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General Category >> Rubber Side Down! >> It STILL wont go above half throttle
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Message started by DrT on 04/19/10 at 10:44:43

Title: It STILL wont go above half throttle
Post by DrT on 04/19/10 at 10:44:43

About a year ago I posted a message about an old savage that I had rebuilt. The problem was that it started fine, pulled well but then died every time the throttle was opened more than half way - anything over 40mph in top gear. As soon as the throttle was returned to less than half it picked up and ran well. In the end I abandoned it because it also had an oil leak due to stripped threads in two head cover bolt holes. Over the winter I stripped it down again and helicoiled the stripped threads. It is now oiltight but still has the same problem. This doesn't seem to be a carb problem.

So far:

I have tried two carbs, including one that had been ultrasonically cleaned in a heated ultrasonic bath - problem persists. There are no holes in diaphragm and slide moves freely

I have tested the petc0ck - on prime it flows very freely but the problem persists even if it is run on prime

New fuel line and vacuum line made no difference

New spark plug - no difference

New coil/HT lead – no difference

Changed CDI unit – no difference

Changed main jet for larger sizes - no difference

I am starting to run out of ideas. Originally I thought that it must be a carb problem but I think I have eliminated that option. I am starting to think that it might be electrical - the only things I haven't yet tried are replacing the stator and replacing the plug cap. However, before I go any further I would welcome any helpful suggestions about what could be the problem. Just to be clear - we are not talking about a bit of chugging or hicupping at WOT - if I crack the throttle open completely it coughs, splutters then dies almost immediately. Close down to half throtttle and its OK again.

Title: Re: It STILL wont go above half throttle
Post by Reelthing on 04/19/10 at 11:28:20

Man, given all the things you've been through... - maybe a different thought - how about the kill switch on the throttle handle is has an issue - bypass that and retest

Title: Re: It STILL wont go above half throttle
Post by BurnPgh on 04/19/10 at 18:13:29

CDI and wiring harness match the engine year?

Title: Re: It STILL wont go above half throttle
Post by Sandy Koocanusa on 04/19/10 at 19:33:36

If it was one of the many old cars I've owned over the years, I'd say maybe the vacuum advance on the distributor wasn't up to snuff. Our timing doesn't advance though, does it?

If it was slightly out of time ALL the time, would it act the way you describe?

Title: Re: It STILL wont go above half throttle
Post by jef.savage on 04/19/10 at 19:40:27

After I put a '96 engine in my '86 I started it up with the original CDI. It Ran fine at less than half throttle wouldn't run at more than half throttle. It didn't die but just sputtered and backfired like crazy and made no power. Installing the marching '96 CDI fixed it. Are you sure your CDI matches?

Title: Re: It STILL wont go above half throttle
Post by DrT on 04/20/10 at 13:02:27

Now THAT is a VERY interesting thought! I bought the bike in bits several years ago and I just assumed that the CDI would be the right one for the bike. The bike was registered in 1989 so its pretty old. Its quite possible that both CDIs are for later models (I think the engine is original as it is the 4 speed version). I will acquire an earlier CDI and see if that does the trick. Watch this space...

Title: Re: It STILL wont go above half throttle
Post by T Mack 1 - FSO on 04/20/10 at 13:09:17

The ignition module does change the timing at a certain RPM.

Sorry, I can't remember if it retarts or advances it and at what RPM.

That said, it could be why the wiring connector changed from the early years to the newer bikes.