Topic: My pilot screws have no impact on RPMS

[Vintovka]

I've recently got my 1978 CB750F back on the road but there seems to be a problem with the pilot screws on the carbs.  The bike runs great but its fouling plugs after about 25 miles and when I adjust the pilot screws I get absolutely no RPM response from my digital multimeter or on the main tachometer of the bike.

 The bike has brand new D7EA plugs I installed because the D8s were fouling as well.  PO installed kerker exhaust, pods, 120 main jets, 35 slow jets, and a Dyna S ignition system but coils are still stock.  I'm not sure if the PO has made any other changes to the bike as far as boring or cam changes or anything in the actual engine.  The bike will idle fine at 1k rpms but its like the pilot screws have no effect whatsoever on RPMS not even on the digital multimeter.  The only time I get any change is if I screw them all the way in, the bike very slightly revs.  I keep them at 1 turn out I know factory is 1 and 1/2 but it has D7 plugs, pods, exhaust, and 120 mains which should LEAN the bike out but it appears to be too rich.  The pilot screw is on the engine side so IN leans and OUT richens.

 The carbs have been completely cleaned, dipped, carb cleaner blown through every orifice.  I even just put in 4 new small o-rings and washers on the pilot screws and each one has a spring on it.  The fuel is clean and the fuel tank has been re-lined with POR15 and its holding strong.  Petcock is clean fuel system is clean the bike seems to run perfect until the plugs are fouled no backfiring no sluggish response at all even though sooty plugs are signs of running too rich.  The plugs are gapped correctly.  About a week ago even synched the vacuum on the carbs with a vacuum gauge.  Another sign of running too rich is the fact its getting horrible gas mileage.  

[Spanner 1]

I think your float levels are wrong ( too high) and fuel is being sucked out the main jets, making the 'idle' mixture too rich, which would take-away any response of turning the pilot screws in or out. Probably need to drop the needles too, oh wait, 78F carb has fixed needles ?
Correct float height is 14.5mm btw, did you check it ? and if so did you check it with the carbs orientated correctly ?

[Mandic]

I am right there with you.  My 77 750F doesn't change a bit no matter how I adjust the pilot screws.  I need to reset float levels I think.  I set it to the 14.5 the book said to but when I crack the throttle it falls on its face unless I ease into it and the idle jets not doing anything leads me to believe things just aren't right.

[Vintovka]

Im really not sure about the float levels I just did it so they were at 90 degrees with the carbs in the correct position on the bench.  Another thing is even though I pull the choke out it has no impact on the way the bike runs at all and it is for sure closing the choke flaps underneath the air filter pods.

[Kong]

Massive air leaks (rubber boots) combined with misadjusted floats.

[octagon]

guess you've checked your air filters carefully - you could try taking them off temporarily and see if that makes a difference

[TwoTired]

There are three fuel metering restrictions on these carbs.  The pilot, the side needles, and the main.  Which one dominates depends on throttle position.

The pods change carb throat pressures which effect the fuel draw through each and every one of the metering restrictors.

What makes you think 120 mains are correct?
What makes you think the slide needles have the correct position and taper for the throat vacuum changes made by the pods?

Have you checked the fuel level in each carb bowl?  High levels will make it easier for fuel to rise/reach the carb throat.

Do all spark plugs show the same deposit patterns for each cylinder?

The shop manual requires a tach with 50 RPM sensitivity to fine tune the pilot screws.

[TwoTired]

I am right there with you.  My 77 750F doesn't change a bit no matter how I adjust the pilot screws.  I need to reset float levels I think.  I set it to the 14.5 the book said to but when I crack the throttle it falls on its face unless I ease into it and the idle jets not doing anything leads me to believe things just aren't right.

You need to get your accelerator pump and circuit working properly to cure the throttle crack issue.

[Duke McDukiedook]

Pods, pods, pods...

[Vintovka]

There are three fuel metering restrictions on these carbs.  The pilot, the side needles, and the main.  Which one dominates depends on throttle position.

The pods change carb throat pressures which effect the fuel draw through each and every one of the metering restrictors.

What makes you think 120 mains are correct?
What makes you think the slide needles have the correct position and taper for the throat vacuum changes made by the pods?

Have you checked the fuel level in each carb bowl?  High levels will make it easier for fuel to rise/reach the carb throat.

Do all spark plugs show the same deposit patterns for each cylinder?

The shop manual requires a tach with 50 RPM sensitivity to fine tune the pilot screws.

I thought it was common knowledge to go up a jet size for exhaust and a jet size for pods which would put the F model at 120 mains.  I didnt put the 120s on the PO did so apparently he read the same information.  Yes each spark plug shows the same deposit pattern black and sooty.  I think Im going to pull the carbs again and just start over not sure what else to do I still haven't gotten a straight answer from anyone a few are saying float levels so I'll fine tune that this time.  The digital tach I have is actually a multimeter and it does read changes in 10s of RPMs.

[TwoTired]

I thought it was common knowledge to go up a jet size for exhaust and a jet size for pods which would put the F model at 120 mains.  

It is not common knowledge.  It is folk lore, with all the reliability that entails.  A dyno fuel map or a test track with recursive spark plug reads will tell you what your specific engine set up (inlet and exhaust) needs.

Stock jets were 105, making 120 several sizes larger than stock.   ...And this jet only dominates at 3/4 to full throttle positions.  Have you done a "plug chop" with clean plugs at this setting?

Good luck finding your magic "straight answer" to a bike uniquely configured.  Do you even know what brand pods you have? ("PODs" is not a specification, btw)

The digital tach I have is actually a multimeter and it does read changes in 10s of RPMs.

How many volts equals 10 RPM?

[Vintovka]

I thought it was common knowledge to go up a jet size for exhaust and a jet size for pods which would put the F model at 120 mains.  

It is not common knowledge.  It is folk lore, with all the reliability that entails.  A dyno fuel map or a test track with recursive spark plug reads will tell you what your specific engine set up (inlet and exhaust) needs.

Stock jets were 105, making 120 several sizes larger than stock.   ...And this jet only dominates at 3/4 to full throttle positions.  Have you done a "plug chop" with clean plugs at this setting?

Good luck finding your magic "straight answer" to a bike uniquely configured.  Do you even know what brand pods you have? ("PODs" is not a specification, btw)

The digital tach I have is actually a multimeter and it does read changes in 10s of RPMs.

How many volts equals 10 RPM?

The 1978 F model bike had 110 main jets stock and the 1978 K model had 105 main jets stock I know because I own both and its straight out of the 1978 Honda shop manual page 4-2 under fuel system.  I've read plenty of other sources say 105 was stock but I'm going with the Honda shop manual unless you can verify it was misprinted information.  Just pulled the plugs and they were again black as all hell so I'm referencing this thread   http://forums.sohc4.net/index.php?topic=76757.msg885927#msg885927  and shimming the needle with one washer to try leaning it out.  I don't plan on racing the bike so I want my mid range to lean out and not be so fouled.  The "pods" are K&N air filters.  I never did a "plug chop" just normal everyday cruising at the throttle ranges I'll be riding at after the changes like I said I'm not racing the bike.  If it was up to me the bike would be completely stock it was the PO that installed the air filters, exhaust, and jets. 

The stock F slide needles don't have clip positions as you can see in the pictures in the linked thread.  Don't get me started on the multimeter I'm not trying to get an accurate reading I'm just trying to adjust it through the rise and fall of the RPMs.  I also understand the main jet is roughly 3/4 to WOT thanks to your chart which is why I'm referencing the mentioned thread above and attempting to shim the needle and lean the bike out that way.  Now that the carbs are back off even though I didn't correctly measure the float levels before, I just did and they were in fact in the correct position (14.5mm +- 0.5mm).  I appreciate any help, constructive criticism, and your extensive knowledge of the SOHC bikes, but I don't need your sarcasm.

[Spanner 1]

Well then, if your float levels are O.K. it might be time to start going down a size or two on the main jets..... also shimming the slide needles will tend to richen the situation , not lean it out any.

[bollingball]

I might be way out in left field here but have you measured your plug caps it could look like it is firing good but is it really. I farted around with my carbs for awhile just to find out PO had put 0 ohm caps on it so I got the 5K ohm caps and the plugs cleaned up nice. However my bike is stock airbox.Just something to check

[TwoTired]

The 1978 F model bike had 110 main jets stock and the 1978 K model had 105 main jets stock I know because I own both and its straight out of the 1978 Honda shop manual page 4-2 under fuel system.  I've read plenty of other sources say 105 was stock but I'm going with the Honda shop manual unless you can verify it was misprinted information.  

The 77-78 F models had a more restrictive 4 into 1 exhaust than the K model's 4 into 4, which is part of the reason why the F had smaller Main jets.  The other reason was that the F engine drew deeper vacuum than than the K model due to Cam and valve differences.  That's probably why this chart:
http://www.motorcycleproject.com/motorcycle/text/specs.html
...And the Honda Shop manual supplement  page 282 says the PD 42A Carb have 105 mains.  Whereas the 78K models with PD42B carbs had 110 Mains. (pg 275).

The stock F slide needles don't have clip positions as you can see in the pictures in the linked thread.  

The 77 F model model PD41A carbs DO have notched slide needles.  If you insist on running this bike with the pods and changed exhaust.  You will probably need the adjustment range provided by these needles.  (or you could machine extra grooves in the needles you have.)

I don't need your sarcasm.

And, I don't need your attitude and false accusations.  Besides, it is clear you believe you know way more than I do about your bikes, so I will just wish you well on your project and refrain from further posting to it.  Dictate terms of the help you need to someone else.  And have fun with your struggles.

[Vintovka]

The 1978 F model bike had 110 main jets stock and the 1978 K model had 105 main jets stock I know because I own both and its straight out of the 1978 Honda shop manual page 4-2 under fuel system.  I've read plenty of other sources say 105 was stock but I'm going with the Honda shop manual unless you can verify it was misprinted information.  

The 77-78 F models had a more restrictive 4 into 1 exhaust than the K model's 4 into 4, which is part of the reason why the F had smaller Main jets.  The other reason was that the F engine drew deeper vacuum than than the K model due to Cam and valve differences.  That's probably why this chart:
http://www.motorcycleproject.com/motorcycle/text/specs.html
...And the Honda Shop manual supplement  page 282 says the PD 42A Carb have 105 mains.  Whereas the 78K models with PD42B carbs had 110 Mains. (pg 275).

The stock F slide needles don't have clip positions as you can see in the pictures in the linked thread.  

The 77 F model model PD41A carbs DO have notched slide needles.  If you insist on running this bike with the pods and changed exhaust.  You will probably need the adjustment range provided by these needles.  (or you could machine extra grooves in the needles you have.)

I don't need your sarcasm.

And, I don't need your attitude and false accusations.  Besides, it is clear you believe you know way more than I do about your bikes, so I will just wish you well on your project and refrain from further posting to it.  Dictate terms of the help you need to someone else.  And have fun with your struggles.

Not sure what your problem is I don't have any qualms with anyone on this forum I just joined and I've done enough reading before joining to know you know your #$%*.  I also don't think I know more than you or I wouldn't be asking for help.  I specifically asked if it was a misprint in my original shop manual.  

Here are the pictures you can clearly see on the right hand side of the specifications box is the 750F with PD42B carbs and No.110 main jet size.  The left side of the specification box is the 750K model with PD42A and No.105.  I'm not crazy or making this up the publish date is DEC1977 copyright HONDA MOTOR CO LTD at the bottom of the page.  Horrible quality taken with my old phone but I still think you can clearly make out what I'm talking about.  This is an all original 1978 official Honda CB750 shop manual for the 750k8 and 750F3.

 My 1978 750K is all original and unless the PO put in 105 mains and the manual is wrong then damn thats quite the coincidence.  I cant comment on the 750F mains because the PO put in 120s but the manual says 110s were stock.

[bollingball]

Vin have you looked at the stamp# on your carbs they should be (PD42a for the F model and PD42b for the K model the manual has it listed both ways section 1-3=110 for the K and 4-2=105 for the K I have a bone stock CB750K8 with PD42b with 110 mains but from years of (PO) who knows what has been done inside mine or yours. I'm leaning toward TT being correct key word (supplement) looks like Honda trying to correct a misprint for the main jet size which is what you are looking at IMHO

[lucky]

I am right there with you.  My 77 750F doesn't change a bit no matter how I adjust the pilot screws.  I need to reset float levels I think.  I set it to the 14.5 the book said to but when I crack the throttle it falls on its face unless I ease into it and the idle jets not doing anything leads me to believe things just aren't right.

When it stalls or quits on quick opening of the throttle from idle it means the idle jet is too small. If you putt in larger idle jets then open the mixture screw one turn to start.

[Spanner 1]

What's up with folks dragging-up old threads and posting replies more than A YEAR LATER.. 

[TwoTired]

My 77 750F doesn't change a bit no matter how I adjust the pilot screws.  I need to reset float levels I think.  I set it to the 14.5 the book said to but when I crack the throttle it falls on its face unless I ease into it and the idle jets not doing anything leads me to believe things just aren't right.

When it stalls or quits on quick opening of the throttle from idle it means the idle jet is too small. If you putt in larger idle jets then open the mixture screw one turn to start.

I don't believe this is strictly true. Particularly with PD carbs and accelerator pumps.  If it stumbles on throttle twist, the accel pump isn't working to its potential.

For models that don't have accel pumps, stumble on throttle twist is to be expected if the sudden change is too drastic, as in to full open from idle position.
  The pilot circuit, which includes the pilot jet, may easily become too lean with air induction changes that affect the production designed throat pressures, and may or may not be compensated for with pilot screw setting changes.  If not, THEN a jet size increase may be required.  However, off idle mixtures are the domain of the slide cutaway shape/angle, which is largely ignored by many bike modifiers.

Tuning a carb (without accel pumps) to respond to full throttle whack from idle makes it highly probable that they will foul spark plugs with any sort of prolonged idle as the id;e mixture will be far too rich.  That may not be a problem for machines used only on the track.  But, street use is another matter, what with stop lights and traffic.

Cheers,

[grcamna2]

Those accel jets on all 4 carb throats and the pump itself in # 2 carb always need lots of work to get them to perform as well as can be...they get kind of gummed up and take a bunch of cleaning to get them all "squirting" evenly into all 4 carb throats .

[Spanner 1]

TT and grcamma2... y'all are more than a year LATE responding  thats 2011 not 2012.. hehe.

[TwoTired]

TT and grcamma2... y'all are more than a year LATE responding  thats 2011 not 2012.. hehe.

Spanner, the threads don't get deleted with time, and are available for those that search.  Are you saying that correct information has an expiration date?
If a thread is dredged for whatever reason, I still think the information in it should remain informative.

BTW, I also responded in 2011, in case you didn't notice.

[Spanner 1]

The 'dredger', 'lucky' ( see reply # 17 ) confused Jan. 2011 for Jan 2012... no big deal... but the OP's questions are more than a year old!... he's 'prolly sold the bike, got re-married and moved to Mongolia, Outer that is.... my comment was 'why do folks pull-up old threads and comment months/years later.'..  it makes NO sense 

[Kong]

Actually it makes lots of sense.  The reason to put new information into an old post is that the answer is being given to the hundreds and hundreds of people who will read this post who come later.  Some of them may have similar problems, some of them are just trying to lear as much as they can about how the systems work for future reference.  So the answer is directed to a larger audience than just the person who posted the original question and spans a broader time period than just the moment of the original problem.