dropping exhaust valves 1999 400

[Old scooterist]

Hi all, Peter here from Estepona, Spain.
I posted this on the greetings 'Hallo' section and it was suggested I mention it here too.

I have an AN400X I bought when I decided to move to Spain in September '99, "Any colour as long as it's black" got me a discount and 850 part ex for my old Honda CB72 cafe racer, sight unseen.

No problems for 18 years them it dropped both exhaust valves whilst cruising at 80Kph. Just managed to keep it on the plot and miss all the cars that roared past indifferent to my plight and stop on the side to call the tow truck, so glad I had the recovery option.

Usual carnage ensued, Smashed head, cylinder, snapped the con rod and stuffed it out the case into the rear tyre.

Bought a second hand engine complete less carb' and with CPU and checked it out and stuffed it in, not long after it did the same thing to that engine.
Both engines were untouched as built by Suzuki and serviced.

I have rebuilt the second engine with another second hand head and a re-bore using a Vertex piston and all seemed OK until l I started it, fired up straight away but died. in the end I took the head off and this idiot had left the paper towel in the inlet to keep the crap out in the inlet and it sucked it into the valve seat. so easy fix, no damage. had to hit Amazon for a new compressor kit as I seem to have either lost mine or lent it, (same thing really)

Has anyone had dropped exhaust valves? I didn't over rev it, I was just cruising at 80Kph at the time so well under the limiter.

[Deleted User 18446]

Had you adjusted the valves if exhaust gap too tight can get too hot .Never heard of being s problem on others that age

[Old scooterist]

Valve adjustment was as book. 017-022mm

[Old scooterist]

Well, back together and after a boost to the new battery it ran, lot of white smoke, a puddle of oil by the back wheel but it runs.
Had to bleed the brakes of course, rear is a sod as it includes the front as well but all good now, I don't like my brakes too hard and lacking 'feel'.
Bodywork went on not too bad. main problem is finding where the hell they all were after all this time. At least it will not fall apart when I ride it to the test station.
I was quite surprised by 43€ TP, recovery and legal, (they don't do TPFT here) insurance, I expected nearer 80€, even at my age.

Going to ride it about the building a bit before I insure it and g et it tested as if it goes wrong I am not wasting time and money.

I just hope the smoke disappears as it never used to smoke, I hope it is just the re-bore settling in.

[Old scooterist]

Well, got it back together but would not turn over, just clicked, so bunged on the booster pack and still 'clicked', so suspecting hydraulic lock from flooded carb' I pulled the plug, it was dry and still no go so had to strip off the left covers to turn the crank by hand and it then turned over on the button. reassembled but so pissed off I went home, hope it starts when I return to it this week.

[Rocketman122]

interesting read. thanks for the update. I hope you can get it sorted. I do my own wrenching on scooters, though not all, though its above my level of expertise here.

[Data]

Hello Old scooterist, glad you are making progress. But I am a little concerned your first engine dropped the valves, only to have the same thing happen to the second engine. That's a rarity! The only connection I can see is that the bike was using the very same carb on both engines. You now have your second engine rebuilt and you say it's smoking. You can of course expect some smoke after an engine rebuild but that should pass very quickly. I am suspicious that your carb may be part of or all of the problem causing the valve problem. Trust me, it can happen. I've worked on Burgmans over a considerably long time, both the 400's and the 650's. Some of the carbed bikes too. Of course it might just be coincidence that both your engines were running very worn cam-chains or a worn cam-chain tensioner let go or some such issue that caused the problems.

My questions to you are: did you definitely find out the cause of the original engines valve dropping problem? Did the cam-chain break? Was the camshaft broken? Did you identify the problem causing valve dropping on the second engine? What mileage was on the first engine? Do you know the mileage on the second engine? Incorrect mixture, contrary to popular belief can also cause these mechanical maladies. I'd be looking at your carb very closely. They don't last forever. But you may know all this anyway, and probably do. Interesting and I'd like to know more. Thank for posting. Come back with more info if you can.

[Old scooterist]

Thanks for the interest.
The cam was perfect on both as was the chain, very little slack and adjuster was at 50% when removed, (I removed spring first). Rockers and other upper parts good too. Just the valve heads snapped off where the bimetal joint it, as you know the head and stem are two bits welded together.
Only fault I found yesterday was the choke stuck shut, (extended to 32mm) so pig to start. Trying to buy a new one
Usual goo from bloody ethanol had to be cleaned out after standing for ages and yesterday it ran fine after I got it going, even very little smoke but a little oil by the back wheel. usual exhaust header glowing red of course due to no air flow, it is a heavy duty stainless one.
The plastic cap is cracked on the 15F2 carb' but I plastic sealed that on the inside.
So no obvious reason for either blow up.
I was doing about 55mph, 90kmh the first time and about 70mph, 120 kph the second time. Rider only.
As I said, I binned the first engine as the crankcase was smashed and only check service parts one the second used one which lasted a few days.
Mystery, just hope this one last longer.
Peter

[Data]

Yeah, I'd say your motors blew because of a serious carb/mixture problem. I'm factory trained on these bikes. The headers hardly ever glow red so yours is seriously out of wack I'd say. There could be other issues too! I personally wouldn't run your latest motor on that same carb. Get a new one. Check for other air leaks too especially on the intake side. I'll bet you'll blow that motor again using the old carb. Let us know what you find. Good luck!

[Old scooterist]

Thanks, I will sonic clean the carb this time.
Buying a new 15F2 is impossible and a used one will most likely mean buying trouble and a later one cost as much as the engine is worth. In fact a new carb' and a new exhaust would right the bike off.
Don't khow an engine with an exhaust temperature of about 900 degrees could fail to run red hot when static, I did say it was Stainless.
Currently trying to find out if the Artic Cat choke is the correct extended length as they are about for £20, none for the Keihin
15F2.