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

Back again with more 206 issues… At this point, I am becoming more and more frustrated with this engine. I think it might be time for just a new fresh start and a new engine. But, in the interest of not giving up and a deep need to understand what is happening, I’d love some help diagnosing the issues I was plagued with this weekend. I know the more info, the better so here is a wall of detail! :slight_smile:

Background Info:
My previous head was warped and I kept blowing head gaskets. So, I picked up a new “race prepped” head from JRPW recently and installed it before this weekend’s race. I torqued the head bolts to spec, checked valve lash (.004 exhaust, .002 intake), installed the header studs and a fresh gasket, and checked the intake and carb gaskets.

At Saturday practice, I spent a good amount of time warming up the engine for the first time with the new head. Basically I started it and kept the revs low for 10 minutes or so until the head temps (IR gun) stabilized. Turned it off, returned it to TDC and let everything cool back down to ambient temp. Once cool, I checked the head bolt torque (all good) and rechecked the valve lash (all good). I also checked the header bolts (all good, slightly tightened them). I did this procedure once more and then set the idle on the stand to 1800-2000rpm. The needle clip was in my default position of 3 but I noticed I needed to open up the idle air bleed quite a bit more than I was used to. Of note, it was hot and the rel. air density was 95. So, I figured it was a bit rich and needed some more air, which tracks with the higher temps and RAD. It seemed okay though so I left it and went out.

I go out for the first session and take it easy the first few laps keeping the revs below the limiter. However, I started to notice a few laps in that the engine wasn’t revving correctly. On the straights, even with the throttle flat out, I couldn’t get it to rev higher than ~4500 rpm. Odd, kept driving another lap and noticed it was getting worse - nearly dying out of corners and max revs were decreasing on the straights. I barely made it back to the pits.

I checked everything related to the head install (torques, header leaks, valve lash, etc) and everything looked okay. I also checked the obvious issues like the throttle cable coming loose, pinched fuel lines, clogged fuel filter, slipping clutch, dragging brakes, etc. Everything looked good. So, I then figured it must be the carb. My default carb settings are 0.865" float height and P3 on the needle clip. That float height is a bit rich and figuring the hot temps (over 80), Rel air density of 95 and the fast, flowy track, the engine was too rich. I decided to make a significant change in order to better tell if it was the right direction or not. I set the float height to .880". While in there, I also checked float drop to make sure the needle wasnt getting bound up or falling out. When setting the idle, it was still having a hard time idling (too low). It seemed to want more air, so I moved the needle clip to p2 to lean out the idle a bit more. That seemed to help and it idled better but I did notice that it seemed to surge off and on. While idling, the engine will just randomly rev up 400-700 rpm higher than where the idle settles. The engine has also never been very easy to set idle because of things like this so I didnt think much of it.

That next session, the engine seemed to have run fine. I was back near the pace I was expecting and felt pretty happy. Checked all the head bolt torques, valve lash and header studs before packing up for the day and everything was solid.

Race Day:
In the morning, there was two practice sessions. I did an oil change (needed). I ran both sessions and the engine seemed to run properly. It was a bit cooler than the day prior’s practice sessions but everything seemed to be running well.

Then, in qualifying issues came up again… It was a bit warmer now than in the earlier practice sessions but still cooler than Saturday’s practice (mid 70’s). The Rel air density was 96. The same thing happened as the first Saturday practice where it ran fine for 2-3 laps and then I started to notice I was being gapped on the straights by others. Then it slowly got worse, started doing the weird hesitating thing out of corners and max revs on the straights kept getting worse and worse until I brought it in. I stopped before the scales in the pit and kept it running so I could see if the choke was open or something obvious was wrong. It idled fine while checking before I decided to call it and tried to throttle over to the actual scale and I hardly moved… I turned it off.

With qualifying ruined, I needed to figure out what was going on. My understanding is that hesitation and failing to rev on straights were indicative of starving for fuel. I figured my relatively large float height adjustment Saturday was too much for the slightly cooler weather (though we are mincing hairs here). I figured it would be best to set the float height back to a more normal range and decided on .870 (richer than it was, but leaner than my standard setting slightly). While the carb was open, I also checked all the jets for obstructions and did notice a small bit of something inside the emulsion tube. I used a bit of wire to get it out and figured that was part of the issue. Though, if it was the only problem, I figured that shouldn’t only cause an issue 2-3 laps in. Put it all back together and fired it up. Engine ran okay but still had a really rough idle, seemed to want more air from the idle air bleed and would surge up randomly at idle.

In Heat 1, the engine seemed to be running okay. I noticed it a bit down on power in the first few laps but then it seemed to be running fine. I was having a good race and made up 3-4 positions. Then, while making a pass, the other driver swept into me like I wasnt there on the corner exit and we crashed and spun. That bent the hell out of my sidepod and my front end but that is a different story.

In Heat 2, other than “straight” being 10 degrees out of center, and turning right a bear, had a good race. I made up good positions and the engine seemed to be okay. Before the final, I put in a new spark plug so I could get a more accurate reading off of it.

However, in the final race my day went from bad to worse. On the first lap, I avoided some chaos on the outside of a series of fast flowy corners by staying to the inside on the exit of turn three. However, there was a kart to my left who’s front right tire was probably around my rear left bumper area. I guess one of the guys who went off rejoined the track unsafely and the guy to my left had to swerve to avoid him. This put him into my rear left and all of a sudden, I had a kart driving up on top of my left shoulder and side… We went off the track into the grass and that jolt finally pushed him off of me. Somehow I kept it straight in the grass and managed to get back to the track. However, almost immediately there was something wrong with my engine. It was doing the same thing…no revs on the straights and stuttering on the exits. I came in for a DNF.

I have no idea what was going on with my engine. I checked the spark plug and it looked fine. Normal I think? The center electrode was on the white side and there was some soot on the outside of the four triangular fins around it. Their points seemed about the same as the electrode. I was hit on the left side so I cant imagine it being related to the crash. My axle wasnt bent, my brakes were not dragging, my chain, sprocket and clutch all look fine. There is nothing wrong with the rear of the kart. Head bolt torques are all fine, valve lash is fine. I have a dual sided catch can for the vent and carb overflow so it isnt back-pressure. One unique symptom I noticed was that it seemed to hesitate and studder the most throttling out of the first right hander after two high speed flat out corners with longer straights.

Here is what I am thinking and I’d love some help thinking through these symptoms and seek advice on what to do next:

  • Could be an issue with fuel getting to the carb - fuel pump, fuel pickup, fuel filter, etc. Nothing looks wrong with these components but I am noticing bubbles in the fuel line. Is that normal? Should I replace the pump, filter and pickup? I run my tank lower on fuel for weight reasons but I have always done that. Fuel levels were not any lower than I normally run. I also would imagine if the fuel sloshing around starved the carb in corners, it would stabilize on the long straights and rev fine until the next corner. It also wouldnt make the kart fail to accelerate after coming into the pits to check on things while it idled stationary.

  • I know the different between the engine low on revs on exit due to chassis binding and that isnt it. I put my foot down and there is just nothing there, it studders, then it begins to rev again but nowhere near what it should rev to.

  • Should I just buy a new carb to remove that from the equation?

  • The lower end of this engine is old. Unknown age exactly (bought used with the kart) but its head didnt have the cooling fin if that says anything. It has orange seals but old style cables. When I pulled the head to replace it, I noticed a ton of carbon build up on the piston head. Should I just get a new engine at this point?

  • The fact that it seems to run fine on the stand (other than the surging) and then starts to have issues a few laps in tells me it has something to do with heat and fuel. But the fact that it ran fine in two practice sessions, then had issues in Qualifying, ran fine in Heat 2 and then had issues in the Final, just makes no sense to me. I also don’t know what to think about the surging. I checked for leaks in the carb and intake gaskets too.

Sorry for the wall of text! Just figured I would get ahead of as many probably follow up questions as I could. I have really tried to think through all of this as best as I can!

Ugh, I’ve chased similar issues and it’s not fun. In my case I binned the (Warlbro) carb that was on my $1000 kart and it seemed to solve it… Until the float jammed, but that’s another story…

I agree that your issue likely heat soak related. Riddle me this… does the RPM on your datalogger show any irregularity when the weird behavior starts?

I have not had a chance to download the data from the Mychron to take a look. I’ll do that tonight and report back.

What is super hard about this situation is that so many factors are at play (new head, old lower, old carb, high heat, crashes, sessions that went well, sessions that DNFd, etc. Hard to sift through the noise to isolate the root cause.

From what I remember hearing, the loss of revs happened slowly. I then realized that I wasnt getting full revs on the straights and then shortly after that it started almost dying out of a few slower corners.

Do you happen to know what the flow rate of fuel is on the 206? Like is the fuel line from the tank to the carb cycling every second? handful of seconds? Knowing that might inform where the issue might actually be occurring (in the corner it hesitates out of or in the previous corner for example).

Oh one more data point: The engine still feels like it has normal compression. TDC is easy to find and there is a strong “pop” feeling when it ticked up into TDC position. So, I am pretty sure the head isnt leaking anywhere (gasket or valves). I think that also says the piston rings are not leaking significantly either.

Sorry if you mentioned this somewhere and I missed it but do you have a picture of your catch can setup? I’ve seen countless issues come from just not have the proper ventilation there. It seems like you have gone through a lot of effort here so I don’t think it would be something that small, but worth a look (I see now you mentioned you’ve checked there)

Not an expert, but I like to learn things the hard way… I’d agree it sounds fuel or carb related. Is your float inlet needle assembly in good shape? I had one that the springy thing got sticky and gave me fits.

I thought you said you checked out your fuel system too, but have you checked out the vent line for the tank? Kind of a “no brainer” system that gets overlooked sometimes (ask me how I know this lol). Even a little restriction on that line can mess with you. This might be a reach too, but is there any way a line or something is getting pinched with you in the kart or while driving that wouldn’t show on the stand?

Hope you get it sorted out!

I was just about to say this. Catch can issues potentially.

This is the catch can I use. I have used it for over a year with no issues so I really don’t think this is the issue. The side the valve cover vent goes to is entirely separate from the side the carb overflow goes into.

This is an interesting theory. I did not make any changes to this. Its a short length of fuel line up to a standard plastic catch can attached to the top driver panel support. How restricted would this need to be for this to be causing issues? Like a tight bend or a pinched off line?

When I used this one I actually drilled more holes into it. But if you’ve had no prior issues than it’s probably something to rule out then.

Two things, first make sure the gasket between the two sides of your catch can is functional so there is not crossover from one side to the other, second, make sure your intake manifold is not bent from a previous accident. Usually some carb cleaner or brake clean sprayed at the gasket/oring will increase the idle and indicate a leak.


Sorry, I misread one of the posts above!

Both items appear intact and not leaking. I’ve ran the catch can many many times for over a year. I am confident it isnt related to that. The intake gaskets are all new and the bolts are snugged up appropriately. Nothing is bent there either.

Okay, so I pulled my carb apart last night and did not find anything indicative of being the source of the problems. The float height was still exactly 0.870", float drop was to spec (1.050") and the inlet needle isnt binding. There was no obstructions found in the emulsion tube, jet or idle jet. Confirmed the choke mechanism is functioning properly and I run a lever spring so it can’t be rattling open (I guess “closed” technically). The bowl gasket and the manifold gasket are in good shape, torqued down appropriately too. Carb was level too. There was no significant amount of fuel in the catch can. No significant amount of oil in the valve cover breather side either.

I also confirmed that there is a great amount of compression in the engine. Turning it over slowly, there are prominent “bumps” ticking over to TDC. No indication of an exhaust leak around the gasket (no soot or anything obvious). The spark plug was tight, cable pressed on fully and no nicks or issues found on the spark plug wire.

I drained the fuel tank and inspected the pickup tube. It is in perfect condition. If it helps, the weighted end of the pickup sits at the bottom of the tank and rests pointing to the right side of the kart. Fuel filter looks clean and no obvious obstructions. It is also the filter I have used for awhile now without issue. The only thing I did notice was perhaps a small crack at the top of the fuel pump. I’ll upload a photo shortly. It is a small ~1cm section of the outside case at the top. I don’t see any signs of fuel leaking from there and no suction could be felt while cycling the engine.

This is the only thing out of normal I could find. No idea when it happened or if it matters.

You can turn the pump around a little to have the pulse line real short and direct. I know some guys preach the short line (definitely not the answer just showing another way)

I also have read some people have issues with their coil did you check that out at all?

Kind of OT but that Tygon fuel line is worth every penny over the standard fuel line :drooling_face:


Random thought but I’ve seen it happen - check the carburetor plenum between the carb and the head. Sometimes these warp, get bent, or don’t mate to one surface quite right and can cause the issues you’re describing.

It’s a simple part but could be the thing causing the issue.

Some people also call this the “carb spacer”

Often they get bent in transit or when someone drives over the engine etc.

Good thought, and to add to that, get the engine running, and press/pull on the carb and see if that effects the idle. If there is a tiny crack in the plenum that might be letting air in when things flex. I had a similar issue when my son raced box stock honda. Couldn’t get the engine to run smooth on track, but it idled fine. Eventually discovered a crack in the phenolic spacer between the carb and head. Once the kart was on track and things were bouncing around, the crack let air in and ruined the air/fuel mix.

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