Topic: 78 CB750F Performance Issue

[FJ40toy]

Hey folks, been around for a little bit, but haven't posted much. I have done some searching for similar issues and haven't had much luck finding anything. I am having a performance issue with my rebuilt CB750F. Before I get to the issue, I think I'll list what has been done to the engine so you can have an idea of what I'm working with.

-Bored to an 836 with Wiseco pistons with 10.25 CR
-head rebuild by MRieck w/ stage 1 port job
-CX-2 cam from CycleX, degreed to the cam specs
-29mm Keihin CR carbs, with new carb boots and pod filters
-Dyna-S electronic ignition with new coils
-Ripple Rocker Hindle 4-2-1 exhaust

My issue is that it stumbles from approximately 3300 to 5500 rpms if I roll the throttle on quickly. If I go to WOT it acts like I hit the kill switch until I back-off to ½ to ¾ throttle. I think it is leaning out. Above 7000 rpm to red line it pulls well. Another sign I think it is lean is that the engine runs better as it warms up more. If I roll on slowly it progress through the rpms but doesn't pull hard. It feels flat. I have checked the plugs and there are white. Another reason I think it is leaning out. The CR Keihin’s came with the follow jets and settings:

Main Jet: 120
Slow Jet: 60
Air Jet: 240
Needle: YY8, clip position 4 (7 total positions)
Air Screw: 1 turn

I have taken the carbs off and removed the spigots, checked the o-rings, and installed and tightened them with a strap wrench as they were only hand tight before and I think it was leaking. A test ride was better, but still not right. I have made single changes along the way and have done test rides after each change. Lots of test rides. The plugs are always white/ clean looking, which tells me it is lean, at least during cruise which is around 3000 to 3500. I have installed richer needles and smaller air jets to try and richen up the mid-range area, which helped but the bog/ stumble is still there. Here is what I have for carb settings currently:

Main Jet: 120
Slow Jet: 55
Air Jet: 200
Needle: YY6, clip position 5 from top
Air Screw: ½ turn
Float level: 14mm (stock setting for this carb)

I am thinking of ordering larger main jets, but pretty sure it won't help the mid-range. I have the next, and last, richer needle that is available, YY5. Keihin only has one emulsion tub for these carbs, so can't change those. Pretty Sure that is what the Air Jet is for. I haven't sprayed the carbs to check for a vacuum leak yet, should probably do that next to rule out. I may try putting the 60 slow jet back in to see if that would help. Any suggestions on what else to check? Thanks for the help and sorry for the long post.

[scottly]

You might want to move this here:
http://forums.sohc4.net/index.php/board,113.0.html

[FJ40toy]

Will do, thanks.

Sent from my moto z3 using Tapatalk

[Maraakate]

The pods probably aren't helping and there's always going to be a compromise with pods.  With that said, have you simply tried running with the choke on or putting the choke on or partially on when you test the ranges that have bad performance?  If it gets better when you do this then it is too lean.

Doing plug chops have never seem to be too reliable for me.

FYI: The air screw is actually the pilot adjustment.  This only affects the idle range.  If the bike is idling OK you shouldn't have to play with it.

[FJ40toy]

I was thinking of installing the stock airbox to see if that changes how it runs. May do that this weekend.

On the choke, yes I have turned it on while having the stumbling issue. Thanks for the reminder on that Maraakate. It did flattening the stumbling, so I would say it is lean. I only have one needle size left to richen the problem area. I can try going down a few sizes on the air jet again to see if that helps. I'll try the stock airbox first and go from there. Thanks for the input.

Sent from my moto z3 using Tapatalk

[Maraakate]

Yes, unless this is a bike strictly for the track trying to get the jetting right for the pods across a ride range of RPMs and elevation can be more trouble than it is worth.  Put that airbox back on with the stock filter and see what you end up with.

You've probably seen it before, but attached is a picture of the throttle ranges the jets control.  Also, this site may help to explain some of it more in detail: http://www.dansmc.com/carbs2.htm

Keep us posted of the progress!

[Maraakate]

Forgot to mention that conversely, if you suspect things are too rich you can shut the petcock off and if the bike gains much power right before it runs out of gas then you will know it is too rich.

[FJ40toy]

Forgot to mention that conversely, if you suspect things are too rich you can shut the petcock off and if the bike gains much power right before it runs out of gas then you will know it is too rich.

I'll keep that in mind when for my next test ride. Thanks for the help!

This bike is for the street, so no racing for it. It needed a rebuild so why not add a little extra zoom to it!

Sent from my moto z3 using Tapatalk

[FJ40toy]

Well, put on the stock air filter box and it didn't seem to help. I am thinking I may have an ignition issue since it now sounds like it is breaking up at cruise.
Going to put in new plugs first and see how that goes. I have a spare set of new coils I can swap in after the plugs. I did have some loose connections for the coils before, but have fixed those. Will double check though, just to make sure.

Sent from my moto z3 using Tapatalk

[Maraakate]

I have never used the dyna ignition.  Have you verified timing with a timing light (if possible assuming dyna lines up to the same marks)?  If you did have loose connections it's possible you damaged the cdi unit.  If you have the points you can put that back on and verify.

Plugs are probably OK if you said they were reading a bit lean.  As long as it's not burned.

[scottly]

The pods probably aren't helping and there's always going to be a compromise with pods.  With that said, have you simply tried running with the choke on or putting the choke on or partially on when you test the ranges that have bad performance?  If it gets better when you do this then it is too lean.

Doing plug chops have never seem to be too reliable for me.

FYI: The air screw is actually the pilot adjustment.  This only affects the idle range.  If the bike is idling OK you shouldn't have to play with it.

The Keihin CR carbs are not stock carbs, but are racing carbs, and were not used with a stock air-box.

[Maraakate]

The pods probably aren't helping and there's always going to be a compromise with pods.  With that said, have you simply tried running with the choke on or putting the choke on or partially on when you test the ranges that have bad performance?  If it gets better when you do this then it is too lean.

Doing plug chops have never seem to be too reliable for me.

FYI: The air screw is actually the pilot adjustment.  This only affects the idle range.  If the bike is idling OK you shouldn't have to play with it.

OK, never used them.
The Keihin CR carbs are not stock carbs, but are racing carbs, and were not used with a stock air-box.

[turboguzzi]

like scottly said here, CR are easy to tune, regardless of pods or not.

you already raised the needles a bit and plugs still white, so sounds like bigger mains needed, it's a 5 mins job to change in CRs, just unscrew the drain plug. I would go for 130. if it starts being rich, then 125.

[Old Scrambler]

+1 with TG............Race Carbs = Race Tune..........130 jets and lower the needles.

[scottly]

The main jet has the most influence, hence the name. Also, do not confuse RPM for the amount of throttle opening: set the main and air jets for WFO first, then adjust the needle for part throttle. Put a piece of masking tape on the throttle, and make reference marks for 1/4, 1/2, 3/4, and full. All of your symptoms so far indicate a lean main jet.

[FJ40toy]

Thanks for the input guys. I have ordered 130 main jets and few other sizes to boot. I'll give an update after I install them and go for a test ride.

Sent from my moto z3 using Tapatalk

[FJ40toy]

I installed the 130 main jets and is better. Have a set of 135 mains I'll install and take for another test ride.


Sent from my moto z3 using Tapatalk

[FJ40toy]

The main jet has the most influence, hence the name. Also, do not confuse RPM for the amount of throttle opening: set the main and air jets for WFO first, then adjust the needle for part throttle. Put a piece of masking tape on the throttle, and make reference marks for 1/4, 1/2, 3/4, and full. All of your symptoms so far indicate a lean main jet.

Thanks for the tips. What is WFO?Not familiar with that acronym. I know WOT, just never heard of WFO. Thanks again for the help.

Sent from my moto z3 using Tapatalk

[Jerry Rxman Griffin aka MuthaF'er]

Wide Fucking Open. Of course if you don't have your account set to show "bad words" you won't #$%*ing see this 

[FJ40toy]

Wide Fucking Open. Of course if you don't have your account set to show "bad words" you won't #$%*ing see this 

got it! Learn something new!

Sent from my moto z3 using Tapatalk

[scottly]

Wide Fucking Open. Of course if you don't have your account set to show "bad words" you won't #$%*ing see this 

Hmm, it appears that if you underline the first letter, the "bad words" filter doesn't see it??

[FJ40toy]

So I have made some progress on this.

 Found out my advance unit was advancing too soon so I ordered one from 4into1.com. The unit looks good, but the F timing mark did not line up with the mark on the case. I aligned the old advance unit before removing it, that is how I noticed it was off. So I swapped the new weights and springs. Reinstalled the original unit with the new weights and springs, checked the timing and full advance is coming on around 2400-2500 rpms. Was going full advance at 1900-2000 rpms before. I don't think that was the root cause of my issue, but needed to be addressed.

Decided to set the carbs back to the out of box settings; 120 main jet, 60 main jet, 250 air jet, YY8 needle, air screw at 1 turn out.
It came originally with 55 slow jets. Did a test ride and seemed to be leaning out about 1/8 to 1/4 throttle. The needle was set to the middle position, position 4. Raised the needle by moving the clip down to position 6, one up from the bottom. It runs a lot better.

Will work to dial it in a little better, but at least it is rideable now.

Thanks for the help in trying to isolate the issue. In my research I read I need to set the slow speed circuit first, then move to the mid-range, then finally address the main circuit. This is contradicts what I used to do, which was dial in the main circuit first. It makes sense to start with the slow speed circuit first as the circuits are cumulative.

Sent from my moto z3 using Tapatalk