Topic: Yet another Carb Tuning Question

[Harper]

Hi everybody,

I'm trying to tune rebuilt carburetors, but don't have a vacuum gauge, so it's going slowly. I have a 1975 CB 750K, all stock except for a 4 into 1 exhaust. My current issue is that the engine lugs from about 1/4 to 1/2 throttle (I don't know if there is an official term for my engine's misbehavior. When I get to this throttle position it loses power, drops in rpm and tone, and feels like it would die if I don't release the throttle back under 1/4 or give it over 1/2). Additionally, if I snap the throttle open from idle, the engine misses and dies. The bike performs better (the area of stumbling is smaller) if I slowly apply throttle, and after completely warming up. Finally, unless completely cold, the bike needs almost no choke to start, and in fact, will sometimes die if I try to start it with the choke.

I read the carb FAQ and tried some of the diagnostics. I tried removing and covering the airbox and comparing performance to see if it was running rich or lean, and the engine seemed to perform better with restricted airflow. Because the problem area was in the middle of the throttle range I raised the needles one notch. This has not solved the problem; performance seems unchanged. I've done further testing, but there is no noticeable difference between rich and lean set ups.

I used very thin wire to bench synch the carburetors when I was reassembling them. Could my problem be that the slides are too low? I read that having too little cutaway open on the airbox side can cause too much vacuum, and result in the kind of "snapped-open-throttle death" that I'm experiencing. This might also explain the rich starting mixture (why I don't need the choke)?


Thanks,


Harper

[SOHC Digger]

Did you rebuild the carbs yourself or have them rebuilt? Was the bike running poorly before the carbs?

[Harper]

I rebuilt them myself. The bike was running fairly well before I rebuilt. A friend who also rides CB750s said it felt underpowered, and the plugs were black and sooty.

further description of the problem: while there definitely is an underpowered band of throttle (from a little under 1/4 to around 1/2), the bike dislikes sudden application of throttle more than that particular area. That is, if I creep my hand back, I can have fairly constant acceleration throughout the entire spectrum of throttle. But, if I rush it too much (especially if RPMs are low), I get hesitation, and if I ever whack open the throttle, the engine almost dies.

Thanks

Harper

[Harper]

Hi everybody,

Does anyone know what might cause the engine to die whenever I snap on a lot of throttle?


thanks,

Harper

[KB02]

Sounds like it is running too rich. That would explain the dying out with a slap of the throttle - basically choking it out. '76 and earlier bikes don't have the accelerator pumps, so slower twist is needed instead of the quick twist.

Do a search for a home built Manometer. Syncing the carbs will be a big help in getting them tuned.

(And remember to put gas in the tank... )

[Tretnine]

How's the inside of your gas tank? Rust will cause your floats to stick and open a world of problems.

Your cheapest/easiest bet would be to keep trying different clip positions. If you're clever, you can do this while the carburetors are still on the bike, though it's still tedious. My opinion is different as the bike definitely sounds lean to me. After that you can adjust a little bit with different spark plugs, but if the clip settings don't do it, you'll need to rejet. (and check for problems/plugged jets, which could easily be your answer...)

It's all explained better than I can do in the FAQ http://forums.sohc4.net/index.php?topic=5410.0

Also, you should invest in the tools. They'll make your life wonderful to live. The morgan carbtune comes very strongly recommended, by myself and many others here.

[bender01]

If your plugs are sooty its likely the emulsion tubes didnt get clean enough when you cleaned the carbs.Did you get them out of the carb bodies? Take them apart again clean and bench sync them with a wd-40 straw as a guage. I would put the needles back to stock setting.  If the carbs are really clean and problem is back then move the needles. What size jets are in your bike now?

[the architect]

Harper --  I'm having almost the exact same problem with my '75, following my full rebuild.  At this point, I've tried every single clip position.  It seems to prefer full lean or just one position above full lean (these two positions behave almost the same).  It didn't care for the stock middle groove, and the richer I went, the poorer it performed.  I'm at a complete loss for what to do next, so I'm hoping someone will give you (and me  ) some brilliant advice. 

I'm running individual pod filters (installed by a P.O.) and the OEM 4-4 exhaust with the mufflers removed (yeah, it's loud  ) with stock carb jetting from the Partnmore rebuild kits I used (Keyster branded).

Somebody help Harper (and me  )!!

[Harper]

The architect: That's where I have my clips as well, but I think that, like you, this setting is less bad rather than better. It seems to me that there is an issue beyond needle position.

Bender01: my plugs were sooty before the rebuild. Now they have a little soot around the outside lip (not sure what it's called), but the insulator tip (right by the electrode) is tan like it should be. Yes I took out the emulsion tubes, and I thought I cleaned them well. I'm really not happy with the rebuild kit I got, so I think that I will do as you suggest: take them apart, clean again, return to stock settings and stock parts, and bench synch to a spray tube. My only deviation from stock is the main jet. Right now I have 130 mains and 40 pilot. 120 is stock, but I think that the PO rejetted to accommodate the 4-1 exhaust.

Tretnine: you've struck fear into me with your talk of rust. I id notice a little discoloration in the corners of the tank when I put it back on, but I didn't fully investigate. I did clean and reinstall the filter on the petcock). And I guess I'm not clever enough.. would I loosen the carbs from the rack a little, just enough the get the top of the slide out from the throttle control? And then unscrew the slides right there? And I really think I ought to just go for the tools. There's just a ride I really want to go on this Sunday, so I'm trying to get it safely rideable by then.

KB02: that's a good point about the accelerator pump. I always noticed that there wasn't crazy throttle response. But this is much much worse than before. I've seen the homemade plans, and I might do it. I might just spring for the real deal however, because Im not sure how exactly I can execute them.


Thanks everybody! I'll go do some work, and post later about what happens


Harper

[Harper]

I just had a thought. Yesterday I took off one of the float bowls to show my sister what the pilot jet looked like. The gasket that lies on the top of the float bowl was swollen, and popped out as soon as I released the band that holds the  float bowl on to the underside of the carburetor. After a few minutes it had shrunk back down to size. Does this mean that it was swollen with gas? And if so, does that mean that there is too much gas in the float bowls? I thought that the appropriate fuel level was below the seam between the float bowl and the rest of the carburetor. I tried the plastic tubing test, but had difficulty getting a clear reading. I'll try again


Harper

[Tretnine]

I've never done clip position on a 750 in frame, but I just did it on my 550. If the layout is the same you should be ok, but you might run into problems with positioning.  Pull off your tank, first. (Obviously.) then you take the cap off the top of the carb, pretty easy for 1&4, but trickier for 2&3. Then you unscrew the Really, there are only three screws (On my 550 anyway) before you have access to the clip and the needle, you disconnect the arm from the rod that connects all the carburetors together and you also unscrew the connecting arm from the throttle valve. (The grey thing in the throat) the Arm should swing out once you've got it all disconnected and then you can just stick your finger in there and pull out the throttle valve. no big deal. Even if you lose screws along the way they should be contained on the top of the throttle valve, just don't drop anything in there after you have all your parts out or you'll be disconnecting the whole rack. Bada bing reset your clip.

The tricky part is going back together. The biggest thing is that you place your tiny little screws back into the arm and push it slowly into the opening in the top of the carb - evenly-  until you the screws barely touch the threaded holes. It works like a little crane for you. you don't want to push the arm down too far or they'll pop back out of the hole they seat into and then you have to start over again. You start one screw about 2 rotations, then get the other one seated and you're all set. It takes a pretty skinny screwdriver, so get a good one and don't strip anything or you'll regret it. It sounds really tough, but you've only got 3 screws. I managed to do all 4 in the engine and have everything put back together in less than a half hour. You also only need like 2 tools. Simple pimple.

[Harper]

haha, simple pimple, nice
This sounds like what I envisioned. I gave it a try, but I got worried about bending slides, so I just took the whole thing off again. Thats a really good description of the process of reattaching that weird little piece with the two screws that sits on top of the needle. It's always a pain.

So today I took them all off again, shot them with carb cleaner, replaced the rebuild kit needles with my old needles (just because the rebuild kit was starting to piss me off. I left the clips one below center, which was their setting from the PO), and tried to more accurately bench synch it. I think the only thing that made a difference was the increased accuracy of bench synching. But that really made a difference. I still have some problems with idle smoothness and bottom end response, but overall it made a huge difference. Which also convinced me that I need to finally just go ahead and get a manometer, to really get this right.

The guy at the NAPA store put the fear into me today by telling me that if I continue to tool around and test ride without properly synching the carburetors, I run the risk of doing serious damage to the engine. If one cyinder is running lean, might melt something important, etc etc.  I really want to go on a longer group ride this Sunday (50+ miles). The motorcycle is running a lot better, but I won't be able to check with a vacuum gauge before then. Am I flirting with catastrophe, or was his an overstated warning?

The Architect: I think the answer to our problems just lies in synching and tuning. Just a little adjustment today made a huge difference. have you officially synched them yet?


Thanks


Harper

[bender01]

 Youll be fine with a bench sync for that ride! It probably needs to be ridden a little anyway. Was there any crap in the bowls from your tank? Flecks at all?  It does sound like the floats might be set to high if the gasket was wet. If you leave the petcock on  does the fuel come out the overflow when its not running after a few minutes? Just some thoughts.

[Harper]

bender: thanks, that's the response I wanted! Couldn't find any junk in the bowls. I drained each bowl into a mason jar to look for stuff, but couldn't see anything. I also checked straight out of the tank, and that looked clear as well.
About the floats: I have noticed an occasional wetness in the overflow tubes. I'll check to see for certain tomorrow. The float height was at the correct height when I reassembled them, and I didnt change anything on the floats during the rebuild process. If there's a problem, maybe it's related to new spring needles (or whatever that little part with the spring that regulates fuel flow to the bowls is called) that I put in. Would high fuel level manifest in bike performance? How?


thanks,


Harper

[bender01]

Well if theres too much fuel flooding the bowls, jets I think would be getting too much fuel and create a  numb feeling.Possibly not burning all the fuel during ignition? Thats my best guess.