About to burn this bike to the ground

[CarloFreeze]

Let's figure this out. Tell me which Keihin carbs you are running and we'll get them dialed in. Float heights are important. If you have the carbs with the spring clips, that's good because you'll be able to drop the bowls pretty easily. You may have to pull them to change things depending on what you've done.

Don't start messing with jetting or changing needle positions just yet, in order to get it right, you have to start with getting it to idle properly, everything else for tuning is useless unless you get that right.

Now stock sizes and setting for the jetting on the "roundtop" carbs is 40 idle jets, needles in the middle position, and usually 105 main jets. These numbers will probably change, but it's better to change little things at a time. Make sure you have the baseline things working first. The jetting of the midrange and main jets doesn't matter at idle. You should start with your air mixture screws at 1 turn out on each carb. Float levels on the carbs are measured at 26mm from the notch just above the gasket surface. This is the most important. You have to make sure you get that part right.

Here's a great visual guide.

http://www.hondachopper.com/garage/carb_info/float_levels/float_levels.html

[Crowe138]

Done this over n over again jetting cb750 chops. If you had stock jetting in there before then it's definently gotta change or will run totally wack with pods n straight pipes. It is possible to get it right tho so don't give up . The idle jets stock are #40 n that's usually good at 1 turn out on pilot screws or one size bigger if you can't get the idle good with all the wide open stuff ur runnin, the mains gotta go up almost 4 sizes most the time. That means you need to get something between 120-130 on the main jets( Kiehin main jets in that number range go up in weird increments usually by 5) lotsa of sites online sellin whole sets of mains for cb750's , honda dealers will have them to but usually expensive.

Also double check that the coil wires were put on the right cylinders (long lead coil is for the outside cyl. 1 & 4 , short lead coils for the inside 2 & 3) when it was put back in the frame and make sure the centrifugal advance behind the ignition plate isn't sticking or broken n double check timing with a timing light should be idling on the "F" mark. You can feel the pipes to right after it starts up to make sure all cylinders are firing if u got one or two cold pipes u got a problem due to ignition or fuel, don't burn your hand, good luck

[jthomasbtm]

I'm gonna swing over to the garage after work and check all these things and I'll let everyone know about everything as soon as I get there. Thanks for the help guys.

[Freewheeler]

I'm gonna swing over to the garage after work and check all these things and I'll let everyone know about everything as soon as I get there. Thanks for the help guys.

I cant wait to hear!

Nick

[JuanCarlito]

I cant wait to hear!

Nick

Yea me too. Following your thread hoping to figure out my own 650 issues. Test run after work today. . . .gonna be a long 9 hrs lol

[CarloFreeze]

The other thing to do when you take the carbs off is to make sure you bench sync them so that all the slides are at the same height when no throttle is applied. Once you get them dialed in jetting and setting wise, you can vacuum sync while the bike is running with a set of gauges and really see how smooth the engine gets.

All the carb tuning info you need is right here. http://www.hondachopper.com/garage/c...carb_info.html

Also, like Crowe 138 said, mark the position of your ignition plate with a Sharpie at the screw holes. Pull the plate off and lube the spark advancer if you haven't done it already. It's important that it doesn't stick because it can cause a hanging high idle if it sticks at full advance.

This is what it looks like with the points plate removed. Just pull it off the shaft and lube the cam with some white grease and reinstall. There are small springs that allow the lobes to open up, make sure these slide freely as well.

[jthomasbtm]

So I went to the garage after work, pulled her carbs and brought them home. Pulled them apart and the slow jet is a number 40 and the main is 110. Float height is at 26mm. Throttle needle is on the dead center notch. Screws were 1 turn out. A few people said they thought I needed bigger jets, but would that be the case since the plugs are fouling out? That's the info I have right now, let me know what you guys think.




These are the carbs I'm running. Well trying to.



These are what the plugs look like.

[Sky]

Suck, squeeze, bang, blow. All in the right amount at the right time. If your having trouble tuning, better rule out mechanical issues before continuing.
Sooty plugs can be an indication of weak spark. You said the ignition is new? Are you running the coils recommended by the ign manufacturer? 12 volts at the coils? Wires, plugs and gaps? Don't forget good grounds for all electronic components.
I'd start with making sure the ignition timing is statically correct, and that the advance unit (if present) is moving freely.
Double check your compression, and a leak down if low comp is indicated. Valve timing is paramount, cam chain timing ok?
Intake leaks and/or worn slides will foil any fuel metering.

[jthomasbtm]

I'm using, dyna coils and electonic ignition. Getting 12 volts at the coils, all the wiring is new and good, leads and plus are new and plugs are gapped correctly.timing is set, compression is really good, and I believe cam chain timing is good.

[CarloFreeze]

Okay good, it sounds like you have the ignition side of things pretty set. There's nothing to time with the cam chain unless you did a top end job and changed anything there. Right now you are basically at the stock settings for those carbs. I run the same ones. You may find yourself changing those needle clip settings and main jet sizes once you get the bike running right at idle, but let's do one thing at a time. In your video it sounds like all 4 cylinders aren't firing but it could just be the pipes. I guess double check that the ignition is wired correctly to the coils. Yellow = 1 & 4 cylinders, Blue = 2 & 3 cylinders. A reversal there would make the timing fire all strange. Did you check the spark plug caps? They should be NGK 5K ohm resistor types. Make sure they are all showing proper resistance by sticking one meter probe in the plug side and the other probe where it screws onto the coil wire. Should be +/- 20% from 5000 ohms. What plugs are you using as well? The factory manual recommends NGK D-8ES or Denso X24ES-U. I actually prefer the Denso's after trying both types, but NGK D-8's are easier to come by.

Go through these steps and report back. I wish I could say it was easier to come help in person, but you're about 4 hours away from me haha.

[Crowe138]

You're seeing slightly sooty plugs now but haven't got past quarter throttle so I wouldn't take it as to solid info yet on jetting, 110 is likely to small to get a good transfer from the low range to mid range with straight pipes and pods, pick up 120 , 125 , & 130 main jets.
The plug can be sooty from using the choke to start it or from it still not getting up to running temp since your not able to really get a good ride, also if you put new rings in then they will still be seating and reading won't be extremely accurate at this point. Like sky said , weak/intermittent spark can also cause the plugs to look like that, the plug boots could have to much resistance (super common on old cb750's) retarded ignition timing , stuck centrifugal advance, and excessive exhaust valve gap or to tight of an intake valve gap. Visually confirm your sparks good on at least one or all of the outer and inner cylinders if you haven't yet should be crisp blue spark.

[turtlem1969]

One thing not yet mentioned- check your carb to intake boots, if they are loose or cracked or not sealed tightly they will give off all sorts of weird readings and make it run like crap when the right hand is twisted.

[jthomasbtm]

The ignition wiring is good and I'm using dyna boots and leads along with dyna coils. The plugs are also gapped to spec. It was firing on all 4, while the bike was running all the pipes were hot.

[jthomasbtm]

I get a super bright blue spark from every plug. That's what I initially thought when this was happening. Still trying to figure this out.

[jthomasbtm]

All the boots are good and cranked down tight.