DS7 Exhausting Out of the Carb!?

[Halmic]

I'm looking for possible causes why a basket case DS7 isn't getting fuel into the combustion chambers.

The carbs are cleaned and seem to check out fine, the floats are adjusted, bowls are full, pilot jet checked for obstructions, and I raised the needle all the way up just to help get fuel moving.

Compression isn't great at just over 100psi, but should be enough to push air around.

The plugs are bone dry after kicking it over. 

When placing my hand(s) over the exhaust (no pipes on yet) and back of the carb it actually feels like more air is being exhausted rear-ward through the carbs than forward out of the exhaust.  I also get a little fuel spatter on the hand behind the carb, and none out of the exhaust.

This bike is a mess, the PO was a severe hack and gave up halfway through destroying this thing. 

He said he put in some new(er?) pistons (no boring was done it sounds like), so I am thinking he possibly installed some incorrect  pistons that are blocking the transfer ports?  That's the only reason I can think of for this, but please advise if you've got a different idea!

[Striker1423]

Since the DS7 is a piston port and not a  rotary valve motor... Probably wrong pistons. Or not enough vacuum being created. Air leak?

If it was a rotary valve motor is say check that... But it's not.

[sav0r]

Most 2t's will have some blowback carb side, especially so piston port engines. It's a feature.

[m in sc]

if you are getting 100 psi its not a piston issue.. unless he put in rd250 windowed pistons. then.. you'd def have an issue.  pull the carb put piston at tdc and look in there for skirt windows on the intake face. IF they are there, then yeah, he really fucked up.

[Halmic]

if you are getting 100 psi its not a piston issue.. unless he put in rd250 windowed pistons. then.. you'd def have an issue.  pull the carb put piston at tdc and look in there for skirt windows on the intake face. IF they are there, then yeah, he really fucked up.

Mark,
I will check for the windows today, I suspect they are there.  Thanks!

[RdsOnly]

Carbs spit= normal
Timing is key on 2 strokes
2.0mmBTDC if stock [run ave gas 100ll@ 50/50 no ethanol fuel] - old bike needs old fuel.
#1 timing 2.0 btdc- 1.8mm if piped [chambers]
Fuel:
Carbs- clean all passage ways- Emultion tube [main jet screw into] remove and clean
Air passage ways
Mick-VM28 carbs are not complicated
Pilot= 25-32.5 in size
Air screw [AS] 1.0 - 1.5
Main 125-280 depending
- choke if it does not work check float bowls- bottom of carb- chock carb has a though hole none chock has a blak.
100+ psi compression it will start:
IF you have 12vdc at points- coils
- idles high air leak - intake rubber manifold #1 - #3 crankseals
#3 right side smokes way more that left.. Left sucks air, right sucks oil

Have fun
JT

[m in sc]

any updates on what pistons are in there?