honda gx390 stock timing, what are you getting?

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KartFab

Member
So, first time measuring timing. Using a decent timing light and spinning the engine over with a drill, valves out. I have a jeggs 11" precision degree wheel, and measured tdc with a piston stop (used lock tite to fix it into position so the inside adjustment threads would not budge). So i did the whole counter clockwise, clockwise thing, divide by two, then that is what you set your degree wheel to, then cranked it down. Turned engine counter and clockwise and got the same degrees from TDC as my measurement so i know for a fact i have actual TDC.

Next marked the block, and the flywheel at a perfect 90 degree angle, hooked up the timing light and gave it a spin with my drill. Marked where it was firing (used feeler gauges to gap the magneto to exactly .4mm).

Long story short, is that timing read at EXACTLY 22.5 degrees before top dead center no matter how many times i did it, or how fast i spun my drill. Now, i double checked TDC again with the piston stop, got precisely the same number rotating counter and clock wise, and then rechecked timing. still the same 22.5 degrees BTDC. Now factoring in human error etc, i can see being off by a half a degree or so, but i was thinking these engines had timing of 25 degrees BTDC stock.

Am i just being weird, or is this normal?
 
Fashion a positive stop so it stops the piston about .020" to .030" from the top of the stroke. Install your degree wheel, fashion a pointer of some kind. Have the pointer pointing at 0 degrees with the piston up against the positive stop. Rotate the engine in the opposite direction until it hits the positive stop. Observe what degrees the pointer is now pointing at on the degree wheel. Just for demonstration purposes, let's say it's pointing at 30 degrees. Top Dead Center (TDC) will be halfway between 0 and 30 degrees, or what ever degrees it is pointing at. Adjust your degree wheel so the pointer is pointing at the number of degrees halfway between there and zero. For instance, let's say it's pointing at 45. TDC is halfway between 0 and 45, so 22.5 is TDC. Rotate the engine so the pointer is pointing at the number halfway between zero and where ever the positive stop stopped it. Making sure the crankshaft doesn't move, loosen the degree wheel and rotate it so the pointer is pointing at zero. Tighten the degree wheel and rotate the engine back and forth to the stop. The pointer should be pointing at the same number on both sides of zero.
Now these numbers are just for demonstration purposes, whatever number comes up when you rotate the engine the opposite way, TDC is halfway between there and 0. Measuring TDC with a depth mic, or caliper, you could be off 2 to 4 degrees.
 
Another thing; I have heard that some of these ignitions retard the timing at real low rpm's, like when you pull start them, so, if this is true, spinning it with a drill motor may not be an accurate way of checking the timing advance.

For my Yamaha KT, I made a permanent pointer and marks on the flywheel so I can start the engine and check the timing at any rpm.
 
KartFab said:
So, first time measuring timing. Using a decent timing light and spinning the engine over with a drill, valves out. I have a jeggs 11" precision degree wheel, and measured tdc with a piston stop (used lock tite to fix it into position so the inside adjustment threads would not budge). So i did the whole counter clockwise, clockwise thing, divide by two, then that is what you set your degree wheel to, then cranked it down. Turned engine counter and clockwise and got the same degrees from TDC as my measurement so i know for a fact i have actual TDC.

Next marked the block, and the flywheel at a perfect 90 degree angle, hooked up the timing light and gave it a spin with my drill. Marked where it was firing (used feeler gauges to gap the magneto to exactly .4mm).

Long story short, is that timing read at EXACTLY 22.5 degrees before top dead center no matter how many times i did it, or how fast i spun my drill. Now, i double checked TDC again with the piston stop, got precisely the same number rotating counter and clock wise, and then rechecked timing. still the same 22.5 degrees BTDC. Now factoring in human error etc, i can see being off by a half a degree or so, but i was thinking these engines had timing of 25 degrees BTDC stock.

Am i just being weird, or is this normal?
Click to expand...

No, you are not being weird and yes, this may be normal. Most drills are not capable of spinning the engine fast enough to get a true reading (usually over 1K RPM). A better way to do it is start the engine and use your timing light to see where it is firing, on the mag, at about 1.5-2K. Then put your degree wheel on and see how many degrees it is. A reading of 22.5 degrees seems a bit low, but maybe????????????
 
Is this a new engine? I ask because if it is used, someone could have put a different or clone crank in it which would explain the difference in timing you are seeing. The predator 420cc cranks do fit in the Honda blocks, but have a different crank journal size and different flywheel taper I believe
 
If it is a new engine the Honda GX390 has variable ign timing and you are not spinning it fast enough to see the electronics at work.
 
Engine came off a dewalt power washer and I have had it for 3ish years, bought it used. I do know it takes the thinner piston rings (found that out the hard way). It isn't a UT2 coil. As far as precision vs accuracy goes, I know all the measurements were precise (repeatable), not sure about accurate ahaha. So every time i took i timing reading and re-did TDC measurement I always got 22.5 btdc as the time the timing was showing. Just for fun, after I get the engine running and a tach attached, ill go ahead and see if the timing mark changes at higher rpm, as i painted a little strip of white paint on the block/flywheel.

I went ahead and advanced timing to 26.5 btdc (by 4 degrees) and will keep it there for now as this build is not going to need a lot of advance. The engine should max out at 5000 - 5500 rpms as I put in the NR Racing "marine grind" cam 280-0211. Just welded up a stainless exhaust for it and it looks sweet! Fired it up for a second and it idles perfect. Just gotta break it in and will get back to you all on the timing
 
Ok, so i checked timing at 3000 rpms and it was right at 26.5 BTDC, right where I set it. Engine does in fact top out at 5500 rpms and im using a .043" jet with stock carb. Has a lot of torque and actually unseated one of the rear tire beads :D

Prior to that the engine had a little chirp to it, which was a leaking head gasket. The copper .016" head gasket wasnt working out so well, so switched back to the stock .050" gasket with no issues.

Im going to play around with the torque converter drive unit (40 series) to modify the rpm engagement with medium cam rollers and red springs, bumping it up from 2000ish engagement rpm to 2600 engagement rpm
 
You usually have to anneal the copper gasket to get them to stay sealed up right, if you just buy the gasket and put it on with no sealer and don't anneal it, I have never seen one stay sealed up more than a couple races that way. I use copper on my open animal and honda's with up to 13.5:1 compression and have never had a problem keeping them sealed after annealing and using permatex copper gasket spray on both sides.
 
Hmm okay. Ill see about trying the head gasket again. How much clearance does the head need from the piston? Same for the valves. I am thinking ill just use a straight edge, and feeler gauge at tdc to check the piston height. No idea how to do that... thoughts there?

Next up is annealing. So i bought some MAP gas, any special techniques for that? From what I am seeing, you heat to cherrry red, then drop into water?

And finally jetting. There really isnt any buildup on the plug that I can see, and ive gone up to a .043 jet with the stock carb. When at full throttle, i back off the pedal just a tiny bit and it lurches forward just a little bit. Im assuming this means its lean? Ill post pictures later of my findings.

Also found a yamaha R1 ehxuast, chopped off the tubing and routed some walker 1" flex exhaust to step up the exhaust flange. Also found out that the r1 ehxuast tubing is titanium, which I tig welded up. It is acutally really really easy to TIG weld. I had to chop off a sliver of exhaust tubing to use as filler metal. Sounds nice now :) Added header wrap for looks because the flex tubing looks kind of ghetto, and header wrap is totally worth it for looks right?
 
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