Changed main jet now wont start
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Enthusiast
Vespa P125X
Joined: 30 Aug 2017
Posts: 62
Location: Dubai
Tue May 11, 2021 11:07 pm quote
Hi all,

Trying to tune my set up and changed the MJ from 128 to 120 on advice from another forum, now the scoot wont start.

I am running:
1978 P125x
DR177
SIP2 exhaust
24.24 carb
MJ BE2 with 120
P200 air filter
New spark plug

It appears I am getting fuel to the carb
I am getting a spark
compression strangely feels low on the kick

The last time I rode it 2 weeks ago it was fine and went well, I was just trying to get a bit more out of the set up. Its strange that it now will not start.
Molto Verboso
One or two fun scoots....nothing too precious
Joined: 17 Jul 2013
Posts: 1679
Location: UK (South East)
Tue May 11, 2021 11:53 pm quote
Won't be related to the change of MJ size. Is there fuel getting from the carb to the crankcase? Take the air filter off and check for spraying of fuel into the venturi, with and without throttle. Also check that your choke is operating correctly
Molto Verboso
2007 Stella 225
Joined: 02 Nov 2019
Posts: 1692
Location: Rochester, Minnesota
Wed May 12, 2021 3:04 am quote
Not running isn't the main jet, you should still start and idle using the choke and idle jet or slow run jet.

My two cents on that huge main jet change, from someone who has soft seized 1 cylinder, and melted a second. Measure cylinder head temperature at the very least and see what your getting now. Don't try to keep going leaner and leaner in the search of power. You'll end up walking home. "Go Lean and Walk Home" Measure cylinder head temperature and see where you're running, slowly step down on main jet sizes, get a variety pack. Try to keep you CHT in the 300F range.

On the scooter I'm tuning now for example using your 128,

128 Runs CHT max 260F
125 CHT max 295F
123 CHT max ? Never found out, saw temperature easily climb above 325, pulled plug and it was white. Keeping in mind it was black from running rich at 128.

I put the 125 back in and happy so far with plug color and temperature.
Hooked
PX
Joined: 28 May 2020
Posts: 131
Location: Bognor Regis
Wed May 12, 2021 3:43 am quote
Re: Changed main jet now wont start
craigyc wrote:
Hi all,

Trying to tune my set up and changed the MJ from 128 to 120 on advice from another forum, now the scoot wont start.

I am running:
1978 P125x
DR177
SIP2 exhaust
24.24 carb
MJ BE2 with 120
P200 air filter
New spark plug

It appears I am getting fuel to the carb
I am getting a spark
compression strangely feels low on the kick

The last time I rode it 2 weeks ago it was fine and went well, I was just trying to get a bit more out of the set up. Its strange that it now will not start.
I have exactly the same rig, i have an AC140 BE5 and 122 MJ and this took quite a while to get right combo thanks to this forum. It runs slightly rich on idle but rather have the wide open throttle running right.
Hooked
2005 Stella 2T
Joined: 14 Jun 2020
Posts: 124
Location: MA
Wed May 12, 2021 7:21 am quote
That BE2/MJ120 combo is way too lean for your setup. The MJ120 is in the ballpark, but needs to be paired with a richer mixer tube like a BE5 or you will blow something up.
Hooked
PX
Joined: 28 May 2020
Posts: 131
Location: Bognor Regis
Wed May 12, 2021 12:07 pm quote
I would Put in the be5 ac140 and a 124 jet to be on safe side but have a 122 to hand. To confirm I have dr177 24mm carb, p2 drilled filter, matched carb box to inlet , road 2.0 which is exactly the same as yours. I run a 7 plug on 20 Mile trips but step to an 8 temperature for long runs. The guys on this forum helped me to get this right so you should be right in the ballpark without the stress of having to experiment.
Addicted
Old douglas ..smallie with polini 115..super with nasco 177
Joined: 07 Nov 2013
Posts: 640
Location: New Zealand
Thu May 13, 2021 1:44 pm quote
95%of carb problems are electrical
Checky your lecky

Points/condensor or CDI depending on what you have is more likely
good luck
Enthusiast
Vespa P125X
Joined: 30 Aug 2017
Posts: 62
Location: Dubai
Thu May 13, 2021 8:08 pm quote
Cheers
Thanks gents for all the input.

@bluecati, I am getting a spark. Would it still be the condensor? If so where do you start? Any tips?
Enthusiast
Vespa P125X
Joined: 30 Aug 2017
Posts: 62
Location: Dubai
Thu May 13, 2021 8:10 pm quote
swa45 wrote:
Won't be related to the change of MJ size. Is there fuel getting from the carb to the crankcase? Take the air filter off and check for spraying of fuel into the venturi, with and without throttle. Also check that your choke is operating correctly
It appears I am getting fuel. How do I check the choke? Thanks.
Enthusiast
Vespa P125X
Joined: 30 Aug 2017
Posts: 62
Location: Dubai
Thu May 13, 2021 8:12 pm quote
Re: Changed main jet now wont start
ferriswolf wrote:
I have exactly the same rig, i have an AC140 BE5 and 122 MJ and this took quite a while to get right combo thanks to this forum. It runs slightly rich on idle but rather have the wide open throttle running right.
Thanks, I'll try that once I can get the thing started!
Enthusiast
Vespa P125X
Joined: 30 Aug 2017
Posts: 62
Location: Dubai
Fri May 14, 2021 11:56 pm quote
Getting a spark
I am getting a spark. Does this look strong enough? If not where to look next?
Video is here.

Enthusiast
Vespa P125X
Joined: 30 Aug 2017
Posts: 62
Location: Dubai
Fri May 14, 2021 11:57 pm quote
CDI Looks like this.
Alll good with the CDI?



Enthusiast
Vespa P125X
Joined: 30 Aug 2017
Posts: 62
Location: Dubai
Sat May 15, 2021 12:00 am quote
Getting fuel to carb.
It appears that fuel is getting to the carb. Does this look ok?
Enthusiast
Vespa P125X
Joined: 30 Aug 2017
Posts: 62
Location: Dubai
Sat May 15, 2021 12:10 am quote
Head off
Apart from a little bit of a leak on the cylinder head (I assume lapping and tightening a bit tighter). How does the head look and the inside of the cylinder? Looks OK to me but obvioulsy not an expert.
Why is compression low?
Is this usual?
Is this the problem?
How do I fix it?









Ossessionato
76 Sprint V, 63 GL, 62 VBB, 05 Stella
Joined: 16 Jun 2011
Posts: 3204
Location: Nashville
Sat May 15, 2021 4:59 am quote
I'm going to guess you don't have enough compression for the motor to start and run at this point.

Sadly, you have all kinds of detonation based on those photos of you piston crown and cylinder. The leaky head (the natural state of a DR, btw ) probablyl caused enough damage that you eventually killed it.

Funny thing about killing pistons is that a lot of the time, you don't realize it (I don't, anyway...) until it won't start again, because it had juuuust enough compression to keep running with everything hot, but once it stops, it's dead until you sort things out.

If you can pull the cylinder off entirely, you'll most likely find that the piston has heat damage over the exhaust port area and is preventing the rings from seating properly. Depending on how extensive the damage, you might be able to fix it.

First, free the rings and remove them, then sand/file the piston back into something resembling proper shape.

for the rings and cylinder, they most likely have aluminum smeared on them. Get your hands on some muriatic acid and that will burn the aluminum off, but won't effect the cast iron. Wash it all off good, re-assemble, then fix your jetting before you get back out on the road.

As others have noted, your current jetting is way too lean.
Hooked
PX
Joined: 28 May 2020
Posts: 131
Location: Bognor Regis
Sat May 15, 2021 6:02 am quote
chandlerman wrote:
I'm going to guess you don't have enough compression for the motor to start and run at this point.

Sadly, you have all kinds of detonation based on those photos of you piston crown and cylinder. The leaky head (the natural state of a DR, btw ) probablyl caused enough damage that you eventually killed it.

Funny thing about killing pistons is that a lot of the time, you don't realize it (I don't, anyway...) until it won't start again, because it had juuuust enough compression to keep running with everything hot, but once it stops, it's dead until you sort things out.

If you can pull the cylinder off entirely, you'll most likely find that the piston has heat damage over the exhaust port area and is preventing the rings from seating properly. Depending on how extensive the damage, you might be able to fix it.

First, free the rings and remove them, then sand/file the piston back into something resembling proper shape.

for the rings and cylinder, they most likely have aluminum smeared on them. Get your hands on some muriatic acid and that will burn the aluminum off, but won't effect the cast iron. Wash it all off good, re-assemble, then fix your jetting before you get back out on the road.

As others have noted, your current jetting is way too lean.
Not so great news for poster Chandlerman, is there anyway of preventing leaky heads on a DR or is this a question of just checking once in a while that the cylinder head bolts are correctly torqued? I would have assume you would visually see a leaky head?
Molto Verboso
2007 Stella 225
Joined: 02 Nov 2019
Posts: 1692
Location: Rochester, Minnesota
Sat May 15, 2021 6:17 am quote
chandlerman wrote:
I'm going to guess you don't have enough compression for the motor to start and run at this point.

Sadly, you have all kinds of detonation based on those photos of you piston crown and cylinder. The leaky head (the natural state of a DR, btw ) probablyl caused enough damage that you eventually killed it.

Funny thing about killing pistons is that a lot of the time, you don't realize it (I don't, anyway...) until it won't start again, because it had juuuust enough compression to keep running with everything hot, but once it stops, it's dead until you sort things out.

If you can pull the cylinder off entirely, you'll most likely find that the piston has heat damage over the exhaust port area and is preventing the rings from seating properly. Depending on how extensive the damage, you might be able to fix it.

First, free the rings and remove them, then sand/file the piston back into something resembling proper shape.

for the rings and cylinder, they most likely have aluminum smeared on them. Get your hands on some muriatic acid and that will burn the aluminum off, but won't effect the cast iron. Wash it all off good, re-assemble, then fix your jetting before you get back out on the road.

As others have noted, your current jetting is way too lean.
Something like this, those long streaks are most likely grooves down the cylinder wall or deposited aluminum. In picture #2 those specks of metal around 12:00 o'clock on your piston are most likely where you were to lean, overheating and detonating and melting aluminum.

While it's apart and your cleaning it up check the squish between the cylinder head and piston. It usually pretty large on these over 2.5mm and you can clean that head up and take some of that squish ring off and it will run better. Something closer to 1mm is good.





Molto Verboso
One or two fun scoots....nothing too precious
Joined: 17 Jul 2013
Posts: 1679
Location: UK (South East)
Sat May 15, 2021 8:29 am quote
ferriswolf wrote:
Not so great news for poster Chandlerman, is there anyway of preventing leaky heads on a DR or is this a question of just checking once in a while that the cylinder head bolts are correctly torqued? I would have assume you would visually see a leaky head?
My solution was to scrap the DR head. I used a CNC milled VMC head with zero squish, so that I could lift the cylinder with a 1.5mm base packer, thus achieving much better port timings, a tight head/cylinder seal, and a respectable squish. Alternatively, just machine the sealing surfaces or sand them on a piece of fine sandpaper on a sheet of glass, using a figure of eight motion.
Ossessionato
76 Sprint V, 63 GL, 62 VBB, 05 Stella
Joined: 16 Jun 2011
Posts: 3204
Location: Nashville
Sat May 15, 2021 8:43 am quote
swa45 wrote:
Alternatively, just machine the sealing surfaces or sand them on a piece of fine sandpaper on a sheet of glass, using a figure of eight motion.
This is what I used to do. I'd lap both the head and the deck of the cylinder, then use Permatex 80967 when I assembled it.

When you lap them, color the surface with a sharpie so you can track your progress and you'll be amazed at how uneven it is when you first start.

To really get serious, I've done similarly to swa45, only I used a BGM 177 head, which actually sits into the bore, so I had that additional sealing surface, plus it makes it super-easy to get the squish to exactly what you want since it's just a matter of using the correct thickness head packer.
Enthusiast
Vespa P125X
Joined: 30 Aug 2017
Posts: 62
Location: Dubai
Sat May 15, 2021 8:19 pm quote
Oooh Noooo
Thanks for all the input fellas. Very much appreciated. Although the bad news no so much appreciated!

It all makes sense. I will try and get the head off today and see how it looks and report back.

With regards to detonation issues, does this relate to timing? And if so any advice on setting the timing correctly, specifically for this cylinder and the reduction in squish?
Ossessionato
62 VBB1T Round Tail W/ leaner sidecar
Joined: 26 Jan 2019
Posts: 2245
Location: california
Sat May 15, 2021 10:14 pm quote
When you have a leak at the head - it lets air in - and it creates super lean mixture /detonation.

There are lots of ways to address it - as the guys noted above.
I would recommend, start by pulling the cylinder. You are this far, sliding the cylinder off isn't that much more.

If you feel comfortable doing it - take the piston off - and you can examine it nicely on the bench. A sharp razor blade, a small piece of sand paper folded, etc. to reshape the ring grooves if they are melted and holding trapping the rings from expanding - like on Christopoher's picture - will get them back in order.

Often, you can clean up the piston, put new rings on, make sure they float freely and don't bind in their slot, clean up the cylinder walls with the muriatic acid - and get back on the road for not much money.

Of course, you would need to lap the head as the others suggested - to get it nice and flat before you reassemble. If you decide to slide the cylinder off - shoot some pics of the piston and see how it looks. As noted - pay close attention to the area near the exhaust port. It is a common place for them to get trapped.
Molto Verboso
2007 Stella 225
Joined: 02 Nov 2019
Posts: 1692
Location: Rochester, Minnesota
Sun May 16, 2021 6:11 am quote
After, cleaned up fairly well, while your cleaning up the head, polish the bowl up and get rid of the casting crap (technical term).

Here is a head that has been cleaned up some, and the squish band removed to almost "0" to get a respectable squish and compression ratio. That's not done being cleaned up but it's what I could find, been a few years. Couldn't do much about the porous casting and air bubbles but did get anything sticking up cleaned off.





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