throttle response vs brake air pressure

[ewo1]

So I finally rebuilt the engine in the second bus I have, see my milkshake oil thread, and it is doing something I have never seen before.

When I initially start it up, if the green needle (secondary air tank ?) is below 30 pounds the bus will idle but there is no throttle response at all.
throttle response starts to come back somewhere between 35-40 pounds of air pressure.

then in order to release the brakes I need to firmly step down on the brake pedal and then I can push the yellow brake valve down in order to release the brakes.

The second part of this I believe is the brake interlock for the wheelchair lift that I am keeping operational in the bus.

So is it normal to have no throttle response until the brake pressure rises up?

[rossvtaylor]

If you have an air throttle, then yes...it requires the air pressure to move the air piston connected to the governor arm on the engine. I've only seen air throttles on rear engine buses, so I'm guessing that's what you've got? Air throttles are great and reliable...and they don't have a long cable to lube or get sticky. The only "downside" is a bit of response delay, between pedal to governor. Lots of folks are so used to it that they don't even notice, but there is a slight difference between air and cable throttle response (if the cable linkages are tight).

[ewo1]

Originally Posted by rossvtaylor

If you have an air throttle, then yes...it requires the air pressure to move the air piston connected to the governor arm on the engine. I've only seen air throttles on rear engine buses, so I'm guessing that's what you've got? Air throttles are great and reliable...and they don't have a long cable to lube or get sticky. The only "downside" is a bit of response delay, between pedal to governor. Lots of folks are so used to it that they don't even notice, but there is a slight difference between air and cable throttle response (if the cable linkages are tight).

Thanks, where can I find the throttle response unit?

In a way, I kind of think that this initial delay is not a bad thing, forces me to wait 1/2 a minute or so to let the bus engine settle, lube up, in before I put it in drive.

[rossvtaylor]

The pedal will have a short plunger through the floor to a valve, which should have two air lines coming off it. They're usually smaller lines, like 1/4-inch...not the bigger 3/8 or 1/2 inch lines you find for the brakes.

At the other end, on the engine, you'll find a piston assembly which will have a short rod (like 6-8 inches, usually) that's got some adjustment nuts on it...so you can adjust the length. That rod usually has a Heim joint on each end. One end will be on the governor (throttle) arm on the engine. The end closer to the piston will often be on a short swing arm with holes along it, so you can move the rod to different positions and adjust the throw.

I wish I had pics, but I recently rescued a stranded Blue Bird. Their throttle quit responding, suddenly, and they were stuck. A "mechanic" told them they needed a new accelerator pedal assembly. I went over the system and found that the actual issue was that the 1/4-inch bolt that connected the Heim end to the governor arm had come off. 50 cents in parts later, and they were all fixed up.

While I was at it, I had the owner mash the pedal all the way down and while he did so I tried to move the governor arm, to see if the pedal was giving them full throttle. It wasn't. Even with the pedal all the way down, I was able to push the governor arm a bit farther and rev more. I moved the other end of the rod down one hole on the swing arm, at the piston, and that gave them full range of motion and actual full throttle.

I'm no longer surprised by how many rear-engine buses don't get to full throttle, because of worn linkages or parts which are never adjusted. It's worth checking every bus.