Author Topic: Bike stutters (Read 10006 times)

jimbojangles · « **on:** November 10, 2010, 11:00:19 AM »

I've been searching the forums for days to find out why my 750 K3 stutters worse than porky pig, but so far everything I've found is about 550 flat spots or topics where people didn't post what finally solved their problem.

What can cause this bike to stutter and gurgle when I just barely give it gas going down the road? It idles great at about 1100 RPM and has tons of power when you grab a fist full of gas, but cruising at 30-40 miles per hour is miserable. Also, it runs like a dream when cold and about 40 degrees outside...until the engine gets warm.

Quick history:
Ignition all new: battery, dyna ignition, new coils, wires caps and plugs.
Carbs have been off the rack and cleaned 3 times now. synced even more.
I've tried messing with needle position which helped a little, but eventually leaned it out too much when trying to go highway speeds. I've replaced the needles and they are now in the middle notch.
Intake is stock, but exhaust appears to be a mac 4-2.
105 mains & 40 pilots.

THANKS

dave500 · « **Reply #1 on:** November 10, 2010, 11:09:47 AM »

sounds like a rich idle/off idle condition,try leaning the air screws or even using a 38 slow jet,you could even try a little more advance from standard.,do one change at a time though.

jimbojangles · « **Reply #2 on:** November 10, 2010, 11:16:22 AM »

All great ideas. Idle screws are all at 1.25 turns out. I'll try them at 1.5 on my way home from work today. It was just idling so well I didn't want to mess it up.

Would fuel octane have any effect on this? I've been running the cheap stuff the last couple of fill ups, but will try premium next time.

THANKS

TwoTired · « **Reply #3 on:** November 10, 2010, 11:26:51 AM »

Why do you have 105 mains? Pod filters?

Fuel octane only matters if you are getting pre-ignition/detonation for the effective compression ratio on your engine.
All the gas at the pumps have the same energy density, unless blended with a lesser fuel (ethanol, methanol, etc.)

The higher octane ratings only effect the flash point of the gas, so it will ignite with spark rather than embers or compression heating.

Cheers,

jimbojangles · « **Reply #4 on:** November 10, 2010, 11:37:48 AM »

I read that 750s had 105 mains in 1973. Is this wrong?

Just wishful thinking on the octane thing.

TwoTired · « **Reply #5 on:** November 10, 2010, 12:37:05 PM »

Sorry, I got confused. I thought you were discussing a Cb550.

But, I might suggest you put in clean spark plugs, then go out and while it is stuttering, hit the kill switch and clutch-in at the same time. Then stop and yank the plugs to observe deposits.
1. Are all the plugs showing the same deposit patterns? If not why not?
2. Are they sooting or is it actually going lean during the "stutter" phase?

You could get this info on a dyno with an exhaust sniffer, too.

Um, did you replace the stock slide needles with stock ones or aftermarket?

Are your new coils 3 ohm? If so, did you increase the spark gap?

Cheers,

jimbojangles · « **Reply #6 on:** November 10, 2010, 12:42:48 PM »

New coils are stock replacements from Z1. The ones with the wires fused to the coils. (I failed to mention I'm pretty broke). Slide needles are from North City Vintage Honda. (Killer fast shipping) I couldn't find the brand. Either way the symptoms are the same with the original honda needles and the replacements.

I did a plug chop. There are several miles of 30 MPH road to get to my house. Stuttered all the way. All 4 plugs were solid black when I killed it in the driveway.

tango911 · « **Reply #7 on:** November 10, 2010, 01:05:57 PM »

one of my plugs keeps fouling and all my settings are the same, im thinking my plug cap is bad, I replace the plug and it runs great for about 50 miles.

ill keep watching your thread. see what happens. sounds like your 4 are rich.

TwoTired · « **Reply #8 on:** November 10, 2010, 01:28:30 PM »

How old is your air filter? "Looks clean" I heard many times. But, it doesn't help air flow through it or lessen the pressure drop across it. At any rate, carb tuning should be performed with a brand new air filter so there is no question about that component. And tuning to a dirty air filter is not recommended as once they become restrictive the restriction increases pretty rapidly. See pic.

I looked at this chart:
http://www.motorcycleproject.com/motorcycle/text/specs.html
..if you have 105 mains I assume you have either 657B or 086A carbs. Is this right?

You will want to verify the float setting and the actual fuel level in the carb bowls. Too high a level can cause rich running.

Be aware that these mechanical slide carbs have two fuel circuits and three metering devices.
1- the pilot circuit, 2 - the slide needle/throttle valve, and 3 - the mains.
Each one dominates the mix depending on throttle position.
So, reading the plugs after general riding, doesn't isolate any of the 3 devices for specific fuel flow contribution, even though black plugs do indicate a rich mixture from one or more of them.

Have you tried running without the filter element as a diagnostic test?

Cheers,

Duke McDukiedook · « **Reply #9 on:** November 10, 2010, 01:54:14 PM »

Like TT said check your static float fuel level and your air filter.
You basically have the same setup as I do on my K3.

jimbojangles · « **Reply #10 on:** November 10, 2010, 02:09:42 PM »

Thanks guys. I at least have some ideas now. I thought I was running out.

My air filter is new. Bought it in June or July.

I'm pretty sure my floats are all at 26 mm, but I will measure the fuel inside to see if they are all the same.

I'm going to try to lean out the idle circuit a bit on the way home and will then try running it without the filter maybe this weekend. It's supposed to start raining tonight and I'm not crazy about doing test runs in the rain.

Thanks for all the help. When I figure this out I will be sure to post for all.

Brown Bomber · « **Reply #11 on:** November 10, 2010, 02:14:21 PM »

Just subscribing here.

ekpent · « **Reply #12 on:** November 11, 2010, 05:33:08 AM »

Is the gas tank and petcock totally clean and fuel line routed correctly. Sounds like with that set-up you should be lean enough,my concern would be bowls running low at constant speed maybe. Have you put in any aftermarket slow jets? If you get a chance to check them,should be #40's,look for a stylized "K" logo stamped on them to see if they are stock Kehin. I have seen junk aftermarkets that were big enough to drive a bus through. Let us know how it all works out for the next guy down the line.

jimbojangles · « **Reply #13 on:** November 11, 2010, 06:43:04 AM »

OK, I leaned out all of the idle screws before driving home last night. All at 1.5 turns out. The bike drove great at highway speeds. It never stutters under load, just when cruising at 30 mph or when I let off the gas completely and just barely roll it on again. This makes going through the sharp S curves near my house difficult because it stutters rather than accelerating out of them.

I got home and checked plugs. 1&2 were maybe a little too lean. #3 was a bit dark and #4 was pretty black. The stutter was just the same as it was when all 4 were black. I'm thinking Dave might be onto something with the timing. I'll check this weekend. I'll also keep messing with idle screw to see if I can get things right.

Jets are all stock 40s.

So this is my question. I've had trouble in the past with #4 being rich, but thought I had it worked out with my last carb clean and sync. Is it possible that 1 cylinder being rich can cause the stuttering and stumbling?

tango911 · « **Reply #14 on:** November 11, 2010, 06:56:50 AM »

on mine the one plug kept going bad, was really rich, after 40 miles it would stutter bad, my other 3 were brown, I put a new plug in it, its was fine, i think on mine the plug cap was bad. But i had already ordered new coils and wires a week before it was doing this. Come to find out my wires were spliced with extra wire on 2 and 3 , and the 1 and 4 wires were original.

i would replace your plugs, check valves again, set your timing and go for a ride, see if your stutter is gone.

Duke McDukiedook · « **Reply #15 on:** November 11, 2010, 07:23:25 AM »

You sure you cleaned the carbs thoroughly- ALL orifices with carb cleaner and wire AND pressurized air (~90psi)?
Have you run a compression check on your cylinders to see what kind of compression you have?

jimbojangles · « **Reply #16 on:** November 11, 2010, 07:52:52 AM »

I think You've got me on that one. I completely disassembled the carbs and let them soak in carb dip before blowing them out with pressurized carb cleaner, but I don't have access to an air compressor, nor the money to buy one.

Also, I bought a compression gauge last weekend, but it didn't have an adapter to fit our size spark plug holes. Any idea what size those threads are and where I can just get an adapter??? I think I was hesitant to buy the compression gauge because I don't know what I'll do if that's the problem.

tango911 · « **Reply #17 on:** November 11, 2010, 07:53:56 AM »

the threads are 12mm if i remember right

dave500 · « **Reply #18 on:** November 11, 2010, 11:30:02 AM »

i made an adaptor for mine,i smashed the porcelain out of an old plug then ran a tap to suit the larger plug thread in the hex area.

Skoti · « **Reply #19 on:** November 11, 2010, 11:37:35 AM »

When you fix this problem please post how you done it.
I have exactly the same problem, just as you describe it.

I have renewed spark plugs, points, plug caps, air filer and carb slow running jets.

Set timing with strobe lamp, balanced carbs with mercury gauges, checked valve clearances, tested ignition coil ohms, checked and set carb float levels and throughly cleaned out carbs.

Was thinking of renewing needles and needle jets but note you've already done that to no effect.

will keep watching this thread

Duke McDukiedook · « **Reply #20 on:** November 11, 2010, 12:00:11 PM »

Unless needles are bent they rarely ever get worn out. The tapers on the Keyster needles are suspect, only get Honda or Heihin OEM needles.

tlbranth · « **Reply #21 on:** November 11, 2010, 12:43:32 PM »

If you have brass floats, make sure they're not leaking. I had holes in 3 out of four of mine.

jimbojangles · « **Reply #22 on:** November 11, 2010, 02:21:08 PM »

It's good to hear that I'm not the only one to experience this, but I was seriously hoping someone had that miracle cure I was looking for. You know, one of those "duh" suggestions that costs nothing to fix. I plan to raid the neighbors garage for an adapter and test compression. I want to find the problem, but really hope it's not compression.

Hey Skoti, what do your plugs look like? any problem cylinders?

tlbranth- my floats are the black foam stuff...do these ever cause problems?

Stick with me. I promise this bike won't be on craigslist any time soon. I'm pretty broke and hoped to use this bike as a daily commuter. I'm also extremely stubborn. I will try any and all reasonable ideas! Just not on Saturdays when the Razorbacks are playing. GO HOGS!

twincityrider · « **Reply #23 on:** November 11, 2010, 02:42:08 PM »

When you had the carb rack off did you pop out the emulsifiers from the carb bodies and clean those? Sometimes they get missed. Hope you get back on the road soon.

TwoTired · « **Reply #24 on:** November 11, 2010, 04:20:18 PM »

I expect the stuttering is due to spark plug sooting. The soot shunts the spark away from the gap. New plugs will probably stop it for a while, until they soot up again.

However, now that you have some clean plugs, swap them around with the sooty ones to clean them and test the diagnosis.

Cheers,

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Author Topic: Bike stutters (Read 10006 times)