75 CB 750K Not Firing on Number 1

[CB750Winger]

   I've already had a couple of rather long posts on this issue but #1 is still not firing all the time. I put in a hotter plug but still loads up. The pipe gets warm but not hot like the others. The compression is 142 - 148 in all of them, I have new coils, plugs, caps, points, condensers and new wiring all the way up. Carbs have been rebuilt and synced, fuel mixture set, timing is right on with cam chain tight, valves set, on and on and on. I'm thinking I need to pull the carbs and check #1 float valve and make sure everything is clear. I have not checked but maybe I don't have the cap screwed into the coil wire good enough. Ohms are good on the caps. It also seems like I have good spark when I hold the plug on the head so I really don't get it. I'm at wits end with this thing but want to get it running perfect. Help wanted from you Gurus.

[Tech78]

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[CB750Winger]

  Hey Alan, good to hear from you again. The plug has been wet so I put a hotter one in but still didn't fire consistant. Funny thing on the way to work this morning, about 1/2 way there, whatever it was broke loose and all four firing normal and bike got all it's power back. It idled real good so maybe something stuck in the mid or high speed curcuit?  I forgot about this problem but now with the power back, I have a sputter shifting up through the gears like I had before. Everytime I think I have it dialed in, there is another issue. Any thoughts? I'm baffled, I've worked on tons of motors but this one is, let's say - special. Thanks

[Tech78]

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[CB750Winger]

   I've checked the screen a couple times and has been clean and no junk in the bowl. I did install clear in-line fuel filters and they take a slight dip and then back up over the carbs and back down to the fuel tubes. The filters are not ever full, gas just seems to trickle through so maybe I have some sort of air lock there. I may put clear fuel lines on and re-route them. For more info, I have MAC 4 to 2 baffled exhaust with stock air box and new filter, stock size slow jets, 110 main jets and have the needed jets set at mid position. Could it be I have a fuel delivery problem with the lines going down and then back up? I would think gravity would take over but I might be wrong.  It fired good all the way home yesterday but still have the sputter on acceleration  Paul

[Tech78]

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[CB750Winger]

   I put a new gas cap from Honda on it this spring (my old one was crusty and bad chrome). I'll double check the vent with a broom bristle. On the main jets, 110's came in the stock rebuild kits I got but I'm not for sure if these or 105's are right. It had 115's in it when I took off the MAC 4 to 1 header. Any body want to buy this header? It's in great shape and new black paint. The bike runs great when you really crank it on so I don't think it's the mains, it does not bog. I guess it could be, I'll try about anything right now. It seems to really sputter in the low to mid speed transition. Thanks for all the suggestions, we'll get this thing right somehow!

[Tech78]

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[CB750Winger]

   Thanks Alan, I might try the 105's but I picked up some clear tubing yesterday so I'll put this on first and re-rout them so the fuel flow is running downhill all the way to the carbs. Right now, the little fuel filters take a dip down along the right side and back up over the carbs and then down again. I'll check fuel flow first, the filters are never close to being full so may be holding air? Anybody else had this problem?      Paul

[CB750Winger]

  I pulled my carbs off AGAIN yesterday and dropped the #1 float bowl thinking there was something plugged in there, only to find out the cast aluminum pipe that the main jet screws into has about 3/8" crack around it just above the jet. This has to be the problem with it loading up and not firing. You can see light through this pipe in that area so there is probably the equivelent of 4 or 5 main jets in the same one pulling fuel. The crack is at or below fuel level in the bowl. I mixed up some JB Weld and covered all the way around the casting so I'll let it set up a day or so and fire it up. I'm hoping my problem is solved, I just can't believe I didn't see this before! After all I've done to the bike, I'll feel really stupid if this was it the whole time. Thanks for all the help from you guys and I'll let you know how it turns out. The bike might be worth all the trouble after all.

[Cemoto]

Hello - How are you making out?

 I have the same problem with my 1980 CB750F.  I've changed the air filter, coils, wires, plugs, igniters, rotor, and stator (had a charging problem too).  The bike still does not fire the #1 cylinder consistantly.  It has spark and seems to run fine at high RPM.  Very low on power, however, very fuel efficent only running on 3 cylinders.

We measured the temp of the pipes at the cylinder and all were 415 degrees F except the #1, which was 117 degrees F.

Really have no clue where to go from here.

BTW, there is a gut on Ebay who rebuilds rotors for $69.  - mine came back measureing 5.2 ohms, works great.  Tim Parrot is his name.

Regards,

Al

[CB750Winger]

   I found the carb casting that the main jet screws into had a big crack in it just above the threads of the jet and it was pulling too much fuel and fouling the plug so I put JB Weld all the way around it. I took it for a good run yesterday and it was firing and running more like it should. It might take a while to clear that cylinder out. You may want to switch the plug leads on yours to see if it makes any difference. You might not have a good contact from the plug cap to the wire. Bad cap maybe? Have you held the plug on the head and crank it to see if you have a good spark? Float set properly? Sounds like you've tried about everything too.   Paul