Topic: 75 CB750 Carb/ignition problems

[68grandprix]

Hi, I recently bought a 41,000 mile '75 Cb750 that is stock except for a 4into2 exhaust system. It sat outside all winter. To get it running I completely disassembled and cleaned out the carbs, changed the oil and plugs, got a battery and fired it up. My problem is, the bike seems to run fine all the way to redline, but only if I ease into the throttle. If I hammer it at low rpms it will die(if I keep the throttle twisted), and at higher rpms it doesn't seem to be as powerful at full throttle as it should. It has 110 mains and unmarked? pilots. The needles are one step above the bottom notch. The air screws are one turn out. It has the factory airbox. What should I change to solve this(mains, needle, etc)? Also, I set the timing for the 2/3 cylinders by moving the point plate, but when I try to set the 1/4 timing, I can't get it to the mark and it barely changes by moving the small plate. Why is this? Thanks, Jimmy.

[Mikeshonda750F]

1-4 are on the backplate, and 2-3 sit on their own plate. The point gap must be set just right or it will throw the advance/retard off. Hook up a dwell meter (20$US at sears). Black to ground, other lead to the blue or yellow wire and set the dwell. Dwell is how long in the crankshaft rotation the points remain closed and will give you a PRECISE gap... no guessing "well... it just barely touches.. crap.. i messed it up when i tightened it up".

Once the dwell is set, your advance/retard will be very close and should be in position easily with the adjustments on the plate. Adjust 1-4 1st, then 2-3

[Mikeshonda750F]

Also, the pilot jets are marked on the side and are probably still 40's. You should be ok with that jetting.

[TwoTired]

I understood that the 77 and 78 model CB750s were the first to have accelerator pumps on the carbs.  Before these, snap opening the throttle would routinely just make the engine wheeze and die.  These carbs are NOT CV type and suddenly opening the slide all the way causes the carb throats to equalize pressure to atmospheric levels.  The sudden loss of all vacuum meant it couldn't draw fuel from the carb's jets and it would starve the engine for fuel.  Accelerator pumps shoot raw fuel into the carb throats so the engine doesn't starve for fuel.  The 77-78 carbs ran much leaner that the older carbs also, particularly at idle speeds.  So, the accel pumps are really necessary on these.

For the older carbs I wouldn't expect a good engine pickup with more than a 50% change in throttle position.  Much depends on how over-rich your low speed mixtures are, though.

Mikes advice on the timing is spot on.  Believe it.

After your timing is correct, and you have performed all the other routine tuneup procedures, air filter, tappets, cam chain adjust etc., if you still feel there isn't enough power at high RPMS, then do a plug chop during those conditions, pull the plugs and compare them to the pics at:
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html
This should tell you if your high speed mixtures are correct.
Have you verified your plugs are good and of the correct heat range?

Um, have you done a compression check yet, to establish that the cylinder conditions warrant good power?

Good Luck,

[68grandprix]

When I had ax Xs650 chopper I used the old cellophane trick to set the points. I checked the point gap and it is on. I'm waiting to adjust the rockers until I can get the hoda feeler gauge. I did adjust the cam chain,. The plugs are Champion RA8HC. These are what came in the bike when I bought it. I got the proper(NGK D8EA(S)) plugs to replace them but I couldn't fit a socket in the center two holes so I just bought new Champions which use a smaller socket. I think I need the original toolkit with the preesed socket to put in the correct plugs. For the carbs, I assume they are running lean since I tried running then engine without the airbox and it idled very high, but fixed itself when I put on the airbox. Regarding the flat spot, wouln't raising the needle one notch or stepping up to 115 mains help? I will check the compression soon, Thanks, Jimmy.

[CHUNG]

The timing needs to be dead-nuts perfect on these bikes. So does the Carb Sync. It's a simple trick to use the gap to set time.
2 and 3 thou on the valve lash. (2 In, 3 EX) I use standard feelers but remove them from the pak to make it a little easier. Also you can put a bend in the last 3/8 inch of the gauge to make it a little easier to make the number. A specialy ground screwdriver helps to hold the adjuster while you lock the nut. It's a real Zen thing to lock them while holding the proper setting. After a gazillion times, I still usually miss at least one on the first attemp.
A #110 Main Jet is almost always light, even in the heat of the desert, they seem to like #112.5- #117.5 regardless of muffler or filters. A 120 is fat on most bikes with a muffler unless it's a colder climate. Whacking the throttles WFO should negate the needles as the problem. This throttle setting tends to only indicate a problem in Main Jet size. (if it's a carb prob) 1/4-3/4 is where the needles do their thing.

[68grandprix]

Thanks for the replies. I can't believe I didn't think of bending my cheapie feeler gauges. I was all ready to go out and buy the special ones. I'll try the 115 main jet and see if it help. That's a lot easier than taking the carbs off again to raise all the needles. I would have done all this yesterday, but my friend rolled his car with me in the front pass side(the side that got the roof crushed down to the headrest, which my head sits above)and I'm kingd of hurt(neck and headwise) right now. I should be good to go soon. I just want to feel the full potential of this bike. It sounds awesome, and it has to be a lot faster than my 82 Yami XS650, which is sold(and slower than my car). Thanks, Jimmy.

[Einyodeler]

. The plugs are Champion RA8HC. These are what came in the bike when I bought it. I got the proper(NGK D8EA(S)) plugs to replace them but I couldn't fit a socket in the center two holes so I just bought new Champions which use a smaller socket.

Hi Jimmy The plugs you are running are too hot of a heat range and they are resistor plugs.
The correct plug that cross references with the NGK D8EA is the Champion A6,the A8 is equivalent to the D7EA.Anytime the number starts with an R it`s a resistor plug.This happens often when you go to an Autozone or other automotive parts store, the kid behind the counter usually doesn`t have a clue. Remember that your bike already has resistor caps and if you use R plugs that means you`ll get an even  weaker spark than before.
   With NGK and NDenso plugs - the higher the number the colder the plug. With Champion and Bosch - The higher the number the hotter the plug.
      For the D8EA You can go with Champion A6 , Denso X24ES-U  or Bosch X4CS.

[Bob Wessner]

I remember reading this post yesterday, it caught my eye because when I went to the Honda dealer here last fall for new plugs (trying to get my K0 back on the road), when I specified the plugs I needed, the parts guy brought out a set and I distinctly recall him saying "these are resistor plugs, but that should not make much difference."

The post has me wondering now. With all the variables associated with a 35 year old bike and my skill (mostly the latter probably) how much of an issue is this? I have no problem getting new plugs if it will make a difference, but if all the dealer carries is resistor plugs, what would be the best alternate say, Champion or something? Oops, never mind, I see the cross-reference. I would still be interested in opinions as to how much of an issue the resistor plugs would really be. Thanks.

[dusterdude]

i understand the deal with resistor or non resistor plugs.i am runnig 3 ohm coils and resistor plugs(at least for now,until i change them again)but,i have not had any ignition related issues with this setup.i ran 75 mph down the interstate a couple of weeks ago and the bike ran great.just food for thought i guss.

[MRieck]

Just as a side note I'd check and make sure there are no vacuum leaks. Those carb boots are old and stiff based on the age of the bike and the mileage. Spray some brake cleaner around the boots when it is idling- if RPM's pick up you have leaks. Rubber parts don't last forever especially after a million heat cycles and exposure to the elements.

[68grandprix]

Today I put on some D8EAs and 115 main jets. It didn't fix the stumble, but man it runs a lot better(as in faster)on full throttle runs down the road. I can't wait til my tach and speedo cables come from Dennis Kirk. I'll try the carb cleaner trick, but if that's not the case I'll try raising the needles. I would think this would bring the fuel in slightly faster. I also tried turning the idle screws 1/8 turn in from one out. I don't think that helped. The plugs look good. When I put the choke half on it revs cleanly when I cracked the throttle wide open. Thanks, Jimmy.

[Mikeshonda750F]

The A6 and A8 Champion plug is no longer avaliable (as in, no longer made). The new stock number is RA6HC and RA8HC which are resistor plugs ~ If you happen to get them, they are simply old stock (This is according to my 2004-2005 champion book).

[68grandprix]

The boots are fine. I moved the needles up(now on bottom notch)and it helped but the hesitation is still there. Before if I cranked the throttle and held it the bike would die. Now it just cuts out huge but then recovers. My plugs are mostly white. Would a bigger pilot jet help? Thanks, Jimmy,

[TwoTired]

My plugs are mostly white. Would a bigger pilot jet help? Thanks, Jimmy,

Sounds like it.  But, white plugs would scare me.  That's pretty lean.  And there is risk of detonation. I hope you're near sea level.

  You could try turning in the pilot screws to richen the idle mixture, too.  Turn them in an 1/8 turn at a time.  Idle pickup should improve.  Keep turning them in until there is no improvement.  That will be the best you can do with that adjustment.  If it's not good enough, then the idle jet should go larger.

Then see if the plugs are still white after driving around for10-20 minutes.  If they go tan.. woo Hoo!

I admire your persistance!