Motorcycle throttle sticks / idles too high & too low, Ideas?

Question

Hey. 1982 Yamaha Virago 750cc. I recently rebuilt and cleaned the carbs and flushed the fuel system. Now it either idles really high, like 2500 rpms, or it bogs at 500 and dies. Nothing in between. When I hit the throttle, same problem, either no power at all, or it gets stuck at the top of the twist and screams at 5 or 6000 rpm (making a ton of noise).

My current thoughts are - 1 re-clean the carbs, and lubricate the throttle lever. 2 - remove old throttle cable and replace with new, lubricated cable and adjust the limits. 3 - adjust the return spring on the throttle lever.

Your thoughts?

JMB

Answer 1

When you cleaned the carbs, did you check the throttle cable and twist grip for smooth operation? One way is to disconnect the cable from the carbs. You should be able roll the twist grip (throttle) back and forth and the cable will move in and out of the housing. The throttle should twist smoothly without binding. If this is not the case, dismantle the twist grip then clean and lube. The cable should be replaced if damaged, either the housing or the cable itself. If it looks OK, then use a cable lubing tool (such as MotionPro Cable Luber part 08-0182) to inject cleaner then fresh lube into the cable housing. Since the bike is a 1982 model, there is no telling when this might have been done last.

The throttle linkage on the carbs also should be clean and move smoothly. This should be done before reattaching the throttle cable. After assembly and hooking up the cable, the throttles should open smoothly and return to the closed position when the twist grip is released.

You didn’t say anything about preliminary carb settings. After you dismantled a carb, the air screw and throttle stop (idle) screw both need to be adjusted, this part of synchronizing your carbs. With two carbs with mechanical linkage, the linkage adjustment will be part synchronizing process.

You mentioned the throttle lever in your question; perhaps you mean the choke (cold start) lever. The choke lever and cable should also be cleaned and/or replaced as needed.

After you have done all of this, then you can begin to trouble shoot the carburetors themselves. That bike uses Hitachi HSC40 carbs. These are CV (constant vacuum) carbs where the slide moves up and down depending on the vacuum in the manifold. The throttle plate (butterfly) is moved by the twist grip. Common problems with CV carbs are leaky diaphragms at the top of the slides and worn throttle shaft pivots. Either both or one of these could cause the problems you described.

Above all, if you haven’t got Shop Manual, get one.

Answer 2

You can try the throttle cable first. But with it at low idle or high rpm and nothing in between you should take a look at the Carbs.

Answer 3

First make sure there's no vacuum hoses that are disconnected because that will cause eratic idling. On my bike, rust formed between the handlebar and the twist grip causing throttle problems, I had to remove is, sand the handlebar, greased it and that was the end of that problem. When you open the throttle and let go, it should snap shut. If it doesn't, look for control problems like dry cables, throttle cable kinks of the twist grip.

Answer 4

If the carbs bolt on you may have tightened it to much. My buddy did this on a 1980 honda dirtbike. It actually deforms the carb and the slide gets stuck.

Answer 5

its an 82 and your skeerin me with the carb Qs , first if there coroded and look bad they are. hit ebay, and if your throttle is sticking due to a worn slide, hit ebay, if all looks good a good cleaning and sync. usually does the trick, but never take your ign. sys. for granted, Ive seen that mistake too many times, you do know how to sync a set of carbs right?

Answer 6

throttle cable or air intake leak.

Answer 7

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