Thanks James, noted. I suspect I’ve been told that a few times before (notwithstanding my brain fade)
As to the pop-off pressure, I’m pretty sure the OEM inlet spring is a 42 grams, but will need to check. I know Peter has spoken to taking it up to 46/48 grams, but maybe that’s more for tight sprint tracks (?).
This may need an entirely new topic, but can someone please explain pop-off mechanics and settings (coming from someone who, until very recently, has only ever used a slide carburettor)?
Pop off is somewhat akin to float level in some ways. When the valve “pops”, it allows fuel into the metering chamber where the jet tapers and orifices are.
A lower pop off pressure means that this valve is opened at a lower vacuum. Ie, more fuel flows.
Higher pop off means that the demand/vacuum/fuel signal has to be higher before the inlet needle is opened to allow fuel in.
Noted, thank you. So what are the pros and cons? In other words, why has a higher weight inlet spring (and thus presumably a higher pop-off pressure) to say 46 grams helped with Peter’s problem above?
Understood! I never liked the Tillotson, especially in an application like this where you need progression and not an on-off switch. Nice choice chassis-wise! Later on as you burn through the black pads, try the reds as a comparison, you’ll love the brakes even more!
On the jetting my small piece of advice: when you have a new engine or carb with no parameters and you are going in completely blind…start super rich. Warm up engine on the stand in 1st until you read 115F. Once warm, accelerate to 10K RPM then from there full gas for a couple of seconds. In a KZ, you normally read around 15.5K rpm. If more, you are lean, if less, you are rich on the max. In your 175 that has a rev limiter, like I said start super rich, it may stall and run rough let’s say at 12K rpm. You then lean a little until you read more rpm…you keep doing that until you just reach 14K by the rev limiter without the engine sputtering. That setup should be roughly close to optimal.
If I recall, his issue was a common one, where the lower spring rate was popping when encountering certain lateral g-loads, switching between lean & rich conditions & messing up throttle response.
Noted Andy. Thank you.
Noted Spencer, thank you. And presumably, as Evan said above, if increasing the inlet spring weight, one needs to run richer via the needles (high only?) otherwise a lean issue awaits.
I realized I never got back with the results. Just like Peter, switching the 42g spring for a 46 or even 48 made a huge difference. The engine feels completely different especially in 2nd and third in low to moderate speed corners, the power is much more progressive and there is much less bogging. I agree that switching to a higher g spring is definitely worth it in my experience.
One thing to check now and then is the small O ring on the throttle shaft. I suspect worn O ring has negative impact on how the carb works. Tillotson starts to puke more and more fuel out from the right side of the carburetor if the O ring doesnt seal any more. Thats my observation
To check dissassemble throttle mechanism, take out locking plate, throttle plate and pull the throttle shaft out. You should feel resistance from the O ring. If its worn theres zero resistance
The O ring is something like 5x0.7mm so its really thin. Havent measured it. It doesnt have much material to compress - even when its new.
New oem spec O ring wont seal if the carburetor is old and the bore is worn. Machining the shaft groove a bit wider so it takes thicker O ring might make old carb to seal again.
On my motor new carb lasted about 100 liters of fuel until the O ring was done.
Mine started to work great after fitting 46gr spring. Before I was always making the low side too lean to make the engine run clean. By making the low side lean the engine seized, every time, right after the main straight - there was not enough fuel going in when I lifted the throttle
so let me understand this one…once you install a stronger spring, you are in fact limiting the fuel intake on both high and low circuits, or better said you need a stronger depression signal to open. That prevents the bogging, but also leans out by reducing fuel intake, which in turn you should fix by compensating with the screws to balance it out. The danger I see here is that at the end of the straight you may already be at the edge of starvation on the max circuit, as soon as you brake to downshift and engage the min circuit, you are out of fuel and stick…which is a different scenario than let’s say a single speed, because in a shifter you may stay at partial throttle for longer, as you brake/downshift
Notice what many do in KZ…loop the gas line above the carb, put an extra reservoir on top etc every trick to allow for a reserve of extra fuel ready for long corners at high G or after a long straight, in case for some reason the pump can’t cope. On a membrane carb by adding spring tension you are doing exactly the opposite, increasing the chance to starve the engine in order to solve bogging, as you don’t have a bowl/float system.
If it was me, I’d approach it this way: if you are limited by a regulation so you have to use the tillotson, then stick to it but it also means you have engine builders, teams etc running it so heavily rely on them for advice on setup. If not, then get a dellorto.
I happen to have an Aluminos with a 175 and tillotson from a famous racing team who ran them at national level sitting in my trailer, with the base setups written on it…I know specs are not the same, but would it help if I take a pic? Maybe as a reference?
Andy, back to my issue above, I’ve now been told it was a massive lean (not cold) seize. The strong guidance from my engine builder is to start with a full extra turn (60 mins) on the high speed needle to circa 2.45, and then work back from there. Also, not to touch the inlet spring for the moment (work with the base 42 grams). So, certainly consistent with your thoughts. 2.45 sounds super rich, but we’ll see what happens. Long circuit is certainly demanding on fuel (but perhaps not such an issue on the pop-off, but let’s see on the tight(er) turns).
Thank you Mikko. I’ll keep a close look out on that O ring risk area.
Hi, sorry it took me a while to read your comment, I’m very busy.
The 175 doesn’t come with the Dellorto anymore here in Europe, it’s been a while since they’ve been selling the engine with the Tillotson.
I do have a limiter (14150Rpm) I can’t imagine that engine at 17000Rpm, you would be rebuilding the engine constantly, and I don’t know anyone that doesn’t have the limiter, 14000Rpm is more than enough for sprint racing imo.
Hey!
If you buy the Super Shifter in Europe now, it will come with the PVL ignition, I replaced that one and I’m using Selletra, which is really nice!
And yes, they’ve been selling the engine with the Tillotson for the last two years here in Europe.
I have two radiators, winter and summer, small and big, if you use a big radiator during the winter, you’ll get cold water at the bottom of the engine, generally at the end of the longest straightaway and you’ll probably seize the engine.
My settings are 1:30 low, 1:45-1:50 high, more than 2:00 seems very rich for any sprint circuit that I know of, but for road racing maybe, I don’t know tbh.
I’m impressed you reached 140km/h at 12000Rpm, I normally get to 145km/h when I’m above 13500Rpm, in 6th gear of course.
EGT never above 680-690, 700 would be borderline I think, no need to run that lean imo.
I’m using Castrol 747, at 5%, 95 octane fuel, I’ve never used 98 octane.
Sorry for the seize, make sure you warm up the engine at 55 degrees before pushing, if it’s winter and it’s cold, below 20 degrees (Celsius) then wait till the water is at 58-60 degrees, the ambient temperature makes a difference, it’s not the same 55 degrees with 25 to 35 degrees of ambient temperature, than 55 degrees with 10-12 degrees of ambient temperature, a 175cc needs proper warming up, otherwise you’ll seize it, warm up accordingly and you’ll be fine, and try to keep the EGT at 680-690, occasionally 700 but I would not go above that, besides there’s no need, at least for sprint racing, the engine is a beast, although I wish it wasn’t so, so heavy.
I would say 48gr is perfect for any sprint racing, I don’t know anyone using a 42gr for sprint racing here in Europe, they should sell the Tillotson with the 48gr so divers, like me, don’t have to go crazy with that 42gr, it drove me nuts, the engine was bogging and…it was terrible, literally, I still have nightmares about it 
I’m glad you noticed the difference, it’s a complete different engine when you use a 48gr, I don’t know why the sell the Tillotson with the lazy 42gr
Hi Peter
Thank you for your comments.
I had a second run at Phillip Island over the weekend. This time I didn’t blow-up the engine (thank goodness), but it was un-drivable with the high speed needle set to 2.30 plus. I did back it off and the engine became somewhat better, but then the EGT was getting high - photo below; it went to 690C, so I backed off and came in.
All rather frustrating and I’m maybe fronting more cash for a Dellorto (36 mm, which we’re allowed to run) for long/road circuit running, but I don’t want to give up yet
Now, I know you’ve moved to increase the weight of the inlet spring (42 g to 48 g). I’m wondering if I should go the other way (lighter) in order to get more fuel in while at the same time I can lean out the high speed needle closer to the 1.50 to 2.00 mark. Any thoughts (or Andy)?
BTW, I was running a 19T engine sprocket and a 20T rear sprocket on the 175 SS. As a benchmark, my Stock Honda that day hit 180 km/h (112 mph) into T1, which we go flat through. Just ignore the times on the image below - the blasted Mychron is picking up the kart track next door, not the road circuit, so they are meaningless. And yes, I know the water temperature is a bit low, but it was impossible to get the temperature up (it seems I need to also get a smaller radiator).
I’d definitely advocate moving to a “softer” inlet spring. It’s harder to blow stuff up with more fuel vs less.
That’s a huge difference in top speed and the RPM looks low even for a 175. But it’s hard to say if it’s geared too tall, or engine is just not running right yet.
The closest I’ve ran to a road racing application was Battle at the Brickyard where we ran the following with the 175 SSE:
1.16 ratio (19/22)- just shy of 100mph in draft
2 turn + 15’ H
1 turn + 15’ L
1 turn for air screw
42g inlet spring
I would think on a track like Philip Island you’d be closer to a 1.00 ratio, and I would advise against going leaner. If in doubt, replace the carb kit and check for no leaks.
Also might be of interest, I’ve just uploaded the various gearcalc sheets I have here, it includes one for the 175 from IAME:
