Carb Puzzler - Get the answer win the prize!

[GT Mills]

Wednesday's puzzler:

You have before you a rack of low-mile carbs on (for the sake of the puzzler quiz we'll use) a good-running RX1 with 4 carbs.

One day, you go out and start the perfectly-running RX1. This day you notice the exhaust note seems to be hitting on 3 of the 4 cylinders. You wait for the motor to warm, clear its throat and fire on all four, which it does.

Next day and each day thereafter, you notice it takes longer and longer to fire on all four. You eliminate plugs and coils as the issue, then pop the airbox lid off, set the throttle at 4000RPMS and cover the inlet boot of each individual carb in succession, noting that cover carbs 1, 2 & 3 result in a drop of 1000 RPMs as expected, but when #4 is covered the revs INCREASE by 1000 RPMs!

Note: this engine runs perfectly in every other respect.

What is the problem?

 

[STAIN]

cracked carb boot on #4 cylinder

 

[Yamajon1]

Wednesday's puzzler:

You have before you a rack of low-mile carbs on (for the sake of the puzzler quiz we'll use) a good-running RX1 with 4 carbs.

One day, you go out and start the perfectly-running RX1. This day you notice the exhaust note seems to be hitting on 3 of the 4 cylinders. You wait for the motor to warm, clear its throat and fire on all four, which it does.

Next day and each day thereafter, you notice it takes longer and longer to fire on all four. You eliminate plugs and coils as the issue, then pop the airbox lid off, set the throttle at 4000RPMS and cover the inlet boot of each individual carb in succession, noting that cover carbs 1, 2 & 3 result in a drop of 1000 RPMs as expected, but when #4 is covered the revs INCREASE by 1000 RPMs!

Note: this engine runs perfectly in every other respect.

What is the problem?

Plugged or pinched carb vent line

 

[STAIN]

This carb had me stumped for a while.....

 

[Macheater]

Could be a few different things.

My money is on a plugged pilot/idle fuel jet. Those orifice are some of the smallest in the entire carb. When things start to get a little varnish or gumed up due to age, fuel quality (ethonol), etc, they are usually the first to get plugged.

With that said, and given the age of the machine, cracks in hoses, and carb boots could also come into play.

 

[GT Mills]

Liking these offerings so far! The connecting boots actually are cracked on #2 & 3, but 1 & 4 are still good. The boots are going to be replaced, but the cracks are not affecting performance yet. I have an AFR but the sensor went on the fritz just before this problem popped up, so it's of no use today but while still working it indicated normal O2 readings and the boots showed these cracks long before it went out.

The plugged/pinched vent line is interesting, you are talking about the vacuum chamber vent, not the bowl overflow, right? I'm thinking that if this is the case then covering the intake stack with my hand would still result in a drop of RPM, not an increase. But I should check for one of those dang bugs in there that plugs up small tubes with a mud nest, and look for maybe another fault preventing the slide working in sync with the other 3. I will check by manually raising the slide with the engine running, see if I get different results moving this slide than the others.

The plugged jet seems like a good bet, since plugging the throat would increase manifold vacuum, pulling hard enough on the gas in the bowl to increase fuel flow and raise RPM.

 

[STAIN]

But I should check for one of those dang bugs in there that plugs up small tubes with a mud nest,

I had that happen on an older Kawaski 4 wheeler...ran like chit

 

[GT Mills]

Turns out the NEEDLE JET HOLDER (photo) had come loose. As mentioned above when fast idling at 4k, covering each carb with my hand resulted in only #4 having an INCREASE of RPM instead of a decrease. Pulled that vacuum cap off, and the holder came out with the slide spring! Put it all back together properly, and BINGO, everything back to normal.