Head Gasket Backfire Help Needed!

[Manmelvin]

    This is my first post on ROV so please bear with me and I appologize in advance for the wall of text. So I went on a short ride down to the store today to pick up a few things and after starting up my bike it ran for a few seconds and then there was a loud bang, followed by exhaust and a bit of engine oil coming out of the sides of the engine. What I believe happened was that there was some sort of backfire in the top of the head casing causing all of the hemispherical plugs (4 per cylinder, 2 per side) that are part of the head gasket to pop out. These are the plugs that cover the space needed to remove the bolts on the ends of the cam shafts I believe. Anyway I was able to push them all back in enough to get the bike home were I took off the cylinder heads, repositioned the gaskets correctly and put them back on. Later on today I went back out on a short ride just to check that everything is working normally. I do not notice any difference in how the bike runs or in its performance. I stopped for a few minutes in another store and when I came out and started my bike the whole thing happened all over again. I have no idea what would be causing this kind of problem except perhaps some sort of fuel or exhaust leak getting into the top of the cylinder and igniting. Any help or suggestions on what would be causing this would be much appreciated.

Thanks,

Evan

[Jimustanguitar]

If you're getting a backfire in the valve cover, you may have bad valve stem seals. The other (more likely) possibility is that your crankcase vent is plugged and pressure is building up inside of the case and blowing these seals out. Does your bike have a hose coming out the top of both valve covers that goes to the airbox? Check these hoses.

Have you tuned your carb lately? Backfires tend to happen during starting, but you shouldn't be getting one while it's running if everything is dialed in.

Welcome to the forum!

[Rikugun]

If it were blocked vent tubes, wouldn't it take some time and higher engine speeds to develop enough pressure to eject the cam plugs?

Couldn't it be just the backfiring at start up pressurizing the crankcase and blowing out the gasket/cam plugs? This of course assumes it is backfiring through the intake, not exhaust.

How does it run and idle otherwise? Is the YICS canister still in place? Are there any missing plugs/hoses causing vacuum leaks that might cause backfiring at start up? Does it start hard and require lots of cranking time?

edit: just wanted to confirm that you are talking about the cylinder head cover gasket, not the cylinder head gasket as stated.

[Manmelvin]

Yes Rikugun you are right, I am talking about the cylinder head cover gasket not the actually cylinder head gasket. It is hard to tell if it is actually a backfire or more of a pressure buildup. When it happened the bike was running for about 2-3 seconds with no choke or anything. When I got the blowout it blew a lot of "smoke" or something similar that seemed to be exhaust vapors and there is quite a loud bang, but those plugs are set in there pretty well so I imagine just a large pressure build up could cause that sort of noise. My gut feeling is that it is acutally an ignition backfire not a pressure blowout just because it doesn't happen all the time and after i jammed the plugs back in the engine ran completely normally and didn't pop them out again. Another interesting thing to note is that it blew out the plugs on both cylinders at the same time which I guess means that the pressure traveled through the exhaust? I cleaned and synched the carbs right after I got the bike about 2-3 months ago. The bike runs great otherwise, YICS works fine, no vaccum leaks, almost no stumble. It takes maybe 3-5 seconds of turning it over plus a little bit of throttle to start it cold. I haven't had it backfire through the exhaust since I got it running well a few months ago. The only thing I have changed recently enginewise is that I leaned out the mixture a bit because the weather got a lot warmer. I will check the hoses etc. and report back.

[Rikugun]

Try putting the mixtures back where they were before the backfiring (if that's what it is) started happening.

I doubt it has anything to do directly with the exhaust. Since both cylinders are connected to the crankcase, an intake backfire on either cylinder will pressurize the crankcase as previously described and potentially blow the cam plugs from both cylinders.  Whether it's obstructed vents or backfiring, I think what is being ejected (along with the gasket :D) is "crankcase breath" which will have oil mist and byproducts of combustion from normal blow by. Exhaust in a sense, but technically not "from the exhaust" per se. 

Have you checked to make sure no vacuum plugs, hoses, etc. have also been dislodged? Check the fuel pump, YICS, petcock too. I'd also have a look at the air box to make sure it hasn't been partially dislodged from the carb airhorns.  Anyone else have any theories or things to check?

[Rick G]

I would bet on a plugged /pinched breather hose , causing a build up of pressure in the crankcase.

[Manmelvin]

UPDATE: So i checked the vent tubes and the attachment points on both the cylinders and the airbox they are both clear. When I opened the airbox there was a bit of oil in the bottom that probably got blown up through the vent tubes into the airbox itself, not sure what this means. I also noticed that one of my mixture screws had jiggled out so I guess the rear cylinder has been running very rich. I put that back to its original spot and the bike seems a bit happier now with no backfires on the last few rides. I am having some trouble getting the gasket to seat itself correctly after the blowouts and there is some oil leakage and spray from one of the cam plug covers. Any ideas on how to fix this? I think the plugs themselves may have gotten a bit torn up from the blowouts they experienced.

[QBS]

A little oil in the air box is to be expected.

[fret not]

To get the "used" parts to seal the surfaces need to be clean, and a coat of Gas-ga-cinch on the gasket (or seal, or whatever you want to call it) should stop any leakage.

[Rick G]

The Valve cover gasket and plugs are one piece. You need to install new gaskets .  Your not going to seal the plugs that blew out , there part of the gasket.

[Rikugun]

If the gasket is still intact and the cam plug portion merely has been dislodged, you may be able to reseal it. As has been suggested, remove the cover and clean the gasket, head and cover surfaces thoroughly. If new, I would install it dry and fully expect it to seal. Since it's old and possibly less pliable it may be advisable to use a gasket sealer in moderation. This also assumes there are no cracks or tears in the gasket and even then resealing these old valve cover gaskets can be hit or miss. There are a minimum of fasteners to tension the cover so it relies on a new soft gasket to conform and seal.

[Rick G]

We've been through this , on this forum , many times over the years. If there damaged you will need  new ones . If you can't get new ones , I have usable  used ones.

[Rikugun]

We've been through this , on this forum , many times over the years. If there damaged you will need  new ones . If you can't get new ones , I have usable  used ones.

I don't think anyone is disagreeing with you Rick.  :)

[Manmelvin]

The plug part of the gaskets were just popped out. They are a little worn from being blown out but I will see if I can make them work. I took the heads and gaskets off, cleaned them up repositioned everything and tightened it all back down. So far no leaks and no blowouts so I will just have to wait and see if it holds. Thanks for all the help everyone. If these gaskets don't seal I have a parts bike that I can scavenge the gaskets off of if it does come to that.

[sunburnedaz]

I got by for a while till I could find new gaskets by smearing a small bit of black RTV on the plugs.  They blew out after using a little too much starter fluid when I was reviving her for the second time.

[Rikugun]

....They blew out after using a little too much starter fluid when I was reviving her for the second time.

Which caused a backfire?