Topic: 76 CB750F Running Lean or other issue?

[operasinger750]

Hey all.  I'm a newb working on my first bike.  Last fall I bought a 76 F with pods and RC exhaust, Dyna S ignition, and I'm running NGK D8EA plugs.  I perceive a lean condition based on what happens mid-throttle in 4th or 5th at highway speeds.  I twist the throttle to say 50% and the bikes gets louder (sounds bogged down) but doesn't increase in RPM until a second or so later, and then it's a slow acceleration.  the same happens on WOT, but less so if I twist slowly.  I think it runs hot, but I can't give you a temp reading.  I've done all the usual stuff: changed oil, set valve tappet clearances, adjusted the timing on the Dyna plate with a strobe, worked through all my electrical issues (holding a nice charge now), and rebuilt the brake masters and calipers.  It runs well at startup, but this condition seems to get worse as the bike warms up (maybe overheating), and I would say that it ran better when the weather was in the 40's and 50's too, which indicates it's running rich, but I'm not sure. 

Last fall, I dropped the bike off at a shop to have them clean the carbs and sync them, just to get to ol' girl running (it wasn't running when I bought it for a song).  I know that you're supposed to do that last, but I wanted to know that the bike would run before I sank more money into it.  Then I did all the maintenance I mentioned, so I'm kind of working backwards.  The shop had bench synced the carbs and set the bleed screws to 1 2/3 turn out.  I got higher RPM with them turned to about 2/3 out, so I have them set there.  It starts easily and idles well at 1600. 

These plug shots are with plugs that are at about 800 miles (not a WOT plug chop, just current condition).  I was doing maintenance over this 800 miles, so I'm not sure if this reading really means much.  I just adjusted the timing about 200 miles ago and that was the last thing I did.  I have this sneaking suspicion that the PO put the pods on without rejetting.  I do have the airbox, so I could try that.  Let me know what you think about the setup or anything I've done and/or about the plug pics.  Thanks!

[operasinger750]

Closer shots if that helps.

[operasinger750]

I should also mention that I'm getting lots of tiny pops on decel, even at 2/3 turn out!  This and the other info is what has me thinking that it's lean.  Any ideas?

[c(b)hris]

What size main jets / needle positions are you using?

  I do have the airbox, so I could try that. 

might be the simplest quick fix?

[Trav-i]

It does sound like you have a lean condition.  Pods can be pretty tricky to tune, I would put the stock airbox back on.  You will notice a major improvement in the way it's runs.  Stock settings I think are air screws 1 to 1 1/8 turns out needle clip is 2nd from the bottom, 110 main jets, and 35 slow jets.  I tried pods once, and ended up putting the airbox back on and am so much happier because of it.

[Retro Rocket]

It does sound like you have a lean condition.  Pods can be pretty tricky to tune, I would put the stock airbox back on.  You will notice a major improvement in the way it's runs.  Stock settings I think are air screws 1 to 1 1/8 turns out needle clip is 2nd from the bottom, 110 main jets, and 35 slow jets.  I tried pods once, and ended up putting the airbox back on and am so much happier because of it.

+1   Airbox will fix the popping and even out your "uneven" running condition. These bikes were engineered for an air box with the correct velocity stacks and after saying this a million times on here even the die hard pod guys are starting to get it, that is , the stock airbox will give you the best performance overall, pods are a compromise. Most guys that persevere with pods only do it because they prefer the look, i prefer reliable performance and not fiddling with the bloody carbs.....

[david 750f]

If you think the bike is running lean try a little choke and see if it makes it worse or better.

+1 on putting the stock airbox on to get it tuned properly.

What is the current needle clip position, main and slow jet size?

FYI, stock carb parts should be 105 main, 40 slow jet and the centre clip position on the needle.

The addition of the RC header may need larger mains and/or moving the needle clip.

[Trav-i]

If you think the bike is running lean try a little choke and see if it makes it worse or better.

+1 on putting the stock airbox on to get it tuned properly.

What is the current needle clip position, main and slow jet size?

FYI, stock carb parts should be 105 main, 40 slow jet and the centre clip position on the needle.

The addition of the RC header may need larger mains and/or moving the needle clip.

Thanks for the correction David, I was trying to remember those settings without looking them up.  Guess I shoulda just looked 'em up.

Travis

[operasinger750]

Thanks all and thanks David750F for the choke idea.  I ran a little bit around town today out in the country (I'm in Iowa for the summer) and got to test response with the choke.  It definitely runs better with some choke.  I would say the choke lever was at about 45 degrees when I was getting the best response. 

As far as carb internals, I don't have a clue.  I'm really hesitant to rip into them since I'm not familiar with that-I'm gun-shy of carbs.  I think my plan of attack is going to be to try the stock box and hope that the PO rejetted after he added the pipes, coils, and ignitions.  That would theoretically improve the overall performance.  Hopefully that's the case-that it's just the pods-which I'm not a fan of...  I notice that at highway speeds with a strong side wind, the bike is fussy and doesn't want to maintain speed as the wind gusts!  I much prefer the idea of having the stock intake...  Now I gotta read those posts about putting the thing back on.  I did try to do that earlier, but those boots aren't as flexible as I would like. 

Thanks for the input.  I'll update after I get the stock box on.  The box is one I bought off Ebay that has a paper filter.  I'll try that first.  Should one have a cloth filter with my exhaust?

[Spanner 1]

There is no cloth filter.... the stock or aftermarket filters are paper ( O.K.... foam type pod ones excepted)...... Cuore Ingrata; very challenging , right? ( Saw Carreras doing it live, wow ! ).

[david 750f]

Check to make sure that you have no air leaks around the intake rubbers  and air filters.

When you checked the ignition timing, did you also check if the advance mechanism was advancing properly?

[operasinger750]

The advancer works just fine.  I cut one ring off the springs as per Hondaman and that made it idle much better since the advancer was jumping around a bit even when idling at 1000.  I believe it reaches full advance around 2400 or so. 

Spanner-OK.  I thought K & N made a stock replacement cloth filter to fit in the airbox.  Thanks!

More as soon as I get the airbox on.

[operasinger750]

An update on the situation. I took off the pods and wrangled the stock air box into place. I also re-attached a vacuum hose from the breather to the stock box-no more blowby on the back of my pants. With no other mods, the bike certainly runs better and has smoother response. I was pleased. I went to a shop in Des Moines called Vintage Cycle DSM and i had them ride it to see if they thought it needed more tuning. They were chopping two CB's when i walked in, so i felt confident they might be able to help.  The proprietor came back with a big smike on his face and said "I wouldnt do anything!". That was great to hear and definitely a satisfying feeling. Thanks for the input, y'all!

[operasinger750]

An update on the situation. I took off the pods and wrangled the stock air box into place. I also re-attached a vacuum hose from the breather to the stock box-no more blowby on the back of my pants. With no other mods, the bike certainly runs better and has smoother response. I was pleased. I went to a shop in Des Moines called Vintage Cycle DSM and i had them ride it to see if they thought it needed more tuning. They were chopping two CB's when i walked in, so i felt confident they might be able to help.  The proprietor came back with a big smike on his face and said "I wouldnt do anything!". That was great to hear and definitely a satisfying feeling. Thanks for the input, y'all!

[Jerry Rxman Griffin aka MuthaF'er]

Do a plug chop at WOT and see what you get. The exhaust might afford you 110's at the least (provided you have the stock 105's in). The main jets are a piece of cake to replace and can be had from your Honda dealer. Shut off the petcock/fuel, drain the gas through the drain screw on the bottom of the bowl, pop the "spring/holder" off, wiggle the bowl off, take the long tool with the U shaped head from your tool kit and unscrew the main jet holder with the jet in place, check the jet size that's marked on it's face, remove it with a flat blade screw driver, then reverse what you just did with the next larger size jet. See how it feels and then take another plug reading. Decide from the before and after.

Mains are relatively easy, it's the needles that suck.

[operasinger750]

Will do Jerry.  I have a new air filter on the way.  The box I put on was an Ebay find that had an old filter-not filthy, but it needs to go.  Once I get that in, I will do the WOT chop.  Thanks for the advice, especially with the U shaped head! 

[operasinger750]

Today I dropped a new air filter into the bike and it actually ran worse (lean) than with the old cruddy filter that came with the box when I bought it off of ebay.  I suppose I just need to turn the air screw in 1/4 turn or so-the old filter was probably more gunked up than I thought.  I pulled the main jets today-120's.  Should I go to a 115 or 110 and then do a WOT plug chop?  I'm running an RC pipe and electonic ignition.

[operasinger750]

Today I went for a longer ride to see what how the bike responded.  It was best with the air screws at 1/2 turn out.  The bike ran a little hot and was smoking a bit by the end of the ride and certainly demonstrated lean conditions in general.  I feel like I'm running round in circles a bit.  First the bike runs predictably lean with pods, I go back to the stock box with a plugged up old filter and it runs great.  I also discover that I'm running 120's, but when I put a new air filter in, I go back to running lean!  So, I guess the PO did at least some rejetting (120 instead of 105 mains), but going back to stock air box still creates too much air in the mixture?  What am I missing?  What would you do?

[Retro Rocket]

The blocked filter would restrict air so it makes sense that a new less restrictive filter is drawing more air. I think there is something wrong with your carb settings. What position are the needles in.? I would start with all standard settings and go from there....

[operasinger750]

That follows my reasoning as well.  I'm using an Emgo filter, rather than a standard Honda.  I wonder if there's a difference in terms of mixture.

You're also seconding the idea that I'm probably going to have to adjust the needles at some point in this process.

[Retro Rocket]

I also know that emgo are relatively cheap filters and their pods are crap. K&N make a good free flowing filter for the 750....