Supertuner, I just read a post by Ted in the Turbo section which to my mind may help explain the top end loss experienced by some people when installing the filter kit. I wanted to know what you think. Because the filters have low restriction, there is less vacuum to lift the slides which explains why the softer springs improve performance. This is seen on the dyno with a stationary sled. I am wondering what this means to a sled travelling 100+ mph. Could the increased underhood pressure cause the slides to close. The faster you go the less they are open. Furthermore, even though the needle is attached to the slide, the float bowls will become pressurized and proportionately more fuel will flow past the needle causing the engine to go fat. What do you think?
SUPERTUNER
TY 4 Stroke Master
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low flying missile said:
First off, Ted Jannetty who has his performance auto and truck shop about 1 mile from my shop is a very smart man.
I do believe this might have something to do with it. I do however think that people need to address their clutching more than this. I did some tests last winter on a lake with and ECP Air Kit equipped RX1 where I duct taped the hood vents off. It went about 1 to 2 MPH slower as before with the vents open.
Freddie
RX1Jim
TY 4 Stroke Junkie
Regarding increased float bowl pressure at high speeds, the Vmax-4 guys found they got better and more consistent performance out of these sleds by routing the float bowls vents out from under the hood and endingthem under the handlebar cover. You can read more about it on the vmax4.com website.
underhood pressure
I have always found it interesting that for years Polaris sleds had a check valve on the carb vent lines. My 93 V max 4 came with 136.5 mains, was a pig on gas and didn't have a good top end. I installed those Polaris check valves and ran 142.5s . Performance and mileage improved and a set of plugs would last a whole season. I am using one of those check valves on my EVAC setup now. I think I'll try installing them on my vent lines and see what happens. At least half the hypothesis will be tested.
I have always found it interesting that for years Polaris sleds had a check valve on the carb vent lines. My 93 V max 4 came with 136.5 mains, was a pig on gas and didn't have a good top end. I installed those Polaris check valves and ran 142.5s . Performance and mileage improved and a set of plugs would last a whole season. I am using one of those check valves on my EVAC setup now. I think I'll try installing them on my vent lines and see what happens. At least half the hypothesis will be tested.
RX1Jim
TY 4 Stroke Junkie
Low Flying:
Which direction did those check valves allow air to flow? into or out of the carb float bowls. The CV carbs on the RX-1 appear to have 2 types of vents on the carbs: there are 2 clear plastic tubes on the front of the carbs and 2 black "U" shaped tubes on the back of the carbs. If I ever get a chance to ride I'm going to route all of these vent lines the same way the VMax-4 guys suggest to see if there is a difference. If the underhood pressure rises even slightly due to the high speed air flow, it would have a definite effect effect on jetting.
Which direction did those check valves allow air to flow? into or out of the carb float bowls. The CV carbs on the RX-1 appear to have 2 types of vents on the carbs: there are 2 clear plastic tubes on the front of the carbs and 2 black "U" shaped tubes on the back of the carbs. If I ever get a chance to ride I'm going to route all of these vent lines the same way the VMax-4 guys suggest to see if there is a difference. If the underhood pressure rises even slightly due to the high speed air flow, it would have a definite effect effect on jetting.
I dont think check valves will work guys the air pressure has to be able to go up and down so the check valve would stop it from going one way.
check valves
The check valves are oriented so they close when you blow into the vent line. This prevents underhood pressure from entering the carbs. Rather than place a check valve on every line, I connected all the lines to a manifold I made from copper pipe and tubing. I then used one vent line with a check valve in it to vent the manifold. I tried running the vent lines to the same area as the airbox opening and also to the area behind the windshield under the handlebar pad with good results. The problem with this is that the carbs will drain into the motor if a float or needle sticks. The vent should be lower than the carb.
The check valves are oriented so they close when you blow into the vent line. This prevents underhood pressure from entering the carbs. Rather than place a check valve on every line, I connected all the lines to a manifold I made from copper pipe and tubing. I then used one vent line with a check valve in it to vent the manifold. I tried running the vent lines to the same area as the airbox opening and also to the area behind the windshield under the handlebar pad with good results. The problem with this is that the carbs will drain into the motor if a float or needle sticks. The vent should be lower than the carb.
one more question for Freddie
I have a question about the operation of the CV carbs. Is the vacuum signal that lifts the slides derived from a completely different vent system from the float bowl vents? If so I am wondering if there might be a way to use the low pressure area behind the winshield to create a stronger vacuum signal to the slides? If this is possible, then it would be appropriate to leave the float bowls and air filters at underhood pressure. I just want to add so that everyone who reads this knows. I have Freddies kit on my RX1 trail sled and would recommend it to everyone. The throttle response, acceleration and driveability have improved dramatically. As an unexpected bonus, my fuel mileage improved by between 2and 3 mpg. I really can't say whether my top end is down and don't really care that much. If it is, the trade off is still worth it to me. I'm more interested in seeing if I can help someone who is having a problem.
I have a question about the operation of the CV carbs. Is the vacuum signal that lifts the slides derived from a completely different vent system from the float bowl vents? If so I am wondering if there might be a way to use the low pressure area behind the winshield to create a stronger vacuum signal to the slides? If this is possible, then it would be appropriate to leave the float bowls and air filters at underhood pressure. I just want to add so that everyone who reads this knows. I have Freddies kit on my RX1 trail sled and would recommend it to everyone. The throttle response, acceleration and driveability have improved dramatically. As an unexpected bonus, my fuel mileage improved by between 2and 3 mpg. I really can't say whether my top end is down and don't really care that much. If it is, the trade off is still worth it to me. I'm more interested in seeing if I can help someone who is having a problem.
Re: check valves
I am not an expert but by doing this I believe you could cause a lean condition. I was told when I put this eram on it will boost pressure up 1 psi and they said to get the proper fuel I need to vent the charged air box to the float bowls to equalize pressure. With your set up you are stopping the extra pressure to reach the bowls and cause a lean condition. Now if your under hood pressure increases your bowls wont see it. Check into it further.
low flying missile said:
I am not an expert but by doing this I believe you could cause a lean condition. I was told when I put this eram on it will boost pressure up 1 psi and they said to get the proper fuel I need to vent the charged air box to the float bowls to equalize pressure. With your set up you are stopping the extra pressure to reach the bowls and cause a lean condition. Now if your under hood pressure increases your bowls wont see it. Check into it further.
RX1Jim
TY 4 Stroke Junkie
Low flying,
The vacuum to operate the slides is developed between the intake valves and the buttrefly in the carb. When you punch the throttle, the vacuum level in this area essentailly drops to zero and rises as the engine speed increases. As the engine speed increases the vacuum increases and causes the slides to open. This is the whole principal behind the CV carbs, the vacuum level determines the opening size in the throat of the carb which keeps the response crisp. This is the same principle that actuate the secondary openings on a vacuum actuated 4 barrel automotive carb. Essentially the engine only uses the amount of throttle it needs. The vacuum developed by the engine should only be used for actuating the slides, you cannot use an external source.
In your last post you said you got good results when you routed the carb vent lines out from under the hood. Could you elaborate on these good results: more top end, gas mileage, etc.?
Thanks!!
The vacuum to operate the slides is developed between the intake valves and the buttrefly in the carb. When you punch the throttle, the vacuum level in this area essentailly drops to zero and rises as the engine speed increases. As the engine speed increases the vacuum increases and causes the slides to open. This is the whole principal behind the CV carbs, the vacuum level determines the opening size in the throat of the carb which keeps the response crisp. This is the same principle that actuate the secondary openings on a vacuum actuated 4 barrel automotive carb. Essentially the engine only uses the amount of throttle it needs. The vacuum developed by the engine should only be used for actuating the slides, you cannot use an external source.
In your last post you said you got good results when you routed the carb vent lines out from under the hood. Could you elaborate on these good results: more top end, gas mileage, etc.?
Thanks!!
vents
Sled Dog, you are absolutely right. That would create a lean condition. I thought about it a little more and remember that I had one vent hose per carb connected to the little copper pipe manifold and check valve in the belly pan. This was to allow the carbs to drain if a needle stuck or I flooded the motor badly. I joined the other vent lines with T's and ran one vent line up to above the opening of an airbox (there are 2 on the Vmax 4) so the float bowls and carb bells were drawing air at the same pressure. Sorry about the confusion but it has been more than 10 years since I did this to that machine.
RX1Jim, The Vmax4 statements were only made to show the big effect these small pressure differences make on carb behaviour. The Vmax had carbs vented to underhood pressure and the air intake from the low pressure area behind the windshield. The Rx1 has the better situation of having both in the high pressure area underhood. There will be a slight ram effect at high speed. If you remember Freddie said he lost 2 mph by taping up the hood vents.
I think that what this boils down to is that the CV carb is probably a bad choice for a snowmobile. I have this image of the slide being held up like a ping pong ball being suspended by the air from a hair drier. I'm sure the slides bounce up and down with every bump in the trail. The V max4 had mechanical slides and had great throttle response. I think the only reason we got CV carbs is because they were already in use on the R1 motor cycle.
Sled Dog, you are absolutely right. That would create a lean condition. I thought about it a little more and remember that I had one vent hose per carb connected to the little copper pipe manifold and check valve in the belly pan. This was to allow the carbs to drain if a needle stuck or I flooded the motor badly. I joined the other vent lines with T's and ran one vent line up to above the opening of an airbox (there are 2 on the Vmax 4) so the float bowls and carb bells were drawing air at the same pressure. Sorry about the confusion but it has been more than 10 years since I did this to that machine.
RX1Jim, The Vmax4 statements were only made to show the big effect these small pressure differences make on carb behaviour. The Vmax had carbs vented to underhood pressure and the air intake from the low pressure area behind the windshield. The Rx1 has the better situation of having both in the high pressure area underhood. There will be a slight ram effect at high speed. If you remember Freddie said he lost 2 mph by taping up the hood vents.
I think that what this boils down to is that the CV carb is probably a bad choice for a snowmobile. I have this image of the slide being held up like a ping pong ball being suspended by the air from a hair drier. I'm sure the slides bounce up and down with every bump in the trail. The V max4 had mechanical slides and had great throttle response. I think the only reason we got CV carbs is because they were already in use on the R1 motor cycle.
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