Please help me! Here's a little bit of the history behind my '99 Scrambler 400: Having good low end but basically no top end power I naturally wanted to enhance the performance. So I purchased the Hot Seat Pipe, Head, clutch kit and then a UFO for the stock carb. I put the head, pipe, and clutch kit on right away and had very good results. It was a few weeks later and I put the UFO into the carb. All I can really say is it is a tuning nightmare, with a little better response. I went up one needle jet size with the stock needle, 240 main jet, 25 pilot. Performance was pretty close. Then all of sudden, while running about half throttle, it coasted to a stop and died. The engine restarted right away, but didn't sound right and was low on power. I disassembled it and the upper ring had caught the bottom of the razor sharp exhaust port, bending the ring and breaking the upper land of the piston. Oh well, needed a Wiesco anyway. Now with a .020 overbore, new piston, much cleaner ports with radiuses, and Boyesen Pro Reeds it was running again. With the same carb setup, the motor now sounded very lean in the midrange. I went an additional 3 needle jet sizes larger and put in a 260 main. Since I was still breaking in the new piston I wanted to go easy on it, so I was primarily running 3/8 throttle and less. I got about a 1/2 of a tank of fuel burnt when it coasted to a stop and died again. This time it didn't restart, no compression. The exhaust side of the piston was burned far enough to loose compression and there was scuffing all the way around the bore, but the skirts were clean and loaded with oil. I had been checking the plug quite a bit before the failure and it was always black, still was after the failure. So I rebuild again, this time .040 overbore and I replace the stock carb and UFO with a 38mm TMX from Hot Seat supposedly set up for their pipe and head, figuring that the UFO had caused a very lean area at 1/4 throttle. I put the richest main that came with the carb in just to error on the safe side, since I am now very paranoid. I also put an exhaust temp gauge on for no surprises. Sure enough, about have way through the break-in, it coasts to a stop and dies and then restarts right away and sounds fine. Exhaust temps never got above 950F. I drop the pipe and there is no burning to the piston, but I can see some light scoring by the rings and the exhaust skirt looks pristine. The plug is black and wet. I then put a water temp gauge in and went through the fuel supply system looking for restrictions. I then run some more at about 1/4 throttle, still breaking in the engine, and it coasts and dies again. It restarted and sounded OK, but upon inspection the piston was starting to burn again and more scoring on the piston. Again the plug was black and wet, the exhaust temps were around a max of 900F, and water temp was around 185F.

I don't understand why it, with out warning, just coasts to a stop like it was just shut down, and then restart just fine. I'm fairly positive that all the failures, including the first one, are related. But, the symptoms just are not adding up for me. I've checked timing, verified fuel supply, etc… I pretty much to the point of just wanting to light the thing on fire. This machine has been a lemon since I got it new, I'm sure it was assembled late Friday afternoon, but I love how the 4X4 performs and rides.

Sorry for the long winded explanation, and I know I missed things, but I figure I need to get as many facts out as possible in order for anyone to put together a good diagnosis. I know I'm running out of ideas.

Thanks in advance,

Jason

 

Just an idea, my friends sport 400 would do the same things. The vent line for the gas tank was melted shut. Check the vent line by the filler neck and make sure it is not pinched or plugged.

 

I can relate brother, if you ever want to throw a bonfire party I'll be first in line. I wish I could help, but it seems like you covered most of the angles. On thing is for sure, they do ride great but if you happen to get that monday morning quad/friday evening assembly your screwed.

 

You didn't mention if using premix or the factory oil injection. I had a similar problem with my scrambler 400, I went to pre-mix and never looked back. I was told that my oil injection pump might have lost it's prime. You're supposed to bleed the system anytime it's apart.

1996 Scrambler 400 - Full RCR treatment

 

I've checked the vent line, that was one of my early thoughts. I actually had the gas cap loose the last time it burnt down, just making sure the tank was vented. As far as the oil, I still have the oil injection. The pump is turned up alittle bit to match the added fuel of the TMX and I was running a 40:1 pre-mix for the breakin.

I still question the water pump, though it didn't over heat it still seemed pretty warm for as light a load as I was running. I know these engines have a history of the impeller nut not being tightened properly from the factory.

I also wonder if it still is just miss carburation, but all other indications suggest a rich condition, except for the exhaust side of the piston.

Thanks for the help,

Jason

 

If you have all the stock parts still I would try re-installing them. Just to find out if it's something to do with the aftermarket stuff. Just go back to a baseline that worked for sure before the problems. Then see if it happens again.

 

What does the new head bump the compression to? If you rebuild the engine again, you may consider completely disabling the oil injection and going to a 32:1 premix. Also, have you contacted the manufacturer of your aftermarket parts to ask them for troubleshooting help? Maybe they have run into this problem. About the impeller being loose; if the water temp did get too high, it seems to me that the overheat light would come on and you would have a blown head gasket before the engine would sieze.

 

Scramblin,

Without going into too much detail I had essentially the same set up as you except all my components were RCR except for the head which was also the lower compression Hot Seat Head. My bike received plenty of oil (I premixed and ran oil injection the same as you) I am on my 3rd rebuild. I know however my first seizure was because I was way too lean on the main. My second seizure however I have no idea what went wrong. All my jets were very rich. I had a Wiseco and port job installed by RCR. I was running at best half throttle and the bike shut down. RCR took the bike apart and noticed the top of the piston was scuffed and the the top ring was off because of the melted top of the piston. We thought it could possibly be a "cold" seizure but I always let the bike warm up and it was 75 degrees the day this happened. I rebuilt again and after 4 tanks of fuel everything appears to be running fine. I am gun shy like you are and rarely tap into the top end.

I think the problem might be the hot seat head because we have similar problems but only one common part. If you test with the stock head and lose the problem please let me know. My engine mods are as follows:

RCR Midrange Pipe
AAEN Roller Clutch
Hot Seat 10.5:1 Head
V Force Delta Reeds
34mm Mikuni Carb bored to 35.5
Port and Polish Trail Mod RCR
Shaved Flywheel
K&N Filter with Outerwears
RCR Airbox mod
35 Pilot, Stock Needle set on Middle clip, 280 Main

 

I do have the lower compression head. I did have quite a bit of time(4-5 tanks) on it and the pipe with the stock piston and carb with out any problem. Anythings possible though. As far as going back to stock, so of the modes required cuting parts. Like the air box for the TMX and the stock slide for the UFO. I'm a little leary about rebuilding it yet without having a good idea what the problem is. It gets quite expensive if I have to replace a piston each time for trouble shooting.

I'm going to try and call Hot Seat tonight and get thier opinion on the problem. Hopefully they have heard of this before. I think my next step it to completly tear down the motor and inspect every inch of it, crank seals,water pump, stator, etc...

A few more questions, anyone else have a similar set up with a 38mm TMX or 34mm UFO? and what is the jetting that you run? It also lost spark briefly after inspecting the piston once. Checked the wiring a little bit and it was fine since. Is there any known problems with the CDI boxes, stators, coils, or wiring on these machines?

Thanks for the info,

Jason