Topic: setting your airbox on fire... help pleeze

[celebutante]

Apparently I'm not the only one dumb enough to do this. I had Valley Cycles work on my '74 CB750 K4 recently, but they didn't do a full carb rebuild (I think he just unclogged it a bit), thus it continues to spit out gas sometimes when it's not running. Outside of being VERY cold blooded (10-15 minutes before it'll idle without me holding the throttle open), it runs fine and makes decent power, no dead spots.

Couple nights ago, I forgot to turn off the petcock, and went to start it. It made gurgly sounds, and after a couple seconds it stalled. Tried again, more gurgle and stall. That's when I noticed the smoke rising from the right side (!). After a couple of seconds of panic, I shut off the petcock and ran inside my apt and grabbed a fire extinguisher in case it was gonna blow up. A minute later, there's still a small amount of smoke coming from the ride side of the airbox and it's warm, so I'm imagining some kind of small inferno in there. Being somewhat new to the world of old CB750, I didn't know you could open up the airbox with the two big wingnuts at the bottom, but looking in there I saw 'em and undid them quickly (one was warm!).

Once I got the airbox apart, fortunately nothing was on fire. It did appear to melt the plastic "outline" on one side a little where the top and bottom mate (very minorly though, didn't deform it). I'm thinking maybe it caught fire for a brief second and the smoke was just from the plastic burning. The bottom of the airbox had a very small amount of gas at the bottom, about a half a teaspoon maybe. I took the bottom off, and it's sitting and drying out.

My question: since I don't want to try and rebuild the carbs myself (apt carport not good work environment and sync'ing is probably over my head), I want to take the bike someplace to do it.

Do you guys think the reason it happened was because I left the petcock on for two days?

Think I'll be ok if I put it back together once all the gas dries so I can ride it to a shop instead of having to get a trailer? It's been riding fine- I rode it all night Saturday. Thanks in advance!

[moham]

I think the reason your bike caught on fire was that it's leaking highly flammable gasoline. You need to address the fuel leak before you do anything else. One fire should be enough warning to fix what's wrong before trying to ride again. You need to pull the carbs and probably start with checking float/float valves. For some reason they are not turning off the flow of fuel. Also, what shape are the bowl gaskets in?

There is myriad, step by step info on this board for cleaning/tuning your carbs if you are willing to learn. Good luck and keep the extinguisher handy if you chance it again...  

EDIT:

I just read your post again and realized that my advice about the bowls may not exactly be your issue. Is gas anywhere else besides the airbox? Is it coming from the bowls or overflow? 

[Spanner 1]

If I lived in L.A. I'd offer to help you with your bike, at least to keep it away from that Shop you used already....!! Any local members want to help this guy sort-out his bike...?

[celebutante]

@moham: it's coming from the overflows.

@spanner: thanks... as I said, I have a less than ideal work area (run back and forth between apt and carport), and outside of this, it's a clean and nice running low-mileage bike that I don't want to take off the road for an extended period of time (or mess up!).

[Gorms]

I would suggest trying to rebuild the carbs.  I had never even seen the inside of a carb I rebuilt them quite easily with little fuss...and my carbs were about as bad and gunked as you can imagine...inside and out.  I laid out a towel, had a can of carb cleaner, and a small bucket with simple green and a brush.  As I took apart the carbs, I took pictures and laid all the peices out on the towel.  I methodically made sure every part was clean and put them back the way they came apart.  Really not that hard.  Could be done at a kitchen table if need be.

[TwoTired]

It should be routine for you to turn the fuel valve off whenever the engine is stopped.
You have to adapt to these old bikes, particularly if you are unwilling to make the carbs work as new.  Your shop couldn't or wouldn't do it.  Finding someone conscientious enough to do the carbs right is probably worth the extra coin it will take.  

If your air box filled with gas, there is just as good a chance the engine filled with gas, too.  Check you oil level, if high and smells of gas, you better do an oil change right away.  Carbs are relatively easy to fix compared to rebuilding the engine.

Check your gas tank for rust/debris, and also the filter between tank and carbs.  Doesn't matter how clean your carbs are if you feed it sh!t with the gas.  One little spec lodged between float valve and seat, will allow the gas to drain from tank and pool in engine and/or air box.
Given your situation, I would check carb overflow tubes for blockage.  Overflow is supposed to pool on the ground under the bike rather than in important bits of engine.  There are four attached to the bottom of the carbs.  Insects like to build nests inside them.

[Don R]

On a V8 the idea is to turn off fuel and crank the motor to suck the burning fuel into the engine. I assume it would help here too. If you are hydro-locked that won't work. Scary aint it?

[moham]

Celebutante: Just re-read my first post and I have to apologize for sounding like a douche. Wasn't intentional. I still think you need to check the float valves to make sure they are shutting off the fuel when the float reaches the proper height. You are much better off both in the short and long run to take a crack at this stuff yourself. I think even finding a shop that will look at an old bike like yours might be tough, let alone the sheer expense when they get through with you, with no guarantee of results either. As far as not having a space to work, a buddy of mine liked telling me his story of rebuilding/hopping up his 850 Norton Commando in his studio apartment kitchen/living room back in 1975. Where there's a will there's a way, I suppose. I have pulled carbs and a couple of engines in various states of disassembly into whatever work space I could muster. Even had the carbs for my 78 750K sitting on my desk at work for a few days.

This forum has been the key for me to learn how to take the plunge and dig into whatever system on the bike that has needed attention. The folks on here have a ridiculous amount of cumulative knowledge that you can't find in any single shop or book...

This is all just my opinion, of course, based on my admittedly muddy thinking and self-delusion. Take it for what you will...

[Queeg]

If I lived in L.A. I'd offer to help you with your bike, at least to keep it away from that Shop you used already....!! Any local members want to help this guy sort-out his bike...?

hell i ain't doing anything on Sunday. bring that baby over to my shop. we"ll smack the crap outta that guy and make him nice and new.   and I'm cheap i work for beer or diet drpepper.  lately diet drpepper cause well i aont getting any younger and neither is my liver.

  ohh and stay tuned I'm feeling really bored and like fire.

[celebutante]

@moham: No offense taken. I tend to agree with you that it's hard to find someone who will care enough to do it right, which is why I took it to Kurt at Valley Cycles- he's considered the super guru of CB750 (he did the fork seals for me and some other stuff), but he is crazy insane expensive and didn't really give me an estimate, just sort of gave me the bill. Grrr.

I suspect I will have to tear into this myself...