Moving up to 38mm carbs, which manifolds should I use

[Dvsrd]

 This thread makes me wonder how much is gained by using larger carburettors on RDs.
As far as I understand, one of the main limitations/restrictions in all the 64 mm bore RD350/ 400/LC/YPVS engines is the transfer ports/ ducts. This due to the fairly tight bore spacing.
Next restriction would be the reed valve cavity. It can only be widened so much without a major effort.
So where does carb size become a limiting factor? After all, the RZ had well over 50 rwhp with stock 26mm (?) carbs. And some claim up towards 70 hp with 30 or 32 mm carbs.

I do realize the TZ350F came with 38 mm carbs, but had no reed valves.

[m in sc]

up high it matters, i know that there was a guy int he uk that pulled 70 hp out of an r5 with 28s. saw the vid of the dyno run, it was VERY radical and ported to the moon, and did not last long.

however, i can definitely feel the difference between 30mm and the 34mm carbs. There have been times where i have considered going down to 32's or 30s again, but if they are tuned right the large carbs work really well IF the mods support it, however, typically with minor mods, slapping 34s on a stock motor with chambers will make it pretty terrible for street riding.   

That being said, hes doing a track bike, so the mods need to be there as well, and the main theme after we all figured out what reed he actually had was: go with reeds that flow more.

[Jspooner]

John is a little over an hour from me.  I forgot to mention, the new chambers going on are Swarbricks.  Lyn was nice enough to sell me a set he had for an RD400.  I just need to weld on the mounting tabs and they are ready to go.

Rob

You'll be happy with those Swarbricks.

[1976RD400C]

The 34s work real well on my bike that has the modified TZ cages and transfer port work done. I'm surprised how they can be jetted right on. If you get it up on the pipe holding it at 3/4 throttle and then open it up you notice it for sure. It's like a 4 barrel carb kicking in on a V8. A have a stock RZ350 that seems to run as it should, revs up to 10000, power valve working good, and the RD accelerates faster.

[Plasticman]

This thread makes me wonder how much is gained by using larger carburettors on RDs.
As far as I understand, one of the main limitations/restrictions in all the 64 mm bore RD350/ 400/LC/YPVS engines is the transfer ports/ ducts. This due to the fairly tight bore spacing.
Next restriction would be the reed valve cavity. It can only be widened so much without a major effort.
So where does carb size become a limiting factor? After all, the RZ had well over 50 rwhp with stock 26mm (?) carbs. And some claim up towards 70 hp with 30 or 32 mm carbs.

I do realize the TZ350F came with 38 mm carbs, but had no reed valves.

Keep in mind, what I'm putting together is a race only machine.  I'm hoping it spends a LOT of time in the middle and upper end of the RPM range.  If I was building something for the street then I would stay around a carb size of 30-34mm.  But the other RD's I'm racing against are running 38's and have engines built by Harry Barlow.  Once this upgrade is complete and tested, I fully expect fuel supply and traction to be the next two hurdles to overcome.

Rob

[1976RD400C]

Are the brakes nice and strong on this bike that is going to carburate it's way to the lead?   

[teazer]

I would suggest that you look at any reed cage set up you might be tempted to use and compare the effective reed area when the reeds are wide open and compare that to the carb cross sectional area.

Another useful approach is to look at say a late model RM250 or YZ250 to measure port area and reed area to see if you're in the ballpark.  No point in fitting huge crabs if the inlet restriction is tiny reeds/cages. 

Another place you could look is Arctic Cat triples and Polaris triples with 200-300cc cylinders, and then look at their transfer port area to ensure that they are all in balance.

Polaris XCR9800 reed blocks are reeds are probably a touch large, so try a set from an Arctic Cat triple ZRT Thundercat or whatever they called them.  That's what I want to weld onto a set of RD400 cylinders with lift plates to get enough port-time area. 

You could just buy a set of OW31 reed valve TZ750 cylinders if the rules allowed it, or a set of TZ350G 6 port cylinders and add reed blocks and boost ports.

[Jspooner]

You could just buy a set of OW31 reed valve TZ750 cylinders if the rules allowed it, or a set of TZ350G 6 port cylinders and add reed blocks and boost ports.

With all the crazy shit people do these days you'd think someone could take an OW31 cylinder, remove the water jacket and fabricate/weld some fins on it so it would be legal to race against the RD400s. 🤔

[Plasticman]

Are the brakes nice and strong on this bike that is going to carburate it's way to the lead?   

I sure hope so.  FZR600 disc, stock RD400 front caliper, Cannondale motorcycle master cylinder.  All stock, OEM rear setup, for now.

Rob

[m in sc]

talk to me about this master cyl. 

[85RZwade]

I think they used Magura components...?
Wade

[racerclam]

Yes V force reeds would definatly be a good choice , more reed tip area plus more reactive material , more pedals doing less work , every part of power band will improve . I have another recomendation for you , the carbs I build with venturi dividers , makes for a linear power band  , its like opening 4 small carbs in series . Air velocity stays high throughout , Stronger pull out of corners and basicly  any time the throttle is rolled on .  You might be interrested in checking out my web site  richstaylordporting,com

[racerclam]

And always  the use of UFOs is a must . They make a round slide out flow a flat slide carb up to 13/16 throttle opening ,

Rich

[Plasticman]

And always  the use of UFOs is a must . They make a round slide out flow a flat slide carb up to 13/16 throttle opening ,

Rich

The UFO's I ordered showed up yesterday.

Rob

[Plasticman]

Yes V force reeds would definatly be a good choice , more reed tip area plus more reactive material , more pedals doing less work , every part of power band will improve . I have another recomendation for you , the carbs I build with venturi dividers , makes for a linear power band  , its like opening 4 small carbs in series . Air velocity stays high throughout , Stronger pull out of corners and basicly  any time the throttle is rolled on .  You might be interrested in checking out my web site  richstaylordporting,com

I've always wondered if those work.

Rob