Unknown Ignition Timing and Piston Facing (long)

I am rebuilding an old Romanian moped, which I purchase for my kids some 2 months ago for 125 $. The machine is 20+ old, and for sure, it has seen better days. So far I had to re-build the crankshaft threads, and the flywheel key channel. I also had to replace the crankshaft main bearings, including making custom cages as the original bearings were not available anymore.

As for the electrical part, I have set the timing arbitrarily at 2 mm before TDC, as I have no documentation available for this engine. After cleaning the carburetor, I give it a kick, in fact several kicks, and the engine finally started. The idling speed is way too high, and whenever I want to adjust it the engine dies. I am pretty confident there are no air leaks on the suction side, as I have replaced all gaskets, seals, etc. However, I do not have its original 14 mm carburetor, which has been replaced by a more common 17 mm carburetor.

In order to isolate the problem, please provide some timing advise:

1. What would be the best ignition advance to be set for an unknown engine?. I had a look on the mopedriders.org manuals, and the advance for various mopeds is between 1.3 and 3 mm before TDC. It seems that a higher compression requires a lower advance (e.g. for Tomos A35 the compression ratio is 9.1, and the advance is 1.3 mm, while Mobylette AV 7 S engine has a compression ratio of 6.5 and the advance is 3 mm).

2. How the engine behaves if the advance is set higher or lower than normal (e.g. 3 mm instead of 2 mm, or 1 mm instead of 2 mm respectively) ?.

3. I know this engine has many miles on it, and it has low compression capabilities. However, I did not have a compression gauge to test it. With this respect would it be better to set a low advance, say 1 mm before TDC, to make it easier to start? My idea is to initiate the spark when the fuel-air mixture pressure is as high as possible.

The second issue is piston facing. I have been told that piston ring locating pins shall face downward, towards the exhaust port, and I have installed the piston accordingly. Reading Franco Morini M1 Service Manual, I found that the piston shall be installed so that the piston ring locating pins shall face upward and away from the exhaust port (Service Manual M1 Reed Valve Engine, page 30). Should this type of facing be associated with a reed valve engine only? (My engine is piston ported).


Iulian Tatu

Re: Unknown Ignition Timing and Piston Facing (lon

2mm BTDC is ok as a starting point.

The goal of spark advance is to produce peak cylinder pressure a little bit After TDC... somewhere around 15 degrees ATDC. Peak cylinder pressure should happen ometime after the entire load of fuel/air mix has burned and has had an opportunity to contribute to heating the gases in the cylinder.

It takes time to burn the entire load of mixture in the combustion chamber. Spark advance ignites the fuel mixture early, while the piston is still rising.

Fast burning conditions require less advance than do slow burning conditions.

Premature spark wastes power and can cause detonation and engine damage.

A late spark wastes fuel and wastes power.


A more compressed mixture (higher compression ratio) burns faster.

A mixture less polluted with residual exhaust gases burns faster.

An expansion chamber (performance exhaust) will contribute to both of the above when engine RPM reaches the pipe's powerband..

An engine with poor compression requires more spark advance.. (but what it really needs is to fix the cause of low compression.)

Better combustion chamber designs allow faster burning.

A lean air/fuel mixture burns faster than a richer one..

But a carburetor that is too large for the engine cannot atomize fuel droplets as efficiently as a smaller carb. Poorly atomized fuel mix burns slower and wastes fuel.

Too-large carburetors cause other difficulties regarding smooth running and power production throughout the range of speeds. I would use a stock carb if the rest of that bike is stock.

Humidity in the air slows combustion since it both richens the air/fuel mix ratio (water replaces some oxygen) and separates fuel and oxygen molecules with non-burning water molecules.


Piston ring ends must not travel past a cylinder port opening or the ring's end will pop out into the port and be broken off. There is no universal standard for piston ring locating pin position nor for cylinder port position. Install the piston so the ring ends do not come near a port..

It's common for a piston to have an arrow or some other casting mark on it's crown.. and it's installed so that mark is on the exhaust side.

Re: Unknown Ignition Timing and Piston Facing (lon

_would it be better to set a low advance, say 1 mm before TDC, to make it easier to start? My idea is to initiate the spark when the fuel-air mixture pressure is as high as possible_

Just to be clear about this question.. Ignition timing does not contribute to hard starting to anywhere near the degree that spark strength, plug gap and proper air:fuel ratio does. Adjusting ignition timing for the sake of easy starting is a misguided effort.

I think the hard starting and idle problems can both be traced to that carb.. Cleaning the fuel system and adjusting float level and a careful idle adjustment (while the engine is hot) may help some, but the easy efffective cure is probably gonna be a stock 14mm carb.


Whatever timing you decide on will be in effect over the entire RPM range.

A late (retarded) spark may not give the fuel mixture enough time to burn. Retarding ignition timing to 1mm BTDC would be suitable only if the fuel mix were able to burn very quickly.

A highly compressed, lean and clean mixture can burn faster, and power output can then benefit from a relatively retarded ignition timing.

But with a worn engine suffering from low compression and burdened with a carburetor that is very much larger than stock, my estimate is that timing should be more advanced rather than retarded.

Re: Unknown Ignition Timing and Piston Facing (lon

Iulian Tatu /

Ok, thanks for information, this forum is a great resource.

Summarizing, my next steps would be:

First (short term and no costs involved):

1. Reverse the piston position. (hopefully the rings are not broken yet).

2. Set the timing to 2 mm BTDC, and test a possible and increase to 3 mm, considering the lower burning speed of this engine.

3. Make the mixture leaner (lower the carburetor float level, lower the needle jet position, adjust the air screw), in an attempt to increase the burning speed. However, there is a limit in doing this.

4. Wait for nice, sunny days. No joke, I found that the engine starts consistently harder in rainy days. It must be the higher air humidity which richens the mixture.

Next (long term, cost may be consistent):

1. Continue to look for a stock carburetor, or another smaller carburetor. (not successfully till now, keep looking).

2. Test the engine for compression (no gauge available yet, borrowed or purchased). However, this is not mandatory, I know this engine is worn.

3. Measure the piston diameter / cylinder bore, piston / cylinder clearance, rings gap (I can do that).

4. Look for a matching piston / piston pin / rings. It will be difficult to find what I need. An alternative would be to ask my machinist to make custom rings for me.

Thanks again,

Iulian Tatu

Re: Unknown Ignition Timing and Piston Facing (lon

..."4. Wait for nice, sunny days. No joke, I found that the engine starts consistently harder in rainy days. It must be the higher air humidity which richens the mixture."...

Higher humidity air has water, water does not atomize like fuel. This excess water in the air replaces the fuel molecules. (LEANS THE MIXTURE) Clue that the fuel may have some water in it already; the added humidity adds more water making it more difficult to start.

Re: Unknown Ignition Timing and Piston Facing (lon

I was drawn to the way you went about this project.. I too make my own stuff and repairs whenever i can.. like welding up a cracked flywheel or spending days on a jig to straighten a frame or something.. and turning small parts on a lathe. It's all part of the fun for me.

you obviously know what you're doing. If i had any advice it would be to thoroughly follow through on measuring bearing clearances as well as bore and piston, etc. before putting a lot of time into it. Then go nuts and have fun and spend money only after knowing all the vital elements are still within specs.

Re: Unknown Ignition Timing and Piston Facing (lon

..."However, I do not have its original 14 mm carburetor, which has been replaced by a more common 17 mm carburetor."...

The larger carb has a larger opening, hence more air at idle. More air at idle pulls more fuel, faster running idle. You try to close the air valve to reduce air but it increases the speed of the air over the needle jet causing a rich mixture and it stalls due to not enough air. Second thought is that the you close the air valve and the air speed increases going through the smaller opening but slows down by the time it reaches the needle jet pulling less fuel causing a lean mixture and it stalls.

I do not know what type of carb you have. Some have an Idle speed screw that only raises and lowers the throttle slide. The first synopsis is the answer since the throttle slide is over the needle jet. Some carbs actually have two settings, the idle speed and a fuel adjustment screw. You need to adjust both screws to synchronize idle fuel/air mixture.

Re: Unknown Ignition Timing and Piston Facing (lon

wait a minute..

water vapor is in the air. By volume, humid air contains less oxygen than dry air.

Gasoline delivery through the carb is not affected by humidity. Humid or dry, the amount of fuel delivery is constant.

If less oxygen is available the mix is richer.

Water in the air displaces oxygen, not fuel.

Re: Unknown Ignition Timing and Piston Facing (lon

For instance.. take a 12:1 air:fuel mixture. dry air is 21% oxygen.

So, 12 pounds of dry air contains around 2.5 pounds of oxygen. The burning portion of a 12:1 mixture is 2.5 pounds of O2 to 1 pound of fuel.

Now make the air very humid. 12 pounds of humid air has a lot of water vapor in it. Now, that 12 pounds of air certainly contains less than 2.5 pounds of O2.

The ratio just got richer [ less oxygen : same amount of fuel ]

Complicating all this is air temperature.. atmospheric pressure. But all things being equal, dry air contains more O2 per pound, and humid air results in a richer air:fuel mixture.

Re: Unknown Ignition Timing and Piston Facing (lon

I am corrected. I was thinking that the humidity as a gas when it is more like a solid. I still wonder about this though. Humidity is two parts hydrogen and one part oxygen. It is effected by temperature. When it goes in the engine and hits the hot walls of the cylinder the hydrogen and oxygen turn back into a gas and the hydrogen is highly flammable. I would think that this would be good for the fuel and actually richen it due to the hydrogen gases. I do know it burns in there with the oxygen, it is responsible for some of the toxic emissions. Humidity is confusing! Thanks for correcting me Joew. :>)

Re: Unknown Ignition Timing and Piston Facing (lon

there aint nothing easy or intuitive about the concepts.. i always have trouble with them and have a lot of respect for anyone who can get their brain around it.

As far as burning water.. Merle Haggard wrote a song..

`````````When they find out how to burn water, And the gasoline car is gone. When an airplane flies without any fuel, And the satellite heats our home. One of these days when the air clears up, And the sun comes shinin' thru. We'll all be drinkin' that free bubble-ubb, And eatin' that Rainbow Stew.``````

Unfortunately, the hydrogen/oxygen water molecule bond is very strong. To separate them so they can be rejoined (burned) and produce power requires an energy input greater than the strength of that bond (and greater than the power produced)

The reaction would absorb energy from the system. So if hydrogen/oxygen actually split apart in the cylinder it would cool the cylinder and the gases in it... counter productive.

imo, economically feasible methods of splitting water pretty much dictate the use of solar energy or some other source of free/cheap energy. New studies with exotic catalysts and large scale methods show some promise..

but i'm not gonna hold my breath until my free bubble-ubb and Rainbow Stew arrives.

Re: Unknown Ignition Timing and Piston Facing (lon

LOL, :>) I will agree, there is allot to it. Thanks for the info.

Re: Unknown Ignition Timing and Piston Facing (lon

Iulian Tatu /

My carburetor has both screws, that is the idle speed screw to rise the throttle slide, and the air adjustment screw, located downstream of the throttle side. This carburetor belongs to a 50cc, 4 HP, 4 speed/gear motorbike and it is practically identical with the original carburetor, except its bore and slide diameter.

I would not care that much about idle speed, if the moped construction would include a classic gear box with a neutral point. But in my case the centrifugal clutch engages due to high idling speed, which makes me uncomfortable when the moped is up on its stand. The situation is further complicated by the difference in idling speeds between a hot and a cold engine. What I need for sure is to set a safe engine start-up/shutdown procedures, before passing this machine to my kids. I have already installed a kill switch connected to the points. However, I will leave this at a later stage when all other issues are solved.

As for the measurements, I intend to use for comparison some figures depicted from Moped Riders Association manuals (another great resource too). That is:

- Ring gap: normal 0.3 mm / service limit 0.8 mm.

- Piston to ring side clearance: normal 0.05 mm / service limit 0.1 mm.

- Cylinder / piston clearance: normal 0.06 mm / limit 0.1 mm.

Yes, I do like getting my hands in all kind of stuff. In fact I believe I enjoy more to fix the moped than to ride it. Yeah, that is just another thing that came with the age…

Thanks again,

Iulian Tatu

Re: Unknown Ignition Timing and Piston Facing (lon

You could make a restrictor plate for the carb. This may help with the idle and also curb the top end till the family gets use to handling the ped.

Re: Unknown Ignition Timing and Piston Facing (lon

Out of curiosity, what is the make of this moped? Is it a Simpson?

Re: Unknown Ignition Timing and Piston Facing (lon

Iulian Tatu /

It's make is MiniMobra. It has been fabricated in Romania some 25 yeard ago, and it has not been sold abroad, as far as I am aware.

A restrictor plate for carburetor is something new to me. Some new questions arrises:

1. Where this plate should be installed? Upstream or downstream of throttle slide?

2. If upstream, this would not induce additional chocking effects?

3. If downstream, it would not lower the required vacuum at full throttle?

4. Should it be eccentric or concentrinc with respect to the carburetor bore?

Iulian Tatu

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