I've got a cylinder that wont ground out and a crazy carb!

[herrhead]

I just made myself a set of sparkplug extensions for synchronizing my carburetors using the “shorting method.” I used a pair of sparkplug collars crimped onto a set of modified spokes, a design recommended by various online sources. Long story short, my left cylinder does not seem to short out when I ground it with a screwdriver. The right cylinder cuts in and out just fine, but not the left. I’ve switched the extensions, the plugs and the plug wires to see if the problem would go to the right cylinder, but it didn’t.

I also noticed that the left carb wasn’t responding quickly to any throttle input on the throttle arm. I would lift the throttle arm and the idle wouldn’t increase and then after a few seconds, suddenly rev up and stay there even if the throttle was released back down. I suspected that the slide was sticking and sure enough it was. I opened up the top of the carb, checked the diaphragm and closed it back up. I checked the slide operation with my finger and it seemed fine, until I fired the bike back up and tried shorting out that cylinder again…same thing would happen: the cylinder wouldn’t cut out and when I fiddled with the throttle arm on that carb, I got the delayed throttle response, sudden high revving and then sticking.

So my questions are: what would cause the left cylinder not to short out? What are some causes (and fixes) for a sticky carburetor slide? And, would there be ANY chance that these two issues are related?

BTW, this is a ’72 R75/5 that was recently purchased from the original owner’s friend. The bike has 20K miles and had been sitting for 20+ years. I’ve recently rebuilt both carbs replacing all o-rings, jets, needles, etc. I changed all of the fluids, retorqued heads, checked valves, set timing, replaced points, condenser and plugs…the bike fires right up.

Any help or suggestions would be greatly appreciated. Thanks.

[Polarbear]

I am not a fan of grounding the plug firing with a screwdriver to start with. I've never done that and though it may work for many, its shadetree mechanic tech! I assume the /5 still has the points, or? Is it electronic ignition? Dyna 3 is the one to use for this bike, if not already in there. Many have suggested the shorting out damage occurring to the system, when disconnecting wires from the plugs and the spark not grounded.Hmmmm? I've done "hundreds" and never had this occur, by simply pulling a wire from the plug "before" starting the bike and starting the bike on one cyclinder. The loose plug wire must be free from grounding itself to anything, so keep it safely away from metal parts on the bike. You can pull the wire as the bike is running, but had better make a quick separation to avoid spark jumping. I like the one cylinder start up and reverse for the other side carb adjust. Just set the idle screw high enough to start on one cylinder and adjust after running. Turn it out until the one cylinder nearly quits running and then go do the other side, using the same plug wire proceedure for running the other cylinder. Its better to do this with the engine fully warmed up, as a cold engine carb will perform differently from a warm one. After all this, you can adjust your cables, with sufficient slack. Where the cable attaches to the carb on top at the dome, you have an end to the cable sleeve that fits nicely into dome, where the cable becomes exposed on its way to the actuating throttle arm. This is where you can find your slack for comparison to the other carb. Use enough slack to keep the cable sleeve end from dislodging itself and nothing more. This is usually around 3-5mm, guesswork:). Using a carb synchronizer is always nice,BUT one can get soooo very close if not perfect with some practice without it. It takes some experience to get good at it, but its possible. Using your finger as a synchronizer, you can actually touch one throttle arm and watch the other as you move the throttle grip for comparisons. It works, but practice makes perfect. Your carb sticking issue is probably a dirty something in there. The carb slides have two slide areas of concern, if a domed carb is at hand! The main slide in the venturi and the dome slide up top. The dome slide(above the diaphram) goes unnoticed by most and is subject to grit and or gummy residue and will make headaches for those trying to figure out why? Go see again if in fact the dome slide is very nicely cleaned up with no scratches, burs, etcetc...Good luck with the /5 as its a fine machine indeed and will carry you far. I'm fairly nearby in Clovis,CA and have been on Airheads for 35 years and the fun still remains. I ride the famed R100/7,'78 and its been so fine all these years. The Dyna 3 ignition system will make your/5 a better running bike, too and its still available. Happy Trails, Randy13233"Polarbear":D

[benway]

" Long story short, my left cylinder does not seem to short out when I ground it with a screwdriver. The right cylinder cuts in and out just fine, but not the left. I’ve switched the extensions, the plugs and the plug wires to see if the problem would go to the right cylinder, but it didn’t. "


If you short out the LHS, it will be running on the right only.

when you short out the RHS, it will be running on the left only

are you saying that when you short the rhs, the bike dies? amd when you short the lhs it doesnt?

if so, it seems possible that your LHS is not running at idle, which is why when you short it out it makes no difference.

as you have lhs carb issues, I would check the idle circuits for blockages etc. and re-check the bits I rebuilt. I believe you can get the chokes "backwards". check snowbums site for more info

hope that helps

[20774]

Randy -

The shorting method will not cause any harm to the electrical system when done correctly...the biggest problem is shocking yourself!! Matt Parkhouse demonstrated the whole process at the Spokane rally and writes about it in his tech articles. All you are doing is providing a more direct path for the spark energy to find its way to ground. Which is also what you should do when removing a spark plug wire as you suggest. My recommendation would be to put a spare plug into the end of the cable and ground the threads of the plug to the engine fins. This allows the spark energy to find its way to ground just as if it were screwed into the head. Having no place for the spark to go creates additional resistance in the ignition circuit which can feed back to the coils. These additional pulses to the coils can weaken or damage the coils in the long run.

If you want to run on one cylinder, maybe it would be better to create a fuel/air mixture that can't possibly run, like turning out the idle mixture screw (it's a gas circuit IIRC) so that it's very, very lean. Tune the opposite carb. Make notes of the settings for the newly tuned carb, and turn the mixture out. Then reset the other side back to a nominal setting and start the bike on the other cylinder. Once tuned, return the other carb back to the settings you identified initially. Or get even crazier and pull the float bowl from one side and force the float up against the stop so that no gas will flow out that side. Now that cylinder can't run because there's no gas...seems a little dangerous if the float should accidently drop and gas start flowing or you could somehow damage the float mechanism.

Seems like a lot of trouble, going back and forth like that. The shorting method makes short work of this. Having a differential pressure measurement system allows you to eliminate the need for shorting the cylinders altogether.

[herrhead]

Thanks for the suggestions guys.

You’re right Benway, I think the left carb isn’t idling correctly, hence, the engine speed not being effected by shorting out the left cylinder and then dying when I short out the right one.

I’ll open up the left carb and see what is going on in there. I think checking the condition of the slide in the dome, like Randy suggested, is the first thing. Then continue by cleaning all of the idle circuits, etc.

I would use my vacuum gauge to synch the carbs, but my /5 Bings don’t have the ports for them so that is the reason why I’m trying out the shorting method…it also can’t hurt to know how to sync carbs using other methods when a gauge isn’t around. I know there are ways to retrofit vacuum ports on the older carbs, but it doesn’t seem worth it to me. The shorting method seems like the most logical way when thinking about quickly comparing and accurately matching cylinder speeds without a gauge.

Randy, you are probably right about the Dyna III making the /5 run better. I was thinking about fitting one to my /7, but only because the cam nose is slightly bent and I would be able to compensate for it by using one. But I don’t think I would change something that is so simple, cheap and already works if I don't need to. :)

Thanks again for all of the help. I’ll let you all know what I end up finding out.
Andrew

[Polarbear]

Good stuff here and good luck. Kurt has valid arguement too, so just keep hammering away and the problem will pesent itself soon enough. I like the Airheads best because one can usually find a problem without too much hassle and they are so simple. The old bikes are throwbacks of another time and surely are still very capable touring mounts, so keep up the spirit and ride 'em hard. Randy13233:D