IAME / Tillotson HW33A Carb question

In rebuilding my KA carbs this season I have noticed that one of the flappers in the upper fuel pump diaphragm is becoming deformed. Most recently I had difficultly starting and found this.



AS you can see the one flapper is deformed to the point where it is likely not sealing off the hole it is intended to cover. What I don’t know is why or what effect if any thins has. Anyone else have this issue? Solutions?

Was this a one time thing? (defect from the start) Or has this happened before? What does the machined sealing surface of the carb section look like?

How long ago did the carb have a new gasket kit installed and the type of fuel and oil used? I put a new carb kit in before each race weekend.

This has been observed on previous rebuilds on 2 separate carbs (my son and I race in the same class). In both cases the machined surface is smooth with no defects.

While this particular diaphragm has been in use for 4 or 5 race weekends I have noted this happening in shorter rebuild intervals like 2 races.

We run 110 Sunoco with Elf 909 oil

This year we’ve run VP98 fuel with ELF HTX oil and have not seen this. Assuming that everyone else in your class runs the same mixture, is anyone else having this issue?

I haven’t asked, but that is a good idea. AS far as I know, we all run the same fuel, with several different oils. I used Motul at the beginning of the season and had the same issue so I don’t think thats it. I also have used non-oem gasket sets and it was actually worse. The OEM Tillotson is what is pictured.

James McMahonKartingIsLifeFounder

Aug 16

Most of the racing fuels used in karting in the US contain lead. It definitely helps reduce wear as it acts as a dry lubricant for bearings, cylinder, piston etc. It’s also a neurotoxin, so there’s that too.

I should add that Sunoco 110 had a formulation change a little over a year ago that resulted in it being no longer recommended for two strokes. That information is buried on Facebook somewhere unfortunately.

What’s the basis of pump fuel vs race fuel jetting difference? Specific gravity or something else?https://forums.kartpulse.com/t/sunoco-standard-110-race-fuel-vs-vp-110-racing-fuel/5979/23

Checking the Sunoco fuel selector, 110 doesn’t show up. Try changing fuel, easy to do.

Are you using Sunoco Standard Leaded? (110 octane)?

Yes. This and a lower octane 95 (I think) are the two sold at the track. I posed the question to my fellow karters at the track and those with IAME / Tillotson carbs have seen this same situation, but not to the degree noted in my case.

Everyone is pointing at the fuel but I am also suspicious as to what force caused this to deform (vacuum or pressure). My pop off is at 10psi.

The mention of Sunoco Standard not being recommended for 2 strokes is a bit puzzling. The online information mentions karting as an application, but not 2 or 4 stroke karting. However, the 95 Optima does mention easy blending with 2 stroke oils whereas the Standard does not mention that.

As a reference, our pit group (and the club) all run VP MS98 L with Motul 2T and haven’t seen any issues like what you are experiencing nor have I heard of any others having issues running this fuel and oil.

I don’t think the pop off would be the issue, but mine is around 12 -14psi. I’ve never seen anything like that, and I’ve gone up to 8 race sessions on a set before rebuild. This year, we ran MS98L with ELF 909, but I have also ran C12 with ELF or Motul, with no issue.