2500 rpm stumble

KaZooCruiser wrote:

...am not going to assume that you actually took out the bitty pilot jets and cleaned them. They are located under another plug that has to be removed to gain access to them. Sneaky buggars. Have to use a really thin screwdriver to get to them, because they are buried in the well....

Yikes! I assumed he did.

You know what they say about ASSuming! :laugh:

When I cleaned them, I did not plan to kit the carbs, so I left the pilot screws alone. Big mistake, probably. Later, when I kitted them, I did not spray out the pilot passages as I had forgotten to get carb cleaner. :pinch:

Now I have the rack apart on my bench and I will get a can of carb cleaner and am going to very carefully spray through every passage and pay particular attention to the transition port that lies between the main jet and the inside of the front of the slide. I am also going to make sure to work compressed air through everything.

Using my makeshift level gauge, I found that carbs 1, 2, and 4, were right at the high side of the 2.5 to 4 mm below the main body the manual specifies. Number 3 was even with the main body, definitely too high.

As for the advancer, it was a bit gummy and it cleaned up nicely and works smoothly with new grease. In fact, after doing that, the running was worse. :S

One thing I did notice was the inlet throats of the carbs were wet when I took the airbox tubes off. No gas was in the airbox itself. Perhaps that came from the engine running rough.

If after setting the float heights and cleaning there is still no improvement, you'll probably hear a scream heard around the KZR world!

I think I found an "issue." Maybe I found the problem source.

Last night I was idling the engine in a dark part of the yard, and happened to see flashes of light emanating from under the gas tank.

When I looked closer, I saw rings of fire around 2 of the spark plugs where the insulator is supposed to be gripped by the metal shell. Minute compression leaks. Couple that with fire crossover where the spark was leaking to ground paths on three plug wilres.

It all makes sense, but I haven't swapped out the parts yet.

Low engine speed results in high compression with little amperage to fire the mixture. So narrowing the gaps masked the symptom.

Something else to check for those dealing with the same issue. Crossfire, random misfire.

Okay I got the project '78 SR sorted today. It wasn't easy and at one point had me all but loading it up and pushing it into the river. :whistle:

I went through everything with a fine tooth comb. Carbs, ignitions, compression, timing, the whole bit. Everything was in spec. Put it all together this morning and still the same. After some ritual language and then resisting the urge to kick/kill/maim something, I decided to do something stupid--I turned the pilot screws all the way in, yes all the way and it still ran! Not well but #1 was pulling everything along. So I backed out #4 until it picked up and then 2 and 3 and finally #1.

Then I set the dwell. Now I had been following other advice, including the FSM, and just setting to a spec on the dwell dial face. I realized last night as I thought about things that the point gap was too narrow. So I went for half the spec value on my dwell meter and that netted me a gap of 0.025". Which was a bit too wide as the timing wound up being about 2 degrees advanced and the parts were at the end of their adjustment slots. So I backed 1/4 down to 0.020", noted that reading on the dwell meter and then set 2/3 to match. I was able to just get the idle timing on the F marks. I noticed that with a wider gap the points aren't arcing. Bonus!

So twiddled with the pilot screws a bit more, but I think they are backed out so far that they're having little effect. I suspect the stock #15 pilot jets are still in the carbs and 17.5s would probably work much better.

It now has low-end grunt and runs perfectly above 3000 RPM. There is still a slight stumble off idle, but I think this is due to the pilot jets. If I work on it again for him, that's one thing I'll address. I've already gone wayyyyy over budget and I want it out of my garage. But due to weekend plans it may stay there until early next week.

Out on the road it is a pleasure. Acceleration everywhere and the slightly louder than stock mufflers sing a great song.

So, I think my problems were too narrow of point gaps, the pilot screws were too tight, and #3 float was a bit high. I also put the coil supply relay on it. I don't really know if it made too much of a difference, but it won't hurt anything. There is 1.6 ohms between the battery and coil wire connections. I wonder if that is normal or if the ignition and kill switch are just getting that resistive with age.

N0NB wrote:

Okay I got the project '78 SR sorted today....

That's good news!

Regarding points, I've never used a dwell meter. But have always has good success with points by first setting the gap with a feeler gauge, and then using a strobe light to set the timing.

I would say that if for no other reason, the dwell meter can help match up the pair. Once one set is at the desired gap, the dwell meter seems an easy way to get the gaps even.

Patton wrote:

N0NB wrote:

Okay I got the project '78 SR sorted today....

That's good news!

Concur on getting your issues sorted out.

Mine took a different tack this weekend.

Changed out the plugs for some new, NGK B8ES (one range colder). Gapped at .030. I found that as I suspected, lack of low rpm fire was the main issue. Now find that the engine pulls smoothly off idle, now matter what gear is selected, even under what could be considered lugging. These 650's seem to have a bunch of torque. And it's nice to be experiencing it again.

The thing that threw me off was the fact that the plugs only had about 3500 miles on them. Oddly enough, one of the brand new plugs started demonstrating the "ring of fire" the night after I installed it. The same night the problem showed up, AGAIN.

I looked at the running engine at idle last night, with the new, new plug in place, and saw no flashy-sparky.

I kind of understand heat expansion acting on the metal shell of a plug over time, but a new plug? QC at NGK? Or lack of same, perhaps.

KaZooCruiser wrote:

...lack of low rpm fire was the main issue....

Saw the reference to Dyna ignition and could have missed seeimg in thread but wondering if also using Dyna coils and Dyna plug wires (solid copper core with built in plug caps). Old plug wires and plug caps sure can cause a lot of grief.

Good hearing your progress.

Was remiss in failing to also mention --
Assure fully charged battery and full battery voltage at coils. Also can't hurt to use dielectric grease on all electric connections to keep out water and avoid corrosion (includes coil ends of plug wires and inside plug caps, and all connectors, bullet and oherwise).

Post edited by: Patton, at: 2007/05/29 19:22