Saxon 500 stutters when revving from idle

[TheFlyingPolak]

I have a 1997 MZ 500 Saxon Tour for over a year now and I have had one persistent problem with it pretty much since I got it. Whenever the engine idles for about 15 seconds or more and I give it some throttle, it stutters as if it was starved of fuel or misfiring. Sometimes it picks up from the stutter, but sometimes the engine dies. If I gently give it throttle every couple seconds this doesn't happen. This means every time I stop at a traffic light I have to be really careful with the throttle and rev gently every couple seconds or I may stall when the green light comes on. I set the idle speed higher than normal to counteract that and it helps most of the time but it's not a proper fix.

The carburetor on this bike is the Dellorto PHF 34 GS. I have cleaned it multiple times and all jets should be free of clogs. The accelerator pump is set to only kick in at about 1/4 throttle. To change that I would need to buy a new throttle slide with a different pump cam profile, which would cost over £80. Is this the right way to fix it or could something else be wrong with the engine? I have also recently replaced the air filter and checked for vacuum leaks.

[DAVID THOMPSON]

mine is very poor on power at low idle speeds and runs poor below 3000 rpm

my only electrical problem was the big black wire from battery ground to the engine ..
it got loose at the engine
this caused a brown wire to smoke when tring a start up
tightened loose ground at motor and replaced brown burnt wire and all has been OK

the b plus at the tach needs to be good
and i have had lot of problems with loose spade connectors and bad fuses
over the last 10 years

my carb is a Bing CV like a lot of BMW bikes
some one had removed the slide return spring
which was not helpful in making it run correct

[breakwellmz]

I had a Saxon 500 but don`t remember it having that type of carb.
I`m surprised that your pump actuates that late, the pump on my CVK40 actuates from idle up to about 1/4 throttle.
As an aside i fitted a Rotax additional flywheel to mine.

[ralf]

Have you tried changing the needle position? Are your jets, needle etc to original spec? I have a friend who had his Rotax set up on a Dyno and to get the correct fueling it needed the needle raising so the clip is in the bottom groove. It's easy done just to try and costs nothing. I'm currently trying my 500R on the bottom groove and it does seem better but more miles are needed to be conclusive.

[DAVID THOMPSON]

link i wrote several years ago about spark plugs in the 500

viewtopic.php?f=26&t=827&p=4620&hilit=chain+gang#p4620

read it
different size wrench required with new plug

[TheFlyingPolak]

Have you tried changing the needle position? Are your jets, needle etc to original spec? I have a friend who had his Rotax set up on a Dyno and to get the correct fueling it needed the needle raising so the clip is in the bottom groove. It's easy done just to try and costs nothing. I'm currently trying my 500R on the bottom groove and it does seem better but more miles are needed to be conclusive.

I believe all my jets, needle and throttle slide are to original spec, according to the Saxon 500 manual. I may try moving the needle, though does it not start affecting carburation only from 1/4 throttle?

[DAVID THOMPSON]

on the old 1950's and 60's BMW twins we had a lot of carb problems
till we found the secret to getting the magneto tuned proper

once we had good spark they ran OK

[TheFlyingPolak]

I checked all my jets and it turns out the idle jet is slightly bigger than original spec. My jet is 50 while the original spec is 45. To what extent would this affect the fueling? Should I replace the jet?

Everything else on the carb is to original spec.

[TheFlyingPolak]

It's probably also worth noting some other symptoms I'm experiencing on the bike.

When I coast down to a low RPM, especially in a high gear, the engine begins to sneeze or sputter until I pull the clutch or give it some throttle.

If I close the throttle above about 4000 RPM and begin to coast, the engine tends to sputter or bang once while coasting. Especially if I accelerate aggressively and quickly shut the throttle at 5000-6000 RPM it can bang violently.