Ac advice

[veg]

So I’ve started out buying various parts originally with the intention of building a flat tracker, a 250 ac bottom end then Lc top end in an ac 250 frame however I’m really starting to think about doing an ac special using the 250 engine, fitting 400 top end, is that possible? My 250 frame is an early one but like the Daytona bodywork again an easy fit or not possible?
Thanks in advance 👍

[yamark]

Hi Veg, as far as I know, the 400 cases are different from the A/C 250's.
The stroke is different on the 400's, so cranks, cases, cylinders and cylinder heads fit the 400's only.
That's my understanding- Dusty's your man on this- might be worth a PM

HTH, Mark 

[veg]

Yes wasn’t sure mark I’ll see if either dusty or Pete pop in, otherwise I’ll pm them.
Hope you’re good matey 👍

[dusty350]

Mark is correct - 400 crank wont drop in a 250 unfortunately. However, 350 ac barrels will go on a 250 bottom end, and wont be far off the 400, performance wise. Issue being, 350 ac barrels are hard to find for that very reason.
I think the Us tank will fit the Uk frame. Pete is the man that will know all the minor details for sure

Dusty

[yamark]

Nov 14, 2021 23:05:53 GMT 1 veg said:

Yes wasn’t sure mark I’ll see if either dusty or Pete pop in, otherwise I’ll pm them.
Hope you’re good matey 👍

Dusty to the rescue , I'm good mate, thanks.
Good luck with the project

[veg]

So it’s looking increasingly like getting a 400 motor or a Ypvs then. A pv leads me down a very different track. Cheers Mr Mod 😁👍

[steve h]

Ask Ron if he can 370 some 250 windy cyls.

[veg]

I’ve got a set of std bore Lc 250 barrels that I did think of giving him. Dusty posted an incredible looking ac with Daytona bodywork on that I keep revisiting.

[dusty350]

400 engines are like rocking horse poo. I looked for over a year for one for the Cafe build, and ended up going with the Ypvs engine which was a lot easier to source. That doesn't mean they dont come up though, and last one I saw was 1600 quid - pre rebuild Pv engines are the same money of course, but you will have the added cost of fabrication and welding for the engine and rad mounts that you wont incur with an ac motor. Not trying to dampen your enthusiasm - just pass on what I found doing mine. Ac stuff has gone ballistic, price wise, in the last 18 months and watching ebay is quite depressing for peeps wanting to build them. But, Ac specials and hybrids are a lot thinner on the ground than other models, and I think they look awesome when done well.



Lots of nice specials on Youtube for inspiration

Dusty

[veg]

Thanks bud
I’ve got the basis of a good bike, ac 250 frame and bottom end, Lc barrels newly built tdr wheels, tzr forks yokes etc, dresda box section swing arm etc it’s just what direction do I go in? Flat tracker, road bike? I’ll figure it out. Sounds as though my bottom end and barrels will help bankroll the motor if I go for a pv.

[reedpete]

Feel touched you guys think I should know something about this ! 😁
Most points already covered above..
Only things I’d add is that the latest spec aftermarket 350 barrels are very usable and can be very reasonably priced. Taking them directly to 1mm OS or 2mm OS isn’t the sacrilege it would be for OE barrels.
Conversely, taking the 250 barrels to +2mm (56mm) and building a screamer is also entertaining and not to be overlooked if you are trying to build something fun to a budget.

If you do still look at 400 based stuff then worth noting that c/d stuff is significantly cheaper and more available than e/f, but for no other reason than that’s what you need to restore the 2R9 models.

[ajh]

just my opinion but if you are used to LCs, then I would deffo go to a 350 or 400 engine. I found my ac 250 is just a little bit too underpowered and not as much fun alongside an LC, consequently I did something I didnt want to do and sold it! After an LC really need a good kick of a powerband to make the ACs fun

[reedpete]

Nov 15, 2021 10:48:44 GMT 1 ajh said:

just my opinion but if you are used to LCs, then I would deffo go to a 350 or 400 engine. I found my ac 250 is just a little bit too underpowered and not as much fun alongside an LC, consequently I did something I didnt want to do and sold it! After an LC really need a good kick of a powerband to make the ACs fun

agree a stock ac250 compared to LC or anything else just feels underpowered….but the point I was making was they can be livened up substantially and surprisingly with track tune and an overbore. The barrels are even still cheap enough that you can have a go yourself and follow the diy tuning guides from back in the day. Still plenty of rideability unless you are condemned to 100% surburban riding, but out on the B roads…usual RD grins per mile resume .
Of course a visit to Mick Abbey Etc will have best results and those type of guys have the nice tools that can put boysen and auxiliary exhaust ports where they should have been all along. Accompanied by matched pipes a hot AC 250 will embarrass standard tune bigger bikes easily…

[veg]

Pete you’re far too modest. Didn’t know you can get aftermarket 350 barrels? Interesting and interested.

[steve63]

Nov 15, 2021 12:42:41 GMT 1 veg said:

Pete you’re far too modest. Didn’t know you can get aftermarket 350 barrels? Interesting and interested.

A guy on the 2 Stroke forum bought a set or two for his 350B. I think they went well but believe he had issues with the liner moving as they were pressed in rather than cast around. It sounds from the above post like they may have sorted the quality issue out now though.

I rode  a 350B once and remember it pulled really well from very low down, something like 4 or 5,000rpm but it was all over by about 7,500. My LC would be just getting into the best part of the power by then. A 350 AC with some pipes and a bit of fettling would be nice and save a bit of money compared to a YPVS.

[ajh]

Nov 15, 2021 12:24:43 GMT 1 reedpete said:

Nov 15, 2021 10:48:44 GMT 1 ajh said:

just my opinion but if you are used to LCs, then I would deffo go to a 350 or 400 engine. I found my ac 250 is just a little bit too underpowered and not as much fun alongside an LC, consequently I did something I didnt want to do and sold it! After an LC really need a good kick of a powerband to make the ACs fun

agree a stock ac250 compared to LC or anything else just feels underpowered….but the point I was making was they can be livened up substantially and surprisingly with track tune and an overbore. The barrels are even still cheap enough that you can have a go yourself and follow the diy tuning guides from back in the day. Still plenty of rideability unless you are condemned to 100% surburban riding, but out on the B roads…usual RD grins per mile resume .
Of course a visit to Mick Abbey Etc will have best results and those type of guys have the nice tools that can put boysen and auxiliary exhaust ports where they should have been all along. Accompanied by matched pipes a hot AC 250 will embarrass standard tune bigger bikes easily…

youre bang on about the surburban riding. For me it was the lack of power below 6K. Once it was on the powerband, smiles all the way.
I'd of liked to try to squeeze more low down power/torque but not sure how realistic that would of been to get as much as I wanted. Would of liked to try but my AC was just a little bit too nice as standard to start hacking into

[dusty350]

Sidney81's aircooled build is based, at the mo around a 250e engine. I believe he had a tune on it, plus pipes, and had it dyno'ed, so it would be interesting to see what power it's making. He has a pair of ac 350 barrels to go on which should aid performance to get close to a similar spec 400. A lot of weight can be saved if building a special as the use of a lot of metal parts made the ac's a fair bit heavier than the Lc's. Add a modern ignition, carbs and pipes and it will be a lot livelier than a standard one
I do love the idea of an Lc top end though

Dusty

[reedpete]

Nov 15, 2021 12:42:41 GMT 1 veg said:

Didn’t know you can get aftermarket 350 barrels? Interesting and interested.

Yes, produced originally as spares for the Rajdoot RD350. Now selling via all the usual auction sites etc….but there is a long history of evolution. 
Bottom line is you can get the most recent and acceptable versions with ‘overcast’ liners for a price that’s relatively sensible (£175-200). 
But there are still loads of the earlier style out there and I guess the price of those will drop as people just want to get rid and dump them.
current shipping costs are making the delivery times and delivery costs to the uk a bit wobbly but there’s more than one way to skin a cat.
if it’s something you want to take a close look at then happy to chatter about the details.

[muttsnuts]

don't forget you need the cylinder heads for the 350, or the 250 heads machining out, plus the 350 heads use the B8HS/B9HS spark plugs (1/2" reach) rather than the B8ES/B9ES (3/4" reach) plugs that the 250 C/D/E/F and 400 C/D/E/F used, don't get it wrong otherwise you'll have a few issues lets say !

BTW - I happen to have a pair of 350 A/C cylinder heads that I could be persuaded to sell.......

[muttsnuts]

Also, I happen to know where there is a 400 water cooled engine....RD400F bottom end with modified 350LC barrels....... haven't decided if I am going to sell it though !

[reedpete]

Best heads are the (rare) 521 heads from the uk 350B, use the longer reach plug, more meat in the dome to aid heat transfer out to the fins. Next best is 250 C/D/E profiles to suit and with outer fin removed or left as is, depending on taste !