Back firing moped

Michael Miehle /

My grande won't start well, I just put the engine together from spare parts of two engines and to start I have to pedal like hell and juice the throttle. Then it will back fire and run. It runs fine after that, I think the timing is off a little. Tomorrow I plan on advanceing the spark a little to see if that helps. Any sugjestions?

Re: Back firing moped

when you say 'backfire' do you mean that you get actual backfire through the carburetor? Or do you just hear a POP in the exhaust?

Timing does seem like the first thing to check, especially if its backfiring through the carb..

Re: Back firing moped

Michael Miehle /

Both, one was so bad it blew the airbox off. Funny as hell but a bad sign of trouble I think.

Re: Back firing moped

Magneto may have sheared the key, but usually that retards timing. don-ohio

Re: Back firing moped

Michael Miehle /

No that's fine I adjusted the points a little and it starts without backfiring, but now it won't idle. It doesn't slow down to idle very well after releasing the throttle and when it does slow down it sounds could and then will stop with out even a sputter. I rest the plug and points to factory settings and turned the idle screw all the way in and it will idle but very fast. As I ease the idle screw out it slows down but won't idle slow enough to disengage the clutch. I'm using a 12/12 dellorto carb I think I will put my 12/7 on and see if that fixes things. Is there an idle jet on the dellorto carbs? Maybe that's dirty.

Re: Back firing moped

You mentioned turning the idle screw .. the idle screw adjustment determines the size of the idle "jet". So, yes, there's an idle jet. You suspect that the carb may be dirty? My first step would be to clean the carb.. Not slowing down to idle after releasing the throttle can be a mechanical problem but i'm sure you already eliminated a sticking cable and related things.

i dunno.. carb problems are just carb problems.. If everything is tight and no fuel or air leaks, the bike should idle and run half way decent.

I was wondering about the spare parts from two engines that you used to build this one. Were the engines identical? Is the cylinder you are using factory stock? Is the compression ratio still somewhere near normal? What i'm geting at is the questions of port timing.. wondering if someone cut into the ports of the cylinder or milled the head or did any other modifications to the parts you are using.

Re: Back firing moped

Michael Miehle /

I cleaned the carb once already, so I'll pull it and clean it again. And as far as modifications to the parts, dunno? They look stock to me. I thought the idle adjusment just opened the barrel up to let more air in. Isn't there a small hole ti assist with idle somewhere?

Re: Back firing moped

there are 2 idle adjustments .. one is idle RPM.. the other is idle fuel/air mixture.

The idle speed screw mechanically pushes on the throttle linkage a bit.. It does the same thing as slightly twisting the throttle on the handle bars.

The other little screw is the air/fuel mix. At idle, a engine requires a rich mixture and this screw regulates how much air is mixed with the fuel at idle. The combination of this air-adjustment screw and the hole whose opening it regulates is your idle jet.

These two adjustments work together. The object is to lower the idle speed as much as possible. i'll paste something from another thread..

-----------

Regarding crappy idling in general:

your idle air/fuel mixture is wrong for some reason. It could be because of a vacuum leak or it may need nothing more than a simple adjustment of the idle speed screw and/or idle air/fuel ratio screw.

Vacuum leaks can happen anywhere inbetween the carb and the engine block. I recommend using a coating of Permatex 2 (non-hardening gasket sealant) on all the joints.. the carb to the intake manifold, the manifold to the reed valve block, and the valve block to the head. Use a small brush and put a thin coat on both sides of all mating surfaces.

As far as adjusting idle screws, drive a mile or so to warm up the bike to normal operating temperature.. then turn the speed screw in until the motor spins fast enough to idle. (this speed screw simply mechanically opens the throttle linkage.. the other little screw down there is the mixture screw)

Now, slowly turn the idle air/fuel mixture screw in and out until you notice a point where the engine speed increases. Next, back off the speed screw and slow the motor down until it threatens to die.. Then turn the mixture screw in and out till the motor speeds up..

At this point you may be able to slow the motor even more with the speed screw. If so, again adjust the mixture screw until the engine speeds up.

Test ride the bike and see if it the engine rpm drops to a smooth, slow idle. If not, repeat the adjustment proceedure.

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i hope that helps.

Re: Back firing moped

There's only an idle speed screw on Dellortos,Joe. He can't adjust mixture.

I guess you've checked and eliminated any air inleakage past the carb,right? Well, one tihng that can make them idle fast is a worn throttle slide. Also if the gas is loading up some before you start it because of a leaky needle seat, they can idle fast, but should quiet down after a minute or so. don-ohio

Re: Back firing moped

Thanks for the heads-up, Don .. jeeze.. i larn sumfin everyday.. Now i need to know why someone would build a carb where you

can't adjust the idle mix. Or do these Deltortos actually have removable idle mix jets?

Re: Back firing moped

They have a fixed amount at idle. You can drill the emulsion tube holes for richer idling and mid-range, but it's tricky. don-ohio (:^)

Re: Back firing moped

Make sure the cylendar head is tight. Torque each nut to about 15-18ft.lbs. in the criss-cross pattern, you can check the gasket as well, but I would think it could possibly be your timing. If you got another set of points, you might wanna toss them in there.

-A.J.

Idle jet

Michael Miehle /

I didn't see an idle jet were I expected too, were is it located on the dellorto carb? It should be just a small hole right?

Re: Idle jet

Re: Idle jet

Re: Idle jet

Michael Miehle /

Thanks Zippy, I had seen this post before but couldn't find it again when I looked. I thought the idle jet was on the first lip mine was so clogged I couldn't see it on the second lip.

Fixed

Michael Miehle /

Cleaned the idle jet with a small piece of wire and she runs like a charm. As many times as I read it and I still messed it up. Thanks everyone for helping.

Re: Fixed

Good job Michael, man over machine!

Pass on the knowledge someday to fellow mopeder.

Happy Trails

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