Hawk 250 fuel boiling/vapor locking.

[Megadan]

Took my Hawk out for the longest ride yet, a good solid hour or so of non stop running around randomly. Thought I had the idle set good, and then suddenly my idle started dipping at lights to 1k rpm or less. Blip the throttle and it would respond immediately. Ride it for a bit, come to a stop, idle back to normal...then it dips again. If I sat at idle without feathering the throttle long enough it would stall. But would start right back up.

I experimented with the idle speed, but I would either get 1krpm or less or 2k and wavering. Completely different to how it would run after warming up for 10 minutes or so of riding, which would always be nice and steady.

Well, after parking the bike I decided to inspect a couple of things. and see if there was any obvious causes. Nothing jumped out at me with exception to when I looked at my fuel line and filter. I installed a clear fuel line and a small fuel filter from the parts store, and this made me glad I did. The fuel line was mostly "air" with only a smidgen of fuel to be found.

I thought that was strange, so I opened the petcock with the bike sitting on a jack stand on the right side, and fuel flooded in and filled the bowl, which then started to bubble bubble boil and trouble through the line and fill it all back up again with gas vapor.

Does anybody else have this type of problem? The proximity of the carb bowl to the head AND the exhaust with the insanely hot cat in it is kind of...bad. I would rather not waste my exhaust wrap on the stock exhaust, so I am thinking about using some of that gold heat reflective foil I bought and sticking it to the bowl to see if it helps. It will look a little goofy with a gold fuel bowl, but I have always been a form over function kind of guy. The bowl is hot enough that you can't really touch it after shutting off the bike, so it will definitely help.

[JerryHawk250]

Mine did that a few times until I removed the cat. The cat gets super hot. I though I caught my pants leg on fire one time it got so hot. Once I removed it the exhaust runs so much cooler.

[2LZ]

First I've heard of this here, I think. That would cause a wicked lean condition under the right conditions.
I wonder if this had anything to do with Entropy's buddy who cooked his bike blazing down the freeway wide open?

[Weldangrind]

X2 on 2LZ' comment and question.
Lifanda never had a cat, so I never faced that issue.

[Megadan]

A part of me is considering just doing all of my mods do the bike and just doing a finished dyno run... I guess I could waste a few feet of header wrap and simply wrap the cat and exhaust near the carb though.

[2LZ]

Originally Posted by Megadan

A part of me is considering just doing all of my mods do the bike and just doing a finished dyno run... I guess I could waste a few feet of header wrap and simply wrap the cat and exhaust near the carb though.

Ah man! If you have access to a dyno, ya gotta get us a before and after! ;-p

[JeremyC]

Gut it! That'll eliminate the need for header wrap.

[Ariel Red Hunter]

Originally Posted by Megadan

Took my Hawk out for the longest ride yet, a good solid hour or so of non stop running around randomly. Thought I had the idle set good, and then suddenly my idle started dipping at lights to 1k rpm or less. Blip the throttle and it would respond immediately. Ride it for a bit, come to a stop, idle back to normal...then it dips again. If I sat at idle without feathering the throttle long enough it would stall. But would start right back up.

I experimented with the idle speed, but I would either get 1krpm or less or 2k and wavering. Completely different to how it would run after warming up for 10 minutes or so of riding, which would always be nice and steady.

Well, after parking the bike I decided to inspect a couple of things. and see if there was any obvious causes. Nothing jumped out at me with exception to when I looked at my fuel line and filter. I installed a clear fuel line and a small fuel filter from the parts store, and this made me glad I did. The fuel line was mostly "air" with only a smidgen of fuel to be found.

I thought that was strange, so I opened the petcock with the bike sitting on a jack stand on the right side, and fuel flooded in and filled the bowl, which then started to bubble bubble boil and trouble through the line and fill it all back up again with gas vapor.

Does anybody else have this type of problem? The proximity of the carb bowl to the head AND the exhaust with the insanely hot cat in it is kind of...bad. I would rather not waste my exhaust wrap on the stock exhaust, so I am thinking about using some of that gold heat reflective foil I bought and sticking it to the bowl to see if it helps. It will look a little goofy with a gold fuel bowl, but I have always been a form over function kind of guy. The bowl is hot enough that you can't really touch it after shutting off the bike, so it will definitely help.

I would figure out a way to mount the gold foil close to, but not touching the carb. The idea is to have an air layer between the carb and the foil. Maybe a piece of wire coat hanger made into a rectangle for the foil at one end, and a curlique on the other end to make it possible to bolt to something. Perhaps a carb mounting nut...ARH

[pete]

Originally Posted by JeremyC

Gut it! That'll eliminate the need for header wrap.

then turn the cat chamber into a power bomb.....
to reduce the retuning pressure wave so the next
exhaust charge dosn't have to fight so hard to
get into the header pipe...



...

[Ariel Red Hunter]

Originally Posted by pete

then turn the cat chamber into a power bomb.....
to reduce the retuning pressure wave so the next
exhaust charge dosn't have to fight so hard to
get into the header pipe...



...

I think he wants to do a dyno run on it dead stock, and then with the various tuning mods..ARH

[Sir kayakalot]

Do the dyno run and then just gut the cat. To me it's a no-brainer

[enforcer89]

Originally Posted by Megadan

Took my Hawk out for the longest ride yet, a good solid hour or so of non stop running around randomly. Thought I had the idle set good, and then suddenly my idle started dipping at lights to 1k rpm or less. Blip the throttle and it would respond immediately. Ride it for a bit, come to a stop, idle back to normal...then it dips again. If I sat at idle without feathering the throttle long enough it would stall. But would start right back up.

I experimented with the idle speed, but I would either get 1krpm or less or 2k and wavering. Completely different to how it would run after warming up for 10 minutes or so of riding, which would always be nice and steady.

Well, after parking the bike I decided to inspect a couple of things. and see if there was any obvious causes. Nothing jumped out at me with exception to when I looked at my fuel line and filter. I installed a clear fuel line and a small fuel filter from the parts store, and this made me glad I did. The fuel line was mostly "air" with only a smidgen of fuel to be found.

I thought that was strange, so I opened the petcock with the bike sitting on a jack stand on the right side, and fuel flooded in and filled the bowl, which then started to bubble bubble boil and trouble through the line and fill it all back up again with gas vapor.

Does anybody else have this type of problem? The proximity of the carb bowl to the head AND the exhaust with the insanely hot cat in it is kind of...bad. I would rather not waste my exhaust wrap on the stock exhaust, so I am thinking about using some of that gold heat reflective foil I bought and sticking it to the bowl to see if it helps. It will look a little goofy with a gold fuel bowl, but I have always been a form over function kind of guy. The bowl is hot enough that you can't really touch it after shutting off the bike, so it will definitely help.

My bike did the same thing next time open your gas cap I bet you hear vacuum release . I ordered a new gas cap from txpowersport and haven't had a issue since . If I put stock gas cap on it vapor locks after it's warm there 20 shipped so try that out and report back they shipped it fast too

[Megadan]

No need to mod my exhaust or gut the cat. I actually have the famous ebay exhaust already. I held off on installing it when I first got the bike because I decided I wanted to dyno test a more or less stock bike with a properly jetted carburetor. Then do mods and dyno again. I don't really want to dyno it until it has a few hundred miles on it to properly break in and seat the rings. It's getting there. Up to 170 miles now lol.

My main concern with wrapping the cat with exhaust wrap would be overheating it. It's not advised to wrap cats because of how hot they can get, and melting the substrate and causing a blockage is not a result I would like.

I would say I could blame it on the 90+ temps we have been having for the last 3 weeks now, but this happened at night, in low 70's to high 60's. With the bike in motion most of the time.

Originally Posted by Ariel Red Hunter

I would figure out a way to mount the gold foil close to, but not touching the carb. The idea is to have an air layer between the carb and the foil. Maybe a piece of wire coat hanger made into a rectangle for the foil at one end, and a curlique on the other end to make it possible to bolt to something. Perhaps a carb mounting nut...ARH

That would be more ideal yes, but it's more work than is necessary. Even on the surface it is contacting it will reflect more than 80% of radiant heat energy. It's a thermally reflective foil applied to a fiberglass substrate insulation, with adhesive backing. I have experience with this stuff on turbo vehicle intake pipes, wrapped directly on to the pipes, and the drop in IATs was significant. Also used it on a firewall that used to get well over 200 degrees in a car I raced, and after applying it, and it went from almost too hot to touch, to just slightly warm. like a heated blanket.

My main goal is to just reduce/eliminate it temporarily. Who knows, maybe I will bother to make a little heat shield that bolts on the front of the carb flange and shields it from the jug/head and exhaust.

Originally Posted by enforcer89

My bike did the same thing next time open your gas cap I bet you hear vacuum release . I ordered a new gas cap from txpowersport and haven't had a issue since . If I put stock gas cap on it vapor locks after it's warm there 20 shipped so try that out and report back they shipped it fast too

I checked, zero vacuum buildup in the tank. That was actually the very first thing I checked for. It is literally fuel boiling in the bowl. With the fuel filter I have on it, it gives it a chamber to build up enough pressure to reduce flow, then the bubble eventually gives, fills back up, then does it again. In fact, the reason it goes back to running fine if I blip the throttle is, and I watched this happen, it literally pulls the bubble out and fuel rushes in. Then it idles fine for a few seconds, then the bubble builds again. This is a classic sign of vapor lock on a gravity feed system. The vapor being formed in the boiling fuel in the bowl escapes UP the fuel fitting and into the line. The loss of density actually stops fuel flow from the tank until something causes the fuel to force past or get drawn in.

One thing I may do is pull the tank and put that same foil on the underside. I did that on a previous air cooled bike I had and it also helped quite a bit. The fuel tank definitely gets pretty warm, so cooler fuel going in will reduce it's likelihood to boil when it hits a hot bowl. If I can get the temps down in both places, it can also slightly help with charge cooling. Not going to give some huge boost in power or anything, but even if it was .05hp, I'll take it lol.

[Sir kayakalot]

Dan, if you have a temperature gun I'd love to know what that carburetor reads when it's at its hottest. Kind of a bad design feature with the cat being located so close to the carb

[Megadan]

I never thought about getting that reading honestly. I should have. I already put some of that foil on the bottom, back, and right side of the bowl and went for a test romp just like I was riding it the night it happened. I rode around for a good solid hour and a half purposefully at low speeds with some nice brisk accelerations to heat the exhaust up mixed in. The good news is, no evidence of the fuel boiling. It did make a difference on my idle.

The bad news is, that wasn't the culprit...or should I say the only culprit. It still idle dropped and died. repeated this a couple of times while I am scrambling to check what I can to see if it will stop doing it. Removing the gas cap, checking setting screws to see if maybe they backed out (Marked them with a grease pencil) checking for a wire that might have burned through... anything.

Then, after the third time it died it would not restart. The motor would crank and crank, but never fire. Pulled the plug and grounded it to the head, super weak spark while cranking the starter. I decided to see what the spark was like with the kick starter... nice and strong. Put the plug in and used the kick start and it fired right up. Still didn't want to idle, but I was able to at least drive it home.

In short, even though I did everything right with the battery. Filled it to the lines, let it sit an hour, hooked it up to my itty bitty trickle charger and let it sit 24 hours. I have a crap battery. When I got home I hooked it up to my smart charger and it was at half capacity. Still 12 volts, but hardly any amps to speak of I am going to try the desulfide mode, but more than likely I am going to go get a battery from Walmart, or maybe just spend the money on a LiFePO4 battery. Once I know I have a good battery I can see if the problem persists.

EDIT: Just to mention this, I am planning to test the charging system next. I don't believe the reg/rec or stator to be an issue, but who knows. Maybe I have a bad connection somewhere.