389 or 376

[dansofield550]

I have the 376 carb fitted to my bike, engine number is 14**** csr so guess it should have a 386, my question is will the bigger carb make much difference? Will it be better? So far I get good mpg (85) and a fair speed from the old thing however I have a persistent flat spot from just off closed throttle and wonder is it the small carb doing it, all jets are by the book so far.

[Ozmadman]

Pilot jet??

[dansofield550]

I thought so too, have fitted bigger and smaller to no avail,

[Rob Harknett]

mod 14 was 376/99 180 main jet 25 pilot, 14csr398/82 200 main 20 pilot, ( most years )
but needle position, slide type, and bore should also be considered.

[dansofield550]

I have the needle set second groove from the top. Slide is 3 1/2 , is the slide dia the same for 389 and 376?

[Rob Harknett]

Ah needle position is wrong, should be 3rd central notch, assuming you have no airfilter fitted. neadle jet 106. 3 1/2 slide is OK. If you also have the jets I mentioned, you have everything OK for a 376 carb. except the needle position. Expect to find a marked improvement with the needle in the correct position, which is 3, central, in almost all carbs unless an air filter has been fitted. Also expect a drop in MPG. The 376 is 1 1/16" bore, the 389 1 1/8" bore, it would have jets mentioned earlier also a 3 cut away slide, due to the slightly larger bore. PS I noticed earlier I miss typed 398 should be 389.
The CSR should have the 389 carb, but the 376 should be OK if it has correct internals and settings

[iansoady]

In general, a smaller bore will give less top end but will have faster gas flow at low speed so will be more "rideable".

[dansofield550]

Hello rob I do have a pod type filter fitted, the needle position has been in all slots lately and to be honest I've not noticed any difference to the problem of lazy pick up from closed throttle, I would love to fix it as its spoiling the ride and I use it all week 25 miles a day, I did wonder if they're like it anyway and I'm expecting to much from an old carb, almost all parts in the carb are new with exception of the needle, always a guessing game with old unknown origin or parts I guess, I couldn't see a letter on the needle where would it be on the needle? Anyone know? Thanks Dan

[Rob Harknett]

There may be nothing wrong with the bike. I have lightweights, they are not that powerful, in fact my 1936 prewar 250 would beat them easy. Do not expect twist and go reaction from them, they need to be coxed along. A modern 50 cc scooter may beat it, but get caught up eventually. I have 3 of them, 2 fitted with ace bars, fun to ride along the country lanes where their top speed is all you'll really want.

[dansofield550]

Do you know if the parts would all fit from a 376 to 389 as theres a 389 body going cheap on eBay at the moment, I was surprised by my bikes speed I managed down the a2 down hill from Bluewater in Kent 85 on the Speedo, on knobblies it was interesting for sure, cruising at 65 is easy for it,