Topic: 71 cb750 running rich no matter what I do

[kangbang]

Don't use the choke as any kind of a 'guide' on the old round top carbs on the 750. Yes it will die when you close them.That would be normal. It is a very primitive affair with no fast idle cam built in etc like more modern chokes. I don't even leave them on when cold,just for initial start up and then feather them on and off a little as needed while I keep the rpm's up a little with the throttle till warmed up.

Ah... I see. Thank you. Also, do you have experience with MAC 4-2-1 exhaust system? From the looks of it, it definitely looks like MAC. Its not one of those straight thru with glass pack exhaust so it has quite of bit of back pressure. I was wondering if that could be causing the richness.

[ekpent]

I have some bikes that have the stock headpipes and aftermarket mufflers, no MACS though. Are the baffles removable on those that you have ? Some are not. Did any chud fly out when you first started it up when getting it. I have bought many bikes where mice, chipmunks and other little beasts had filled up the pipes with nuts, seeds, nest etc..
   If it truly is still running rich then maybe you need to downsize your main jet or fiddle with your needle.
  What are your engine vin numbers again ?

[kangbang]

I have some bikes that have the stock headpipes and aftermarket mufflers, no MACS though. Are the baffles removable on those that you have ? Some are not. Did any chud fly out when you first started it up when getting it. I have bought many bikes where mice, chipmunks and other little beasts had filled up the pipes with nuts, seeds, nest etc..
   If it truly is still running rich then maybe you need to downsize your main jet or fiddle with your needle.
  What are your engine vin numbers again ?

Sort of.. its has a single rivet which I can drill out, but I would rather avoid that if possible. Maybe i will just do a test ride with the muffler section off...?

CB750E-1101490 is my engine vin.

[harisuluv]

Did you ever figure out if you have aftermarket needles or OEM ones??  This isn't something that you want to be "up in the air."

Spark plugs are going to stay fouled.  You're going to need to replace them when they get fouled.

[kangbang]

Did you ever figure out if you have aftermarket needles or OEM ones??  This isn't something that you want to be "up in the air."

Spark plugs are going to stay fouled.  You're going to need to replace them when they get fouled.

Yes the needles are Keihins. After getting the Jets from you. All the brass parts are OEM. Wouldnt cleaning them with wire brush and sandpaper then regapping make it unfouled?

[harisuluv]

you may get some limited use after cleaning them, but you really need new plugs. 

[kangbang]

you may get some limited use after cleaning them, but you really need new plugs.

Okay. I will get some new plugs and report back

[Don R]

I had similar issues. My jets had been drilled by a PO we replaced them.  I went to the clear tube method of setting  the float levels, the springs in the float valves were so soft you couldn't really tell when they touched. I replaced them and  adjusted the float level with the clear tube. Then I  went up from 105 (stock ) to 110 jets in my F1. It goes a lot further on a tank and even needs the choke to start now. Runs soooo good.

[ekpent]

Yes please throw the old plugs in the garbage when they go bad. Cleaning them sometimes just makes them pretty on the outside but its what's up inside that counts. Try some NGK'S for kicks, I get them cheap at Advance Auto around here and start your new tuning with some good fresh soldiers in the barrels   

[kangbang]

Yes please throw the old plugs in the garbage when they go bad. Cleaning them sometimes just makes them pretty on the outside but its what's up inside that counts. Try some NGK'S for kicks, I get them cheap at Advance Auto around here and start your new tuning with some good fresh soldiers in the barrels   

Yea I got some new sparks last night and I got the one suggested by the gentleman over the counter... I was suspicious but I was in a rush so I just bought it and left. Once I got free time, I measured the resistance and of course he gave me a 5k ohm resistor plug. So Im gonna go back today and change them for D8EA.

I also got around taking the baffles out of my MAC muffler. It was a little quieter than open header but still not much better. I have attached a pic the baffle that was inside. It seem very restricting but who knows?

Since I took the baffle out, the bike idles much better. I have only ridden it once since then, but it just idled very well and it didn't seem like it was going to die. Also the idle did not hang anymore. Maybe that was the gremlin in my bike. Hoping it will running even better after getting new fresh sparks.

[bjbuchanan]

Those mac's are restrictive. You still should have been able to tune the restriction tho, usually only like a half turn leaner on the airscrew tends to clean it up for most guys

Guys on here, myself included have modded those baffles to reduce that horrible choke point

[kangbang]

Those mac's are restrictive. You still should have been able to tune the restriction tho, usually only like a half turn leaner on the airscrew tends to clean it up for most guys

Guys on here, myself included have modded those baffles to reduce that horrible choke point

Hmm. maybe I have other problems that make the bike rich in general. Do you have some pictures of the modded baffles?

[bjbuchanan]

Top left search bar, all types of good stuff can be found.

http://forums.sohc4.net/index.php?topic=106651.0

[kangbang]

So I finally got the bike started again today. I have been busy working on the exhaust, fuel tank, fuel system, and headlight mount.

Some things I have done in the last couple weeks:
-I did a glass-pack design inside the MAC muffler, and it actually sounds really good. Got rid of the obnoxious "open header" sound.
-Re-did the spark system, replaced the spark plug wires with new copper core 7mm, NEW spark plugs, checked resistances in the coils.
-Cleaned out the fuel tank (it was already clean), rebuilt the petcock. Also, sandblasted the tank, dented it for fork clearance and repainted. body work was a PITA...
-Machined out a new headlight mount. Doesn't relate to the rich problem, but it looks so much better than previous attempt at mounting the headlight 

Just took it out for a ride.... still the same. I have attached a picture of the sparkplug, right after the ride. (this was a brand new spark plug prior to the ride). Upper-end felt good, but it kinda slums alittle when I go from stop. I also have the idle screw out 2 turns (on 71 model, it adjusts air).

any more suggestions??

[harisuluv]

looks like you've done some fine fab work.  I looked back in the thread, i didn't see any mention of syncing at all?

Looks like you've addressed most of the fuel system except for the main component of fuel metering:  the carburetors.

I see you cleaned them via ultrasonic.  Ultrasonic can be ok with some specialized solutions.  Without them, they won't do anything to varnish, and will remove plating before they remove lacquer.  You can use compressed air to confirm passages, but all that does is confirm that air flows through them.  It does not confirm that they are partially blocked, or have been cleaned improperly making them too big, etc. 

Also that's just one plug, have you checked all four of them?  This is important, if you have a sync problem you will get another clue from looking at all four plugs.

[kangbang]

looks like you've done some fine fab work.  I looked back in the thread, i didn't see any mention of syncing at all?

Looks like you've addressed most of the fuel system except for the main component of fuel metering:  the carburetors.

I see you cleaned them via ultrasonic.  Ultrasonic can be ok with some specialized solutions.  Without them, they won't do anything to varnish, and will remove plating before they remove lacquer.  You can use compressed air to confirm passages, but all that does is confirm that air flows through them.  It does not confirm that they are partially blocked, or have been cleaned improperly making them too big, etc. 

Also that's just one plug, have you checked all four of them?  This is important, if you have a sync problem you will get another clue from looking at all four plugs.

Thank you . Nice to have access to university machine shop.

I have checked all 4, stupidly did not take a picture of all of them. However, they were all dark, one was slightly lighter but still darker.

I have also tried loading the air passage hole from the intake side to the pilot jet with carb cleaner and blasting with compressed air. Not sure if that did much.

The current setup is only bench synced, currently do not have a syncing tool handy. Once I sort of figured out this problem, I was gonna go to a motorcycle shop and just sync it there. Could this make the whole system run rich?

Would you recommend SeaFoam??

[ekpent]

Synching helps out the idle and low RPM mostly unless they are very far off. I usually run my air screws at one turn out I believe. Next step may be going to a smaller main jet like a #110 if the carbs are know to be spotless.
   Looks like a tough problem there.

[kangbang]

Synching helps out the idle and low RPM mostly unless they are very far off. I usually run my air screws at one turn out I believe. Next step may be going to a smaller main jet like a #110 if the carbs are know to be spotless.
   Looks like a tough problem there.

Seems weird having to go leaner when I have PODs and 4 to 1 :/ maybe it's time I start messing with different jets

[ekpent]

Really thought getting those stock brass bits would help you out. Several years ago I sold a fairly stock 75 750F to a friend. Cleaned the round top carbs well etc but it did not run well with a load on the road. After He bought it he tried various different jetting configurations, worked on the advance etc all to no avail. Finally he called to report it was running very well when he simply changed out the ignition condenser's for new units.  May not be your cause but just reporting his experience.

[kangbang]

Really thought getting those stock brass bits would help you out. Several years ago I sold a fairly stock 75 750F to a friend. Cleaned the round top carbs well etc but it did not run well with a load on the road. After He bought it he tried various different jetting configurations, worked on the advance etc all to no avail. Finally he called to report it was running very well when he simply changed out the ignition condenser's for new units.  May not be your cause but just reporting his experience.

Yea it is running better but not much better. By condenser, are you referring to the coils?

[Kevin D]

My setup:
115 mains
38 slow
1 1/2 turns idle
needles at stock height
foam air filter/stock air box
341 pipes with end caps pulled
bench sync only
26mm float height (IIRC)
Denso plugs

My experience:
When the bike was new it had the stock 120/40/D8EA/300exhaust setup and the plugs would carbon foul regularly on roundtown riding, and clean up with a freeway blast for 10 minutes at slightly illegal speeds. I put the 38's in a long time ago with no noticeable effect. More recently, the 115 mains and Denso plugs and.... no more plug fouling. I might have leaned the needle clip 1 notch, but thats more work. These days I'm mostly a 45-50mph backroad cruiser, with occasional redline blasts through 1st-2nd-3rd. The bike starts good, runs great, idles OK, 45-48mpg.

I'm not an expert, just sharing my experience
and my ride is a:

[kangbang]

That gives me more hope. Maybe I will try going to a smaller main. What do you mean by 341 pipes?

My setup:
115 mains
38 slow
1 1/2 turns idle
needles at stock height
foam air filter/stock air box
341 pipes with end caps pulled
bench sync only
26mm float height (IIRC)
Denso plugs

My experience:
When the bike was new it had the stock 120/40/D8EA/300exhaust setup and the plugs would carbon foul regularly on roundtown riding, and clean up with a freeway blast for 10 minutes at slightly illegal speeds. I put the 38's in a long time ago with no noticeable effect. More recently, the 115 mains and Denso plugs and.... no more plug fouling. I might have leaned the needle clip 1 notch, but thats more work. These days I'm mostly a 45-50mph backroad cruiser, with occasional redline blasts through 1st-2nd-3rd. The bike starts good, runs great, idles OK, 45-48mpg.

I'm not an expert, just sharing my experience
and my ride is a:

[Kevin D]

The 300 pipes were the stock 4 into 4 exhaust on the K0 and K1. 341's were on the K2 and later. The have the same footprint but the 341's are more restricted/quieter, part number 18300-300-030 vs 18300-341-907.
 

[ekpent]

Really thought getting those stock brass bits would help you out. Several years ago I sold a fairly stock 75 750F to a friend. Cleaned the round top carbs well etc but it did not run well with a load on the road. After He bought it he tried various different jetting configurations, worked on the advance etc all to no avail. Finally he called to report it was running very well when he simply changed out the ignition condenser's for new units.  May not be your cause but just reporting his experience.

Yea it is running better but not much better. By condenser, are you referring to the coils?

Went back to your opening statement so you can scratch that suggestion as I see you are running a Dyna and no longer have the stock condensors. The K0 and K1 were jetted up fairly rich when stock.  Kevin offered some pretty good real world insights about his bike.

[Henning]

My setup:
115 mains
38 slow
1 1/2 turns idle
needles at stock height
foam air filter/stock air box
341 pipes with end caps pulled
bench sync only
26mm float height (IIRC)
Denso plugs
...

Kevin, thanks for posting that, it's really close to my setup, which is:
115 main, 40 idle, 1 3/8 air screw, needle clip middle position, stock airbox, 3 341's and a 300, 26mm float, Dyna ignition and 3 Ohm coils.

It's still a tad rich/running too cold; I know because I get cream in the tank if the weather is cold. Plus sometimes a slight stumble at very small throttle openings. So I can turning turning the air screw out a bit plus maybe the smaller idle jets.