Rejetting after expansion chambers

[GHtokyo]

I have a 1973 RD 350 engine.  Working on adding Power Pipes by J&R expansion chamber exhaust pipes.  What size main jet should I start with for tuning?
I have the stock 130 main now with pod air filters.
No other mods....yet.


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[m in sc]

hey! welcome.

you should get rid of the flat backed pods, fit either a y boot with k&n or unis. Jetting will jump up to 190-220 mains. j&rs don't require a huge jet amount. IF you stick with the flat back filters, you might be around 150. (stock is actually 140 with an unrestricted airbox 73/74 350) If it was running ok on 130 mains, that shows you how restrictive those filters are. 

Good luck. those j&rs are very cool indeed.   

[GHtokyo]

Yes, I actually think it is running rich on the 130 which also shows how poor the flat bac
.  Its even the larger  pod filtersod

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[GHtokyo]

Sorry, was gonna say, yes it's crap with even the larger, off-center pods.  Looking to add a y boot next.  How important is the crossover tube after the carb?

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[m in sc]

are the filters joined? in the back? i've seen that done but it doesn't really do anything. (if that's what you mean by crossover. ) 2 out of the 3 yamaha twins i have are running all foam unis, the other one is running a y boot.  id go y boot and k&n on your bike if it was mine.

its more about the intake runner length.

[GHtokyo]

Ok, I'll go with the k&n.  They make reed valve carb holders that have a hole in the bottom for a crossover tube between the carb and the engine.  Just wondering what that does.   Mine are starting to crack, so...

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[m in sc]

oh, you are speaking of the crossover on the intakes at the top? it helps, but were not used (stock) until later.  I have crossover intakes on both my rds, they help the power transition.

[SoCal250]

John at Economy has everything you need. And don't forget the forum discount code
Economy Cycle - air filters
Economy Cycle - intake manifolds
Economy Cycle - Mikuni jets

[RDFL]

I've always liked the look of the upswept J&R's especially the chrome ones.

[85RZwade]

Yeah, those are sweet lookin pipes!

[quocle603]

Do what Mark suggested.

If you decide to either go the y boot to a uni or K&N, or two UNI filters to the carbs, be prepared to drastically upjet your main jet and increase your pilot jet as well. The carb will need to compensate for the amount of air that it will be getting. L You can do your plug chop but try to listen to what the bike is trying to tell you, document your jetting and how your bike ran with those settings.

[Diablo007]

My friend who's had his shop since '68 has told me stories of RD350's seizing when individual pods were run.  The competitive racers here during the '70's went back to the y-boot and the seizures stopped.  Could be that our track was at sea level, but I won't risk individual pods on my bikes due to his stories.

[rodneya]

My friend who's had his shop since '68 has told me stories of RD350's seizing when individual pods were run.  The competitive racers here during the '70's went back to the y-boot and the seizures stopped.  Could be that our track was at sea level, but I won't risk individual pods on my bikes due to his stories.

Love hearing stories like that. Blaming poor workmanship on parts. Pods can not cause seizing, but incorrect jetting for pods definitely can.
One of the biggest markets for two stroke performance parts is the Banshee and there are a huge number of them running pods with no issues. Many of them putting out over 100hp.

Every piece of advice I have been given ( not asked for) about my bike has turned out to be incorrect. Like.
Dont modify your motor because then factory jetting and airscrew settings wont work anymore - My personal favorite.
34mm carbs are too big and will kill performance.
Pods are bad
Pipes with more than 4 inch dia bellies will not work.
Exhaust stingers mote than 1 inch dia will not work.
Twin front disks will kill handling.
Modern sportbike forks are too stiff and will kill handeling

[m in sc]

you can tune around pods, of course! unis are better than anything flat backed though.   
And, of course, if you mod the intake that much and dont rejet, of course it will be wrong.  Your '68 shop guy should have advised the customers to jet properly.


I will say, an rd will handle lighter with a single disc. been there, done it. does it kill the handling? no. does it make it heavier back to back? yes. however, if you go to modern forks with less offset on the trees, this will counteract some of the dual disc 'heaviness'  on the handling.

Advice is only good when its complete.  when its spoken in absolutes, its usually incorrect on some level.   

[Diablo007]

Almost went on a diatribe.  I'll leave it at:

I wasn't offering advice, it was meant more as a word of caution.  It seems jetting should have been mentioned also.  I try not to be too verbose.  I guess I should have been a bit more...

Never said it was happening to customers.  As far as I know it didn't.  I was happing to racers, all of whom tuned their own motorcycles.  Again, maybe I should have been more verbose.

I will add though, When I was frequenting the track to participate in race schools and watch races.  The top 2-stroke guys did talk about the temperature and humidity changes over the course of the day and how it would affect their choice of jetting.  To the point that I do remember the discussion of changing jetting during a race day.  Can't remember if they actually did it, it's just the conversation that stuck in my head.  One of the 'joys' of living on the most isolated, populated land masses on Earth.