Loss of power high rpm

GoldenMotor.com
S

scribling

New Member
Mar 31, 2014
36
0
0
Simi Valley
Had it running great then the nut came off the small gear and a mount broke. Took the whole thing apart, built it better than before, new gas tank ... now when I gun it, at high rpm it just doesn't have the power it used to and sputters a bit. I've checked everything.

Wait, maybe not everything ... the new tank! I didn't check that. Maybe it's not allowing fast enough flow! That would definitely do it. Right? The new tank has the angled petcock and a filter inside.

Can anyone think of anything else that would cause this? I'll report back after checking the tank.
 
GearNut

GearNut

Active Member
Aug 19, 2009
5,104
11
38
San Diego, Kaliforgnia
The new fuel tank can certainly cause issues. Did you flush out the dirt and crud that the factory left in it? Are you using an in-line fuel filter?
Is the engine fully broken in?
Check for loose wiring, sour fuel, low float height setting, partially plugged jet or wrong jetting, plugged exhaust, bad spark plug or CDI.
Does it vibrate alot at high RPM's? Too much vibration will cause the fuel inside the carburetor to foam.
 
2door

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
63
Littleton, Colorado
And if you're using it, get rid of the kit spark plug boot. They are notorious for bad electrical connections especially under vibration. Replace the boot and use a plug with the top cap installed. The kit boot relys on making electrical contact via the threads at the top of the plug. That's just stupid.

Tom
 
S

scribling

New Member
Mar 31, 2014
36
0
0
Simi Valley
I removed the in-tank fuel filter. It's better, but not quite back to normal. I'm going to remove the inline filter and see if that helps.
I don't have excessive vibration at high rpm, actually the higher the rpm the less vibration I get. I'm interested to see if I can get it back to top speed how smooth it'll be.
I did replace the plug cap on reassembly, so I'll check that too. My wiring is a bit shoddy right now. I wanted to make sure it ran right before re-soldering.
Thanks for the help.
 
GearNut

GearNut

Active Member
Aug 19, 2009
5,104
11
38
San Diego, Kaliforgnia
To avoid future headache, definitely run an in line fuel filter, even if it is a better quality one than the one you suspect now.

Also, some of the petcocks can be drilled wrong and the resulting fuel ports (where they make the sharp 90* bend) inside them are restricted. I am not suggesting you go hog wild and drill out the petcock, just look closely at it's internal ports to see if things look right.

Shoddy wiring you say? That sounds an alarm to me.
 
Davezilla

Davezilla

New Member
Mar 15, 2014
2,705
10
0
San Antonio Texas
I agree... definitely run a decent fuel filter, just get a good inline one from an auto parts store, or even walmart as they sell the Fram ones which are better than the stock one.
Briggs makes a nice one that's really compact and see thru for about $3 in the lawnmower section at Home Depot, Advance Auto, pep boys, or maybe even walmart. I like that one because I got limited space and this filter is nice and compact.

If you got the speed carb with the built in petcock, that one can be problematic since it can shed off rubber and clog up the jets or fuel inlet at the needle and seat valve, the bad part is there's no way to put a filter in between that one and the carb since it's built in, but if you got that one, either just leave it on and only use the one at the tank, or replace it with a regular banjo fitting since you have a petcock at the tank there's really no need for the one on the carb.

I also agree with the shoddy wiring... if you got a poor contact it can lead to misfires and cutting out, fix that as soon as you can.

Here's a quick shot of my fuel filter, this is the Briggs inline filter that costs around $3 if I remember right...

 
Davezilla

Davezilla

New Member
Mar 15, 2014
2,705
10
0
San Antonio Texas
It's a stock intake pipe for a Yamaha PW80, I got it because that's what type of carb I'm running. My carb is a PW80 carb but I found out that it's the same carb as the CNS carb so it'll bolt right onto a CNS carb as well
 
S

scribling

New Member
Mar 31, 2014
36
0
0
Simi Valley
Getting closer ... GearNut was right with the vibration idea. I can't get the air out of the fuel line with this tank and the inline filter was vibrating like crazy - the fuel was basically froth. I tied it down with a rubber band and got about 80% of the power back.

Ok, ok, I'll solder the connections today ...
 
S

scribling

New Member
Mar 31, 2014
36
0
0
Simi Valley
Looks like it's all because of vibration. I think my motor inside, is really out of balance so I get a ton of vibration. I removed the rear rubber bushing I had between the mount and the frame, removed the inline fuel filter and power is restored! Also my intake gasket had disintegrated ...
 
Last edited:
2door

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
63
Littleton, Colorado
Do you have any rubber in the front mount? If so get rid it it too. The engine should be mounted solid to the bike frame. No cushioning or resilient material between the engine and frame. Rubber mounting doesn't eliminate vibration but just makes things worse.

I'd keep the in-line fuel filter. It is essential for keeping trash out of the float valve and main jet in the carburetor.
Buy good quality gasket material from an auto parts store and make your own intake manifold gasket. Don't waste your money or time ordering a replacement which will be the same quality that came with the engine. Same for the exhaust gasket. Make your own.

Tom
 
Last edited:
crassius

crassius

Well-Known Member
Sep 30, 2012
4,032
158
63
USA
Slight disagreement here, as I never run a bare mount. Had to fix too many frames that cut through after a few years of metal vibrating/rubbing there. For the noob, a piece of PVC pipe works great, for those with time & patience, a piece of hard leather works better (once it has seated and cured in which takes a week or so).
 
SuperDave

SuperDave

Member
Sep 24, 2011
179
0
16
Panama City Beach, Fl. USA
Hand me my banjo, willya? Time for another chorus of China Girl Blues!

Got a floppy fuel filter, there's foam in the mix
Its running too lean, now my bike can't do tricks
It won't pull a wheelie or white smoke the tire
The exhaust is so hot that my leg is on fire
 
2door

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
63
Littleton, Colorado
crassius said:
Slight disagreement here, as I never run a bare mount. Had to fix too many frames that cut through after a few years of metal vibrating/rubbing there. For the noob, a piece of PVC pipe works great, for those with time & patience, a piece of hard leather works better (once it has seated and cured in which takes a week or so).
Click to expand...
We really aren't in disagreement. PVC pipe isn't what I'd call a resilient material. There is little if any give to it. Same for leather unless it is very very thick leather in which case it will compress over a short time so that it too will have lost any degree of resiliency. Of course, as you said, "once it is seated" which will require close attention and tightening until that happens.

My issue is with soft/resilient rubber mounts that allow the engine to move and transfer the vibrations to the mount fasteners. The more the engine is allowed to move the more vibration will have an effect on things.

Tom
 
You must log in or register to reply here.
Top Bottom