lots of blowback out the carb

I just recently bought a 1980 Peugeot 102 sp U3 for $120. I was able to get it running. It has a new spark plug with the correct gap, the compression was 150psi and it had a Dell'Orto 15.15 SHA on it. It spits a ton of air/fuel out the carb intake when I open the throttle under load and has no power. The emulsion tube holes had been drilled out way too big so I purchased a cheap carb to see if that helped the problem and it didn't. I am convinced it is not a problem with the carb and probably not the spark. I am also sure that my engine does not have reed valves which is what I was hoping it could be. From what I could gather it might be worn piston rings? (I don't quite know why this could be if the compression was so high.) I ran searches through this forum for pertinent threads and found none so here is my cry for help. I am brand new to mopeds and engines but I am wanting to learn and already know a fair amount. Thanks for the help!

Re: lots of blowback out the carb

How does your cylinder look? Specifically inside around the different ports.

Re: lots of blowback out the carb

♣Slew Foot♣ /

Rings are worn out too big a gap...

Reed flutter...

Re: lots of blowback out the carb

picture of your motor might help. So if its a 102 then the motor is solid mount and doesn't move, but you have the special edition sp, hmm. well if its piston port intake off the cylinder, then timing could be off, or if its case inducted like the 103 then your reed block might be jacked.

Re: lots of blowback out the carb

Tri-ped Dave O.D.B. /

102SP U3 is piston port so reed flutter has fluttered out of the possibilities. 150psi seems like really high compression so here is what I think. Maybe a previous owner put a kit on it and did not space the cylinder correctly. A lot of the Pug kits need really thick base gaskets that a newbie would not catch.

Re: lots of blowback out the carb

Tri-ped Dave O.D.B. /

So I was basing my previous statement on the assumption that you have tried jetting correctly. Or maybe you just have way too big a jet? What size jet are you running with the 15SHA? What does your spark plug color tell you? Is the plug wet?

Re: lots of blowback out the carb

Tri-ped Dave O.D.B. /

If you have done all the due diligence on jetting, then I still think an improperly installed kit may be the reason. High compression, non-stock carb, spitting fuel, a lot of circumstantial evidence points to my theory of incorrect port timing which is a common issue with a lot of the Pug kits. One of mine required 3mm of spacing at the base to get the port timing correct. Someone without the knowledge might just install with the lone paper base gasket supplied with the kit thinking that is OK.

Re: lots of blowback out the carb

Check that the ignition timing is spot on .

Re: lots of blowback out the carb

Indeed - excess reversion can be caused by too much advance on the ignition timing

Re: lots of blowback out the carb

Wow thank you all for the help! The relevant suggestions are:

1. piston improperly spaced/bad ports (over-bored?)

2. timing

3. carb/jetting

I think #1 is the last thing I'll check because it requires the most disassembly. If numbers 2&3 don't fix the problem I'll return to 1. I will be making sure the timing is stock when my flywheel puller comes in the mail this week. As to the carb/jetting/spark, my spark plug looks good, it is the stock requirements, is gapped correctly, not too wet, and coffee color on the white ceramic. Seemingly no signs of running too lean or to rich. My jet is a #60. I have not messed around with the jets, because I wanted to fix this problem first. I try to follow the rule of only changing one thing at a time so that you can figure out which thing is the culprit. The bike has definitely been messed with by the previous owner because the exhaust, and carb are aftermarket, and the timing had sharpie marks on it when I exposed the flywheel. Sorry this is kind of a brain dump.

Here is more description of the problem. When idling there is a slight puffing sound from the carburetor. When the throttle is opened up this gets louder and turns into a muffled pop that shoots sensibly cold (as in not hot like exhaust) fuel/air mixture out. This problem is very bad when there is resistance against the motor, and is accompanied by bogging down. However when the bike is on its stand it can "work though" the puffing by getting to higher revs. The same effect can be achieved by pedaling furiously and variating the throttle on and off on level ground to get to a decent speed with no puffing or popping. Unfortunately though, there is little to no power.

Re: lots of blowback out the carb

if you have the two coil ignition, i would instantly suspect that junk rigjt off the bat. For more info take a look at myrons page, now if it has the 5 coil then those are pretty solid.

Re: lots of blowback out the carb

UPDATE: I pulled off the flywheel and I have the five coil magneto. I reset the points to .4mm and set the timing to stock. The problem is exactly the same. There is however a new finding, IT RUNS BETTER WARM BUT NOT HOT. Once hot it will not run because of so much blowback out the carb at all rpms. It is running rich but this is due to the air passing the emulsion tube twice as much as it should (both directions, out and back in) and therefore picking up twice the fuel. I ended up disconnecting everything and pulling the engine off the frame and taking the top end apart. Could this be port timing? Is it that there needs to be a thicker gasket between the crank case and top end? I'm happy to take pictures let me know.

Thanks!

Re: lots of blowback out the carb

> James Rea Wrote:

> -------------------------------------------------------

> UPDATE: I pulled off the flywheel and I have the five coil magneto. I

> reset the points to .4mm and set the timing to stock. The problem is

> exactly the same. There is however a new finding, IT RUNS BETTER WARM

> BUT NOT HOT. Once hot it will not run because of so much blowback out

> the carb at all rpms. It is running rich but this is due to the air

> passing the emulsion tube twice as much as it should (both directions,

> out and back in) and therefore picking up twice the fuel. I ended up

> disconnecting everything and pulling the engine off the frame and taking

> the top end apart. Could this be port timing? Is it that there needs

> to be a thicker gasket between the crank case and top end? I'm happy to

> take pictures let me know.

>

> Thanks!

Have you tried running the bike without an exhaust? If the pressure created after combustion can't escape the exhaust cause it's clogged or something it would make sense that it then pushes out through the carb

Re: lots of blowback out the carb

Good you have the five coil. Always post pictures, that would tell a lot about your setup without you even attempting to describe it. In the beginning you said your emulsion tube holes were drilled out, is that correct? Do you have an air filter and box and metal filter screens? What do you expect to acxomplish by taking the motor off the bike?

Re: lots of blowback out the carb

Here are the pictures. I'm sure that it is not the carb because it happens even with it off and me cranking over the engine by hand. The exhaust is clear clogged so there is a different problem.

IMG_008139.JPG
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IMG_008148.JPG

Re: lots of blowback out the carb

IMG_008151.JPG
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Re: lots of blowback out the carb

Here are the results of my port timing with my home made degree wheel. Could that near opening of the exhaust port at 197 degrees be the culprit? There is only about half of the upper piston ring keeping that port closed. Maybe too thin of a base gasket?

C83B6700-34FC-44C9-AD2C-B442131E4430.jpeg
73982B99-C970-4EEE-810C-67214586EA4C.jpeg

Re: lots of blowback out the carb

so looks like a stock cylinder, i wouldnt worry about your port timings. Did you say compression is good? I see some scoring on the exhaust side of the cylinder.

Anyways, stock jetting for a 15 sha im guessing should be a little bigger than 60. Are you using the airfilter/cap as having it on/off will affect jetting

Triple check your timing with a gun. As rebel said earlier, too advanced and itll blow fiel outta the carb.

Re: lots of blowback out the carb

So I originally thought that I had set timing correctly and because it was still acting up I pulled the engine to see if someone had tried to port it and messed something up. When it looked stock and in decent shape I looked at the port timing and was messing around with the intake port and seeing how a little air escapes past the skirt out the intake when the piston starts compressing the crankcase on the downstroke. However, @Pat Splat I think is right in saying that my port timing is ok. So then I went back to the timing and realized that I think it was firing very late. I don't have a timing light but I accidentally set it .75 inches too late as measured on the flywheel. Shame on me. I'm waiting on new rings and a new bottom gasket and then I will assemble and try it. As far as my carburetor jet/air filter I probably should get a jet set and mess with the mix, I also had been running it with and without the filter. Thank for all the help and I will get back with the results. I'm learning a lot and enjoying it!

Re: lots of blowback out the carb

Downhill Harvey (OFMC) /
two-stroke.gif

Re: lots of blowback out the carb

Alright sorry for the delay! Finally I figured out that while I thought my timing was good, it was actually much to retarded. I now have it advanced almst 1.5 times what the peugeot wiki said but that's the only place it will run and I dont think i'm hearing detonation (I know just like I didnt think the timing was off) Ive gotten it running and can reach about 27-30 on flat ground. It has a terrible initial acceleration but now it runs! Thank you all very much for the help.

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