driveability issues on moby w/dellorto 15/15

First off, let me give you guys a little background. I bought this ped after it having been sitting in a shed for 7 years. I was thrilled to find that the engine still turned over, and the tank had little/no rust in it. It had no spark, which I resolved by putting a condenser in it and cleaning/adjusting the points. I re-set timing at this point as well, and it is dead nuts on. I went with a dellorto carb as the old carb had snapped off at the intake and I read good things about that setup.

I got done with the swap and it ran fine for about 3-4 miles. After that, I got a pretty brutal backfire through the carb which blew the steel mesh filter off... Ever since then, I have a RIDICULOUS lack of power. It will accelerate on the stand, but not as quickly as i'd like, and if I'm riding it, it will often times barely even get up to speed unless I pedal it. If I get it going with the pedals, it will climb to 20-22 VERY SLOWLY and it feels like it may die the whole way.

The petcock is new, I have adequate fuel flow and the gas is very clean. Plug is brand new, I've also already tried swapping it. Plug wire, coil, both new, nice blue spark, plug is gapped correctly. Timing is perfect. Adjusting it makes it no better. All the wiring on the bike is intact and connections are tight/clean/good. Ground(s) are good. I took a look at the variator (pulled the clutch bell off) and both of the spring loaded weight things move freely and the friction material is all good (I sanded it anyways.) I lubed the variator while I was at it. The belt is new and doesn't slip. Exhaust has been removed from the bike - I had to clean it out as it had a hornets nest in it - ha. No exhaust restrictions at all. Head gasket is intact, base gasket is new (I know this because I pulled the motor apart yesterday to ensure that it wasn't a compression issue.

Here's the question. Would missing that mesh filter be the root of all of this? I could see it causing a lean condition - but so lean that the bike won't move? It's equally as bad hot as it is cold. I am running, i think, a 59 jet, which is what was recommended to me.

I attached a pic of the plug after a couple miles, it's a sandy color. It's a shitty pic, but if i get the plug closer to the camera it blurs up. Cell phones.

So. Anyone have any input?


Re: driveability issues on moby w/dellorto 15/15

To clearify, are you running with a pipe on or not? Depending on how I read that line it could be taken either way.

I think that missing the little mesh screen is not that big of an issue, although are you missing the black cover as well? A 59 jet on a stock engine is on the rich side or right on (58 on a stock cylinder was perfect for me). You may want to try putting a sock of other minor restriction over the carb and trying it then, but I am inclined to think that the bigger issue has to do with the fact that it backfired at all, but I am not sure what that means either.

Sorry not of much help, good luck!

Re: driveability issues on moby w/dellorto 15/15

Wait - you have no exhaust at all? nothing - an open port? That is your problem.

Re: driveability issues on moby w/dellorto 15/15

No, no, sorry. I removed the exhaust to clean the guts... It is back on, no exhaust leakage at all.

Re: driveability issues on moby w/dellorto 15/15

find hippy

Re: driveability issues on moby w/dellorto 15/15

) Cupermcnewbster ( /

sounds like the timing slipped causing the backfire. I remember hearing (could be wrong) that tightening the novi nut can twist the cam a bit, thus moving the timing and causing issues. Maybe just too advance/retarded?

backfires are almost always timing related.

Re: driveability issues on moby w/dellorto 15/15

i have rechecked the timing 2 different times - it hasn't moved. i'm not sure about a 2 cycle engine, but on a 4-stroke (I've worked on cars for a living for years and years at this point, but never touched a moped/scooter/motorcycle/lawnmower/whatever) backfires through the intake/carb can also be caused by an excessively lean mixture. Anybody else?

Re: driveability issues on moby w/dellorto 15/15

whered you get your condenser from? the NOS ones are notoriously crap. most people prefer condensers from auto parts stores.

when i had a bad condenser on a moby, my symptoms were similar.

Re: driveability issues on moby w/dellorto 15/15

mine is an NOS unit, i bought from mopedshop. i couldn't find an internal one anywhere - do most people run an external condenser? i guess i could have weak spark at higher rpm but i have BRIGHT blue spark while spinning the motor with a drill, seems pretty intense.

Re: driveability issues on moby w/dellorto 15/15


Sparks can be deceiving. I once had a perfectly runing bike, and then it died. I checked everything; the spark was strong and blue when I kicked the engine over. Come to find out I needed a new spark plug, it fixed the problem instantly.

Not to say that you need a new splug persay, but that blue sparks can be filled with trickery.

New condensor: $3.60

New Spark plug: $2.14

Having a motobecane that runs right all the time: not possible

Re: driveability issues on moby w/dellorto 15/15

so i think i took care of it. i guess those d/o carbs or whatever suck a shitload of air without the filter. if i restrict the airway with my hand i get a TON of power both under load and with no load. i've gotta order another mesh filter i guess. bummer!

Re: driveability issues on moby w/dellorto 15/15

Or you could get creative with some other filter material (another foam filter, cut down, socks, kittens, etc)

Re: driveability issues on moby w/dellorto 15/15

58 jet is WAY too lean if ur running sans air filter on a 15:15 carb. I used to run my 15:15 with out the filter with the stock 68 jet it came with and that seemed appropriate.

here is my stock moby jetting guide lines:

56-57 stock all filters slow cylinder

58-61 its all stock, not a slow cylinder and your using all the filters the dellorto came with

62-64 w/o the black cap or mildly ported with the black cap

65-66 if its mildly ported w/o the black cap or unported with no filter

66-68 heavy porting and matched with black cap, or mildly ported with a high flow filter

68+ ported, high flow filter etc.

kinda confusing but it always works for me

Re: driveability issues on moby w/dellorto 15/15

You may have blown out a head gasket. I did this today on a moby that i'm building for a friend.

Re: driveability issues on moby w/dellorto 15/15

Model post. This post is a good example of what happens when you do your troubleshooting before asking for help, then write a detailed and readable explanation, and check back regularly with updated info. You get lots of good input from good folks. The author even came back and told us what he did to fix the problem! I hate it when I find a post with the exact problem I have in the archives, to find that once the solution was found, it was never posted.

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