Jetting '86RZ 350F2 - I think I'm close but not quite right.

[hilltj]

I've just rebuilt my pride & joy, an '86 RZ350F2. It has a 4mm stroker crank (TSS), Vforce reeds & reed blocks, ported Barrels, 0.9mm squish, Zeeltronic programmable CDI/PV controller, straight cut primary gears (TSS) & overdriven oil pump. Carbs are standard powerjet but with powerjet blocked, Standard idle jets about 1 1/4 turns out, 290mains, Banshee needle (second clip from top) & Banshee emusion tubes, Toomey replica pipes & "Y" tube air filter. Bike runs Ok at all revs, but my problem is thus; after running @ what appears to be a constant speed of say 90-100km/hr for some period of time it feels like it is "Bogging" down, that is the motor loses power, becomes unresponsive to throttle input, but idles ok, symptoms clear after a minute or so, then it goes ok again. I cannot be certain of the set of conditions that brings this on, but upon arriving home, plug removal shows a mid-coffee brown colour plug. I've rebuilt several 2strokes in the past, but never one that has deviated from standard, so have little experience with jetting. I'm deadly afraid of nipping the motor up. From my reading of this excellent site & others, I think I may be rich. I would appreciate any advice & recommendations

[seahorse]

Sounds like a fuel vent issue, fuel filter, float level low ? Probably that simple.

With regards to the Jetting, im not a fan of the PJ, I run non PJ, with a y boot, chambers, boysens, ported and 6525 Pistons, currently on a 25 pilot, 320 mains and needle middle. From what iv seen "my plugs"read and what others are using on RZ forums I could go much higher on the mains ( if I road every where at 9000 rpms ). I gues the only way to be absolute is get it dynoed by someone who will hook it up to read fuel air ratios.

[seahorse]

Further to my last, if its not the fuel vent, id be pulling the carbs for a thorough clean. Then whilst the are off, re adjust the float levels ( easier off the bike, sit em side by side in a soft vice, run the clear lines, open the drains, small funnel some fuel into the inlets, and the levels should stop at the fuel bowl gasket lines. ( Make adjustments to the tab on the float bowl arm to get it right up or down in micro bends. ( I did this off the bike myself yesterday its much easier and you can get them with in a bees dick of exactly the same in no time.

[Loud]

Shouldn't you be plug chopping at the time the symptoms are present, not leaving it until you get home? I know I've heard time and again that 2-strokes don't like constant rpm. Sounds like a great bike!

[hilltj]

Guys, thanks. I too agree that the problem sounds like a tank venting problem. I'll check the tank/cap for venting. Carbs were dismantled & thoroughly cleaned as part of the project & I checked the float heights (but not by measuring the height of the fuel level in the bowl, which is clearly the best way). If it were float height problem, wouldn't it affect running at all revs?. I'm really frightened of running too small main jets & i've seen reference on this site to people running 270 mains with a similar engine set up to mine. I was thinking I may be a bit rich, but Seahorse is running 320s!. Anyone know someone in the Mackay region who knows 2 strokes (unless you wear a Crusty Demons tee shirt, have a mowhawk & wear a back to front baseball cap, the local bike shops seem not interested in helping a balding middle aged bloke approaching 50, who's just built the bike he lusted for when 18!)

[g0zer]

i figure you have gone 240mainjet * 421cc / 350cc = 290 mainjet, thats pretty reasonable.

dont forget the needle taper controls fuel flow at all throttle open positions given the main jet is big enough to supply it, you have increased the main jet size but are running a standard needle position?

personally i would run c5 for first start up and run in, i run c4/c5 on a banshee needle to match a 270main with pods on a 350... and i note you are running a 421 without an airbox.

[hilltj]

What concerns me is that I'm running 290mains, but banshee needles second clip from the top (C2). I found this setup overcame a flat spot when I ran same jets but needle in C4 position. My concern is am I running too big main jets & restricting the flow by having my needles too low or would I be better going, say 280 or 270 jets with the needle raised to say C4 or C5?.

Bike seems to run Ok, pulls & revs well, but I don't have anything to compare against. Maybe it's performing well below what it should!

[g0zer]

from what ive seen of dyno curves for those motors it is normal for them to lose a couple of horses just before they come on pipe. are you leaning it out trying to smooth a flutter that is not caused by rich a:f?

roger at wickedATV has dozens of dyno printouts for these things floating about on the net and iirc hp drops off prior to coming on.

also remember its a carb not a powercommander, you want to lean it to correct a flutter at a certain rev range.. but the method you are using (dropping needle) leans it across the entire rpm and throttle position range! and i am not convinced that the flutter is even caused by richness... (correction: roger says it is, but i note he is leaving it rich in the hole and considering his resources and tuning experience i am sure there are good reasons he compromises here)

found a dyno curve for you

[hilltj]

g0zer, I guess that's possible. Do you think that with my current jets that I should raise the needle a clip or 2 (that is to c3 or C4). Could this be contributing to my "bog down" that I wrote about in my initial thread.

I appreciate your patience, as you've probably guessed I'm a newby to tuning 2 strokes although I've built several in standard trim. I consider myself comfortable with spanners.

[seahorse]

Guys, thanks. I too agree that the problem sounds like a tank venting problem. I'll check the tank/cap for venting. Carbs were dismantled & thoroughly cleaned as part of the project & I checked the float heights (but not by measuring the height of the fuel level in the bowl, which is clearly the best way). If it were float height problem, wouldn't it affect running at all revs?. I'm really frightened of running too small main jets & i've seen reference on this site to people running 270 mains with a similar engine set up to mine. I was thinking I may be a bit rich, but Seahorse is running 320s!. Anyone know someone in the Mackay region who knows 2 strokes (unless you wear a Crusty Demons tee shirt, have a mowhawk & wear a back to front baseball cap, the local bike shops seem not interested in helping a balding middle aged bloke approaching 50, who's just built the bike he lusted for when 18!)

I still think as 1st reply thats its fuel vent issue.

Float height levels are important for example if they were way low, there is a real chance the fuel bowl may not be able to keep up/supply a main with the correct amount at WOT hence, also if one is low and one is high then you will have similar issues and have major problems tuning the bike to get the correct fuel /air ratios.

I think the PJ were designed to deliver more fuel at the right time working in sinc with the pilot and mains to overcome this flat spot and feed the almost turbo like power band of these donks in the upper rev range on the standard engine with standard pipes.
I know that some people have had disasterous results with modded bikes and the PJ.
I now nill about the PJ as I have the non PJ carbs. Power curves, fuel/air ratios, riding style, riding conditions? I can tell when mine is running hot, and lean. the Temp goes up, and the plugs are white that to me is a bad thing. If the temp stays fine, and plugs are tan then im happy.

I just leaned mine off today ( I run the original 5 k needles which are shorter then the banchee, hence I feel safe with the factory middle position, which from what I have read is erred on rich side of a 240 main non PJ. As I run a 320 main i was a bit wet and brown on the middle clip, now second clip. But ill be keeping a close eye on things and finger on the clutch. I like the transition smooth and from experience this positioning makes the most difference, If I run bottom or second bottom my bike has a massive stumble, flutter from round 4 to 6000 rpm, this improves as I come up the clips, but obviously I have to pick the safe middle ground on the plugs.

if you have the space and time do some chops at 1/2 throttle in say 3rd gear for a couple hundred meters, err on side of caution start at a richer setting.

Sort the venting fuel starvation issue and get your float levels right befor you do anything.
Your current setting are probably OK for the street, temp and plugs will tell you when there not.

Good luck and and lots of stuff written on rd/rz500.com forum on these bikes, you will find a few from here hiding there as well.

[Richie Rich]

Wish I could offer some constructive advice to assist you but alas the dark art of carb set up alludes me.

What I am keen on seeing is sone pics of your bike! Sounds awesome. I have a restored '83 350K but also owned the last of the RZ's the '86 FS back in the day so would love to see your bike.

Good luck.

[Sean'o]

You say you are sitting on a constant 90-100kph, but at what rpm? I went 1 clip position up, leaner, it ran like a charm, sharp and crisp, but, after sitting on a constant 7-8k rpm for a few mins, things started to slow down, it nipped up. I put the clip back one spot lower, richer and havent had an issue since. Main jet more comes into play at wide open throttle, sounds like you are on a partial throttle just cruising where it would mainly be the needle, richen it up a clip position (lower the clip) and see what happens. The symptoms you describe i wouldn't imagine would be coming from an already rich condition, that will just make them feel doughey all the time.

[smeghead]

This may help point you in the right direction

http://www.perthstreetbikes.com/forum/f64/carb-tuning-basics-141343/

[g0zer]

sorry for some reason i thought you were running athena 66mm barrels, so used to seeing build threads about +4mm cranks with athena barrels these days so that dyno chart (for a non powervalve athena based 421) is not applicable to your problem.

i still think you are way low on the needle, c2 is stock 350cc clip position and you have removed the airbox and have more cc.. both these mods will require more fuel. lift it to c5 and ride it. make sure its at operating temperature before worrying about bogging. like i said before, i run c5 with 5n7 needles with pods on a 350YPVS and there is no bogging imho my motor pulls like a train at low rpm the bike is used mostly for tonking about the burbs offpipe ~4000rpm.

[hilltj]

What I'll do is raise the needle a couple of notches, that is C4. Then see what happens. I ran in this position initially & there was a significant flat spot upon acceleration. But I''ll redo the test & see.

Cheers

[seahorse]

Howd we go Hilt, was it the fuel vent, did you sort that clip pos.

[hilltj]

I raised the needle one clip (back to centre position) & lost some of the crisp throttle response @ lower revs. It felt rich. I lowered it to C4 & it ran worse, clearly rich (which I was not unhappy about, it proved the point). I didn't experience the "bogging down" type effect though, but it was only a short ride as the raising of the needle gave obvious effect. Incidentally, at C4, I had to increase the idle speed stacks (another sign of being too rich!). What I noticed though was @ full throttle, the slides were not syncronized well so I adjusted this too. Motor goes well once the revs are up & pulls like a train from say 5,000rpm onwards. My next step is to ditch the banshee needles & try again with standard ones. I'm beginning to think that the taper is wrong for my set up. Stay tuned!!.

[seahorse]

Good stuff Hilt,I seem to have to sync the the slides every time I pull the carbs, sometimes they dont move, and other times they seem to move a bit, I make sure I visualy bottom them right out with the throttle stops before I begin the sync process, I think I have caught myself out in the past by not doing this. Adds advised one time ages ago that you should also see that they are fully opening as well ( food for thought )Those banchee needles are noticeably longer than the standard, but they do match those miracle tuning kits that you could buy years ago. Its sounds like your progressing, thats a good thing.

[g0zer]

I cable tied the cable splitter so it cannot move to correct the wandering sync

[seahorse]

did that in the early stages as the splitter would always pull off the rubber housing every time I pulled the carbs off, made a difference, but still would occasionaly be a bees dick out every time, probably old cables and old eyes playing tricks, when you get it close listening to them bottom out at the same time is the confirm, cheers gozer might see at the shoot,and just realised your avatar shot is gold, poopoo got you eye.