Runaway idle on cold start
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bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5653
Location: So Cal
Mon Feb 01, 2021 3:03 pm quote
Before I pull the smallie engine and start chasing air leaks, help me make sure Iím not overlooking something simple.

Itíd been sitting for a few months unridden, so last week I thought Iíd take it for a ride. It fired right up but the idle shot through the roof. It was revving pretty much out of control. Choke wouldnít kill it. Throttle had no effect.

Throttle cable wasnít binding.

Slide wasnít stuck.

Carb was tight on manifold.

Killed it and started it again. Same thing. After about a minute the idle dropped down and everything was fine.

Tried it again the next day, same thing. Screaming idle at cold start, then settled down after about 15 seconds.

Normally Iíd say leaky seal but there isnít a lot of smoke. Oilís not gassy. No sign of leaks around flywheel.

Ideas?


Polini CP24 carb, Polini 130 kit, cases matched, PM40 pipe. ET3 ignition, 18į.
Enthusiast
Vespa 125 VMA, Vespa SS 180
Joined: 14 Feb 2018
Posts: 64
Location: Minneapolis, MN
Mon Feb 01, 2021 4:35 pm quote
Iíve had my smallie do that if thereís not enough fuel. Usually if I shut off the tap and run it out of gas. Maybe a fuel line or fuel tap issue?
bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5653
Location: So Cal
Mon Feb 01, 2021 5:23 pm quote
Yep, thought it might be fuel starvation. Sounds exactly like the float bowl is emptying. But it only does it on cold starts. And after itís revved like a madman for a few seconds, it settles down and all is fine. Tank is new, tap too, and just put on a new fuel line.
Moderator
VNB VSC VBC VSX
Joined: 28 May 2008
Posts: 4698
Location: Hustletown, TX
Mon Feb 01, 2021 5:41 pm quote
Spitball:

Put some grease or rather a little 2 stroke on that stupid felt ring at the carb manifold.
bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5653
Location: So Cal
Mon Feb 01, 2021 5:59 pm quote
No felt ring with the CP24 carb. It uses a rubber connector between the intake manifold and carb. Good idea though.

AD66AF76-1800-4313-9FBD-C577C3D8B64B.jpeg

Moderator
VNB VSC VBC VSX
Joined: 28 May 2008
Posts: 4698
Location: Hustletown, TX
Wed Feb 03, 2021 7:53 am quote
Sorry! You had typed that above and I just read right past it.
Ossessionato
2015 GTS300, 1974 Primavera, 04 Ninja 250
Joined: 04 Apr 2013
Posts: 4589
Location: San Diego, CA
Thu Feb 04, 2021 7:46 pm quote
Maybe our paper exhaust gasket has failed... barbecue beef mmmm
bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5653
Location: So Cal
Thu Feb 04, 2021 8:02 pm quote
Replaced it... but kept as a souvenir. That gasket earned us a Peopleís Choice Award.
Ossessionato
Joined: 26 Oct 2015
Posts: 2923

Thu Feb 04, 2021 8:14 pm quote
I bought a new tap before and the rubber seal behind it chunked out, which caused a partial blockage of my fuel line.
bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5653
Location: So Cal
Sat Feb 06, 2021 1:53 am quote
Iíll check the tap. It only does it on first start up ... engine screams like a banshee ... then back to normal after about 15-20 seconds of racing. Dunno. Slide sticking? Plugged jet? Maybe carb just needs a good cleaning.
Ossessionato
2015 GTS300, 1974 Primavera, 04 Ninja 250
Joined: 04 Apr 2013
Posts: 4589
Location: San Diego, CA
Sat Feb 06, 2021 10:13 am quote
You can never go wrong cleaning your carb.
Addicted
ET3
Joined: 05 Mar 2013
Posts: 569
Location: OC
Sat Feb 06, 2021 11:35 am quote
^This. I would also check your choke.

I'm not sure if you set up a remote or have it on the carb body itself but I did find a time that mine was binding up and not fully closing.

Try not go gorilla tight on the rubber manifold, they can cause an air leak too. I've had my CP fall completely off!

Reseal the cylinder head with some copper RTV. I'm not sure if your kit has the metal copper gasket but I smear a thin coat of RTV on mine for 'belt and suspenders' precaution.

You could check your gear oil for gasoline, if present = blown clutch seal.

Check your spark plug that its snug.

Just suggestions...

*side note...you need to resurrect your Honda NC50 Express
Hooked
'04 Genuine Stella, '03 Vespa ET, '62 Motovespa 150S (with a P125X engine), a '71 Rally 180, '57 Lambretta LD125 Mk. III
Joined: 22 Jan 2008
Posts: 471
Location: Lehi, UT Utah
Sat Feb 06, 2021 5:03 pm quote
Maybe the throttle slide is sticking, and the carb heats up enough to expand enough to let it settle back into place.
bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5653
Location: So Cal
Wed Feb 10, 2021 8:51 am quote
mr.j wrote:
^This. I would also check your choke.

I'm not sure if you set up a remote or have it on the carb body itself but I did find a time that mine was binding up and not fully closing.

Try not go gorilla tight on the rubber manifold, they can cause an air leak too. I've had my CP fall completely off!

Reseal the cylinder head with some copper RTV. I'm not sure if your kit has the metal copper gasket but I smear a thin coat of RTV on mine for 'belt and suspenders' precaution.

You could check your gear oil for gasoline, if present = blown clutch seal.

Check your spark plug that its snug.

Just suggestions...

*side note...you need to resurrect your Honda NC50 Express
Dang, the goofy Honda Express may the Bitcoin of my scooter investments!

Back to the Primavera ... good suggestions. Thought the choke might be the issue. Had a cable on there and swapped it for a pull choke. Didnít help.

Iíll pull the carb this week and give it a good cleaning. Hopefully thatíll solve it.
Member
PK50XL,PK100S,ET3
Joined: 26 Aug 2020
Posts: 32
Location: UK
Tue Feb 16, 2021 8:02 am quote
is the float needle shutting off ok/ not sure what the symptoms of leaky float and over-full float chamber would be but if it had a very gradual leak it could explain why its fine after youve run it for 10 mins or so.
bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5653
Location: So Cal
Tue Feb 16, 2021 11:14 am quote
Well crap... carb cleaned, float and needle seem to be fine, headís torqued tight, spark plug too, chokeís working, still screams out of control on cold start up. WTF. Time to pull the engine and check for leaks.
Ossessionato
2015 GTS300, 1974 Primavera, 04 Ninja 250
Joined: 04 Apr 2013
Posts: 4589
Location: San Diego, CA
Tue Feb 16, 2021 3:21 pm quote
pressure check it
Ossessionato
62 VBB1T Round Tail W/ leaner sidecar
Joined: 26 Jan 2019
Posts: 2036
Location: california
Tue Feb 16, 2021 7:03 pm quote
This says failing seal all over it to me.
Assume any clamps or studs related to carbs and intake have been snugged.

Two ways to make a motor race.
Starve it lean.
Turn the throttle wide open.

I would be tempted to disconnect the throttle cable - just for my sanity.
If it raced - I would start hunting the leak.
Crank seal and carb would be my go too-s.
Might hold a can of brake cleaner handy to spray at areas to see if it killed or changed the race.
Maybe as it warms the seal is seating.
Spray cleaner that chokes it would give you a clue?
Member
VBA
Joined: 12 Mar 2018
Posts: 47
Location: Texas
Tue Feb 16, 2021 9:29 pm quote
SoCalGuy wrote:
Tank is new, tap too, and just put on a new fuel line.
Were these replaced before or after you started having high idle cold starts?

I saw you planned to check the tap, I'm guessing you did and it yielded nothing?

My brain is stuck on the fuel for some reason.

Going a different route, what part on the bike could heat up and be happy in 45 secs?

I acknowledge it could be an air leak, but just trying to think of other possibilities.
bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5653
Location: So Cal
Wed Feb 17, 2021 8:41 am quote
Thanks for the input.

Mystery solved maybe?

Rubber connector is a little mangled where it attaches to to the manifold. Got a new one ordered. Fingers crossed.

60C2867F-C895-44C7-9745-56901F0C1DB4.jpeg

Addicted
2010 S50 Modified "Punkin"; 2003 ET4 Malossi 187 "ISO"
Joined: 18 Feb 2017
Posts: 671
Location: Deepinnaharta, Texas
Wed Feb 17, 2021 3:43 pm quote
Greetings:

I think we have a winner! Johnny, tell him what he's won!! Yeah it doesn't take much of an air leak to produce the symptoms you describe. I'm confident the replacement will fix the problem.
Addicted
GL, PK, PE200 with hack, Sears Rust Badge
Joined: 24 Apr 2012
Posts: 956
Location: Deep in the heart of Texas
Wed Feb 17, 2021 6:33 pm quote
Iím glad u may have found the problem. Iíve chased an air leaks for months with the carb manifold connection. Think I got it right with Motoseal and crab not turning but when I spray behind the carb the motor dies. After so many years I would think someone would come up with a better way to connect a carb. I wish I could use that setup on a stock 19/19.....or can I?
bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5653
Location: So Cal
Thu Feb 18, 2021 12:34 am quote
Havenít tried it but I bet you could use something like this in place of the alloy sleeve/felt ring to mount a stock 19/19 carb.

https://www.treatland.tv/LBMopeds-BLUE-woven-silicone-rubber-mount-PHBG-p/lbmopeds-blue-coupler-phbg.htm
Addicted
GL, PK, PE200 with hack, Sears Rust Badge
Joined: 24 Apr 2012
Posts: 956
Location: Deep in the heart of Texas
Thu Feb 18, 2021 4:39 am quote
SoCalGuy wrote:
Havenít tried it but I bet you could use something like this in place of the alloy sleeve/felt ring to mount a stock 19/19 carb.

https://www.treatland.tv/LBMopeds-BLUE-woven-silicone-rubber-mount-PHBG-p/lbmopeds-blue-coupler-phbg.htm
Iím ordering that. Itís just mesmerizing how the scooter west video makes it look so easy. The guy doing the carb rebuild and install while Robot talks looks strange and sus but he def knows what heís doing.

bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5653
Location: So Cal
Sun Feb 21, 2021 5:32 pm quote
Just a wrap this upÖ

New rubber carb connector did the trick. Thanks all. Smallieís back in biz.
Moderator
VNB VSC VBC VSX
Joined: 28 May 2008
Posts: 4698
Location: Hustletown, TX
Sun Feb 21, 2021 5:44 pm quote
fucking smallframes


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