139QMB Bogging Down Under Load after 15 minutes running.

[fangoes]

Bogging down at WOT after 10 minutes running....seems to be able to handle WOT for the first 5 minutes of riding.??
Idles great, starts great, take-off great all the way to past 1/2 throttle.  Backing off throttle to "catch the bog" restores the revs and usually I'm able to limp home with 1/2 throttle.   

2009 Pulse Scout - has a 139QMB in it.  
Bought it second hand - about 12000 kms on the clock

Was running OK for a bit on and off, lacked power but was mostly reliable, then noticed the exhaust had rusted and broken off...this is when the bogging first began to occur, after the original exhaust broke off from the shock of a violent bounce over a speed bump - got caught out and didn't see it.  
 
Purchased and installed a new Exhaust and soon realised I would need to upjet.  I had backfiring and 1/2 - 3/4 throttle hesitation so watched a few of 90GTVert's videos and learnt all about that - played around with bigger jet sizes and needle heights for a while and finally got rid of 3/4 throttle hesitation and backfiring I was experiencing but no matter what jet size or needle position, after around 10 minutes running time (5 min idle, 5 min cruise up the road) I have consistently been getting bogging down at WOT.

Things I've looked at and fixed up so far:

• Installed New Exhaust
• New Gear Oil
• New Engine Oil
• New Battery
• New Spark Plug
• Checked Airbox and removed the air restrictor - haven't replaced air filter though....
  
• Checked Spark Plug colour and condition, also checked for spark - nice blue sparks when the bike is cold....haven't checked the sparks after bogging though (read in another post in troubleshooting about Coils going bad after they heat up and can cause weak spark... 
• Cleaned Carb, and up-jetted from a stock 79 jet to a 95 jet, very top needle position.  Tried 80 and 85 jets but still looked lean on the plug and lost power...re-tuned the air-fuel mixture screw after each new jet install.  Diaphragm and needle all look good and work as expected.  Not sure how to check float valve height?
• Checked for Vac leaks, all looks ok.
• Checked Air Intake Manifold for cracks, all good, however the O-ring inside of the mani was perished so I replaced that
• Checked Fuel Petcock, fuel is running
• Checked Fuel Filter, fuel is running
• Checked and adjusted valve clearances to .05mm (first time I've done that so hopefully am doing it right - seemed to go ok - no tapping or anything.  I read in another post about valve adjustments correcting bogging down similar to what i am experiencing.
• Checked CDI connection - looked OK

Just starting to get to the end of my tether trying to work this through.  I've learnt so much already though in my attempts at getting it to run reliably so has still been a fun and interesting experience but yeah...please someone help me to get this thing running loudly and proudly and reliably.

Should I think about ordering a new Coil and Lead?  Try another valve lash adjustment?  Float level in bowl?
What else could it be?

Starts great
Idles Great
Good pickup off the line
Bogs down at WOT at the same time into my run consistently...very mysterious.

Any assistance or advice is greatly appreciated.

Thank you very much 

[humanshield]

I would try another coil.

Classic symptoms of a bad coil...even though it could be something else



Could also be the float bowl going low for whatever reason.

BTW...

You have set your valves too tight.   .05mm = .001 inches  (TOO TIGHT)

When it gets hot they are probably tightening just enough to keep the valves cracked open.

[stevec]

bad coil possibly, the stock ones are prone to failure.
double check your valve adjustment, improper adjustment can cause the engine to lose power or stop running when it gets hot.

[Fox]

Another possible suspect would be tank venting. Next time it happens remove the gas cap and see if the symptom disappears. If you hear a rush of air when you loosen the cap then that is a sign the tank is not vented properly. If not vented right as the gas runs out of the tank there is no air coming in to take it's place so over the course of 10-15 minutes it creates a vacuum in the tank so fuel stops flowing freely starving the caruretor and you get that whoosh sound when you loosen the cap. Some tanks have a vent on top that is run to an emissions canister via a rubber tube that has an in-line check valve. Those check valves can go bad. Others use a vented gas cap.

[walleye1963]

You say needle at top?been a while sence I watched Brents vids but I thought the needle was suppose to be on the bottem? I might be wrong .

[niz76]

I usually adjust both valves to .003. I have had similar problems in two different instances: 1- exhaust happened to be bad- no matter what I tried I finally changed the exhaust and it worked, but that was on a 2t and I kinda doubt that's your problem.
#2- Makes more sense- You may be on the right track suspecting float height. You may be using fuel from the bowl faster than it's filling which would explain why it cuts out at the same time every ride. Float should be level when you tip your carb upside down with the bowl off. Also, make sure your fuel inlet to your carb is clear of any debris- I use a piece of wire to shove up into the fuel inlet from below with the bowl off... Good luck!

Backfiring is usually from an air leak somewhere or lean condition (possibly caused by the leak) Check your exhaust gasket for leaks (main suspect since you installed a new exhaust), Intake boot, vac lines etc. also try adjusting your a/f screw richer (turn clockwise).
Also try re-installing your snorkel (air box restrictor- Actually you may want to try this first as it may just solve your problem...

[humanshield]

Mar 5, 2015 0:11:01 GMT -5 Fox said:

Another possible suspect would be tank venting. Next time it happens remove the gas cap and see if the symptom disappears. If you hear a rush of air when you loosen the cap then that is a sign the tank is not vented properly. If not vented right as the gas runs out of the tank there is no air coming in to take it's place so over the course of 10-15 minutes it creates a vacuum in the tank so fuel stops flowing freely starving the caruretor and you get that whoosh sound when you loosen the cap. Some tanks have a vent on top that is run to an emissions canister via a rubber tube that has an in-line check valve. Those check valves can go bad. Others use a vented gas cap.

Good one Fox.

[fangoes]

Thank you to everyone that responded, I really appreciate it. And my apologies for making that cry for help the first post I've made on these forums...I'm an Australian living in the UK for a while, pleased to meet you all. Anyway...

Questions...

What is the correct ballpark clearance values?
I read in the GY6 Service Manual to set the valves both intake and exhaust to 0.05mm (0.00197") which is why I was aiming for that but yeah humanshield i've never adjusted these things before so have no idea what the ballpark even is. So cheers for bringing this up and making me investigate further. I've read quite a few different numbers around the place, so will have to keep reading and adjusting. niz76 you said 0.003 - thats inches right? so that would be 0.08mm? my gauges only go up in 5's and 10's from .05 to .8 so will have to maybe go get an imperial set too...

Is this normal?!?
I grabbed a quick 30sec video of something i noticed this morning getting ready for a quick test run.
I've got a hell of alot of smoke - oil for sure coming from the exhaust - but also from another weird place. Looking towards the rear of the scooter on the bottom RHS of the rocker cover I noticed smoke puffing out of here. Could this be a cracked head or something dramatic like that or more a seal or gasket?

Piston rings seen better days??
From all the posts I've read along the way seeing all that smoke out the exhaust being burnt I can almost assume the piston rings are shot, top end needs an overhaul if thats the correct terminology lol. Unfortunately I am really broke right now so may not have the chance to get this running like I want as immediately as I would like. But yeah...it's a bit of a smoke machine right now. Am I beating a dead horse hahahah?

Does this mean I could do with an even larger main jet??
On my run I noticed the bog at the exact same spot on the run again like always...Returned home, and put a piece of gaff tape over the airbox intake - covering the hole by around 40% maybe. Went out for another lap of my test circuit, and at the same place it would usually always bog and threaten to stall it powered confidently away. This place in the run is a very conservative incline up a hill, and for some reason the gaff making the mixture more rich by restricting air is preventing the bog.?? It was slightly hesitant when coming back on throttle out of a turn with the tape over half the intake hole whereas without tape was very smooth throughout 1/4 to 1/2 throttle but continued to consistently bog down at WOT. I have a 95 jet in there now, maybe go out to a 100 or even 105?

IS that Vapor Lock you speak of sir??
Thanks for that tip Fox, I have heard my brother talking about vapor lock before with his classic V8 Commodore back in Australia. Today when it bogged down I went home and took off the cap (only had 1/4 tank) and went for another burn without the cap on, it didn't seem to improve anything unfortunately.

Should I try to get a new gasket for this??
Also Im a bit confused as to whether or not there was an Exhaust Gasket...when I changed it I didn't notice one on the old flange, and couldn't see one up in the engine...it was difficult to get to though without a stand, so I might have missed it....but yeah - could possibly be no exhaust gasket on the bike right now...yes im a backyard amateur for sure. I thought that might have something to do with the above smoke from weird place problem but when i looked at it closely from underneath i couldn't see gases escaping from that direct area? It feels like the exhaust is seating well to the engine. Definitely no air leaks anywhere else that I can find.

Sorry for the novel, I hope what I'm saying makes sense, and if it doesn't, I hope it's comic relief for some of you at least. Cheers for the help again. I'd love to get this sorted before I have to sell it when I return to Australia but after seeing that smoke, I fear it might have gotten the best of me.

Thanks again everyone.

[niz76]

Have you gotten in there and taken a really good look? The video isn't too helpful since we just see some smoke but never see a camera angle that shows where it's coming from. From the wet black looking stuff on your exhaust that we see for a split second it seems as though that's a big ol' exhaust leak. Which would explain the backfiring and probably running lean and overheating... hard to tell from that vid...

[fangoes]

I can't say I've taken as good as look as I would like, it's difficult for me cause I'm a n00b and the fact i don't have it up any higher than the centre stand, so is difficult to get right underneath to inspect, even with my small head.

Yeah sorry about the video, i thought it might be a little hard to tell without perspective. ITs like coming from the LHS of the engine, down low towards the rocker cover. But yeah cant tell exactly where, I might have to attempt taking the shroud off to see I can gather more info for yas or at least get a camera under the bike where the exhaust connects and see what i can see.

Thanks.

[humanshield]

I set my valves at .003 inches

Exhaust and Intake.

Some advocate .004 on the exhaust. That keeps it ever so slightly cooler by increasing the time it's in contact with the seat.



For the flat spot, you could try raising the carburetor needle by lowering the clip position.
I cure hesitation often like that.

[fangoes]

Is it alright to try instant gasket for the exhaust? Other problem is I don't have a socket wrench (yeah yeah judge me...it's ok). I have the sockets I need and a good hand driver so figured i could torque it up fairly adequately with that...is it dumb to believe this? I've done a lot of great work on the bike and learnt a heap but I cant say Im setting the best example coming in here asking for your expert advice when i cant even purchase the right tools for the job...Am doing the best I can - I love riding 50's so will keep on trying as long as the machine is willing.

[humanshield]

Mar 5, 2015 18:20:52 GMT -5 fangoes said:

Is it alright to try instant gasket for the exhaust? Other problem is I don't have a socket wrench (yeah yeah judge me...it's ok). I have the sockets I need and a good hand driver so figured i could torque it up fairly adequately with that...is it dumb to believe this? I've done a lot of great work on the bike and learnt a heap but I cant say Im setting the best example coming in here asking for your expert advice when i cant even purchase the right tools for the job...Am doing the best I can - I love riding 50's so will keep on trying as long as the machine is willing.

Personally, I wouldn't use Instant Gasket.  Do you have a scooter store anywhere near to get a proper gasket?

When you're tightening those exhaust bolts be VERY VERY careful.   I've snapped off quite a few of them.  It's surprisingly easy to do.

[fangoes]

Cool thanks humanshield. Ill adjust to a tight 0.1mm's (0.004") and see how that goes. AS for the needle position and hesitation hers the story so far....

Had a 85 in there - mid to high needle position
- hesitant low throttle ranges
- backfired throttle off

90 jet - bottom needle position
- still hesitant
- less power but less chance of backfiring

95 jet - top needle without tape over half of airbox intake
-no hesitation
-no backfire
-more power
-bog after 10 min

95 jet top needle with that gaff tape
-no bog
-slightly less power
-no backfire
-small amount of hesitation, less than what i experienced with the needle positions i used with the 85 and 90 jets

[fangoes]

Na i'm up in the sticks in the north-west of England so have been doing everything mail order unfortunately. Ill have a hunt around tonight and if I can make an order soon. Thanks for the advice, I thought people might say that, convenience usually makes things worse... I am really hoping its the exhaust and not something crazy. The large amount of oil being burnt - am I correct in saying its a sure sign the piston rings are on their way out?