Topic: 1977 CB750 Jetting carb after modifications

[rkubin]

yes eurban, I was referring to the clear tube method as a sanity check- thanks for the info!

as an update, I adjusted pilot screws by "high idle" method - all are now 3-4 turns out.  did a test ride and it idles reasonably well without choke, but has a major stumble (almost a decel) as throttle is cranked- then there is a major rush from 5-7k rpm (front wheel was getting light in first). this is improved if i keep the choke on some.

I am thinking that moving needles up one position (currently 2nd from top) could be next step. will also check the float levels as described.

any other recommendations?

[eurban]

3-4 turns out on the idle screws is just about at the end of operating range of the mixture screws and is uncharacteristic of a properly functioning bike.  I still think that you have some fundamental issues that need to be corrected before you should worry about choosing your main jet size or needle position.  Confirm proper fuel levels in the bowls, confirm actual squirting of fuel from of all 4 accelerator pump nozzles (when you whack open the throttle), and check for air leaks if you haven't already.  Consider ordering up a set of new press in idle jets from Sirius Inc (sp?) and in general going through the idle circuit in the carbs again.  How's your tank /Are you feeding your carbs clean fuel?

[rkubin]

thanks for the info eurban!  I also thought 3-4 turns was out there.  I will pull pods and check fuel levels by tube as well as accelerator functioning.  I don't believe there are any leaks, and tank has been cleaned and repainted, with an aftermarket "pingle" style petcock installed.  If no revelations, will pull carbs.

[eurban]

Question unrelated to your issues . . .Is there an available adapter to install the aftermarket "Pingel style" petcock or did you have to do some fabbing?

[rkubin]

the petcock I used was designed for older harley tanks with a 3/8" NPT connection.  You can get an adapter plate that can be mounted with 2 screws and gasket, but I had local shop weld a piece of 3/16" flatbar over the petcock holes then drilled and tapped for 3/8" NPT.  In retrospect, the adapter would have been easier and cheaper, as the drill bit was a weird size (over 1/2") and expensive as was the tap, both of which I am unlikely to use again.  The petcock though was only $12 off of ebay and looks like it should hold up well.

[eurban]

You must be using a non stock tank as the stock tank has a male metric threaded bung on the underside.  Thought you might have found an adapter for the late tank's setup but I guess not. . . . .

[rkubin]

you are right- I forgot that the tank is from an early model ('70 I think) cb750...

[rkubin]

ok- either I am doing something wrong or maybe found the problem...

tried the clear tube float level method using instructions from eurban above - I am getting fuel leaking out of the air inlets on all 4 carbs.  I have the gas tank slightly elevated (off the bike to one side). tried using a longer clear tube on one carb (others with drains closed), ensuring that the top of the tube was higher than the gas tank, with same result- gas coming out of the air inlet.  when I shut off the petcock, the level in tubes does drop, down to near the bowl to body seam.

am I doing this wrong?  or does this indicate an issue with the floats?  what next?

[wsup55]

I'm interested to see what you find on this.  After a carb rebuild I'm having the same problems (also a 77K).  Seems to want the screws out 3.5T which i find odd.  Stumbles until 4K then screams.  It will actually start filling my airbox with gas if I let it sit.  I'll let you know if I make progress as well.

[rkubin]

Still not sure what is going on with float levels- any ideas out there?

I checked accel pump function- it is not working... Guess the carbs are coming off again!  Any help on troubleshooting accel pump much appreciated. I did rebuild carbs and replaced all seals, gaskets, o-rings and checked all passages.

Thanks

[rkubin]

Another question on floats - manual says 12.5 mm, but how is this measured on the '77/'78 carbs?  I can find info for earlier carbs, but not these.

[rkubin]

first off, apologies to Tim for hijacking his original thread, but seems to make sense to keep the relevant info together.

Pulled carbs today- turns out I did not check all passages, as accelerator pump passages were clogged.  From looking at the design, looks like fuel is fed into the pump from the #2 carb bowl through 1 passage, then is pushed up to the jets through another when the plunger is depressed.  The rubber diaphragm looks to be in reasonable shape (no cracks), but appears to be missing one hole (or it is plugged solid), which aligns with the passage to feed the pump from the bowl (see pics below). This would explain why it's not working (still need to check the passages to each jet nozzle).  I will punch out/clear the "missing" hole in the diaphragm unless I hear different.

Now on to the floats- I am a little confused on how/where and what dimension to measure the float level for the '77 carbs.  Pics below show the float and ruler.  I understand that you measure the distance from the gasket surface to the float bottom at the point when the arm touches the float valve, but have questions:

- is the float valve supposed to be pushed in (seated) to measure?
- where along the float base do you take the measurement? center? end? what if it is different on each side (one float seems to have a twist- one side is different height than the other)?
- the '77 K7 supplement to the honda manual I have says the level should be 12.5 mm; I located a honda manual for '78 K/F, which I understand have similar carbs to the '77, and it says 14.5 mm; as you can see from the pic, the current measurement for this carb appears to be around 14.5 mm; I can't see how this could be reduced to 12.5 mm.  Is it possible the 12.5 is incorrect?  or that I actually have '78 carbs (the casting number is PD 41A 01 for anyone who might be able to verify)?

Any help would be greatly appreciated- hoping to fix tomorrow...

[ChortlingSandcaster]

I have recent experience jetting for those mods:

- from stock airbox to individual pods

- from stock exhaust to 4-into-1

You are flowing a LOT more air into the motor and will , perhaps need to lower the needle clip one notch to raise the needle upward in the carb body for midrange.

You perhaps will need to go up on the pilot circuit jet.

You already have done the main.

Although mine was on a different bike, same era Mikuki, late 70s.


I yanked my carbs and soaked them in Berrymans carb cleaner after complete disassembly.

When I put it all back together I had exactly what you think have -- 'lean lurch' or miss.

After going with a bigger pilot, chaning the needle clip to 4th position, going to bigger mains -- run like a top.

I cannot advise you on the jet sizes but in general, you are flowing a LOT more air through that motor, and when you add more air -- you gotta add more gas to compensate.

If you turn on the choke at idle to temporarily enrichen her and SHE THEN IDLES that'll give you a clue that bigger jet down there will help.

DISCLAIMER: I just read the accelerator pump bit.  My bike didn't have one, dont know how/if that affects my jetting experience I gave you above.

[rkubin]

Thanks ChortlingSandcaster.

Anybody got input on the float settings or accel pump questions?

[eurban]

rk,
77 750k carbs are pd41a, 77f pd41b, 78k pd42a and 78fpd42b
The pd41a's are the only ones specced at 12.5mm float heights, the rest are 14.5mm.  The lower the float height setting the higher the fuel level in the carb and I honestly think that one can consider setting between 12.5 and 14.5 mm as a range to play with for fine tuning.  I would start with the 12.5 height for your 41a's.  Your point of measurement for the floats seems to be correct in your picture but your ruler isn't very square to the bowl mounting surface.  Measure at the point where the float turns at an angle.  Carbs should be on their side and you are looking for the point of first contact between the metal tang on the float and the pin on the needle with no compression of the pin into the needle.  Make sure that the needle pins are free to plunge and replace if not.  Make sure the needle seat is completely clear of contamination.
The accelerator pump diaphragm should have holes in both of "mickey mouse's ears" so you certainly need to correct this rather common problem.  Make sure the pump passages are clear and that mechanically its linkages and external springs are working OK.  Set the pump rod to linkage "air gap" according to the manual.
Things that you can now do with the carbs off . . .Make sure that the pilots are clear and that when you spray carb cleaner in the empty hole where the pilot was that it squirts out each of the multiple holes involved in the idle circuit (cover up one or two of the holes to observe spray out of the other exit(s)  Make sure that all four carbs behave the same way in this manner.  Consider setting up a temporary fuel supply so that you can fill the carbs (leveled out on the bench or somewhere outside) and check for leaks/overflow issues and correct fuel level in the bowls with the clear tubing technique.  You can also check for squirting from the accelerator pump nozzles and correct any problems before reinstall.  Without a functioning acell pump you will most certainly get a significant stumble when you rapidly open the throttle.  Bench synchronize the slides while you have the carbs off too
Chortling, as I said before in this thread, my experience with the PD carbs on the 750 is that the 35 pilots are up to the task of handling most exhaust, intake and engine performance mods.  The pilot system must be properly clean however and this is sometimes quite a chore to achieve on a set that has been gummed up significantly.  Certainly before one makes any attempt at sizing the mains or pilots or picking a position to set the main jet needle at, they needle to make sure that their carbs (and all the other bikes systems) are actually functioning properly.  I think that DK still has a bit more work "baseline" work to do on his carbs.  If once everything is up to snuff, one finds that idling and just off idle performance continues to improve the more you open up the idle mixture screws and you have reached the end of their effective operating range of about 3 turns out, then it would indicate that you need to go up to the next larger size in the pilot. . .

[rkubin]

thanks eurban!  I will check/fix the float levels and accel pump before doing any further changes.  It was suggested earlier to move needles up one more position, but I think I will wait until I can check the effect of these fixes as a good baseline.

wsup55, I will post results, but you may want to check float levels (clear tube method as described by eurban) and accel pump function, both easily done without removing carbs.

[wsup55]

Thanks, it will be another week before I tear into mine but i am suspecting the accel pump.  Will check the floats again while i'm in there.

Also, just in case you need it, check this out.
http://www.hondachopper.com/garage/carb_info/accelpump_valve_repair/page1.html

[rkubin]

wooo hooo!!!! floats set, accel pump passages cleaned, pulled the slow jets and re-cleaned while I had things apart, put everything back together, turned on the gas and checked accel pump function- jets from all 4 nozzles.  Put K&N pods back on and started it up - sounded really good as it warmed up.  Took it out for a test ride once warm and it is running very good now (no choke).  The only potential remaining issue is a slight flat spot around 5-6k rpm.  It is not really bad (compared to running without accel pump!), but thinking maybe the needles could be raised one slot.  I will put a few more test miles on before I decide- taking it a little easy to break in new rings.

wsup55, thanks for the link on accel pump repair (they also have an excellent section on float measurement)- luckily, I didn't have to go to this length, although it did take soaking and blowing through of carb cleaner.  I suspect that the valve is cruddy (the rest of the carbs were) but it is working.  Took a lot of work to clear the accel jet for carb #4.

Overall, very happy at this point, considering the bike hasn't run in 14 years!  Here are a couple of pics.  My first MC build (and believe it or not, my first Honda - have been a yamaha/kawasaki (and one Montessa) guy), but won't be the last...

thanks eurban and others for the help.

Rick

[rkubin]

forgot to mention- I optimistically set the IMS to 2 turns out.  It is running so good at idle that I didn't even think to check- may play with them tomorrow.

[eurban]

Nice work!  Now that you seem to have things baselined its on to fine tuning for the exhaust and intake . . .Have fun!

[wsup55]

sweet!!!  I had to go for the full valve repair but the accel pump was definitely the issue.  She's running great now.  Thanks for posting your results!  I can't wait to ride this weekend!!

[SteveCB750]

Good day,

Well, this all explains my problems..... I bought a set of K&N pods and increased my jets two up.... Apparently I still have home work to do.. Should I still be looking into aftermarket coils and cables or should I finish tweaking and see where that takes me?

Cheers
Steve

[Dawks]

I've a K7 with starting and fuel problems since full rebuild of motor and carbies.
Rebuilt with 836 and stage 2 cam and springs
changed mains to #120 and lowered clip on needle (assumed it would need more fuel)
Problems
Hard to start from cold (runs fine when warm)
Pinging under load at lower revs
Dyno test showed extremely lean to 4000RPM and extremely rich 5000rpm to 8000
after reading these post will go down on mains and lower needle back to standard.
Can I check the Fuel pump operation with the carbies in the bike?.
I've ordered #35 slows/pilots from Sirius inc, are they easily removed and replaced,do i need to press them out?
made around 72HP at 8000 on dyno is this good/bad?   

[eurban]

Dyno numbers vary widely but 70hp from an 836kitted bike that hasn't had serious flow work is pretty much what I would expect.  The late heads typically breathe OK.  I would guess that your top number and the meat of your power curve will improve with proper carburetion.  Go over this post and search a bit for the ins an outs of getting the PD carbs to a proper "baseline state". . . .  Your accelerator pump must function properly with these carbs and yes you should see fuel squirting from the nozzles (air box removed, brass nozzles are next to the choke plates) when you rapidly whack open the throttle.  Pad your plyers with leather if you like and twist/pull out the pressed in pilots.  . . .Use high octane pump gas with the 836 and once you have gotten your carburetion functioning properly, consider swapping to a D9EA plugs with the higher compression motor and consider backing off the total spark advance slightly.

[wsup55]

are you running a stock airbox?  I'm not sure dropping your slows is going to help since you're showing extremely lean in low RPM.  What are your fuel screws set to?  I was also having starting problems on my K7 but after resetting my valve clearances and adjusting the choke cable, starts right up now, at least it's a place to start.