Topic: Carb rebuild... problems, need some help

[written_black]

Hey everyone. Got my carb kits today, rebuilt the carbs. I bench synchronized them, and set the floats at 26mm. It didnt want to fire right up. After some coercing she fired up, but didn't want to idle. After it warmed up it idled, but poorly and wanted to die. The same problem that it had still existed from when I got the bike. It wouldn't idle right, it wanted to idle high after being on the throttle. It's doing the same thing. Also when you open the throttle up it lags hard for a few seconds then it wakes up, once it's above 3000 rpm it acts like it should. I also noticed if I put my hand over one of the left two cylinders it would still run, suck my hand really hard, but run. The right side would choke it out. So I am thinking there is all kinds of stuff up. The valves were suppost to have been adjusted, but I'm going to check them tomorrow. Could not having them sync cause it to run that poorly.

[eldar]

your pic looks like a pre 76 750K. So I will go with that.
Did you set your idle screws? Those comtrol your low speed mix. From the look of your bike, they should be on the airbox side of the carbs. 

Make sure the choke is set correctly. What was in the carb kits you got? Did you clean all the jets completely using a carb cleaner and dip?

How about the spark plugs? Did you check them? They may need to be replaced if they look old or at least cleaned and regapped.

Check your ignition timing as well.

So in this order:
Set valve tappets
check timing
do complete clean on carbs and set everything to base settings. Your idle screws should be set to 1.75 turns out I think.

[written_black]

Oops.. Yeah it's a 73 500. The idle screws i turned out 2 turns, and it ran a little better as I turned them in. Tomorrow I am going to check the valves and the plugs. I may get some better gas too, it was older gas that I had in a gallon can. I also forgot, before I tore into the carb's it smoked from being to rich, it doesnt now. The kits were K&L. They came with new jets and rods. Thanks again.

[kghost]

Yeah, what Eldar said.

and sync them it makes a difference.

[eldar]

Yeah sync then I think you do the idle speed final adjustment

[written_black]

I've got a sync tool that I am getting here with in the week. So hopefully that's the problem. This motorcycle business sure is different than hot rodding. I've got a Nova with a 327 in it, and it when I rebuilt it, it ran perfect, with very few tweaks. I guess it's what you grow up with, and grow with.

[74cb750]

Once you have it dialed in it feels much better. I got my 1974 CB750 running fine, then blew a head gasket. Now I can't get the 1975 CB750 running right  Oh well....good luck with it. Did you put any fuel line filters on?

[written_black]

Yup fuel filters installed, and going the right direction. I went out and messed with it again. It fired right up this time, which is good. I think I just need to get those carbs synced. I've got plenty to do in the mean time. Gotta get it painted. I primered the tank the other night, and it looks great. Hopefully my fork seals will get here tomorrow and I can do that. I also got some tires coming this week also. It's up on jask stand now. I cant ride it really till the end of the month when I have money for tags and insurance. I'll let everyone know how the plugs and valves look tomrrow.

[Sam Green Racing]

Make sure you get fresh gas.
Drain all the old gas out, and make sure there's no crud in the tank.

Good luck  Sam.

[TwoTired]

I bench synchronized them, and set the floats at 26mm.
Also when you open the throttle up it lags hard for a few seconds then it wakes up, once it's above 3000 rpm it acts like it should. I also noticed if I put my hand over one of the left two cylinders it would still run, suck my hand really hard, but run. The right side would choke it out. The valves were suppost to have been adjusted, but I'm going to check them tomorrow. Could not having them sync cause it to run that poorly.

The CB500 carbs are supposed to have the floats set to 22mm.  Probably run on the lean side at 26 mm, as the fuel level will be rather low.  Also, the book value for the idle screws is 1 turn out +/- 1/8 turn. This works well for the stock airbox/filter and stock exhaust system.  Probably be a different setting if these items aren't stock.  Otherwise, keep turning them in an eigth turn at a time until the throttle response is good without a wheeze response to half throttle changes.  Be aware you cannot snap the throttle wide open from low speed and expect it to go, as there is no accelerator pump.

Check your intake runners for leaks.  Do all the other tuneup items before tweaking the carbs; plugs, points, timing, tappets, cam chain, and air filter.  Sync the carbs and then set the idle mixture.  The order is important. Or, you'll be doing things repetatively to get things right.
Anyway, if it doesn't respond to this standard proceedure, then it's is time to troubleshoot.
Cheers,

[dusterdude]

hey written black,i know what you mean,i did a duster with a 340 and did not have the problems ive had with these damn 4 freaking carbs on my 750.

[turnenwheels]

Elder has the right idea, The idle screw was an issue, once that was dialed, I was able to set the air on my 74'750.  Still have difficulty with the sync, my elevation is at 3600ft.  i've set the air screws at 11/3 turns, still requires choke though... any suggestions out there?  New plugs, dipped and cleaned carbs and tank.  what is the suggested idle?  I've heard 900rpm's to 1100rpm's.

[written_black]

I rebuilt the carb's first mostly because I thought they needed the most attention. They were i nsad shape, the bowls were leaking, previous owner had them siliconed on, took forever to get them cleaned up. I'm not certain how to check the timing. I'm assuming since it fires right up that it's in time. I barely have to touch the starter and it starts. I'm going to go check the valves, and put some new gas in it today. The tank was pretty clean, and I have new filters on it. I dont have an airfilter in, would that make a difference?

[TwoTired]

I rebuilt the carb's first mostly because I thought they needed the most attention. They were i nsad shape, the bowls were leaking, previous owner had them siliconed on, took forever to get them cleaned up.

Of course.  The carbs have to work before you can tune them.  But, the carbs aren't tweaked during a normal tune up.  They are pretty stable, adjustmentwise once they are dialed in.  But, tappets wear, points wear, and timing changes occur during normal operation.  Restoring these at tune up time usually means your can ignore the carb adjustments.  But, if you tune your carbs with these other things out of whack, they won't be right when they are set properly and you will likely have to re tune the carbs.  It's your time.  Do what you want.
Did you replace the orings that seal the main jets to the carb bodies?  Leakage there can make things problematic.

I'm not certain how to check the timing. I'm assuming since it fires right up that it's in time.

Bad assumption, I think.  It's harder to solve an equation with unknown variables.  And, until you know... Anyway, you can download the shop manual from elsewhere on this site.  All the tune up proceedures are explained in there.

I'm going to go check the valves, and put some new gas in it today. The tank was pretty clean, and I have new filters on it. I dont have an airfilter in, would that make a difference?

The slow jet orifices are .016 inches in diameter.  Does "pretty clean" means the bits you saw were smaller than that?

Air filters do make a difference, sometimes huge.  And, unless you are going to run the engine like that 'til it dies, it's pointless to tune carbs until one is installed.
Some background.
Fuel flows through an orifice when there is a pressure differential one side to the other. The more pressure differential, the more flow.   In your carbs, one side of the orifice is always at atmospheric pressure.  The other side is subject to the suction from the running engine and the venturi effect in the carb throat.  Operation of the choke increases the suction effect, drawing more fuel through the jet orifices.  An air filter presents partial restriction (like partial choke) and therefore effects fuel air mixture ratios.  Carbs should be tuned to compensate for this restriction.
FYI, air filters keep dust particals from being blasted onto the cylinder walls, eroding them.

Good luck!

[written_black]

I guess I am a total newbie. In the car world, usually if it fires up easily, doesnt ping, or lag the timing is usually pretty close. When I first got the bike and road it around it did none of those things. I assumed the valves were adjusted since the guy told me that a bike friend of his did them. Thats why I tore into the carbs. I also assumed they werent in bad shape, same guy was suppost to have rebuilt them. I was just going to replace the bowl o-rings, but when I got in there it was a different case, so I decided to redo all of them. I'm going to run an air filter when I get on the road. I can't track down a filter right now, I was just trying to get it where it would run; something to do while I wait for tires and fork seals to come in. I appreciate all the help and advice you guys are giving me. I'll get it all sorted out. It runs better with the adjustment of the air screws, and I got new gas today, 91 octane. The tank is clean, it just has a little bit of surface rust on it, no chunks, gas flows clean.

[eldar]

With these old bikes, just stick with 87 octane. You will not get anything noticable from the 91 other than a higher fuel bill.

[written_black]

Well I got it running better. As it turns out, the filters were shot, put new ones on ran better. I think once I get the sync tool I should be able to get it running well. I haven't checked the timing yet. I took the cover off and I found more stupidty from previous owners. The screws that hold the points onto the plate were completly rounded out. I can still adjust it luckily. I also checked the valves, they were adjusted properly. I think the new gas I put in helped a lot also. I just have to wait for the fork seals, seat cover, carb sync tool, and new tires to come in now.. I guess I can finish sand the primer on the tank so I can shoot it this weekend.