Piston port reed valves ... Atomic and MB

[corrado]

As there's been a few post recently re fitting aftermarket reed blocks to piston ported cylinders I thought I'd add these pictures on a separate thread rather than on one of the individual other threads. The pics show the MB shorty reed valve and the Atomic small and large manifolds

MB shorty v Atomic large [though suspect the Atomic is the small block version ...ie small block 125/150/175]


This shows the Atomic large V MB shorty.


This shows the Atomic large v Atomic small [big block version of the small .... ie 200cc +]


This shows the MB shorty


This shows the Atomic large


This shows the Atomic small


The MB shorty shown is the old version, there is a newer version that has a different carb mounting, instead of the one piece rubber it now has an alloy plate fitted and is fitted with the bolt on rubber adapter that comes in various sizes to suit different carbs. Though this is immaterial to the point of the thread.
This is the new set-up..


Both the MB shorty and the Atomic large require removing a fair bit of fin area from the cylinder you're fitting it to. I've got the Atomic small version fitted to my 225 Mugello and hardly removed any fin area, though I did remove some material from the manifold itself.

The MB one and the Atomic small both angle the air flow in a fairly straight formation into the cylinder whereas the Atomic large seems to direct at least 50% of the flow into the solid area at the rear of the manifold which may or may not obstruct the airflow, merely a semi educated guess as I've not got sufficient knowledge to say that for a fact.
At the time of fitting mine I didn't want to go to the extra expense of the MB one over the Atomic one and the removal of fins and the perceived poor air flow made me choose to fit the smaller Atomic one.

[CHRIS in MARGATE]

Very comprehensive and I for one struggling to make my mind up find it really useful.
Can I clarify: The 200 stud spacing Mini Atomic has a larger inlet passage, is that right ?
What is the internal rubber size that fits over the carb spigot ?

Great info Martin.

[corrado]

Hi Chris, I didn't have a large Atomic reed manifold for big blocks available which makes the pics slightly mis-leading, I suspect [from memory] that they'll both have the same sized opening [where it fits onto the cylinder]. My point was that there seemed little point in fitting the larger one if the air flow was no better than the small one. Also the small one accepts different sized carb rubbers like the updated MB one. The Atomic rubber is designed to fit the PWK type carbs but PHBH carbs fit too, although the PHBH is 1mm wider. From memory think the PWK is 35mm and PHBH 36mm, though check those figures.

Pre reed valve I was trying to get my Mugello 225 to run right with a TM24 Mikuni carb but leaned it off too much and holed the piston. I didn't have a replacement piston but I did have a spare TS1 piston [Pro-Porting I think] so I thought I'd try fitting the reed block. My tuning skills are less than basic, I'm ok at matching stuff up but I'm not up to making a boost port or touching inner ports as I wouldn't know how or why I was doing it. Inlet / exhaust duration etc figures go above my head. So basically I fitted a TS1 piston to my cylinder without touching the cylinder apart from matching up the inlet manifold. I was told it should run ok but might loose a bit of top end but gain a bit of low down power. Deciding not to continue with the Mikuni I tried [against my better nature] to try a 26mm PHBH, but struggled to get it to run clean low down. After lots of fecking about thinking I had an air leak somewhere I stripped the top end down and found the crank was out of line so rebuilt it with a new expensive crank. I was told that I probably didn't need the TS1 piston and that another Mugello piston would have worked but with a half moon shape on the piston skirt on the inlet side, so in for a penny I added the half moon to the piston skirt too. After rebuilding it I could get it started and was convinced that it was the PHBH carb that was the problem so I took the Mikuni TMX27 off my Imola engine and fitted that on it. Result. Yet again the Mikuni is spot on throughout the rev range and the engine was transformed. Fitted with ADS3 [the torquey one] exhaust and Li150 gearbox with 4.7 final drive it pulls up any incline in top. Tempted to go taller [just to see what it'll pull] but also thinking don't fix what isn't broken. On a dyno it showed just shy of 21 bhp and 19 lbs ft torque. Best engine I've ever run, it'll sit happily at 60 on the motorway with plenty left for overtaking. I've no speedo so no idea of top speed [the 60 mph is based on overtaking HGV that are electronically governed at 56 mph]. Despite the exhaust it shows no signs of hitting any wall and has plenty of rev on capability. Probably won't top out a TS1 but when the TS1 has to drop into 3rd on a hill I'll still be accelerating in top. Well I would have if my main bearing track hadn't decided to spin on my expensive crank [subsequently replaced by dealer but no faith in using the replacement], 58 x 110 btw. So it's currently in bits again. When it was running it had one failing and that is spit-back, lots of it even though it does 55 mpg. Not sure whether to live with it or get the porting checked out by someone who knows what they're doing .. boost port maybe ... it's that don't fix what isn't broken mentality kicking in again [forgetting the fact that it is currently broken]. Fitment wise the issue I had was with the manifold contacting the frame. I filed, refitted, filed, refitted until I filed through to thin air, sorted that with JB weld until it just clears. Fin wise I think I had to make a fin thinner but didn't have to remove any as I found I could remove excess thickness from the manifold in the contact area.
If I didn't spend so much time on computers I'd get my a**e in gear and rebuild it, no doubt with taller gearing, and I'm still tempted to retry the Mikuni TM24.

[Warkton Tornado No.1]

This is a great ‘Post’ & I agree with CHRIS in MARGATE in that you have been very comprehensive with your initiative. Thank you.

As I understand it, there is more to reed valves than meet the eye. You mention the airflow between the valve & the rest of the manifold. The flow dynamics immediately after the valve is something difficult to assess, I suppose, without the knowledge or equipment to do so. My awareness is limited to tuner’s attempts to optimise the function by ensuring that the cross sectional areas are not drastically different as the inlet charge gets from the carburetor to the cylinder. Hence ‘stuffers’ exist in many instances. It would appear ‘bigger’ is not always ‘better’ if it means the inlet tract is like an over-inflated inner tube with a bulge...

Having got this far with the worthy comparison of what looks like a new, more compact configuration of reed valve manifolds compared to those long, up-draught efforts of old, it might serve some purpose if they could be further analysed.

Back to back tests on a dyno may spring to mind, but, there may be a computer program that may simulate the flow, in which case the manifolds would need modelling if none exists.

Sorry if this all sounds like gibberish, but hopefully you won’t object to my response. I’m just trying to open up what should be a healthy debate by throwing in a few thoughts even if I don’t have the answers!

[Norrie Bodge]

There's also the LTH version manifold too?

[corrado]

The LTH is more or less the same as smaller Atomic, I think one "borrowed" the idea from the other. Early LTH ones were CNC machined and latter ones cast, presume the CNC was too expensive.

I've not it here to check but I think I may have used the reed out of the LTH in my Atomic manifold. Think I may have swapped them over to see if the carbon petals in the LTH one helped with the spit-back, alas they didn't.

[coaster]

Fantastic write up Martin, plenty to think about there. two questions, does the smaller Atomic have a smaller reed block than the Ultimate one and if so, how do they both compare with the reed block that MB use (Polini?)

Ta

Colin

[CHRIS in MARGATE]

Martin you've started something here and I think there is a wide audience. Frame clearance worries me which is why I was trying to gather as much info as possible before committing. I want to do it in the next week or so before Shipston if I can be confident of the fitting issues.

Chris

[skinnerwill]

i fitted a LTH reed to my small block. i had to remove a fin and i also found that carb was fouling on cowling. i made a couple of 5mm ally spacers to help with problem, one to lift it higher and one to bring it out a little further.
i now dont suffer any spit back at all.
crank is 60 x 110, rd 350lc piston,JL3, PHBH 30mm, i also ground out a boost port and opened up inlet to almost cover both windows in pistons. seems to pull well especially low down but havnt opened it up yet as running in. initially just used a 175 cylinder head but dome shape of pistons is different, i have just had one re-profiled at chisel but not yet fitted it. bike is now going for paint so be a while before i know if im truly happy with it

[corrado]

I wish that I'd have realised that the RS ones had different names, ie, Mini and Ultimate, it'd have made life easier.

Colin, the Ultimate has a full sized RD reed block, the Mini has [wait for it] a mini reed, the same size as the LTH, though LTH do at least 2 versions. The MB one is about 25% bigger than the mini one but still a fair bit smaller than the RD block. Mini has a single [looks like fibreglass] petal on each side. The LTH one I used is carbon and has 2 flaps on each petal, though it probably is one single petal. The MB one has 3 flaps on each side. None of them are W type they're all V type.

Chris I'd say that the LTH one sits at least 2mm lower than the Mini RS one so a better bet if you're worried about frame clearance. The MB one is more angled so imagine that frame clearance is better than all the others. As you know frames and fit vary so much from frame to frame so clearance on one frame doesn't guarantee clearance on another.

[a.lo]

the mini RS and the LTH use the same reed block, minarelli horizontal,they are widely available in different materials and prices

[coaster]

I had my MB 'shorty' fitted to an SR225 Ally barrel (110 rod so 3mmpacker fitted) in my Series 80 Jet and had no frame clearance problems at all. Just wish I could fit it to the Avanti but it's said they can't be fitted Perhaps I'll offer it up at the weekend and see if I can't see what the issue is.

[corrado]

I'm using a Stage 6 carbon reed block from an LTH kit in my mini Atomic manifold.

[Warkton Tornado No.1]

The fact that you’re getting spit-back is, obviously, the fly in the ointment & it will be something that I’m confident you will resolve.

Surely, it can only be a minor failure in some way for the reeds to seal which is surprising given that they are the smaller, more responsive type than you may have otherwise chosen

I have read on other two-stroke articles that carbon reeds are not necessarily the best option, but you no doubt are aware of that