Topic: 1975 cb750f no idle jet adjustment

[mechanic3]

I know this has been done to death but am at  the end of my rope. The idle jet screw does nothing. You can screw it in all the way or even remove it with little change of idle. When doing this I idle it down to about 900 to 1000 rpm but is rough running. This has been a problem with this bike since I got it. I tried the things that wear mentioned before and some that I researched but to no end. Have not had the bike on the road yet just trying to get it to run right at idle. I mentioned on an earlier post that the cylinders have some pitting but comp. is 140 psi on all cylinders. Runs ok above 1200 rpm but not so good lower. If you add fuel (choke) it smothers out and it smells rich at idle. Plugs are black . Also checked for vacuum leaks around carb to head boots with brake cleaner and idle did not change. Before all this I changed the plugs, points, condensers, checked timing (static and dynamic), cleaned the carbs. (3 times)new low speed jet and synced carbs. Do not have an adapter to do the clear hose float test but instead set float at spec. (26mm) same thing  and tried 28mm (lower)  no difference. Low speed jet is a #40 and main jet is a 120. Any thoughts on this would be appreciated.
 

[kerryb]

I did all kinds of stuff trying to make my 550 idle better and stop fouling plugs.  I made some progress with carb cleaning, adjusting, and synchronizing.  Timing with a timing light and dwell meter helped a little too, but I couldn't get it smooth at idle and clean burning...till I measured the plug caps, they were all over 5k ohms.  Replaced caps and all the problems I was chasing were cured...smooth, calm, and non-fouling.

Just a thought...to consider.

[mechanic3]

thanks kerryb I will check that but I don't think it explains the lack of jet screw adjustment.

[evinrude7]

Vac sync the carbs. Before that clear tube method. Buy some clear hose small enough to warm up the end with a lighter and screw it into the bowl drain hole. No need for an adaptor.

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[mechanic3]

already synced the carbs. with gauges and needed no adjustment. I will try your trick with the hose. Best stay away from the gas with the lighter . LOL. Thanks evinrude7

[evinrude7]

already synced the carbs. with gauges and needed no adjustment. I will try your trick with the hose. Best stay away from the gas with the lighter . LOL. Thanks evinrude7

For sure. Good luck. How's your cam chain? Adjusted?

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[my name is nobody]

120 mains are too big, stock is 105

[mechanic3]

One of the first things I did was adjust timing chain. My book calls for a 120 main in this bike and that's what was in it. Also at idle its not really used is it?

[pjlogue]

The idle air circuit can become partially/totally plugged with dirt and deposits in a carb of this age.  You want to be sure all internal passages are real clean for this circuit.  The only way to do this is to remove the idle air needle/spring and slow jet and run a small wire into the intake side into where the needle resides.  The intake side of the carb has two small holes. The hole without the brass insert is the air idle circuit.  The one with the brass insert is the air feed for the high speed circuit.  ( Do not damage this brass insert or change/enlarge this hole.)  Flush each passage with plenty of carb cleaner.  Do the same with the passage from the air idle needle to the slow jet.  Plenty of carb cleaner!  I included the high speed circuit even thought it does not affect the idle circuit because as long as you have the carbs apart you need to be sure this circuit is clean.  You also need to be sure the holes in the emulsifier tubes are clean with no deposit buildup on the cross holes.  The slow jet fuel metering hole is very small and difficult to clean if plugged.  Running a wire through the hole can damage the passage. 

The deposits left from oxidation of old fuel, gum deposits, white powder from dried fuel stabilizer can be quite stubborn to remove.  Mechanical cleaning with a wire is the best way to brake them up so the carb cleaner can flush them out.

I also know that many of the aftermarket rebuild kits have new springs and they are crap.  The ends of the springs are not a smooth 90 degree end to the length of the spring and have sharp edges that can gouge into the brass needle scraping off pieces of brass which then become lodged downstream inside the carb body.  I will also mention that using OEM Keihin brass in these carbs is the best way to insure proper function.  A good idea is to take the brass in these carbs and look at the holes under a jewelers loop to see if they have been damaged.  Gouges, scrapes can indicate a previous owner's heavy hand at trying to clean them.  After 40+ years of unknown history yyou can find all sorts of damage or modifications (enlarged holes)

BTW/ a 120 main jet is to large for a stock '75 750F.  I believe they came with 105 or 110 mains.  Once you are satisfied the carbs are clean and timing/ignition is right you should do a series of throttle chops at various speeds to inspect the plugs for proper mix. 

-P.

[david 750f]

Follow pjlogue’s advice, the carbs have to be spotless..Stock main jet is 105, 40 on the slow (pilot) jet..

[Jerry Rxman Griffin aka MuthaF'er]

+1 on the 105's UNLESS it has pods vs stock air cleaner. You better check your book too if it calls for 120's.

[mechanic3]

hi guys. Thanks for your input. As suggested by pjlogue I double checked the ports and changed to 105 main jet and also raised the float to 23.75mm which clear tube tested at around 4mm from bowl seam. Re-installed and runs the same (no low jet screw adjustment and poor idle below 1200 rpm. Only clear tubed the outside carbs but both were the same. Hate to raise floats any more when spec. is 26mm.

[mechanic3]

Also I was wrong about the jets being 120 they were 115 . Sorry

[mechanic3]

Can the carb sliders leak vacuum? Did not have them out.

[pjlogue]

hi guys. Thanks for your input. As suggested by pjlogue I double checked the ports and changed to 105 main jet and also raised the float to 23.75mm which clear tube tested at around 4mm from bowl seam. Re-installed and runs the same (no low jet screw adjustment and poor idle below 1200 rpm. Only clear tubed the outside carbs but both were the same. Hate to raise floats any more when spec. is 26mm.

The idle circuit consists of the idle air needle and the slow jet and its emulsion tube.  Fuel passes through the slow jet into the emulsion tube where it mixes with air from the idle air needle.  If you can't run below 1200 rpm and you know the timing is spot on, cam chain adjusted properly and valve lash adjusted properly I would suspect a clogged slow jet.  The tiny hole in the main jet is VERY small.  Pull the slow jet and make sure you can see daylight through it.  Make sure the cross drilled holes in the emulsifier tube are clean and not restricted by deposits.  You can run a micro drill (by had using a pin vice) to make sure the cross holes are clean.  You basically want to use a drill size just below the hole diameter and use the flutes of the drill to carefully scrape away any varnish/gunk. 

If the holes in the slow jet are restricted/clogged you will not be able to adjust the air idle screw to change the idle mixture.  This is also true if the passage from the idle air screw to the slow jet emulsifier chamber is restricted/clogged. 

With an unknown bike history, you need to make sure all parts of the system are correct before attempting to mess with carbonation.  Check that your ignition system is in good condition.  High tension wires are in good condition, plug caps are not cracked and plugs are good.  Check the resistance of the plug wires, plug caps and plugs.  The total resistance from coil output to the center electrode on the plug should be around 5,000-8,000 Ohms.  If you show one or all set of wires/plugs up at 12,000 or higher you need to replace the bad components. Plug caps may be bad, wires may be bad or resister plugs may be bad.

-P.

[mechanic3]

kerrby had mentioned plug caps and they are at 9300 OHMs which is on the high side. I found a post by HondaMan in 2010 that said new caps around 7500 and worn out ones 9500 OHMs. My wires and coils look original and are all one piece  Guess im spending more money on coil set and caps.LOL Went through jets and tubes with the small hole in them multiple times an am sure all is clean. Can't tie my shop up any longer. Need the space. I run a small automotive shop. When I get some more ignition parts  I will let you know if there is an improvement. Thanks everyone for your help (Week end is over)

[pjlogue]

You should check your intake manifold rubber boots for air leaks if you haven't already done so.  The rubber gets rock hard and can develop air leaks.  Spray carb cleaner around the boot to carb body and boot to intake nipple.  An air leak can make the bike run rich on these pulse carbs.  If you are running rich there may not be enough air needle adjustment to get to the sweet spot where you can tell the difference between a lean mix and a rich mix.

These carbs are tempermental and can be hard to trouble shoot but when they are clean and set up right they work well

Good luck.

-P.

[mechanic3]

thanks pj

[dave500]

the caps unscrew from the wires,your coils will probably be fine,you can use a guitar string wire to probe out tiny holes through the idle jets.

[mechanic3]

jets are clean and idle screw and jet is new (#40)

[evinrude7]

jets are clean and idle screw and jet is new (#40)

keihin jets? and TEC points? 

[david 750f]

Are you sure that you are running on all 4 cylinders?
Touch the pipes at start up to see if all the exhausts are warming up evenly. Have you tried the idle screw on all four cylinders?
What do the plugs look like?

[Bankerdanny]

Did you replace any of the other brass in your carbs (the needles and main jet). Do you have actual Keihin needles and jets? Non-Keihin are very unreliable. What notch is the needle clip in? If it is on too low of a groove then fuel could be escaping past the main jet at idle, which will mess with the idle screw's ability to impact the idle.

Before you synched the carbs did you bench synch? Are you sure that the slides are closing sufficiently? If the throttle cable is binding and the slides don't fully come down that will also mess with the idle mixture in a way that can't be fixed with the idle air screws.

The idle screws having no impact would say that the idle circuits are not clean enough or the needle is not completely seating in the main jet emulsion tube because (1) the slide isn't fully lowering (2) the taper is wrong (3) the clip is in the wrong groove (4) the slides are incorrectly adjusted and are not sufficiently closed at 0 throttle or some combination of these things.

You haven't said what your air cleaner style is.

[mechanic3]

to  start with I left the fully cleaned original jets in and it ran the same. Yesterday I switched out the 115 main to a 105 and the # 40 idle jet to an aftermarket set. Please don't sue me for that, it is all I had. It ran the same as it always has with idle screw making no difference. But like I said yesterday the spark plug cap tested on the high side for resistance (9300 ohms) so have ordered  new coils and caps. Two of the wires had worn spots on them and are part of the coils. What Bankerdanny says makes sense about the rods  possibly being to high . Has to get fuel somewhere. Not doing any more carb work until ignition is 100%. I do not have any noticeable misfire and runs grate at 1200 and up rpm. Would like to drive it to see if it pulls ok but we still have snow on the ground and O degrees C (32F) is a warm day. Still need a good fuel tank ant tires. Running it on a bottle right  now. Sort of like me, one pint at a time.  Am running stock air box.

[mechanic3]

Managed to get time to install new ignition coils, wires and spark plug caps. and made no difference on  the poor idle and lac of idle jet screw adjustment. On a more important note I hope everyone in Canada and across the world can stay safe with all that is going on with the  covid 19 virus. In reality I know that not everyone will be that lucky, just try to do the best you can and stay at home. GOOD LUCK EVERONE