Rough running '85 Honda VT700C
in [Tech]
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From: drsuaveiguana on 2 Aug 2007 23:40 I would appreciate any advice on this. I have an '85 VT700C with about 32K miles. When I got the bike about 4 years ago there were acceleration and top end deficiency issues. Otherwise the bike ran fine. I had not previously done much work with motorcycles, so I ignored the problems for about a year before deciding to do something about it. After many diagnostic tests (hoping I wouldn't have to get into the carburetors), I found the source of the performance issues - two torn carb diaphragms. I replaced the diaphrams and regained the missing power. But, as luck would have it, another problem revealed itself. The bike would hesitate when quickly opening the throttle. I let that issue go for another season and dug back into the bike to find the cause. After another series of tests, I ended up checking the carb synchronization and found them off. While the carbs were off the bike, I did a thorough cleaning of the jets and ports. I reassembled and synced the carbs and that did the trick. Rapid throttle response, good acceleration, etc. The bike ran wonderfully that way for about 3 months. Now problems again. After about a month of not riding, I took the bike out. After about 20 minutes, I was on the interstate going around 70 mph. I accelerated to pass a truck and the engine suddenly tripped over itself, cutting out instead of accelerating. I took an exit and managed to get back home, but the bike sounded bad and wouldn't break about 45 mph. Now the bike will start fine and idle alright, but it is stumbling again on rapid throttling. It also runs roughly as the rpms climb. There is a lot of hesitation and backfiring. I have checked the fuel, fuel filter, diaphragms and slides, compression, and sychronization. All appear good. The plugs are fairly new, but I did check them out also. With the bike running, I removed each of the 4 spark plug wires (each cylinder has two) one at a time to see if one of the plugs was perhaps not firing. I noted decreased performance when each plug wire was removed. I figured that meant that each one was actually firing alright. One other thing happened today as I was testing compression. The battery seemed suddenly drained. This is odd because the battery was new as of the spring. It has not given me any trouble at all. Then, as I was starting to crank the engine for the compression test, it weakly turned once or twice and gave up. I used a set of jumper cables to get through the test and I am charging the battery with a charger overnight. I'm not sure what to look for next on this bike. I also don't know what the deal is with the battery, and the timing of the battery failure seems a very odd coincidence. Any suggestions? Thanks, Darren
From: caraway.r on 3 Aug 2007 11:21 On Aug 2, 10:40 pm, "drsuaveiguana" <u36331(a)uwe> wrote: > I would appreciate any advice on this. > > I have an '85 VT700C with about 32K miles. When I got the bike about 4 years > ago there were acceleration and top end deficiency issues. Otherwise the > bike ran fine. I had not previously done much work with motorcycles, so I > ignored the problems for about a year before deciding to do something about > it. After many diagnostic tests (hoping I wouldn't have to get into the > carburetors), I found the source of the performance issues - two torn carb > diaphragms. I replaced the diaphrams and regained the missing power. But, > as luck would have it, another problem revealed itself. The bike would > hesitate when quickly opening the throttle. I let that issue go for another > season and dug back into the bike to find the cause. After another series of > tests, I ended up checking the carb synchronization and found them off. > While the carbs were off the bike, I did a thorough cleaning of the jets and > ports. I reassembled and synced the carbs and that did the trick. Rapid > throttle response, good acceleration, etc. The bike ran wonderfully that way > for about 3 months. Now problems again. > > After about a month of not riding, I took the bike out. After about 20 > minutes, I was on the interstate going around 70 mph. I accelerated to pass > a truck and the engine suddenly tripped over itself, cutting out instead of > accelerating. I took an exit and managed to get back home, but the bike > sounded bad and wouldn't break about 45 mph. > > Now the bike will start fine and idle alright, but it is stumbling again on > rapid throttling. It also runs roughly as the rpms climb. There is a lot of > hesitation and backfiring. I have checked the fuel, fuel filter, diaphragms > and slides, compression, and sychronization. All appear good. The plugs are > fairly new, but I did check them out also. With the bike running, I removed > each of the 4 spark plug wires (each cylinder has two) one at a time to see > if one of the plugs was perhaps not firing. I noted decreased performance > when each plug wire was removed. I figured that meant that each one was > actually firing alright. > > One other thing happened today as I was testing compression. The battery > seemed suddenly drained. This is odd because the battery was new as of the > spring. It has not given me any trouble at all. Then, as I was starting to > crank the engine for the compression test, it weakly turned once or twice and > gave up. I used a set of jumper cables to get through the test and I am > charging the battery with a charger overnight. I'm not sure what to look for > next on this bike. I also don't know what the deal is with the battery, and > the timing of the battery failure seems a very odd coincidence. > > Any suggestions? > > Thanks, > Darren Darren, I have no V-twin experience, but can give an idea or two to try. I'm not sure where you are geographically, but if you can find the product called Seafoam, pour about half a can in your fuel tank when it is down close to reserve territory. Crank the bike and let run for 10-15 minutes or better yet go for a 10-15 minute ride. This should have provided plenty of time to allow the gasoline/Seafoam mixture to reach the carbs and engine to warm up. Let the bike set overnight. Next day, top off your fuel tank and go for another spin. If you notice improvement, another stiff treatment or two may finish cleaning things up. If not, it's still a good habit to repeat every month or so to keep the fuel system clean. Next check and double check your fuel filter, replace if older than a year. Check to see if your fuel cap is supposed to be vented or not. These last two items will allow startup and idle but cause fuel starvation at higher RPM's for extended periods. It sounds as though you did a good job of cleaning and synch'ing to begin with, though on a Yamaha Maxim 650 I had, the carbs had to be removed 4 or 5 times before I finally got all the gunk out of every possible cranny. Another possibility is that a component of your electronic ignition may be heating up and failing, but the fuel end is a bit easier/ cheaper to address first. Hope this helps, Ron
From: Albrecht via MotorcycleKB.com on 3 Aug 2007 11:37 caraway.r(a)gmail.com wrote: >I have no V-twin experience, but can give an idea or two to try. >I'm not sure where you are geographically, but if you can find the >product called Seafoam, pour about half a can in your fuel tank when Yannow, so many people have been ranting about how wonderful Seafoam is, I had to go and look up the MSDS and find out if there is anything to the claims. Auto parts stores around here don't even sell that stuff. If you want to store your outboard motor or get some oil on the cylinder walls in the spring, Seafoam would be okay for that use. That's what it's really for. But it's not much of a carburetor cleaner. I haven't found anything that works like Berryman B12 Chemtool Choke and Carburetor Cleaner. 4 or 5 ounces in a tank full of fresh gasoline will clean all the gum and varnish out of the carburetors. -- Message posted via MotorcycleKB.com http://www.motorcyclekb.com/Uwe/Forums.aspx/tech/200708/1
From: drsuaveiguana via MotorcycleKB.com on 3 Aug 2007 14:11 Dave wrote: >I went through this a few years back, also on a '73 CB, a CB750K. Few >things to check: Dave, Thanks for the comments. The day was warm, bright, and sunny. The bike was near the low fuel warning light level when the initial problems occurred on the interstate. First thought was that the bike was running out of gas, and that perhaps the warning light was burned out. At the bottom of the interstate exit ramp, the low fuel light came on. That meant I had somewhere near a half gallon of fuel left. I filled the bike up immediately with fresh fuel, but have noted no change in symptoms after close to 45 minutes of run time since. I replaced the fuel filter with a new one as a first step, but it made no difference. I think your questions about a vacuum-controlled fuel petcock are not applicable to this bike. My petcock is simply a shut off valve (the warning light serves as the "reserve tank"). The fuel pump follows immediately after the filter and supplies the carbs with gas. You may be onto something with the power supply to the coils, though. I hadn't made the connection about the ground fault, but it makes sense. If something shorted, it may have pulled substantial load from the coils causing an immediate loss of power. This would also explain the drained battery. I will try the fuse isolation trick when I get the chance. The plug wires are only about 2 years old and they still look great. If I keep hitting walls, though, I'll check them out. Thanks for your time. Darren -- Message posted via MotorcycleKB.com http://www.motorcyclekb.com/Uwe/Forums.aspx/tech/200708/1
From: drsuaveiguana via MotorcycleKB.com on 3 Aug 2007 14:14
Thanks for the suggestions. I will let everyone know what I find out. Darren -- Message posted via http://www.motorcyclekb.com |