Grande problems again...Zippy, Fred.

Alright...I just got my new proma circuit exhaust, and 13mm carb, with a new airbox. All brand spankin new. I ride it for about 10 minutes and it is fine, then all of the sudden it will die. When it dies, it is almost like it isn't getting gas. It kinda bogs, then just goes kaput! Then it won't start, but the engine turns over. I walked it about a half mile, then it started up...I rode it for like a quarter mile, and it died again...started bogging then just stopped. Man, I just love walking my moped over a mile back to my house. At least this time I knew how to disengage the engine! :) Does it sound like a fuel issue, or a timing issue? I am leaning towards fuel, but I just dunno. I shined a flash light into my cylinder with the plug out and the top of the piston is all charred and black. I know the carb exhaust setup isn't the deal, because it is doing the same exact thing that it was doing, before I got the new parts. I also bought a 70cc kit for it, but I don't want to install it until I figure this problem out.

Re: Grande problems again...Zippy, Fred.

Could it be the rings?

Black and 'charred' is good

What color is the plug while its running good ?

Sure sound like fuel to me.

Pulling the gas line from the tank... you get good flow continuously ??

You have an inline gas filter ??

Take the carb back off and blow it out with compressed air (especially the float needle and seat)

Are you capable of checking the timing ?

Do it if you can.

It wouldn't usually be a 'timing issue' if it just ups and dies... (unless its getting REALLY hot... which incorrect timing can cause).

Do you know compression is good ?

If its not fuel or overheating... then it can sometimes be an electrical component that quits working when it gets hot.


As far as the piston being black and charred.

its when the piston starts to go gray that indicates detonation (bad bad bad) and overheating.

If its black around the edges and going gray under the spark plug you are heading to disaster (holed piston)

(same as a pllug going white)

Re: Black and 'charred' is good

Plug is still I know it is running lean.

When I pull the line from the petcock, the gas flows steady. But it just doesn't poor. The dellorto carb, has a built in gas filter on the fuel banjo.

I am not sure how to check the timing.

I have a feeling that it is getting super hot, cause I can smell it. But I don't think it is seizing either because it still turns just won't start.

I know someone told me how to check the timing, but I can't find the post.

Re: Black and 'charred' is good

as far as compression, i haven't checked it. what are symptoms of bad compression?

Re: Grande problems again...Zippy, Fred.

Hmm you have boldly gone where Zippy has not gone before.

But lets use logic to see what we can do. You say the the top piston looks all charred and black. From that we could deduce that ;

1. it isn't exhausting all the burnt gases or

2. is being overwhelmed by fuel so it can't exhaust all the gases.

So basically it is choking to death on smoke ( no oxygen left in the cylinder to support combustion)

To test this theory take the exhaust off. See if it will start and ride it around the block (so you are close to home) or a big parking lot so you don't have to push it home so far. Keep riding to see if it runs better and doesn't stall.

Check the port and clean it along with checking the exhaust pipe and muffler.

The piston acts as a vacum pump on it's downward stroke pulling air out of the cylinder. The piston on it's downward stroke first pushes exhaust gases out the exhaust port then as it continues downward it sucks the air out of the cylinder causing gas/oil mix to rush in to fill the void. Now the piston starts back up again to compress the gas/air mix till it gets just before TDC and the spark plug fires. If the cylinder is still full of exhaust it will be become like a stopped up fireplace.

Now for why it is being overwhelmed or not exhausting properly. I suspect a relationship between the new carb providing more air/gas/oil mix.

Maybe you need a hotter plug. the lower the number the hotter the plug. I use NGK B6 or 7HS on stock Grandes. I would recommend trying a B5 or B4 gapped at .020.

Also take it a step at a time. Take off your cylinder head and clean the domed top and the pistion top (decarb). Also clean the exhaust port before you put the stock muffler back on.

Then put your stock muffler back on and ride for awhile (a day or two) with the new carb and old stock muffler.

I would recommend you contact your supplier because they should have some experience with this.

I hestiate to recommend timing changes or even a hotter plug for fear of overheating or burning a hole in your piston as Fred pointed out.

Try decarbing, go back to the stock muffler and take it a step at a time. Don't introduce too many variables into theix at one time. Then work with your supplier to see how they can help.

Give us a report back on you progress.


geeez doood... I wish someone would write a nice guide that contained advice and how-to about checking compression and setting timing.

We'll have to find somebody to do that.

Do these ------>

1... Why does the petcock just 'trickle' ?... no good.. take the petcock off and clean it till it flows good... check the filter on the new carb for crud buildup.

2... Do a crude check of compression (finger over the spark plug hole while somebody else pedals)... do it properly - read the guide.

3... This ped has points ?... then set timing per my 'guide'.. (I hope your ped has T and F marks on the flywheel.. use the cig paper method)

4... If the plug is white... Then you better put a richer main jet in now... you can go back down later.

5... The plug is white ?... Then DO NOT put a hotter plug in it !

Do all of those and then answer if it doesn't get better.

Re: More

Fred... I know that your guide has a timing section. :) Thank you very much, but I looked for the T and F and couldn't find it. I am probably going to take it to Ike's shop so that we can work on it this weekend. He has jets. I just don't get it. Casey put pretty much the same pipe on his grande and the exact same carb and airbox, and his ran perfect. That is why I am confused. I looked at it again after I posted last night, and here is what else I noticed...There is oil around the base of the cylinder. Maybe I need to replace those gaskets too.

Zippy...I have all kinds of oil comming out of the hub on my rear wheel, where you put in the gear oil. Is there a gasket for that? That couldn't cause my problem could it? At any rate, if there is a gasket in there, I am going to want to replace it, right? Or is this normal?

Re: More

Ron Brown /


I am in favor of you taking it to Ike before you have a disaster.

Everything Fred says looks good to me except for the fuel flow. One man's trickle is another man's satisfying piss. Imagine the rate at which your ped can use fuel, like how long would it take to consume a pint, then let the fuel flow into a pint jar and see how long it takes to fill, it should fill at about 10 times as fast as you can use fuel. This gives you a better idea of adequate fuel flow and will also show a clogged tank vent.

If you can easily drain the float bowl, Turn off the gas, drain the bowl and see what comes out. Then ride til it quits and turn off the gas and try again. If you get a lot less fuel, you have a fuel flow problem, which could be causing lean running and you need to fix it before you can jet correctly.

You also must have the timing correct before you can jet correctly.


Re: More

Now Ron, you just confused me. Should I run it once, till it dies, then drain the bowl...Then run it again, till it dies and drain the bowl again to see if less comes out?

Re: More


The oil on the rear hub is not related to your engine problem.

There is a seal or fiber washer that should go on the access screw or hex head bolt , whichever you have on your Grande. There is also an oil seal (Black rubber "O" ring) that goes on the shaft that the rear clutch rides on. Either of these could be causing the excess oil.

As far as the engine issue, i agree with the other guys, to take the fuel petcock off and clean it. take the fuel line off at the petcock end or you'll have heck a of a time getting the petcock out.

The petcock has a plastic tube covered by a wire or nylon mesh filter that sticks up into the frame. Pull the petcock out carefully and slowing so the mesh filter comes out with the petcock. Once out, put it on the bench and carefully unscrew the mesh filter. Then you should see a plastic tube. You can remove this to, it just pulls out. Use some WD40 to loosen the gunk around the tube before you pull it out. Note which hole or side you took the tube from. This is the Full ON hole, the other hole is the reserve hole. You don't have to take the ON/OFF stem out and I would not recommend it.

With the tube out, take the petcock and squirt some strong detergent into the holes on top and gas line fitting end on the bottom Then run very hot water into the holes to try to melt and dislodge some of the old varnish. You can also use acetone but flush it with water immediately. Squirt WD40 in the holes and then use compressed air to blow through the fuel intake stem. Turn the valve to the UP position and blow air into the fuel intake stem and the repeat for the DOWN position. You should be able to get both holes free this way.

Before you put the petcock back on the bike get you a skinny funnel to stick up under the fuel stub on the frame and then flush the tank with about a cup of acetone, swish it around and follow with a couple of cups of gas until it runs free.

Then put the petcock back on the bike. Stick a fuel line on the petcock end with the other end free and put some gas/oil mix in the tank and test the petcock in all three positions to make sure you have free fuel flow and that it will shut off.

Remember you have to have enough fuel in the tank to go above the plastic tube for the DOWN position to work properly.

You can also run hot water and soap through the old fuel line if it is the heavy plastic Vespa original. This will make it a little more pliable and easier to reinstall. If you use this fuel line, use in in your fuel flow test to make sure it works OK.

Re: More

Ron Brown /


Before riding, when the gas has been turned on for a short time, then when it quits.


Re: Grande problems again...Zippy, Fred.

Got it running perfect! I decarbed the cylinder and piston, and put on a new base and head gasket. Now my chops are golden brown!

Re: Grande problems again...Zippy, Fred.

Ron Brown /

Way to go Chris!

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