OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk honda gv 400

hi all, my mate bought a greenfield ride on with a 11 hp gv 400 honda on it. it would not idle, hard to start and would backfire through the carby to the stage it has burnt the foam around the aircleaner element. After checking the fuel side of things, i thought things pointed to valve problems. I removed the head, but have not checked the clearances yet. What i found was on the intake stroke, naturaly the illet valve is open, but on the compression stroke you can put your thumb on the inlet valve and turn it all the way up on compression, the compression stroke has nearly hit TDC before the valve actually seats firm. on the exhaust stroke there seems too much over lap with the inlet opening well before the exhaust has closed. The inlet valve bit explains why its backfiring as the air fuel mix is escaping through the inlet manifold. Like i say i havn't checked the valve clearance yet, but it all seems too excessive. Can these engines run being a tooth out on the timing, as thats just what it seems like. I havn;t had much time and have to go to work again now but will look at it on the weekend. I think the way it looks we will have to go inside the engine, cause things just dont add up, thanks micka

Most 4 strokes will run if they are a tooth out, but depending on the direction of the error, they will either lose power and overheat, or spit through the intake pipe.

It is normal for the inlet and exhaust valves to be open simultaneously at the end of the exhaust stroke: they are typically both open for about 10 to 15 degrees of crankshaft rotation - 35 degrees for a high performance engine, and 70 degrees for a racing engine that is not expected to idle, even badly. The overlap is roughly symmetrical about top dead center, so you can usually check for a jumped timing belt or incorrectly set timing gear just by seeing whether the position where the two valves are equally open is quite close to TDC. Of course there is no substitute in the end for checking the manufacturer's timing marks.

The problem is much more likely to be a tight inlet valve tappet than incorrect timing, unless the engine has been overhauled.

Is this engine a OHC model? I have seen the timing belt slipon those before otherwise as grumpy said if the engine has been overhaulled its quite possible it was not correctly assembled.

no joe its a side valve, i'll check the valve clearance tomorrow and see what all looks like and go from there, i'll let you know

the inlet valve had no clearance, so i took a bit off the stem, it now has about 8thou clearance.the inlet valve is starting to close around BDC, the valve is still closing as the piston is coming up on compression and still backfiring through the carby, its now trying to start but wont, its a lot harder to turn over, i checked the flywheel key, its ok, something is out of wack somewhere, i'm starting to think i'll be stripping it down for the answer to it all, i've never seen one break a tooth unless its a major hemorage. this one was running but now i cant get it started at all it just sounds like the timing is out by a mile. anyway i'll see what tomorrow brings, cheer micka

It would normally be at least 35 crankshaft degrees past bottom dead center before the inlet valve was completely closed, so that is not a very convenient way to check the camshaft timing. Can you find the crankshaft position, near TDC, where both valves are open the same amount? That is a more useful way to check whether the timing is wrong.

Did you lap the inlet valve? If it was leaking it will have needed lapping. Most Honda mower engines have a decompressor, and this makes it more difficult to tell what is going on with compression. You have to check the tappet clearances in the right place rotationally, or you will get misleading answers. You now have more compression, and that is good if the inlet valve was leaking, but if you have adjusted the tappet clearance with the camshaft in the wrong position, you may have simply disabled the decompressor.

If it now has good compression but fires through the carburetor, either the inlet valve is still leaking or the ignition timing is seriously wrong. You can probably check the ignition timing roughly by holding the spark plug lead in one hand and rotating the flywheel with the other. When you feel the pain, note where the flywheel is. Start off turning it slowly of course - you'll soon find how fast to flick it to feel it without spoiling your day.

hi grumpy, i set the valves at TDC on compression, i didn't lap the inlet valve, but filled the port up with petrol and it held it ok. i havn't done as you said with spark timing yet but will as soon as i get a chance. i have checked the flywheel key thinking it may have sheared, but all is good there. i'll get back to it soon and recheck what you have said and let you know how i go. Thanks Grumpy

micka, Honda specifies that the tappet clearance has to be measured at "BTDC on the compression stroke", with the engine cold.

Edit note: this has been corrected to quote the Honda manual. I'm not too clear on what "BTDC" means. It sounds as if they meant "TDC" and this is a typographical error.

thats probably where its all gone wrong, iv'e been doing back the front, i'll check that before i go to work, cheers grumpy, thanks

micka, I misread the Honda manual, it says "BTDC", and because that didn't make sense without specifying how far before top dead center, I somehow read it as "bottom dead center". In practice the inlet valve always stays open in the early part of the compression stroke, so you can't measure inlet valve clearance at bottom dead center.

Did you know you can replace the "valve adjuster" (the little cup part on the end of the valve stem) to change the tappet clearance on that engine? Unlike Briggs and Stratton engines, you do not grind the end of the valve stem, you either replace or grind the valve adjuster.

The method B&S recommends to ensure you are in the right place on the cams for measuring tappet clearance, is to go just past top dead center on the compression stroke, so the piston has moved down the bore about a quarter of an inch.

Honda specifies a compression pressure of 30-60 psi at 600 rpm, and 120 psi at 1200 rpm, when pulling the starter cord. You said you had an inlet valve tappet clearance of 0.008" after you had adjusted it. Honda specifies 0.003 to 0.006", so you are on the high side. In your situation I recommend that you lap the inlet valve. I do not think being able to hold petrol at atmospheric pressure, tells you that it can hold air at eight times that pressure. As a further bonus, lapping the valve will reduce your tappet clearance.

The Honda manual says the ignition timing is 20 degrees BTDC, which would kick back awesomely unless they have variable ignition timing. This is the reason I wanted you to try to check the actual ignition timing - in case something has gone wrong with the variable ignition timing system.
Post Edit: These engines do not have variable ignition timing: Honda relies on heavy flywheels to prevent kick-back.

micka, if you have checked the camshaft timing, I think we'd all like to know what you found.

There are only three things that seem able to cause steady puffing back through the carburetor: wrong ignition timing, wrong camshaft timing, or leaky inlet valve. However when I re-read your description, you may have meant that occasionally, at unpredictable intervals, it pops back through the carburetor, and if the air cleaner is removed, it may be a sort of ball of flame that is expelled out into the air. This is quite different from what I thought you meant up to now.

Those occasional fireballs from the carburetor are called "spits" or "lean spits", and are a symptom of lean mixture. They mainly happen when you have just applied load (i.e., just after the throttle has been opened by the governor).

If you have spits rather than steady puffing, there is an intake leak or a fairly severe carburetor problem, calling for a strip and clean, with new gaskets where necessary. The intake leak is the more likely on an old engine that may have had amateur maintenance along the way.

micka, I had a further thought about this whole case and edited my previous reply. Please read it - we may have been travelling in the wrong direction right from the start.

hi grumpy, sorry i havent up dated my findings. i could find nothing out of order inside the engine and i'm about to close it up again. i read your post this morning and will check it out and post the outcome of it. thanks grumpy

hi grumpy, i checked the valves again today and have the carby in pieces soaking. i had to spin the engine pretty fast to get a tingle off the spark plug lead, by then the piston was moving too fast to really tell where it was firing. one thing i did notice was watching the piston comming up on compression, there are two marks on the flywheel, one marked F and the other T, i thought firing and TDC, there is a cast line on the housing i presume these line up with. what is happening is the F mark lines up right on piston tdc and the T mark lines up after tdc and the piston is moving down about a quarter of an inch, if that cast mark does mean anything then the timing is not right, it seems the only that can vary is a sheared key. we have a few honda engines and i think to be sure its time to get a manual or diagram to sort it out, cause i'm really only guessing, does that sound like a plan, thanks micka

micka, the flywheel normally rotates clockwise when viewed from above, so the firing mark has to be clockwise on the flywheel from the TDC mark for it to fire before TDC. So far as firing is concerned, the GV400 owner manual says:
"Adjust contact breaker points so that they just start to open when F mark on flywheel aligns with the mark on cylinder barrel." It also says:
"IGNITION TIMING: 20 degrees BTDC (fixed). The CDI type does not require adjustment."
So, does your engine have breaker points? If it does, they are in the center of the flywheel, together with the coil. Whether it has points or CDI, I cannot see any way the TDC mark could be wrong unless the flywheel key or keyway is damaged. Incorrect camshaft timing cannot cause the flywheel timing mark to be wrong. I think you need to pull the flywheel and have a close look at the key and keyway. When you have them in good condition so the TDC mark is correct, you need to adjust the points in line with the instructions above, so it fires at the F mark. (I can't understand how they can have it fire at 20 degrees BTDC without kickback, but apparently they somehow have that taken care of.)

For tappet clearance the owner manual says:
"Measure at TDC on compression when engine is cold."
I think this is what you did in the first place, before I confused you with information from the small GV engine's manual, which turns out to be quite different for the GV400. So, it sounds as if your tappet clearances are now OK, following you grinding the stem of the inlet valve.

So, a lot of the confusion up to now has been due to me reading the wrong manual. The GV400 manual does not seem to be available on this site, but you can download it free, from here:
http://engines.honda.com/pdf/manuals/3189120.pdf