Can't figure out compression problem -2 stroke HELP!!!

[DT RE Chris]

 hi, I have a 50cc 2 stroke moped I can't get started, was working fine then gradually got really hard to start and had no pulling power and lost top end speed, now wont start at all, everything seems to check out fine apart from low compression which is 40psi, thinking the piston rings had worn I just replaced the barrel with a new one which came with new piston & rings, did a compression test and its still 40psi.

So, took the barrel back off and checked the seals/gaskets which are perfect, checked the head in case it was warped which looked fine, I even flatted it on some glass after inking it up and cant see anything wrong with it, put it all back together and still 40psi!!

Checked the crank cases with a pressure test and no leaks from there, even fitted new crank seals just to make sure.

It just doesn't seem to be pulling any air through the carb, if I squirt some fuel manually into the barrel it almost tries to start for a couple of seconds but only with the help from the starter motor turning

There's no leaks from around the reeds.

It just doesn't make sense, where else can I be loosing compression?

 

[slice]

Your problem is that 2Ts don't have compression the same as 4 Strokes, the only way to test it is a leak down test and you have done that. So my guess would be fuel flow as if it "almost" starts with fuel poured in then you have a blockage somewhere, I would suggest that you whip the carb off and check to see if it's full of crap or not and clear the jets before you do anything else. Others on here will know MUCH more than me about 2Ts than I do so just wait till one of them turns up otherwise..!

[DT RE Chris]

Everyone else who's done a compression test on the same model moped is getting at LEAST 90psi some even 120psi. (see link)

Carbs not the issue, been checked multiple times, if I suck on the vacuum pipe fuel flows into the carb but when connected the air passing over it isn't enough to open the diaphragm, tried easy start and carb cleaner when cranking but it doesn't even pop,  

[nayruf]

Hi

I don’t know of a specific answer to the problem,

Compression testing two strokes in the conventional way is not necessarily a true indication of the condition of the engine, my 2t bike reads around 115 psi under a conventional test,

But, I wonder if you have the correct size piston and or rings for the bore,

Did you check the piston ring gap at the top and bottom of the bore (least travelled areas of the cylinder wall) and comparing the same in the middle of the bore (most travelled area of cylinder wall) prior to fitting them? Perhaps you have an oval shaped cylinder, or may be though unlikely a crack in the cylinder wall or cylinder head,

 

[Cynic]

Ok, first and most important bit.

Stating everything is perfect is wrong and not helpful.

First thing that springs to mind is the reeds, if they are not sealing properly you will have issues.

Also the exhaust, is that clear, a 2stroke pulls air back from the exhaust during the cycle, if that is badly restricted or blocked by a build up of crap (on my daughters bike the 20mm pipe on the exhaust was so choked a pencil wouldn't fit) it will reduce the draw.

 

 

[DT RE Chris]

Reeds are fine, and exhaust is clear,   it's just not making any sense, I literally tried everything I know and have never come across an engine I can't get running, i'm baffled

 

[DT RE Chris]

Barrel and piston came as a kit brand new and fit very snug and tight, plus the original barrel was giving the same psi reading so doubting there's a fault there

[Airhead]

40 psi is very poor, you did the test WOT ?

[Cynic]

This may sound odd but it would not be the first piece of test kit to go south.

Have you tried the compression tester on your DT.

[DT RE Chris]

Not recently, but was used on my DT last and read fine, it could be faulty but it doesn't explain why there's not enough air being drawn through the carb, the plug is always dry

 

[blackhat250]

Carb issue,,   - clean , then bump start it,, should start  even if low comp,,   the ring Gap,,,,, you never checked it, ,,

[DT RE Chris]

Its a moped

 

[Cynic]

What condition was the original piston/bore in? Did you see any reason for the lack of power. 

The new top end will have the compression to run the engine, even the complete garbage Chinese 25quid knock offs run for a while. 

40psi will wet the plug, by generating 40psi it IS pulling air into the engine and should be wetting the plug. Quite drastically with such a lack of air it wont atomise cleanly.

 

I think yo have a fuel issue, maybe the vacuum tap diapram has perished, fuel pump, if it has one? Tank breather? That's my feel from what you have said. I'm going with fuel.

[DT RE Chris]

Original barrel and piston looked fine, didn't need replacing I just did it to rule it out, i'll have another look at fuel but even when I suck on the vacuum and fill the carb it still doesn't fire up

[Cynic]

That's the thing, its a 2 stroke, even with really really worn rings a 50 will run, hell without rings they will run. Most of its compression in such a little piston comes from piston speed. A lot of the big bores only use 1 ring.

Unless these is a hole in it the compression is not the issue, same with the bottom end. Unless there is physical damage it will run, granted badly.

Fuel should be drawn in. If you have changed the crank seals and the top end even half way properly then with fuel in it and the spark properly timed it should run, even backwards quite happily.

Are you kicking it or using the starter, if using the starter is the battery up to the job. Power drill with a socket on the end of the flywheel nut starts most things LOL.

You don't say where you are in your profile, maybe one of us are near enough to help for a brew.

[finnerz89]

Have you tried starting it with the fuel cap open? 

[DT RE Chris]

 I know, I've run FAR worse condition engines with holes and leaks all over the place, its so frustrating as it seems everything is ok, I did wonder about the timing, I know its a CDI unit but I've heard the timing can alter, I have tried two different CDI units on it, however, after I did find the flywheel had worn through the pick up wire and was making contact, could this have damaged BOTH CDI units? unlikely but not ruling anything out now

[DT RE Chris]

Yes battery power is fine as connected to my car via jump leads so starter motor is turning over better than usual.

Not tried removing fuel cap yet but I did turn the tank upside down and it leaked from the cap so I don't think a vacuum is the issue. 

[blackhat250]

How is the Spark,,,, bright .blue.  - or yellow,  

[DT RE Chris]

So...I was barking up completely the wrong tree, turns out the compression gauge I borrowed was faulty  

However, I still can't get it running, it seems there's not enough air passing through the carb as it doesn't actuate the vacuum petcock when cranking so no fuel comes out(replaced petcock and no different) If I suck on the vacuum pipe fuel pours out, I even filled the carb with fuel but still nothing, I've tried spraying fuel through the carb when cranking and no good either

Reeds and intake pipes and seals have been replaced so its not sucking in air from somewhere else as far as I know.

It does almost start but only when holding the starter motor on,  you can here it taking a bit of the load off the starter and it turns over a bit quicker but wont quite fire up on its own.  

Had the carb in bits and again all appears fine, not sure about the auto choke on the carb but even if this has failed it should still run surely?

I don't know what to try next as there isn't anything left

Any suggestions?