Topic: 1977 CB750 Jetting carb after modifications

[jaguarjx2]

Hello, I recently finished a rather drawn out project with my 1977 CB750.  It was essentially a frame up restore, Engine was rebuilt, cylinders honed, rings replaced, valve job with new valves, replaced springs with single coil springs.  I also port and polished the head based on specs from Branch Flowmetrics.  Installed a dyna S ignition pickup, Accel Coils, pod air filters, and a 4 to 1 exhausted.  I sandblasted and repainted everything, as well as replaced the wiring harness.  I cleaned and rebuilt the carbs and reset the floats.  I increased the main jet size from 115 (stock) to 130 due to the add ons, idle jets and needles are still stock.  After double and triple checking everything I pulled the choke and fired her up.  I set the idle high to allow the engine to warm up so I could adjust the carbs.  It was running a little rough at about 2500 rpm but i was unable to keep it running at a normal idle.  I synchronized the carburetors although idle did not improve.  I also tried to adjust the idle circuit with no success.  As far as I can tell I have a lean misfire.  

Any input would be greatly appreciated.

Thank you
Tim

[TwoTired]

When you cleaned the carbs, did you pull out the pilot jets and see light through them?

[jaguarjx2]

Yes I did, I also sprayed carb choke cleaner through all passages to ensure there was not blockages.

[shorterdanny]

I am currently looking at the same issue you have, and reading around the forum thus far...the general rule is increase you pilot jet one size for every 3 size increase to the main jet, might be worth a try..how does it run above 2500rpm???

[Inigo Montoya]

Getting ready to get into jet troubles huh. First off, what pods did you get? If they are the cheap emgo ones, you might have no end of trouble.
Also, on the 77, you cant just go up in idle jet sized as they are pressed in. In many cases though, you dont need to.
Have you tried to adjust your low speed mix or just decided that increasing jets is what you need?
the mix screw offer a very good range of adjustment. In is lean, out is richer. Also, have you changed the needle height at all?
I would most likely turn out the mix screw a little and lower the clip to raise the needle. I know some here have done pods and never changed the idle jets.

Also, do not assume your idle jets are clean just cause you saw light and sprayed carb cleaner. you have to make sure EVERY hole in them is clean and the best way is with a strand of copper wire to poke gently through the holes.
Dont just assume that because you made these changes that you need to increase your jet sizes.

Also, have you checked what color your plugs are after you idle for a bit? That can tell you a lot.

[eurban]

Good info from the previous poster!  My experience with the 77/78 carbs is that changing the idle jet to a larger size is unnecessary in most applications.  I would hold off on making tuning type changes (main jets, main jet needle position etc) till you have eliminated malfunctions in the carbs and ignition areas. 

In addition to Inigo's suggestions (particularly the proper cleaning of the pilots). .  .You can use clear tubing on the overflow nubs to see the actual level of fuel in the bowls if you want to confirm your float adjustments.  Have you considered air leaks?  Are your intake rubbers new or in decent condition?  Spray the the intakes down and listen for changes in idle speed. How confident are you that the ignition wires and caps are making a good connection and are the caps matched to the spark plugs (resistor caps with non resistor plugs or less ideally the opposite)? Is your Dyna ignition properly installed with the timing set at full advance and the mounting plate screws tight ensuring a proper ground of the plate to the engine?  Does your advancer move freely?  When you did your carb synch, were you able to get the vacuum adjusted evenly?  If not did you bench synchronize the carbs prior to install?

[Lee in Ottawa]

I'm looking into pods and exhaust on my 77 and this thread is making me think otherwise

[jaguarjx2]

Well first off I would just like to thank everyone for the new posts.  I am hoping to begin working on this bike again this week since it's finally warming up here. I'll try to keep everyone updated. 

To address a few questions, Shorterdanny,  It seemed to run ok above 2500 but was still lopey, and appeared to have a lean miss.

 To Inigo, The pods I have are the emgo ones, do you have any suggestions on a better option?  Currently I have not tried adjusting the idle mix screw but that is the first think I am going to attempt.  I also have not changed the needle height.  I have already used the copper wire though the idle jet to ensure it is unobstructed.  I also have not tried reading the plugs. 

To eurban, I have already confirmed float level using the clear tube method.  The intake boots appear to be in good condition, but I will try checking for leaks the next time I run it.  The ignition cables are a typical automotive resistive cable, the spark plugs are OEM spec, I can't remember the exact number off the top of my head.  The Dyna ignition was installed properly using the directions given.  It is properly secured and grounded, and the advancer does move freely.  I first synched on the bench, and while running I was able the synch the carb with no issue. 

I really appreciate all the help I have received and I hope we're able to solve this issue.

[Freaky1]

While I will admit without any hesitation that I have far less knowledge that most people on here, I'd figure I'd shar my experience. While I didn't do any work inside my motor on my '77 750F I did painstakingly make sure the cam chain and valves are adjusted right and that it was good and warm before setting the idle and timing. I am running earlier carbs, 74 or 75ish (sorry, I got them out of a box of stuff, but completely rebuilt) with UNI pod filters that are thoroughly oiled and a more or less wide open 4 into 1 exhaust. As for jetting, living up here where the air is thin, I didn't change a thing, I figured I'd see how it ran first and so far so good. While oiling the pods kinda sucks (I think it's more glue that oil, that stuff is a pain to wash off) I would recommend oiled pods, if you spend the time and money to get this far ditch the cheep pods. 

[Inigo Montoya]

As freaky said, the uni pods or the k&n pods. You will always have best results with the airbox though pods can be made to work ok.
Part of your issue may also be the spark plug wires. the stock wires were not a resistor style, only the plug caps.

[jaguarjx2]

Well I understand what you guys are saying about the air filters, unfortunately I purchased the bike without the airbox, and due to some modifications It would  be difficult to install it now.  Either way, from what I read this causes a problem at cruise due to wind or air disrupting the flow of air enter the carburetor.  So this shouldn't be an issue while attempting to tune in my garage.

[eurban]

At this point, checking for air leaks would be a good next step.  I would then go back into the carbs and confirm absolute cleanliness of the pilot circuit.  You can actually order a set of replacement #35 press in pilots from Sirus Inc (sp?) if you want to throw $25 towards insuring that your pilots are not the issue.  BTW adjusting the IM screws should make a noticeable difference when the engine is idling and when it doesn't idle circuit impairments are often to blame.  Not that this should affect idling but you might as well confirm proper function of the accelerator pump if you haven't already. . . I agree with Inigo that your spark plug wire/cap/oe plug combo may not be correct and it is possible that it may be causing problems.  Still you shouldn't have to spend anything more than time to check for air leaks and idle circuit issues.  Once eliminated move on . . .

[Inigo Montoya]

Yeah always check the simple stuff first before throwing money at something. Air leaks have buggered a lot of people here.

[rkubin]

hi there- as it turns out I am in a very similar situation, so will post here rather than start new question.

I have a 1977 K7 with just under 10k miles that I have restored/customized, frame-up over last 1.5 years.  I put on K&N pods and a yoshi style 4-1 (minimal baffle), so went up 2 main jet sizes to 125 (stock is 115) and have them set at second position from top.  Pilot screws were all set at 1.5 turns out per manual (they were originally 3-4 turns out when disassembled).  Carbs were completely disassembled and cleaned.  Stock ignition.

Bike started right up, but will not run/idle without choke on, even when hot.  I am assuming that indicates a lean mixture.  I do not have carb sync gauges, so have not checked sync.  Will check plug condition tomorrow.

Any suggestions on next steps?  Will adjusting the pilot screws help with the mixture?

thanks!

[Inigo Montoya]

As your setup is no longer stock, the baseline settings no longer apply. Turn out the screws 2 turns I would guess.
Also, I would drop the clip to the middle position.

[shorterdanny]

rkubin: my setup is the same as yours pods and 4 into 1. i have increased my mains to 125s and left the rest stock. one thing to try is starting the bike with the throttle 1/4 to 1/2 open. if the bike revs up obviously shut the throttle, this would indicate you float levels are wrong or sticking, as bypasses the idle circuit, cavet I NO MECHANICAL BACKGROUND. this is just one of the problems i came across when tinkering. at the time i had the same symptoms you describe. (when you had the bowls off the carbs the main jet is lower than the pilot so low float level your bike would just keep running out of fuel.)

My pilot screws are out 1.5 turns, and bike idles nicely at 1100rpm.

edit i have just sync my carbs using a morgan carb tune..massive diffenerce on the smoothness of the engine and my over run backfire is now cured..

[Inigo Montoya]

Remember though, the mains are not the biggest part of the system. The idle jets operate up to 75% of the throttle range.
Larger mains only affect the upper range and nothing more. So at idle and even up into the higher rpms, idle tuning is most important, the mains wont do a thing.

[shorterdanny]

completely agree, certainly didn't mean to discount the idles, just a way of helping to find the problem. but on the K7/8 750 the idle setup is pretty good as standard so not much needed just tweeking the screws, then increasing the mains to help out on the WOT.

[rkubin]

thanks for the suggestions.  I turned the pilot screws out 2 turns (now 3.5 turns out), and that helped - bike will run without choke once hot, but still runs better with choke partly out, which means to me that it is still running lean.  Plugs look fine.  I did bench "synch" carbs.  Am trying to locate/borrow synch tool.

shorterdanny: not sure what you mean about starting the bike with throttle cracked- why wouldn't it start and rev anyway?  what are you checking for?  also, what position did you put the clips in on main jets?

could timing also be an issue?  I put in new points/condensors, but don't have a timing light, so adjusted by sound.  I could take it into a local shop that specializes in old Hondas, but I have done everything myself so far...

[eurban]

I would guess that you still have more cleaning work to do on your pilot circuit.  Pressed in pilots removed and cleaned along with a systematic cleaning off all the idle passages in the carb bodies. Sometimes this is a process that needs to be done a few times particularly if it is your first attempt.  Also make sure your intake rubbers aren't leaking air and that your float levels are actually correct (clear tubing trick)

[rkubin]

hi eurban- I was very anal with the carb overhaul.  pilots were removed, all orifices cleaned and checked.  bike has under 10k original miles, but did sit for many years with gas in the carbs.  intake rubbers are fine, but I have not checked the float levels - that could be a relatively easy next step, to check anyway.  If levels are off though I guess I will have to pull carbs again.

[rkubin]

so I plan to check the float levels today- what is the correct level?  does this have to be done with the engine running?

thanks

[eurban]

For the clear tubing check you want the fuel levels identical and at or just a bit below the bowl to carb body seam.  Test should be done with the engine off and the bike on the center stand.  Install the clear tubing on the drain/overflow nubs; petcock off, drain the bowls (using the tubing); once drained, hold the tubing up along the carb bodies and open the petcock; the final level of the fuel in the tubes is the actual level of the fuel in the bowls. . . .

[mick750F]

so I plan to check the float levels today- what is the correct level?  does this have to be done with the engine running?

thanks

   Carbs have to be off the bike to adjust the float levels. You can download a manual here if you don't have one....

http://www.sohc4.net/index.php?q=node/133

Don't forget to make a donation.

Mike

[eurban]

I think he is talking about using clear tubing on the overflow nubs to do a reality check on the fuel levels in the carbs.  I could be wrong but . . . .