idle jumps rpm's without throttle

bucannon · 02-17-2014, 07:55 PM

okay so after a rebuild i put my Mikuni 30mm carb back on my 200cc bike with about 900 kilometers on the clock. it runs and idles but not perfectly...

any help would be appreciated!

here are the issues:

1. I can only start it with the choke closed even when it is warm.

2. ocassional backfire (and i did adjust the valves once)

3. it will start and idle around 1500 rpm. with my hand off the trhottle the whole time it will go up to 4000 rpm until it backfires then will slow down, over and over.

zingshoen · 02-17-2014, 08:19 PM

sounds like multiple problems! is the carb actually clean? did you blow compressed air through all the passages, and did you get a flow of air through them?
also check that the choke operates the way you think it does and that the slider isn t stuck. and make sure you get free flow out of the petcock.

if all that is okay, i would check the idle mix screw, it may not let enough fuel through. that explains why you need the choke to start, and then you get surges coz you have too much air and too little fuel, which results in backfiring. don t let the motor run for long that way, it s way too lean.

bucannon · 02-17-2014, 10:02 PM

Quote:

Originally Posted by zingshoen

also check that the choke operates the way you think it does and that the slider isn t stuck.

oh dont worry i only had to make that mistake once!

FastDoc · 02-17-2014, 08:47 PM

Induction air leak. Almost certainly.

bucannon · 02-17-2014, 09:59 PM

so i pulled the throttle assembly off since its too rusty to adjust, noticed an improvement on the fast idle, but its hard to tell from how shortly i kept it running since doing that. im ordering a new throttle cable tonight.

I don't have an air filter on it now but i put what remains of the one i had(rubber "neck" with smaller opening) to offer some restriction in air flow like a filter would. this may be part of the issue.

i know the intake manifold is in good shape and the nipple on it has had a new vaccum hose cap thing put on.

Weldangrind · 02-18-2014, 11:07 AM

No air filter is certainly compounding the issue. I also agree with Doc that you could have a vacuum leak. Stick a filter on it (that is properly oiled) and try again.

bucannon · 02-18-2014, 03:25 PM

what mm filter do these carbs take?

Weldangrind · 02-18-2014, 11:06 PM

42mm is a good fit, if you're using a pod.

zingshoen · 02-19-2014, 05:19 AM

here in oz they don t make a 42 mm unifilter, though they have 40mm and 44mm.

but that s another topic...while you certainly need a filter i don t think engine will run differently with or without filter. BUT the filter rubber bottom you have on there may restrict the two venturi tubes at the front of the carbie, so you could try without it, perhaps that will make a difference to your problem. then you need a filter which does not block those venturis.

bucannon · 02-21-2014, 03:23 AM

Quote:

Originally Posted by zingshoen

BUT the filter rubber bottom you have on there may restrict the two venturi tubes at the front of the carbie, so you could try without it, perhaps that will make a difference to your problem. then you need a filter which does not block those venturis.

it provides space for those, but i have a new filter in the mail.

once i put the new throttle cable and air filter on it I will let yall know how it runs...

Weldangrind · 02-19-2014, 03:40 PM

If it's jetted for a filter, it will run lean when the filter is removed.

02-17-2014, 07:55 PM	#1
bucannon Join Date: Nov 2011 Location: Galveston, TX Posts: 63	idle jumps rpm's without throttle okay so after a rebuild i put my Mikuni 30mm carb back on my 200cc bike with about 900 kilometers on the clock. it runs and idles but not perfectly... any help would be appreciated! here are the issues: 1. I can only start it with the choke closed even when it is warm. 2. ocassional backfire (and i did adjust the valves once) 3. it will start and idle around 1500 rpm. with my hand off the trhottle the whole time it will go up to 4000 rpm until it backfires then will slow down, over and over. __________________ 2008 200cc dirtbike
	0

02-17-2014, 08:19 PM	#2
zingshoen Join Date: Jun 2009 Location: Brisbane, Queensland, Australia Posts: 648	sounds like multiple problems! is the carb actually clean? did you blow compressed air through all the passages, and did you get a flow of air through them? also check that the choke operates the way you think it does and that the slider isn t stuck. and make sure you get free flow out of the petcock. if all that is okay, i would check the idle mix screw, it may not let enough fuel through. that explains why you need the choke to start, and then you get surges coz you have too much air and too little fuel, which results in backfiring. don t let the motor run for long that way, it s way too lean. __________________ 1971 Garelli Gulp 50cc with open ports and 16 mm Bing Carbie + 1980 Honda XL 223 ZS + 2007 Zongshen LZX200G + DR400SM
	0

02-17-2014, 08:47 PM	#4
FastDoc Join Date: Mar 2009 Location: Southeastern Washington desert Posts: 14,761	Induction air leak. Almost certainly. __________________ Happy to serve.
	0

02-17-2014, 09:59 PM	#5
bucannon Join Date: Nov 2011 Location: Galveston, TX Posts: 63	so i pulled the throttle assembly off since its too rusty to adjust, noticed an improvement on the fast idle, but its hard to tell from how shortly i kept it running since doing that. im ordering a new throttle cable tonight. I don't have an air filter on it now but i put what remains of the one i had(rubber "neck" with smaller opening) to offer some restriction in air flow like a filter would. this may be part of the issue. i know the intake manifold is in good shape and the nipple on it has had a new vaccum hose cap thing put on. __________________ 2008 200cc dirtbike
	0

02-18-2014, 11:07 AM	#6
Weldangrind Join Date: Mar 2009 Location: Sardis, BC, Canada Posts: 25,977	No air filter is certainly compounding the issue. I also agree with Doc that you could have a vacuum leak. Stick a filter on it (that is properly oiled) and try again. __________________ Weldangrind "I figure I'm well-prepared for coping with a bike that comes from the factory with unresolved issues and that rewards the self-reliant owner." - Buccaneer
	0

02-18-2014, 03:25 PM	#7
bucannon Join Date: Nov 2011 Location: Galveston, TX Posts: 63	what mm filter do these carbs take? __________________ 2008 200cc dirtbike
	0

02-18-2014, 11:06 PM	#8
Weldangrind Join Date: Mar 2009 Location: Sardis, BC, Canada Posts: 25,977	42mm is a good fit, if you're using a pod. __________________ Weldangrind "I figure I'm well-prepared for coping with a bike that comes from the factory with unresolved issues and that rewards the self-reliant owner." - Buccaneer
	0

02-19-2014, 05:19 AM	#9
zingshoen Join Date: Jun 2009 Location: Brisbane, Queensland, Australia Posts: 648	here in oz they don t make a 42 mm unifilter, though they have 40mm and 44mm. but that s another topic...while you certainly need a filter i don t think engine will run differently with or without filter. BUT the filter rubber bottom you have on there may restrict the two venturi tubes at the front of the carbie, so you could try without it, perhaps that will make a difference to your problem. then you need a filter which does not block those venturis. __________________ 1971 Garelli Gulp 50cc with open ports and 16 mm Bing Carbie + 1980 Honda XL 223 ZS + 2007 Zongshen LZX200G + DR400SM
	0

02-19-2014, 03:40 PM	#11
Weldangrind Join Date: Mar 2009 Location: Sardis, BC, Canada Posts: 25,977	If it's jetted for a filter, it will run lean when the filter is removed. __________________ Weldangrind "I figure I'm well-prepared for coping with a bike that comes from the factory with unresolved issues and that rewards the self-reliant owner." - Buccaneer
	0