Running lean... rejet? What else to tune?

[pasteurized]

My '79 GS750 has run lean ever since a buddy ripped out the innards of the tailpipes on my bike. Yes, it is louder, but it has run lean ever since. It stackfires and barely idles, and when ridden it bogs off the line and hits a wall and leans out at about 6k. Which jets should I replace the stock ones with? What are the stock sizes?

Also, after rejetting, what adjustments should I make to the pilot screw and air screw? They have been messed with and are way off I am sure. Also, I have heard that raising the needle may be needed... I understand the idea but do these carbs have provisions for doing that? How do I know i need to do that?

 

[KEITH KRAUSE]

What mod's do you have? What brand name of these mod's? (Pipe, filters, air box mod's, etc.)
What have you done to try to make the bike run better?
Anything else the bike does wrong? Smoking, engine noise, poor gas mileage, fuel/oil leaks?

 

[pasteurized]

No other mods. Stock airbox, filter, etc. I have messed with the air screws but not understanding at the time what I was doing, I'm sure I probably screwed it up even more. First of all, I know I need to clean out my choke tubes/passages because it will not start unless I spray some starting fluid into the inlets for the carbs. Then once it is started, it barely runs, and if I try to rev it, it wants to die unless I SLOWLY turn the throttle. If I do, it very very slowly raises the rpms but then starts to stackfire and just runs crappy.

 

[nabrams]

Was the bike sitting for more than six weeks? If yes, then your carbs might need cleaning.
If your carb insides are dirty, then you have no chance of getting your bike to run properly.
Also, modifying the pipes will definitely require jetting changes. Your bike will run lean without the jetting changes. In some cases (like yours) the bike may be SO lean that it won't start without starter fluid.
When you say:

Then once it is started, it barely runs, and if I try to rev it, it wants to die unless I SLOWLY turn the throttle. If I do, it very very slowly raises the rpms but then starts to stackfire and just runs crappy.

That clearly indicates a severely lean situation.
I don't know the specifics of your bike, but I had to go up from 102 to 110 (mains) on my 1980 Honda CB750 with a non-stock 4-into-1 MAC exhaust. If you've changed nothing else (air box, air filter, etc.) then experimenting with larger main jets (progressively going up in jet size) will probably get your bike running well, as was the case for myself and many others who've done the same thing.
Also, you must be absolutely certain that there are NO leaks where the pipes join the header - this will make your bike run like crap.

 

[KEITH KRAUSE]

A good carb cleaning should be done first. You should get a good manual so you can service the carbs, etc. When the carbs are apart, check the condition of the various o-rings. Robert Barr, a member here, sell o-rings for your VM carbs. Then check the float heights. If any other parts look worn to you, ask us.
You should also replace the o-rings in the rubber manifolds (engine side). If they leak, you'll never get the bike tuned. Apply some hi-temp bearing grease to them to help them last. Torque to 6 ft/lb. All these o-rings are pretty cheap.
After the carbs are cleaned up, try adjusting your pilot fuel screws (underneath and engine side of bowls) and your side air screws. Since they've been moved, I can only suggest a ballpark setting to get you started with the pilot fuel screws. Turn them out 1 full turn from LIGHTLY seated. Never tighten these sharp-tipped screws. Be careful to use a good fitting tool on both screws to avoid stripping the heads, they can be stiff.
The side air screws are to be adjusted to achieve the highest rpm. The bike will have to hold an idle of course first, and there are other things to check that I'll mention later. Start the side air screw adjustments by turning them all to 1 1/2 turns out. Once the bike is on the centerstand and warmed up, adjust one carb at a time by slowly turning the screw in or out while listening for the rpm's to max out. When the rpm's max out, stop turning. You should find the sweet spot somewhere close to the 1 1/2 turn setting you started with. Now adjust the idle to 1,100 rpm's by adjusting the idle adjuster knob. Go to the next carb and repeat until all four are set and the bike is idling at about 1,100 rpm's. That should get you close with the screws. The sensitive pilot screws may have to be adjusted further by testing and getting plug reads at slower speeds (30/35 mph) in 4th gear will work. Turning the pilots out, will richen the mixture, and vice-versa.
After the carbs are rebuilt/floats adjusted/cleaned and the above is done, the carbs need to be synched with a vacuum tool. If you did the work correctly, the carbs should be good.
You also should do basic maintenance for a good spark. Be sure the plugs are NGK B8ES or equivalent and gap them correctly. Clean all connections at the coils/leads/plug caps. Be sure the battery is charged and filled. Clean up the battery posts too. Adjust/clean the points if they're still there, adjust timing. Be sure the air filter is clean and all clamps are tight around the carbs. This is lots of stuff, I know, but it's really just basic things the bike has to have to run right.
As for the jetting, a stock bike with just a muffler change shouldn't need big jetting changes. I would suggest 1 full size up on the main jets. I believe your stock mains are 102.5 (?) If so, try 107.5. The needle shouldn't have to be changed if it's still in its factory position, which I believe is with the e-clip in the 3rd position from the top. The pilot screws will have to be adjusted upon test results like I said. The side air screws will always be adjusted by the "highest rpm" method. If you do take 1/3 throttle position plug reads, and the plugs look lean, you may have to get some jetting spacers to make 1/2 position needle changes. The spacers for your carbs are about .022" thick. Remember, if you disturb the jet needles on VM carbs, you must re-synch the carbs every time. Hope this helps.