Wierd Nikki problem while autocrossing... and something odd also!!
05-01-11, 08:59 PM
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Wierd Nikki problem while autocrossing... and something odd also!!
My Nikki has been relatively problem free. I autocross race a lot and most of the times I have NO issues.
Every once in a while I will get this "flat spot" or "hesitation" that occurs. It is funny because it does not matter what the autocross event is, this "hickup" would happen at the same place. Some events, no problem... some events problems. Seems to happen after a lot of transitions.
Today all five runs, right after some technical slaloms and then a quick "bump" in the elevation (made the car want to jump off the ground), the car acted like it had no fuel or the engine bogged. It had NO power for probably 3 seconds and then the power came back.
I got back to the staging area and noticed the back rotor primary part of the carb was wet with fuel all over the top of the venturies and booster. I even pulled both plugs and found the back plug wet with fuel.
It seems like I am getting a fuel slug from "slosh" in the carb.
NOW HERE IS THE WIERD THING!! My friend has a 1st gen RX7 with a 12a and a Nikki carb. His car "normally" acts perfect also, but today autocrossing his car did EXACTLY the same thing at exactly the same place on course!!!
Right after some hard slaloms... quick right turn...jump up a high spot (elevation change) and bam!!! No power!!
Any ideas?? I have been fighting this problem off and on for about a year. Every time I think I have solved it, it will rear its ugly head!!! This time it cost me a win!!!
Every once in a while I will get this "flat spot" or "hesitation" that occurs. It is funny because it does not matter what the autocross event is, this "hickup" would happen at the same place. Some events, no problem... some events problems. Seems to happen after a lot of transitions.
Today all five runs, right after some technical slaloms and then a quick "bump" in the elevation (made the car want to jump off the ground), the car acted like it had no fuel or the engine bogged. It had NO power for probably 3 seconds and then the power came back.
I got back to the staging area and noticed the back rotor primary part of the carb was wet with fuel all over the top of the venturies and booster. I even pulled both plugs and found the back plug wet with fuel.
It seems like I am getting a fuel slug from "slosh" in the carb.
NOW HERE IS THE WIERD THING!! My friend has a 1st gen RX7 with a 12a and a Nikki carb. His car "normally" acts perfect also, but today autocrossing his car did EXACTLY the same thing at exactly the same place on course!!!
Right after some hard slaloms... quick right turn...jump up a high spot (elevation change) and bam!!! No power!!
Any ideas?? I have been fighting this problem off and on for about a year. Every time I think I have solved it, it will rear its ugly head!!! This time it cost me a win!!!
05-01-11, 10:08 PM
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Seems like a quick visit to Sterlings Website shows how to fix this problem!
Read the quote from his site!
The Legendary 'Left Hand Stumble'
When I discuss racing applications with regards to Nikki modifications, I usually use auto cross as my primary example. This is not just because I have several customers who auto cross with Sterling Nikkis, but also because the demands of auto cross racing are the highest. There is a great need for good mid range acceleration in auto cross, as well as for the carburetor to decelerate quickly and corner without stumbling. As I've stated many times, in any case, running at WOT can be optimized just the same no matter what the performance application. So the auto cross racing Nikki must meet the demands of all types of racing, with the exception of drag racing.
The Sterling Nikki is as optimised for the street as it is for auto cross. The only difference between a Sterling Street Nikki and a Sterling Racing Nikki built for competition with few modification regulations is the float set that I use. In auto cross and track racing there is a problem with the Holley, and to a lesser extent the Nikki, where upon exiting a left hand turn, there is a stumble. This is a known annoyance that most SCCA racers learn to "drive around". But any time that can be shaved off the track, and any distraction that can be eliminated is going to be good for the racer.
People are usually surprised when I explain to them what is actually happening inside the carburetor in these instances. The series of events that follows a hard left corner is rather simple but the problem caused by them is compounded and unique. Understanding what's really going on inside the carburetor will help those racers that are forced to drive around the issue because their class rules do not allow for one very simple modification...
Because of the orientation of the carburetor, the fuel is forced to the back of the float bowls with great force during left hand turns. As the fuel collects at the back of the float bowls, the primary emulsion tubes, and in severe cases even the primary fuel jets, are uncovered and starved of fuel for an undetermined, but minute amount of time. Two things happen while the carburetor is in this state; The main circuit delivers a lean condition, though the driver may never notice a decrease in power associated with it if he's feathering the pedal through the turn, and the floats drop, letting in more fuel.
When the car exits the turn and the G-forces acting on the fuel let up, the fuel returns to a fairly level position, but there's far too much of it. The emulsion tubes are completely submerged, and the carburetor is flooded. Because there is usually a down shift in the turn and a "power out" attempt upon exiting the turn, the driver is pressing on the throttle, trying to accelerate. At this point, the flooded carb would normally drop down to the appropriate fuel level fairly quickly on it's own. But the accelerator pump is adding even more fuel to the incoming air for a duration, and the engine bog that drivers experience is actually due to a rich condition from the combination of the siphoning of fuel from the over-loaded emulsion system and the action of the accelerator pump.
I've had many racers tell me ( in some cases, insist ) that they want baffles installed in the air horn of their race Nikki. I try once to explain why that won't work, and if they insist I do it anyway, I charge extra and I make it clear that I take no responsibility for the fact that it will make no difference. This is because the air horn gasket already traps the fuel and keeps it from climbing back up into the void at the top of the airhorn as much as any baffle ever could. No baffle installation is going to keep the fuel from going where artificial sideways gravity dictates. This baffle installation modification is a carry-over from the 70s when the gaskets used on the Nikki did actually allow for terrible fuel slosh. The current replacement gaskets are the same design for both the earlier Nikkis and the later ones, so there is no reason not to use an early Nikki.
A very simple and much more effective solution to this problem is to install the floats designed for early Nikki carburetors. These floats are only found in pre '79 and some '79 - '80 Nikki carbs, as far as I know, and they are full sized, as opposed to the later design that look like they are diagonally cut in half. I have not measured the volume of each float yet, but it's inconsequential. What actually matters is that they displace as much fuel as possible inside the bowl. The larger floats work by taking up room that would otherwise become a void that would fill with fuel when the float drops from the sideways G-force that throws the fuel to the back of the carburetor, when the float no longer has anything supporting it to keep it closing the needle valve. The carburetor will still go through the first stage of the cornering issue where the primary main emulsion tubes and fuel jets are uncovered, though it might be less likely that the fuel jet would be uncovered, depending upon your fuel pressure. But the second stage of the corning issue, where the float normally drops down allowing fuel to enter in will be much less dramatic, if it even happens at all. I have yet to test side by side comparisons of this issue. Testing such things is always difficult for me as I don't have easy access to a track, and racing around on roads is just never a good idea to check these types of things.
Now I need to figure out where I can get the older oversized floats!! Any ideas?
Read the quote from his site!
The Legendary 'Left Hand Stumble'
When I discuss racing applications with regards to Nikki modifications, I usually use auto cross as my primary example. This is not just because I have several customers who auto cross with Sterling Nikkis, but also because the demands of auto cross racing are the highest. There is a great need for good mid range acceleration in auto cross, as well as for the carburetor to decelerate quickly and corner without stumbling. As I've stated many times, in any case, running at WOT can be optimized just the same no matter what the performance application. So the auto cross racing Nikki must meet the demands of all types of racing, with the exception of drag racing.
The Sterling Nikki is as optimised for the street as it is for auto cross. The only difference between a Sterling Street Nikki and a Sterling Racing Nikki built for competition with few modification regulations is the float set that I use. In auto cross and track racing there is a problem with the Holley, and to a lesser extent the Nikki, where upon exiting a left hand turn, there is a stumble. This is a known annoyance that most SCCA racers learn to "drive around". But any time that can be shaved off the track, and any distraction that can be eliminated is going to be good for the racer.
People are usually surprised when I explain to them what is actually happening inside the carburetor in these instances. The series of events that follows a hard left corner is rather simple but the problem caused by them is compounded and unique. Understanding what's really going on inside the carburetor will help those racers that are forced to drive around the issue because their class rules do not allow for one very simple modification...
Because of the orientation of the carburetor, the fuel is forced to the back of the float bowls with great force during left hand turns. As the fuel collects at the back of the float bowls, the primary emulsion tubes, and in severe cases even the primary fuel jets, are uncovered and starved of fuel for an undetermined, but minute amount of time. Two things happen while the carburetor is in this state; The main circuit delivers a lean condition, though the driver may never notice a decrease in power associated with it if he's feathering the pedal through the turn, and the floats drop, letting in more fuel.
When the car exits the turn and the G-forces acting on the fuel let up, the fuel returns to a fairly level position, but there's far too much of it. The emulsion tubes are completely submerged, and the carburetor is flooded. Because there is usually a down shift in the turn and a "power out" attempt upon exiting the turn, the driver is pressing on the throttle, trying to accelerate. At this point, the flooded carb would normally drop down to the appropriate fuel level fairly quickly on it's own. But the accelerator pump is adding even more fuel to the incoming air for a duration, and the engine bog that drivers experience is actually due to a rich condition from the combination of the siphoning of fuel from the over-loaded emulsion system and the action of the accelerator pump.
I've had many racers tell me ( in some cases, insist ) that they want baffles installed in the air horn of their race Nikki. I try once to explain why that won't work, and if they insist I do it anyway, I charge extra and I make it clear that I take no responsibility for the fact that it will make no difference. This is because the air horn gasket already traps the fuel and keeps it from climbing back up into the void at the top of the airhorn as much as any baffle ever could. No baffle installation is going to keep the fuel from going where artificial sideways gravity dictates. This baffle installation modification is a carry-over from the 70s when the gaskets used on the Nikki did actually allow for terrible fuel slosh. The current replacement gaskets are the same design for both the earlier Nikkis and the later ones, so there is no reason not to use an early Nikki.
A very simple and much more effective solution to this problem is to install the floats designed for early Nikki carburetors. These floats are only found in pre '79 and some '79 - '80 Nikki carbs, as far as I know, and they are full sized, as opposed to the later design that look like they are diagonally cut in half. I have not measured the volume of each float yet, but it's inconsequential. What actually matters is that they displace as much fuel as possible inside the bowl. The larger floats work by taking up room that would otherwise become a void that would fill with fuel when the float drops from the sideways G-force that throws the fuel to the back of the carburetor, when the float no longer has anything supporting it to keep it closing the needle valve. The carburetor will still go through the first stage of the cornering issue where the primary main emulsion tubes and fuel jets are uncovered, though it might be less likely that the fuel jet would be uncovered, depending upon your fuel pressure. But the second stage of the corning issue, where the float normally drops down allowing fuel to enter in will be much less dramatic, if it even happens at all. I have yet to test side by side comparisons of this issue. Testing such things is always difficult for me as I don't have easy access to a track, and racing around on roads is just never a good idea to check these types of things.
Now I need to figure out where I can get the older oversized floats!! Any ideas?
05-01-11, 10:25 PM
#3
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Autozone.com has the floats listed seperately, however I don`t know if they are the larger ones since there is no picture. http://www.autozone.com/autozone/par.../N-imn4tZ8vd20
05-02-11, 10:45 AM
#5
The 1979 Rx-7 floats are not the right ones. You want the OLLLLDDD round style floats that were in the Rx-3. Try a 1977 Rx-3 on the Autozone website. It should give you this part number: 779-8034.
They only list one of the two floats now. But I believe that you can use that same float in both float bowls and it will work just fine. Give it a try and report back. I know others have done this with success. I think you will have good luck too.
They only list one of the two floats now. But I believe that you can use that same float in both float bowls and it will work just fine. Give it a try and report back. I know others have done this with success. I think you will have good luck too.
05-02-11, 02:29 PM
#6
Keep in mind there is ONE class of Nikki that the older floats will not fit in... that being any equipped with 'vacuum power valves.' They are the reason for the notch in the modern floats that makes them differ left and right, too.
I know, naturally, because I have just such a carb.
Pics what show why. and how to tell if you have one.
With:
Without:
Easiest way to tell, though, is if you have TWO solenoids on the right side of the carb:
I know, naturally, because I have just such a carb.
Pics what show why. and how to tell if you have one.
With:
Without:
Easiest way to tell, though, is if you have TWO solenoids on the right side of the carb:
05-02-11, 02:40 PM
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Glen, thanks for the pics! My carb does NOT have those spring looking gizmos next to the floats. Now that I see that pic, I understand why they have left and right floats for newer carbs.
I guess this part number 779-8017 could be the right one from autozone.
Anyone else know for sure if this part number is right or where I can get these floats?
I guess this part number 779-8017 could be the right one from autozone.
Anyone else know for sure if this part number is right or where I can get these floats?
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05-02-11, 02:42 PM
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Another thing I dont understand is WHERE the fuel slosh comes from. I have read a bunch about this problem on auto X cars, but it looks like the top gasket would separate the fuel from the bowls from sloshing in the throat of the carb!
05-02-11, 05:22 PM
#15
This is why older carbs (before the smog people got all nasty about evap systems) used to have those tall angled bowl vent tubes opening above the throats.
05-02-11, 06:36 PM
#16
Your answer has already been posted. The notches are for these two spring looking things, which I'm betting that your carb doesn't have. I know mine doesn't.
You should be able to use two identical floats without any problems. I know other people have successfully done it. Give it a try. I'm 99% sure that it will work great, and eliminate your stumble.
You should be able to use two identical floats without any problems. I know other people have successfully done it. Give it a try. I'm 99% sure that it will work great, and eliminate your stumble.
05-02-11, 10:35 PM
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Looks like here is the answer!!! It is off of Sterlings Back Yard Mechanic forum....
http://sterlingmetalworks.com/bymc/i...hp?topic=283.0
http://sterlingmetalworks.com/bymc/i...hp?topic=283.0
05-02-11, 10:36 PM
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Your answer has already been posted. The notches are for these two spring looking things, which I'm betting that your carb doesn't have. I know mine doesn't.
You should be able to use two identical floats without any problems. I know other people have successfully done it. Give it a try. I'm 99% sure that it will work great, and eliminate your stumble.
You should be able to use two identical floats without any problems. I know other people have successfully done it. Give it a try. I'm 99% sure that it will work great, and eliminate your stumble.
05-02-11, 11:07 PM
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Nope... the open bowl vent system provides a path, from those arc-shaped recesses near where the VPV spring-pistons go, over the top of the gasket, thru the opened path provided by the vent valve, and thence into the throats . You'd have to get a good loft & slosh to splash it up there, but once in there, nowhere else to go. You can see it clearer if you look without the gasket in place.
This is why older carbs (before the smog people got all nasty about evap systems) used to have those tall angled bowl vent tubes opening above the throats.
This is why older carbs (before the smog people got all nasty about evap systems) used to have those tall angled bowl vent tubes opening above the throats.
Glen do you know anything about these "old school" vents you speak of?? I don't have to worry about emissions!
From what I am reading, these oversized floats should take care of business!!
05-03-11, 10:04 AM
#21
If you look at a number of old-design carbs, including pre-RX-7 Nikkis I've seen photos of, you'll see that the bowl vents are nothing more than an angled tube that runs from the inside top center of the bowl to a position over the top of the carb throat. They are usually cut diagonally at the top, so that the open end of the tube is parallel to the throat opening. The height of the tube, and it's angle and position in the bowl, is what prevented corner slosh from letting fuel escape that way.
Grabbed this pic off the web; serious old-school bowl vent tubes easily visible.
Recall that the non-emissions functions of a bowl vent are two-fold: one function is to keep vapor pressure from building in the bowl and forcing fuel out, but the main function is to ensure that the air above the fuel in the bowl is at exactly 'ambient' atmospheric pressure.
Carbs function off of pressure differentials. you want the atmo pressure acting on the fuel in the bowls to be equal to the pressure right above the carb throat.
In the first-gen Nikkis I'm familiar with, the overall height of that vent pathway is a lot lower, and not nearly vertical; it weaves back and forth to let it run past the Vent Solenoid. I'm theorizing that it's easier for fuel to slop up into the vent system & then get dumped down the throat.
It's all guesswork on my part at this point, though; just from thinking about the shapes of things and what accelerations would do to the fuel slopping around in there.
Grabbed this pic off the web; serious old-school bowl vent tubes easily visible.
Recall that the non-emissions functions of a bowl vent are two-fold: one function is to keep vapor pressure from building in the bowl and forcing fuel out, but the main function is to ensure that the air above the fuel in the bowl is at exactly 'ambient' atmospheric pressure.
Carbs function off of pressure differentials. you want the atmo pressure acting on the fuel in the bowls to be equal to the pressure right above the carb throat.
In the first-gen Nikkis I'm familiar with, the overall height of that vent pathway is a lot lower, and not nearly vertical; it weaves back and forth to let it run past the Vent Solenoid. I'm theorizing that it's easier for fuel to slop up into the vent system & then get dumped down the throat.
It's all guesswork on my part at this point, though; just from thinking about the shapes of things and what accelerations would do to the fuel slopping around in there.
05-03-11, 10:19 AM
#22
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If you look at a number of old-design carbs, including pre-RX-7 Nikkis I've seen photos of, you'll see that the bowl vents are nothing more than an angled tube that runs from the inside top center of the bowl to a position over the top of the carb throat. They are usually cut diagonally at the top, so that the open end of the tube is parallel to the throat opening. The height of the tube, and it's angle and position in the bowl, is what prevented corner slosh from letting fuel escape that way.
Grabbed this pic off the web; serious old-school bowl vent tubes easily visible.
Recall that the non-emissions functions of a bowl vent are two-fold: one function is to keep vapor pressure from building in the bowl and forcing fuel out, but the main function is to ensure that the air above the fuel in the bowl is at exactly 'ambient' atmospheric pressure.
Carbs function off of pressure differentials. you want the atmo pressure acting on the fuel in the bowls to be equal to the pressure right above the carb throat.
In the first-gen Nikkis I'm familiar with, the overall height of that vent pathway is a lot lower, and not nearly vertical; it weaves back and forth to let it run past the Vent Solenoid. I'm theorizing that it's easier for fuel to slop up into the vent system & then get dumped down the throat.
It's all guesswork on my part at this point, though; just from thinking about the shapes of things and what accelerations would do to the fuel slopping around in there.
Grabbed this pic off the web; serious old-school bowl vent tubes easily visible.
Recall that the non-emissions functions of a bowl vent are two-fold: one function is to keep vapor pressure from building in the bowl and forcing fuel out, but the main function is to ensure that the air above the fuel in the bowl is at exactly 'ambient' atmospheric pressure.
Carbs function off of pressure differentials. you want the atmo pressure acting on the fuel in the bowls to be equal to the pressure right above the carb throat.
In the first-gen Nikkis I'm familiar with, the overall height of that vent pathway is a lot lower, and not nearly vertical; it weaves back and forth to let it run past the Vent Solenoid. I'm theorizing that it's easier for fuel to slop up into the vent system & then get dumped down the throat.
It's all guesswork on my part at this point, though; just from thinking about the shapes of things and what accelerations would do to the fuel slopping around in there.
My flooding must have sloshed over some other way or came from totally submerging the emulsion tubes and flooding through the boosters.
Anyhoo, hopefully the oversize floats, properly setting float levels, and optimizing fuel pressure will mitigate this problem. Other than this problem, the carb performs flawlessly!!
05-03-11, 12:52 PM
#24
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Any hints??? Anything I can do while I have the carb torn down this weekend?
05-04-11, 10:43 AM
#25
just start by cleaning every thing .emulsion tubes , all the jets float bowls ,float bowl jets , my solution is you start with a carb that is clean and works normal. Can you install new needles and seats as well as a full gasket kit, do not misplace any of the chrome ***** in the carb when you do your cleaning.