, weed eating, and fixing things I got to ride some bikes today.
First was up to the Dragon and breakfast with the boys at Deal's Gap Resort.

0510150918.jpg

Then after a run on the Dragon on the Ural back to the cabin to crank up the DF and take her for a ride.
Gonna leave her stock as she has enough juice for the speeds here in the mountains on two lane roads.
0510151803.jpg