Won't maintain power after warm up

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Dymaxion

New Member
Jan 1, 2010
66
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Michigan
I recently finished my first build. Due to weather and lack of time I really havent ridden it except a few short jaunts, but there seems to be a problem after it warms up. It's a Grubee GT-5 Skyhawk angle head.

It starts and seems to run fine (given the oil-rich break in mix and brand new engine). After a minute or so I can shut off the choke and it still runs great. After about another minute or two it starts to hesitate and not want to maintain power, then power quickly falls off. As speed drops you are forced to pull the clutch lever and feather the throttle to get it going again which it usually does, then try again. As soon as you start running up the RPMs again, it will happen again. The only way to get it back to normal performance is to shut it down and let it cool.

I am not a two stroke guy so I am not sure where to start looking. Based on searching this site my guess is maybe gas tank not venting which restricts gas availability, or possibly an air leak but I would think that would cuase problems right away, not only after the engine warms up. Not really sure what else to look for.

When I get home this afternoon I am going to try to figure it out. Any tips from the guru's here would be greatly appreciated!!

Thanks.
 
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M

marts1

New Member
Sep 18, 2009
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Oshawa Ont CA
Sounds much more like a fuel problem but even tho it runs fine cold it still could be an air leak due to expansion. Another carb would be very handy to see what happens.
 
2door

2door

Moderator
Staff member
Sep 15, 2008
16,302
175
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Littleton, Colorado
Heat could very well be allowing an air leak to increase in size causing a lean mix. Double check the intake manifold attachment to the cylinder and the carburetor where it slides onto the intake. These two areas are the most common places to find leaks. Did you use a good silicone sealer on both?
Head bolt torque is important also. 120 in/lbs, not more. Spark plug, 10 to 18 in/lbs.
Remove or loosen the fuel tank cap and see if this effects the stalling.
What fuel/oil mix are you using for break-in? The kit instructions typically call for 16:1 which most of us feel is a little oil rich, which incidently causes a leaner fuel/air mix due to the oil, which is not burned, throwing the actual mix off. I know that sounds confusing but its a fact that has been proven. Too much oil can actually cause a lean, or over temp condition. Many of us like to shoot for 24:1 for break-in. Get back to us after you've had a chance to explore your problem a little.
Tom
 
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Dymaxion

New Member
Jan 1, 2010
66
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0
Michigan
Thank you everyone for your ideas and suggestions. I have not had much time to tinker as life keeps getting in the way, but here is the latest update:

I replaced the orig plug with a ngk b5hs. Original plug was black and oily but that may be from the original start up trouble I had with a very oil-rich mix. I also drained all gas and mixed up a new batch at 25:1 Quaker state 2 stroke air cooled. I put black rtv on the intake tube and gasket and checked all bolts.

I tried it out and it seemed to run better. Still hesitates at near full throttle and seemed spuradic but definately better. I rode for maybe 10 minutes without it dying off like before.

Now my question is how rough do these thing run during break-in? It was having throuble pulling me up a modest hill - if I gave it more throttle it would bog down but it did make it. Is this normal?

Thanks all.
 
Sydneysider

Sydneysider

New Member
Mar 20, 2009
189
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Sydney
Hi Dymaxion, if it still hesitates check out the choke cover inside for play as sometimes the nut holding it to the carb most likely has loosened. Take off the air cleaner cover and filter and look inside to see if the choke is tight and clears the barrel completely when the choke is off. Tighten the choke nut securely if its loose and see how it goes.. Good luck :)
 
S

sellouma

New Member
Jan 8, 2010
39
0
0
New York
Dymaxion said:
I recently finished my first build. Due to weather and lack of time I really havent ridden it except a few short jaunts, but there seems to be a problem after it warms up. It's a Grubee GT-5 Skyhawk angle head.

It starts and seems to run fine (given the oil-rich break in mix and brand new engine). After a minute or so I can shut off the choke and it still runs great. After about another minute or two it starts to hesitate and not want to maintain power, then power quickly falls off. As speed drops you are forced to pull the clutch lever and feather the throttle to get it going again which it usually does, then try again. As soon as you start running up the RPMs again, it will happen again. The only way to get it back to normal performance is to shut it down and let it cool.

I am not a two stroke guy so I am not sure where to start looking. Based on searching this site my guess is maybe gas tank not venting which restricts gas availability, or possibly an air leak but I would think that would cuase problems right away, not only after the engine warms up. Not really sure what else to look for.

When I get home this afternoon I am going to try to figure it out. Any tips from the guru's here would be greatly appreciated!!

Thanks.
Click to expand...
Hi Dymaxion,
i have the same engine, i just started it this morning for the second time , drove it for 5 blocks, and it died on me. probably the spark plug is glogged with gas+oil mixture.(i put 7 ounces of oil for 1 gal of gas. 1 ounce less than the required for the 16:1 ratio).it was running fine the night before.(i rode it for 20 minutes).
it could be something else.any idea?
 
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Venice Motor Bikes

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
7,271
1,810
113
Los Angeles, CA.
16:1 is too much oil! That ratio was used in the old days when they mixed standard motor oil in the gas... Now we have 2-cycle oil, & a good mix using that for break in is 24:1... ;)
 
S

sellouma

New Member
Jan 8, 2010
39
0
0
New York
Venice Motor Bikes said:
16:1 is too much oil! That ratio was used in the old days when they mixed standard motor oil in the gas... Now we have 2-cycle oil, & a good mix using that for break in is 24:1... ;)
Click to expand...
fix it!!!
I took off the spark plug and cleaned it, shut off the gas, pedaled for 4 blocks, replaced the spark plug and the engine came alive right away!!!I was relieved, man dance1.
so i let it run for 20 minutes, no problems.
I noticed something else though; the back wheel wobbles little bit, and i think that might lead to problems later on, so i probably should fix that before riding my bike.
I can't wait to enjoy the .trk
 
H

hybrid541

New Member
Jan 25, 2010
3
0
0
US
I have a RAW engine on my bicycle which I built two months ago. I rode it on and off for about 1 tank of gas. I have the same problem. Although the engine starts no problem and will not die on me, it only gives full power for a few minutes then I can feel the power degraded to a point that I have to pedal to maintain the speed. Because the limited space I had to take the air filter cover out to fit the engine on the frame. I leave the foam filter on with steel wire to hold it in place. Does it matter? I still use 1:20 mix. will try 1:24 later. Any suggestion is greatly appreciated.
 
Brent436

Brent436

New Member
Feb 14, 2010
55
0
0
Massachusetts
This is similar to a problem i had getting my 48cc Grubbee skyhawk going. it would start up and then die after a few minutes. i closed the spark plug gap a bit and then it would stay running but it would be rough. whats the carb needle height at? i have mine at the 2nd notch from the top and thats what mostly everyone on the forum recommends. it was originally at the 3rd notch and it was losing power at 3/4 to full throttle. i moved it to the 2nd notch from the top and it ran like a top. hope this helps. good luck!
 
BarelyAWake

BarelyAWake

New Member
Jul 21, 2009
7,194
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Maine


and when assembling, be sure to align the idle set screw's nubbin with the lil ramp on the slide;




Another tip - should it run well cold & lose power, you should check the head and base gaskets for seepage/leaks... it may not be the problem, but it's not unknown.
 
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Brent436

Brent436

New Member
Feb 14, 2010
55
0
0
Massachusetts
If you look at the pics BarelyAwake posted the little brass needle is what your trying to get to. the black c-clip is how you adjust it. be careful taking it on and off the pin though because they have a habit of flying off in random directions. i get a pair of needlenose plyers and grab the c-clip pull it off and push it onto the desired notch. its worked well for me as i havnt had the clip fly away from me using the plyers. just a suggestion. hope it helps.
 
GearNut

GearNut

Active Member
Aug 19, 2009
5,104
11
38
San Diego, Kaliforgnia
Also, the C shaped washer goes on top of the needle/ clip assembly. This allows the spring to press down on top of the C shaped washer, which in turn presses down on the clip and needle and prevents it from slipping up into the slide and randomly buggering up the fuel mixture.
 
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