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jaxon55

Engine revs high

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I am a new member (forum and club) and bought a 1962 LI 125 with a Deanspeed 190 top end, Ducati electronic ignition and a Mikuni 30mm flat slide carb. It has a small mesh air filter. It starts great but if I ride it at a higher speed (40+) for a few minutes it will either stop, and I have to grab the clutch lever to stop the rear wheel locking or if I come to traffic lights or other stop, it will rev at full revs. I use 90 octane gas from my local marina and best quality fully synthetic oil. It sounds like a carb issue.  I am a total 63 year old novice so any suggestions are greatly appreciated.    

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You are having much more than a carb issue.

Get ready to rebuild that engine. 

You have been experiencing heat seizures and the engine is racing at a stop because of an air leak somewhere... check cylinder head, intake manifold, loose carb, worn or holed air hose, loose exhaust header, magneto flange leak, oil seals, or cylinder base gasket.  

time to get busy tearing that engine down  

My guess is the piston, cylinder, possibly the crank and bearings and seals are no longer any good after the heat seizures  

Get yourself a new Sticky manual and you will learn a lot about what your are going to be working on  

good luck with this  

where are you located?

 

 

 

 

 

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Thanks for this reply Lambrettayeroc. Hope you are wrong, as the bike was just rebuilt! But your reply is greatly appreciated and worth me looking into. I am in Central Ohio. 

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He's not, this is definitely siezing. First thing I'd do is a leak down test. From there you'll probably be on the hook for piston and bore at the minimum but at least you'll know where the issue that caused it came from so you can address that on assembly.

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After doing what has been suggested,  I would also check what plug is in it, what your timing is set at at. Check it yourself, don't take someone else's word for it. Also, are you sure it's a mikuni 30mm? A flat slide tmx or is it a 28tm? There is no 30 tm. A 30 tmx can be very tricky. The needles, power jet and needle jet can only be ordered from europe. They do not stock them in the states. At least I have not been able to find the 622 series, or power jets here. If it is a 30 tmx,  I would suggest getting an easier carb to tune. If you have checked your timing and it's 18 degrees or less and you have at least an ngk b8es and are committed to using the 30 tmx, I would increase your main considerably.  If it's at 220, I'd jump to a 240 and increase till it runs rich and back off from there. Hell of a hobby. Did the engine have the symptoms you described since you started riding it or did it steadily get worse? Did you make sure there's oil in the case? 

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30 carb is also WAY too big for a standard cylinder deannspeed 190.

WAY TOO BIG!

25 carb is the biggest I’d go for that set up. 

We also don’t know what exhaust you have. 

I’m not a fan of dean speed piston set up for many reasons.  I’ve seen SO MANY fail. 

In my opinion the cylinder walls are too thin on a 190 from a 150 or 175 cylinder to dissapate heat properly. I do not recommend going over 175 on a 150 cylinder  (I know I’ll hear feedback on this... I’m entitled to my opinion though. :)  )

But you have what you have. 

Take the cylinder off and inspect it.

Hopefully you can get away with a new piston. 

There is not much room for oversized ones in that set up. 

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As the "King of Soft-seizes" I have found (no seizes in over six years) you must break the engine by the book.  First gear 15 mph max; 2nd gear - 20 mph; 3rd - 30 mph; 4th - 37.  Driving  over 40 for any length of time will fry it.  Do not maintain any speed for long periods.  Do not lug it.  No hills at full throttle.  Do not accelerate at full throttle.  All of this is intended to keep the engine as cool as possible.  Heat will cause seizes and damage pistons and barrels.  (I have been there at least four times)

WW

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Did you do the rebuild or someone else?  If someone else get in touch with them. 

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I'll go a bit against the grain on this one.  Start simply.  If it is an air leak, start with the intake manifold and boot holding the carb to the cylinder.  Tighten those first.  Go for a ride.

If problem persists, move on to the 4 head bolts.  Then maybe tighten the mag flange, but to do to do this, you need to pull the flywheel.

Does engaging the choke calm your out of control revs?

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Thank you all so much for these replies. Its a steep learning curve but all what you are saying makes sense. The whole scooter was built by a highly reputable person and I was given strict running in instructions which I followed. The exhaust is a Taffspeed. The only things I have adjusted are the air screw and the tickover screw. I have just looked at the carburetor and think that it is not a flat slide, possibly a vintage Mikuni or a VM series. The air /fuel screw was out three and three quarter turns. I have adjusted back to two and a half turns. I have attached photos and maybe one of you will know it. The only letters I can find on it are W5 and underneath 2  2?IMG_3417.JPG.747c32ace60750651b7a3755ca454777.JPG 

IMG_3416.JPG

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This highly reputable person who built the engine... did you have them do this work for you?

Why are they not stepping up and getting this dealt with for you?

I don’t  need / want an answer to these questions but I’m putting this out there  

 

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OK, well if the choke engaged slows things down, you most likely have an air leak.  Now you need to find out where.

Looking at the carb, this screw is an air mixture screw because it is in front of the slide.  If it was behind the slide, it would be a fuel mixture screw.  Turning this screw counter clockwise allows more air, clockwise less air.  3.75 turns out for such screws is a bit much.  But, overall, this screw is most-likely not the issue.  I'm just letting you know what the screw is for.  You want a fairly slow tick over at idle, and you want a fairly quick return to idle after full throttle.  You don't want the bike taking forever to return to idle.

I'd move on to tightening the intake.  Easier when the front of the motor is facing down.  You remove the rear shock, push down on the bike, and the front of the engine goes down.  This will allow you to more easily get to the two intake fasteners.  You may have to remove the cylinder shroud to access one of the two fasteners.

If you want to work on these things, you must by the Stickies book.  There's no other option.  ?

IMG_3417.JPG.747c32ace60750651b7a3755ca454777.JPG

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Thanks again guys. I will do/have done as suggested. I got the spark plug out today (wrench arrived yesterday) and it was black but not over wet. I need to get Stickies book!!

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Sorry for the delay in replying guys. The problem was the Mikuni carb (fuel starvation). I found someone who specialized in vintage cars and bikes etc. When he stripped the carb he found silicon in places there shouldn't have been. It was basically crap. The carb was replaced with an appropriate Dellorto, everything was gone through and now she runs as she should. Engine was checked re bearings but all was well (I appreciate the info to check this). Now just gaining my confidence in it again. Much nicer to ride without listening for the engine cutting out!

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