Topic: Runs good! Sometimes.....

[cb750f-2010656]

I just don't get it.  Most days she feels like she's lagging on a cylinder.  Other times its just smooth consistent power.  I checked the carbs over and tuned everything else.  She must be mad as hell at me for not spending enough money on her.  Typical woman.  Don't she know I LOVE her??

[kerryb]

Only ride her when she's in the mood...wait a minute, where did I hear that before?

[cb750f-2010656]

Gotta get to work, attitude or none.

[ekpent]

Only ride her when she's in the mood...wait a minute, where did I hear that before?

How long have you been married ? ,may never ride again 

[kerryb]

Is there evidence that the same cylinder is the culprit each of the down days?  Is the fuel fresh?  Does ambient temperature or weather seem to make a difference? 
Have you checked ground connections for the coils, checked plug caps for looseness &/or general condition?

Tell us more about the bike and special parts or changes you've made to her.

Try getting her a new hat, they always feel better with a new hat...or shoes, yeah, they like new shoes!

[BomberMann650]

Get purses and jewelery if she's that type of gal.

[jonda500]

Try proving your faithful love and affection for her - polish her up from head to toe, squirt some seafoam in her tank and warm her up for a good five minutes or so before riding her long and hard...
John

[disco]

I just don't get it.  Most days she feels like she's lagging on a cylinder.  Other times its just smooth consistent power.  I checked the carbs over and tuned everything else.  She must be mad as hell at me for not spending enough money on her.  Typical woman.  Don't she know I LOVE her??

Just went thru a similar episode with newly rebuilt 836cc. It would run beautifully one moment with crisp throttle response, then as you say felt like lagging on one cylinder. I finally narrowed it down to No1 cylinder (pipe was cooler than others). Removed the spark plug cap & held the plug wire up to the spark plug terminal......vrooom, back to her old self! Measured the old spark cap resistance with multimeter & they're knackered. New NGK spark plug caps & spark plugs on order!

[Jerry Rxman Griffin aka MuthaF'er]

Make sure the tank vent in the cap is not clogged. Next time it starts that loosen the cap and check the response. Make sure the fuel tank filter is not clogged and flush the tank. Simple checks that don't hurt to quickly verify and check off the list.

[Deltarider]

Most days she feels like she's lagging on a cylinder. 

Plugs should show which one. Common causes are: (intermittent) arcing of HT leads to ground and hindered fuel flow by non standard fuellines that are kinked. Iinline fuelfilters can be the culprit. Tank ventilation? Sticking float needle?

[cb750f-2010656]

She is a stock 76 750F with the exception of the new long-@$$ Jardine slip on muff.  Carbs overhauled, though still I believe I need the top plates with the rubber grommet in the center for the slide rods (old round top).  I have installed new manifolds on both sides of the carb a few years back.  Three plugs are sooty, number two being the oddball and it looks fine.  I have noticed number three doesn't run well at idle, and my compression reads 1-4: 130, 127, 125, 110.  My plug wire cap on number three is in fact broken.

Also to note: When she acts up, I have this flat spot at low throttle.  cracked it is fine, but after that and for about a 30 degree twist of the throttle engine seems to load up.  When it runs okay the flat spot is still there, but much smaller.  This flat spot makes navigating corners at speed very dangerous.

I ran an entire can (not at once) through her when I first got her out again.  I have checked the screen inside the tank, and have a new inline after that.  The tank vent is clear.  Still need to go through and check all my electrical connections, though.

Thanks everyone for the help.  She is a good old girl and forever has a place in my heart and my front yard.  Sorry, but the 86 FWB de'Elegance has the garage.

[cb750f-2010656]

I've been wondering about my compression readings.  I assumed that the steady decline from left to right was just the oil film being washed away from the cylinder by fuel.  Is this normal?  I took those readings without draining fuel from the bowls, and only removing one spark plug at a time to reduce the risk of FOD in the cylinders.  Also, I only used the kick starter.

Also, is 130PSI good?  Low?  I'm asking because my 16 valve GS750 makes in the 150's across the board, however it is electric start.

[flybox1]

Carbs overhauled

Please define this...and be detailed.

[cb750f-2010656]

She was stored for 3 years with all fuel drained after this operation.  Prior to returning her to service, I opened up the carbs and checked them over again, and again after riding for a couple of weeks.  The overhaul I mentioned proceeded as follows:

I stripped them down.  Put the bodies and bowls into some of the newer B12 Chem Dip.  The dark green stuff that really doesn't work worth a $#!T.  After soaking for a couple of days I thoroughly rinsed them and dried with compressed air.  I installed new pieces including float needles/seats, jets, air screw, the spring and washer for the air screw, all O-rings and gaskets, and fuel line.  I checked my floats before I put them in.  Adjusted the floats, and bench synced the carbs.

I did not change the round part at the top where the slide lift rod comes out.  Those were like $100 a pop when they WERE available, so I passed up on that.  All the rubber manifolds in between airbox/carb and carb/ head are new.  The fuel petcock is new.

[flybox1]

She was stored for 3 years with all fuel drained after this operation.  Prior to returning her to service, I opened up the carbs and checked them over again, and again after riding for a couple of weeks.  The overhaul I mentioned proceeded as follows:

I stripped them down.  Put the bodies and bowls into some of the newer B12 Chem Dip.  The dark green stuff that really doesn't work worth a $#!T.  After soaking for a couple of days I thoroughly rinsed them and dried with compressed air.  I installed new pieces including float needles/seats, jets, air screw, the spring and washer for the air screw, all O-rings and gaskets, and fuel line.  I checked my floats before I put them in.  Adjusted the floats, and bench synced the carbs.  What aftermarket carb kits did you use?  Jet brand?  Using the clear tube test, did you confirm the float valves are maintaining a proper bowl fuel level?

I did not change the round part at the top where the slide lift rod comes out.  Those were like $100 a pop when they WERE available, so I passed up on that.  All the rubber manifolds in between airbox/carb and carb/ head are new.  The fuel petcock is new.

[cb750f-2010656]

I used the k&l kits.  I have clear tubing installed all the way.  I haven't gotten around to checking the fuel levels in the bowls.  I need to remove the screws from the bowls as they are frozen, and would like to wait until I have some more cash set aside and a second set of wheels in case I break something.  I'm picking up a motor for my GS500F tomorrow morning but it'll be a few weeks before I can start riding it.  Then I can afford to have the Old Girl down for a while.

[flybox1]

If you still have the OEM Keihin jets....install those.  Aftermarket jet quality control is less than desirable.

[cb750f-2010656]

I don't but I can buy some.  I'll replace the spark plug caps and try one more time to tune her, also.  But if anyone has those carb tops I mentioned before, that I didn't replace, I'll buy them.  When I open the throttle, I can see the rods being pulled slightly forward, right about where my flat spot is.  Gotta fix that too.

[cb750f-2010656]

Just curious, how much can one expect to pay for a new rack of carbs, if available?

[flybox1]

Just curious, how much can one expect to pay for a new rack of carbs, if available?

New?  Not available.  Used? Ebay, but buyer beware.
Unless the corrosion is so bad the jets wont stay in place, most carbs can be put into service.
PM member harisuluv...he can provide you all the parts you need, and in good condition.

[cb750f-2010656]

Cool.  I'll give him a shout.

[evinrude7]

+1 on harisuluv. 

[cb400f/20yo/semi-broke.]

I rode my bike (CB400F) to college 10 km's, then rode it home. It took me a semester to fill it up with carbon deposits (i believe so). I would describe it as lagging on a cylinder. After checking everything and cleaning carbs over the winter, i tried to just ride 40 km's. And the last 20 were great. Then i rode another 35 km and they were all great. I believe, (but i dont know) that it was to many short rides that caused my problems. especially since the school rides were low rpm's. I am contemplating to use additives regarding all my short rides. Just an idea?
The bike has new dyna electronic ignition, new plugs, new coils, correct resistance on coils, and clean carbs, and appears to have your problem. With sudden #$%* perfomance.

The above is a personal experience, the above is tried on a bike that i hope will last throughout college (no funds for proper repairs/ only means of transportation) . I will restore it when i get a job.

[cb750f-2010656]

I thought about pulling the top end and decarbonize as I have a gasket set already, but I'm afraid of what I might find.  My GS750L ran great, but just pulling the head cover and wouldn't you know it needed an exhaust cam and rockers.  Pretty ate up.  I'm not the type to simply put it back together with parts that need replacing.

[flybox1]

^^ youre opening a can of worms 

Get your carbs sorted out.
Get through the complete periodic maintenance schedule.
Get it running, safely, and ride it a bit.  Put some miles on it.
Then re-do your compression test....Its too early to decide right now what it REALLY needs.