LO206 - Surging at idle and lack of revs 3-4 laps into race

I have also had this happen when the grub screws didn’t grab well and it felt very much like the engine was bogging down. BUT, the big missing symptom this doesn’t catch is that @Spenny also mentioned a surge (right?). Now, it could be that the surge was the result of chasing a mechanical issue through carb tuning. Maybe the fueling was dead-on at first but now is out in left field because it wasn’t the original issue?

Also, I am sorry we are chasing ghosts! I do appreciate the help though!!! This is why I love Kartpulse.

If the axle isn’t 100% locked down, binding can be intermittent

And I sympathize with you. I would hate the idea of swapping the engine and not knowing why that fixed it OR swapping the engine and still having the problem. Even if it is just academic at this point, I’d want to know the problem’s source

I know you said you checked the pick up and fuel line, but it does sort of sound like a fueling issue to me. I had two odd issues in the past with fuel pick up:

  1. Leftover plastic flashing from the tank flaked off and got sucked up into the fuel line. Kept obstructing the pick up and I couldn’t figure out why the engine kept starving during the race. Eventually it starved itself of fuel enough to seize the engine.
  2. I had blue masking tape on the hose of my fuel jug to mark it as mine, and over time it degraded and started to peel off and fall into my fuel tank without me noticing. I was having intermittent fuel starvation and carb rebuilds or adjustments did nothing for it. Eventually I did notice small specks of blue in my fuel filter and I pieced together what was happening.
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I pulled the tank, pickup line, filter, and all fuel lines between the tank and the carb. I am not seeing any debris or indication of potential issues. I think I am going to replace the pump and filter just to be safe as I do notice a lot of air bubbles in my fuel lines prior to the fuel pump (never noticed bubbles between the carb and pump.

The hard thing with issues like this is that it could all be due to one thing or a bunch of little things. Hard to tell what is “noise” and what is the root of the issue!

Yeah, I had an issue with my daughter’s kart recently where it just fell flat during one session and wouldn’t rev hardly at all. After turning it off and back on (IT Special), it revved just fine on the kart stand. I tore the carb apart and saw no issues other than a couple specks of dirt. My guess is some got in the bowl and blocked the jet. When running, there was enough suction to keep it blocked. With the engine off, the dirt was free to fall back down in the bowl, and the engine ran fine. After I cleaned the carb out, the issue never happened again.

Before you toss a bunch of $$ at it, replace ALL the fuel tubing. Replace the in-line fuel filter. Bubbles are usually not as evident AFTER the fuel pump, as that section of line is positive pressured. Bubbles are usually noticeable BEFORE the fuel pump. Another observation is does the fuel line keep fuel up to the pump after shut down, or does it seem to drain back relatively quickly? Sucking air into the lines from a cracked fuel filter or hose is pretty common and can make the carb inconsistent to tune. Some air can make you go a bit lean, more air can actually affect the fuel pump function.

Personally I would also put a full carb kit in it, floats, needles, etc; or an entire new carb and a fuel filter and go test. I tossed a race weekend for my son last year trying to diagnose a motor issue. Finally put a new backup carb on it and she ran fine. Turns out the float was hanging in left hand turns only. On stand it was fine, in rights and on straights it was fine. Put a rebuild in the pulled carb when we got home and all was good.

Last item I have seen, never on ours, was a coil. When motor got heat soaked the coil would no function consistent. Again, never in the 38 race weekend we ran a 206 on my son’s kart.

Good ideas!

Yeah, I am leaning towards getting a new carb, new fuel pump, new fuel lines and new fuel filter. If that does not fix it, I’m binning this engine lmao.

I pulled the flywheel cover off last night and inspected the flywheel and coil, including the gaps. Everything looked to spec, tight and in good condition so I cleaned it all up and put it back together. I also sanded the clutch drum and shoes, replaced the springs (2 black, 2 white) and added a lightweight weight to each shoe. I hope this brings down the engagement rpm slightly to try to add some headroom above the slowest corner’s RPMs.

I checked on this last night. Axle is exactly where I left it and centered. All the grub screws are tight as well. The gaps on either side of the rotor to the pads are exactly the same. So, I don’t think it is the axle shifting. Good idea though!

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Very curios to see what the cause is.

@Spenny just to drive this into the ground, can you pick up each back tire one at a time and see if the pad spacing to the rotor changes? Grasping at straws here, but a broken chassis weld, for example, could cause it to drag without seeing a problem on the stand. Still doesn’t explain away the surge though unless the surge was just a consequence of chasing a non-carb issue through carb tuning.

Okay just got my kart chassis back from the shop and they were able to straighten it all out which is great. As for the engine, I decided to pick up a new carb and set that up with a mostly stock, conservative setup:

  • Float Height: 0.865"
  • Drop: 1.050"
  • Clip: P3
  • Air Bleed: 1.5 turns out

It is looking to be pretty warm this weekend so we will see what happens!

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Okay so update after race day! TL;DR I had no issues all weekend with the new carb…

With the new carb setup as noted above, I did not have any issues all weekend. That was despite rather warm temps where I figured the engine would become rather rich. But, I stuck to my plan and did not make any changes to the carb other than minor air bleed and idle speed adjustments. However, I did notice a few things:

  1. The engine idled very low all weekend but got lower and lower as it got hotter. Opening up the air bleed did not seem to do much so I had to screw in the idle speed screw to get it to idle around 1900 rpm or so. But it wanted to “idle” at like 1500-1600 before the idle screw was tightened up significantly.
  2. This was the first time I’ve let it idle that low and I had to be careful waiting to leave the pits and while on the starting grid for the standing start to keep it from dying. I had to keep my foot on the throttle slightly so the engine would stay at 2000rpm or so and wouldnt die.
  3. This weekend I had the best standing starts I’ve ever had! I now believe that I was either setting the idle too high or the low end mix way too lean and robbing myself of power off the line in previous races where I’d get left behind off the line.
  4. Top end felt in line with other drivers. I was not getting gapped on the straights anymore and seemed to be hitting the same top speed and hitting the rev limiter at the same spots as others who I knew were on the same gearing as me.
  5. In the final race Sunday, it was the hottest temps all weekend and I did feel like I was lacking a bit out of the slow corners. But my tires were also toasted and the track gripped up a lot so, it could have just been binding / driving issues rather than tires.
  6. The engine would still surge at idle from time to time which was odd. It seemed to randomly just rise up to 2500+ RPMs without me touching the throttle or anything. Still not sure what is causing that or if its an issue. The surging was more significant than with the old carb but happened much less often.

So, I am not pretty confident that something was wrong with the old carb which was causing the issues at the last race. The only difference in carb settings was that the needle clip was at P2 but I don’t think that would have caused all of the major engine DNF issues noted above. Not sure what though because I ultrasonic cleaned it, inspected everything and didnt see anything wrong. Oh well, the new carb seems to work, and I had a great race weekend without any issues (finally)! Thanks for everyone’s help in this thread! I love Kartpulse!

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