-The tz250racing Message Boards
+--Forum: TZ Carburetion & Jetting
+---Topic: Poor pulling at 8500 to 10000rpm started by wizz954
Posted by: wizz954 on June 27 2010,9:15
My bike has been quite poor out of corners and not sure what is causing it.
I've run the bike for 2 seasons and this is the 3rd season.
I always run avgas at 30:1 and 330/340 mains, R6 nozzles (up from R4 that I ran for 2 seasons, this is what came with the bike). Going up to R6 nozzles has fattened up the midrange from 5500-8500rpm and it feels strong here (relatively speaking), but falls into a hole from 8500rpm to 10000rpm. Once it hits 10000rpm it flies again and revs out to 13000rpm.
What affects running at this rpm range? Is it the needles/nozzles or powerjet. I'm sure I am on the correct main jets as the EGTs are perfect, no higher than 1150 deg F.
Posted by: wizz954 on June 28 2010,10:05
If it feels flat and lethargic under WOT at 9500rpm, does that suggest too lean or too rich? Lots of inductino noise but no go. To me it sounds too lean if it was a 4 stroke but 2 strokes seem to defy the usual rules.
Posted by: vince24x on June 28 2010,1:46
the feeling is too similar imo. assume it is too lean and fatten the jetting; then test. if worse, head the other direction.
mind that the relevant jet circuit is related to throttle position, not really RPM (excepting the PoJ circuit which opens/closes based on both).
make a paint mark on the edge of the throttle grip and like tick marks on the housing that will let you see at a glance how far open the throttle is. this can help w/ testing.
hth,
vince
Posted by: wizz954 on June 29 2010,12:27
Does 70 PJ sound about right?
Also would reed valves cause loss if power in this rev range if too soft/bendy?
Posted by: vince24x on June 29 2010,1:12
i wouldnt point to the reeds initially. mine shag out at the tips long before they lose their stiffness.
i assume this is a 4DP/4TW engine? what fuel? 70 sounds a little large to me, but take that w/ a grain of salt.
also, be wary that you have the correct mains based on EGT. from your description, it does not appear you are not using the EGT as i would. i suggest shooting for the highest reading you can get. a problem is indicated when the temp drops down unexpectedly.
q: why did you go from an R4 to R6? was the R4 not working? if nothing else has changed, maybe that is something to consider.
vince
Posted by: wizz954 on June 30 2010,10:56
3YL engine with 4DP pistons that have been shaved by 0.3mm to be somewhere close to 3YL piston spec.
I changed to R6 because the engine always sounded dead low down and hard to get off the line without bogging. It's feels a lot stronger really low down now. Trouble is the punch is too low down and hasn't improved my drive out of corners when revs are up around 9-10,000rpm.
Interested about your theory on EGTs. Can you expand further why you would want as high as possible. I set them so the pistons don't melt, I've been told 1200 F is dangerzone. I try to run to 1150 max, sometimes reaches 1180.
Posted by: vince24x on July 04 2010,1:17
| Quote (wizz954 @ June 30 2010,7:56) |
| Interested about your theory on EGTs. Can you expand further why you would want as high as possible. I set them so the pistons don't melt, I've been told 1200 F is dangerzone. I try to run to 1150 max, sometimes reaches 1180. |
do a little search on the site; it's been discussed.
think bell-curve with temp on the ordinate and fuel/air on the abscissa; stoich is at the point of max temp. so, i'd want to run ideally just on the rich side of this. any further down and you're giving up power. regardless, if you are too rich or too lean, the temp is down compared to its peak at stoich.
vince
Posted by: wizz954 on July 07 2010,8:45
Hey Vince, I searched the forums at length before I posted. I had another look and found one post that I couldn't understand. I'm afraid I don't have an engineering degree and get totally lost when you mention words like abscissa and stoich
My jet kit has PJs of following sizes. 50, 60 and 70 that I am currently running. That sort of suggests I can only go down from 70, so I will try 60 at the next practice day and see how it goes.
Posted by: wizz954 on July 28 2010,9:17
I put a new motor in as I am doing an endurance race this weekend. I left the powerjet at 70, so I could compare both engines. I upjetted on the mains by 2 sizes to run in the motor and what I found is that the response is great with small throttle openings but as soon as I wind it up, it feels really sluggish with anything from 1/2 to WOT. All this below 11000rpm so powerjets still open.
This is pointing to the powerjet so I will definitely go down to 60PJ this weekend and see how it goes.
Posted by: RS Addict on July 29 2010,2:15
Dear me, I'll have to get my old slide rule out
Stoich I knew but abscissa got me thinking so hard that I dragged out my old study books....from quite a few years ago!
When y is plotted against x, the x-values are abscissa.
When specific volume is plotted against temperature, the temperature values are abscissa etc.
Stoich is the ratio at which all oxygen is used up and all fuel is completely burnt.
You gotta love 'ginger beers'!
Stevo BEng (Mech)
Posted by: wizz954 on Aug. 02 2010,9:30
Whatever that all means, changing to a 60PJ did the trick, now the motor pulls hard out of corners on part throttle and full throttle.
The main jets are now the problem. The motor wouldn't pull above 11500rpm and to cut a long story short, we jetted down after each session and ended up down from the normal 330/340 to 240/280 and EGTs were still under 1000 deg F.
Very cold weekend, 8 deg C at 700m altitude.
What I have now is a sweet running motor from 6000rpm upwards but lacking in top end power. Plugs look dark brown except the tip of the electrode which is slightly lighter over 3mm from the centre of the electrode outwards.
These main jet sizes are ridculously low and well outside the range of my jet kit, but the lower we went, the better it went.
Anyone come across this before?
Posted by: BruceLind78 on Aug. 02 2010,11:08
In all of this, have you check your float level and made sure the float needle and seat are sealing?
Posted by: wizz954 on Aug. 02 2010,6:28
Good point. Not specifically but the carbs haven't changed between engines and being separate cylinders, the odds are low both are afflicted.
I have a rebuild kit ready to go in, so I will fit that to be sure.
Posted by: far601 on Aug. 02 2010,7:43
("i have a rebuild kit ready to go in, so I will fit that to be sure).
just out curiousity could you also post what needles you have in the carbs?if 240/280 m.j. are working i'd suspect you would have to have a mighty rich needle.we ran stock TZR 's with 32mm carbs in the 240/280 range. please confirm and post needle designation. c..........
Posted by: BruceLind78 on Aug. 02 2010,11:31
| Quote (wizz954 @ Aug. 02 2010,6:28) |
| Good point. Not specifically but the carbs haven't changed between engines and being separate cylinders, the odds are low both are afflicted. I have a rebuild kit ready to go in, so I will fit that to be sure. |
Also make sure that the Fuel enrichment plungers(choke) are on the seats. I had a rider one year at Daytona, who had rebuilt his complete bike. in the rebuild, he had changed carb bodies from top to bottom and not realized it. In the change, the cables and plungers were installed so they were held off the seats. Dont have the lever and cables installed. Still make sure that the plungers are seated. you are getting to much fuel from somewhere.
Posted by: wizz954 on Aug. 03 2010,9:52
The previous owner removed the chokes and glued it all up with Araldite or something.
Needle is the standard 3YL needle as per the manual which I don't have to hand, but something like 6FH0-62. Nozzle is R6 which is the standard nozzle as per the manual. I recently changed to R6 from R4 which were in the carbs since I have owned the bike.
One thing I noticed when downjetting all weekend is that the EGTs only changed marginally for fairly large changes in jet sizes.
Posted by: wizz954 on Aug. 30 2010,8:49
I found a possible cause of the richness. The front cylinder powerjet solenoid wasn't connected, this will explain why the front cylinder was on much smaller MJ.
end