Engine won't rev / HELP!

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antero
Cucciolo - the Lil Pup
Posts: 8
Joined: Sun Apr 16, 2006 12:39 pm
Location: Finland

Engine won't rev / HELP!

Post by antero »

Hi!

I bought my -72 450 SCR cylinder and head apart and now I'm trying to get it on the road.

Renewed parts:
- wiring (except the ones coming from the alternator)
- contact breakers (Zanussi)
- condencer
- spark plug + lead
- carburettor (PHBH 28 AS)
- gas pipes
- air filter
- gaskets

The ignition is checked million times with and without the automatic advance, which seems to work. Spark is ok, one grade hotter and colder plug has been in test. Battery is weak but functioning. There is gasoline coming from the tank to the carburettor. There is no excessive smoke and compression is above 9.

The problem is that the engine starts up nicely and rews perfectly up to approx 4000 RPM, but then nothing; the engine stalls and cuts the revs. I can ride the bike, but only on really low rews. The postition of the throttle does not influence on the running after that certain amount of RPM.

The plug is carbon black but I don't think it is because of rich mixture; I have tried two carburettors; the original with standard settings and the new one with factory settings.

Everything is double checked except the coil, regulator and alternator.
Finally I'm starting to doubt the function of the breaker points. Although they're new they can fail? Should I try another points? Or move to electronic ignition?

Please give me some kind of advice, I have been struggling with this for too long now. I really don't have a clue anymore.

Greetings

Antero
450 Scrambler -72
Nick
Diana
Posts: 63
Joined: Wed May 10, 2006 3:40 pm
Location: On da Rock

Post by Nick »

Try it without the muffler. If the muffler is plugged up, or blocked with a wasps nest or other junk (or if you have some sort of aftermarket exhaust which is too restrictive) the bike will run OK at low rpm, but not run at high rpm.
If this doesn't fix it, take the carb apart and check that the main jet, needle and other parts are clear and are set to the specifications in the manual.
From probably the only person in the world who rides a Ducati 350 Sebring for daily transportation.
antero
Cucciolo - the Lil Pup
Posts: 8
Joined: Sun Apr 16, 2006 12:39 pm
Location: Finland

Engine won't rev / HELP!

Post by antero »

Muffler! That's one of things I newer suspected. There is an aftermarket muffler installed by the previuos owner. Thanks Nick for advice, I really hope that is the problem.

I've tried two carbs with different settings. I belive that even though the settings wouldn't be perfect, the engine would rev, altough poorly and accidentally.
450 Scrambler -72
Nick
Diana
Posts: 63
Joined: Wed May 10, 2006 3:40 pm
Location: On da Rock

Post by Nick »

If removing the muffler doesn't fix it, install the main jet as per the manual. From your description it sounds like the main jet has either fallen out of the needle jet, the main jet is plugged up, or the main jet is the wrong size. Also check that the needle has not fallen out of the slide.
Changing carbs will not fix anything if both carbs are incorrectly jetted.
Also, be sure to use a new spark plug. Once the plug has been fouled it will never work properly. Throw it away. Until you get the bike running properly, it is best to simply buy a box of new plugs and change them frequently.
If the plug is coming out covered with soot, the bike is running way too rich. The three most obvious causes are: overly restrictive muffler, overly restrictive air cleaner, jetting too rich.
From probably the only person in the world who rides a Ducati 350 Sebring for daily transportation.
antero
Cucciolo - the Lil Pup
Posts: 8
Joined: Sun Apr 16, 2006 12:39 pm
Location: Finland

Engine won't rev / HELP!

Post by antero »

Okay,
after checking the muffler, carburettor and jets again, I replaced the powerfilter with the good old aluminium funnel. And it works. Still not perfect around needle jet throttle, but much better.

Now I just have to find working settings for the new carb with different kind of air cleaner.

Good luck with Sebring and thanks for advice!

Antero
450 Scrambler -72
User avatar
Craig in France
Paso 750
Posts: 923
Joined: Tue Mar 23, 2004 3:58 pm
Location: Montpellier, France

Top end performance

Post by Craig in France »

Hi Antero

Just a thought ...

Which silencer do you have fitted? I have the short one, but I did once try a medium length one, just to see ... Agh!! Horrible! Top end went totally flat.

Quickly went back to the short one!

HTH

Craig in UK
450 Scrambler
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