Author Topic: Scooter backfires when starting or slowing down  (Read 101 times)

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Offline Mat1998

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Scooter backfires when starting or slowing down
« on: May 30, 2018, 01:04:09 PM »
Hey guys, so recently I noticed my rct170 backfires while starting and occasionally when I slow down at a traffic light or stop street. Any idea why it's doing this all of a sudden?


Offline SA SCOOTERS

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Re: Scooter backfires when starting or slowing down
« Reply #1 on: May 30, 2018, 01:24:49 PM »
Vacuum carbs has a tendency to do that when they get a bit worn.
The vacuum slide is slow to return down and its leads to a short temporary over-rich moment.
The backfire you hear is the un-burnt fuel exploding in the zorst.
Once the engine heats up....and the carb also heats up a bit....it tends to go away.

on my old gs1000g with vacuum carbs i had flames coming out the rear when i closed the throttle at 160.
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Re: Scooter backfires when starting or slowing down
« Reply #2 on: May 30, 2018, 01:33:13 PM »
on your scoot the throttle cable controls a butterfly valve that opens and closes. The more open the butterfly valve the more suction/vacuum is created and this 'sucks' the slide open
There is lag once you open or close the throttle as the slide depends on vacuum to open and lack of vacuum to close.
That is why slide carbs are better. Or more responsive.

To see that process....you need to hold a perfectly tuned carb in front of you and softly blow through it. a good slide will move....and bad slide will require you to blow your lungs out...and some require the heavy vacuum only the engine can create before it slides up.

So, you close the throttle...the butterfly shuts but the slide is still on its way down. This give you a very rich mix until the slide has reached the bottom. The unburnt fuel that exits the engine ignites due to the very warm header pipe. Or combusts with the next cycle of hot air.
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In the beginning God made the heaven and the earth. The rest was made in China.
Loncin 150 Racer, Honda 600XR, BMW R1100S, HD 1340 Electra Glide and a Fishing Kayak...What more can a man ask for?


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Re: Scooter backfires when starting or slowing down
« Reply #3 on: May 30, 2018, 01:37:12 PM »
The real problem starts when the slide is so slow to return that the damn thing backfires into the air cleaner. Some of us used to take air cleaners off from our bikes back then....lack of funds (used for beers)...and you better not have any combustible stuff/petrol/oil etc under your seat, or your bike goes up in flames.
« Last Edit: May 30, 2018, 01:38:11 PM by SA SCOOTERS »
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Re: Scooter backfires when starting or slowing down
« Reply #4 on: May 30, 2018, 01:39:38 PM »
on any other engines than scooter engines, which has fixed timing and stock cams.....back firing could be a b***ch to trace
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In the beginning God made the heaven and the earth. The rest was made in China.
Loncin 150 Racer, Honda 600XR, BMW R1100S, HD 1340 Electra Glide and a Fishing Kayak...What more can a man ask for?

Offline Stanthescooterman

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Re: Scooter backfires when starting or slowing down
« Reply #5 on: May 30, 2018, 01:41:42 PM »
The backfire you hear is the un-burnt fuel exploding in the zorst.

Correct.

There are several possible contributors, even a combination:

Certainly, the carb is running far too rich - you will need to investigate and find out why, but off the bat, check to see whether your auto choke is misbehaving.
Once having eliminated that as a possibility, you need to set your air screw correctly, which is best done by an experienced and competent person.

Lastly, because its been running rich, you may well have carbon in your exhaust valve seat, meaning that the valve will not fully close when it should, which will exacerbate backfiring.
Previous bikes & scooters:
BMW C600 Sport
Kymco Grand Dink 250 (300),
BMW F650GS Dakar,
Gilera Runner 180,
Kymco Agility 125,
Kymco TB100,
Kymco Heroism KH125,
Honda CBX550FII,
Suzuki RG50 Gamma

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Re: Scooter backfires when starting or slowing down
« Reply #6 on: May 30, 2018, 01:43:35 PM »
Sometimes a cracked intake will also do that...but then the scoot will battle to idle.

and what Stan said.. :yes:
www.sascooters.co.za 
  
In the beginning God made the heaven and the earth. The rest was made in China.
Loncin 150 Racer, Honda 600XR, BMW R1100S, HD 1340 Electra Glide and a Fishing Kayak...What more can a man ask for?

Offline Mat1998

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Re: Scooter backfires when starting or slowing down
« Reply #7 on: May 30, 2018, 02:39:04 PM »
Cool, thanks guys, it doesn't do it often but I'll get it looked at.


Offline Stanthescooterman

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Re: Scooter backfires when starting or slowing down
« Reply #8 on: May 30, 2018, 05:58:26 PM »
You mentioned a while back that you've had starting troubles with both your scooters....

This backfiring symptom may just be related, in some way.

Either way, there sounds like there is something not lekker with your carb, and has not been for some time.
Previous bikes & scooters:
BMW C600 Sport
Kymco Grand Dink 250 (300),
BMW F650GS Dakar,
Gilera Runner 180,
Kymco Agility 125,
Kymco TB100,
Kymco Heroism KH125,
Honda CBX550FII,
Suzuki RG50 Gamma

Offline sharkcontrol

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Re: Loose manifold nuts
« Reply #9 on: May 31, 2018, 08:23:14 AM »
Mat also check the fastness of the exhaust pipe flange-with the continuous vibration my flange nuts would vibrate loose until i put a drop of loctite on them.
« Last Edit: June 01, 2018, 04:17:48 PM by Stanthescooterman »
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Offline Mat1998

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Re: Cracked exhaust
« Reply #10 on: May 31, 2018, 08:53:22 AM »
Shark, im taking the entire exhaust off this weekend. I noticed my exaust is mounted in three places and two of the three mounts have cracked from vibration so I need to do a bit of welding. Im also respraying the entire exhaust anyway so I guess it's not a bad thing.
« Last Edit: June 01, 2018, 04:17:27 PM by Stanthescooterman »

Offline Stanthescooterman

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Re: Loose manifold nuts
« Reply #11 on: May 31, 2018, 12:51:12 PM »
Mat also check the fastness of the exhaust pipe flange-with the continuous vibration my flange nuts would vibrate loose until i put a drop of loctite on them.

Just a drop, but pref a spring washer.
I've found that loctite can cause the manifold nut and stud  to turn out together, next time you unscrew it.
« Last Edit: June 01, 2018, 04:17:13 PM by Stanthescooterman »
Previous bikes & scooters:
BMW C600 Sport
Kymco Grand Dink 250 (300),
BMW F650GS Dakar,
Gilera Runner 180,
Kymco Agility 125,
Kymco TB100,
Kymco Heroism KH125,
Honda CBX550FII,
Suzuki RG50 Gamma

Offline Stanthescooterman

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Re: Cracked exhaust
« Reply #12 on: May 31, 2018, 12:55:47 PM »
...  two of the three mounts have cracked from vibration ...

Very common occurrence on Chinese scoots.

If you have a gas torch of some sorts, then once you've welded the cracks, heat up those areas until they're starting to glow, then allow to cool.
This relieves stresses in the metal and reduces the relatively high likelihood of future repeat cracks at the same spots.
« Last Edit: June 01, 2018, 04:16:38 PM by Stanthescooterman »
Previous bikes & scooters:
BMW C600 Sport
Kymco Grand Dink 250 (300),
BMW F650GS Dakar,
Gilera Runner 180,
Kymco Agility 125,
Kymco TB100,
Kymco Heroism KH125,
Honda CBX550FII,
Suzuki RG50 Gamma

Offline Mat1998

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Re: Cracked exhaust
« Reply #13 on: May 31, 2018, 05:30:24 PM »
Cool, thanks for the tip Stan
« Last Edit: June 01, 2018, 04:16:27 PM by Stanthescooterman »

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Re: Cracked exhaust
« Reply #14 on: June 01, 2018, 11:36:22 AM »
...heat up those areas until they're starting to glow...

... but obviously not if the mounting is the anti-vibration type, i.e. a rubber bush sandwiched between two concentric thin metal sleeves, because then you'll be sorry.
« Last Edit: June 01, 2018, 04:15:42 PM by Stanthescooterman »
Previous bikes & scooters:
BMW C600 Sport
Kymco Grand Dink 250 (300),
BMW F650GS Dakar,
Gilera Runner 180,
Kymco Agility 125,
Kymco TB100,
Kymco Heroism KH125,
Honda CBX550FII,
Suzuki RG50 Gamma

Offline Mat1998

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Re: Scooter backfires when starting or slowing down
« Reply #15 on: June 01, 2018, 04:57:58 PM »
It's just the metal bracket and a bolt, it mounts in three places. The one bracket is completely cracked and the second one has a small hairline crack where the bolt goes through it. The third mounting is still perfect

Offline Mat1998

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Re: Scooter backfires when starting or slowing down
« Reply #16 on: June 03, 2018, 10:38:40 AM »
Looks like auto choke is working, it idles a bit high for 3-5 minutes and then settles down, handy feature of the 170 is the rpm gauge, so I suspect it's running too rich , dirty exhaust although there's no smoke and the oil level has remained the same so it's definitely not burning oil. I'll adjust the carb to make it slightly leaner and see if that does the trick. My dad's decided to rather send my exaust to a guy that specialises in fixing exhausts and once he's done I'll respray it with the heat resistant paint I picked up yesterday from Chamberlains. I'll be making a quick review of both my rct 170 and the retro and post the YouTube link on the forum. Thinking of also doing one on my friends vespa but I'll see how far I get

Offline Stanthescooterman

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Re: Scooter backfires when starting or slowing down
« Reply #17 on: June 03, 2018, 01:57:19 PM »
Cool, thanks for the feedbak, Mat.

Looking forward to your reviews!  :yes:
Previous bikes & scooters:
BMW C600 Sport
Kymco Grand Dink 250 (300),
BMW F650GS Dakar,
Gilera Runner 180,
Kymco Agility 125,
Kymco TB100,
Kymco Heroism KH125,
Honda CBX550FII,
Suzuki RG50 Gamma