Topic: Carb/Fuel issues again!!! Calgon take me away...

[jdpas29]

i've been working on this bike for months and it's ridable but has a couple of issues that i'm wondering about. 

i have noticed after doing a carb rebuild/cam chain adjustment/ valve clearance adj/ and pretty much everything else adjustment is that my throttle and rpms are great when the bike is sitting still and in neutral.  i only have issues with the throttle response (bogging on quick accel) when i'm on the bike moving.  does that mean anything?  does the resistance of the bike have something to do with it.  it isn't air turbulence. i know that for a fact after fixing that problem earlier. 

if i back off of the throttle after goosing it while moving, the bike seems to "catch up" with itself and pickup smoothly throughout the rpm range up through 6500 RPM (which is about as fast as i ever want it to go).  does this mean anything? 

i'm starting to think that my bike is flooding on quick accel just from instinct of what flooding feels and sounds like. 

[TwoTired]

Yeah, some Kawasakis are like that. 

[HondaMan]

What kind of bike? None of the SOHC Hondas had accel pumps, so it's not drowning. More likely, it's going dead lean when you pop the throttle.

Two things can cause this on an otherwise properly-timed (ignition) bike: too lean of an idle screw setting or one or two carbs' idle circuits are plugged up. The other thing is too tight of clearance on the intake valves. Honda calls for .002" intake and .003" exhaust on all SOHC fours with stock cams, but I recommend .003" intake and .004" exhaust. You'll see why when you get up to 5000 RPM.

If your ignition timing (points type) is late, or the gap too small, a similar symptom can occur. Also, if your gas is old, it can act this way: today's fuel loses 10 octance after about 2 weeks, so keep that in mind...

[TwoTired]

None of the SOHC Hondas had accel pumps, so it's not drowning. More likely, it's going dead lean when you pop the throttle.

I have a 77 and 78 CB750F and they both have accelerator pumps on their carbs.  I think the K models did, too, for those years.

 

[750goes]

Hondaman

why do you need to adjust the intake and exhaust valves further than recommended ??, does the 5000 rpm produce some differences at that motor speed ?? does this let the engine breathe better or a bit tighter?? seems like operating both inlet and exhaust valves at at higher clearances, leaves less movement on the valve openings?? i'm not sure what this means as regards performance or the fuel question??

thanks for the information you are about to impart..

[jdpas29]

sry....  1978 CB 750K..

[eurban]

jdpas29-

Have you visually confirmed that your acelerator pump is working?  With the air box off and with gas in the bowls, you should be able to see a squirt from each brass nozzle (rear side of carb, bottom of the bore)  when you quickly open the throttle.  When you tear down the carbs and rebuild it can take a bit to reprime the acell pump so whack open the throttle a number of times if the nozzles are not squirting.  Some nozzles squirting others not, indicates a blockage, none ussually indicates a problem with the pump on #2 carb Check for this operation as the problem you describe sounds a bit like a the pump is not working.  Also confirm that your ignition advance mechanism is working.  Good luck

[Bob Wessner]

today's fuel loses 10 octance after about 2 weeks, so keep that in mind...

From Chevron site...

"Storage, including storage involving gum formation, does not change the bulk properties and most of the performance characteristics of gasoline (excluding the characteristics affected by the gum itself). For example, storage does not change a gasoline's anti-knock index (octane) or energy content. However, these properties will change if the storage is accompanied by evaporative loss. The evaporation of light ends decreases a gasoline's antiknock index and increases its energy content."

Doesn't seem likely that two weeks would cause much of a problem.

[jdpas29]

ok... speaking of the carb/fuel advance springy thingy...  is there supposed to be an O-ring around where that fuel squirts out inbetween the float bowl and carb body of the #2 carb.  there is a little square brass pin and i have noticed that if i push the spring back and forth while the bowl is off that gas comes out of the hole there.  when i did a carb rebuild i found an O-ring that would fit the hole, but seemed a little big.  do you think that if this O-ring was too big it would be stopping enough of the flow of fuel to cause a problem?

[eurban]

That carb springy thing would be part of the acelerator pump.  You do need an oring to seal the fuel supply hole for the acelerator pump.  If the oring was a good bit too big then it could reduce fuel flow to the acelerator pump but I would tend to think that this is a long shot.  The "bowl" for the acelerator pump stores a  bit of fuel and the pump only operates when you wack open the throttle so unless your fuel supply is severely blocked I would think you should look elsewhere.  . . . .Like I said, visually confirm that there is or is not a problem.

[HondaMan]

today's fuel loses 10 octance after about 2 weeks, so keep that in mind...

From Chevron site...

"Storage, including storage involving gum formation, does not change the bulk properties and most of the performance characteristics of gasoline (excluding the characteristics affected by the gum itself). For example, storage does not change a gasoline's anti-knock index (octane) or energy content. However, these properties will change if the storage is accompanied by evaporative loss. The evaporation of light ends decreases a gasoline's antiknock index and increases its energy content."

Doesn't seem likely that two weeks would cause much of a problem.

Locally, the refinery is required to add a lot of ethanol and, I have been told by one who works there, that hydrogen is sometimes added (bubbling with heat) when supplies are low and must be stretched. The ethanol, once exposed to air, quickly loses volatility. If you have half a tank of fuel, in less than 10 days the lost "light end" will leave the fuel and enter the air in the tank, never to be recovered. The hydrogen can leave faster than that, especially when it's warm. That's the basis for this observation.

I also know from experience with my Ford FE (1967 390 CID "H" engine, about to be rebuilt) that 2-week old fuel will knock like a #%&!* until I add some more premium to it. It can't sit more than a week without running poorly. In the 1960s and 1970s this simply was not the case with this car. (I've had this a VERY long time...)

[HondaMan]

Hondaman

why do you need to adjust the intake and exhaust valves further than recommended ??, does the 5000 rpm produce some differences at that motor speed ?? does this let the engine breathe better or a bit tighter?? seems like operating both inlet and exhaust valves at at higher clearances, leaves less movement on the valve openings?? i'm not sure what this means as regards performance or the fuel question??

thanks for the information you are about to impart..

The extra clearance issues revolve around several things:

1. Wear of the rockers on their shafts. This causes a slight misalignment after about 10,000 miles or so that causes the rockers to open the valves erratically during the beginning of lift. This imparts a sideways moment to the valve stem, causing a slight bind and resulting cam load and vibration. Delaying the valve's opening by .001 or .002 inches "waits" while the slack in the rocker shaft area is removed, then the valve opens smoothly, like when it was younger. The lack of initial bind also reduces friction losses in the valve train, putting more of the power on the ground. It's a small amount, but measurable.
2. Temperatures of the valves. The longer dwell on their seats let them run cooler, especially when they are a less-than-perfect face fit.
3. Sealing time. When the valve sets against its seat a bit earlier with a worn guide, it has time to seal a little better before compression starts.

Technically speaking, increasing clearance reduces "valve open" time, and theoretic racers will say this reduces power. In practice, I've always seen quite the opposite. And, I learned it from Yoshimura, who I figured ought to know. When I built 16000 RPM (that's not a typo) versions of the "big four", we used .006" intakes and .008" exhausts!

[750goes]

Hondaman,

thank you for the explanantion, it makes sense if the rockers are worn, you say after 10,000..
If I increase my inlet/exhaust openings by .001 or .002 to your recommendations, then I should notice..
1) a slight increase in power - not that I would notice anyway
2) a better chance of not burning a valve face


Is there any down side to this increased adjustment??
would there be more "tappet" noise...??

thanks for your input..

[mrbreeze]

This does make sense to me and yes,I am also interested in your response. It also sounds like a good way to extend the time between making maintenence adjustments. If there is no noticable power loss or valve train noise,I'm game to try it.Any other input on the subject?

[eldar]

depending on how you rejeted, You could also be running rich. Engines will bog if too rich also. Now this is not usually a problem if running pods but it could be. you need to make sure the accel pump is adjusted correctly as well. At idle, there should be a gap between the actuator and the rod. If ther eis none, you could be pumping gas in all the time.

[Buffo]

I have spent many hours on my 750K8 carbs to get it to run right...I still tweek it from time to time...the K8 is a real #$%* to jet properly if you change the intake and exhaust...from My experience you may need as big a main as #125...that is what I run with a K$N factory replacement drop in the stock airbox and a open 4 to 1 header

have you done your plug chops at Idle...1/4, 1/2 3/4 and full throttle?

[jdpas29]

depending on how you rejeted, You could also be running rich. Engines will bog if too rich also. Now this is not usually a problem if running pods but it could be. you need to make sure the accel pump is adjusted correctly as well. At idle, there should be a gap between the actuator and the rod. If ther eis none, you could be pumping gas in all the time.

i've done things in a horrible order mainly because i found out about this forum after i began toying with the old lady.  just the other night i did a bench synch  to kinda start from scratch with the carbs and now it idles really rough with the current setup which is..   4-2 mac exhaust, pods (with foam in them), valve clearance corrected to stock .002 and .003, 130 mains, standard slow jets, floats corrected to 14.5 mm).   when i first got the bike running without any air filter at all, it ran pretty good wide open but crappy at low and inbetween throttles so i am going to revert to a 110 jet (new ones) and start all over with my carb synching deal.  i have a hunch that the foam in the pods is going to make it possible to run this bike with 110 jets in it just like it was stock.  it may not be perfect, but i think i can get it close.  i first went up to 120 jets and it was not really any better.  went up to 130 jets thinking i was still lean because that was the common logic with pods.... it ran even worse and really feels and sounds like it's flooding.  so i think the foam makes enough of a difference to maybe not have to worry about jetting larger. 

and, Buffo, thanks for the reply.  i'm going to wait until i get the 110s in the bike and in some sort of semi working order and then i'm going to do a plug chop and take some photos and post them on here for you guys to look at.  i really appreciate all the help from you fellas... not to mention the endless comedy and wittiness from the frequent poster.  who knew that a bunch of jap bike riders would be such comedians?!?!?!   

i'm about to invest in a carb synchronizer.  do you guys think that having all four vacuum gauges at one time is really necessary or is the 2 gauges ok?  basically, i'm wondering how much back and forth over and over crap that the two gauges is going to make me have to do vs. just going ahead and getting the 4 gauges.  also... is the morgan carbtune overrated or is it worth the dough.  it looks like it would be a whole lot more accurate than a bunch of bouncy needles on these standard kind of vacuum gauges.  it's 100 bucks shipped versus 60 shipped for the 4 vacuum gauge deal...  is there a knockoff of the morgan carbtune that's more affordable and more prevalent in the states vs. Britain?

i love my bike and i just want to make it run right or even close to right at this point.

[Buffo]

I got 2 carbs syncs for X-mas...I want to sell one of them...it is plastic bodied one in the dennis kirk book and retails for $100...it is a 4 vacume tube one...it needs murcury but you can get that pretty easily...I want $50 plus actual shipping if you want it

Buffo

[TwoTired]

I bought my 4 gauge set in 1975 for $50.  It included the valve dampers the keep the needles from bouncing.
Cheapest will be mercury sticks.  These work well if you don't mind dealing with mercury.

The Morgan thing I haven't used.  Looks nice$$$.  I suppose if I was going to do a whole bunch of bikes and didn't have anything, it would be worth the extra expense amortized over several bikes.

I really don't think that adding foam obstruction in your pods is a good tuning technique.  How can you be certain you've added the same restriction to each individual carb? It's certainly not a permanent fix, as you can get the same information by simply adding choke to determine if your mixture is out of whack and then address the real issue instead of just putting on a bandaid.  Further, you can add choke at any throttle setting if you disable the fast idle cam and individually address the slows, needle, or main jets for improved mixtures.

More foam adds drag to the inlet air and the drag increases nonlinearly with airspeed.  Therefore, foam stuffing for balanced restriction at idle speeds will have much higher restriction at mid throttle and even greater at higher throttle settings.    How much do you want to juggle slow jets, needle settings, and main jet selection along with varying amounts of stuffed foam?  It's your time and energy, though.  Good luck!

Also, unless you have access to Dyno equipment, learn to read your spark plugs.
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

Cheers,

 

[jdpas29]

i used to run the bike with the choke halfway out and it ran pretty good.  i heard that it was bad for the bike so i stopped doing that.  how do you disable the fast idle cam and ... what is the fast idle cam?

Buffo, thanks, but i'm gonna hold off on the mercury just yet.

[jdpas29]

and wouldn't disabling the fast idle cam change the way the bike behaves during a warm up?

[Buffo]

dont disable anything...fix it right...

the first thing that you need to do is set your cam chain tention...then the valve clearences...then your ignition timing...then sync your carbs...then start doing plug chops...(the proper way) then rejet in small increments...keep in mind that every time you change the jets you have to do a complete set of plug chops...

the real problem is that running with the choke on only really restricts the amount of fresh air for the carbs...it does not really make your bike suck all that much more gas from the mains...on the flip side bigger mains deliver the  proper amount of fuel with the proper amount of air.

Buffo the Magnificent

[jdpas29]

i don't think my pods are the same as yours, Buffo.  they are cheaper than the K&N ones.  yours may provide a little more baffle against high speed turbulence than mine.  do you have any stuttering or hesitation at higher speeds due to the pods?

[Buffo]

I don't run "Pods". I have a K$N factory replacement drop in filter that goes inside the stock airbox right where the paper filter used to go...

keep in mind that at high speeds the air that is supposed to go into your carbs is sucked out of your carbs in "bursts" because your pods have no protection...the suction of the carbs can not over come the velocity of the turbulent air rushing by them at varying speeds and directions

[TwoTired]

and wouldn't disabling the fast idle cam change the way the bike behaves during a warm up?

Absolutely!  It's only a temporary diagnostic technique.

The fast idle cam keeps the carb linkage from going to the idle stop while the choke is actuated.  You would only need to disable it (by backing out the adjustment screw) while you wanted to check for idle improvement at idle settings.   This would be done after the engine is warmed up and would have to be readjusted for correct fast idle after your experimentation with the choke restriction that forces more fuel to draw from the jets and enrichens your mixture.  If warm idle and throttle response improved with some choke, then you know that the slow jets need to be increased in size.

the real problem is that running with the choke on only really restricts the amount of fresh air for the carbs...it does not really make your bike suck all that much more gas from the mains...on the flip side bigger mains deliver the proper amount of fuel with the proper amount of air.

Sorry Buffo, I disagree.  Restricting the air inlet (which is what the choke does) lowers the pressure in the carb bore and allows the outside barometric pressure to push more fuel through the jet orifices.  The very same effect larger orifices will provide.  Using the choke in this way is a diagnostic tool used to determine whether the engine is currently running lean or too rich already.  If applying partial choke improves the engine response at your current throttle setting, then you know going to a larger jet size or higher needle position is the way to proceed.  If applying partial choke makes engine response poorer or burbly, then leaning that metering device is indicated if the plugs then show the proper color deposits.

I do not suggest that the bike be operated routinely with the choke applied after warm up or the fast idle cam be permanently disabled.

Cheers,