Topic: '77 CB750F carb tunning

[mireparo]

So...I've got a 77 cb750f with mac 4-1 and K&N pods. Below 4-5k rpms anything above 1/2 throttle bogs down. I'm running 120 mains and the needle one notch below the top (one notch richer than stock setup). My accelerator pump seems to work fine and my carbs have been completely gone through and cleaned. Sparks plugs look good (light tan). I also get popping/ coughing from the carbs at low to 1/2, this is worst when cold. Also, my idle is 1000 after 10 mins ridding but 2000 after 20 mins ridding. My largest concern is to fix the bogging. Where should I go from here?

[Redman]

 The rpm changes you describe make think that you might have an intake leak. Make sure you have solid connections.

[uptworedline]

i read a trick on this forum a while back; it was to run your bike, do whatever it does that makes your bike bog, and as it's bogging, kill the bike and do a plug check. that will tell you what the deal is. other than that check for vac. leaks

[mireparo]

My intake boots are new from Honda and I am careful to seat them properly each time the carbs are reinstalled. Also the RPM changes are consistent each ride I take.

[jeanhank]

Did you check your plugs after a plug chop?  If it was just after riding it around for awhile, the color of those plugs would depend on where you spend most of your time on the throttle.

Is there a baffle in your exhaust?  If you have a pretty open exhaust, and pod air filters, it seems to me that 120 mains are a bit too small, especially if you have the needle at that position.  Your needle affects mostly in the 1/4 to 1/2 range of throttle opening and your main jet affects about 2/3 and up.  There's a lot of info about this around here.

Coughing from the carbs and a popping exhaust indicates a lean condition, or possibly out-of-sync carbs.  Try going down to the middle notch or below on the needle and see if that fixes that small-throttle coughing.  If you're still bogging down at more open throttle positions, go up a main set size.  You might have to then fiddle with the needle a bit more to find that 1/2 throttle sweet spot keeping in mind that at that range, both the needle and to a smaller extent the main jet affect the mixture.

Oh, and as for your cranky idle, well, I used to have a similar problem.  Do the carbs on the 77 CB750F have an idle mixture adjustment?  Lower your idle to ~1200 rpm with the idle screw when the bike is nice and warmed up.  Start with one carb and turn that screw 1/4 turn at a time until you find where the idle is the highest.  Lower the idle back down again with the idle screw, and repeat the procedure until there isn't a real difference in idle speed if you turn that screw 1/8 turn either way.  Do that for the rest of them. 

What you're going to find is you'll probably be richening up your idle mix quite a bit.  This will improve your low-throttle running as well as help your bike idle while cold, so that it idles at a speed comparable to what it does when hot.

At sea level on my 70 CB750K with pods and an open Kerker, i have 125 mains and the needles in the middle...

Wow, carbs, such amazing chunks of metal... can you tell I've been spending a lot of time on them lately?

[mireparo]

JeanHank, I have yet to try a plug chop and will soon. If get to bog for a few seconds and then cut the engine will that be long enough to show a change on the plugs? It's hard to do it longer than 10 sec as it slowly accelerates and quits bogging.
As for needle position, I stated the wrong position used, I have tried every possible position and found it runs best on the second from the bottom notch (one below stock position).
I've also tried larger mains and it runs to rich (Exhaust popping and dark plugs).
What makes me curious is that all throttle positions are run good above 5k. All info I've found on carbs seems to refer to throttle position regardless of rpm. Vacuum leak?, But where? boots are new and seated well.
I did improve carb coughing by adjusting idle screw out.

[TwoTired]

If you put on pods and increased the main jet size, why don't you think the slow jet size doesn't needs increasing, too?

[joeb]

Did you repalce the clamps around the intake boots or are they old ones. On my  836  I have the idle screws backed out almost 3 turns and experience no bogging or hesitation. Are the carbs sycronized. 

[eurban]

If you put on pods and increased the main jet size, why don't you think the slow jet size doesn't needs increasing, too?

In my experience with the 77/78 carbs: no air leaks + clean idle circuits with clean stock #35 pilots + properly functioning accel pump + properly oiled KN tapered, oval pods = Runs fine.  My 836 is actually happy with around 1 turn out on the IM screws. Main jet is larger and needle position is a stage richer. Before the big bore, bike was happy with just bit more than 2 turns (richer setting than with the big bore) out on the IMSs. . . .  I would guess that most of the 77/78  carb out there, have never had their pressed in pilots pulled and have at least some idle circuit impairment after 30 years.  This impairment may be more obvious once PODs are installed.    That, and these carbs just don't work right without the accelerator pump functioning properly. Again, this may be more accentuated with PODs.

[TwoTired]

An 836 sucks harder on the carbs than a 736, given the same air filtration type and cam grind.  This means an 836 will draw more fuel from the same jet size relative to the 736.  It is not clear that the Idle mixture screw has enough adjustment range to accommodate the vacuum loss with pods, and an engine that hasn't had a displacement increase/ cam change to increase the vacuum.

I'll bow out of the discussion now and let the experienced experts provide more meaningful help.  Hopefully, they are right, and mireparo is wrong.  The carbs simply aren't clean enough, the accel pump isn't working right, and you just need to adjust your Idle screws to 836 settings.

The only other observation I'll make is this:

I'm running 120 mains and the needle one notch below the top (one notch richer than stock setup).

According to :
http://www.motorcycleproject.com/motorcycle/text/specs.html

- The second clip position from top IS the stock setting and you have not made the midrange richer. Maybe, just maybe, making the midrange richer means you can go down a step or two on the main jet size.  I'm done speculating for this thread.  I'll let the knowledgeable provide further input.

Best of luck!

[mireparo]

TwoTired: My idle jets are pressed in. Can these be replaced or do people just drill them out?

JoeB:I put new clamps on from cycleX, the stock ones are crap.

Eurban: I've spent many hours cleaning these carbs and clearing passages for the accelerator pump. With the pods off I can see fuel being pumped by the accelerator pump, so to best of my knowledge everything is clean and clear.

Carbs are synced and I adjusted my idle screws out 2.5 turns (up from 1.75, stock), this improved my idle cut down on carb cough and gave me a more consistent idle speed.
I did a plug chop during bogging and found my plugs to be darker than normal. Since it bogs at 1/2 throttle below 5k rpm I'll try lowering my needles one notch to lean out mid throttle and see what happens.
Thanks everybody for all the feedback!!!!

[eurban]

An 836 sucks harder on the carbs than a 736, given the same air filtration type and cam grind.  This means an 836 will draw more fuel from the same jet size relative to the 736.  It is not clear that the Idle mixture screw has enough adjustment range to accommodate the vacuum loss with pods, and an engine that hasn't had a displacement increase/ cam change to increase the vacuum.

Like I said, prior to the big bore, my 78 required a bit over two turns out of the IMSs.  This was with pods. . . . .  There really shouldn't be the need for too much speculation here.  Experimenting the with IMS settings is relatively easy.  You can either tune for highest idle speed or move the screws in or out (1/4 turn increments)  and go out and test for off idle performance.  If your idle speed is still climbing and or your performance is still improving as you approach 3 turns out then you would go up in pilot jet size.  I do doubt you will find this to be the case . . .Mireparo, in reference to your questions to TT asking about the pressed in pilots.  Based on what you say here I wonder if you have actually pulled your jets and cleaned them as well as the passages behind them?  If you haven't done this yet, do so before you make any more attempts at carb tuning.  Otherwise you are just wasting your time!

[mireparo]

Like I said, prior to the big bore, my 78 required a bit over two turns out of the IMSs.  This was with pods. . . . .  There really shouldn't be the need for too much speculation here.  Experimenting the with IMS settings is relatively easy.  You can either tune for highest idle speed or move the screws in or out (1/4 turn increments)  and go out and test for off idle performance.  If your idle speed is still climbing and or your performance is still improving as you approach 3 turns out then you would go up in pilot jet size.  I do doubt you will find this to be the case . . .Mireparo, in reference to your questions to TT asking about the pressed in pilots.  Based on what you say here I wonder if you have actually pulled your jets and cleaned them as well as the passages behind them?  If you haven't done this yet, do so before you make any more attempts at carb tuning.  Otherwise you are just wasting your time!

I pulled my float bowls to check on the idle jets and it all came back to me. I did pull them a clean them last year when I initially rebuilt my carbs. For good note, I pulled them again and they were all clear.
I adjusted my idle screw out to 3 turns and saw improvement in 1/2 to full throttle (I did not realize the idle screw would have any significant effect off idle)

Is it possible to buy those press in idle jets? and where?

[TwoTired]

TwoTired: My idle jets are pressed in. Can these be replaced or do people just drill them out?

They can be drilled/reamed, etc.  I know of no direct replacement sources.  However, in past threads people (Buffo) have posted good results by getting screw in jets and forcing them into the soft metal bodies.  I believe this was suggested by the people at: http://www.motorcyclecarbs.com/Keihin_Pilot_Jets_C72.cfm?UserID=1301531&jsessionid=aa3094c9224b1d112257
You could call them.

If I ever find the need to use pods filters, I expect I would modify the carb bodies to accept screw in jets, by tapping the bodies for threaded insertion. Sudco also offers replacement jets:
http://www.sudco.com/onlinecat33.html
I'd pick a style similar to what is pressed in now.  Phone them for threaded dimensions critical for acceptable fit.

Or see,
http://www.keihin-us.com/list.htm
You need a jet component that has the emulsion tube and overall length to reach into the float bowl fuel supply, and a threaded portion that exists about where the pressed in section on the ones you have now are.

Pity none of them posts dimension drawings to make a good selection online.  But, they should help with that over the phone.

http://www.cyclewareables.com/pages/keihin_main_and_slow_jets/keihin_main_slow_jets.htm
http://www.carbparts.com/keihin/needles_tuning/keihin_jet_list.htm
Cheers,

[eurban]

Like I said, prior to the big bore, my 78 required a bit over two turns out of the IMSs.  This was with pods. . . . .  There really shouldn't be the need for too much speculation here.  Experimenting the with IMS settings is relatively easy.  You can either tune for highest idle speed or move the screws in or out (1/4 turn increments)  and go out and test for off idle performance.  If your idle speed is still climbing and or your performance is still improving as you approach 3 turns out then you would go up in pilot jet size.  I do doubt you will find this to be the case . . .Mireparo, in reference to your questions to TT asking about the pressed in pilots.  Based on what you say here I wonder if you have actually pulled your jets and cleaned them as well as the passages behind them?  If you haven't done this yet, do so before you make any more attempts at carb tuning.  Otherwise you are just wasting your time!

I pulled my float bowls to check on the idle jets and it all came back to me. I did pull them a clean them last year when I initially rebuilt my carbs. For good note, I pulled them again and they were all clear.
I adjusted my idle screw out to 3 turns and saw improvement in 1/2 to full throttle (I did not realize the idle screw would have any significant effect off idle)

Is it possible to buy those press in idle jets? and where?

Kind feel like I am banging my head against a wall here with TT egging me on . . .While it is possible that the idle mix screws will have some small effect from 1/2 to full throttle, the idle circuit is not even close to being the dominant player in these throttle positions.  The needle clip position, the needle profile and as you approach WOT the main jet size will dominant the mix.  When I mentioned "off idle performance" I meant performance at very small throttle openings and things like transition from 0 to 1/4 throttle. . . .Bottom line is you do not evaluate your IMS settings or your idle jet size based on the carbs performance from 1/2 to full throttle.  I really think you are barking up the wrong tree! 

[TwoTired]

Kind feel like I am banging my head against a wall here with TT egging me on . . .

I do hope it is a soft wall, or you wear your helmet.

While it is possible that the idle mix screws will have some small effect from 1/2 to full throttle, the idle circuit is not even close to being the dominant player in these throttle positions.  The needle clip position, the needle profile and as you approach WOT the main jet size will dominant the mix. 

This is true when air velocities through the carb develop enough venturi effect to create the vacuum needed to pull fuel through the main jet and throttle valve.  Below 4-5K these velocities simply aren't there when the throttle is suddenly opened.  If the accelerator pump isn't providing enough fuel boost (as it should), then engine pickup from below those engine speeds is going to depend on an "over-rich" condition, such as what the older carbs employed for throttle response.  (Older carbs don't have an accelerator pump.)

When I mentioned "off idle performance" I meant performance at very small throttle openings and things like transition from 0 to 1/4 throttle. . . .Bottom line is you do not evaluate your IMS settings or your idle jet size based on the carbs performance from 1/2 to full throttle.  I really think you are barking up the wrong tree! 

Reading mireparo's original post...

Below 4-5k rpms anything above 1/2 throttle bogs down.

I interpret this as a complaint about transition, not steady run conditions AT or above RPMs of 4-5K where main jet and slide needle ARE the determinant factor.   

If mireparo is confident that his accelerator pump is working to maximum potential, then there is little choice but to increase fuel delivery from the pilot circuit and "pre-rich" the mixture in anticipation of a sudden throttle opening.  If he can't achieve this goal by IMS adjustment, then increasing the pilot jet is the remaining avenue to address.  If you force the Main and Throttle needles to "pre-rich" the mixture below 4-5K when the carb throat vacuum is diminished, then it will be way over-rich when operating *AT* engine RPM's above those.

I also get popping/ coughing from the carbs at low to 1/2, this is worst when cold.

This is another pointer to pilot circuit lean-ness, IMO.  Increasing main and throttle valve setting will have minimal effect on this. Choke application should help this, as it increases carb throat vacuum pulling on fuel from jets.

I'm trying to post help relative, to what mireparo is relating.  If he is relaying "wrong tree" info,  Then I have it all wrong, and this is just barking at the moon (above the tree).

Cheers,

[eurban]

Good analysis TT.  However, I would still guess that he is barking up the wrong tree with the focus on larger pilots.  It sounds to me that his carbs are having "functional" issues rather than tuneable issues.  My money would be on one or a combination of the following: acell pump not fully functioning or not properly adjusted (check the spring and throttle linkage associated with the pump, check for essentially 0 clearance between the previously mentioned linkage and the pumps actuating rod), impaired idle circuit(s) inspite of the cleaning attempts, and or air leaks. . . .  Sirius Inc can supply the #35 pressed in pilots and they would be a good place to start with queries about larger options.

[mireparo]

Eurban: I hear what you are saying and I do realize the idle screw has less effect than the needle and main jets. However, I was under the asumpition that since my bike idled okay I shouldn't mess with my IMS. Also since I spent considerable amounts of time messing with needle position and main jets I was looking at other possibilities.
The last adjustment I made was to richen the needle setting (raised it up to the bottom notch) and this did show improvement. I am going to atempt to raise the needle even more (at this point I'll need washers as all the notches have been utilized) to see what happens.
FYI: I'm not changing main jet size at this point, because I remember using larger mains when I was tunning last year and my bike was running noticeably too rich (dark plugs and stinky exhaust)
Overall bike is running better.
Idle is between 1k and 1.5K depending on how long I've been riding and it only bogs at 3/4+ throttle position when RPMs are below 4-5k and this is cured by adding choke. If going up in needle position does not solve this I will try a small increase in Main jets from 120s to 122s for starters.
Thanks again for the feedback!!