is your bike losing power at top speed?

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Goat Herder

Goat Herder

Gutter Rider
Apr 28, 2008
6,237
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N.M.
In some but has been very rare cases a fuel hose that is rattling hard enough may foam up causing a short but temporary fuel flow issue. I have only seen or heard about maybe two or three folks here have this prob. Just saying its something to consider. Just use a well placed zip tie.
 
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bitsnpieces

New Member
Dec 9, 2010
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Thanks Goat Herder, but was thinking more about the difference in flow between the stock valve, to a 'proper' valve. If there's a big difference in flow, maybe I may need a change.

But from that post you quoted on 2 Door, I'm guessing the basic flow is 'apparently' enough to run the engine, and if that is the case, I think it may very well be the vibrations on the bike - which means at the end of the day, change the valve for more fuel flow.

My guess is, we may not be able to feel the vibrations, but maybe even the smallest of constant vibrations eventually shake up the fuel, which as you say, temporary loss of fuel being supplied to the engine. Unless you slow down a bit to ease the vibrations, the fuel just keeps rumbling up.

Kind of like a bowl of sand or grain, and you constantly hit a side, the contents move towards that side, or away, forgot which it goes - Whatever the smallest of vibrations is doing, it's moving the fuel up.
Having a bigger flow may add weight to keep the fuel pushed down and running, but then by this theory, if you had really bad vibrations, even a larger flow wouldn't solve the issue since there would be enough shaking to shake up the fuel, upwards.

So I'm guessing two things come from this, eliminate as much vibrations as you possibly can, and use a valve with better flow.

That's just my 2 cents thrown in. :p
 
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Black_Moons

New Member
Oct 25, 2010
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I had a long clear fuel line on my bike, half full of gas, and with enough vibration it actualy would 'foam' up and eventualy reach the top of the fuel line! Kinda amazing really, climbing 3" up just from vibration.
 
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Don P

Member
Sep 1, 2010
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indiana
Hi all i have had the same problem, 66cc 44 tooth gear hits 25mph about 6000rpm. shuts down then picks up after 3 4 sec. i changed tank had a (new spair) filter ,hose, carb is clean still dose it.

i even drilled out the pit cock help some has good flow. but after reading the post here i think it is a fuel foaming. it is doing it when the engine starts to vibrate. just put two and two together.

so i need to get some hose and mount my carb to stop vibration. we shale see. \
i will report back. soon Don P
 
corgi1

corgi1

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Aug 13, 2009
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I rescued a riding mower long ago on trash day and the motor mount bolts were loose and it shook so bab it would not run,,,tightened the bolts and it ran great,so my conclusion was that the fuel was not able to handle the vibs
 
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Don P

Member
Sep 1, 2010
234
0
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indiana
Hi all

i fixed it. i had my engine mounted in rubber for vibration and it let my engine move.
after removing the rubber and mounting to frame (no rubber) works fine 30+ mph where 24mph was tops before.

i was foaming my fuel.
 
Dogtown Burner

Dogtown Burner

New Member
Sep 15, 2011
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Los Angeles
Don P said:
Hi all

i fixed it. i had my engine mounted in rubber for vibration and it let my engine move.
after removing the rubber and mounting to frame (no rubber) works fine 30+ mph where 24mph was tops before.

i was foaming my fuel.
Click to expand...
I have soild mounts, and after adding the manic mechanic head, at a certain rpm ( guessing 6k and 25-7 mph) it begins vibration bad, and the power flattens out. the fuel in the line foams up like a cappuccino.and it loses power.. it also changes pitch prior to this. it goes from running super sweet-spot 2stroke blissful clean power to a more wacky powerless loudly vibrating power loss situation.

anyone share this problem ever? any solutions?:-||
 
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nightcruiser

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Mar 25, 2011
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I have always thought the fuel flow from the tank might be a bit restricted at WOT for whatever reason. After reading this thread I pulled apart a stock petcock I have here, what a POS! See the attached picture.
There are two small holes in the base, and a horseshoe shaped channel in the lever side of the valve that will allow fuel to flow from one hole to the other. There is a cheap little compression ring that puts pressure on the unit to make it seal (hopefully). I am definitely going to look for a better valve to use before spring rolls around...
If anyone finds a valve that threads directly into the tank please post the info here...
 

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wayne z

wayne z

Active Member
Dec 5, 2010
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louisiana
nightcruiser said:
I have always thought the fuel flow from the tank might be a bit restricted at WOT for whatever reason. After reading this thread I pulled apart a stock petcock I have here, what a POS! See the attached picture.
There are two small holes in the base, and a horseshoe shaped channel in the lever side of the valve that will allow fuel to flow from one hole to the other. There is a cheap little compression ring that puts pressure on the unit to make it seal (hopefully). I am definitely going to look for a better valve to use before spring rolls around...
If anyone finds a valve that threads directly into the tank please post the info here...
Click to expand...

If yer talkin about the peanut tanks, most are standard 1/8 pippe thread.
I have replaced several petcocks on them with one that NAPA sells for mowers. All plated steel, knurled knob, screen strainer, and 1/4/' hose barb, for about 10 bux.
I did find one tank that had metric threads. I just chased the threads with a 1/8" pipe tap.
 
Dogtown Burner

Dogtown Burner

New Member
Sep 15, 2011
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Los Angeles
nightcruiser said:
I have always thought the fuel flow from the tank might be a bit restricted at WOT for whatever reason. After reading this thread I pulled apart a stock petcock I have here, what a POS! See the attached picture.
There are two small holes in the base, and a horseshoe shaped channel in the lever side of the valve that will allow fuel to flow from one hole to the other. There is a cheap little compression ring that puts pressure on the unit to make it seal (hopefully). I am definitely going to look for a better valve to use before spring rolls around...
If anyone finds a valve that threads directly into the tank please post the info here...
Click to expand...
maybe this?

HARLEY PETCOCKS,FUEL VALVE,REPLACES HD # 62125-55B,FITS 66/74,"STRAIGHT" | eBay
 
GearNut

GearNut

Active Member
Aug 19, 2009
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San Diego, Kaliforgnia
nightcruiser, that cheap kit petcock is internally designed to operate the same way that motorcycle petcocks have been for years.
Although motorcycle petcocks have changed internally in the last few years and that old design in now woefully outdated.

As for the aftermarket Harley petcock, it is threaded 3/8 NPT.. The petcock bung on the kit tank is waaay too small. :( If it were to fit it would work very well..... if it actually does not leak like most of the cheap aftermarked Harley petcocks do these days. They all need the internal face of the lever milled flat, ironically exactly like the cylinder heads on these Chinese MB engines need to as well.
 
Dogtown Burner

Dogtown Burner

New Member
Sep 15, 2011
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Los Angeles
well,
I was able to fix the problem i was having. good old detonation. too hot in the head. I had put that manic high compression head on it, but kept the bh6s plug. too hot. I put the stock plug back in and its screamin again! fuel is still foamy in the line, but i guess its irrelevant to performance. it cuts out over large bumps momentarily, and the carb eventually works its self off the intake, (which is a fun little noise,) but its working great now!
 
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gera229

New Member
Sep 4, 2011
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USA
Foaming fuel is the result of the gas tank not "breathing."
But not necessarily.

If the gas tank was not breathing, then the fuel would not even get to the engine very good.
I mean I opened the gas cap on my tank and it still foamed.

I do think that if the petcock was at the end of the fuel line rather than at the beginning then there would not be any foaming since the fuel would go and fill up the fuel line.
 
happycheapskate

happycheapskate

New Member
Nov 26, 2009
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Rockwall TX
Yes, that one works great!
http://www.sickbikeparts.com/catalog/images/Valve.jpg $10

Mine was hard to thread into the china tank, though. I wonder if it was actually a different thread. Oh well, I wrenched it on, and it hasn't leaked in a year.

Re: petcock at end of the line. Some of the NT carbs have a little on on the carb. It sucks, so I just leave it open. If you have it on the tank, you can shut off the gas if the line ever gets yanked off, and it's in an easy to remember place if you start riding and realize you left it shut off.

timboellner said:
End the controversy and just order this one from Sick Bike Parts.

Sick Bike Parts

TiM
Click to expand...
 
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2door

2door

Moderator
Staff member
Sep 15, 2008
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Littleton, Colorado
The kit tank is a machine thread, 10mm and most aftermarket petcocks are 1/8" NPT, National Pipe Thread. The thread pitch is close but the machine thread is not tapered like a pipe thread. That's why they are hard to thread in. If you want you can run a 1/8" pipe tap into the tank fitting but you must be careful and not thread the tap in too deep or it will leak.
The kit tanks fitting requires a gasket to seal. Pipe threads seal because they are tapered and seal tighter as they are tightened into the fitting.
Tom
 
happycheapskate

happycheapskate

New Member
Nov 26, 2009
1,989
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Rockwall TX
Either the SBP petcock or the stock one came with a red fiber washer. It worked.
I threaded it half way, then backed it out, then put thread seal tape (teflon tape) on it and finished the job. It's been very permanent!
 
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Edd

New Member
Jan 25, 2012
19
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0
Cypress, TX
All this info is really helpful. I am new to MB and have owned engine for less than a month (and signed up literally minutes before posting this!). I have very bad foaming problems in the fuel line. I shortened the fuel line so that it is almost stretched and my RPMs increased significantly. Have not tried riding w/o the fuel cap to check for venting problems. Still don't feel like the engine's turning full RPMs at WOT. Anything else I can try (besides replacing petcock, I'll do that as a last resort, short on funds right now)?
 
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