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I'm in the process of getting my 1982 Honda Magna back in running condition. I've cleaned out the carb and put in a new battery. It now runs, but dumps a TON of coolant out the tailpipe. Almost a constant stream. It blows a bit of white smoke, has trouble idling, and overheats rather quickly.

What steps should I take to diagnose and fix this issue?

  • Can you describe 'a ton of gas out the tailpipe'? Is it liquid and actually dripping out? A misfire wouldn't cause this condition. Since the petcock is operated by vacuum you could still have a sunk float. The gas only flows when the bike is running. Don't turn the petcock and leave it on the prime position, if you do have a bad float needle/seat the gas will run into your intake and fill the cylinder creating a hydro lock condition. – DucatiKiller May 3 '16 at 5:42
  • Any additional information you can add to your question. Have you pulled the plugs and checked them? How do you KNOW it's fuel coming out? What Engine do you have? They come in 500/7501100 flavors? Anything at all will be helpful. Welcome to the site. Cheers! – DucatiKiller May 3 '16 at 5:44
  • @DucatiKiller Yes, it is liquid dripping out of the tailpipe. Like I said, it is pretty much a steady stream. I was simply making the assumption that one cylinder is not firing and, therefore, expelling that gas. My petcock doesn't have a prime position. Only 'on' and 'off'. I replaced all the plugs a few weeks ago. Engine is a 750. – jordan May 3 '16 at 21:12
  • I have a belief that it's not fuel and that it's coolant. Could you verify? – DucatiKiller May 4 '16 at 2:22
  • @DucatiKiller I was actually about to comment that same thing. I think you are correct, although I'm not sure where the coolant would be coming from. What would this mean? – jordan May 4 '16 at 2:24
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You can spend time identifying which cylinder is bad but in the end you need to replace both head gaskets

If both head gaskets aren't failing the other one will fail soon. You may not know it yet but you don't want to replace just the single head gasket. You want to do both.

If I were in your position I'd just order two new head gaskets and get on with it but I've worked on a ton of these things so I have a different outlook on getting from point A to point B.

Identifying the bad head gasket

  • You could do a leak down test to identify the bad cylinder(s).

  • You could perform a compression test to see what cylinders have poor compression and which do not.

  • You could turn the engine over a few times with the starter and pull out your spark plugs to see which ones are wet and which are dry.

Here are a couple of links that may be helpful to you.

Conclusion

Clearly you have a bad head gasket if you have coolant coming out of your exhaust.

IMO consider replacing both head gaskets.

Get a factory manual for your bike.

Spec

184 +- 28 PSI with a maximum difference between any two cylinders of 57 PSI. This from the Honda factory shop manual.

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