Racing Throttle

[sam]

Just a quick question what could be causing the throttle to race to about 4000 rpm when clutch is engaged or the bike is in neutral. When the bike is in gear it seems fine.

Thank for all your wisdom.

[sam]

also just started backfiring when bike is shut off. thanks

[grizld1]

Sam, this is about the umpteenth post on this question so I'm gonna sticky yours and hope the next new guy looks around just a little before he asks it again.

There are two common causes for the elevated idle you describe (apart from a sticking cable or having the throttle stop set too high). One cause is air leakage at the carb mounts or throttle shafts. See the Carb Guide for diagnostics and fixes. The other common defect occurs on pre-'80 bikes with breaker point ignition. Timing is advanced and retarded on those by the automatic timing unit (ATU).It lives under the chrome cover on the right side of the head. When it's working properly, it moves a rod attached to the breaker point cam into the advanced mode when rpm's rise, and back to retarded mode when they drop. Often the rod sticks, either because it has not seen fresh grease for 20 or 30 years or because the springs that draw the bobweights on the ATU (and the rod with 'em) back to the retarded position are worn out.

To service, don't remove the bobweights or anything else on the ATU. You'll notice there's a disk in the unit that is moved by the tips of the bobweights. It should be scribed with a line which aligns it with the ATU rotor. If you don't see alignment marks, make some. Also remove the left side chrome cover and mark the position of the points cam so you know, uh, which end is up. Don't turn the engine over after this point. You'll notice that the ATU is attached to the camshaft by a stepped nut. Tap it loose with a drift and turn it off, pulling the ATU out on the shaft as you go and removing it as a unit. You'll notice there's a pin in the timing rod. Remove the pin and pull the rod out to the left. Flush the inside of the camshaft with contact cleaner or similar solvent, clean the rod and lube it with moly grease (I like chainsaw bar sprocket grease for the purpose). Reassemble, making sure that the disk and rod are aligned as they were before you removed them. Be sure to smack down the stepped nut firmly. If it loosens, the alignment pin in the camshaft will deform the ATU sleeve, timing will advance abruptly, and most of the time a piston will hole before you can shut down.

[garyh]

Hi Grizld1 -

I appreciate your reply to Sam - and thanks for posting for me...the "Next guy" looking for The Answer I have a stock 77D with same problem.

I followed your well worded instructions and got to the point of having a loose stepped nut. My problem is that the ATU does not come out on the shaft as a unit - in fact, it dose not budge at all. I was thinking of getting out my Big Hammer and smaking it a few times - but thought it would be best to seek professional help. Do you have any suggestions on how I can get the ATU to break free?

Thank you -

GaryH

[grizld1]

Don't apply that BFH; you could damage the camshaft bearings. See what you can do with a bit of PB Blaster (or other penetrating oil) and a few light taps.

[5twins]

If all you're trying to do is clean and lube the advance rod, you can do that without removing the advance unit. Just remove the disc on the rod's end that engages the advance unit weights. The rod with the points cam still attached can then be withdrawn out the left (points) side.

If you want to clean and lube the advance weight pivot pins, that can also be done with the unit still in place. Just unhook the springs, remove the e-clips, and slide the weights off the pivot pins. After the initial cleaning and lubing, I keep mine lubed with an occasional (small) shot of foaming chain lube applied preferably when the motor is still warm so it penetrates better. Be aware that there is a very thin shim washer under each weight. You may not even notice it if it comes off stuck to the back of the weight. Take care not to lose those.

[garyh]

Grizld1 & 5twins -
Thank you for your time - and for sharing your knowledge with all.
You guys are appreciated.

Gary H
Long Beach, Calif.

[jeffery]

I have a similiar problem with my 77D which don't have bob weights but rather a boyer ignition, magura throttle, and 34mm roundslides. I used to get the throttle speed set with the drill side of a drill bit but can't get the throttle speed to stabalize all of a sudden. Do you guys use goop on both sides of the carb to head gaskets or one side? I think a drill bit that is about .200 is about the right size to get the idle set but mine is irratic. Up till now is has been running fine. I guess I could have suddenly developed an air leak. Carb clamps are tightened all the way down. Any suggestions before I remove the carb boots? I'm getting some popping at the exhaust to as it set and idles.
Jeffery

[dillrepair]

ok...i'm not really sure how to describe this but....if anything my bike should be running rich right now... i've added a heiden tuning kit from mikes... for my bs34 (81 special) so that means 145 mains and something else larger for idle, whatever the kit comes with... needles set to middle position.. running pod filters and a supposed aftermarket exhaust..... from back in the day aftermarket stock looking exhaust.... before i did this carb work the idle would race after engine was nice and warm.... after this carb work and i balanced carbs with meters and running new holders and lines... does not do it as much now but still is noticable ... aka now it races to 2500 vs previous 3000/3500 when warm..

any ideas???.... sorry for the disjointed post... just curious if i've even put it in the right place.

i've read carb guide but am willing to read it again if you point me somewhere specific i missed.

thanks in advance for the advice
-Pat

[Mat]

I've had a similar problem for years with my '79 Special II. I wouldn't describe it as a racing throttle...more like a creeping throttle. The longer I ride my bike, the more the idle RPM creeps up, generally from around 1000RPM to 3500RPM. I now use a Boyer ignition, have stock headers with Sportster mufflers, and knock off cone filters. Nothing seemed to make it better or worse--including running without filters. The guy that rebuilt my engine remarked something about a dirty alternator being a potential culprit, but I don't know how to check that.

[grizld1]

Mat, if power supply to the Boyer igniter box is weak it won't retard. I don't know what your mechanic means by a "dirty alternator;" if the battery stays charged at 12.5 volts or better the alternator's doing its job. If it isn't, alternator inspection isn't rocket science. Get a good multitester (not a $10 piece of Chinese junk) and follow Curly's Charging System Guide. If the battery is healthy, check connections on the black and red wires and the white ground wirte from the Boyer box; heat can increase resistance in a marginal connection. Clean up or replace the connectors; it often helps to install a ring connector on the ground wire and run it to a motor mount or other frame ground, sanding for a clean contact. If that doesn't help, try using switched power from the harness to trip a relay drawing power for the igniter box directly from the battery on a separate power wire, via a 10A fuse.

[Mat]

The 10A fuse test sounds familiar--I remember it because I accidentally let the fuse hit my fuel tank and it popped due to touching earth. Battery is healthy (spendy AGM) and I have no idea what else to check. Only thing that seems off is that my "custom" exhaust solution leaks at the header "bleed" holes near the head.

[xsleo]

If your "custom" exhaust solution has a leak it can cause exhuast popping.
Did you just clamp your muffler on the outside of the header pipe? This won't work well on the later exhuast where they are one piece. When you saw the stock muffler off the head pipe, you will find that the head pipe is a double wall pipe. The inner pipe carrys the exhaust, the outer pipe is just for looks. You will need to get an adapter from the parts store. On end fits in you muffler, the other fits snuggly over the inner pipe.
5twins or Pamcopete has a pic and shows the part number.

[pamcopete]

[grizld1]

Mat, I wasn't suggesting a "test" with the 10A fuse. I was suggesting that you permanently install a "normally closed" 30A or 40A 4-prong relay to power your igniter box, trip the relay with switched power from the harness (brown leads), and install a 10A fuse between relay and box.