Topic: '73 CB750 Sluggish issue and more....

[youssa]

Good Morning y'all, I've been reading up alot of other post that I could come up with on a google search, but I would like to see any advice on my specific setup.

 So I have a '73 cb750 with a 4 to 1 exhaust and velocity stack (plan on getting pod filters). No other mechanical mods on her. As for the issues I am having, the bike start up fine and idles without any problems. As soon as I take off the response from throttle can be a bit sluggish and sometimes "lurches", this is only experienced at low rpm/low throttle. The second issue with throttle is at higher speeds, when I open her up (usually after 1/2+ throttle) to gain speed it would sound as if the clutch is pressed in (sounds louder without moving any quicker).

From my understanding, the fundamental issue is that it is running lean. I will be taking the carbs out to inspect and most likely clean it out, but I'm not sure what jet sizes should be on it with the current exhaust and air filter (or lack there of with the stacks).

Are there any steps I can take to really pin point the issue? Should I just do a plug chop or are there other things to help out with trouble shooting? What jet sizes should I order for a 4-1 exhaust and velocity stack/4-1 and pod filters?

I would greatly appreciate any and all information/tips/guidance!

p.s. Any recommendation for a reliable shop in NoVA would be awesome.

p.p.s. Just read through the recent Throttle Issue post and it seems identical. Will use that to kinda offset any of my own confusion. Still would like any input, thanks!

[calj737]

Sluggish throttle is no doubt a lean fuel condition, which will only worsen with pods after velocity stacks. But, you still need to change your carbs settings with both setups.

Your 1/2 throttle acceleration sounds more like a slipping clutch issue.

As for reliable shops in NoVA, I know of none, but I do know of a very good one in Richmond (got a trailer?). But the reality is that both of these issues are easily addressed by the owner of the bike with a good manual, some medium intellect, and a keyboard (to ask questions on this Forum). Know anyone that fits this description?

Heres a link to the section of the forum for manuals and technical details for your bike:
http://manuals.sohc4.net/cb750k/

And heres an online link to the factory parts fiche and part numbers you can use to understand the assemblies and components for your bike:
http://www.manassashonda.com/OEMpartfinder.htm#/Honda/CB750K3_A__MOTORCYCLE%2c_JPN%2c_VIN%23_CB750-2200001/CARBURETOR/CB750K3-JPN-A/2Y143413AE14341325C

Just navigate back to the full listing of different sections to view other components.

[MCRider]

what Cal said.

Be aware that a low throttle lean condition canot be remedied with jets, in the usual sense. Not main jets anyway. They are for 1/2 throttle plus. The off idle is the slow speed jet and/or the needle position.

You'll be learning this soon, I expect. Moving from velo stacks to pods will richen it up a tad. You may never get it right with Pods v the stock airbox. But you can get it better.

[youssa]

Appreciate the quick response and information, calj! The sites should help out alot in terms of figuring how to order.

I thought the throttle issue was also because of the lean environment. I'll try to explain what happens when I ride, as soon as I get going the bike tends bog down (almost as if it were to die) and/or lurches. Once past that part it rides like "normal" but if I were to attempt to go WOT the bike just gets louder without actually going any quicker. I didn't think it was a clutch issue, mainly because I tested it by cruising then holding in the clutch while maintaining the same throttle input to see if it would catch up, so to speak.

I'll be pulling the carbs today to definitively see what's what inside. As for the jetting, what would be best sizing?

[calj737]

I didn't think it was a clutch issue, mainly because I tested it by cruising then holding in the clutch while maintaining the same throttle input to see if it would catch up, so to speak.

Actually, thats not the way to test it. If you get the "engine acceleration without speed" situation, downshift promptly and see if it happens again. Or, does it only happen at a certain throttle position, regardless of gear, engine load, or land speed? Variables in when it happens help to identify the exact cause. It could also still be a carb issue, too lean on fuel for the throttle position.

I'll be pulling the carbs today to definitively see what's what inside. As for the jetting, what would be best sizing?

Can you identify which carb models you have? Theres a number stamped on the flange (side of carb body). It should be: 657A, 657B, 086A... That helps verify what your jets and settings should be.

Most of these models use a 40 Slow jet, a 105 or 110 Main jet, screw out 1 turn, and a needle clip in slot 4. This will get you close. Take lots of pictures and post them up. Learned eyes will pipe in with great answers (not me, but theres others on here).

[youssa]

I'll get the carb models on here as soon as I get home along with as many pictures.

It only happens with throttle position, if I were to downshift it would go back to "normal" but as soon as pull more throttle it goes back to screaming with no pull.

Sorry if this is not making sense, I'm trying to explain it as best as I can.

[evanphi]

I'll get the carb models on here as soon as I get home along with as many pictures.

It only happens with throttle position, if I were to downshift it would go back to "normal" but as soon as pull more throttle it goes back to screaming with no pull.

Sorry if this is not making sense, I'm trying to explain it as best as I can.

That sounds like a slipping clutch to me.

[calj737]

It only happens with throttle position, if I were to downshift it would go back to "normal" but as soon as pull more throttle it goes back to screaming with no pull.

Sorry if this is not making sense, I'm trying to explain it as best as I can.

As Evan said, perfectly explained and preliminary diagnosis is a slipping clutch. Good news: you've only got a lean issue in your carbs. Bad news: you've also got a little work to do on the clutch  But that could be due to a bike having sat a while, needing a proper oil, a little adjustment of the cable (really simple fixes).

[MCRider]

Appreciate the quick response and information, calj! The sites should help out alot in terms of figuring how to order.

I thought the throttle issue was also because of the lean environment. I'll try to explain what happens when I ride, as soon as I get going the bike tends bog down (almost as if it were to die) and/or lurches. Once past that part it rides like "normal" but if I were to attempt to go WOT the bike just gets louder without actually going any quicker. I didn't think it was a clutch issue, mainly because I tested it by cruising then holding in the clutch while maintaining the same throttle input to see if it would catch up, so to speak.

I'll be pulling the carbs today to definitively see what's what inside. As for the jetting, what would be best sizing?

You say the engine gets louder. Does it actually turn faster per the tachometer?  IF not, (actually gets louder) what you are hearing is likely intake noise. Slides go up, engine sucks harder but tuning doesn't allow more power, just louder sucking noise.

If its actually turning faster per the tachometer, then its the clutch.