Low RPM hesitation & buck

[buggrin]

The 78E has started to sputter when pulling below 3K. It's an intermittent thing and I can't find a way to predict when it will happen (hot, leaning, lots of throttle, etc), but it's about 25% of the time. The buck is pretty violent and as well as I can remember this is the first time it's done this (had the bike since new). I have not pulled the plugs to check for color yet, but am guessing that it's a carb issue. Everything is real strong thru the whole range except for this occasional bucking.

After 46K miles of no carb probs it was maybe the wrong time to wonder why all you guys kept talking about carb cleaning, float adjustment, etc.

Where would you suggest I start looking? I've never been into the carbs & don't own a sync gizmo, but guess there's always a first time.

[nwilley]

Sounds like it's time to dig into the pickup wires. Even if you have checkED/fixed them before, check them again! DAHIK

[Mike Hart]

Only below 3K? Only? At any throttle opening?

Hmmm. Start with simple stuff, I guess - drain the fuel that's in the bowls, see if there's any crud in there... though it could just be the idle circuit, in which case the carbs have to come off to get the idle screws out, there may be crud in there that doesn't show in the float bowls.

The idle screws sit vertically, so it's easy for dirt to get around the passage, but you've never messed with em, so...

Haven't heard of this one before. How does it idle? When it bucks, does it backfire out the carbs? (might be hard to tell with airbox on)

Pickup coil probs usually happen at any throttle transition, but maybe that's a good place to look, since it is a known weakness.

[buggrin]

It's done the bucking only a few times in the past few weeks. Now that I think about it, it is a lot like the hesitation that occurs when it's not warmed up and the "choke" is pushed in too soon "ba-chaa----ba-chaa, but now it's happening well after it's warmed up.

Hard to tell if it's backfiring thru the carbs, but is farting or belching out someplace fer sure. Idle is fine, and it only happens when accelerating from zero throttle after a longish slow down -- for instance: let off throttle for few seconds to slow down, turn corner, then open throttle while still in a high gear at low rpm (2-3K). Then bucks 3 or 4 times. Eventually begins firing normally and accelerates back into normal operating rpms. Normally when accelerating from a slow down like this I will apply the throttle 15 - 30% cause full throttle at these low rpms has never seemed to be productive.

Low end torque was what I loved when I bought the thing back in 78. I had been riding a Kawasaki 750 triple two stroke so you can imagine my shock when I twisted the throttle and no ker-chunk-ring-a-ding-ding.

[MartyA]

Bob, I vote for the carbs. I tested a 78E that was for sale last month that had been sitting for 2 years at least. The guy said the gas could be up to 4 years old. This one had 12K miles on it. I started fine, but stumbled and spit between idle and ~3500 rpm, then smoothed out and launched itself like there was nothing in the world wrong, right up to 6000 rpm. Probably could have kept going, but it was only a test ride.

I took an empty small plastic bottle of that cheap "white gas" and cut part of the barrel off, leaving the neck and cap intact. You can slide this under the carb bowls from each side and open the drain screw. It won't overflow and will catch the screw. Be sure your petcocks do work, that when there is no vacuum they don't flow. Otherwise you will overflow that bottle, your shoes, the driveway....

Clean the bottom of the bowls off very well first, otherwise you will wash any crud or grime from the bottom of the carb into the bottle and the gas will appear dirty. You want to see what comes out of the bowls, not what is on the outside of them.