2500 rpm stumble

I tried to do some research.
"stumble" "stumbling" "bucking"

Anyway, it's off-idle. Title says it.

Kind of maybe where the needles need to be lifted a notch. Looks like a lot of work to do that, so I would like to eliminate other possibilities, since this seems to be a sort of a new development. Only between 2200-2800 rpm.

Resealed the carb holders, and didn't discern a vacuum leak afterwards. Bike runs strong everywhere else.

Last valve clearance check showed good.

Fairly new plugs, replaced coils with 3 ohm Honda CBR coils. Coil relay, Dyna. Working advance.

Going to check the air filter. Blow it out.

What is it about the last 20% requiring 80% of the work?

I'm kind of stumped.

This issue comes when coming off idle; in other words, when you are getting underway? Do you have a stumble when cruising in the same rpm range? What type airbox/filter is in the bike?

Still doing some testing, because the weather here is not being helpful.

Started checking a little closer due to wiredgeorge's questions.

Problem seemed to be worse with the air filter out. I hoped cleaning it would have taken care of it. It's a stock paper filter, (EMilGOmez) and stock airbox.

Becomes apparent at 30 mph in 5th. There are a lot of 35-45 speed limits around here, so with two people on the bike, the stumble is enough of a problem to require a lower gearing to bump up the engine speed out of the unstable range. Seems to rev up fine unless the engine is under load. I'm going to search "flat spot."

Drained the float bowls, in the hopes that it was water from the last bath. No help there.

Progress update. . .

Pulled fuel bowls and flushed / removed bitty particle rust from bottom of bowls, and cleaned pilot jet circuit teeney-tiny holes with guitar wire. Also found that draining bowls with screws does not remove bitty particle rust or weird jelly-looking stuff from bowls.

Bike runs marginally better.

Checked timing with wonder-gizmo timing light that allows checking advance while bike is running. Reset Dyna triggers to fire at correct time. One was advanced / retarded in relation to the other. Reset timing to 30 degrees advanced at 3900 rpm to alleviate pinging. Bike still starts good with slight retard from stock setting of 35 degrees.

Bike runs marginally better.

Closed sparkplug gap to .711 mm.

Bike runs LOTS stronger in midrange. Suspect too -wide gap is allowing fire to get blown out some.

Reset idle fuel screws, by turning out a little, especially #2.

I think I got it with that one.

:lol:

Okay, I'm about at my wit's end with this project '78 650 SR (mixture screws on the lower front of the carb).

It has a very pronounced stumble off idle to 2500 ~ 3000 RPM. Above 3000 RPM it runs like a scalded dog. It idles smoothly. Fuel level is fine as it idles just as well on the side stand as the center stand.

A plug check showed what appeared to be wet fouling of 2&3 plugs. So I worked my way through the ignition system from the condenser to the points and swapped the 2&3 coil. I found the voltage at the coil 2 volts below nominal battery voltage so I used a jumper to power the coils directly from the battery. None of this made any change to the stumble.

My neighbor loaned me a Dyna coil to try, which I have not tried yet.

As far as I can tell, the jets are stock. I did put in new K&L needles and set them to the middle groove. The bike does have aftermarket slip-on mufflers (the stock ones were sawed off under the engine) and is louder than stock.

My next course of action is to try raising the needles one groove. Am I on the right track thinking that it's probably leaning out?

I need some direction. :pinch:

N0NB wrote:

Okay, I'm about at my wit's end with this project '78 650 SR (mixture screws on the lower front of the carb).

It has a very pronounced stumble off idle to 2500 ~ 3000 RPM. Above 3000 RPM it runs like a scalded dog. It idles smoothly. Fuel level is fine as it idles just as well on the side stand as the center stand.

A plug check showed what appeared to be wet fouling of 2&3 plugs. So I worked my way through the ignition system from the condenser to the points and swapped the 2&3 coil. I found the voltage at the coil 2 volts below nominal battery voltage so I used a jumper to power the coils directly from the battery. None of this made any change to the stumble.

My neighbor loaned me a Dyna coil to try, which I have not tried yet.

As far as I can tell, the jets are stock. I did put in new K&L needles and set them to the middle groove. The bike does have aftermarket slip-on mufflers (the stock ones were sawed off under the engine) and is louder than stock.

My next course of action is to try raising the needles one groove. Am I on the right track thinking that it's probably leaning out?

I need some direction. :pinch:

Some thoughts --

Ignition components are probably okay because of good performance over 3000 rpm, but won't hurt to confirm condition of points (clean, unpitted, proper gaps). And also having good fat spark at idle. Assure battery is fully charged and well grounded.

Smooth idle notwithstanding, wet fouling of 2&3 plugs could indicate too rich rather than too lean at lower rpms. Might try adjusting mixture screws on #2/3 carbs (screw in for leaner low speed). Would also confirm that pointed tips of adjusting screws are in acceptable condition.

If not already done, would also assure correct float bowl service fuel level via clear plastic tube method.

Am uncertain why new needles were necessary, but believe needle clip positions not so critical from idle to 2500 rpm, and actually seem okay because of good performance over 3000 rpm. Could be very wrong, but don't believe current symptoms are caused by the needle clip position.

With stumble off idle, would be more concerned with pilot circuit (from idle to 2500), especially clean pilot jets and pilot air passages.

Suppose it's remotely possible that choke enrichener circuit is not completely closing.

Rooting for you!

Post edited by: Patton, at: 2007/05/24 04:52

Seems like if you downshifted and tried riding in a more appropriate gear instead of 5th at 30 mph, it might help... you are likely lugging the engine now that I think of it. I can't ride around at 30mph in 5th in any of my bikes smoothly. That is what 3rd gear is for.

I'm not the one riding in 5th @ 30 MPH.

I'm just trying to get this thing underway in first gear. It has literally no power off idle until above 3000 RPM.

Patton, the reason I put the new needles in is because one was bent at the tip a slight bit. Perhaps I should have replaced that one only.

Reading elsewhere on the Web, the comments lead me to believe that it is lean. I suspect the slip-on mufflers have less restriction than stock and it's leaning out when the throttle is first opened.

Ignitionwise, there are good points on it. I assume the condensors are good. However, it has the stock coils and I've tried two different coils on 2/3 with no change in behavior, so I'm thinking carburetor issues (the reason I focused on 2/3 was popping and misses from the left exhaust). Since this is the first time trying to resolve an issue like this I naturally want to zero in on the issue rather than chase my tail for the next month or two.

I would think that if it were running rich that I would see a bunch of black smoke out of the exhaust. However, the engine bucks and runs rough similar to when it is run out of gas.

BTW, I cleaned the carbs and replaced needed O-rings and gaskets and left everything else stock, except for the needles since, as mentioned, one was bent. The original needles were in the middle groove as well.

Sigh... Checking the level with a tube proved next to impossible on the middle two carbs with the close proximity of the outer carbs and the air tubes in place. I'm thinking of making a jig so I can check them on the bench.

N0NB wrote:

Okay, I'm about at my wit's end with this project '78 650 SR (mixture screws on the lower front of the carb).

It has a very pronounced stumble off idle to 2500 ~ 3000 RPM . . . Am I on the right track thinking that it's probably leaning out?

Hi, do me a favor and link your thread regarding this machine to this thread for me. I remember reading about it, but would like to review the history of what you've done.

Rather than ask questions that may have already been answered in your posts.

N0NB wrote:

... trying to get this thing underway in first gear...literally no power off idle until above 3000 RPM...believe that it is lean... suspect the slip-on mufflers have less restriction than stock and it's leaning out when the throttle is first opened...good points...assume the condensors are good...tried two different coils on 2/3 with no change in behavior, so I'm thinking carburetor issues (the reason I focused on 2/3 was popping and misses from the left exhaust)..... would think that if it were running rich that I would see a bunch of black smoke out of the exhaust. However, the engine bucks and runs rough similar to when it is run out of gas....cleaned the carbs and replaced needed O-rings and gaskets and left everything else stock, except for the needles since, as mentioned, one was bent. The original needles were in the middle groove as well....Checking the level with a tube proved next to impossible on the middle two carbs with the close proximity of the outer carbs and the air tubes in place. I'm thinking of making a jig so I can check them on the bench.

Does indeed sound like carb issue. The wet 2/3 plugs seem inconsistent with being overlean. "Scalded dog" running at higher speeds indicates proper fuel-air mixture where carbs are then operating beyond pilot circuit. So problem really seems to be in pilot circuit. The bucking akin to fuel starvation might possibly result from fuel fouled (wet) 2/3 plugs clearing themselves at higher rpm. Concur it's probably best to use all four new jet needles to be sure they all match. And perhaps I'm focusing too much on wetness of 2/3 plugs. (BTW, do believe that putting around in higher gears at low rpms relies mostly on the pilot circuit.)

My understanding is less muffler restriction can result in less torque, but doubt it's the main culprit for stumble here off idle to lower rpm levels. Freer exhaust mostly aids engine breathing at higher rpms, with less influence at low rpms.

Would consider installing new pilot jets. Also, would spray carb cleaner through air passageway in pilot circuit from intake hole at rear of carb (might be an air jet at entrance - if so, assure it's clean and clear) . Block bottom of pilot jet with finger while spraying, and believe spray should emerge from orifices (bypass outlet and pilot outlet) in bottom of carb throat located before and after throttle slide closing position on engine side. This should assure clear passages throughout pilot circuit.

Is advancer unit operating properly -- springs okay and connected -- and unit not stuck in open "advanced" position?

If ignition components are all correct, and enrichener circuit is properly closed, and float level is correct, and float needle/seat up to specs, with ample clean fuel supply to float bowls, and pilot circuit is perfect, can't think of much else to suggest (am still pondering).

Will be interested in final resolution.

Post edited by: Patton, at: 2007/05/24 12:46

KaZooCruiser wrote:

Hi, do me a favor and link your thread regarding this machine to this thread for me. I remember reading about it, but would like to review the history of what you've done.

Rather than ask questions that may have already been answered in your posts.

Okay, I think you mean my thread in the Projects forum about it.

I've decided that before I tear into the carbs, which I am reasonably certain are clean as I cleaned them and used compressed air on all passages and dirty gas has not be put in them since, I am going to make sure the advancer is lubed and free. While it is working, I have some doubts that it's working correctly. I think it bears closer scrutiny.

Ok, I did some look and see and saw this:

N0NB wrote:

After doing putting the cams back in place I checked the time and found the cams were retarded by one tooth.

Perhaps this is why the starter clutch was worn out and the kickstart spring out of place.

At 32K, I wonder why (unless done wrong somewhere)the cams are late.

You didn't by chance measure the chain for wear?

You are going to check the advancer. Maybe springs are bad, or it's sticky.

Where do you have the pilot screws set at this time?

I 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.

Did you replace the o-rings on the pilot screws, and verify spring length on all of them?

hope some of this helps