how to get extra speed

ok, w/ the bbq5 race coming up, i'm hoping to squeeze a bit more speed out of my bianchi. it runs fine, and at least 35mph or more (my speedometer seems to max out about there). but ... once it hits critical top speed (esp on downhill) it starts to stutter a bit ... like it wants to go faster, but something's holding her back. it doesn't slow down, just briefly accelerates a bit more before dropping down to its original top speed. it's like i hit a wall and the bike just can't go faster.

here's the set up so far ... it's a minarelli v1 engine w/ good compression and timing. it has a dellorto 15.15 carb (w/ large airbox, two wholes facing towards the engine), a polini custom exhaust muffler, and a size 53 jet. oh, and a 10-tooth front sprocket. starts up fine, and seems fine otherwise. any ideas?

i do have an 11-tooth sprocket i might throw on (at the risk of losing some bottom end). but if anyone can think of a reason for the stuttering, please let me know. thanks.

Re: how to get extra speed

Ron Brown /

Miguel,

Have you tried it without the air filter and/or air box?

Ron

Re: how to get extra speed

I recently experienced a similar "stuttering" at full throttle on one of my Vespa Ciao mopeds (Dellorto 12.12 carb). It ran fine below full throttle. I couldn't figure it out. Then, on a whim, I took off the banjo (fuel inlet) and discovered that it had varnish caked on the inside walls of the inlet tube. It was like hardening of the arteries. At first glance, it looked normal, because the hole inside was perfectly round. However, some carb cleaner, compressed air and a piece of wire took it right out. Now it runs 5 mph faster than before and no more stuttering. Clearly, the Dellorto wasn't taking in fuel fast enough.

Chris

Re: how to get extra speed

awesome, i was going to do a cleaning of the carb and change to a new spark plug anyhow as my pre-race prep, but i'll pay especially close attention to the banjo. good tip, chris.

ree, a couple of the decepticon guys also suggested running w/o an airbox. but i'm a bit leery. wouldn't things damage the metal screen? or am i being paranoid? the optimal thing would be a racing airbox (those round ones). but i don't have one ... and the people who do, aren't giving them up.

what about jetting? any suggestions? i can never remember whether to go up or down in sizes.

Re: how to get extra speed

Ron Brown /

Miguel, be careful, you may offend Ree!

The air cleaner/air box question was as a test only. One trip in a non-dusty environment will not do damage. However, if your air cleaner is restrictive, prolonged riding without it could lean out the mixture enough to sieze it.

Just remove it and take a test ride to see if it is any different.

You can also turn off the gas at full throttle and see if your power increases just before the motor dies.

Ron

Re: how to get extra speed

oops sorry, i misread ron as ree.

too late to change anything now. i put in the 11-tooth sprocket, and noticed a sizeable jump in speed. i think it's all i can hope for by tomorrow. i think i need a larger manifold or whatnot. well, the bbq is only hours away. should be great fun!

Re: how to get extra speed

Reeperette /

Heh...I wouldn't be offended in any case.

Running without the airbox for a long time, is a bad idea...but for a short hop or test run, it should be ok - make sure to do a plug chop so you don't run too lean !

One mod you can manage is to remove the metal screen and replace it with a type of foam they use in racing air filters, but others would know more about that than I do.

-R

Re: how to get extra speed

actually ... it's a dead issue. i didn't realize that i was comparing my moped to two 60cc outfitted bikes. i put a new spark plug and tightened the chain a bit (and the 11-tooth sprocket). now it runs like a champ. i'm passing cars at almost 40mph (if not that, my speedo is buried).

oh, and i got 2nd just a notch behind another minarelli bike in the mad cross-town race. ;-)

Re: how to get extra speed

Hi,Miguel! I had the same problem with my Minarelli on my fastest MOTOMARINA and it turned out that it needed the 2 plugs pulled in the airbox and another 7/16 one drilled between them before it stopped that hi-speed stuttering.Has run great now for the last 1000 mi. or so,but it likes the regular Bosch plug better than the Bosch Platinum for some reason.It gets better fuel mileage now since I drilled the airbox.I also fashioned an aluminum snout out of roofing flashing material about 2" long facing rearward to keep out water splashing up.It seemed to run even smoother then!Sorry you didn't win the race `cause I'm pullin' for Minarelli BIG-TIME !!!

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