While going down a big hill WOT....

my moped likes to slow suddenly a few times for like a second or two (soft seizing?) when passing the 35 mph mark. It will also backfire on occasion during these hills.

Can someone tell me what causes this and if there is anything I can do to manage it with the mixture or idle screws?

The bike is an 08 Tomos Streetmate R with 500 miles. Recently added a biturbo, UNI filter and increased main and idle jet. It has done this on hills before and after the mods.

Re: While going down a big hill WOT....

Are you sure its not just the motor kicking back down, it can only spin so fast.

Re: While going down a big hill WOT....

yeah, it's not the motor. that motor can spin a hell of a lot faster than that. Odds are good that whatever jet size you have in their now, it's too small. Check yer plug, if it's not a nice color, get yer self some bigger jets.

Also, when bombing down hills, don't just coast, make sure you're at least giving it some throttle.

Re: While going down a big hill WOT....

No, I'm not sure of anything. When it slows it's very abrupt, like tapping the brakes for a split second and it becomes unresponsive to throttle - so I back off, it becomes responsive suddenly, sometimes with a backfire.

With all these performance guys getting 45+, I wouldn't think it would be spinning too fast only going 35-40.

Re: While going down a big hill WOT....

Yeah I'm giving it throttle down hill all right, wide open... maybe I shouldn't be doing that?

My one and only plug chop looked fine after the biturbo, uni, and upjet. So you are saying to increase fuel (decrease the air) in the mix. I thought backfires were a result of unburned fuel suddenly ignighted.

Re: While going down a big hill WOT....

Only blast it once every so many seconds.

If you open it WOT, you are getting too much fuel and your engine can't compress it as easily?

Just an idea...

Eric

Re: While going down a big hill WOT....

Love 'em ideas...

I just tried the hill again, blasting the throttle every couple seconds... no backfires, just a hiccup once or twice. Once in a while I could find a middle ground where I was applying just enough throttle to keep the motor humming without it hiccuping.

This is my first moped. I was under the impression you could do full throttle down a hill (I'd love to see that speedo needle buried).

Aside from plug chops, how do you guys know when you have the mixture screw just right on the A55 delorto carbs?

Re: While going down a big hill WOT....

I have an '08 Sprint with exactly the same mods. We don't have any really big hills around here, but I always go down hills WOT, and have not noticed anything like that. I jetted and adjusted my carburetor using an exhaust gas analyzer at a motorcycle shop, so it is exactly the same now as it was before I made any modifications. Without one of those, it's strictly guesswork. Peace. Jerry.

Re: While going down a big hill WOT....

Jerry, what size jet do you use?. Mine's a 58 and I'm at sea level and alot more humid and cooler than AZ.

Re: While going down a big hill WOT....

I'll have to check. I ordered like 5 of them, and changed them twice before I got it right. I'll have to see what I ordered, and what I have left. I live at 1300 ft above sea level. Peace. Jerry.

Re: While going down a big hill WOT....

That's not something to worry about too much IMHO. it's just engine braking. (braking, not breaking)

Once the engine gets spinning fast enough, it can no longer help you accelerate er even keep going the same speed. So since your engine can't keep up, and the wheels are still attached to the engine (since the clutch is engages), the engine slows you down.

Truck drivers do that down big hills. Shift into first gear and don't waste your brakes.

I wouldn't worry too much about it.

--Scott

Re: While going down a big hill WOT....

I want a jake brake for my magnum.

Re: While going down a big hill WOT....

Are you sure it is backfiring? If you are running too lean it will pop out of the carburetor which some people confuse with backfiring. The problem with running too lean is that you can damage the engine... maybe even melt a piston.

If it is backfiring you should be smelling unburnt fuel coming out the exhaust and have nasty oily plug.

If the plug isn't oily I would try upjetting a bit more and see what happens. Also check for restriction in the exhaust and fuel systems.

IMHO Terry

« Go to Topic — end of thread

Want to reply to this thread?

We'd love to have you join the discussion, but first you'll need to login (or create an account).