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2010 Nytro Yamaha/MPI 180SC, hanging idle/high rpms when letting off the gas.

jgustman

jgustman

Expert
Joined
Feb 25, 2005
Messages
202
Age
50
Location
Cottonwood, MN
Country
USA
Snowmobile
'10 Nytro RTX
LOCATION
Cottonwood, MN
Sled is running fine but started noticing while trail riding that the rpm’s were not coming down fully when letting off the gas in the corners and would take a few seconds at a stop at times for the ide to come down to normal. It was hanging around 2300 rpm’s for a few seconds when stopping and letting off the throttle on the corners was more like 4500 rpm’s. Temps were 0 deg. F. On a side note I have just installed a Hindle slip on and fuel ratios were running low to mid throttle 12.5-13.5, usaully drops into the 10.5-11 when letting off the throttle at low speed trail riding. It seemed intermittent but seemed progressively worse as the day went on. Any ideas?
 

74Nitro said:
Sounds like BOV
Click to expand...
It does seem to be over boosting as well, easily 6-6.5+ psi at full throttle and close to 9200 rpm on brief full throttle pulls, Ulmer clutching per his setup recommendations
 
Is it equipped with a BOV?
When I bought my kit from Yamaha it didn't come with it.
The BOV is to release built up pressure after releasing the throttle.
 
74Nitro said:
Is it equipped with a BOV?
When I bought my kit from Yamaha it didn't come with it.
The BOV is to release built up pressure after releasing the throttle.
Click to expand...
Yes, non adjustable. It was new in the box Yamaha Oem Kit, came with everything including clutch and gear/chain. Thinking about it I don’t recall hearing the BOV venting like normal when this was happening as well.
 
My kit was the 170 OEM and didn't come with it so I initially drove it without. It did exactly what you are describing.
I would make sure the BOV is functioning as it should be.
 
Feels like no engine breaking letting off the throttle.
 
Now the biggest question is where do you buy a replacement BOV or can it be taken apart and fixed. I haven’t looked, but I assume a vacuum leak cannot cause the BOV to not open?
 
74Nitro said:
My kit was the 170 OEM and didn't come with it so I initially drove it without. It did exactly what you are describing.
I would make sure the BOV is functioning as it should be.
Click to expand...
Mine is probably the 170 kit as well, but the way it’s boosting at below zero, it’s probably 180+, Ulmer racing said these kits were more like 170-175.
 
I don't have the sled anymore so I can't look at the BOV
The 26 drive with 28 driven was 170
28/28 was 180
 
What I am saying is that the 170 hp was at around 8800 rpm. Mine is pulling 9100+., boost pressure was hitting 6+ with temps below 0 degrees. Boost is suppose to be around 4.5 psi.
 
Boost vs RPM doesn't change much. Here's some engine dyno information. This is with a faster pulley combination (29 drive & 25 driven).

8000 - 7.9 psi
8250 - 8.4 psi
8500 - 8.85 psi
8750 - 9.5 psi
9000 - 9.95 psi

From 8500 vs 9000 rpm is about 1 psi of gain. You should be shooting for 9000 rpm or there abouts, especially with an aftermarket exhaust. Colder denser air will make just as much difference in boost as RPM will also.
 
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