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PostPosted: Mon Oct 05, 2009 3:55 pm 
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Location: Oklahoma City
I was back in the Rockies for the weekend and during the trip, a simple mod occurred to me which I've been very happy with over the last 1500 miles (which is actually only 3 days, but hey). It's actually 2 mods that work together. But I've already been using one of them for years:

1. Disable the carburetor secondary
2. Disable the carburetor W.O.T microswitch (This is the one I've had in place for years.)

The idea behind both of these things is to keep the mixture as close as possible to stoichiometric at all times. (14.7:1) And I should probably stop here and explain my reasoning for why I think it is beneficial.

The feedback carb on the mk1's is controlled by the ECU based upon information from the O2 sensor, keeping the mixture stoichiometric by modulating the MCS (mixture control solenoid) in the carb. This only applies to the primary system, however, and not to the secondary, which has no feedback or ECU control. It's just a regular, old-fashioned secondary. Experience has shown me that pushing the accelerator pedal past that little bit of back-pressure that I feel at about the 2/3rds point results in a precipitous drop on fuel economy. I've always assumed that the back-pressure indicates the point at which the secondary opens. (Though I may be wrong. More on that later.) Based on that sharp drop, I've always figured that the secondary delivers a rich (power) mixture. It makes sense, since the pedal is already past 2/3rds of the way down. The primary, with its closed loop system, is going to try to compensate for that by leaning itself out. But as the throttle is opened wider, it is going to be less and less able to do so.

When the pedal is very near the floor, the W.O.T. microswitch operates, and I assume that tells the ECU to go ahead and richen the primary system, as well, to yield full engine power, at the expense of fuel economy.

Thus, my driving technique, for years, has been never (well, almost never) to push the pedal past that back pressure, to avoid using the secondary. (Despite the obvious temptation to do so going uphill into a headwind!) Note that this has the effect of not fully utilizing the primary since it is never fully open. Also note that that long ago, I disabled the W.O.T. switch. (It was later that I stopped ever pushing the pedal further than the back-pressure almost entirely.)

So while going uphill into a headwind this weekend, I got to thinking "wouldn't it be nice to be able to at least use the primary fully?". Well, now I can.

The secondary is vacuum-operated. So you can just pull the hose from the secondary diaphragm on the front of the carb and plug it to leave the secondary out completely. (Isn't that simple? :) ) And the W.O.T. switch is most easily disabled by removing the nut that holds it on, and turning the actuator lever around so that the tab faces away from the carb and does not operate the switch. (Note that I am not sure if the switch is N.O or N.C., so disconnecting a wire may or may not do what we want!)

Now I can push the pedal all the way to the floor whenever I want... even hold it there for extended periods... and I know that the mixture is always stoichiometric. Pumping losses are minimal at that point. Although the vacuum timing advance is likely nil. I get full benefit from opening the primary all the way. But I lose the power boost that the the secondary, and the fuel enrichment gives me. Thing is... that doesn't seem to be a whole lot. Pushing the pedal past 2/3rds still gives me a little more power. Not *quite* as much as is used to. But it was never all that much; The difference was never all that great. The primary seems to carry the little 1-liter pretty well. And the power boost from the mixture enrichment is pretty marginal. Especially when you consider the substantial (20%) fuel economy penalty it exacts.

So far, I am quite pleased. I feel like I have significantly increased my "usable" power. I can give it as much throttle as I like when negotiating hills and headwinds at today's 75mph speed limits. The 4 tanks since I made the mod were:

58mpg (In the mountains. It always does well in the mountains.)
56mpg (In the mountains. Ditto.)
46mpg (On the trip back to OKC. 75mph. Hills. Crosswinds.)
44mpg (On the trip back to OKC. 70mph. Some hills. Crosswinds. Rain. )

This is in line (perhaps slightly better) than what I am used to. And I was operating the accelerator pedal in a way that I would have considered insane before. And making measurably better time than I would have otherwise.

I'm so happy. :D

Though I should caution that more test data is necessary before drawing definite conclusions. A part of me is "so happy", but another part is "cautiously optimistic".

However, I mentioned earlier that I might have been wrong about the pedal back-pressure being when the secondary starts to open. I was thinking like it was the Rochester Quadrajet on my 1968 Cadillac. But the Sprint's carb, as mentioned, has a *vacuum* secondary. So I'm wondering what that pedal back-pressure is? At this point, I'm thinking that the secondary is mechanically prevented from opening before a certain point, and that the extra spring tension occurs at that point. But I'm not sure.

This is a super easy thing to try. And I would be interested in other folks' results, should anyone decide to play with the idea.

P.S. I've been driving since 4am and just got back. So there are probably all sorts of spelling/semantic errors, etc. I'll try to clean this up later. And also look in the FSM to see if I can see what the spring tension is really about.

-Steve


Last edited by sbergman27 on Mon Oct 05, 2009 4:21 pm, edited 2 times in total.

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PostPosted: Mon Oct 05, 2009 4:15 pm 
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Joined: Tue Feb 10, 2009 4:09 pm
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Location: Brinkworth, UK
The WOT switch is to disable closed loop operation because the O2 sensor at max. load, this is because it is temperature sensitive and inaccurate when too hot.
Closed loop operation only operates on one choke because it is only intended to make small changes to fuel mixture.
Disabling the secondary simply reduces the amount of air that can flow into the engine, you could achieve the same result with a block of wood under the accelerator pedal.


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PostPosted: Mon Oct 05, 2009 4:33 pm 
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Location: Oklahoma City
When you say "inaccurate when too hot", what does that mean exactly? Does it read oxygen when there is none? Or does it fail to read oxygen when it is present? And what is the condition when the pedal is at, say, 3/4, and so the secondary is, say, half open, the primary is mostly open, and the W.O.T switch is not actuated yet? (I'm guessing at "3/4" and "half" but you get the idea.)

-Steve

P.S. Also, when you say "disable closed loop operation" I assume that leaves the MCS at full rich?


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PostPosted: Mon Oct 05, 2009 11:57 pm 
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Joined: Fri Aug 10, 2007 9:43 pm
Posts: 490
Location: Everett Wa
sbergman27, could you please post a picture of the work you did on the carb...

Thanks in advance .....................

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Dale

1985 Chevy Sprint Mk1 G10 5 Speed Sky Blue


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PostPosted: Tue Oct 06, 2009 11:13 am 
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Location: Oklahoma City
Here you go, Dale.

In this image, the vacuum line to the secondary diaphragm has been removed and plugged:

image_id: 17796 Click to enlarge

And here, the W.O.T microswitch actuator has been turned around so that the tab faces out, away from the carburetor:

image_id: 17797 Click to enlarge


Of course, Rhinoman's comment throws into question the utility of these mods. But if the O2 sensor is still accurate with the primary completely open and the secondary completely closed, I'm happy.

Nice looking little blue '85 you have there, BTW. :)

-Steve


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PostPosted: Tue Oct 06, 2009 11:51 am 
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Location: Everett Wa
Thank you and Thanks for the photos ............

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Dale

1985 Chevy Sprint Mk1 G10 5 Speed Sky Blue


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PostPosted: Thu Oct 08, 2009 4:40 pm 
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Location: Brinkworth, UK
sbergman27 wrote:
When you say "inaccurate when too hot", what does that mean exactly? Does it read oxygen when there is none? Or does it fail to read oxygen when it is present? And what is the condition when the pedal is at, say, 3/4, and so the secondary is, say, half open, the primary is mostly open, and the W.O.T switch is not actuated yet? (I'm guessing at "3/4" and "half" but you get the idea.)

-Steve

P.S. Also, when you say "disable closed loop operation" I assume that leaves the MCS at full rich?



The output at high temperatures is indeterminate, different O2 sensors will give different readings, you would have to calibrate the ECU to each sensor, which is not practical so the programming only allows the O2 sensor to be used under conditions where its output can be relied upon, i.e. not cold and not hot. At WOT the O2 sensor reading could be anything so its not possible to use closed loop control. If you have no pressure sensor then the WOT switch alone will control open loop/closed loop, the ECU will assume that the secondary is working.
I don't have any specific details on open loop operation with a carb engine, the injected Suzukis that I have hacked all use the long term fuel trim to adjust fuelling under open loop conditions. My understanding is that with a carb an air bypass is used to adjust fuelling, I would expect the default setting to be half way between rich and lean (when new/reset) in the same way that an injection system would operate. The way to test it is to put a scope on the signal and check the duty cycle.

Of course there is the possibility that you can read live data from the ECU, do you have an ALDL connector?


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PostPosted: Wed Oct 14, 2009 12:02 pm 
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Location: Oklahoma City
Thank you for the response. It is appreciated. I've followed your posts, and you seem to know what you are talking about.

I guess my question would be how a too high 02 sensor temperature can be reliably determined from a WOT switch. At 90% throttle you're fine. But at WOT you suddenly have to leave closed loop because the O2 sensor is unreliable? Irrespective of the engine rpm, ambient temperature, etc? (Ambient temperature may not be all that significant. But it seems like there are a lot of variables for it to be likely that a WOT switch would catch it at the right time.) Seems to me that a thermocouple in the exhaust manifold would be a better way to determine. Of course, back then things were pretty crude. They didn't always do things in the best way.

I would disagree that what I have done is equivalent to putting a block of wood under the pedal. The "block of wood" strategy is what I had been trying to do with my leg muscles all these years, by trying to stay out of the secondary. And I never had full use of my primary, because the secondary starts operating before the primary is fully open. (I don't know that for sure. It is an assumption on my part.) I am further assuming, based upon the fact that every time I have indulged myself by pushing the pedal further during a trip it has exacted a heavy toll on fuel economy, that the secondary delivers fuel not just without the benefit of feedback, but actually aiming for a richer mixture than stoichiometric.

I've traveled another 3000 miles since my last post (Yes, really. Shadow the terrier and I are shuttling between here and the Rockies weekly now. :D ) and I've been pushing the pedal to the floor whenever I feel the need. And there has been a penalty in mpg. But not nearly what it used to be. And I still do feel noticeably more power when I push past my old pedal limit to the floor. (Not that much less than when I had the secondary enabled. Though I've had the WOT disabled so long that I can't make a definitive comparison to that. I don't recall losing all that much when I disabled it.) I still don't have enough data to say it's conclusive. Too many variables to say so at this point. after just a few thousand miles. But overall, I'm finding that driving the car in an economical fashion is easier and more fun now.

With the secondary disabled, I'm guessing that O2 temperature would not, generally, go as high as with it enabled.

I don't believe that this ECU has any diagnostic connector. That would be a 5, 10, or 12 pin connector which is normally not connected, right?

-Steve


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PostPosted: Fri Oct 23, 2009 12:46 am 
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Joined: Fri Aug 14, 2009 12:47 pm
Posts: 93
Location: CA
SBergman, I like your analytical thinking, Most people would not contest conventional thinking or factory engineering. In fact some do not like having their ideas chalenged. I know you have made some people unhappy on this board by making asumptions based on reasonable ideas; there are many reasons for doing things a certain way, some of which you may not be aware of, so be careful with how you state your conclusions.
I enjoy your posts, I like to stir up the pot myself.
BTW, stoichiometric is good for cruise, but not for all operational modes. Fueling would be easy if that were the case. The optimal A/F ratio varies with load and cooling requirements.
Now, if someone would just sell me one of their extra Sprint ERs or Sprint Metros, That would really make me happy!
How about a 50$ finders fee.
SA


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