Air pushing through gas cap

[TdotMurderer]

Okay, so basically, my engine starting acting up in the past couple of weeks, i first started to notice it as sub-par throttle response in the low gears 1-3. I also started to notice that the engine was revving higher at idle 1500 vs 1100.
So I thought maybe that I got some bad gas or that the carb was getting a little clogged up - so i put some SeaFoam carb cleaner in the gas tank.
However, now the engine is starting to sound a little louder and starting to sound unhealthy.

Today I had it running in the garage warming up, and i decided to check the oil while the engine was running.
I took off the oil cap and a strong burst of air blew up (and continued to blow) quite strongly from the hole.

I don;t know what to do, im not very mechanically inclined.
Whats wrong with my baby?

1984 Honda Nighthawk 450

 

[GreyGhost]

Higher idle, crappy throttle response and louder bike all could be explained by a leak in a boot between the carb and engine allowing air to be sucked in (without the appropriate extra fuel).

 

[Nyx13]

Why did you check the oil while your bike was running...? It wouldn't be accurate. It's moving through the engine rather than settled to give you a reading.

​

 

[invictus43]

Today I had it running in the garage warming up, and i decided to check the oil while the engine was running.
I took off the oil cap and a strong burst of air blew up (and continued to blow) quite strongly from the hole.

There is crankcase pressure generally..the pistons are going up and down and that creates pressure in the crankcase. You have a breather for this that runs into your airbox. Check to make sure that isn't blocked..it's a hose that runs probably from the top of your valve cover tot he bottom of your airbox. If you have excessive crankcase pressure you may have piston ring blow-by. You can test this by doing a compression test. You probably can't do it yourself (requires a compression tester) but a shop will do it for you for $50 or so. It will tell you whether or not you have a piston sealing problem.

As GreyGhost says, wonky idle could simple be incidental to the blow-by issue and could very well be caused by a loose intake boot causing a lean condition. It will generally increase idle speed and cause stumbling at low to mid-throttle applications.

 

[invictus43]

Why did you check the oil while your bike was running...? It wouldn't be accurate. It's moving through the engine rather than settled to give you a reading.

Absolutely true.

 

[TdotMurderer]

So am I safe to ride it to the mechanic? Am I doing any additional damage to it riding in to work?

 

[stormcat]

I'm a bit confused. You opened the gas cap as per your thread title? Or the oil filler cap? If you did mean the gas cap, then your gas cap is vented. One reason why your bike may be running rough could be a plugged gas cap vent. That's pretty common.

If you meant oil filler cap, then disregard the above.

 

[GreyGhost]

So am I safe to ride it to the mechanic? Am I doing any additional damage to it riding in to work?

First, check the oil while it is on it's centre stand and not running to make sure you don't have way too much in there.

Second, physically pull on the carbs a bit and make sure that things are still securely attached. It's been a long time since I had a nighthawk apart, I don't think the carbs can completely sepate from the engine without some work, but it is worth a quick check. Try it with the engine running and see if the speed changes (could indicate a cracked boot or loose clamp).

The odds that the bike is lean enough to blow up are slim, I wouldn't worry too much about riding it (although get it looked at sooner rather than later). If you are worried about being too lean, just leave the choke on, fuel economy will go down, but you shouldn't be lean.