Stutter and loss of power at high speed and high rpm

[nilsthenomad]

So here's my situation.

I've got an '81 750K that i recently (in the last 1000 miles) replaced all the seals and pistons rings, installed a new clutch (300 miles ago) and rebuilt and sonically cleaned the carburetors . The bike idles and drives perfectly for all intensive purposes, however I've had some stuttering or sort of a jerking and loss of power when I'm in the higher rpms of 4th and 5th gear. This tends to be when the engine is under load, climbing a hill in the high rpms of 4th or accelerating beyond 65-70mph in 5th. I get the feeling that there are many potential things this could be, but does anyone have a best guess or place to start?

[Re-run]

Well sounds like you did some extensive work. It is most likely 1 of 2 things. Either there is a timing or spark issue; or fuel issue.

Start on electrical. Check your timing to make sure it is good. I would use a timing light to be sure. They can be found for pretty cheap, got mine at harbor freight for $12 a few years back. I would also double check the connections and grounds.

Most of those checks shouldn't take too long.

carbs would be next. Are you running pods? What I would start with is running a plug chop. If the plugs are old, replace them. Otherwise, make sure they are decently clean. Get the engine to normal temp and then run it hard through the gears. Get it to the point it stutters. Hit the kill and pull the clutch at the same time.

Pull the plugs out and check the insulator color. his will hopefully tell you the exact issue. New plugs work best for this as the insulator is virgin then. Make sure you look into the base and not at just the tip.

A simple test you can run is to pull the choke out some when it sputters. If it gets better, you are likely running lean. If it is worse, then your carbs are likely too rich.

[TOOLS1]

Are you running pods?
TOOLS

[nilsthenomad]

Thanks for the advice guys. I am not running pods and have a new air filter in the air box. I also just put plugs in 200 miles ago.

I hadn't had the thought to run it through the gears on the center stand and try and duplicate the problem, so I did that and there was no lug in the engine at any point. It flew right up through the gears into very high rpms with no stutter. This seems to make me think that it might be my new clutch slipping. Maybe it wasn't adjusted tight enough. Does that make sense?

[rich]

A slipping clutch would be indicated if the RPM went up and the speed didn't. Get up in fourth or fifth gear and roll on the throttle. You'll hear and see the RPM climb with no or little uncertain speed.

For what is worth...my 79k is missing from about 6,000 RPM too. I suspect a kink in the fuel line which I haven't had a chance to sort out yet. Need a fuel filter with  a 90 degree bend. The straight through one  have now bends the fuel line too much. At least I hope that's what it is. If not, time to start replacing ignition parts.

[Re-run]

The plug chop is done on the road, not the center stand. The engine needs the load to correctly show results.

[nilsthenomad]

i'm guessing you're right Re-run and it's probably related to my carbs. Took the bike out tonight and only noticed the problem in 5th gear around 60-75mph. Pulled the choke out when it lost power and it got even worse. I'm guessing that means it's running too rich? I'm going to try and pickup a timing light tomorrow to test that, but why does this only happen in the high end of 5th gear and not in any others?

[Re-run]

It's the amount of load on the engine. Think back to a 10speed bike. In low gear it easy to pedal(1st gear). You moved up to 10th and while you went much faster, it was much harder to pedal. Same thing is happening here. You engine makes 60HP(or so) no matter what gear it is in, so the engine has to work harder to keep it going.

Back in 74, honda stuck and 18 tooth front sprocket on and the engine ended up lugging down and a person would actually lose speed. People ended up riding in 4th to make up for it. Honda went back to a 17t and the problem went away.

Here is another test you can try. Remove the air filter for just a bit and see if that helps any. Also, what brand of filter did you use? I had an emgo brand that caused me to run rich cause it was too restrictive.

[nilsthenomad]

Ah got it. Thanks for the bicycle example, makes much more sense. I just got the timing light and checked my timing. I'm a little confused though, am I trying to line the index arrow up with the two marks above the "F" that look like this: || or with the " I "?

[nilsthenomad]

Taken me a few weeks to get back to testing this. Took the advice and checked my timing. That turned out to be fine, so I went for a ride tonight and am still lacking some power in the high rams of 4th and 5th. Pulled over on the road and decided to pull out my air filter for a few miles. Boom! More power than I've ever had before, but unfortunately I think it was too much air and the bike stalled out after about a mile. Put the air filter back in and it started up fine. The air filter in my bike is not the typical paper one, but a red foam one, not sure of the brand, but I'm guessing if I order a standard HiFlo OEM one, this might solve my air issues?

[Re-run]

Red foam sounds like a K&N filter. Did you oil it at all? I know most foam filters require oiling to work right, otherwise air just passes right through.

However, I had an emgo filter that had red media and it was way restrictive.

[cblover]

Try to use a new set of spark plug and see what happens. Another is to re adjust the a/f fuel mixture.

[nilsthenomad]

Just put new plugs in 200 miles ago. Grabbed some air filter oil and oiled up the filter. Definitely felt some power improvement on my ride tonight. Under load I still get a few hesitations on the power here and there, but it's dramatically decreased. Still isn't perfect, but I ordered a new stock air filter, so hopefully that sorts our any problems. All in all the bike is driving much better now. Thanks for the help everyone.

[Re-run]

Ever do the plug chop? What color were the insulators? That can tell you a great deal about how your engine is running. You should have a light tan on the insulator. dark brown or black is too rich. White is too lean. Adjust as needed to get to the light tan.

[nilsthenomad]

So I put the new OEM air filter in today and took it out for a ride. Still the same problem. My plugs are only about 200 miles old, so I also pulled one of my plugs per advisement when I got back and this is what I see



What's the diagnosis doctor?

[Re-run]

Well, hard to see the insulator itself, but if that were my plug, I would call that too rich. However, you have cv carbs and so I would check the manual for tips on this issue. Shiny would probably have some ideas for you, too. Maybe drop him a line for his opinion.

[shinyribs]

Let's see...
- high gear/speed stumble
- activating the choke worsens it ( makes it richer, which would lead you to think it needs to be leaned out)
-more oil on the air filter helped ( which means less air, which means it made it richer. Which would lead you to think it needs to be richened up)

Weird. At one time making it richer hurt, then later, making it richer helped.

That plug definitely looks rich, I agree with Rerun.

From the work you've done I think it's safe to assume your engine is healthy and your ignition timing is in good shape. Idle and low speed manners are good, so your carburetors are probably nice and clean,and synched well.

The first things that come to mind would be valve lash and/or fuel delivery.

You can kinda get a good idea on the state of your valve lash without pulling the valve cover off. A compression test is easy to do without disturbing any gaskets/spending much time. If a cylinder seems a little low on compression it's a good sign that a valve is too tight. Especially since your ring seal should be great after that work you've done. If a valve is too loose it won't show up on a compression test (as a leak), but for a loose valve to really have any impact on the engine it would have to be loose to the point of being very noisy, so that's probably not an issue.

On fuel delivery, maybe your fuel just isn't keeping up? Could be a restriction in the petcock, fuel filter,etc. If you unhook the fuel line directly off the carbs do you get good flow? A solid,full line of fuel? If your tank isn't venting properly it can restrict fuel flow. Do you ever open your fuel tank and hear a whoosh of air? Best time to check this is on a hot day after a directly decent jaunt at speed. Maybe 10-15 minutes at 55mph+.

Oh, another thought. The vent lines on your carbs. The rubber hose ( i think it's just 1?) that just runs to atmosphere, if it's clogged or restricted/pinched somehow that can't prevent the bowls from filling properly.

Also, fuel level in the bowls. A clear line hooked to the drain screw is a good test without disassembling anything. Just loop it up beside the bowl and open the drain screw. See how far up the line the fuel raises and you will see your exact fuel level.

Do all four plugs look the same? If one looks vastly different from the rest it can be a clue on what's up.

Sorry I haven't popped in earlier. Life, ya know?  Good luck,bud. Let us know what ya find.

[nilsthenomad]

Alright, so I started digging and decided to run a compression test. All my plugs look the same, but my compression is troubling:

1 = 95
2=125
3=125
4=150

I ran it a few times on all the cylinders and had some fluctuation on 2,3, and 4 but 1 is solidly low. 4 was always the highest. New seals on the whole engine and cylinder rings. Really hoping there isn't still a seal problem, but I've never touched the valves. Should I be checking my clearances next?

[shinyribs]

Yeah , I'd check them if it were mine. Hopefully you'll just find one that is too tight allowing it to bleed off compression. If you don't have one already, go on eBay and order the tool for holding the valve bucket down. It's one of those special tools that is actually needed, unlike some others. But it's cheap, like $12-15, so don't worry. Careful with the valve cover bolts. The bolts themselves are fine, but the cam caps they screw in to are notoriously easy to strip. They don't need to be very tight. Spark plug tight , or less, is all that's needed.

Good luck man, I hope that's all it is!

[shinyribs]

Just out of curiosity, and you probably already know this, but did you make sure to hold the throttle wide open and had all four plugs out while you cranked? That's the only way to get a good reading.