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@rb_prior avatar
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1963 Allstate, 2005 Vespa PX150, 2001 Harley 95 ci Dyna
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@rb_prior avatar
1963 Allstate, 2005 Vespa PX150, 2001 Harley 95 ci Dyna
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I have a 1963 Allstate with a 150 cc upgrade. The complete fuel system was replaced with new parts (tank, fuel tap, line, carburetor). It starts easy and runs very well.

Today I went for a short ride and about at the 3 mile mark while cruising at 30 mph, the bike suddenly acted like the brakes were applied. I quickly pulled in the clutch and it then started to roll freely. The engine was off. When I came to almost a stop I shifted into 2nd gear and let the clutch out. The engine started and I was able to ride back to home. This has happened before so I put on a new carb SI20/17 which has a 98 main jet. I had previously drilled a 1/16 inch hole in the gas cap to eliminate the vapor lock problem. What is the likely cause of this sudden stoppage of the engine while going down the road?

I also notice a slight sluggishness when I turn the throttle from idle to just off idle. It happens more when the engine is cold. I have tweaked the air mix screw for best idle. BTW, when I removed the rubber plug on the back of the air box to adjust the air idle screw, some fuel came out and ran down the engine. This does not seem right. Below is a photo of my spark plug after returning home from the ride where it suddenly quit.

Any help on the sudden engine stoppage while riding and the sluggishness from just off idle will be most appreciated. Needless to say, I do not want to venture too far from home with a ride that is not dependable.
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Sounds like you had a soft seize my friend. The piston heats up and seizes against the cylinder wall effectively "applying the brakes". You pull over, it cools down and then it moves again. From the look of the plug your running too lean. I would be running a 20/20 on a 150 too.

Pull your head and check the cylinder wall. Look for scoring. If you see it, you had a seize. Pull the jug, hone it and replace the rings, or take it to a machine shop and bore it to a second oversize piston and replace the piston.

Soft Seize?
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parallelogramerist
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parallelogramerist
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Maybe 2 stroke oil wasn't added to the tank, or not enough? I've had plenty of soft seizes, but never at only 30mph.
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1980 Vespa P200E
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Most 2t seizes happen at partial throttle...
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'74 50s x3 '78 P200 E '84 Cosa '91 PK50XL2 - & - '58 AllState '68 Sprint '66(?) Super125 and '72 DanMotor Super150
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Stock exhaust? You seem to be jetted lean from the chart and that's your likely culprit, as mentioned. Sprint takes a 102 & most of the other 150cc models take a 100 stock. Go up from these if you have a better pipe of some sort.

You mention it's happened before, assuming this new 150 cylinder, and it'll happen easier again if you don't clean up the aftermath. Minimum is clean all scuffs & aluminum smears in the cylinder, clean up the piston to 'smooth' and make sure the rings are clean and free in the grooves. Next is professional hone, new rings & dress piston. Then bore oversize & new piston/rings.

At least pop the head off & take a look at the cylinder wall.
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@rb_prior avatar
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@rb_prior avatar
1963 Allstate, 2005 Vespa PX150, 2001 Harley 95 ci Dyna
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UTC quote
This is amazing. I have a new 150 cc upgrade with a new SI carb that is supposedly jetted for a 150 engine and with less than 100 miles into the break period, the engine has a soft seize. And this soft seize occurred while cruising just under 30 mph. This does not give me much confidence with this engine and carb system. I have been using 92 octane fuel with a 1:50 mixture of Motul 837415 510 2T Synthetic Motor Oil. While I am waiting for new jets to arrive in hopes of making the fuel richer, I am going to run the bike with the "choke" on. The choke apparently is a circuit for manually adding extra fuel to the carb and it works at any throttle position.

My current jetting is as follows:
Main 98
Atomizer E1
Air 160
Idle 42

I am going to change the main from 98 to 110 and the reduce the air from 160 to 120. I will also increase the idle to 48-160.

I may also put in a cooler running spark plug.

Hopefully, this all will make the fuel richer and reduce the chances of a soft seize in the future. Thanks for all the great advice.
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Why are you running 92 octane? It's always been my understanding with two-stroke engines that higher octane fuel is not the best for them. They actually run better on low octane.

I actually ran 100LL av gas a while back in a tuned scoot just for the novelty, and while the bike ran amazing I did notice that it leaned out a bit when I did a plug chop.

Anyway, you want that plug to be a chocolatey brown color. What is your elevation? Are you sure the BE1 is correct?
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I first bought the pre-mix 1:50 at Home Depot. This fuel is for 2 stroke power equipment and is made with 92 octane ethanol free gas with a 1:50 mixture of synthetic 2 cycle oil. The bike ran great. Then I started to mix my own using the Motul 510 2T synthetic 2 stroke oil with 92 octane gasoline. Whether the 92 octane produces a leaner condition than the 87 octane I do not know.

My elevation is 1000 feet and the mixer jet is an E1, not BE1. My old SI20/15 carburetor has a BE1 mixer in it. I would guess there is no difference between the BE1 and the E1. Should I change the mixer jet? I plan to install a 102 main jet and a 120 air correction jet. I really wanted to try the 140 air correction jet but could not find one. Currently a 160 air is installed. Lastly, I am increasing the richness of the idle circuit by increasing the idle jet from 42 to 48. Any suggestions are always welcome.
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parallelogramerist
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parallelogramerist
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since you just replaced your fuel line, maybe you got a kink in it, or the fuel line is just too long causing a gravity feed fuel starvation? Or both?
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Judging from the symptoms and plug, I think you need a richer idle jet.

Best,
-Slashy
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RB Prior wrote:
...
Any help on the sudden engine stoppage while riding and the sluggishness from just off idle will be most appreciated. Needless to say, I do not want to venture too far from home with a ride that is not dependable.
RB Prior wrote:
....

I am going to change the main from 98 to 110 and the reduce the air from 160 to 120. I will also increase the idle to 48-160.

I may also put in a cooler running spark plug.

Hopefully, this all will make the fuel richer and reduce the chances of a soft seize in the future. Thanks for all the great advice.
I think you ought to pull your head & gauge the damage. Five minutes, ten tops.
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@rb_prior avatar
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UTC quote
As for the suggestion that I need a richer idle jet, I agree and will be installing one soon.

Here is a list from Scootermercato.com regarding the idle jet:
"Below is the jet and number following it. The larger the number the richer the idle jet.

42-160=.2625

48-160=.3000

40-130=.3077 this is the stock Stella jet

38-120=.3167

45-140=.3214

55-160=.3438

45-120=.3750

50-120=.4167

50-100=.5000 If your running this you may have an air leak."

My current idle jet is a 42-160 (the leanest) and I am replacing it with a 48-160. According to this listing, I will be enriching my idle significantly. Whether I have to go to say a 45-140 or 45-120 which would be even richer remains to be seen.

As for pulling the head and accessing the damage, I will do that. Everything I have done to this old Vespa has been a chore. Most of the replacement parts never really fit and had to be modified. Taking it out for a test ride is an adventure hoping I will get back home. I guess working on a 53 year old classic has it problems.
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Quote:
Everything I have done to this old Vespa has been a chore. Most of the replacement parts never really fit and had to be modified. Taking it out for a test ride is an adventure hoping I will get back home. I guess working on a 53 year old classic has it problems.
RB Prior,
Yeah, that comes with the territory when you have older vehicles. Older than '84 or so means you have to understand carbs. About a decade earlier than that and you need to be able to work with points.
Back in those days, drivers regularly tuned their vehicles because they didn't automatically tune themselves.

It's rapidly becoming a lost art, so I just embrace it.

Best,
-Slashy
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Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and '66 Blue Badge Smallframe
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@ginch avatar
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Just be aware that the idle jet has an influence up into the rev range (not just idle), just as the main has an influence down quite low. If you have a particularly lean idle, riding along and then closing the throttle means that for a short while the engine that was running happily on a (say) 100 main is doing the same speed & revs, but all it has to lubricate and cool it down is a puny idle jet... it's just not enough. Not sure if that's how it happened but you get the idea.
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@rb_prior avatar
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UTC quote
My new jets came in and I have several questions for the collective brain trust. I want to enrichen my idle circuit and ordered several different idle jets. The jets I received were for an Si carb but have the air hole in the top of the jet. The old idle jet from my new Si 20/17 carb was a 42 but it was solid on the top, no hole. It apparently uses the air hole adjacent to the jet. Some plug this hole with lead shot and use the idle jets with the air hole. Can I use these new idle jets in this carb without plugging this air hole? I would guess it would run leaner and that is not what I want. Can I drill out the hole at the bottom of my old 42 idle jet and make it slightly larger? This should make it run richer in the idle circuit and I would not have to plug the air hole.

My main stack is now a 140 air, E1 atomizer, and 98 main jet. I have a 102 main I can use but it falls out of the atomizer. Perhaps I can spread the main jet a little so it will stay in the atomizer.

Lastly, does screwing out the idle mixture screw in the back of the carb enrichen the idle circuit?

All comments and suggestions are most welcome. Thanks.
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I am pretty sure you cant use an idle jet with the hole in it if the carb used one without. I dont know about plugging the whole.

Use a screw driver and open up the main a little.

Screwing the mix screw out leans the mix, screwing in will make it rich.
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bodgemaster
63 GL, 76 Super (x2), 74 Primavera (x2), 79 P200, 06 Fly 150
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UTC quote
Just the opposite Joe. On an SI carb, turning the mixture screw in (clockwise) leans the mix, out makes it richer.
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@rb_prior avatar
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Location: Central Ohio
UTC quote
"Screwing the mix screw out leans the mix, screwing in will make it rich." Now I am confused. On another forum they say "Depending on the plug condition, you'll either want to add more fuel (loosen the screw) or restrict the fuel (tighten the screw)." So which is it? Do I turn the idle mixture screw out to enrich the idle circuit or in to do it?

As for the idle jet without an air hole at the top, I can't find a supplier of these jets anywhere. They all sell the newer Si carb idle jets with the air hole in the top. Perhaps I should seal the air hole that is adjacent to the idle jet. Some suggest using lead shot while others suggest using a rubber seal. I would be most concerned about what ever it was being pulled into the carb throat. Does anyone have an easy fix for sealing this air hole in the carb? Once sealed, I could then use the modern idle jets with the air hole in the top and fine turn my idle circuit for a richer condition.

As an alternative, could I just drill the hole in my stock 42 idle jet and make it larger? This should enrich my idle circuit and I would not have to seal that adjacent air hole.
@ginch avatar
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Veni, Vidi, Posti
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@ginch avatar
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UTC quote
SoCalGuy wrote:
Just the opposite Joe. On an SI carb, turning the mixture screw in (clockwise) leans the mix, out makes it richer.
This is correct.


I can't find the picture that shows how to block the hole, but I think it's pretty straightforward. Anyone?

Certainly a better idea than adjusting jets by hand - a friend had a hand made jet that he thought was just right. But he could never make it slightly richer or leaner because he had no way to say what size the jet actually was.

Edit:
It was closer than I thought. Just take the filter off and there it is. You could do a temporary blockage of the hole if for some reason you don't want to move to the newer idle jets with a range of choices.
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Just to add a little to the confusion:

The term "idle jet" is a misnomer. It's actually what American mechanics call the "transfer circuit". It doesn't do the same job as the idle mixture screw and the 2 should not be confused.

The idle mixture screw sets the mixture at curb idle (out is rich), where the throttle is closed. High manifold vacuum below the throttle blade sucks fuel from the idle port.
When you open the throttle, vacuum below the blade is reduced and the idle port stops delivering fuel. The Venturi effect between the front of the blade and the transfer port creates a vacuum and begins to deliver fuel. This circuit is controlled by the so-called "idle jet" and works for part throttle conditions.
When the airflow through the carb is sufficient, the main venturi will begin to operate. This is metered by the main stack (jet/atomizer/bleed). As the blade moves away from the transfer slot, venturi effect is reduced there and the transfer circuit ceases operation.

Best,
-Slashy

Disclosure: I've never seen one of these carbs in person, but I am a carb guy and I've studied the circuits.
@joep avatar
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UTC quote
Ahhhh, I'm sorry! I was thinking the mix screw had to do with air, screwing it out lets more air in and vise versa. This is why I always ask my club mates in or out, lean or rich every time I and messing with a carb. I cant get that one to stick in my thick skull.

Carburetors are still a bit of a mystery to me. I can take them apart, clean them, and put them back together. Dialing them in has always been an issue, and I usually have a helping hand. I still think they work by magic.
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@rb_prior avatar
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@rb_prior avatar
1963 Allstate, 2005 Vespa PX150, 2001 Harley 95 ci Dyna
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Posts: 320
Location: Central Ohio
UTC quote
I had two 120, one 150, and one 160 air jets. The bike had a 160 air jet at the top of the main stack when it had the soft seize. Main jet was 98 and the atomizer was an E1. I really wanted to try a 140 air jet but could not find one so I took one of the 120 air jets and drilled the center hole with a 1.40 mm bit. Looking at all the air jets side by side it looks like the hole size falls in between the 120 and 150 jets. I would guess it is close to a 140. So I installed the re-drilled air jet. This reduction from 160 to 140 should mean less air to mix with the fuel and thus a richer condidtion.

I still have not plugged my carb air hole so I re-drilled the 42 idle jet and made the hole bigger (1.6 mm). I want to enrich my idle circuit.

The idle air mixture screw is out 1 1/2 turns. Timing is 22 degrees BTDC.

The bike started on the first kick and ran great. It had good pull and no bogging. I did not exceed 30 mph but the bike was very responsive to WOT for a short burst. I pulled the plug and it was darker indicating a richer fuel setting in the lower cruising range. I will keep tweaking it but short of plugging that air hole and going to the newer idle jets, I am inclined to leave it alone.
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bodgemaster
63 GL, 76 Super (x2), 74 Primavera (x2), 79 P200, 06 Fly 150
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bodgemaster
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UTC quote
Sounds like you got it all sorted. Nice work.
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