Topic: High Speed hesitation....

[LeoLegendATL]

I noticed a problem today while riding at around 5000rpm at about 70 mph.  At this speed any throttle inputs seem to spuder the engine and it won't rev past 5000rpms.  Feels like it's out of gas.  I checked and cleaned the petcock and all is well.  The problem seemed to go away and come back, any ideas what this could be.

[Buffo]

change the fuel filter. go from there

[chrometank]

A lot of fuel problems are caused by the vent in the petrol cap cloging and creating a vacuum.Might explain the feeling of running out of gas

[memphis3724]

I noticed a problem today while riding at around 5000rpm at about 70 mph.  At this speed any throttle inputs seem to spuder the engine and it won't rev past 5000rpms.  Feels like it's out of gas.  I checked and cleaned the petcock and all is well.  The problem seemed to go away and come back, any ideas what this could be.

I ran into this problem last week with my new '76 Honda CB550F Super Sport. The mechanic took the carb apart and found that the emulsion tube and jet neddle were worn and need to be replaced. Basically what my case was that it was getting to much fuel and floading while driving it. Hope this helps...

[redschwa]

I know this is an old thread but hopefully I'll bring it back into the light for some good info

I have almost the exact same problem.

My 82' cb650sc does the same thing at 5500 RPM but at about 65MPH. In fourth it pulls all the way up to about 65mph when i shift into fifth it does absolutely nothing. I get no extra power.

Once or twice it seams to have disappeared and i suddenly jump up to 80MPH then it bogs back down.

Did you solve your problem?

Does anyone have any ideas? I'd like to get on the highways. BTW my carbs are rebuilt and perfectly clean. I have no inline filter. My tank is rust free My petcock screen is crystal clear and the carbs are professionally tuned.

[w1sa]

What you describe could be caused by low and/or erratic fuel delivery. If fuel flow to the carbs is not adequate for sustained higher engine speed, further increase in throttle/rpm/speed may become unachievable.

Fuel flow can be affected by tank vacuum. This can be the easiest to check by ensuring the tank breather vent/hole is clear and/or opening the cap while riding and seeing whether the symptom recurs or persists.
You could also check whether applying the choke (progressively) improves or decreases throttle/engine response at/above 65mph. Improvement may indicate lean running (at that engine speed/load). Decreased response may indicate rich running (at that engine speed/load)............................

Proper float bowl fuel level is what needs to be maintained by the fuel delivery system to allow throttle/engine response across the rpm/speed range. The standard fuel delivery to the carbs on your bike includes a vacuum controlled fuel flow valve unit between the tank and carbs. If it's not functioning properly fuel flow will be reduced. The floatbowl fuel level itself is regulated and maintained by the float/valve in the bowls. If it's not set correctly, the fuel level in the bowls will be wrong.

...............................
 BTW my carbs are rebuilt and perfectly clean. I have no inline filter. My tank is rust free My petcock screen is crystal clear and the carbs are professionally tuned.

Often professionally done carb tunes are not much more than a carb sync, usually performed in a workshop environment at low engine speeds and without any sustained on road/load/ performance testing

How extensive was the carb clean and rebuild? Who did it?
How extensive was the carb tune i.e. what did they claim to do?
Have all other aspects of regular maintenance/tune-up been performed (plugs condition, filters, cam chain adjustment, valve clearances etc)

Regarding on road performance.
When changing from 4th to 5th at 65mph,
Have you already run out of acceleration at full throttle in 4th?
Change into 5th, do you then immediately open full throttle/slow roll on to full or wider  throttle?
Is there any surging/hesitation under acceleration or at 65 in 5th while trying to accelerate?     

[redschwa]

Thanks for the great response.

My main guess at this time is probably the fuel cap. I've had problems with it before.

My second step would be float bowls. Too bad mine aren't adjustable. They're plastic with no options for adjustment.

As to your questions on tuning and cleaning. I cleaned my carb, and installed carb kits on all. All other maintenance is tip-top. I've been bringing her to life for several months now. The tuning might be a problem. I didn't have the tool so i took it to a shop that i don't really know, or like for that matter, and had them do it. I can't tell you for sure they did a great job or how they did it, but i noticed better response afterwards. The bike pulls properly through all low gears and easily revs clear up to 8000 RPM.

To answer your final questions I have run out of acceleration in 4th at 65MPH. I easily get up there at 1/3 throttle or less then it hits a wall. It feels like a governor I once had on a dirtbike as a kid. The bike just wont go any faster and it all happens so suddenly.

While in 5th I've tried rolling on the throttle and just opening her up. When i role on it just maintains engine speed until about 1/2 throttle then the speed sputters down and up.

I don't think it is the cleaning or carb problems. They are practically brand new. My main suspicions are the tank breather and maybe the float bowls. Also I remeber my bike doesn't have this weird vacuum thing that is between the petcock and delivery to the carbs. It leaked really bad so i had it removed. I'm thinking this might be a problem but I had several people assure me that it was unnecessary and theirs ran fine without it. Also I had the same problem before it was removed but that could be due to it's malfunctioning.

[w1sa]

Removing the vacuum controlled fuel valve won't cause any problems, so long as you turn the tap off when not being used.

When cruising at 65mph, you probably consume somewhere between 60 and 80 ml per minute. The full throttle flow rate required is probably greater than 200ml/minute.

If it's practical you can do a flow test from your tank into a container. At the same time you can check whether your tank vent is causing a vacuum lock. That will possibly eliminate any flow restrictions from tank cap thru tank/tank filter thru fuel tap, to the fuel line connection at the carbs.
With say min 2/3 of a tank of fuel, fuel cap secured on, fuel tap 'Off', disconnect the fuel line at the carbs end and start draining the fuel thru the fuel tap/line into a container. Measure the initial flow rate (was it say 200ml/min), then keep draining. As the level in the tank drops, verify whether the flow rate changes.
If the flow rate reduces noticeably, open the fuel lid to atmosphere and check if the flow increases. If so, the lid vent is a problem. If the flow doesn't increase you may  have a problem in the tank filter or the fuel tap/ screen.

Here's a link to a manual from FAQ section.
http://cosky0.tripod.com/
Does it cover your bike?

If the tank vent/flow test checks out ok, go for a ride and when the symptom recurs, try the choke operation(previously mentioned) to see whether that affects the throttle/engine response.             
                                     

[pampadori]

two words, bowl venting.  my bike did this before.  It would run funny at 60mph, no matter which gear i was in, when i was giving it just enough throttle to maintain that 60.  If i opened up the throttle more, the bike would run better.  If i dropped below 55mph or so it would also run much better. 
The 4 vent tubes coming off the carb bowls were in turbulent air. move them so the weren't hanging down so low and the bike ran better at 60mph. 


The other common issue with these bikes is running pods on them the air hitting the two outside filters causes them to run differently.  Moving your legs you can feel the bike start to behave differently.  Moving to v stacks or back to stock airbox fixes this problem.

[redschwa]

Thanks w1sa. I'll check the flow tomorrow evening and get back to you on that.

Thanks pampadori, however my bike doesn't have carb bowl vent tubes. Never did, don't know why guess it just doens't need 'em and shes on stock airbox already, but thanks anyways.

[redschwa]

The fuel flow is perfectly fine. Somewhere around 600ml a minute through until is gets very low. I did notice that when I put it to reserve i get maximum 80Ml per minute. Doesn't mean much just noticed it for anyone who cares. Unfortunately I'm waiting on a damn main fuse until Thursday so I won't be able to try the choke until then.  One possibility down, hundreds to go! 

[w1sa]

Thanks pampadori, however my bike doesn't have carb bowl vent tubes. Never did, don't know why guess it just doens't need 'em and shes on stock airbox already, but thanks anyways.

The stock carb arrangement on your bike does have carb air venting. It's a common vent arrangement. T joints between carbs 1-2 and 3-4 with a single tube between 2-3. Single airvent tubes run off the T joints to another T, and a single airvent tube running to rear of carbs/positive pressure side of airbox?
Check to ensure the air vent tubes /Ts are connected and clear and free of being blocked/clogged/sealed/pinched. Any restriction in the venting could be a major reason for your problem. Dirt and insects can get into open end over time.

Also, is the airfilter fresh and clean?

[redschwa]

Okay, I don't know exactly what your talking about.

I think I might have a problem because i have no tubes or T joints on my carbs. The only tube i have is about an inch long on the top of the third carb with a screw in the end of it.

Where exactly are they located? Are they plastic near the bottom and rear of the carbs (by rear i mean rear of the bike)? If so I don't have anything there when i got the bike they were already broken and mostly gone with no hoses connecting them. When I cleaned my carbs i removed what remained of the plastic T-joints.

Are we talking about 27 on the diagram?

[w1sa]

Go to motogrid.com        Look under OEM/ Honda/Motorcycles/CB650sc '82/carburetor assmbly

Parts 10, 11, 20, 21 and 8 on the Aircleaner sheet.

[redschwa]

Finally got my fuse and I'm off to the races! 

So far I removed the air filter cover. It helped in response but didn't affect the top speed problem too much.

After running her at 65mph for awhile I pulled over and checked the pipes. Unfortunately, I couldn't think of any other way to make sure they were hot than to touch them but now I have four burned fingers. They were all very hot so I think all four cylinders are going the entire time.

I couldn't test the choke because for some reason my choke stopped responding right before the ride. It just doesn't work anymore(add it to the list right )

Still going to check ignition timing, the choke, and any other suggestions you guys have or ideas I've forgotten. I really wanna get to the bottom of this problem.

[HondaMan]

This minor problem can be fuel or spark. If you can find an ohmmeter or know how to use one, pull each plug cap and check the resistance of the litle brass connector that screws into the wire and clips onto the plug. It must not be more than 10,000 ohms. It also must not be less than 7500 ohms.

What plugs are you running? pull them out and put pix of them here? They will tell you whether it is fuel or spark. If they are pure white, it's fuel, as in "too lean". Then, it can be either poor delivery (like that 80mL you mentioned: that's low) or choked fuel screen in the petcock. If they are dark, it can be plug caps, timing, vacuum leaks on the intake hoses & manifolds, or the wrong spark plugs. What plugs are you running?

[redschwa]

I finally got a multimeter and learned how to use it and these are the results:

Cap #1: 4250 ohms
Cap #2: 4030 ohms
Cap #3: 4680 ohms
Cap #4: 5150 ohms

I'm running NGK DR8ES-L plugs.
This is Cylinder #1

This is cylinder #4


I don't have three and two because I lost my spark plug socket and couldn't get them out. They have 100-200 miles on them. I also noticed that the ends of the plug wires looked awful. I could barely see the copper wire in the insulation. 

[redschwa]

I was looking into Ignition stuff and ran across what I believe to be a problem. My igntion timing seams to be way off. It looks like it is all the way to the retarded side.


I haven't done anything to it. This is how I got it. I would like to do the ignition timing but I really don't understand this stuff. Later today Im hoping to borrow a timing light from a friend who can hopefully help me get it timed right. Is there anything else I should do while I'm adjusting it? And could this be affecting my problem?

[w1sa]

Not a real clear shot, but plug#1 appears to be suffering some oil fouling. Neither appears to be over lean/rich, but that doesn't mean to say you dont have a fuel/mixture problem above 5500rpm in higher gears under load.    Have you checked the other plugs yet?

I think the standard plugs for USA are D7EA. The DR8s you have are resistor plugs, which in combination with the resistor caps (4-5k) you've got might be ok. (per Hondaman advice above).......................... Hopefully Hondaman will chime in and confirm his view.
You want all (like) ign components to be of close/same spec value to oneanother, for best performance.

The 'Static' timing is achieved by aligning the 'rotor lobe tooth' with the 1-4 pulser unit  (the one at the left) 'sensor projection'. The 1-4 SF mark should then align to the index mark.
The 'Dynamic' (strobe) idle timing is observed/correct when the 1-4 Fl  marking aligns with the index mark at say 1000-1100 rpm. Dynamic 'advance' timing is observed/correct when the index mark is aligned between the advance ( ll ) marks at say 2750 or greater rpm

[redschwa]

I finally figured out the ignition and fixed it all up. I greased the advancer which was dry and squeaked when rotated then i properly adjusted my gaps and timing.

I noticed improved performance but it still didn't feel right. I couldn't try for a top speed run because of the local law enforcement but I was running at 75MPH in fifth gear and about 7300RPM. This seams like way to many RPM's and it was sluggish getting there.

I think I'm going to replace my wires and caps because the wires are messed up and I might as well do the caps while Im there. Is this a good idea?

Any other suggestions?

[w1sa]

New leads and caps certainly may help, but I'd be more interested in the condition of 2-3 plugs first(even with only a couple hundred miles). They may indicate something more definite about the other half of the engine's ignition/combustion.

If 2-3 are currently fouled, new leads and caps wont change that unless the only problem was in the leads/caps and you change out the fouled plugs as well. Just like changing to new plugs wont ensure continued good serviceabilty if the condition that caused the bad plug condition in the first place isn't corrected.

Ideally, all plugs should be pulled at the same time (after simultaneuos use in running) so you're comparing apples with apples.

Edit: PS, I agree, those revs are way too high. Looks like you may have some clutch slip going on!

[shizzomynizzo]

My 550 had a similar problem. It would not go over 5500 rpm in any gear.

Turned out to the plug caps. they looked new, but problem cleared up with a new set. If they're old, change 'em.

[pampadori]

I was running at 75MPH in fifth gear and about 7300RPM. This seams like way to many RPM's and it was sluggish getting there.

don't mess with the ignition anymore.  Stop.
the above problem needs to be fixed or i will go insane.  5th gear shouldn't be near 7300 at 75mph.  your clutch is slipping and allowing the engine to rev higher without adding to your forward momentum.
do this test in 5th gear.  the drive train should of your bike is all locked together and is working correctly. the ratio of 5th gear and the sprockets cannot change on their own.  so they are a constant.  the only thing that can change is how much pressure the clutch will hold. tell me what speed the bike indicates at these RPM's in 5th gear.
at 2000 rpm -
at 3000 rpm -
at 4000 rpm -
at 5000 rpm -
at 7500 rpm -

then we will see if the numbers match or not.  4000 should be twice the speed of 2000.  7500 should be twice the speed at 3750, and so on.

[redschwa]

the above problem needs to be fixed or i will go insane. 

Ha ha, no problem. Tomorrow morning I'll get that info for you. 

[Hush]

Good call on the advancer being dry, seen this many times, forgotten part of bikes.