Topic: Lack of power at low revs

[Tdinova]

I had problems with my sons '80CB650 earlier this summer which never really went away. When I first take off the bike is lacking power for a bit then picks up. I had the carbs off numerous time to check-recheck everything. It all looks great

idles great
replaced all plugs, wires and plug caps.
new air filter.
syned the carbs.

Would the bike be running lean? The plugs only have 1-2000 km on them and look perfect.
Should all the gas screws be out the same turns?

If the bike is running lean, should I turn the screws out or in and by how much ?
any help would be appreciated

[worlddrum13]

What does it do when you choke it while accelerating? And changes?

[Tdinova]

The bike works abit better and smoother. I've doing more reading on here and when the choke is pulled it smooths out. That probably means a lean running bike. Is that right? Should I turn out the screw by 1/2 a turn or 1/4?

[worlddrum13]

The bike works abit better and smoother. I've doing more reading on here and when the choke is pulled it smooths out. That probably means a lean running bike. Is that right? Should I turn out the screw by 1/2 a turn or 1/4?

I don't know about your 650. My 75 750 you turn them in to richen them. Like I said I don't know about your carbs. I'm sure there's a guru here who can let you know for sure. My advice is to turn them in about 1/4 and try it. If it's better feeling than go from there. If it's worse than go the other way. But check the manual first

[scottly]

What do you mean by low revs? How low?

[Tdinova]

When I first take off the bike hesitates then takes off

[nick_noblitt]

Check out the carb info under FAQs, it is definitely running too lean if it runs better with the choke on.  I would set the idle to 1500 to 1800 and tighten the screws until the motor start to bog down then back it off .5 turns.  Take it for a 10 minute spin and check the plugs to see if they are white tipped (to lean) or covered in dark soot (to rich)

Good luck

[scottly]

When I first take off the bike is lacking power for a bit then picks up.

Sounds like you are lugging it. These bikes don't make any power until they get up there in revs.
 
idles great

Good!

[Spanner 1]

Un-SOHC riding practice detected..... or it might be something else ... ( Useless post # 6,734).



Stuck advancer unit, maybe?

[scottly]

Un-SOHC riding practice detected..... or it might be something else ... ( Useless post # 6,734).

Spanner, be nice to the noobs; they don't yet know how much these motors love to rev! 

[Spanner 1]

Again, advancer unit , maybe...... that's all I got.
P.S.... the 'useless post' referred to MY reply, not the OP's question....... ! BTW.

[Tdinova]

Stuck advancer unit, maybe?

I had the pulsars off and the advancer was a tad stickie. When I rolled the advancer they were a tad notchy. I lubed then and put it back together.

[/quote]

I don't know about your 650. My 75 750 you turn them in to richen them. Like I said I don't know about your carbs. I'm sure there's a guru here who can let you know for sure. My advice is to turn them in about 1/4 and try it. If it's better feeling than go from there. If it's worse than go the other way. But check the manual first
[/quote]

I turned the gas screw out 1/2 turn and went for a spin. What a difference. It was either the advancer or the gas screws, went back home and turned them out about 1/8 more, even better.. I am used to working at problems.. DO one adjustment and try the bike. I should not jump saying it's fixed, so tomorrow night I'm going for another spin and report back here.

I own a '07 ST1300 and before that I had a 1981 CB750 Custom for 25 years. The ST is flaw less and miss not working on the Custom. But with age we all must move on.

[mystic_1]

I turned the gas screw out 1/2 turn and went for a spin. What a difference. It was either the advancer or the gas screws, went back home and turned them out about 1/8 more, even better..

Glad to hear you're making progress.

I am used to working at problems.. DO one adjustment and try the bike. I should not jump saying it's fixed, so tomorrow night I'm going for another spin and report back here.

Sound advice indeed.  Might I ask where your screws were to start with, and how that compares to the stock settings?  Is your bike stock, btw, or has the induction or exhaust been modified?

I'd suggest doing some plug chops at lower throttle settings, to get a good read on your mixture.

mystic_1

[Tdinova]

Glad to hear you're making progress.


I'd suggest doing some plug chops at lower throttle settings, to get a good read on your mixture.

mystic_1
[/quote]

I'm almost back to square 1. The bike is not 100% fixed, just wishful thinking, but it does work a lot better.

mystic_1

what do you mean " plug chops at lower throttle settings" and how do I do it and what will I look for after the test?

One more thing, before the problem I could snap the throttle fast and the bike would rev up fast, now the bike stalls.

Forgot to add ,

I started at 1.5 turns out now at ~1.85 out on each
The bike has aftermarket 4/2 since 2005

Should all the screws be out the same distance?

[mystic_1]

So, generally speaking a plug chop is done by driving the bike at a consistant speed for a bit, and then suddenly killing the motor using the kill switch while simultaneously pulling the clutch, coasting to a stop, and pulling the spark plugs to read their color.  The color of the plug insulators gives you an indication of the mixture at the throttle setting you were at when you killed the motor.  Do this at, say, 1/4 throttle or 1/2 throttle or whatever, depending on where you're interested in checking.  Make sense?


Here's a thread from hondachopper.com that talks about the process:

http://hondachopper.yuku.com/forum/viewtopic/id/2802

Here's a chart to compare your plugs to:

http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

Oh, and yes generally speaking the air screws should all be set to the same setting across all your carbs.

Hope this helps.

mystic_1

[Tdinova]

Here's a thread from hondachopper.com that talks about the process:

http://hondachopper.yuku.com/forum/viewtopic/id/2802

Here's a chart to compare your plugs to:

http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

Oh, and yes generally speaking the air screws should all be set to the same setting across all your carbs.

Hope this helps.

mystic_1

I done that tonight.
1st I went for a fast burn back the road. The bike acted as usual, revved out-no problem, but once the engined level out the surging started. After a 5 min run I turned the bike around, fumbled at low revs, almost stalled the picked up as usual. BUT I did not do the 5th gear wide up then cut out the engine because I did not have the room OR the neighbors to allow it. So I went back to 3rd gear and kicked er' down (wide open for 5 secs). About 100 feet from my driveway I hit the kill switch-pulled in the clutch at the same time and coasted to my driveway. Push her up the hill and pulled the plugs. # 4 looked the best of all (just a bit of brown on the porcelain) #2 was abit lean(not too much) but 1 and 3 was white (porcelain was  as new)

Now that made me worry because 1 and 3 runs on it's own circuit and 2-4 on it's own,

SO
I opened
#4, 1/4 turn,
#1 and 3, 1/2 to 3/4, and
# 2, 1/4 to 1/2.

Went for the same test run and what a difference..

NOW I'm not saying it's cured until I go for a ride 3-4 days without a problem.
I should be able to do this by the end of the weekend..

I'll post back

[mystic_1]

BTW you don't necessarily need to do the plug chop at full throttle.  The procedure yields a 'snapshot' of the mixture at the moment you killed the motor.  The important thing is that you run at the desired throttle setting for a little while before chopping the ignition, so you give the plugs time to color up as it were. 

Plug chops at full throttle mostly reflect the size of your main jets.  Plug chops at lesser throttle settings will give you an indication of your air screw settings or pilot jets.

The following chart is helpful:



mystic_1

[the technological J]

i say put some seafoam in it and take her for a 200 mile ride(seriously) take the seafoam for when u need gas....  you could have a real touchy clutch... and my bike doesnt have any real power until around 3500.... also while the bike is idling spray some starting fluid infront of the carbs (between the engine and carbs) to see if u have an air leak

[Tdinova]

Went for my 1st test run tonight. I was excited to glide along with ease. Well that was short lived.. same thing as last night. But I did notice something after a few stops, turn off the engine, restart and go's (about 3) The bike came back to being perfect again.. REALLY, smooth, quick snap of the throat and she would rev without any hesitation. It's a real mystery.

 misunderstood how to do the plug chop.... So what you are saying I should..

hold the throttle at the position where it is running bad ,
kill the red switch, close the throttle and pull the clutch at the same time? 
pull over, pull the plugs and adj my gas screws accordingly? whiter plugs (unscrew the gas screw a bit) and try to get all the plugs to look the same

I had tried seafoam about a month ago, 1/2 a can to 3/4 tank of gas. 
More to follow tomorrow night.

Thanks for all the replies

I appreciate it

Blair

[mystic_1]

So what you are saying I should..

hold the throttle at the position where it is running bad ,
kill the red switch, close the throttle and pull the clutch at the same time? 
pull over, pull the plugs

Yup!

and adj my gas screws accordingly? whiter plugs (unscrew the gas screw a bit) and try to get all the plugs to look the same

Not necessarily.  The idle mixture screws only affect the low end of your throttle settings.  At anything above 1/2 throttle or so the idle mixture screws are largely irrelevant.  Main jet selection and needle position is what's import at higher throttle settings.

Note also that we're talking about throttle position here, not necessarily RPM, although the two are definitely related.  Make sense?

Finally, if there are signifigant differences in mixture between your carbs, with similar settings, then something else is wrong.  Some variation between idle mixture screws is OK but if it's more than say 1/4 to 1/2 turn difference between carbs, something else is up.  Main jets and needle positions should be the same across all carbs.

mystic_1

[Tdinova]

So what you are saying I should..

hold the throttle at the position where it is running bad ,
kill the red switch, close the throttle and pull the clutch at the same time? 
pull over, pull the plugs

Yup!

and adj my gas screws accordingly? whiter plugs (unscrew the gas screw a bit) and try to get all the plugs to look the same

mystic_1

So pulling the plugs at the surging and reading them will not help?

Thanks for the reply. # 1,2,3 are about 1/4 to 1/2 higher than # 4.

The needles main jet is clear and needle position was never touched,

The surging happens at ~ 1/8 to 1/4 position. About 45-50 mph or 75-80 kmph. Any nothing higher or lower all is well.

Again thanks for the help.
Blair

[Tdinova]

I give up..

Same thing.. The pilot screws were out 3.5 turns... I put them back to 1.5 and she sucks, It stalls when taking off. Out at 2 sucks too.. My last resort was to remove the carbs,

Checked float height  -  good

Removed pilot screws and checked for plugged jet , O ring good, washer was there -  good

Put a small hose on the slow jet and poured gas in to see if it was plugged    -  no, it was good, gas flowed through

Main jet   -  good, no plugged holes in jet

Checked 2nd carb acceleration pump   -  good

Syn cabs   -  good

Sprayed WD40 around the rubber boots to check for cracks   -  good

Checked timing with timing light  -  good

Advance timing  -  good


Idles perfectly, when the bike is fully warmed I used to snap the throttle and revs , no problem, now it stumbles and stalls. The bike used to have quick pickup during mid range, - no more.

Can some one explain how-to set the air screws?

[Spanner 1]

Didn't know your carbs had an accelerator pump.... if any of the 4 nozzles in the carb throats are not 'squirting' then no/ little acceleration .   Pump comes into action just above idle. Look at the nozzles as you twist the throttle ( bike does not need to be running ) airbox, obviously needs to be off! The nozzles are the small brass post @ 5 o'clock in each carb throat ( airbox side ) . They must squirt gas towards the intake each time the throttle is twisted.... good luck.

[Tdinova]

Didn't know your carbs had an accelerator pump.... if any of the 4 nozzles in the carb throats are not 'squirting' then no/ little acceleration .   Pump comes into action just above idle. Look at the nozzles as you twist the throttle ( bike does not need to be running ) airbox, obviously needs to be off! The nozzles are the small brass post @ 5 o'clock in each carb throat ( airbox side ) . They must squirt gas towards the intake each time the throttle is twisted.... good luck.

Thanks for the reply.

yes his bike has an accelerator pump.

When I had the carbs off, I laid them upright on my bench and filled them with gas to ensure I had no leaks, and then I used the throttle to ensure they worked. Each of them was squiring towards the intake. I noticed the rod that came from the pump was pretty close to the stop like almost no clearance. His is at the 6 o'clock postion.

I also checked the level of the gas by connecting a clear hose onto the bowl drain and ran it up along side the carb and the gas almost reached the top of the bowl.

[Spanner 1]

Well I'll be darned!... guess it's not that then....... on a sidetrack; you's guys up there in Nova Scotia have the closest thing to a full blown Irish accent I've ever heard outside the Emerald Isle