HL360 modifications

Both the 360A and the 355 has a low side needle hole that is below the level of the bottom of the L.S. well, it's gonna get very thin in that area when you bore any bigger than the 1.060 throttle bore they have now along with the floor of the well, so if converting a 360a to alky after increasing throttle bore you'd have to be very careful not to go any deeper into the well than necessary so you don't break out into the bore. If you can get 1.125 shutter in there, you'd what to go with about a 1.00 maybe a 1.025 venturi at most so you have enough of a drop to the T. bore to have a strong venturi effect so you have a good signal for the high side nozzle. For maximum airflow you would want to reshape the radius in front of the venturi after boring to narrow the band created when you bore it to 1.00. Jon
 

arc100

Member
Both the 360A and the 355 has a low side needle hole that is below the level of the bottom of the L.S. well, it's gonna get very thin in that area when you bore any bigger than the 1.060 throttle bore they have now along with the floor of the well, so if converting a 360a to alky after increasing throttle bore you'd have to be very careful not to go any deeper into the well than necessary so you don't break out into the bore. If you can get 1.125 shutter in there, you'd what to go with about a 1.00 maybe a 1.025 venturi at most so you have enough of a drop to the T. bore to have a strong venturi effect so you have a good signal for the high side nozzle. For maximum airflow you would want to reshape the radius in front of the venturi after boring to narrow the band created when you bore it to 1.00. Jon
I'm not converting to alky, they are for gas. I was hoping to go to a 28mm venturi but this seems impossible. Any other ideas, other carbs?
 
If you can find an HL 380 gasser , you should be able to bore it to 28mm but then you will need to bore the throttle bore up to about 30mm. You will need to make a shutter for that size., an HR 181 has a 27.5 mm venturi with a 33mm throttle bore, an HR 191 has a right at 30 mm venturi with a 34 mm t. bore
 
If you can find an HL 380 gasser , you should be able to bore it to 28mm but then you will need to bore the throttle bore up to about 30mm. You will need to make a shutter for that size., an HR 181 has a 27.5 mm venturi with a 33mm throttle bore, an HR 191 has a right at 30 mm venturi with a 34 mm t. bore
Yea what he said ! Jon's tha man for tillys.
 

azrcr

New member
Jon, please check me if I mis speak, sometimes I can't put what i'm thinking in words too well, typically a small venturi/tb ratio will tend to work for more WOT applications, such as road racing, ( you will lose fuel signal, in the lower/mid ranges).

On the other hand a larger venturi/tb ratio will gain you back the fuel signal on the low/mid range, with the addition of a attomizer ring, you will have additional mid/high gains.. (short dirt track or sprint racing) In addition you will gain overall by allowing you to use a larger overall carb size.

A good example is a 181 is preferred on small tracks where most prefer a 191 on bigger tracks..

I doubt a 380 with a 30mm t/b and 28mm is going to be much good on a small track..

A stock 360a has a 3mm ratio, a stock 181 has a 5mm ratio., A 380 has a 4mm ratio, We have been doing some experimenting for 5-6 years, and found you can over carb the engine if you increase the venturi/tb ratio.

If the engine can handle a 380, then it can handle a 181 with a larger venturi/tb ratio.. How much we go I'd rather not say, our sizes might not work for others apllications..

Bernoulli law is a good theory to check out.. one thing to remember is the low side metering holes will have to be redone to augment the better fuel signal. you'll know they're big enough when you don't run lean with the low side needle 2 turns out.. (ask me how I know this!!!)

Mike C.
 

azrcr

New member
I might add, for 100cc opens, and 116's I prefer the dual 360c's (which you can't get anymore) they also work well on mild sudam opens, where the dual 360a's work better with the 131-150cc opens as well as the dual 380's..

I use the 348's for a replacement, enlarging the throttle bore and using a 360a shutter.. (basically, making a 360c..)
 

PD Power

New member
Good work AZRCR.
That type of carb work gets to the heart of making one of these piston engines to perform.
 
Mike , I feel like the 181 is preferred over the 191 on the shorter tighter circuit's not because the throttle bore to venturi ratio is greater, but because the venturi is .100 smaller keeping velocity up all across the rpm range. It's interesting that when you look at actual T.bore to V. ratio's , Tillotson's largest ratio is in it's smallest modern carb the 357. Even though the 357 , the 348, 360A and the 380 all have a 3mm difference in the V. and T. bore, their sizes are different so the ratio's are not the same. The 357 has a 1.3 to 1 ratio, the 348 has a 1.2 to 1 ratio, the 360a and the 380 very close to a 1.12 to 1 ratio. The HR181 has the same T.bore to V. ratio as the 348, 1.2 to 1 and the 380 and the HR 191 share the same ratio as well with a 1.12. The 360C you talked about has a slightly larger 1.22 ratio. I believe that as long as the throttle bore can flow more than the venturi, with the butterfly and throttle shaft in place, that being any bigger is not necessarily an advantage. It would depend on what you are dumping into downstream of the butterfly. If your feeding a big volume reed cage then a larger ratio would be good , but if your feeding a short manifold that tapers down to a small port then being larger there can be counterproductive. On a side note a WB3a carb has only a 1.2mm drop to the T.bore for a ratio of 1.05 to one and we can't really say it only works at WOT. As far as that bored 380 goes , I'd prefer to have a bigger ratio but he is limited to how far he can go without seeing daylight, whether it will work good on a short track depends on what he's gonna put it on. Jon
 

arc100

Member
Mike , I feel like the 181 is preferred over the 191 on the shorter tighter circuit's not because the throttle bore to venturi ratio is greater, but because the venturi is .100 smaller keeping velocity up all across the rpm range. It's interesting that when you look at actual T.bore to V. ratio's , Tillotson's largest ratio is in it's smallest modern carb the 357. Even though the 357 , the 348, 360A and the 380 all have a 3mm difference in the V. and T. bore, their sizes are different so the ratio's are not the same. The 357 has a 1.3 to 1 ratio, the 348 has a 1.2 to 1 ratio, the 360a and the 380 very close to a 1.12 to 1 ratio. The HR181 has the same T.bore to V. ratio as the 348, 1.2 to 1 and the 380 and the HR 191 share the same ratio as well with a 1.12. The 360C you talked about has a slightly larger 1.22 ratio. I believe that as long as the throttle bore can flow more than the venturi, with the butterfly and throttle shaft in place, that being any bigger is not necessarily an advantage. It would depend on what you are dumping into downstream of the butterfly. If your feeding a big volume reed cage then a larger ratio would be good , but if your feeding a short manifold that tapers down to a small port then being larger there can be counterproductive. On a side note a WB3a carb has only a 1.2mm drop to the T.bore for a ratio of 1.05 to one and we can't really say it only works at WOT. As far as that bored 380 goes , I'd prefer to have a bigger ratio but he is limited to how far he can go without seeing daylight, whether it will work good on a short track depends on what he's gonna put it on. Jon

Its for 100cc open reedjet type engine running on gas. No alky allowed and only single throat carb. Got a Tilly HW-12a (V30/TB34) coming so will try that in the next few weeks as well.
 
ARC , the 380 would work, but the big carb will be better. Work with the HW if your not happy with it , find you an HR181 or 191. Jon
 

arc100

Member
ARC , the 380 would work, but the big carb will be better. Work with the HW if your not happy with it , find you an HR181 or 191. Jon
I think the HW-12a will be ok, its about the sizes that were used in FSA, I was just hoping to find a carb that didn't cost $350. However it should be just a straight bolt up item with no modifications needed so I guess its a time saver if it works out.
 
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