Strange Problem - Engine slows down after mile

[Toadmund]

Does it happen all the time, or some of the time?
Once in awhile for me.

Gas cap?

Seems that like you I can go fast, then it bogs and if I don't go to idle or low throttle it will stall, if I disengage the clutch I can rev it no problem.
Then the problem disappears as mysteriously as it appeared.

Anyway, going to change my stock oil filter and fuel valve soon and perhaps I will put a piece of wire between the gas cap and cap hole to see if it's a tank venting problem.

It is definately a fuel delivery related problem. At least it runs!

 

[crassius]

there's always the obvious - are you using enough oil?

had two bikes this last couple weeks that were slowly seizing from lack of oil in the mix - both were ok after lubing them well inside, then running a quart of 8:1 thru

probably shortened the engine life tho

 

[Toadmund]

In my case it is certainly a fuel thing, my motor runs as smooth as smooth peanut butter.

MMMMmm, PB&T, the perfect emergency meal!
I suppose I shouldn't use peanut oil as mix.

No, just being weird, I use Opti2.

Just wondering though, would a larger ID fuel hose cause more gas bubbles when at WOT causing insufficient fuel?
I just put a bigger hose on just recently, but I cannot remember if the problem was apparent before and after or just after the hose swap.

 

[maniac57]

Sounds like a fuel starvation problem to me now...possibly a clogged fuel filter or non-venting cap. Could also be a clogged strainer in the cutoff valve inside the tank.
My first thought was vapor-lock, but it would quit completely if that were the case, and I can't imagine a ht carb getting hot enough to cause this anyway....
Check the air filter as well. Couldn't hurt. *shrugs*

 

[vinnie]

Toad & Maniac,

My problems seem to happen after riding the bike for a long period of time. By long period, I mean 20-30 minutes. I use the bike for quick trips that usually take no more than 5 minutes. After the bike engine gets really warm, it will start to bog down and once I slow down to 10-15 mph- I can keep going by pulling very lightly on the throttle to maintain that speed until the engine cools off and then I can throttle up again. I also took the gas cap off to make sure that there wasn't any vapor lock while riding. Would that be a good way to check for vapor lock?

Also, the fuel filter that I have is the clear one that came with the bike kit. It doesn't look clogged and it is full of fuel, which leads me to believe that there isn't a lack of fuel issue. Let me know your thoughts...I appreciate the help!

 

[maniac57]

Might be a marginal magneto coil. Do you know another rider who will let you swap coils and try it?
Then again, a new coil is cheap enough to try anyway.
If the new coil does not fix it, you have a spare of the most common failure point of the ignition.

 

[crassius]

most of these engines take at least a mile to warm up enough to run reasonably and have a fairly steady idle - they will also continue to warm up for the next 6 or 7 miles to reach a stable temp (big, cold, steel crank cheeks in the intake path are slow to warm) - the engine will run differently at this temp, so you may want to tune for the riding you usually do, but split the diff so it will be acceptable thru the whole range

 

[vinnie]

Maniac, I do not have another magneto coil to try using. I may order one to try and have as a backup. Crassius, I see what you mean, but after today I really think it might be an air leak.

I rode the bike at a constant 25 mph for about a mile and it started to bog down. Typically I leave the clutch engaged and just stop pulling the throttle. Today, I disengaged the clutch and it start to rev high and low back and forth, which I believe indicates a vacuum air leak. I have carb spray that I can look for it with, but I can't imagine where else it could be coming from. As you know from previous posts on this thread, I have an o-ring in-between the carb and the intake pipe. I also have a fresh new gasket between the engine and intake pipe. I also checked the head gasket for black burn marks, but it doesn't have any. Where else should I be checking? Thanks for you help, everyone!

 

[Toadmund]

Toad & Maniac,

My problems seem to happen after riding the bike for a long period of time. By long period, I mean 20-30 minutes. I use the bike for quick trips that usually take no more than 5 minutes. After the bike engine gets really warm, it will start to bog down and once I slow down to 10-15 mph-

Now, just wondering here, my bike is a small mountain bike and the engine is a tight fit, therefore my fuel line runs about 1cm away from the head as it comes off the fuel valve attached to the tank.
Could my fuel be heating up causing it to vapor lock?

Perhaps I should insulate or shroud my fuel line?

Is your situation similar to mine?

Does anyone know if this could be my issue?

 

[vinnie]

Toad,

I am not an expert in the matter, so I can't give you an answer about the engine heat causing vapor lock. I cannot dismiss the possibility yet, but I want to run my bike today or tomorrow and recreate the bogging down problem and spray the gaskets with carb cleaner. If it revs higher, it will indicate that I have an air leak there. If I can't find it, then I will have to look into vapor lock issues further. I'll keep you posted

 

[vinnie]

Sorry for the delay everyone, but I spent my day diagnosing this problem. I definitely have an air leak with the exhaust gasket. When it started revving by itself while idling, I sprayed the exhaust gasket area with carb spray and a cloud of some mist came out of the exhaust pipe end. I did it several times and it was consistent. I decided that my gasket (original from my kit) must need replacing. I tried several materials that were lying around.

First: cardboard box - Fail. The air leaking was far worse.
Second: aluminum foil - Fail. It was better than the cardboard, though.
Third: original gasket, which was undamaged - Fail. Just wanted to compare with the first two tries.
Fourth: Thick white paper that I used to make the gasket for my intake pipe with. I figured it wasn't nearly thick enough, so I made 6 or 7 cutouts of the gasket and put them together. I would say it was the best performance all day, but still leaks.

Tom, I read on another thread that you recommend Seal All. Would you recommend it for this particular case and how would you suggest to apply it?

Thanks so much everyone!

 

[Toadmund]

Well me, having the same problems and realizing the properties of metal under heat expansion, assumes that my head bolts are de-torqued (have loosened)
The head gasket seal is good and I still have power under load, then as the motor heats up, expansion happens and combustion gases escape out from the gasket.
This would cause bogging under load (with warmed up engine) because of lack of compression, but would allow the motor to rev OK under no load.

I re-torqued my head bolts back up to 10 foot pounds last night, they were at about 2-4 f/lbs previously.

Going to test this out later, I'll be back.

 

[crassius]

for an exhaust leak, the first thing to do is make the end of the muffler REALLY straight & flat - that flange is often welded crookedly or bent

 

[vinnie]

Well I spent a lot of my day trying to fix the air leaking. Let me first say that it was not this bad until I started messing around with the exhaust gasket the first time.

I went to home depot and got Seal All. I applied it to the exhaust gasket and put it back together. Tried riding it...it seemed better, but still revs out of control at running temp. I then applied the Seal All to the carb where it connects to the intake pipe. I tried to make a video to show you all, but I ran out of gas while making the video.

When it is revving out of control at running temp, here is what happens:

Spray the carb connection with the intake with carb spray: Engine revs go down
Spray the exhaust gasket with carb spray: mist came out of exhaust - now it doesn't seem to be doing that.
Spray intake manifold to cylinder: nothing changes

I don't understand how all of a sudden my carb is leaking air when I know that the exhaust gasket was originally leaking. It seems like the carb leak has gotten much worse since I started addressing the exhaust gasket leak.

 

[vinnie]

I also filed the exhaust pipe contact surface level

 

[Toadmund]

Yep, that was my problem. (head gasket leakage when warmed up to operating temperature)
I was noticing a slight power loss, And I also noticed an unburnt gas smell for some time now.

 

[vinnie]

Would your engine sound a lot louder when it started bogging down as you increased the throttle?

 

[Toadmund]

No.
My gasket is still good.

 

[Herdo]

vinnie, did you end up solving your problem? I don't even have a motorbike yet, just curious.

 

[vinnie]

Just got the new exhaust gasket that is aluminum. It has rained since thursday morning, here in DC, so I haven't been able to try out the bike. It's also out of gas. Hopefully today or tomorrow I can fill up since I really want to see if the new gasket did the trick. I'll keep you posted.

Some upgrades since my last post:
Aluminum exhaust gasket
SBP fuel valve
Seal All for the carb connection with the intake pipe