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80's Honda 10HP Timing

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GregGrimes
Deckhand

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USA
11 Posts

Initially Posted - 04/20/2005 : 09:57:46

Can someone tell me how to read the valve timing marks on an old (1984?) Honda 10HP?

I suspect mine jumped a tooth or two but I can't remember which marks to read. There are two different marks on the flywheel and the cam wheel has one under the rubber cap.

Thanks!
Greg Grimes

Bama Breeze (aka Moondance)
84 SR/FK 4309
Lake Guntersville Sailing Club

Edited by - on

Leon Sisson
Master Marine Consultant

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USA
1893 Posts

Response Posted - 04/21/2005 : 00:19:51

Greg,

See if this helps. I'm going to assume you have a BF100 series engine (or BF75, or similar). I think most of them were dark blue with a white cowl, but some of them may have been all white. I'm looking at a Honda factory service manual (c)1984. What follows is in my words, not Honda's. They rely heavily on their excellent drawings to get their point across.

To check cam timing, turn the flywheel until TDC (Top Dead Center, pistons closest to the sparkplugs) with the punch mark on the top surface of the cam pulley out near the rim turned closest to the flywheel. With the "T" mark on the rim of the flywheel lined up with the recoil starter mounting bolt closest to the cam, the dot on the cam pulley should line up with a raised mark on the cylinder block below the rim of the pulley and pretty much in a line between the center of the cam and the crankshaft.

Those are some mighty long sentences. I hope some of that made sense. Let me know if it's still not clear after you've tried it.

-- Leon Sisson

Edited by - on

GregGrimes
Deckhand

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USA
11 Posts

Response Posted - 04/22/2005 : 10:04:04

Leon,

Thanks for the info, it made perfect sense. I was looking for a smoking gun, but it seems the valve timing is still OK. Do you happen to have the valve clearances listed?

There is a definite hard spot at TDC each rotation. It's there with the plugs out and it's a good bit more than I would from just valve spring tension.

Thanks again!

Edited by - on

Leon Sisson
Master Marine Consultant

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USA
1893 Posts

Response Posted - 04/22/2005 : 22:41:55

Greg,<blockquote id="quote">quote:<hr height="1" noshade id="quote">Do you happen to have the valve clearances listed?<hr height="1" noshade id="quote"></blockquote id="quote"> Book says intake=0.002", exhaust=0.006". That's probably cold, but on such a compact OHC head it wouldn't much matter. In light of the troubleshooting situation, write down the existing clearances before you adjust them. I make it a point to adjust valves on the heel of the cam when possible. In other words, for 360* firing order of this engine, set each intake when the other intake is at full lift. Same for exhaust, set one while the other is fully opened. That assures you that you aren't setting the clearances on the dampening ramps of the cam lobes.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">There is a definite hard spot at TDC each rotation. It's there with the plugs out and it's a good bit more than I would [expect?] from just valve spring tension.<hr height="1" noshade id="quote"></blockquote id="quote">Given the 360* firing order of these engines, at precisely TDC there should be no valve spring tension. More accurately, the rotation resistance caused by the valve spring tension of the opening and closing valves should exactly balance each other at TDC. Since these are small, slow turning, OHC engines, the valve springs aren't all that strong anyway.

I'm assuming this hard spot occurs sharply and exactly at TDC, and doesn't exist even a couple of degrees either side of TDC. If so, that indicates the piston is hitting something in the combustion chamber. Other than squinting down the plug holes with a pen light, the next step is very likely to be removing the cylinder head. At that point, you really should have a factory shop manual open in front of you.

If it comes down to that, be very wary of frozen bolts. If you wring off one in aluminum, the job becomes a whole lot less fun. On a saltwater Honda motor, I had a head bolt so stuck that even after I ground the head off it, I couldn't slide the cylinder head on the bolt. In trying to pry the cylinder head off (with all other bolts removed, and after many cycles of heating and soaking) I broke off a big chunk of the cylinder block at the stuck bolt. Just so you know how bad it can get.

-- Leon Sisson

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GregGrimes
Deckhand

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USA
11 Posts

Response Posted - 04/24/2005 : 19:02:53

I pulled the head and found the top (#1 ?) exhaust valve seriously bent. Fortunately, there was minimal damage to the piston (just a scrape) so it looks like I can stick a new valve in and go.

By the way, the problem was caused when the throttle suck wide open in neutral. Once I put in a new valve, I still have to figure out why the throttle stuck open.

Do you know of any online sources for outboard valves?

Thanks again!
Greg Grimes

Edited by - on

Leon Sisson
Master Marine Consultant

Members Avatar

USA
1893 Posts

Response Posted - 04/24/2005 : 20:43:41

Greg,<blockquote id="quote">quote:<hr height="1" noshade id="quote">...there was minimal damage to the piston (just a scrape) so it looks like I can stick a new valve in and go.<hr height="1" noshade id="quote"></blockquote id="quote">Maybe. Depends on how badly bent, and how it impacted the piston and head. Here are some things to think about. If the valve got hit in such a way as to put a twist, or spin, on the piston, the aluminum connecting rod could have gotten candycane twisted. The piston could have a hairline fracture too small to see through the carbon build up. If the piston got hit out near the edge, the top ring land could be pinched or fractured. The valve guide could be cracked. The valve seat could be nicked. However, if the valve is just barely bent, and the piston is just barely dinged at least 3/16" on from the cylinder wall, then you probably don't need to worry about all that other scary stuff. By the way, be sure to check the other piston and exhaust valve for similar damage. When an engine free revs to failure like that, there's often matching damage on all cylinders.

To check a valve for damage with the cylinder head off, pour a small amount of gasoline into the port with the valve spring installed and the rocker arm (adjuster) not touching valve. Count the number of seconds between when you first start pouring, and when you first see dampness around that valve seat in the combustion chamber. Anything over about 4 to 5 seconds is probably not worth worrying about. If the valve is bent or the seat is nicked or burned, the gas will come pouring through.

Another thing you could do (and I would), is remove all the valve springs, and lightly lap the valves. This will immediately highlight any bent valves or damaged seats. If you are having trouble "reading" the lapped surfaces, start by coating them with magic marker ink, then lap. You will quickly and positively see where the lapping compound has worn away the colored ink.

<blockquote id="quote">quote:<hr height="1" noshade id="quote"> I still have to figure out why the throttle stuck open.<hr height="1" noshade id="quote"></blockquote id="quote">You're right -- that's a problem! (Now how hard would it have been for Honda to have built an electronic rev limiter into the ignition black box?) Check the throttle cable for rust, kinks, and friction. Study and carefully inspect your neutral throttle limiter linkage -- that's supposed to prevent this sort of thing from happening. Be sure the throttle butterfly in the carb moves with no friction, and that it's return spring works. Look for bent or binding throttle linkage rods between the carb and that bell-crank thingy that the throttle cable from the twist grip connects to. As I recall, the bell-crank is supposed to have a return spring on it too. Did you get a factory shop manual yet???

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Do you know of any online sources for outboard valves?<hr height="1" noshade id="quote"></blockquote id="quote">No. But there must be some. Fire up the search engine of your choice, and surf away! You could also ask Honda questions over on [url="http://bbs.trailersailor.com/forums/trailersailor/index.cgi"]Trailer Sailor BBS[/url].

-- Leon Sisson

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