Gx390 honda issue - Snowblower Forum : Snow Blower Forums
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post #1 of 12 Old 10-01-2017, 06:22 PM Thread Starter
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Gx390 honda issue

I got a used, but ok running gx390 that im going to put on my ariens 924044 (32 inch). Compression was only 60 lbs, so I replaced the piston and rings. While I was in there, I installed an electric start kit. It ran great, easy to start and no smoke. The muffler studs were stripped, and after failing at getting helicoil inserts to work, I put on an aftermarket head from Amazon ($45). Now, it's impossible to pull start. And the electric starter barely has the power to turn it. It runs ok, but it's burning oil and smoking. Valves are adjusted to .020 intake and. 015 exhaust. Valve seals are on there right. Any ideas?
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post #2 of 12 Old 10-01-2017, 06:46 PM
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so it was running great until you put the aftermarket head on it?

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post #3 of 12 Old 10-01-2017, 06:59 PM Thread Starter
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Yes, no smoke and started easily. There doesnt appear to be anything wrong with the new head visually. It's almost like it has waaaay to much compression now when I try to start it.
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post #4 of 12 Old 10-01-2017, 07:39 PM
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Would you be able to drill out the muffler bolts for the original to a slightly larger stud or bolt?

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post #5 of 12 Old 10-01-2017, 07:46 PM
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Did the old head had a decompression valve system and the new one does not (I'm not familiar with them, but I know Honda has been using them for a long time). If so, that may be part of the problem, but the starter barely being able to turn it does not sound right.
Are the valves maybe too tight...?

If it was me I'd deal with the studs on the original head and re-use it. A genuine Honda head is always going to be better than an aftermarket IMHO.
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post #6 of 12 Old 10-01-2017, 09:09 PM Thread Starter
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The original studs are 8mm. Somebody already drilled them out to 1/2 inch and tapped them with 1/2 inch threads. So, i tried putting 1/2 inch helicoils in, and then 3/8 helicoils inside those, so I could run a bolt that would fit thru the exhaust manifold. It seemed to get tight, but after running for a few seconds they loosened up from vibration. I can try filling the holes with extreme temp jb weld, then drilling and tapping those I guess. But I don't think I can drill larger. Not much metal left as it is.

Last edited by FredGraham; 10-01-2017 at 09:12 PM.
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post #7 of 12 Old 10-01-2017, 09:11 PM Thread Starter
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As far as the decompression system, yes the engine has one, and i know very little about it, other than it exists haha. I know the heads were identical in appearance.
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post #8 of 12 Old 10-01-2017, 10:34 PM
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has the cam timing been verified?

if it is proper, is it possible that the new piston has a higher dome or the new head has a smaller chamber volume? either would raise the compression by a good amount.

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Last edited by tdipaul; 10-01-2017 at 10:40 PM.
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post #9 of 12 Old 10-02-2017, 06:14 AM Thread Starter
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I have not verified cam timing, but when I replaced the piston I made sure not to remove the cam as to not mess it up. Also, when I set the little arrow to TDC, both rockers have slack and the piston definitely appears to be at top dead center.
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post #10 of 12 Old 10-02-2017, 06:42 AM
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Quote:
Originally Posted by FredGraham View Post
The original studs are 8mm. Somebody already drilled them out to 1/2 inch and tapped them with 1/2 inch threads. So, i tried putting 1/2 inch helicoils in, and then 3/8 helicoils inside those
You seem to be handy with a drill and tap. I that case, this is what I would do to save the original head:

Using red lock-tite install a corresponding size bolt on the threaded holes for the exhaust studs (1/2"...? 9/16",,,,?), cut and grind the bolts flush with the surface of the head, install the exhaust manifold gasket and mark the location of the stud holes, making sure that you are completely perpendicular to this surface drill and tap the installed 'bolts' to 8x1.25mm and you should be good to go (I've done this in the past successfully on an automotive application).
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