Throttle response

[alehman]

ok, so you all will probably hate me for bringing up a pods-on-CV-carbs-related question... but this is more a question about cb750s in general. My 79k has podded carbs with a dynojet stage 3 kit with special slide needles and enlarged drilled air holes on the slide as per dynojet's instructions, and it works really well. When I rev it in neutral, the speedo goes straight to redline with no flat spots, my plugs read a fairly good fuel mix (maybe a little rich because I've been riding at low rpms mostly, 1/4 mile to school every other day, lots of stop signs). I've also peered into carb throats with the pods off and I have full slide function  when I throttle--- someone indicated to me in the past that running pods impairs slide function.


My question is this: throttle response in neutral is great as I've explained, but when I'm at a stop sign and yank on the throttle in first gear, the bike audibly bogs a little but picks up after a moment or two. I want to think that this is a jetting problem, but I'm also considering that this is not only my first motorcycle, but also it is my first cb750... I have no basis for comparison when wondering how throttle response is on these bikes. I've read over on the cb750c forum that they just aren't like modern streetbikes and won't roar off the line, so I wonder what you guys have to say about this. If you guys yank on the throttle in first, does it bog a little/just is slow off the line, or does it sounds like I just have an issue with my jetting? Is this a cb750 thing or a jetting thing?


Also, my accelerator pump function is good.

For the record, my bike is set up as follows:

CV carbs with dynojet stage III kit
pod filters
stock 4-4 exhaust
fuel screws set at 3.5 turns out
Primary main jet @75
Secondary main jet @120 (dynojet recommends 126, which is one reason why I questioned my jetting...)

Thanks!

[Lucky 1]

How would pods impair slide function? Ridiculous.

You need to increase the idle jet size.
Idle is controlled by the idle jet and the mixture screw.
The proof is that you have the mixture screws out so far
 now that they could just fall off of the motorcycle.
CB750 not like modern sport bikes? CB750's WERE the original sport bike.
Do you think that Gary Nixon's bike had poor throttle response?

[shinyribs]

I dont have any CV carb experience at all. I also know of the CV carb w/pod issue you hear so much of. While I can completely understand the ''it'll never work' point of view, I also have pulled off some stuff in the past that others said couldn't work. Anyway,just going off old drag racing experience here(cars). Does your accel pump feature have any way to adjust it? Whether it be factory supplied adjustment,or improvised? When I would have a car that would bog off the line,such as your bike is doing, 90% of the time it was accel pump adjustment. Either too much fuel was being shot in and causing a rich-bog,or too little and it was create a lean-stumble. It's hard to tell a stumble from a big at times. A rich-bog will usually clear up on it's own if you stay in the throttle long enough(feels lazy-then pulls)where a lean-stumble can clear up,but normally require you let off the throttle or at least feather it. Sometimes a bog will feel lazy and smooth,where a lean-stumble will feel uncontrolled and even feel like the engine is vibrating/struggling. Not sure if that makes any sense,hard to explain. If there is any way to adjust the output of your accel pump function you may be able to tune it out of your bike. I dont know if you could modify linkage to activate sooner or later,or if maybe you would have to drill out or restrict the pump nozzle to richen or lean the mixture.

FWIW I really hope you can get this figured out. If you do you'll be a hero to alot of guys. Maybe even myself if I ever get to own a DOHC Good luck !

[alehman]

Yeah, I'm not really sure about why slide function would be impaired... just something someone said, but learning to take things some people say with a grain of salt. Including my clymer, haha. Thing is that throttle response in the idle jet range up to about 3000 rpm is pretty good and it seems lean once the secondary main kicks in. I don't know, I might have just answered my own original question. Regardless, in the spring I'll try out 125 secondary mains to see if it helps but if it didn't, I don't know what I'd do about upping the idle jet because they're press-in on these carbs.

At any rate, what is full throttle off the line supposed to be like with these bike??

[shinyribs]

alehman wrote

 I might have just answered my own original question.

Sometimes talking out loud has great results!

As far a the vacuum signal impairing slide function: your CV carbs dont have mechanical linkage that connect your slides to your throttle cables. The slides are lifted by vacuum. At least that is what I've heard. lack of vacuum created by pods = impaired slide function. Makes sense,but since you've seen your slides lift that doesn't sound like your problem.

[alehman]

Thanks shiny! Yeah, I think the accelerator pump is adjustable because I remember reading in my manual that you could adjust the clearance between the pump shaft and the arm that actuates it. I'll look into it.

To tell the truth, I was lucky with these carbs--- a PO had put in the stage III kit and I discovered it months after rebuilding the carbs, because I had no idea what was normal in these carbs. Again, no basis for comparison. I personally think that they're the reason the cv carbs are accepting the pods.

I will have to ride the bike again to see if I can distinguish whether it's a lean or rich bog. I really hope it's on the richer side of things; plugs are easy to replace, valves not so easy.

[shinyribs]

Adjustable! Sweet!   Good luck and keep us posted. Again,good luck. I really hope it works out for you.

[alehman]

Just wanted to update you all on the pod tuning situation. I upped my needle jets to 120s and i got a measurable increase in throttle response--- no bogging. I'm going to pull my plugs once I get a chance to take it out on a longer run at higher Rpms. That might have to wait til spring though... But at this point I can say that my current pod setup is working just as well as my friends cb750 with airbox. smooth throttle curve all the way to the redline!

[Lucky 1]

If you ask the average person just walking by any car or motorcycle that is not running they will all say "It needs more fuel,carb problem" Before any facts have been given to them.

You do not need to change mainjets .

The midrange is the problem.
Just lower your slide needles one notch before you start changing main jets.

Main jets control the last 1/4 throttle.

[Lucky 1]

Up until 1978 the books do say that, but when you see how thick that metal arm is you will realize no adjustment is possible and it is not needed either because if you look at what the metal rod connects to, you will realize it can only move 1/4 max anyway. Unless you want to push it through the bottom of the pump plate!! LOL.

[Lucky 1]

What do you mean "needle jets?" The needle jet is what controls the flow of gas into the float bowl.

You must mean one of the main jets,primary or secondary.

[Lucky 1]

This myth that keeps getting spread around is NOT TRUE.

The vacuum that operates the vacuum operated slide is from the engine suction when the piston goes down and of course atmospheric pressure that day, and at that level above or below sea level.

The reason for the vacuum operated slide is so that at different
altitudes or weather changes the carbs would not need adjustment .

EXAMPLE: You live in Tucson Arizona (alt.2000ft) and you decide to ride up to the top of Mt. Lemon (8,000) feet.
Normally the air would be so thin that the engine would start running very rich and not want to idle.
 With the vacuum operated slide that condition would be compensated for.

See what I mean?

[Lucky 1]

NOT TRUE. The book IS wrong on this item.

Just look at the parts and you will see that adjustment is not really possible or wanted.
The rubber diaphram only has about 1/4 inch space it can move up or down.

[alehman]

Hey lucky,

I meant that my secondary main is up to 120. I thought that it also went by needle jet, and the part controlling fuel flow into the bowl was the needle valve.

[alehman]

You're also right about the accelerator pump not being adjustable, I've realized. This isn't the first time my clymer has been dead wrong.

Also, do you mean drop the clip on the needle down a notch or move the clip to lower the needle?

[shinyribs]

Lowering the needle will cover up the opening in the jet.

Lowering-leaner
Raising-richer


Sounds like you're making good progress!

[seestheday]

Very interested in your progress.  I might just pick up a stage 3 kit and play with this.  Got lots of other stuff to do first, but this will be on my list.

Question: what exhaust do you have?  What I've read about the stage 3 kits suggest that they should also be used with a "well designed aftermarket pipe"?

[Lucky 1]

Right....I should have said lower the CLIP to raise the needle. Good catch thanks.


This is NOT TRUE.

"My question is this: throttle response in neutral is great as I've explained, but when I'm at a stop sign and yank on the throttle in first gear, the bike audibly bogs a little but picks up after a moment or two. I want to think that this is a jetting problem, but I'm also considering that this is not only my first motorcycle, but also it is my first cb750... I have no basis for comparison when wondering how throttle response is on these bikes. I've read over on the cb750c forum that they just aren't like modern streetbikes and won't roar off the line, so I wonder what you guys have to say about this. If you guys yank on the throttle in first, does it bog a little/just is slow off the line, or does it sounds like I just have an issue with my jetting? Is this a cb750 thing or a jetting thing?"

[alehman]

Seestheday-I have the stock 4-4 (I think it's HM420) that came factory with the 79K. Dynojet gives instructions also on how to rejet with their kits if you have an aftermarket or high-flow exhaust. Keep in mind that if you buy a kit you'll have to drill to a greater width the holes in the carb slides... I believe with a #14 drill bit. It'll be irreversible, but if you decided after installing it that you didn't want to do pods you can revert to a stage I/airbox setup from the stage III kit, which just means changing the secondary main jet back to stock and putting the airbox back on. According to dynojet, even with a stage I setup w/ airbox there's a measurable performance improvement over stock.

Lucky, I think you're right. My throttle cable just got changed and the free play was adjusted really loose, so when I was starting from a stop I sometimes wasn't giving it enough gas. The problem has resolved itself after decreasing the free play.