fml 163 pushrod engine stumble

recracer · 04-26-2010, 10:06 AM

History first . after the muffler install (pz30 carb) tried 115 main jet stumbled badly , dropped to a 105 main and it ran great free revving , thought i mite be a little on the lean side . left it alone for then . two weeks ago started stumbling at high rpms again ,not as bad as the 115 jet tho . so i thought maybe i should raise the needle . well that was fine but no change in the stumble . OK needle won't fix a full throttle lean anyway . put in a 110 main jet . it loves it , still no change at high rpm . plug looks great full throttle chop check . so i tried another cdi ,performance one i had bought awhile back . feels good , but still no high rpm change . now the question . Faulty coil. ground , weak valve springs ? Valves adjusted as well after stumble started .

Weldangrind · 04-26-2010, 10:46 AM

I'm thinking fuel delivery problem. When you last swapped jets, did you clean the carb? Maybe there's a blockage.

recracer · 04-26-2010, 11:11 AM

I did not do any extra cleaning , carb looks very clean and was new this winter . will stumble full throttle only at 7500 to 8000 rpm loaded any gear or neutral . any rpm leading up to that, is fine at full throttle . high rpm plug chop still shows color , but could be misleading because of misfire/stumble.

FastDoc · 04-26-2010, 11:26 AM

Dirty air filter?

recracer · 04-26-2010, 12:05 PM

Can still see daylight thru airfilter K&n Style . In my head ,i'm kinda turned away from it being a fuel problem. unless it's between the tank and carb . havn't looked there. . almost thinking it's a vavle bouncing , hear a snap at each hicup, from the engine . stumble not every firing ,i don't think . about every second or 2 .

Weldangrind · 04-26-2010, 12:42 PM

Ok, have you measured the plug wire? I tossed mine in favour of an NGK wire and boot. The wire unscrews from the coil for testing or replacement.

2LZ · 04-26-2010, 02:07 PM

I saw this on a Lifanoid, new at my dealer. A guy brought it back in for stumbling and cutting out under load. The dealer replaced the carb, fuel filter, swapped coils and a new NGK plug. It still did it.
Ended up being the valves were to tight.

rpowell · 04-26-2010, 04:32 PM

Does it stumble or sounds like it cuts out then back on? I will relay the info to my mechanic and get back to you.

recracer · 04-26-2010, 04:54 PM

Quote:

Originally Posted by rpowell

Does it stumble or sounds like it cuts out then back on? I will relay the info to my mechanic and get back to you.

more like a skip every 1 or 2 sec while at full throttle and near 8000 rpm

Cal25 · 04-26-2010, 05:27 PM

a 115 on a PZ30 will make it way too rich. What jet came on the bike when new?

recracer · 04-26-2010, 07:18 PM

Yes a 115 jet was way to rich . The viva had a pz30 on it already . i didn't realize that ,till i ordered a pz30 to upgrade . thats when i saw it was the same carb . no mods at all it had a 98 jet in it . with the exhaust and k&n style air filter i have been running a 105 . ran great with the 105 , but it started this stumble while it still had the 105 in it . so i put a 110 to see if it would make a difference it didn't .

Wild_Alaskan · 04-26-2010, 10:30 PM

sound kinda like my problem, I rewired it, since AL apparently doesn't have harnesses for these bikes, and it runs now but still breaks up. I will be replacing coil, spark plug and cdi next and let you know how it goes.

TurboT · 04-27-2010, 12:54 AM

At 8000 rpm I expect you're experiencing some vibration through the motor, frame and cabling. My 200cc Beast ATV shakes like a bugger, and rattles the choke cable out a little. You may want to check to see if the vibration from the motor at that RPM is blurping the choke a bit causing you to stumble. Might not be the case, but something to keep an eye on.

Cal25 · 04-27-2010, 09:03 AM

My bike also has the pz30 carb. It had a 105 jet in it when new (hibird 250). I opened the exhaust and deleted the airbox. Started with a fat 115 jet and it was way to rich. Dropped down to a 112 and still had a miss at WOT like you describe although it was better than the 115. Ended up at 2 sizes bigger than stock with a 110. No mis at WOT.

I was pretty sure I was going to be over rich when I started changing jets, but wanted to be safe and not go too lean.
One of the things you mentioned was skipping a stroke. That is one of the symptoms of a rich condition. Another thing to try is observe what happens at WOT in top gear. If you back off the throttle just a bit, It will feel like it gets more power.

Odd if it ran OK after a jet change and later started having the symtems.

Weldangrind · 04-27-2010, 11:20 AM

Quote:

Originally Posted by Cal25

Odd if it ran OK after a jet change and later started having the symtems.

That's the part I kept coming back to. It was running well, and then something changed. That's what's making me gravitate towards a fuel delivery issue or a weak spark.

04-26-2010, 10:06 AM	#1
recracer Join Date: Jul 2009 Location: Charles Town W.V. Posts: 849	fml 163 pushrod engine stumble History first . after the muffler install (pz30 carb) tried 115 main jet stumbled badly , dropped to a 105 main and it ran great free revving , thought i mite be a little on the lean side . left it alone for then . two weeks ago started stumbling at high rpms again ,not as bad as the 115 jet tho . so i thought maybe i should raise the needle . well that was fine but no change in the stumble . OK needle won't fix a full throttle lean anyway . put in a 110 main jet . it loves it , still no change at high rpm . plug looks great full throttle chop check . so i tried another cdi ,performance one i had bought awhile back . feels good , but still no high rpm change . now the question . Faulty coil. ground , weak valve springs ? Valves adjusted as well after stumble started .
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04-26-2010, 10:46 AM	#2
Weldangrind Join Date: Mar 2009 Location: Sardis, BC, Canada Posts: 25,977	I'm thinking fuel delivery problem. When you last swapped jets, did you clean the carb? Maybe there's a blockage. __________________ 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
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04-26-2010, 11:26 AM	#4
FastDoc Join Date: Mar 2009 Location: Southeastern Washington desert Posts: 14,761	Dirty air filter? __________________ Happy to serve.
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04-26-2010, 12:42 PM	#6
Weldangrind Join Date: Mar 2009 Location: Sardis, BC, Canada Posts: 25,977	Ok, have you measured the plug wire? I tossed mine in favour of an NGK wire and boot. The wire unscrews from the coil for testing or replacement. __________________ 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
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04-26-2010, 02:07 PM	#7
2LZ Join Date: Mar 2010 Location: Volcano, Ca Posts: 7,081	I saw this on a Lifanoid, new at my dealer. A guy brought it back in for stumbling and cutting out under load. The dealer replaced the carb, fuel filter, swapped coils and a new NGK plug. It still did it. Ended up being the valves were to tight. __________________ "Light a fire for a man, and you heat him for a day. Light a man on fire, and you heat him for the rest of his life." 2007 Suzuki DRZ400S (SM convert) 2009 Q Link XP 200 1967 BSA B25 250cc Starfire 2022 Royal Enfield Interceptor 650 2023 Royal Enfield Scram 411 1948 Royal Enfield Model G 350
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04-26-2010, 11:11 AM	#3
recracer Join Date: Jul 2009 Location: Charles Town W.V. Posts: 849	I did not do any extra cleaning , carb looks very clean and was new this winter . will stumble full throttle only at 7500 to 8000 rpm loaded any gear or neutral . any rpm leading up to that, is fine at full throttle . high rpm plug chop still shows color , but could be misleading because of misfire/stumble.
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04-26-2010, 12:05 PM	#5
recracer Join Date: Jul 2009 Location: Charles Town W.V. Posts: 849	Can still see daylight thru airfilter K&n Style . In my head ,i'm kinda turned away from it being a fuel problem. unless it's between the tank and carb . havn't looked there. . almost thinking it's a vavle bouncing , hear a snap at each hicup, from the engine . stumble not every firing ,i don't think . about every second or 2 .
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04-26-2010, 04:32 PM	#8
rpowell Join Date: Apr 2010 Posts: 21	Does it stumble or sounds like it cuts out then back on? I will relay the info to my mechanic and get back to you.
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04-26-2010, 05:27 PM	#10
Cal25 Join Date: Feb 2008 Location: Terrell and Grapevine Tx. Posts: 1,585	a 115 on a PZ30 will make it way too rich. What jet came on the bike when new?
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04-26-2010, 07:18 PM	#11
recracer Join Date: Jul 2009 Location: Charles Town W.V. Posts: 849	Yes a 115 jet was way to rich . The viva had a pz30 on it already . i didn't realize that ,till i ordered a pz30 to upgrade . thats when i saw it was the same carb . no mods at all it had a 98 jet in it . with the exhaust and k&n style air filter i have been running a 105 . ran great with the 105 , but it started this stumble while it still had the 105 in it . so i put a 110 to see if it would make a difference it didn't .
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04-26-2010, 10:30 PM	#12
Wild_Alaskan Join Date: Dec 2009 Location: Alaska Posts: 914	sound kinda like my problem, I rewired it, since AL apparently doesn't have harnesses for these bikes, and it runs now but still breaks up. I will be replacing coil, spark plug and cdi next and let you know how it goes. __________________ Dune buggy build http://chinariders.net/modules.php?n...wtopic&t=14470 400cc lifan build vid: http://youtu.be/20HoU8OK_Y0 other fun: http://tinyurl.com/wildalaskan
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04-27-2010, 12:54 AM	#13
TurboT Join Date: Sep 2008 Location: Surrey, B.C., Canada Posts: 2,185	At 8000 rpm I expect you're experiencing some vibration through the motor, frame and cabling. My 200cc Beast ATV shakes like a bugger, and rattles the choke cable out a little. You may want to check to see if the vibration from the motor at that RPM is blurping the choke a bit causing you to stumble. Might not be the case, but something to keep an eye on.
	0

04-27-2010, 09:03 AM	#14
Cal25 Join Date: Feb 2008 Location: Terrell and Grapevine Tx. Posts: 1,585	My bike also has the pz30 carb. It had a 105 jet in it when new (hibird 250). I opened the exhaust and deleted the airbox. Started with a fat 115 jet and it was way to rich. Dropped down to a 112 and still had a miss at WOT like you describe although it was better than the 115. Ended up at 2 sizes bigger than stock with a 110. No mis at WOT. I was pretty sure I was going to be over rich when I started changing jets, but wanted to be safe and not go too lean. One of the things you mentioned was skipping a stroke. That is one of the symptoms of a rich condition. Another thing to try is observe what happens at WOT in top gear. If you back off the throttle just a bit, It will feel like it gets more power. Odd if it ran OK after a jet change and later started having the symtems.
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