Head gasket replacement on early Honda SOHC fours is not that big of a job. Especially compared with similar work on today's liquid-cooled fully-faired machines. However, as easy as it is, there are several techniques that make this job a professional one. Here they are.
First, before condemning the head gasket, do the professional thing. Clean the bike thoroughly. Use Gunk, available at any auto parts store. Ride the machine to warm up the engine, then spray the aerosol Gunk degreaser all over the engine. Let the engine idle, and carefully hose it off with a low-pressure water hose. Keep the engine running. When the engine is sparkling clean, rev it a few times to ensure that no water got into the intake, and shut it off. If you have compressed air, blow it dry. Or, after it has dried itself off for 30 minutes or so, coat the entire top end with aerosol athlete foot powder. Make sure you get the powder, not the deodorant. This is recommended by several of the OEMs, and goes on like flat white paint. When finished, get a strong flashlight or a photography lamp and run the engine until the leak appears on the white powder. Chances are, it's not the head gasket at all, but one of the many o-rings and seals that abound on the early SOHC engine.
If the powder shows there is leakage at the head gasket, it will usually be at the far right or far left end of the cylinder head. The reason the gasket leaks in the first place is because of the oil passages at these points. Like many engines made during the 70s, early Honda fours have camshaft oil passages running through the cylinder block and cylinder head. Eventually, because the heads warp and the o-rings at the passages shrink, the head gasket leaks oil. It's not serious. You won't lose oil pressure or "blow" a gasket. It's just unsightly. And, it is a given on the early Honda SOHC four, the smaller engines (350/400/500/550/650) especially.
As you prepare to do this job, gather the parts you will need. As far as I know, you cannot get the head gaskets from Honda now. You will have to use aftermarket replacements. That is a problem, because the quality of many of the aftermarket gaskets leaves much to be desired. Cometic probably comes the closest to OEM. The others (including Vesrah)are of far less quality. Besides the head gasket, you will of course need several o-rings. These include the breather cover and rocker box o-rings on the smaller fours, the camshaft end-cap o-rings on the 500/650, and of course the oil passage o-rings themselves on all models. For the 750, don't forget the cam cover gasket and the cam tower passage o-rings. Also get the cylinder head sealing wafers, tach drive o-ring and oil seal, and the valve adjustment cap o-rings. Even if they never leak, replace the valve adjustment cap o-rings to prevent the caps from seizing to the engine. If you have a 500, or a 550K whose engine number is under 1083641 or 550F whose engine number is less than 1133172 (these are both 1976 models), pick up an additional complete set (four) of the camshaft end-cap o-rings. I'll show you why later. Lastly, be preapred to also get the cylinder base gasket and its oil passage o-rings. It is very difficult to pull the head without wiggling loose the cylinders and breaking the seal at the cylinder base gasket. If you do, it will leak. Contrary to common folk wisdom, removing the cylinder block will not require the piston rings to be replaced, or the cylinders to be honed, as long as you leave the pistons and rings alone. So don't be afraid to pull the cylinders. The extra few minutes will save you much headache later. For the 500/550, you also want to get the two aluminum sealing washers under the camshaft end-cap hold-down brackets. Be careful to get the right washers. On the same hold-down bolt is a steel washer also. Early versions of the parts microfiche have the part numbers for the two washers switched. You could play it safe by ordering (none of this stuff is in the dealer's stock) all four washers. Also get the six washers under the rockerbox hex bolts. Round up your parts gathering by purchasing the exhaust gaskets (all models except the earliest spigot exhaust 750s) and intake manifold o-rings (smaller fours). The manifold o-rings have a habit of leaking air into the engine after a few years. While the head is off, replace 'em.
This may seem like overkill, but since you don't take the cylinder head off very often (and on the 750 it's an engine-out affair, except for the automatic), consider also replacing the cam chain. This is easily done from the top end, even on the 650 which has the link-plate ("Hy-Vo") chain. Finally, if you are really ambitious, you might consider, while the head is off, replacing the valve guide seals. Age-hardened seals cause smoking in the exhaust. New ones are still available from Honda, and although they are pricey, they are simply better than anyone else's.
There is no need to remove the carburetors on most bikes if you still have the stock airbox. Simply tie the carbs back with a bungee cord after loosening the clamps at the manifolds. The upper parts and the head come off pretty quickly. On 1976 and later 550s, leave the four nuts inside the breather cover alone. It is very important that you find a genuine #3 Phillips screwdriver tip to put in your impact driver when disassembling one of these engines. The Phillips screws used all over these engines are made to fit this special size. That is the way to avoid stripping the screws out. Once the head is off, apply aerosol gasket remover to the stuck-on pieces of the torn head gasket. Every ten minutes or so, scrape using a window scraper (this is a single-edge razor blade held in a special holder, used to remove paint from window panes). Do not use one of those high speed burnishing disks to remove the gasket material. The disc will gouge the soft aluminum of the head. Once most of the gasket is off, splash a lot of solvent (gas is okay, but be careful!) on the head while scraping it with a flat file that has a piece of very coarse emery sandpaper neatly folded around it. This will get the head sparkling clean, and leave it flat and ready to receive a new gasket. Clean the cylinder deck similarly, being careful to avoid dropping debris down the cam chain tunnel or into the cylinders.
On the 750, keep the cam tower caps oriented after they are removed. They are coded to the cam tower. Check the cam tower for wear -- this is a problem area on the 750. Also check its oil passages. Spray them as you did the orifices. If you have an F2 (77 or 78 black engine with oversize valves), you simply must check the valve guides. Remove the springs and see how much the valves wiggle in their guides. Typically, the valve will wiggle like a pencil in a coke bottle. Bad news. For various reasons the F2 wears out its factory-installed exhaust valve guides in 5~10,000 miles. Replacement guides from Honda were modified. They were harder and had to be specially installed. I do not know if these special guides are still available.
Carefully examine the cam chain tensioner. The 500/550 tensioner is prone to rusting up frozen. Make sure it works. Remove the oil orifices (jets) from the head, and from the cylinder base if the cylinders are removed. Spray brake cleaner through them to ensure they are clean. On the earliest 750s, the oil jets in the head are not removable. Still verify they are clean however, with the brake spray.
Once everything is inspected (replaced?) and clean, take the new head gasket and liberally coat it on both sides with Gaskacinch. This is an old Honda mechanic's trick for extending the life of the gasket. After that first coat dries, repeat at least twice for three full coats, allowing each coat to tack dry before applying the next. The gasket should look dull yellow when finished. If the cylinders are off, install the new base gasket perfectly clean, without sealer. Remember the base oil orifices, and their o-rings. It is best to use ring compressors when installing a four-cylinder block, but it can be done without them. The beginner almost always breaks a ring, however, without the compressors. Once the cylinders are on, put the head gasket and cam chain guide in place, and install the head. Remember the unusual cam chain guide arrangement on the 500/550. It goes in the middle of the chain. Torque the head per the manual, and don't forget the upside-down bolts. After torquing, install the sealing wafers -- don't put anything on them. On all the smaller fours, put a new large o-ring in the rockerbox o-ring groove. To get the o-ring to stay in the groove, lightly put some Gaskacinch in the groove (make sure it is clean first) and some on the o-ring. Let them both tack dry, then carefully install and it will stay put. On these same engines, slacken the valve adjuster screws before installing the rockerbox, per the factory manual. This is the best way to prevent bending the valves. On the 750, be sure to put all four of the oil passage o-rings under the cam tower.
If you have a 1971 through early 1976 500/550 whose engine number is under those mentioned above, it has a rockerbox whose rocker shafts rotate while the valves open and close. This is an oversight in design that leads to prematurely worn rocker shafts and rockerbox. A kit was made available back in the old days which locked the shafts to the casting. There is a much easier fix however. Take the extra four cam end-cap o-rings you purchased. After removing an end-cap, place one o-ring in each rocker shaft hole, up against the shaft, then reinstall the end-cap. When the end-cap is reinstalled, it will compress the extra o-rings against the shafts, and keep them from rotating.
After the valves are adjusted, you will need to tension the cam chain. Each model engine has its best method. The 750 is the easiest. It is done without the engine running. Simply turn the crankshaft to TDCC #1, then turn it about 15 degrees farther (in the running direction). At this point, the mechanical advancer's spring retaining pin will be close to the crankcase mark. Loosen the cam chain tensioner locknut and bolt, then carefully retighten both. The 500/550 may be done either running or not running, but the best method is not running. Do the same thing at the crankshaft, but after loosening the cam chain tensioner locknut, turn the tensioner screw with a stubby screwdriver. The factory valve adjusting screwdriver works best, as it allows a lot of leverage to turn that stubborn screw. Do the 650 not running. Turn the crankshaft to the magic place, then loosen the cam chain tensioner locknut, and carefully apply slow, steady turning pressure on the crankshaft while retightening the locknut. Do not ease up on the crankshaft until the tensioner locknut is snug. Turn the crankshaft very slowly but steadily. This is the correct way for the 650, and is usually the only way to quiet a noisy cam chain. The 350/400 is done running, but usually requires a careful, gentle push through the plugged hole at the front lower part of the engine, While the engine is idling, loosen the tensioner locknut and bolt. Remove the special plug bolt and insert a Phillips screwdriver. Gently push upward against the tensioner mechanism while snugging the tensioner bolt, then the locknut. Be really careful. If you push on the screwdriver too hard, you can damage the tensioner. Synchronize the carburetors to polish off this job. They will need to be synchronized because the top end work will upset the uniformity of airflow through the cylinder head.