Topic: 1976 CB750F - who wants to help me with my carb tuning?

[riffman12]

if still light, richen by 1/4 turn or so, and test again.
yes, the 1/4 throttle position chop should be under load, out on the road.  anything flat or slightly uphill, in any gear.

Thanks again for all the help, fly

I know that if I richen the screws by a 1/4 turn in, I will have more hesitation off idle... so even if I get good color on the plug chop, what would I do next? go back to the factory pilot sizes and hope that all my issues from the beginning of the thread were due to floats too low?

I guess first I'll leave the 42s in and see if I can get good color at idle. Then go from there.

[flybox1]

I have this short 6-sided screwdriver bit i use for IMS adjustments.  fits perfectly and I can easily see 1/6th turns.
IIWM, id go richer incrementally by 1/6th turns, until I reached that hesitation you are weeding out, and then go leaner by 1/6th.
Leave it there, and proceed with 1/4 throttle chops to confirm your pilot mix.
We really dont spend a lot of time at idle anyway....or at least I hope we dont 

[riffman12]

I have this short 6-sided screwdriver bit i use for IMS adjustments.  fits perfectly and I can easily see 1/6th turns.
IIWM, id go richer incrementally by 1/6th turns, until I reached that hesitation you are weeding out, and then go leaner by 1/6th.
Leave it there, and proceed with 1/4 throttle chops to confirm your pilot mix.
We really dont spend a lot of time at idle anyway....or at least I hope we dont 

and if I can't find a happy medium where I have no hesitation with good color, then I need to down jet on the pilots?

[Jerry Rxman Griffin aka MuthaF'er]

I've pretty much always had 40's in mine. Whether totally stock, 812, cam, open stock header, stock muffler, Kerker, K & N, 300', 6900', etc. No problems. I've always concentrated on needle position and mains.   

[riffman12]

I've pretty much always had 40's in mine. Whether totally stock, 812, cam, open stock header, stock muffler, Kerker, K & N, 300', 6900', etc. No problems. I've always concentrated on needle position and mains.

hmm and you were always able to get good color on the plugs from an idle chop?

[Jerry Rxman Griffin aka MuthaF'er]

Can't say I ever worried about idle so long as it ran good in that rpm range. If memory serves me (hah!) my first Honda dealer tune up in spring 76 before taking it to Daytona and blowing it up on a missed shift the mechanic actually installed 38's? That is unless it came with 38's and he installed 40's? Just know he changed them. My main concern has been mid range and WFO top range. These damn carbs will drive a man crazy!!
 

[riffman12]

oooook I think I finally figured out what's going on. I had the bike idling really nicely and getting good color on an idle plug chop, then took it out for a ride and it ran really erratically like it was misfiring, stuttering etc. I flipped it to reserve tank just in case it was a petcock issue but it didn't help.

I got back and took a close look at the spark plug wires and found that my number 2 and 3 plug wires were almost or completely broken inside the housing. Must be that I've been moving old brittle wires around so much taking plugs in and out.

So I think it's losing current and stopping those cylinders from sparking properly.

I've never replaced plug wires before. Is there a kit I should buy or do you just buy wire by the foot and cut it to fit? I know how it twists into the plug cap, but how does it attach to the coils?

[Sigop]

I had same problem for a while.  Might want to try a simple fix first. Quarter inch clipping off the end and screw back in might do the trick.  Look in and get back to to the original wire color.

[Jerry Rxman Griffin aka MuthaF'er]

+1 on Sigop. AND do you need to replace the caps too??

Stock coils were not meant to replace wires but it can be done by cutting into the coil. PITA. If you have stock coils replace them with Dyna or other. You have to get wires but you are able to replace wires. Also you can get NGK?? splices for replacing most of the wire. 

[seanbarney41]

If the useable length of your plug wire is shot, for all practicle purposes, those coils are shot.  Sure some have spliced in new cables successfully, but...just get new or good used coils

[flybox1]

oooook I think I finally figured out what's going on. I had the bike idling really nicely and getting good color on an idle plug chop, then took it out for a ride and it ran really erratically like it was misfiring, stuttering etc. I flipped it to reserve tank just in case it was a petcock issue but it didn't help.

I got back and took a close look at the spark plug wires and found that my number 2 and 3 plug wires were almost or completely broken inside the housing. Must be that I've been moving old brittle wires around so much taking plugs in and out.

So I think it's losing current and stopping those cylinders from sparking properly.

I've never replaced plug wires before. Is there a kit I should buy or do you just buy wire by the foot and cut it to fit? I know how it twists into the plug cap, but how does it attach to the coils?

Where exactly were the wires broken?   If too close to the coil itself, you might not have enough to splice.
If you have enough wire for splices.... NGK splice kit #8083.  The kits and 6' of new wire will run you $40ish.
Super simple to install.  Take your coils off the bike and it'll take you an hour or less.
Dont just do 2/3...do all your wires.  No telling what their condition is.
Put the coils back on and then trim the new wire to desired length.  Install caps.  Done.

[riffman12]

quick update for anyone who's not super bored of this 

I had enough length in the plug wires to where I could just cut it right before the break and put the cap back on. The wires are pretty brittle so I'll probably replace the whole set with coils in the near future, but for now there are no breaks and they seem to be OK.

Fiddled more with idle screws and I just can't get these 42 pilots to behave properly. I don't know if it's just because they're too big for my setup or if it's the crappy aftermarket jet quality, but anything off of idle to about 1/2 throttle is just stumbling all over the place and the plugs are coming back very black and fouled.

Sooooo I'm putting the stock 40s back in and going to do my best to get the float heights as perfect as possible, work on plug chopping those and go from there.

If I can get that right I might experiment a bit with richening the needle clip, but we'll see how it looks when I dial the 40s back in.

Yay carbs!

[Jerry Rxman Griffin aka MuthaF'er]

Curious. I once replaced stock jets doing a "rebuild". The air/idle screws were different. One set (kit vs stock) had a hole in the bottom and the other did not. Not sure which at this point. Seriously loaded up at in-town speeds. Kept a spare set of plugs under my seat in case I couldn't clean them off at higher rpms. Put the stock back in and problem solved. And, oh, by the way those stock carbs are on the 1000 engine with 40's. It seems to idle fine with a 315 cam at under 1000rpms. I have not dialed them in yet nor touched the air screws. Luck perhaps??   

My take, don't worry about plug chop color, worry about performance. I concern myself with plug color at higher rpms.

You know, an expensive set of CR's might help you sleep better at night. LOL  But at least you are learning carbs now. 

[jonda500]

 

My take, don't worry about plug chop color, worry about performance. I concern myself with plug color at higher rpms.

+1! Here in Australia I haven't seen a spark plug with that legendary 'light tan' color on it since they stopped selling leaded fuel. The color I have seen on my plugs ranges from white(no color), to slightly blackened white, to black; but the performance is the main indicator I use by far!
John 

[Jerry Rxman Griffin aka MuthaF'er]

Here's a link someone else posted on another topic. Simple and easy to understand. I think this tutorial will help you with the carbs.

http://forums.sohc4.net/index.php/topic,165003.0/topeeicsn.html

[riffman12]

so I've been riding the bike on and off. It's still not as good as it was before this whole "carb fix" project, but it's close.

At the moment these are my symptoms:

1) when I snap the throttle open, the revs drop kind of slowly. It eventually comes back to normal idle, but pretty slowly.

2) if I really get on it on an open piece of road, 3/4 throttle + for a short period of time and then back off to idle, as soon as I start opening the throttle just past idle it stumbles a little bit before it clears up and goes back to normal.

3) After riding for a while, the idle will dip lower than normal before coming back up. A lot less consistent than it was before


I thought some of these might be caused by a vacuum leak, but I really can't find anywhere it would be coming from. Carb boots are new on both sides, clamps are all tight.

I'm going to vacuum sync the carbs again this week and see if it helps.

Any other thoughts?

[riffman12]

Hi guys, Another update as I slowly sort out my carbs. Here's a video showing what I mean by the revs dropping slowly:

I spent quite a while syncing the carbs and that's as close as I could get them with the vacuum gauges.

I've triple checked all 4 carb boots, both engine and airbox side for vacuum leaks.

Can anyone think why else the revs would be hanging like this? Every other video I see of someone revving a 750, the revs snap down pretty quick

[flybox1]

you got tan plugs at idle yet? 

[Jerry Rxman Griffin aka MuthaF'er]

without re-reading everything, does the throttle at the carbs snap shut rapidly when you let off?