is your bike losing power at top speed?

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bairdco

bairdco

a guy who makes cool bikes
Aug 18, 2009
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i've seen lots of threads about people's bike's bogging at WOT (Wide Open Throttle) and i'm pretty sure i figured it out.

what i'm talking about is when you're flat out, then it feels like your bike's stalling out or sputtering, then if you let off the throttle for a few seconds it picks back up.

most people suggest changing the needle clip setting, re-jetting, float setting, fuel mixture, or whatever, but i think the real problem is fuel delivery.

basically, the engine's using more fuel than it can get, so it's sucking the carb dry.

the culprit is the stock petcock.

in my kits, the petcock is either the cheap little brass one, or the even cheaper silver one. the fuel outlet on them both is like 1/16", while the carb inlet is 1/8" on some, and 1/4" on others. (i'm talking "Inner Dimension" or "ID.")

if you take the fuel line off the carb and watch the flow of gas that comes out with the petcock on, (and obviously into an approved container) you'll see it dribble slowly out. that's fine to keep the bike running, but not enough at high speed, when it needs more.

after i rebuilt the engine on my fastest bike, it started bogging at top speed. it was still going fast, but i didn't like being in traffic with the engine cutting in and out, so i went through the whole fuel system, checked the tank, the filter, changed the fuel line, and it still did it.

but when i went over everything, the stock petcock started leaking, so i switched it out with a Tecumseh, 1/8" NPT petcock that i use on my custom tanks.

the problem went away completely, and it's been about 2 months.

you will need to buy a 1/8"NPT (National Pipe Thread) tap for your tank, though, because the stock petcock's have straight cut threads, but you don't have to re-drill the tank or anything. the hole is the right size for the tap.

then just install a good quality petcock (lawnmower shops have good ones for cheap, and so does ebay) and you're set.

while you're at it, get rid of that cheap plastic tube and get some real fuel line...

 
Goat Herder

Goat Herder

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I had the same problem. At the time in a pinch I used one of these while it worked for me just fine. It is not nearly as neat as your solution. I think you make a fair point about the flow.

http://www.capitolsupply.com/ImageS...&g=0x944e8a19b9cb499141fe6c1d08913d4bcf7275df

I had to find a brass barb to go to my tank. Then I coupled it all together with hose and clamps. I shoved my fuel line over the threads on the valve and used squeeze clamps. Not the worm gear clamps it looked nicer. The rubber seals in the valve held up perfectly.

I think another potential prob is a rattling fuel line [fuel might foam up in the line], but I never have had that issue.
 
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WildAlaskan

WildAlaskan

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bairdco i have been thinking the same thing for a while now and i recently took off the petcock completly and went to lowes put three different copper fittings toghether to create a straight flow from tank i still have the same problem but i get plenty of flow now my thinking is the inline fuel filture is jamming it up so im gonna pull a dumb a$$
manuver and run with no inline filture or petcock i just hope i dont have to take my carb apart on the road to clean it out
 
corgi1

corgi1

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Foaming fuel ?,is that why old hondas have a rubber like conector from the carbs to the motor??
 
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FreeWheeler

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Isnt it possible that sometimes it is float level, jets, c-clip? Doesnt the filter act kinda like a buffer if it doesnt empty? I've experienced the "vapor lock" and even though most say dont pay attention to level in the filter, it doesnt cut out when it is full.

????
 
corgi1

corgi1

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if you are refering to the inline filter ,I would think the tank valve was not letting fuel in fast enough to the filter if the filter empties(or out of gas)..............I guess it could vapor lock if the fuel lne was right next to the head,for that I would re-rout it back toward the seat post and back oto the carb,if its right at tank area a little heat sheild would do
 
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FreeWheeler

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If the petcock cant keep up,Yes, then bairdco's suggestion is a perfect solution.

I put vapor lock in quotations to suggest that the condition I experienced isnt exactly a vapor lock, but air in the line which gravity or carb suction isnt enough to overcome. Once the filter fills up and air is out of the line, it keeps up with demand. Easy way to clear it is to pressurize the tank. This is a temporary inability to keep up with fuel demand.
 
corgi1

corgi1

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or de-pressurize for a moment,pulls the air from the tube ,the float valae stops the gas going up the tube
 
bairdco

bairdco

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it could be anyone of those things. my reasons for posting this, is there's a lot of threads where all those things are suggested, but none of them seemed to work.

obviously, mine isn't the one solution that'll solve everything, but it is one that's almost never mentioned, and it's the one that worked for me.

it also seemed obvious to me. if you let the fuel run with the line disconnected from the carb, it dribbles out. with a new petcock, if flows like a faucet.

i also think it's one of those problems where the simple and obvious solution gets overlooked. it was clear to me that my bike was running out of gas at WOT. after ruling out blockages in the system, the only thing left was restrictive fuel flow. and if it's not dirt restricting it, it's the system itself.

as an added bonus, it doesn't leak. every stock petcock i've used either doesn't seal right at the tank and always has an oily/gas residue that ends up on my jeans, or it fails altogether and pours out allover the bike.

every one of the replies above are things that people have tried without the problem going away. if anyone tries my fix and it doesn't work, let me know...

(oh, and i only run the filter that's on the petcock. i've never had a problem with dirt in my tank.)
 
WildAlaskan

WildAlaskan

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ok so i was going on a lengthy journey and didnt want to risk taking out the inline fuel filture i drilled the holes in the petcock larger so the gas flows nicely out i no longer have the metal filtur sticking in the tank cause i felt it restricted flow

i beleive the problem is the krappy inline filtur because about a moth ago i started out fron home just fine then after a sort while started to bog down after taking apart my carb and checking things out i found that everything was clear but a small little cloth cought my eye it was right in the fuel line after inline filter it could only be a peice of cloth from the filter after taking it out all was well and i was to lazy to find a new filter
 
2door

2door

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Sep 15, 2008
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I have mixed feeling about this issue but I do have a warning that my friend Bairdco neglected to mention. If you take his advice and change the kit supplied petcock with a 1/8th pipe thread valve be very careful during the retapping of the threads. Do not run the tap in as far as it will go. Keep your thread depth shallow in the tank. This is because pipe threads seal by use of the taper cut into them. If you thread too deep the new valve will not seal well and you'll end up with a leak.

Now, for fuel delivery. I think of it this way. Advertised milage for the Chinese 2 stroke engines is something like 80 to 100 miles per gallon. We all know this is on the high side but let' say just for sake of argument that you get 75 mpg. How long will it take you to ride 75 miles at WOT? Now with that estimated time frame in mind compare it with how long it would take for your tank to drain if you pulled the fuel line off and drained it into an approved container? I'll guarantee you that the tank will drain in far less time than it will take you to ride 75 miles, or 80 miles or best case, 100 miles. My point being that if the tank will drain naturally in less time that it take to ride the assumed mpg of your engine then it is delivering sufficient fuel. The running engine does not 'suck' the fuel in. It is a gravity flow fuel delivery system and the fuel will only flow as fast as the engine can consume it. It makes no difference if you ride at wide open throttle, or just above idle. The engine can only use so much fuel. If it uses more than the system can deliver because of a restriction then Bairdco's theory is correct. If not, then you'd better start looking elsewhere for your bogging problems. Specifically I'd take a look at your tank venting. As I said above this is a gravity flow circuit and as such the tank MUST be at atmospheric pressure to allow fuel to flow correctly. If the tank can not vent (allow air in) at the same rate as fuel is needed to flow out then there will be a reduction in flow. The kit supplied fuel tank caps are notorious for improper venting. If you suspect that venting is the problem the test is simple. Remove or loosen the cap and give your bike a good WOT run. If the bogging goes away, you've found the problem. If not, then keep experimenting. Maybe a larger jet is in your future. Spark plug color will tell you a lot about where to go from there.
Tom
 
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F

FreeWheeler

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Trimming the tank gasket worked for me. I trimmed 2 sections 180 degrees apart about the same size as the notches in the tank. The idea was to uncover the tank notch and allow a path out. When the tank is locked in place, those notches are usually covered by the gasket and the notch in the gasket uncovers it.

YMMV
 
Venice Motor Bikes

Venice Motor Bikes

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Mar 20, 2008
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2door said:
Now, for fuel delivery. I think of it this way. Advertised milage for the Chinese 2 stroke engines is something like 80 to 100 miles per gallon. We all know this is on the high side but let' say just for sake of argument that you get 75 mpg. How long will it take you to ride 75 miles at WOT? Now with that estimated time frame in mind compare it with how long it would take for your tank to drain if you pulled the fuel line off and drained it into an approved container? I'll guarantee you that the tank will drain in far less time than it will take you to ride 75 miles, or 80 miles or best case, 100 miles. My point being that if the tank will drain naturally in less time that it take to ride the assumed mpg of your engine then it is delivering sufficient fuel.
Click to expand...
I concur... Even the tiniest dribble is more than enough fuel to feed the need of these engines.
Just as 2door said, (Using his math) that tiny dribble from the tank is far more fuel than these engines can possibly draw in @ WOT.

Baird is smart & I'm sure his bike ran better after he did all that work to it, but I've always been able to solve this problem with simple C clip adjustments & (if need be) changing the jet.
 
bairdco

bairdco

a guy who makes cool bikes
Aug 18, 2009
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living the dream in southern california
i don't disagree with 2door, but i know that was what my bike needed. fixed that, and it works awesome.

one major point i forgot to mention though, is my motor is nowhere near stock. it's the reversed barrel with enlarged transfer ports, ported jug, decked head, indexed and filed plug, gutted match-ported exhaust with supertrapp tuned muffler, re-worked carb, match-ported shorty intake, 28t sprocket...uh, i think that's about it...

like any hotrodded vehicle, mine might just need more fuel to reach it's optimum potential.
 
Goat Herder

Goat Herder

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I don't disagree with 2 ither. How ever on my morini it would die sometimes at wide open throttle. Not enough flow was there.


On my china the cheezy filter fell apart on the valve in the tank. Which subsequently a tiny fleck of whatever got into the valve it did not flow well. Cleaned it out on four occasions. Threw it away after that no more drama.

I never did like there valves on the china. I had some that leaked. I had the filters fall apart on them just for staring at them never even had to touch them. I had them show up in the kit all ready broken.

The valve I switched too will not clog and I never put a filter on it just a in line filter later down the line.

When you have a motor that runs perfect all the way to full blown RPM.s and then cut out well that is something to consider.
 
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Venice Motor Bikes

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
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I now concur with both of you... Super highly modified hot rod HT's & Morini's will probably benefit from more fuel flow... but 99% of us here don't have those.
 
epicboy

epicboy

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makes alot of sense. I am going to check mine out as well. What really helped my motor run better and still getting better is that i have made sure my spark plug gap was right (mines at .030). I noticed that if the gap is not right, the engine doesn't perform. I also, made sure my tank and carb are sealed (no air leaks) and make sure your coil to cd1 wires are not diffusing on your steel frame. You will lose spark in you have weak insulation or worse, a exposed wire plug. Anyway, these things helped out alot. I was having a ****ty time for awhile making it work right. cheers...J
 
F

FreeWheeler

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epicboy said:
makes alot of sense. I am going to check mine out as well. What really helped my motor run better and still getting better is that i have made sure my spark plug gap was right (mines at .030). I noticed that if the gap is not right, the engine doesn't perform. I also, made sure my tank and carb are sealed (no air leaks) and make sure your coil to cd1 wires are not diffusing on your steel frame. You will lose spark in you have weak insulation or worse, a exposed wire plug. Anyway, these things helped out alot. I was having a ****ty time for awhile making it work right. cheers...J
Click to expand...
Air leaks/vacuum leak isnt a problem with the tank, quite the opposite. You want the tank to breathe, hence my suggestion of screwing with the gasket.

As for gravity feed only fuel system, you can hear at WFO the system sucking fuel. Just undo the cap at WFO and you will hear it sucking fuel. I dont know what else to call that sound and if bairdco is sucking more fuel than normal, he should really be able to hear it.
 
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