Topic: 1974 cb750 running issues

[piefairy]

My project is coming to a close. Thanks to Calj and all the others for the input and help. 1974 CB750

 I am having a few issues though with its ride-ability. It is idling very high, I do not have a tach on the bike yet, its arriving tuesday along with the speedo, but idle seems to be closer to 2k when warm. If I put the bike in neutral at a stop light or something, I can hear the RPM's changing without any input from the throttle. It will go down to a sub 1000 sound and spike back up to 2k+ at random. I do have the idle screw (the funky looking screw on the carb with all the slots cut in it) backed off all the way, which I would have assumed would cause the bike to not even idle. The bike is also running rich, I am getting a lot of backfire from the exhaust. I will be replacing the exhaust though since its running without any baffles or anything, which may solve this issue or possibly both? Also, throttle is a little slack when not engaged to eliminate it as a possibility.

The last bit which is puzzling, if I go up above about a 1/4 turn of the throttle power seems to wain. The RPM's build slower, which again leads me to it running rich, but the power to the rear wheel seems to slack off, and if I get to about 1/2 throttle bike begins losing RPM's and power. My assumption is bad jetting, but not positive. The clutch is a new set from Barnett, with new plates and springs. Its tightened and torqued to spec. Clutch in first grabs late from standstill, but I have heard thats a bit common on 74's.

Thanks in advanced for the help!

[pjlogue]

Did you do a static sync on your carb slides?  You most likely need to lower your slides in your carbs.  Have you synced them?  Each adjusting nut at the top of carbs (under the rubber boot) needs to be backed off.  How much depends on where they are.  I think ~2-3 threads showing above the lock nut is close to where they should be but if your bike is idling ~2K about 1/2 turn should get you in the ball park.  You should go more though so your idle is determined by the idle adjust so you can stall the bike out if you lower it too much.

-P.

[piefairy]

Did you do a static sync on your carb slides?  You most likely need to lower your slides in your carbs.  Have you synced them?  Each adjusting nut at the top of carbs (under the rubber boot) needs to be backed off.  How much depends on where they are.  I think ~2-3 threads showing above the lock nut is close to where they should be but if your bike is idling ~2K about 1/2 turn should get you in the ball park.  You should go more though so your idle is determined by the idle adjust so you can stall the bike out if you lower it too much.

-P.

No, I haven't. This is a bike I bought, and put around 30 miles on before I started this project. I will pull the carbs and check them out. I'm good with basic mechanics and how stuff works, but never really done much on a motorcycle until this project

Idle I do not think would be effected by the exhaust, but I believe I should get my desired exhaust before going further correct?

[BobR]

 More info would be helpful. Stock jetting? Stock set up (aside from exhaust)? Bob

[cooldrum]

More info. would be helpful.  Stock air box?  Have you checked your points and Timing?  Also, check for any air leaks on your carb boots.  If you have pods, that's another thing.  Exhaust leaks near head clamp area? Check those too. Any type of vacuum leak can present the symptoms you've described here. keep us posted.

[piefairy]

Bike is running pods, I guess, 1 air filter per carb. I am not sure what a head clamp is. Like I said, first time working on a bike. I have no clue on jetting, I doubt its stock given that it has backfired since day 1.

[pjlogue]

For tuning your bike you need to do things in order.  First is to adjust the cam chain tension.  Then adjust your valve lash, then.  These first two steps are done with the bike cold and each has a proper way to do it.  Then you need to check/set your float height.  Best way is tube method.  Be sure your carbs and air/fuel passages are real clean! Once this is done you then need to sync your carbs.  Initially you can do a static sync using a small drill bit as a feeler gauge.  (someone want to chime in with the proper size?)  Once this is done you should be in the ballpark for running.  Then with the engine warm and a hefty fan blowing on the engine for cooling you need to dial in the sync using a manometer set.  it can be a set of dial gauges or tubes.

Once you have the carbs synced and all else adjusted you need to fine tune the mixture of the various circuits in the carbs.  Stock '74 750 have #40 low speed jets and #115 main jets with slide needle in #3 or #4 position and the idle air screw out 1 turn.  With pods you may need to go higher on your slow and main jet and raise your needle a notch. 

All this may sound daunting but it is part of a 44 year old bikes charm and a good way to spend a rainy day in the garage. 

-P.

[BobR]

Bike is running pods, I guess, 1 air filter per carb. I am not sure what a head clamp is. Like I said, first time working on a bike. I have no clue on jetting, I doubt its stock given that it has backfired since day 1.

  Good advice from pjlogue on getting stuff in order first-(cam chain ,lash, float height ). I'll add that if it does have stock mains its going to be way too lean using pods. You are going to need to know what jets are in it and compare notes with other pod users to get a good starting point. Bob

[piefairy]

Yarg... it sound like it will need tools I don't have and knowledge that I also lack... WHY CANT THIS BE ELECTRICAL LIKE BEFORE!!! Anyways, Thanks for all the input, it may be out of my patience grade for now. I'm already over 2 weeks into it. I have a local guy in town that works on some classics, I may just hit him up to look into it. Plus, I've never had an actual mechanic look over it, so it may be a good thing. I already found 1 engine bolt that wasn't attached...