Need data on fuel bowl levels
03-12-10, 12:23 AM
#1
Need data on fuel bowl levels
I need the community's help gathering data on "normal" fuel bowl level behavior after shutdown.
I'm trying to verify if the behavior I now see in my fuel bowl levels on a STOCK setup is correct, or if I have an actual issue any more.
Here's what I see:
Turning on the pump without starting the car gets me correct fuel bowl levels; right on the mark, and they stay there even if I let the pump run for minutes.
While the engine is running, both front and rear bowl levels are excellent, right in the grooves. Rock solid.
After I shut down from fully warmed up, the levels in both bowls will slowly rise, over the course of 15 minutes to a half an hour. The front bowl only rises maybe 1/4 the distance between the level line and the top of the glass window; the rear bowl will rise somewhat more, to a peak at about 3/4 of the difference between the level line and the top.
About an hour or so later, the levels will drop back toward normal.
Overnight, the levels will drop below the level line; the front bowl only a little, the rear bowl almost to the bottom of the sight glass.
Can someone else (or better yet , a couple people) with a Nikki-carbed 12A take the time to check their bowl levels several times after shutdown, over the course of a couple hours and then once overnight, and tell me if they see the same kind of behavior?
I'd be greatly appreciative. I don't have any reference to tell me if what I'm seeing is normal, or if this is a symptom of a problem. I'm trying to prep for a smog check, and such things make me paranoid - - I don't want to end up with a "gross polluter" flag on my record, which here in the People's Republic of Kaliforniski is worse than a 'scarlet letter'!
Thanks so much!
I'm trying to verify if the behavior I now see in my fuel bowl levels on a STOCK setup is correct, or if I have an actual issue any more.
Here's what I see:
Turning on the pump without starting the car gets me correct fuel bowl levels; right on the mark, and they stay there even if I let the pump run for minutes.
While the engine is running, both front and rear bowl levels are excellent, right in the grooves. Rock solid.
After I shut down from fully warmed up, the levels in both bowls will slowly rise, over the course of 15 minutes to a half an hour. The front bowl only rises maybe 1/4 the distance between the level line and the top of the glass window; the rear bowl will rise somewhat more, to a peak at about 3/4 of the difference between the level line and the top.
About an hour or so later, the levels will drop back toward normal.
Overnight, the levels will drop below the level line; the front bowl only a little, the rear bowl almost to the bottom of the sight glass.
Can someone else (or better yet , a couple people) with a Nikki-carbed 12A take the time to check their bowl levels several times after shutdown, over the course of a couple hours and then once overnight, and tell me if they see the same kind of behavior?
I'd be greatly appreciative. I don't have any reference to tell me if what I'm seeing is normal, or if this is a symptom of a problem. I'm trying to prep for a smog check, and such things make me paranoid - - I don't want to end up with a "gross polluter" flag on my record, which here in the People's Republic of Kaliforniski is worse than a 'scarlet letter'!
Thanks so much!
03-12-10, 10:13 AM
#3
Lives on the Forum
Can't say for sure, its been too long since I've dealt with a stock carb. Have you tried over at Sterling's site? www.sterlingmetalworks.com
There is a user there called Divindriver, but I'm not sure if it is you or an imposter.
.
There is a user there called Divindriver, but I'm not sure if it is you or an imposter.
.
03-12-10, 03:11 PM
#4
Yep, that's me - - I'm asking there too, since the best way to analyse for "normal" is to sample as big a population as possible.
So far, I've proven it's not an aspect of bowl venting, nor of tank pressure, by seeing the behavior repeat with both those systems completely disconnected/eliminated through removing the gas cap, bowl vent line, and air-cleaner cover. This isolates the bowls from any external pressure influence as far as I can determine. The behavior doesn't change.
The part where the bowls fill slightly after shutdown I can kind of explain theoretically as being fuel that's in the various feeds flowing back when vacuum is lost; since the metering system raises the fuel above bowl level via vacuum before feeding it, it would make sense that some fuel would flow back down to the bowls from the high point when that vacuum goes away, just like liquid "backwashes" down a drinking straw when you stop sucking on it.
The rear bowl getting more re-fill would make sense under this theory too, since the rear bowl feeds more circuits than does the front: the accel pump and the richer circuit both feed from the rear bowl only.
But I don't have a good theory as to why the levels then drop at different speeds, since that drop should be exclusively due to evaporation, when there's no vacuum to lift fuel out of the bowls. Yet the rear bowl drops more and faster than the front, even though both have the same exposure to evaporation. Closest explanation I can come up with is siphoning within the richer circuit, but the air bleed should prevent that.
That's why I'm anxious to see if anyone else sees the same thing.
I'm willing to bet that even a Sterling-modded carb would not have changed this behavior much if it's truly normal to the Nikki, since it would be driven by the internal fuel circuitry that Sterling does not mod (I think).
So by all means Ken, if you'd care to do a warm-up and then check the bowls a few times as it cools down, I'd be much obliged.
So far, I've proven it's not an aspect of bowl venting, nor of tank pressure, by seeing the behavior repeat with both those systems completely disconnected/eliminated through removing the gas cap, bowl vent line, and air-cleaner cover. This isolates the bowls from any external pressure influence as far as I can determine. The behavior doesn't change.
The part where the bowls fill slightly after shutdown I can kind of explain theoretically as being fuel that's in the various feeds flowing back when vacuum is lost; since the metering system raises the fuel above bowl level via vacuum before feeding it, it would make sense that some fuel would flow back down to the bowls from the high point when that vacuum goes away, just like liquid "backwashes" down a drinking straw when you stop sucking on it.
The rear bowl getting more re-fill would make sense under this theory too, since the rear bowl feeds more circuits than does the front: the accel pump and the richer circuit both feed from the rear bowl only.
But I don't have a good theory as to why the levels then drop at different speeds, since that drop should be exclusively due to evaporation, when there's no vacuum to lift fuel out of the bowls. Yet the rear bowl drops more and faster than the front, even though both have the same exposure to evaporation. Closest explanation I can come up with is siphoning within the richer circuit, but the air bleed should prevent that.
That's why I'm anxious to see if anyone else sees the same thing.
I'm willing to bet that even a Sterling-modded carb would not have changed this behavior much if it's truly normal to the Nikki, since it would be driven by the internal fuel circuitry that Sterling does not mod (I think).
So by all means Ken, if you'd care to do a warm-up and then check the bowls a few times as it cools down, I'd be much obliged.
03-12-10, 04:19 PM
#5
Lives on the Forum
I would be more than happy to do that for you, but currently my baby is in pieces for a clutch replacement, flywheel upgrade, and about another dozen things in preparation for DGRRX.
Do you have a fuel pressure regulator? My thinking is, the FPR could hold pressurized fuel in the line between the FPR and the pump. Then, it might slowly leak past the FPR and into the carb over time.
Best of luck with your quest.
Do you have a fuel pressure regulator? My thinking is, the FPR could hold pressurized fuel in the line between the FPR and the pump. Then, it might slowly leak past the FPR and into the carb over time.
Best of luck with your quest.
03-12-10, 07:32 PM
#6
OK, thanks. Hopefully a few other people can join in the fun & see what their cars do.
No, I have no FPR - - straight stock setup on my car at the moment. Fuel pressure's running just over 3PSI; a little low by spec & I'm working on a replacement, but I don't see any way low pressure could cause this.
No, I have no FPR - - straight stock setup on my car at the moment. Fuel pressure's running just over 3PSI; a little low by spec & I'm working on a replacement, but I don't see any way low pressure could cause this.
03-13-10, 07:42 AM
#7
Nikki-Modder Rex-Rodder
Join Date: Feb 2001
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The pump is a piston pump, so the fuel probably can't bypass it when it's off. It can't dump back towards the tank, so that's the plug at the feed line. At the carb end, when the fuel expands it can dump into the fuel bowls.
When the engine is shut off everything is still warm. The fuel in the lin es expands a bit and drops a bit into your bowls.
Figure the displacement of those floats. We're really only talkling about a cc of fuel +/- per bowl to cause a noticeable change in sight glass level.
As things cool off, the carb, being completely sealed, acts like any other barometer. The pressure decreases significantly, and the fuel is absorbed into the main circuit inside the carb. The fuel level inside the eight upright passages (step jets, air bleeds, etc) rises. Doesn't take all that much to absorb that cc per bowl and then some.
This is my guess as to what's going on.
When the engine is shut off everything is still warm. The fuel in the lin es expands a bit and drops a bit into your bowls.
Figure the displacement of those floats. We're really only talkling about a cc of fuel +/- per bowl to cause a noticeable change in sight glass level.
As things cool off, the carb, being completely sealed, acts like any other barometer. The pressure decreases significantly, and the fuel is absorbed into the main circuit inside the carb. The fuel level inside the eight upright passages (step jets, air bleeds, etc) rises. Doesn't take all that much to absorb that cc per bowl and then some.
This is my guess as to what's going on.
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03-13-10, 02:59 PM
#8
OK, Sterling, I can see that... But I'm still unsure what causes the rear bowl to vary so much more than the front, and drop so far below the level line. Both bowls being under the same pressure and temperature influences, their reactions should be much the same, just as barometers react the same.
I find myself wondering; what if the richer solenoid needle is not seating well? Could it be causing a siphon condition? It's just a brass needle into a metal seat via a spring - - doesn't have any real tightness to it.
Seems like the air bleeds should prevent it, too, and I've verified them clean.
Richer circuit:
Rear bowl entries for the richer circuit (bottom feed, top bowl-side air bleed)
Path of fuel & air thru the carb:
A leaky richer valve would maybe explain the excess bleed-down into the carb from the rear?
If fuel level rises to the point that the in-bowl air bleed is submerged, then that whole line would fill, then leak down... and when all the levels drop back as things cool, that missing volume of fuel could maybe account for the difference in bowl levels.
It'd have to be a very slight leak, though, or else the level in the bowl would never be able to rise above the ' ^ ' point in the fuel path; it'd get that high then drain down.
What d'ya think?
Sure wish I had comparisons from others' carbs & how they are observed to behave after shutdown; It would help a lot.
I find myself wondering; what if the richer solenoid needle is not seating well? Could it be causing a siphon condition? It's just a brass needle into a metal seat via a spring - - doesn't have any real tightness to it.
Seems like the air bleeds should prevent it, too, and I've verified them clean.
Richer circuit:
Rear bowl entries for the richer circuit (bottom feed, top bowl-side air bleed)
Path of fuel & air thru the carb:
A leaky richer valve would maybe explain the excess bleed-down into the carb from the rear?
If fuel level rises to the point that the in-bowl air bleed is submerged, then that whole line would fill, then leak down... and when all the levels drop back as things cool, that missing volume of fuel could maybe account for the difference in bowl levels.
It'd have to be a very slight leak, though, or else the level in the bowl would never be able to rise above the ' ^ ' point in the fuel path; it'd get that high then drain down.
What d'ya think?
Sure wish I had comparisons from others' carbs & how they are observed to behave after shutdown; It would help a lot.
03-14-10, 01:31 PM
#9
Nikki-Modder Rex-Rodder
Join Date: Feb 2001
Location: Trying to convince some clown not to put a Holley 600 on his 12a.
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The richer valve leaking like that is not uncommon. It's also really not a big deal.
The matter of it being inconsistent is probably due to the pressure variables I mentioned above. Maybe sometimes the spring in the solenoid is just having a good day. lol
There's a wieght in the richer solenoid needle valve channel that drops down from the top. It's a pain to deal with on the car, but if you're at least aware of it, it's a help. Remove the solenoid and just pull the spring a bit to give a tiny bit more strength to it.
I completely negate these circuits on my modded Nikkis.
The matter of it being inconsistent is probably due to the pressure variables I mentioned above. Maybe sometimes the spring in the solenoid is just having a good day. lol
There's a wieght in the richer solenoid needle valve channel that drops down from the top. It's a pain to deal with on the car, but if you're at least aware of it, it's a help. Remove the solenoid and just pull the spring a bit to give a tiny bit more strength to it.
I completely negate these circuits on my modded Nikkis.
03-14-10, 11:26 PM
#10
Oh, you'll love this:
I pulled the richer yesterday, working on exactly the same premise, and inspected the needle and the seat closely, flashlight & mirror style.
Didn't see anything utoward, but I figured I'd swap the bore out with a q-tip full of carb cleaner, just on general principles.
When I pulled the q-tip back out, I found this curled around the exact center:
Looks like a drill shaving. May well be from when I had to heli-coil my bowl cover screw holes, back during my last rebuild in '08.
So, first I felt like an idiot for letting it get in there, then I felt all proud that I had quite possibly found the source of my troubles... then I put everything back together and tested, and had no change in symptoms.
Then, I became depressed.
I did notice that the richer bore was completely dry when I pulled it; which means to me that all the gas that was behind the needle had already drained down into the manifold.
I'm going to try your spring stretching trick next.
I'm not aware of any weight involved with the richer circuit on my carb, though... Only two weights I've ever seen are the counterweights for the ball valves on the accel pump inlet and outlet.
I pulled the richer yesterday, working on exactly the same premise, and inspected the needle and the seat closely, flashlight & mirror style.
Didn't see anything utoward, but I figured I'd swap the bore out with a q-tip full of carb cleaner, just on general principles.
When I pulled the q-tip back out, I found this curled around the exact center:
Looks like a drill shaving. May well be from when I had to heli-coil my bowl cover screw holes, back during my last rebuild in '08.
So, first I felt like an idiot for letting it get in there, then I felt all proud that I had quite possibly found the source of my troubles... then I put everything back together and tested, and had no change in symptoms.
Then, I became depressed.
I did notice that the richer bore was completely dry when I pulled it; which means to me that all the gas that was behind the needle had already drained down into the manifold.
I'm going to try your spring stretching trick next.
I'm not aware of any weight involved with the richer circuit on my carb, though... Only two weights I've ever seen are the counterweights for the ball valves on the accel pump inlet and outlet.
03-16-10, 04:53 AM
#11
Nikki-Modder Rex-Rodder
Join Date: Feb 2001
Location: Trying to convince some clown not to put a Holley 600 on his 12a.
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Ok. My garage is filled with Nikkis in various states of assembly & disassembly, and I have piles of Nikki parts & Nikki carcasses scattered about everywhere, and as such I am a complete exception to the usual rotary enthusiast home Nikki rebuilder, but DUDE! -You really put helicoils in your carb body?! REALLY?! lol
I'm sorry, but that officially makes you Nikki-OCD.
No weight, eh? Hmmm...
I'm sorry, but that officially makes you Nikki-OCD.
No weight, eh? Hmmm...
03-16-10, 03:30 PM
#12
Ok. My garage is filled with Nikkis in various states of assembly & disassembly, and I have piles of Nikki parts & Nikki carcasses scattered about everywhere, and as such I am a complete exception to the usual rotary enthusiast home Nikki rebuilder, but DUDE! -You really put helicoils in your carb body?! REALLY?! lol
I'm sorry, but that officially makes you Nikki-OCD.
I'm sorry, but that officially makes you Nikki-OCD.
I truly wish I had a spare appropriate to the car, but after nearly two years of trying to find one, I kind of gave up. It would make troubleshooting SO much easier to have a valid swap.
So, after two of the threaded holes for the bowl cover gave up their soft little ghosts after decades of rebuilds, I just said to hell with it and helicoiled all seven of them. One problem that will never happen again.
Scary, though, drilling holes in your only good part. One screw-up, no car.
Yah; diagram for an '80 Cal MT seems to agree; only two *****, and two weights.
03-20-10, 11:19 PM
#14
Crap.
Now I'm back to where I started; both fuel bowls empty only a couple hours after shutdown. Clearly going down the bores, as the car is nearly flooded while trying to start afterward.
I'm running out of ideas. Absent an overpressure in the vapor space above the bowls, there should be no way for the front bowl to empty after vacuum is lost. There's physically no path for the fuel to take that doesn't require vacuum or pressure to get there. Yet all the bowl and tank vent parts seem to test out fine.
Sugestions, anyone?
I've got about a week left before passing smog becomes more complicated due to my plate sticker expiring.
Now I'm back to where I started; both fuel bowls empty only a couple hours after shutdown. Clearly going down the bores, as the car is nearly flooded while trying to start afterward.
I'm running out of ideas. Absent an overpressure in the vapor space above the bowls, there should be no way for the front bowl to empty after vacuum is lost. There's physically no path for the fuel to take that doesn't require vacuum or pressure to get there. Yet all the bowl and tank vent parts seem to test out fine.
Sugestions, anyone?
I've got about a week left before passing smog becomes more complicated due to my plate sticker expiring.
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