soft seize! stage 6 streetrace carb set up help

[crocman]

Hello, 

Long time lurker, first time poster. I have a moskito mz50 (yamaha axis clone minarelli horizontal shortcase) with the following mods:

- stage 6 street race cylinder kit AC
- .125" spacer under jug and head shaved .100" (1.2mm squish, 190 exhaust, 132 transfers)
- mild porting and case matching
- 21mm phbg
- stage 6 pro rep
- 4.2gram rollers, stiffer contra, and 6000rpm clutch springs
- slightly advanced stock stator
- modified stock air box
- 12mm doppler endurance crank

I ran this all summer with a 92 main jet with no shaved head or spacer under jug (183 degree exhaust and 128 transfers) . It finally 4 cornered (30hrs run time later...) and seized a few weeks ago on a brisk cold morning in Pennsylvania. Figured I was just running it a little too lean. During this rebuild is when I shaved the head and added the spacer. I upped it to a 110 main jet but settled on a 98. It runs great on 98 main jet going to and from work (city driving). However when blasting across the wide open fields for a long period it soft seized on me this past weekend. It will run on a 102 main jet or even a 105 if you roll onto the throttle slowly. 

With 98 jet, from a stand still if you go from idle to wot (step change) it will wheelie off the line
With a 102 main, if you go wot from stand still (step change) it will bog for a second or two then take off
With a 105 main, if you go wot to stand still (step change), it will bog for 5-10 seconds then allow full rpm. Top speed was reduced.

I am not sure what to do. Be happy with the 102 and the minor hesitation? Try moving the clip around to get rid of the bog? Mess with the pilot jet / air flow screw?

[oldgeek]

This may help you find your tune. You really need a temperature gauge if you do not already have one, tuning without one is difficult. Can you tell us the the slide number, needle # and atomizer size?

[crocman]

Thank you for your reply! I read your whole streetrace from scratch thread but I prolly missed some stuff.

I have a cheap Amazon rpm and cht gauge. I turn 10k-11k rpms at wot. The gauge is mounted to a hole drilled into one of the head fins. When running the 98 jet before I le mons it across the fields I would get around 215 wot. But I noticed after shutting off (fan turns off) after running it hard it would rise to 280+.

I ran one day to work with a 105 main jet. Super boggy down low. But I ran it hard up top and took note of temps. I learned at wot the max temp I could reach was 205f. However after shut off. It only rose to 225. This was a 20 degree delta after shutoff versus a delta of 60f. Goes to show I think that the fan running masks a lot of the cooling issues if you never pay attention to shut off temps. When it soft seized. I read 300f on the gauge after it shut off. Ran it the last 10 miles home with the choke on. Stopping every 5-10 min to let it cool. Didn't let it rise above 240f.

I'm running a 102 main, 50 pilot, 7/2 conical needle (don't remember which clip it's set on).

I bought carb from here: www.ruckhouse.com/en/carburettors-sport-midrace/5171-carburetor-dellorto-phbg-21mm-ds-1000000000449.html?search_query=DEL-2632&results=1

I am not sure which atomizer or slide number I have. Is there a standard one? If I take it apart... Is there markings somewhere that let me know?

[aeroxbud]

Having the temperature on the fin, it will always show a colder temperature than what you would see at the plug.
It's normal for the temperature to spike once the engine is shut off. As there is no cooling air.

[oldgeek]

Going by the part # 2632 it should have come with the following.
40 Slide
W7 needle
92 Main jet
262AU atomiser
50 Idle jet
60 Choke jet
9450.1 Float – 4.0 grams
What mods did you do to the airbox?

[crocman]

Aerox,

Yeah that makes sense. The temp is always going to be colder. I concluded that the difference in delta T (same amount of run time, same test road, similar ambient temps) was do to the fact that while running on the 98 jet the fan was providing 60 degrees of cooling at the fin. While on the 105 main, the fan was only providing 20 degrees of cooling. Since the fan was performing less work on the system, this must mean the cylinder was being cooled down by the gas much more effectively. I never realized this before by only looking at CHT while WOT, a difference between 215F (105 main) and 205 (98 main) did not seem that big to me. But a difference in 40 degrees less work being performed by the fan? I thought that was huge.


Oldgeek,

I obviously need to do more research on these carbs. I never even knew you could change out a choke jet. Makes sense. But was not expecting that. I was using a stage 6 foam pod filter but I was trying to make the bike quieter so I wanted to switch to a modified stock airbox. I was wildly unsuccessful at making it quieter. But I will probably take what I learned and try again later.

First I took a 2.25" by 2.5" silicone elbow and mounted the 2.25" side on the carb. Then I test rode the scooter in very short bursts and settled on a 102 main jet. Then I mounted the stock airbox using 2.5" thin wall aluminum tubing that I glued into the airbox. The airbox was fitted with malossi foam. I then mounted (6) uni 1" foam airbox inserts. It was not free flowing enough for the 102 main. Then I removed all the uni 1" filters and just left the holes. That worked great, and acted as though the filter was not there (the goal for max power) but man was it loud.

For my next attempt. I am going to try the uni filters again. But knock out all the plastic mesh that comes from the factory. I am not sure if this is going to make it quieter though....

[oldgeek]

All you should need to do to the airbox is pull the snorkel out. A stock airbox with all those holes and you might as well use a foam filter, which is difficult to tune and will never be as streetable as an airbox will. You dont even need a 21mm carb for that kit. In my experiences a 21mm showed no real gains over the 19mm and the 19mm had much better fuel economy.
If you stick with current air system you might try a W6 needle. If you go with a stock airbox with snorkel only removed a W7 or W9 may work best.
You really should invest in a Trail Tech or equivalent temp gauge that fits under the spark plug, 350°F max is your target at WOT with one of those.

[crocman]

Before I started modifying the airbox I tried it with the stock airbox and snorkel removed and 98 main jet. It felt very very rich. So I would of had to reduce the main jet down till it ran right. This would theoretically I think give less power. Less air and gas at the same stoichiometric ratio as more air and gas would have to make less power right?

Something I noticed in my testing, the stock 90 degree elbow is really restrictive on my build:
- stock air box with stock 90 degree elbow and no filter with large holes drilled and 98 main - wont rev past 8000k - boggy
- stock airbox with silicone 2.5" elbow and no filter with large holes drilled with 98 main jet - ran like pod filter

But what I am hearing is... maybe the controlled restriction of the stock airbox and a reduced main jet would lead to something very tuneable as you suggest and that should be more important. Because with the controlled restriction, outside factors such as ambient air temp would have less effect on the tune. Is that how it works?

Ill have to buy a trailtech. A 19mm dellorto carb is not in the near future but on the wishlist for a later date.

[oldgeek]

I try to look at it like this, a given motor can only use so much air and gas per combustion cycle. Just because we make more air and gas available does not mean it will use it. And yes an air box controls airflow much like a carb controls fuel flow. 2Ts like tightly controlled airflow. Even a small air leak at the intake will drive you insane when you are tuning. Your 21mm carb is fine, I was just mentioning that a 19mm would work too.
Another example would be a glass of water and a drinking straw. Take a sip from the straw, it's flow is controlled and manageable. Change the straw to a 1" hose. It's harder to get a proper drink with the 1" hose, it can be done but it is easier through the straw.
You have done quit a bit to get to where you are, get yourself a needle kit and take it all a step further.

[crocman]

Alright so I got the needle kit. Annoyingly I did not realize it did not have the W6, only W7 and W5. I put the W5 in and a 102 main jet. It was very boggy / rich again on the main jet. I put a 100 main and it seems to run great. I am hoping this new set up will help keep the temps down more.