CB750K5 Has Me Stumped

[CB750Winger]

  Folks, I've done EVERYTHING except coils but my 75 idles pretty good but when go from there on up, it sputters like it's loaded up and has to clear out. Carbs are all new parts, I've had the needles in every position more than once, so you know I've had them off at least 5 times and she won't run smooth. No air leaks, compression is all 150 psi +/- 5, new plugs, points, carb sync, fuel mixture is good. I have the stock air box with 4 into 2 turned out exhaust with baffles. I've had 115 main jets and put in 110's for now. The slow jets are 40's as stock. I'm getting real good at pulling carbs off but I'm geting tired of having to. From a running bike with tank on, I can have the carbs on my bench in 9 minutes. lol  Could it be a coil weak in and out or any other ideas??       Paul

[impaxg]

WHAT COLOR ARE YOUR PLUGS? MOST COMMON PROBLEM WITH CHANGING PIPES OR AIR FILTERS ON CB 750 K'S IS RUNNING RICH OR LEAN. THIS IS A VERY EASY ENGINE TO TUNE. THEY COME WITH GREAT CARBS AND WHEN SYNCED PROPERLY,YOU HARDLY EVER NEED TO DO IT AGAIN. PUT ALL YOUR CARBS BACK TO STOCK SETTINGS AND CHECK YOUR TIMING WITH A TIMING LIGHT AT ABOUT 5000 RPM. AFTER SETTING YOUR TIMING PUT THE VACUUM GAUGES ON THE CARBS TO SEE WHAT IS GOING ON. IF TIMING IS NOT SET CORRECTLY, CARBS WILL NOT SYNC PROPERLY. AFTER MAKING SURE THAT ALL IS CORRECT PUT IN A NEW SET OF PLUGS AND DRIVE AROUND AT DIFFERENT THROTTLE OPENINGS FOR ABOUT 30 -40 MILES. AFTER YOUR RIDE TAKE OUT THE SPARK PLUGS AND CHECK THE COLOR OF THE PLUGS. NORMAL COLOR SHOULD BE LIGHT TO CHOCOLATE BROWN. GREY PLUGS INDICATE TOO LEAN AND YOU NEED TO INSTALL A LARGER MAINJET SIZE, AND BLACKENED OR SOOTY PLUGS INDICATE TOO RICH AND YOU NEED TO GO TO A SMALLER MAINJET SIZE. I HAVE USED JET SIZES FROM 110 TO 140 DEPENDING ON ENGINE MODS. AIR FILTERS AND EXHAUST CHANGES. IN MOST CASES I HAVE ALWAYS LEFT THE NEEDLES IN THE STOCK POSITION. JUST AS AN AFTER THOUGHT, A SIMILAR TYPE OF SITUATION OCCURS WHEN THE GAS CAP VENT IS PLUGGED AND FUEL STARVATION OCCURS.
HOPE THIS HELPS

[CB750Winger]

Good input impaxg, thanks. I have checked points and timing several times and is right on. The advance curve seems to be working real good. I've also greased the shaft on the advance like the book says. I have the needle position in the middle right now and I've tried it everywhere. The inside of the carbs are spotless and the slides have a good fit, no slop. I have the valves set at .0025 and .0035". The #2,3 & 4 plugs are normal tan but #1 is a little sooty but not oil fouled. I also have a brand new gas cap I bought from hdlparts.com. I've had the fuel air mixure set everywhere and pretty much going off the exhaust smell to dial it in. This all kind of makes me think I have an intermittent coil issue on #1 plug or my next thought is an intermittent short at the kill switch? I have not checked that. I can get a new set of coils and wires from partsnmore.com for $30 each. The bike idles real smooth but anything over that, it has a miss. Your thoughts??

[Tech78]

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[CB750Winger]

Hey Alan, good to hear from you. I put all new NGK plug caps on but could possibly be a bad one. I'll put the old one back on and give it a try. I'm on my 3rd set of plugs, the first set had a bad one at #1, NO spark at all so new parts are not always perfect. I'll try the old cap and let you know.   Paul

[Re-run]

I love questions like this! Coils themselves do not usually go bad, if they do, it is bad not intermittent, usually. I do not remember reading if there was a certain rpm that your bike missed. If there is, we can use that to figure out what jet is affecting this the most.

As for plug color, in the 70s, that was correct but then gas back then was a lot different than now. What was considered good then would be on the rich side. There are a lot of false ideas floating around these days. For example, air filter change DO have a big impact....sometimes. Say you go from stock honda to a paper emgo filter, You will likely run richer as that filter restricts more, at least mine did.
If you go to pods or k&n, then you would run leaner. Exhaust changes however, do not usually require any changes unless you are moving to a VERY open exhaust such as drag pipes.
Anyways, lets get to tshooting.
A miss is usually caused by either to rich a mix or ignition. Now you can probably not worry about the coil itself or the points if this is only happening on #1, if it was on #4, then a coil or point could be a culprit.
So concentrate on the wire, cap, plug, & carb. Check the wire, make sure it has no cracks. If you did not do it when you put on the new cap, clip about 1/4 inch of wire off and screw the cap back on. You can also check the cap with an ohm meter. Just put one probe in each side, resistance should be about 5K ohms.
now swap the plug from #1 to #2 and see if the miss moves. If it does, then it is the plug.

That covers ignition. Now you said you put in new carb parts, what brand? Keyster is very common but their needles do not seem to work very well. Most people end up getting honda needles. Something about the taper not being correct.

When you tuned your carbs, I will assume you synced them BUT how did you tune the idle mix? Common practice is to hook up a tach, not the bike one as they are not very accurate, and do each carb. You start with #1 and it an idle of 1200 or so, you tune the air screw till you reach the highest rmp then back off about 100 rpm. Reset idle to 1200 and repeat for each carb.

Another way you can do it is to do plug chops at different rpm ranges. Take plugs out and clean them. Run the bike at idle for a few minutes and pull them and see what the insulator looks like. A light tan is ideal. If they are darker than that, turn out your air screw no more than 1/2 turn and recheck. once done, do a wot plug chop and this will determine if you need a larger or smaller main.

[CB750Winger]

 re-run, I've done all this acording to the procedure you said, and different things that don't seem to help. I think you are right about concentrating on the wire and plug cap. I'll swap it tonight. I did cut back 1/4" when I put the new cap on but I'll also check the ohms on both of them. Thanks for the input and pretty sure I know where to zero in on it.    Paul

PS- I used OEM Honda Jap. carb parts and even swapped out those back and forth with another rebuild kit I had, no change.

[Re-run]

What air filter do you have? how old is it? I had gotten the emgo as it was about a 3rd of the price the honda one was. It was constructed very well just too restrictive. I got a k&n as it was only about 15 more than a paper honda filter.

Only other thing I can think of is one of the fuel passages in the carb body is partially plugged  but lets go with the plug and cap for now. that sounds good.

[CB750Winger]

I have a new OEM Honda filter with stock air box and new 4 into 2 turn out MAC system with baffles. It still runs the same as the 4 to 1 I just took off. The miss happens from about 1500 to 2500 rpms. I would think this could be #1 carb but I've had them off and cleaned so many times I would think I could rule it out. I also blew compressed air through each hole and could feel where it was coming out every time. I'll do the plug cap switch tonight and let you know. Again, it's only #1 that's sooty so like you said, it's probably not the coil or points, got to be the cap or wire to the coil. Thanks again!    Paul

[cbRider]

Paul,
sounds exactly what i just experienced. I found one plug connector open (an infinite reading with the Ohm meter instead 5K ohm) Replaced it and that did it.

Karl

[CB750Winger]

Karl, I sure hope you are right. This thing really hasn't run right since I put the new caps on and the previous owner had everything else so screwed up, it's hard to tell. If it's a bad cap, I'm also going to be real pi__ed off at Old Bike Barn where I bought these, a bad set of plugs and a set of points that sparked around the contacts to the plate. I had to put the old ones back on. So far, everything I've bought from them is junk. Thanks to all here for the feedback, I can usually figure this stuff out but I'm not great on electrical........

[Re-run]

Electrical is a pain. I have never bought from obb but I used to be on a site where we had a LONG thread about how poor their service was.

[CB750Winger]

  yeah, obb service is pretty bad, probably never use them again. Has anyone used partnmore.com? They don't have a big variety but there are things in there I've been looking for and they are dirt cheap. If they are available, I try to stay with OEM parts when possible. My Goldwing gets ONLY OEM Honda parts but they will be hard to come by soon. I use hdlparts.com or call them to order but sometimes service is not great there either, I guess depending on availability. I have also ordered from them using my local Honda dealer and I can get parts in a couple days so I'm sure they have dealer priority. Mine is only a couple miles from where I work so I can pick them up on the way home. I know, I'm lucky.

[impaxg]

Have you checked the carb sync with vacuum gauges?

[brcree]

  OBB sucks!!!!!!!!!!!!!!!!!!! I ordered a cafe racer seat from them 7 weeks ago. When I got the invoice it said 1 to  3 weeks deliv.  I'm on the 7th week. I could not find a phone # and have emailed them 5 times. Finally they email back.................it will be 3 more weeks till I (should) get it!   If they had told me it would take this long in the beginning it would have been better................

[Tech78]

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[Re-run]

I have used partsnmore and have had good luck.
I have also used z1enterprizes.com and they have been excellent.

[CB750Winger]

  Update guys, after sputtering all the way home last night, I took the #1 spark plug cap off and tested it. It was right at 5k but I also tested the old one I saved and put it back on. I also threw in a new spark plug because it was a little sooty. When I took the cap off, it looks like I may have screwed it in too far and twisted the wire some so I trimmed back another about 3/8" and put the other cap on. I ran it around the block but was getting too dark and too much beer. It seemed much smoother through the throttle on the way to work this morning but has a little sputter at 3000 rpms so I think I need to check the other 3 plug caps to see if I did the same thing. I just hope I have enough coil wire left to trim back, it looks like it's been done at least a couple times and they don't give you much extra. It ran good this morning but lately the longer I run the bike, the more sputter I get out of it, we'll see how it acts on the way home. It's been 90 degrees here all week. Thanks for any additional input!       Paul

[Polymer-2]

I've used partsnmore a couple of times and had good luck each time with them.

I got tired of messing with the Honda wires and caps and the price was just nuts so I got a set of 8mm wires from a 2000 or so Cavalier (many GM cars use these same wires).
I cut the coil ends off then carefully ground the outer jacket so that would fit into the Honda coils then used the original Honda plastic nuts that held them into the coils.
On the spark plug side you will need to round up 4 of the threaded caps that screw onto the plugs since the NGK's don't come with them. I found some caps on some Denso plugs that I had laying around for my boat motor but many lawn mowers and older style plugs have removable caps.

[CB750Winger]

 That looks like a great idea but I think the SOHC coils are different, the two wires seemed to be directly attached in the coil. My Clymer manual says the SOHC coils are differnt from other Honda models but similar to auto coils the way they perform, not that I believe everything. So here's what's going on: on the way home from work today, the bike ran good for about 15 minutes then gradually started missing and running like crap again like it did this morning so now I'm thinking this is a heat related electrical issue by the pattern it's been running. It still reminds me of problems I had with a couple cars with points and coils back in the 70's. Replaced coils and problem was gone. Do you buy this theory or is it my imagination?