Air compressor bogs down after starting

[turbodave]

I've got an older Sanborn BlackMax 5hp 60gal single stage compressor. Bought it used so not sure how many hours of use are on it. I've had it for 6-7 years now and had no problems with it until last fall.

The current issue is that the compressor bogs down after it starts up. It starts up normally, but after a few seconds the belt will slip and the compressor will chug at a low rpm until it regains speed. Once it catches up it reaches full pressure and runs fine.

The issue only happens when the compressor has been sitting idle for a while, usually for a few days. Once it runs and gets warmed up the problem usually doesn't happen again that day.

The problem does seem to be worse in colder temps, however it was doing it even on 90 deg days this summer. The garage is heated, so even at this time of year it doesn't go below freezing (thermostat set to 38deg when I'm not out there)

Last spring I changed the oil and cleaned the check valve between the pump and the tank, the problem seemed better afterwards, but after a few weeks it was doing it again. I had a busy year outside of the garage so the compressor hasn't seen much use besides just filling tires and running an impact occasionally. It's often idle for a few weeks at a time.

Any thoughts on what else to check? I've started budgeting for a new compressor, but would rather get some more time out of this one.

 

[pattenp]

Is the belt old and glazed? May just need a new belt.

Edit: Make sure the unloader valve is working properly.

 

[Major Ramifications]

That's what I was going to say, change the belt, clean the pulleys with parts cleaner, and check the tension.

 

[turbodave]

Is the belt old and glazed? May just need a new belt.

Edit: Make sure the unloader valve is working properly.

Thanks, the belts have taken some abuse but I don't think they are the issue. They don't slip on startup, only after the compressor is at speed for a few seconds. The compressor starts to slow down like it's overloaded before the belts start to slip. But they may be slipping before they make an audible screeching noise and burning smell so replacing them will be on my list.

The unloader is working normally, hisses when it reaches shutoff pressure.

 

[Chuck McB]

Interesting you should bring this up! I too have a Black Max Sanborn 5HP and it seemed to slow down and sometimes speed back up this last weekend. Scared the **** out of me as I just knew something was bad wrong. Well for a $70 air compressor from an estate sale I woudn't be suprised but luck was on my side and the belt had become sloppy loose.

 

[turbodave]

That's what I was going to say, change the belt, clean the pulleys with parts cleaner, and check the tension.

Thanks, I think that will be my next step. I'll kick myself for considering a new compressor if that's all it needs.

 

[rlitman]

Is the belt old and glazed? May just need a new belt.

Edit: Make sure the unloader valve is working properly.

If there was a problem with the unloader, it would slip right away, not after a few seconds.

Thanks, the belts have taken some abuse but I don't think they are the issue. They don't slip on startup, only after the compressor is at speed for a few seconds. The compressor starts to slow down like it's overloaded before the belts start to slip. But they may be slipping before they make an audible screeching noise and burning smell so replacing them will be on my list.

The unloader is working normally, hisses when it reaches shutoff pressure.

If the unloader works, there isn't much pressure for the first few revolutions. When it is finally fighting against pressure then it slips. Sounds like a belt. Could be glazed, or too loose. Or perhaps too thick an oil, but I don't think it is that.

 

[turbodave]

Finally had a chance to look at the compressor over the weekend. Took a little work to get the belt cover off (wasn't thinking when I put a shelf 8" above the compressor).

The belt was certainly loose, tightened it up and it runs like new again.

I'll get a new belt for it because the one on there looks old, but surprisingly even after slipping for a year it doesn't look glazed. Thanks for the advice and bringing me back to the simple things when I was thinking I had a much bigger problem.

 

[Chuck McB]

Good news! What was the knocking sound about?

 

[turbodave]

Good news! What was the knocking sound about?

No knocking sound other than my hollow skull. It was chugging when the compressor was turning at really low rpms due to the belt slipping. Purrs like new now.

 

[jeepfan79]

Glad I found this forum, this thread may have just saved me a lot of money! Thanks Guys!

 

[turbodave]

Glad I found this forum, this thread may have just saved me a lot of money! Thanks Guys!

Yep, be careful all the money you save is easy to spend on deals that pop up in the hot deals section...

And since this old there'd got bumped my old compressor is still running strong, no issues since replacing the belt.