Magnum main jet

I am trying to squeeze a little more speed out of my 1979 Puch Magnum. It is in great shape with 600 original miles! It will do about 34 mph. It has a Biturbo exhaust, a 48 main jet, and the needle is set at the top notch. I have tried a 56,58,68, and 70 main jets. None of these work. The bike runs, but not very well with those other jets. Should I try some other sizes? Any suggestions? Also, when I remove the plastic pipe from the air box it runs worse not better (when I did this to my Sport MKII it helped alot). Has anyone found a good combination that will get me a little more top end? Or, is this about all I can expect without doing something radical to it (i.e. eurocylinder)? THANKS!!

magnum.JPG

Re: Magnum main jet

XBrandon EdgeX /

If you're getting 34 mph out of it, what's the problem :)

A few things you could do, if you don't have an 18 tooth front sprocket, put one on and you'll get more top speed, but you sacrifice a bit of acceleration, not much though. If you already have the 18th front sprocket, you can put a 40 tooth on the back and it'll have the same effect.

As for the airbox, I know you can modify the Maxi's airbox, but the magnum has a different type. If it's the round airbox, you can cut the end of it out and jet it a little richer and that should give you more power.

Re: Magnum main jet

Thanks, I am happy with 34 mph. It would only do 27mph until I installed the Biturbo. I just wanted to see if there was anything else that I could do. My Sport MKII goes a little faster and I was hoping to do the same with the Magnum. It already has 18 on the front and 40 on the rear (factory set-up). The other thing is that it has a 12mm intake and the Sport has a 14mm intake. I tried swapping the intakes but I got a high pitched squeeling noise and worse performance. I did not try the carb from the Sport though. It loooked identical to the one on the Magnum though. 48 just seemed small for the main jet. The one on my sport is a 56 and the one on my Maxi is a 58. Thanks for any suggestions. This forum is awesome!!

Re: Magnum main jet

XBrandon EdgeX /

Wow, I've never heard of a 18 tooth 40 tooth combo from the factory! I didn't even know you could get anything exept a 45 tooth stock on the rear.You can get a 36 tooth for the rear end, but then you'll be geared as high as you can possibly go! Oh,wait, I think they also make a 22 tooth for the front as well. The 14mm intake should have worked after re jetting, it just would have not accelerated very well.

Re: Magnum main jet

I am the second owner and the first hadn't riden it in over 15 years. It only had 600 miles on it when I got it. He was an older gentleman and didn't do anything to it so I am just assuming it came from the factory that way. I tried the intake and re-jetted but I used the original carb (tried 56,58,68,72). Is the carb the same or is there a 14mm and a 12 mm carb. How do I tell the difference? They both look the same. It was easy to tell on the intake because they were stamped and the diameter was obviously different. The numbers on the carb don't make any sense. It wasn't obvious that one is a 12 and one is a 14. Also, the squeeling noise that it made didn't sound right. It almost sounded like a vaccuum leak. The gasket was good though. I thought that maybe there was some sort of mismatch with the cylinder opening for the intake. It is round and I have seen some that are square. It just seems like I am missing something simple. maybe I need to try some other jets. Thanks for the advice. Joel

Re: Magnum main jet

XBrandon EdgeX /

The carbs should be the same. If they are different, you should be able to see inside them and notice a difference.

There is a good chance that it is a vaccum leak, either around the clamp on the carb or at the base of the manifold. You should take the manifold to a machine shop and have them see if the base of the manifold is flat, and if not, have them plane it. If it leaks around the carb, I think some Teflon tape should fix it.

Re: Magnum main jet

Joel,try a #52 or #53 jet,do a plug chop and adjust your air accordingly.My Motomarina is awesome with a #52 jet after I got the air box opened a little.Also,are you sure you're not `Maxing Out' on rpm due to an overly big sprocket on the wheel?Probably you're not but it's just a thought.I would think with a bi-turbo exhaust you'd need at least a #52 jet.Don't go too fast though.Be safe!

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