Before the Rebuild

Carburetors really get a bad rap. They stick out, they're an easy-to-grab assembly, right there in front of you, so hey, blame them! Also mysterious to most folks, and somehow representing all that is thought to be the "tuning" of an engine, though that is also false.

Carburetors can make or break the rideability of the bike, especially on some models having certain built-in compromises that careful tweaks to the carbs can overcome. But at the end of the day the carburetor is not the key to a good running engine. The engine itself is. Its cylinder compression, its ignition system health, its correct assembly. Carburetors make a heck of a difference, but they are the icing on the cake, not the cake itself. Not the first thing but the last, in the engine tuning formula. Here then are some suggestions for doing due diligence on the parts of the engine the carburetors support, with an emphasis on issues relative to specific motorcycle models. First the generic stuff.

Cylinder compression: Compression on any 70s through 80s street bike has to be -- has to -- be 150 psi minimum or you are wasting your time messing with the carburetors. This is a fact and a somewhat unfortunate one when it comes to vintage Japanese motorcycles as they tend to lose compression over time due to built-in defects in their cylinder heads.

Valve clearances: Valve clearances are important! No matter the engine, they in large part conttol carburetion. In addition, in certain engines carefully adjusted valve clearances are the only reasonable route to even moderately correct performance.

Fuel tank rust, water, and varnish: Another area needing attention on all vintage bikes. Don't overlook it. This trips up most home mechanics.

Air filters: The best air filter is the pleated paper type that is stock on most Hondas. A few Honda models received a high quality version of a foam filter, the V4s notably. But even that is not as good as paper. Nothing is. Not foam, not gauze, nothing. In fact, foam is the worst, especially the aftermarket foam filters. Gauze is a close second, and in some ways worst of all because it is so hyper-marketed. Now for the model-specific tips.

Honda GL1000: You simply must do ignition inspection and timing way before carburetors even cross your mind. Ignition timing, and just as importantly, the brand of points and the proper installation of the parts, and the condition of the mechanical advancer especially, are crucial. Nothing is as important. Not your carbs or the kind of gas you use, none of that. In fact I find three things wrong on every GL1000 I service. The points are mis-timed aftermarket junk, three out of the four spark plug caps are bad, and the fuel tank is rusty enough to require removal and repair.

Honda GL1100 and GL1200: One thing that supercedes the importance of the carburetors on the 1100 and 1200 is this bike's tendancy toward failing ignition pulsers (triggers). Don't trust any manual, and especially the official one, for the correct tests for the pulsers. No good. Use a dynamic test. I have written about this elsewhere.

Honda GL1500: Before condemning your carburetors, check for the vacuum hose fittings being loose. Extremely common, so much so it is classic.

Honda DOHC and CBX: The big bugaboo on this range of Hondas is cylinder compression. See my articles on this. All of these bikes need valve jobs. A workaround is possible through valve clearance adjustment however.

Honda SOHC fours: Ignition timing again, just like the GL1000 and for all the same reasons. Non-stock points, mechanical advancer problems, and bad plug caps.

Honda V4, including the ST1100: These bikes exhibit issues with their vacuum petcocks. They also have choke problems, most of it linkage related.

Honda CX/GL500/650: The earliest models have ignition issues. The CDI system goes belly up.

70s Honda CB350/450 twins: Ignition timing, right? Again, issues with non-stock points and advancer condition. Add to this improper carb sync, very very common and the responsbility of the vehicle owner because these carbs cannot be bench-synced.

Final thoughts: Do a compression test before sending your carbs off. Don't talk about compression being "okay." "Okay" is not a compression value. Get numbers. If any one of the readings is below 150 psi it is imperative that you find out why. Do this by performing a leakdown test. Do all the cylinders, one at a time. The cylinders that indicated low compression will reveal higher than normal cylinder leakdown, that is, above 10 percent. While the air is flowing, listen at the three places: airbox intake, exhaust muffler, and crankcase (by removing the oil dipstick). Air sounds at intake or exhaust point to valve issues. The same sound at the dipstick hole to ring and cylinder problems. The valve issues may and usually are as simple as the valves being too tight, something that is so common every career mechanic could write a book on it. Loosen the valves that are in trouble and retest leakdown. If no improvement, a special technique is needed. With the leakdown tester still connected, gently tap on the valve while watching the leakdown gauge. If tapping on the valve makes the leakdown go down, good. This means it is only carbon causing the leakdown and this will improve with engine operation. But you will have to readjust the clearances in this case. If tapping does not improve the leakdown, and you're sure there is adequate clearance at the valve, then the cylinder head has to come off. No two ways about it. Similarly, don't believe it when someone tells you a newly-assembled engine can expect to initially have lower than normal compression and/or higher than normal cylinder leakdown. It just isn't so. A properly assembled engine will have the highest cylinder compression and lowest cylinder leakdown within minutes of being first run. It will never get better, and anyone that tells you this is not telling you the truth. There is definitely such a thing as break-in, but not in the area of cylinders and valves, not if the builder used proper engine rebuild techniques.

Last updated January 2022
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