750 Twin Climbing RPM's

I've posted about this before, and I had it working (kind of) last summer. The history is I cleaned the carb, adjusted the valves, and adjusted the air pilot screws. I get it running fine at idle and then take it out on the road. I as soon as a go through a couple of gears and stop the bike start climing in rpms or just dies. When ever I go out its a battle to keep it running.

My question is this. Could this be a leak in the carb holding boots. I'm going to try to get it running this weekend and spray some starting fluid on the boots to see what happens.

An intake leak usually isn't bad enough to make a bike drop stone dead. It will make it run lousy, but you can make it run. It sounds to me like not getting enough fuel. The most likely culprit is a problem in the pilot circuit.

Have you checked the plugs color? Lean or rich?

Did you set the float levels correctly?

How does it accelerate and run in general?

It runs fine while under throttle. The only reason that it dies (i think) is becuase I have to continually adjust the air pilots while stopped and I end up killing it. I don't think that it's a lack of fuel problem. If its anything it's probably too much fuel. I tried to set the bike to manufacturers set points but no luck.
It's a delicate balance with this bike. If it's not right at the sweet spot it will stall out during a stop and if it's to lean the rpm's will climb. I have to readjust everytime I ride.
This is the process:
I start it up and getting it idleing and good response with a crack of the throttle. I'll get a half a block have to stop and it will start to climb for about 10 seconds and then settle down. I also have backfiring after down shifting or a quick release of throttle.

As bountyhunter suggested, I would also suspect fuel starvation. And it might be happening before fuel ever reaches the carbs.

Perhaps from a clogged vent in the fuel tank cap. Experiment by riding while leaving the fuel cap loose.

Perhaps from a faulty diaphram in a vacuum (if applicable) petcock. Experiment by riding while running on Prime position, or in Reserve position if a manual petcock.

If vacuum petcock, perhaps the vacuum line is cracked or leaking at the carb or petcock nipple, which could prevent petcock diaphram from operating as it's supposed to when in run position.

Perhaps from crud inside the tank settling down around and clogging the petcock intake tubes up inside the fuel tank. Test by removing the fuel line from petcock, turning to prime or reserve, and look for free flow of fuel from the petcock. Some petcocks have a screen filter that often clogs, so would check that also.

Is there an inline filter that might be clogged? Same test by disconnecting fuel line from filter (on carb side) and look for free flow of fuel into and from the filter. A fresh inline fuel filter is usually a good idea on each fuel line going to carbs (some carbs are fed by a single fuel line while others are fed by dual lines.).

Petcock may be removed from tank and carefully cleaned.

Fuel tank may be removed, dump old gas through cap hole, add a little fresh gas and slosh around inside tank, dump again.

Maybe the problem isn't due to fuel starvation -- but the above steps, if not already done, should help assure an ample supply of clean fuel, and eliminate that possibility from the list of suspects.

Just some thoughts in hoping for a cheap easy fix.

Good Luck!

al_ndsu wrote:

It runs fine while under throttle. The only reason that it dies (i think) is becuase I have to continually adjust the air pilots while stopped and I end up killing it. I don't think that it's a lack of fuel problem. If its anything it's probably too much fuel. I tried to set the bike to manufacturers set points but no luck.
It's a delicate balance with this bike. If it's not right at the sweet spot it will stall out during a stop and if it's to lean the rpm's will climb. I have to readjust everytime I ride.
This is the process:
I start it up and getting it idleing and good response with a crack of the throttle. I'll get a half a block have to stop and it will start to climb for about 10 seconds and then settle down. I also have backfiring after down shifting or a quick release of throttle.

I've been on the same 750T since 1979 and I have never found mine to be at all finnicky. The only time it ran bad was when I got a gas tank full of water and fouled the fuel system.

Is your bike stock with the Mikuni BS38 carbs and the stock airbox and exhaust? It should be easy to get it to run if gas is getting where it needs to go. Check the size of the pilot jet, mine ran best with a 45 when it had stock pipes.

BTW: if the carbs are BS38, the adjust screws are not air screws, they are fuel feed screws. Opening them riches up the mix, closing leans them out.

On mine it only backfires when the pilots are too lean.

If the idle climbs, make sure you have enough free play in the throttle cables and that they aren't gunked up.

I checked the throttle response last year and it seemed to snap back ok. I'm goign to have to try the prime and reserve fuel and check gas flow.

Bountyhunter,
I'll have to check the jets. Who knows I may just need to clean the carbs again. There should be two adjust screws on each carb. One is for fuel and one is for the air mixture I believe. Or are they both fuel mixture. The carbs are stock. Do you think that it could be a float problem since it idles fine and then backfires when rev'd? Do you know what the float setting should be?
Thx for the help. I'm going to be attacking this tomorrow. If I get in running I'll just mess with the electrical later.
The guy who owned it before me decided to "mod" the starting circuit. Now I can't figure it out.

al_ndsu wrote:

There should be two adjust screws on each carb. One is for fuel and one is for the air mixture I believe. Or are they both fuel mixture. The carbs are stock.

If this is a 1979 750T, the stock carbs are BS38 Mikuni. They only have on adjust it is the fuel feed for the pilot (idle) circuit. Opening gives you more gas.


al_ndsu wrote:

Do you think that it could be a float problem since it idles fine and then backfires when rev'd? Do you know what the float setting should be?

maybe if the fule level is too low. Your fuel level on a BS38 should be 4.5 - 6.5mm DOWN FROM the lower edge of the upper main body of the carburetor (the part the bowl attaches to)

Your inconsistent idle can be from a variety of reasons:

1. throttle cable lacks slack - adjust for visible slack
2. air leak through bad seat on float plunger rubber pads
3. air leak in some other location; such as carb holders or bad orings on mixture screws.

You only have one pilot mixture screw. This screw allows varying amounts or air/fuel mixture. When the screw is turned out, you get more air/fuel which essentially richens the IDLE mixture. Set the screws at two turns and quit mucking with them as they don't cause the idle to vary.