Newbie Tillotson Question

Hey New on this forum but I figured this would be the right place to get some real world advice on my Tillostson HL-334b Carb. Full transparency I have this mounted on a 2stroke50cc vintage french racing engine on a Peugeot Moped. The issue I am having is that when I get the jest dialed in so that the bike runs good and gets on the pipe, head temps are fine at sustained WOT but when I let off the gas and the trottle butterfly closes down the temps jump way up by 35-40degrees into dangerous territory before they settle back down again and this happens fast. I haven’t noticed this with traditional carbs like Dellorto when they are properly jetted. Currently I have the Carb with the idle speed screw set at 1 ful turn in from where it starts to make contact with the lever, pop off is 10psi I am considering runnibg it higher to possibly get more adjustability out of the mixture screws. Right now with an airfilter in place the low circuit is barely over a half turn out to get proper Idle, the high speed screw is just shy of one turn out. Any Ideas about why the temps are spiking so much when the throttle butterfly closes? And any Ideas of how to minimize that would be hugely appreciated.

I will take a shot at this as I don’t have experience with that specific carb or motor, but when you say the temp goes way up, is this 35-40 degrees C? If its F I would think that is insignificant. Either way, I would say open the low needle as that would be the only circuit flowing without the butterfly open. If you are racing, idle shouldn’t be a concern (at least its not in a kart).

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I agree with Bob, sounds like its running lean off throttle. Open the Low Circuit to just below 1 full turn (1-2:30 on a clock face). Also, where are you measuring temps?

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Thanks for the replies here, Robert and Greg. The temps I referenced are in
Farenheit. I am measuring temps at spark plug. When this bike is properly jetted it runs at 380-405f WOT so that little heat spike on deceleration is putting me up into a danger zone of 445F I know these temps seem high but this French motor was designed to run a little hotter then The Japanese engines. I did some more digging on racing tuners about tillotson carb settings and I found mentioned many times to back the idles speed screw out to where it just touches the throttle lever allowing it to close fully. I guess that acts like a choke activating the low circuit and an enriched mix for when you accelerate again. I am going to try this and also richening up the low circuit a tad. The only thing that sucks about this is that in this scenario if it works the bike won’t idle and I am riding it on the street. I guess I could manually crack the throttle at stop signs.

While this is true it doesn’t do you any good if the idle is so low it stalls. I would think you would be better served finding the setting that works for your situation. This may be a higher idle than you want but again you don’t want it stalling at every stop.

As for the temp the CHT is notoriously slow to respond so your temp spike might be on the high side too. I would check the spark plug at a high RPM cut and also be listening or looking for detonation. What fuel do you use?

Generally you should be able to get a Tillotson to idle, some even came with idle stops.

That said, if you find you’re having to choose between a reliable idle and the correct low jet screw setting… it might be worth considering lowering the pop off pressure to allow more fuel across the range with respect to signal/vacuum.