will england :: motorcycle notes and tips
will england :: motorcycle notes and tipsLots of listers have, or have had, or definitely will have someday, carb problems. Problems usually occur when the bike has been sitting for months (or years) without use, and without the gas being drained. When this happens, the gasoline in the tank slowly trickles down into the carbs and dries out and forms gum, varish, and corrosion in the passages of the carb. So, take a hint, and avoid a lot of future headache, and either ride your bike regularly, or drain the gas from the tank and carb bowls for storage. The resulting problems are usually poor idle characteristics, either too lean or too rich, and unstable idle, fouled plugs, and all the other ailments you've read about. Getting these little things clean is a real bear. You generally don't get it all on the first cleaning, or even the second or third! In the hopes that you will not have to do it as many times as I have (and many other listers), I offer the following advice and procedures. I will not get into a detailed description of each carb model and how to dissassemble it--you will need to get the shop manual for your bike and study it. First, do NOT dissassemble the carb bodies from the rail they are mounted to, unless there is some good reason to upset this alignment. Otherwise you will have to carefully align them later. See the manual for how to do this. DO NOT remove the throttle plates and shafts either. Check the throttle shafts for looseness. They should move smoothly, but have no perceptible play. If they have play, more air will get sucked in around the shafts, leaning the mixture and causing poor idle. The only reasonable solution is to replace the carbs (this is the reason why you need to oil the carb shafts from time to time). How much play is too much? I'm not sure, but I'd be suspicious of any play you can feel. Second, get some Gumout carb cleaner in the spray can with the little red spray tube. My experience is that the Gumout brand is at least as good as any other, and far better than some of the off-brands. I find that stuffing that tube up into the passages as far as I can and spraying liberally works better than compressed air. Also get a can of carb cleaner for soaking the carbs in. It needs to be at least big enough to submerge the lower halves of the carb rack into, in an appropriate container, or get two at a time into, or some such arrangement. Remove ALL the rubber parts, choke/enrichment valve, slides, needles, fuel jets, air jets, float valve and float, pilot (idle) screws and their dinky o-rings, etc., and clean them separately (if you let them soak in the heavy duty carb cleaner, it will destroy them). Usually the dunk treatment is not required and should be considered a last resort after you've disassembled and cleaned with spray Gumout. If you dunk, then you may need to separate the carbs, so that the pipes and O-rings between the carbs can be removed. The "emulsion tubes" in the center of the carb are critical for good transition from idle to part throttle operation. These press out the top on the Hitachis (I don't know about the Mikunis) after you've taken out the main jet, slide and needle. They may be pretty well stuck and gummed in there. These have tiny holes drilled in the sides of the tube which typically get clogged--make sure they are clear. In the side of the carb bowl, there is a "well" that a brass tube extends down into. The end of the brass tube has a small jet in it. This is the jet for the choke (or "enrichment" or "starter") circuit. It is pressed into the carb body, and is not replaceable (except maybe by a machine shop). There is also a small hole from the main part of the bowl to the bottom of that well. If the brass tube/jet or the hole to the well is clogged, then it makes for hard starting, but otherwise DOES NOT affect warm running. If you have any problems with normal running, this is not what you should be looking for. If you have problems with cold starting, this is the place to concentrate. Because they are in the bottom of the bowl, these parts clog easily. If you have to soak the carbs, then after you have done so, spray out all the passages with the Gumout. Put the carbs back in the cleaner to get the other half, let soak overnight, and spray out again. COMPARE the amount of spray flow you get through the passages from one carb to the next. If one flows significantly more or less, you're not done cleaning! There are several passages that travel around inside the carb body. Make sure you follow them from start to finish and get them clean! You must check the jets to make sure they weren't drilled or messed with. After 15-20 years and unknown past owners, the chances of this having been done are fairly high. I used numbered drills as gauges to compare mine to known good ones, then I wrote down the diameters in my shop manual for future reference. You need a really small numbered drill set, in the range of #60-#70 or so. Jets do not "wear out." They should remain the same size forever, unless you put acid in your gas tank or something... If they are not original, then you have to make a decision. If the mains have been drilled, but you personally know the bike ran excellent before the latest gumming up, then you can probably continue to use them (I say "probably" because you know it worked right previously, but I won't guarantee the results). If you do not know that, then you must get new ones of the correct size. If the pilot/idle jets have been messed with, get new ones, period. DO NOT think that making the jets bigger will increase performance. These bikes performed about as well as possible for the stock configuration, and the road tests of the '80's prove that out. The objective you SHOULD be pursuing is to get the thing back to stock settings and sizes, because we know it runs right if you do that. ONLY AFTER you have it running right in stock form should you consider changing anything, and then, only if the exhaust or cam or carbs were changed. (if you intend to experiment with different jet sizes, DO NOT drill the original jets--get replacements in the proper sizes. You want to save the originals so you can get back to the original settings when you run into problems--and you will...). Replacement jets for Hitachis: I have used round Mikuni main jets to substitute for the original Hitachi jets. The Mikunis are a slightly different thread (.8mm, vs. .75mm) so you need to put the threads into a die of the correct thread pitch to rethread them slightly (the difference is small enough, over a short enough reach that you won't need to re-thread very much). I discovered that the 107.5 Mikuni is about the same as the 110 Hitachi, at least, they give the same spark plug color at full throttle runs. I found this out by using my drill bits as gauges and comparing the Mikunis at a friendly local dealer. I don't know if the pilot jets will interchange or if the sizes are the same, but I suspect they are. I've heard that Kehin carb jets have the same thread size as the Hitachis. I haven't checked, but you can try them too, use the drill bits as gauges to get the same size holes. Enrichment/choke circuit: This can cause rich running, especially at idle. I had this problem myself on one of my carbs, and I may still be having that problem on another. You have to remove the enrichment valves on the top front (Hitachis) and make sure they are clean. If they are not, the valve won't seal completely and extra fuel will get sucked into the airstream. Basically, it's like you didn't get the choke all the way off on that carb. Those of you who are complaining of black soot-fouled plugs are prime candidates for this problem. After you get the enrichment valve re-assembled, suck on it or spray Gumout from the supply passages to make sure none of it comes through the valve. Slide/diaphragm/needle and air jets: Remove the cover on top to pull out the spring, and slide/diaphragm assembly. Check the diaphragm for tears and cracks. The slide is a simple aluminium pistion, but is coated with anodizing or teflon or something to make it slide easily. Make sure it moves freely in the bore. The needle extends below the slide and pokes down into the main emulsion tube. Check it for damage, compare all 4 for same height, etc. Someone may have tried to raise the needle in the slide, to get a richer mixture at wide throttle settings. The way the needle is held and adjusted is different on the Mikunis and Hitachis, so use your mechanical sense to see if someone has messed with them. If in doubt, get advice. I don't know what the factory needle height is and there is no spec in the books for it. I understand from other list members that small changes in height (on the order of 1mm) make significant differences in mixture. I haven't messed with this, so I don't know for sure how sensitive the relationship is. Pilot Screws: You must get these out, first by removing the brass or aluminium caps that cover them (on the top front). First turn the screws down to bottom, counting the number of turns. When you put the screws back in, put them all the way to the bottom, then back them out the same number of turns. Don't forget the tiny o-rings, washers and springs, in the same sequence, or it WILL run poorly at idle. Be careful when screwing to the bottom. You want the screw to touch bottom, but don't ram it in there, or you will damage the screws/seats, again causing poor running at idle. Use your judgement. These screws have a long tapered needle that is easily damaged--don't drop them on the floor! Somewhere between 2-1/2 to 3 turns is the usual factory setting. Float height: This is critical to correct operation of the carbs at idle and full throttle. Various listers have described using the gas tank hooked up to the carbs off the bike so that they can easily check and adjust the float levels. That's fine and a good idea, but you must also measure the float level with the tube attached to the drain valve with the carbs on the bike and the engine running, to be sure. Set it to shop manual specs. Don't try to make it richer by raising the float level. If anything, you want to err on the side of lower float level than too high. Reasssembly: Follow the shop manual. Also, check the opening and closing of the throttle plates and synchronize them by checking visually and with a strip of paper to make sure they close together. The strip of thin paper is a feeler guage to make sure that one plate isn't hanging slightly open when the others are closed. Put it in the throttle bore on each carb and let the plates close on them. The effort to pull the strip outward should be the same on each bore. If you do this, the carbs will be very closely synchronized before you put the carbs back on the bike, and it will run well. Then do the running sync check to double check and fine-tune. This static sync will also help you find any worn throttle shafts. Finally, lubricate the throttle shafts with oil or chain lube. I like chain lube because I think it stays in there longer. Work them open and shut a many times to work the lubricant into and around the shafts. Vacuum cleaner test: This is something I dreamed up--put a vacuum cleaner hose on the intake port side of the carb so it sucks air through the carb. The slide should move up and down easily. WARNING: Do not do this with fuel or cleaner in the carb bowl, or you may explode your vacuum cleaner (and anything nearby). After you have them re-assembled, float levels set, and back on the bike, then you need to road test and read the spark plugs for correct running. Hopefully you got everything clean the first time and it runs like it should! Reality is, unfortunately, probably not. Does it run good at idle/low RPM and low throttle, but not a high RPM and full throttle? Then the problem is in the main fuel circuit. Since the main fuel circuit is reasonably large and direct and easy to clean out, that is usually not the problem. Does it run good at high RPM and throttle, but not at idle or low RPM? Then the problem is likely in the pilot circuit. This is the most common kind of problem. Is it running rich or lean? TO figure this out, you need to test at both low throttle and high throttle conditions. Low throttle: Run around the neighborhood at low throttle first, not getting above 3000 RPM. Shut it off and check EACH plug. Those plugs that are clean, tan, or medium brown are correct mixture. If the plugs are black and sooty, then you know those cylinders are too rich. You may be able to adjust the idle mixture with the pilot screws, but probably not. Go test it again. If that doesn't work, you will need to remove and re-clean the pilot circuits and choke/enrichment valve. Sorry, that's the only way to do this. High Throttle: Run out on the highway at 1/2 throttle or better for about 5 minutes or more. This will likely put you into the 80MPH range, so pick your test track carefully. After running along for the 5 or more minutes, hit the kill switch, pull in the clutch, and pull over on the shoulder. You cannot slow down with the engine running, or you won't get correct results. Check each plug again and record your results. Chances are the plugs are okay, especially if it ran okay. Same reading method applies. If any are sooty, then you will need to re-clean the main circuits in that carb and recheck the float level. Backfiring, spitting and coughing typically indicate one or more cylinders are too lean. This is the same kind of behavior you get when you push the choke off too soon (and for the same reasons). In fact, you can sort of troubleshoot lean mixtures by adding a little choke to see if that improves the running. If it does, you're too lean. If one or two cylinders are off and the others are okay, according to the plug readings, it will seem like the whole engine is running poorly. I've had that experience myself. Unfortunately, there are no shortcuts. You will have to read the plugs, re-clean troubleshoot, re-check everything, and re-test until you nail it. Or give up in frustration first... Pete K. '82 650 Seca
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