Running woe #3: anemic idle requiring blipping throttle

I still had the annoying problem of the idle slowing down. When I'd close the throttle, the idle would immediately drop to about 1200 RPM, but as I sat at stoplights, the idle would get lower and lower until the bike stalled out after about 10 seconds, if I didn't continually blip the throttle. Turning up the idle adjuster screw, even just a quarter turn, would cause the engine to idle about about 3500 RPM.

I thought this idling behavior might be typical of small motorcycles. My sketchy old Seca idled in much the same way. However, when I took the ABATE Experienced Rider course on the provided little Honda Nighthawk 250s, I was stunned that they idled so well. Sitting on the little Nighthawks felt like sitting on a little happy sewing machine that would happily hum along all day at idle.

Then I rode my brother-in-law's 1968 Honda CB450 twin. It too idled like a little sewing machine. Also it's european-style handle bars made me more determined to get rid of my XS400's semi-ape-hanger-custom bars and replace them with something more sporting.

So, I set to work trying to figure out what was wrong with my bike that would force the idle to die a slow death if I didn't blip the throttle.

I thought maybe the issue was a weak spark at idle, which made the engine not run so well, which reduced the RPM, which weakened the spark, etc...causing the engine to eventually stall out. I checked the coils with a multimeter, and they were fine. I had read on The Used Motorcycle Guide about old Yamaha SR500s that would cut out in traffic until the owners replaced their resistance plug boots with non-resistance plug boots. So I ordered new plug wires, non-resistance boots, and a new euro-style handlebar from Mike's XS. The parts, prices, and service was great. The handlebars transformed the bike:

Unfortunately, the idling problem persisted.

So I thought that maybe the pilot jet circuit in the carbs was still clogged with crud, that I wasn't getting sufficient fuel to idle well, and that the only reason I was able to idle is that I had the idle turned up to the verge of the primary fuel circuit...that would certainly explain the touchiness of the idle adjuster screw. So, I tore the carbs apart again, pulled the pilot jets (Size 42.5 Type BS30/96). Although the brass looked a little corroded, none of the passages were blocked. The ends of the jets near the screw driver slots were a little chewed up, but nothing too bad...I think I'll order a new set in case I ever get around to tearing the carbs down again. I cleaned up the jets, reassembled everything, but alas, I still had the same anemic idle.

Then I had a revalation...CV carbs have a spring that pushes down a slide in the carb body (part # 43 in the diagram below).

If that spring was exerting too much pressure on the slide, then the slide would close like a butterfly whenever there wasn't a lot of vacuum above the diaphram. This would cause the idle to drop slowly as the slide was forced closed by the spring until the bike died. The sensitivity on the idle set screw could be caused because the idle was set so it was on the verge of moving the slides up, and setting it even a tiny bit higher would cause the bike to rev up to the butterfly setting (vs. the forced-closed slides' setting which was limiting the idle speed)

Luckily the bike came with a set of extra, gummed-up parts carburetors...

This weekend, I pulled the slide return springs from my bike, and they were about twice as long as the slide return springs from the parts carbs! I put the shorter springs into the working carbs on the bike, and it ran beautifully! I let it sit there idling happily for ten minutes! I revved it up, and it always quickly returned to its happy and consistent idle.

Unfortunately, I didn't have a chance to go for a ride to see how it really ran...but I have high hopes!

Running woe #2: Hovering high RPMs

The bike still wasn't running just right.

When I'd rev the engine up and then close the throttle, the engine would hover at high RPMs for a while before dropping back down to a more sane level on their own. It was not only annoying to sit at traffic lights with the engine screaming away at 4500 RPM for 15 seconds, it also made riding dangerous because I couldn't smoothly roll off of the throttle in curves.

My first thought was that the problem was that the carbs were out of balance - That one carb was trying to keep the engine at a high RPM, while the other was lugging it back to idle. I double-checked that the carb balance, in case I messed it up after I cleaned out the carbs, but it was still perfect.

I knew that a lean mixture could cause an engine to hover at a high RPM, and that lean mixtures are often the result of vacuum leaks. To test this out, I sprayed WD40 all over the carb bodies, petcock vacuum line, and intake manifold, but I couldn't detect any change in how the engine ran. However, on closer inspection, I saw that the rubber intake manifold bodies that appeared to be in fine shape were in fact riddled with cracks and covered in black RTV rubber. Also the hose clamps on the intake manifolds had seen better days.

I got a couple of NOS intake manifolds from EBay and replaced the flakey Yamaha hose clamps with heavy-duty industrial hose clamps. However, the problem persisted.

I was a little nervous to mess with the idle mixture screws on the carburetors...I left them screwed out the same number of turns as the previous owner had them. However, all signs pointed to a weak mixture, and I knew it couldn't be caused by a simple vacuum leak.

I ended up screwing out the idle mixture screws on each carb about three turns, and the problem immediately disappeared. Over the course of the next few days, I tweaked the mixture screws until it was running really well. I knew I had it too rich if the engine was noticably down on power. I knew it was too lean if the engine RPMs didn't drop crisply when I closed the throttle. I could pull the spark plugs and look at their color after a few days of running to make sure I was still on track.

After fiddling with the idle mixture screws, the bike ran beautifully, and I drove it for several months that way.

I still had the annoying problem of the idle slowing down. When I'd close the throttle, the idle would immediately drop to about 1200 RPM, but as I sat at stoplights, the idle would get lower and lower until the bike stalled out after about 10 seconds, if I didn't continually blip the throttle. Turning up the idle adjuster screw, even just a quarter turn, would cause the engine to idle about about 3500 RPM. I resigned myself to blipping the throttle at lights, drove it, and continued to scratch my head about what could be causing that problem.

Running woe #1 Sticking high idle

Right after I got the bike running, I struggled with a strange engine issue.

When I'd fire the bike up, it would idle at a sane RPM, but when I revved it up, the idle wouldn't come back down on its own. I would be able to pull the idle down to normal levels by leaving the bike in gear as I braked, letting the brakes lug the engine back down to a sane idle speed. I would also be able to get the idle down to a sane level as I rolled along by shutting off the engine with the kill switch, then switching it back on so the bike was push started back to life, idling happily at a reasonable RPM.

I didn't believe that the idle was simply set too high because it was possible to bring the idle back to a sane level by restarting the bike or lugging it back down.

However, simply turning down the idle made this problem disappear. I'm not sure if the fact that I could pull down the idle from its high setting is typical of CV-type carburetors, or if it was because the bike was running lean (see future post), or because the slide return springs were much too long (see future post).

Unfortunately, even after I turned down the idle, when I'd rev the engine up, the RPMs would hover at high RPMs for a while before dropping back down to a more sane level on their own. Also, when idling at a light, the engine RPMs would steadily drop and the engine would eventually stall out after about 10 seconds if I didn't occasionally blip the throttle. On the other hand, turning down the idle made the bike much more more driveable.

Mega Maintenance Post

I haven't updated in a while...I've been thinking of tracking my maintenance and consumable data here in this blog.

When I bought the bike in May 2006, it had about 3600 miles on it. It is now at about 4050 miles. In that time I:
1. Bought a new battery
2. Removed, disassembled, cleaned, and balanced the carbs
3. Got new tires
4. Changed the oil
5. Tensioned and lubed the chain
6. Replaced the plugs
7. Replaced the plug wires
8. Replaced the 5K ohm spark plug boots with non-resistance boots.
9. Replaced the semi-custom bars with bars from a european-model XS650
10. Set the valve clearances
11. Replaced the intake manifold rubber boots (see future post about why)
12. Replaced the slide return springs in the carb (see future post about why)
13. Replaced O-ring around neutral indicator switch

In this time, I have gotten between 49 and 55 MPG in town. I haven't done much highway riding. Strangely, more conservative riding didn't result in better gas mileage.

I plan on updating this blog with future maintenance along with exact mileages as I complete them.