05 RX1 MPI S/C @10 lbs. lean at 6000 rpm ????

[rxboost05]

Hi Guys
Am having trouble getting rid of a lean issue trying to hold a steady 6000 rpm. All air fuel readings are accepable below and above 6000, except at 6000 rpm a steady throttle, readings go to 15 to 1 or higher and it has a lean stumble. In a drag race it blows right through it at wot.
This is my set up now. 137.5 mains, 20 pilots, clips 2nd from bottom, floats at 14 mm. I am running 50/50 mix of 93 oct. and 113 Klotz. Had carbs off and did cleaning this morning (very clean) checked all hoses for restrictions or kinks,(none). Also did Comp. test, 200 lbs. across all cly.
Have had this sled to very knowledgable people who frequent this site and they are at a loose for a fix.
Do not want to damage engine.And we have the best snow year of the last 4 years, very very frustrating.

Thanks in Advance
Bob

 

[Ted Jannetty]

I suggest Contacting Holtzman and talk it over with him, and maybe try a set of lighter springs to get your slides higher at 6000 rpm.

Ted.

 

[BigDog05]

There have been several rear mounts that have this problem, and I don't want to start a major debate.

There are a lot of guys on this forum that know a lot more than I do about turbos.

That being said: I have been told by a person who makes some very powerful turbos, to drive around the lean spot.

Mine has done this since I put the turbo on the sled. Very small window that this occurs, and it is only under low boost conditions. 1-4 lbs of boost.

I have changed pitot jets, raised needles, tried stock, green and silver springs. There seems to be some thought that the rear mounts are getting a type of reversion from the short exhaust if the W/B 02 sensor is mounted between the turbo and exhaust

I spend an entire afternoon running at the bad RPM and shutting the sled off with the kill switch and reading plugs.

While the plugs certainly were not rich, they were showing a small amount of color.

I guess I'll find out when I melt it. Last year I was running straight VP 103 unlead. This year I run straight 100 ll AV gas at 12 lbs at 2500 ft elevation.

The previous year I ran 110 Sunoco leaded at 50/50 with 93 pump. Still same problem.

 

[millstreet]

I've got the same thing with the same thing - sled and lean spot - might try going down all the way with the clips, and see if that helps. If you're already 1 from the bottom I don't know how much of a change it will provide. I've been in the carbs on mine alot lately(not by choice) and the needles do seem to have some bearing 6000-7000 RPM at light throttle(little or no boost). On mine, as soon as you roll into the throttle(add boost) the lean spot goes away. Good Luck,


Andy

 

[hlmrx1]

I had the same thing with a turbo'd 03 RX1, poping at cruizing speed 6000-7000rpm, I just raised the needles up one notch to give it more fuel and it was fine.

 

[rxboost05]

Thanks for the ideas guys. I have had the clips in every position posssible. I am going to contact Holtzman this morning. I will let everyone now the results as soon as i get the springs and do testing.


Thanks again

Bob

 

[rxboost05]

Hi Guys

Did testing this morning with Holtzman spings . Here are the results.

Blue springs, no change still lean stumble.

Black spings. same results.

Red springs, maybe a little better but still lean at 6500 rpms at steady throttle. I am going to sync. carbs as soon as the tool arrives and have two more sets of springs coming from Hlotzman. One softer and one stiffer than what i have now. will let everyone kmow the results of next test

Bob

 

[wheeline]

Thanks for the ideas guys. I have had the clips in every position posssible. I am going to contact Holtzman this morning. I will let everyone now the results as soon as i get the springs and do testing.
Thanks again
Bob

So your saying you raised the needle all the way up (clip in the lowest notch) and still saw the lean spot at 6K?

 

[rxboost05]

clips can go down one more notch. Then needles will be all the way up. Waiting for more springs before springs before any more testing, Work is cutting into testing time.

 

[wheeline]

I hate it when work gets in the way of tuning

 

[dynotechjim]

Just saw this post, reminds me of tuning turbo Harleys when we did carbed Aerocharger kits for the Vtwins with stock CV carbs.

We used to cure lean midrange with CV carbs by stretching the stock springs about 5%-10% more free length. Remember boosted blow-through carbs have air velocity about like stock even with double stock airflow, so keeping slides close to the needle jet opening helps enrichen mixture on those 38mm Kehein CV carbs.

An advantage of the Harley turbo for tuning on the dyno is we could use a plexiglass cover on the boost plenum and observe the location of the slide during all throttle positions/ boost levels and that helped us determine optimal needle settings etc. Sean Ray was a master of watching slide position, and grinding needles on a belt sander to create awful-looking but perfect stepped icicle-shaped needles that gave 12/1 at max boost from 2000-6000 RPM on custom big inch engines that needed special tuning

It was possible with the right components to create over 200 rear wheel HP out of a single 38mm CV carb. On those engines, when the CV carb delivered 100 HP worth of air at 60% of peak revs, the slide may have been between half and 3/4 open, and not fully open until the last 500 revs. This is with stock or stretched stock slide springs to create high enough velocity at the needle jets to get 100 HP worth of fuel (50 lb/hr) at those low revs.

So if you have lean midrange and lots of stock slide springs laying around it might be good to try stretching a set 5 or 10% to see if that helps deliver more fuel at part throttle.

 

[TWIN TURBO]

That kind of explains it, if I get this right. Haven't played much with these CV carbs but I like these tuning problems, fun to try and solve the problem.

If the slide does't open far enough, raising or lowering the needle won't change anything. you will still be on the fat part of the needle or a slow taper with the slide down. You would have to get the slide up farther or change the cutaway to draw more fuel or work with the idle and off idle circuits.

 

[dynotechjim]

on the Harley blowthrough turbo CV carbs it was just the opposite of that--if slides open too high too quickly, there's not enough signal at needle jet to draw adequate fuel even with skinnyest needle. So we had to force the slides to stay low, longer yet be wide open at peak revs. This worked perfect. At 15 psi air in the venturi is twice as dense but velocity is still stock so the tricky part is creating 100% more fuel flow with velocity about identical to stock.

this was surely more critical on Harleys, where time at midrange wot is longer than sleds since Harley riders dont like to downshift (rolling on in top gear at 50mph 2000 revs then about 10 seconds to go from 2000 to 5000) so midrange wot had to be perfect. Midrange part throttle was equally critical, since touring bikes could spend hours at 80mph, 1/4 throttle, slides part way up. Had to get good mileage but not surge, sneeze or detonate. Had to be perfect there. Stiffer springs might not work just like that on the boosted Yamaha but surely worth a try.

 

[rxboost05]

Hi Guys
Did testing yesterday. Holtzman silver spings, needles all the way up. The lean stumble is gone, but air fuel readings still go to 13.8 to 1 at steady thorttle around 6500 rpms The plugs look good, nice light tan. Sure makes this sled alot more fun on the trails.
Heading out the door now to put some miles on.

Bob

 

[TWIN TURBO]

Nothing wrong with 13.8:1 Are the new springs stiffer or lighter than what you had stock? I'm assuming slides are all the way up at full throttle, how about where the stumble was?