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1970 roadster 1600 carb questions


fryanbacon

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The SU type carburetor (also known as a constant velocity carburetor) has a throttle plate behind the slide or piston that is connected to the gas pedal. When the throttle is opened, the air passing through the almost closed slide generates a vacuum that lifts the slide. This allows more air in keeping the vacuum signal regulated and even. As the engine revs up the slide rises in relation to the need for more air, but the air velocity through the venturi below the slide remains.... constant. The SU increases and decreases it's internal size automatically in direct relation to the engine's needs.

 

If it is running and you lift the slide with a finger it will ingest too much air and not enough gas and probably quit.

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So now I am wondering about this, you said you cleaned everything fuel related, does this mean you pulled the carbs apart?

Basically I am wondering if you touched them needles with your fingers, and I mean with anything even a tooth brush, did you remove the needles from the pistons, did you even pick the needle up with your fingers off the counter/shelf, did you touch them at all?

Another thing I want to get back to, you said that line that people on here are saying is the vacuum advance had what looked like fuel coming out of it, tell me more about that, where is the other end of that line connected?

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2 minutes ago, wayno said:

So now I am wondering about this, you said you cleaned everything fuel related, does this mean you pulled the carbs apart?

Basically I am wondering if you touched them needles with your fingers, and I mean with anything even a tooth brush, did you remove the needles from the pistons, did you even pick the needle up with your fingers off the counter/shelf, did you touch them at all?

Another thing I want to get back to, you said that line that people on here are saying is the vacuum advance had what looked like fuel coming out of it, tell me more about that, where is the other end of that line connected?

no i have multiple sets of carbs and i simply just picked the cleanest ones as i didn't want to take them apart, and the ones i put on were practically brand new besides some signs of wear on the outside. The two lines that came from the bowls people were saying was for overflowing and every photo I've seen shows them running to the bottom of the engine.

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48 minutes ago, datzenmike said:

The SU type carburetor (also known as a constant velocity carburetor) has a throttle plate behind the slide or piston that is connected to the gas pedal. When the throttle is opened, the air passing through the almost closed slide generates a vacuum that lifts the slide. This allows more air in keeping the vacuum signal regulated and even. As the engine revs up the slide rises in relation to the need for more air, but the air velocity through the venturi below the slide remains.... constant. The SU increases and decreases it's internal size automatically in direct relation to the engine's needs.

 

If it is running and you lift the slide with a finger it will ingest too much air and not enough gas and probably quit.

so that makes me wonder if its not getting enough air because when i open it and crank is when it sputters and almost runs but when they are closed is when it doesn't even try to start.

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6 minutes ago, fryanbacon said:

so that makes me wonder if its not getting enough air because when i open it and crank is when it sputters and almost runs but when they are closed is when it doesn't even try to start.

Opening the slide will allow way too much air in. The slide is at the idle position where it needs to be for running during a start.

 

Do you have the chokes working? Pull the plugs and examine them, are they wet? If wet try less or no choke till they dry out.

 

 

BTW the fuel system ends with the carburetor and cleaning the piston tops is way too excessive and not needed. The intake and head ports and valves merely deliver the gas and air mix.

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11 minutes ago, wayno said:

I am talking about this line in your photo your finger is touching, does it have liquid coming out of it?

20201009_143012.jpg

when i didn't have i snug it it did a little bit, but im not sure if i still does

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3 minutes ago, datzenmike said:

Opening the slide will allow way too much air in. The slide is at the idle position where it needs to be for running during a start.

 

Do you have the chokes working? Pull the plugs and examine them, are they wet? If wet try less or no choke till they dry out.

 

 

BTW the fuel system ends with the carburetor and cleaning the piston tops is way too excessive and not needed. The intake and head ports and valves merely deliver the gas and air mix.

yes i have he chokes hooked up now and when i pulled the plugs they were covered in something black and slightly chunky, or it could have been buildup over time, bu i cleaned and reinstalled them

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another question,  there was a red oil in one of he carbs and it seemed smooth compared to the other one, is this a carb lubricant? i spilled it and am wondering what it would be because it seemed important.

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Damper oil. Many just use ATF for this or 3 in 1 oil. Factory recommends 20w but anything is better than nothing. Too thin and you may experience hesitation on quick acceleration so try thicker.  As there is no set viscosity rating for ATF make sure both are filled from the same container so they react evenly.

 

The viscosity of the damper oil prevents the slide from suddenly opening when the throttle is floored at low speed. This delay of the slide rising causes an increase in air velocity through the venturi drawing in more gas than necessary and serves as an accelerator pump like a regular carburetor.   

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ok sweet. Got it running by itself finally! but wouldn't call it a victory as it sputters and is puffing white smoke.... what would this mean?

also i realized both carbs were missing the needles by the pistons! so i put new ones in and then came he adjustments on the carbs and i just couldn't get it to run smooth no matter how hard i tried

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No needles would run extremely rich, extremely rich. What size needles as there are dozens of sizes to chose from.

 

White smoke could be just normal steam from the cold. Could be the gas evaporating from inside the exhaust pipes. Give it a while to burn off.

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If it had no needles it likely flooded out right away, I talked about the needles, ZTherapy has a jig to install the needles, it is very hard to get them centered and not dragging on the seat, they even get sticky where you give it the gas and the pistons don't move, another thing I asked is if you had them apart did you touch the needle, or pick the needle up with your fingers off the counter, you can bend them picking them up, it may not be bent enough to see but the seat will know it is bent.

I use power steering fluid in my carbs now, but I have used ATF in the past also.

 

How I adjust a new carb to me is to close the fuel mixture screw, it is a thick washer on the bottom of the carb in the photo below, it's the thing below the spring, then I open both carbs the same amount, I cannot remember how many turns now, try 2 complete turns and see if it starts, if it starts then you need to adjust the air flow using a "uni-sync" air meter, here is a link to one on ebay, I am not saying buy that one, the link is to show you what you need/to look for.

https://www.ebay.com/itm/VW-TYPE-123-POOR-MANS-UNI-SYNC-TOOL-BUG-BUS-CARBURETOR-SYNCHRONIZER-WEBER-EMPI/401470388457?hash=item5d797ff8e9:g:EBsAAOSwL1RcbFv~

You can get them close by eye by adjusting them so while it is running they both lift around the same amount at an idle, but you really need the uni-sync air meter to do it right, you also need to make sure both carb arms move at the same time when you push on the pedal or push on the linkage with your finger, then use the uni-sync again to make sure the air flow is the same.

 

DSCN1007.thumb.JPG.5232955e98a0848aa7af44d83fda4f4f.JPG

 

Now the next step is a little harder, but it has to idle smoothly to adjust the fuel mixture, on the bottom of these carbs is a little pin with a spring on it, when you push up on the pin the piston raises enough to disable that carb, so while it is idling you push up on the pin, if you cannot find the pin then use the tip of your finger to lift the piston, do not touch the needle, now with that carb disabled if the engine dies you need to open the fuel mixture screw(that washer) on the other carb until the engine does not die when you disable the other carb, now you slowly close the fuel mixture screw until the engine just starts to chug but it keeps on running(chugging), then you let the piston drop and it should smooth out, now you lift the other carbs piston and disable it and you do the same adjustment until it chugs exactly the same as the other carb did, now the fuel mixture is almost the same, you then adjust the idle using the idle screw, if you don't have one then you use the air flow meter(uni-sync) and adjust the idle with them, the air flow needs to be the same in both carbs.

Now when you disable a carb and start closing the fuel mixture screw on the other carb and you close it all the way and the engine is still running, then that carb is not serviceable and it likely needs rebuilt as it should die before you get it closed all the way, you want it to chug, chug, chug and not have it die, but if you can close the fuel mixture all the way and it still is chugging away/running that is bad, it's getting fuel when the fuel mixture screw is closed which means it is not serviceable.

 

This is how I adjust my carbs and I learned it in a manual, I took my truck with dual SUs to a guy that races them and he said I had it adjusted very close to perfect, but that was with a set of brand new carbs I bought from Nissan during a clearance sale they had back in the 90s, used carbs are harder to deal with.

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1 hour ago, wayno said:

If it had no needles it likely flooded out right away, I talked about the needles, ZTherapy has a jig to install the needles, it is very hard to get them centered and not dragging on the seat, they even get sticky where you give it the gas and the pistons don't move, another thing I asked is if you had them apart did you touch the needle, or pick the needle up with your fingers off the counter, you can bend them picking them up, it may not be bent enough to see but the seat will know it is bent.

I use power steering fluid in my carbs now, but I have used ATF in the past also.

 

How I adjust a new carb to me is to close the fuel mixture screw, it is a thick washer on the bottom of the carb in the photo below, it's the thing below the spring, then I open both carbs the same amount, I cannot remember how many turns now, try 2 complete turns and see if it starts, if it starts then you need to adjust the air flow using a "uni-sync" air meter, here is a link to one on ebay, I am not saying buy that one, the link is to show you what you need/to look for.

https://www.ebay.com/itm/VW-TYPE-123-POOR-MANS-UNI-SYNC-TOOL-BUG-BUS-CARBURETOR-SYNCHRONIZER-WEBER-EMPI/401470388457?hash=item5d797ff8e9:g:EBsAAOSwL1RcbFv~

You can get them close by eye by adjusting them so while it is running they both lift around the same amount at an idle, but you really need the uni-sync air meter to do it right, you also need to make sure both carb arms move at the same time when you push on the pedal or push on the linkage with your finger, then use the uni-sync again to make sure the air flow is the same.

 

DSCN1007.thumb.JPG.5232955e98a0848aa7af44d83fda4f4f.JPG

 

Now the next step is a little harder, but it has to idle smoothly to adjust the fuel mixture, on the bottom of these carbs is a little pin with a spring on it, when you push up on the pin the piston raises enough to disable that carb, so while it is idling you push up on the pin, if you cannot find the pin then use the tip of your finger to lift the piston, do not touch the needle, now with that carb disabled if the engine dies you need to open the fuel mixture screw(that washer) on the other carb until the engine does not die when you disable the other carb, now you slowly close the fuel mixture screw until the engine just starts to chug but it keeps on running(chugging), then you let the piston drop and it should smooth out, now you lift the other carbs piston and disable it and you do the same adjustment until it chugs exactly the same as the other carb did, now the fuel mixture is almost the same, you then adjust the idle using the idle screw, if you don't have one then you use the air flow meter(uni-sync) and adjust the idle with them, the air flow needs to be the same in both carbs.

Now when you disable a carb and start closing the fuel mixture screw on the other carb and you close it all the way and the engine is still running, then that carb is not serviceable and it likely needs rebuilt as it should die before you get it closed all the way, you want it to chug, chug, chug and not have it die, but if you can close the fuel mixture all the way and it still is chugging away/running that is bad, it's getting fuel when the fuel mixture screw is closed which means it is not serviceable.

 

This is how I adjust my carbs and I learned it in a manual, I took my truck with dual SUs to a guy that races them and he said I had it adjusted very close to perfect, but that was with a set of brand new carbs I bought from Nissan during a clearance sale they had back in the 90s, used carbs are harder to deal with.

holy crap thanks man ill definitely have to do that tomorrow, but thanks for giving such a detailed description and i cant wait to have the thing running right!

You guys have been awesome in helping me and it has been really fun learning about these carbs and how one little piece is so critical to the engine running perfect!

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There are other things that can go wrong like the float in one of the bowls starts to sink or outright sinks, but the over flow will start puking fuel if it sinks.

I failed to mention that once you get it running and driving, then every once in a while you pull all the plugs and check the color, if they are black you might close the fuel mixture screw a quarter turn, if they are white you will open it a quarter turn, your looking for a tan color.

It may be the back two cylinders are black while the front two cylinders are tan, so you only lean out(turn in) the back carb and leave the front carb alone, I had a carb(back one) that was black all the time even when I completely closed the fuel mixture screw, the carb was not serviceable and eventually I started having issues at freeway speeds and had to take that set off and I put a fresh set on the engine that I had bought not too long before that.

Most the time the reason carbs are removed is because they are acting up, yet the guy selling them says the engine ran when I removed them, that does not mean it ran good, it just means it ran, mine ran fine around the city streets, but when I got on the freeway it would start losing power, the more I put my foot in the pedal the worse it got, but when I exited the freeway it ran fine again, but I drive under special circumstances, my 521 weighs 3900lbs, my trailer weighs close to another 2000lbs, with me in it empty I am rolling at over 6000lbs, most the time I have debris in the flatbed that just adds to the weight, that is a lot of weight to get moving and keep moving, so my foot is into the pedal quite heavy all the time, I burn a lot of gas driving that truck, but it is my work truck so it is what it is.

Another thing to keep in mind, I rarely touch my Roadster carbs or my work truck carbs, once set they usually last a long time, they do not need to be adjusted all the time, like I said if you remove the spark plugs and see black close the fuel mixture a little and maybe in a month pull the plugs to see if it is running cleaner, if not close it a little more, you do not have to tinker with it every week, I have went over a year easily without even looking at the plugs, I am a firm believer in if it ain't broke don't fix it, if it is running good I leave it alone.

 

I also forgot to say that you adjust the air flow by opening/closing the screws on the inside back of the carb dome, very little movement can be a lot of difference, a quarter turn or less and check the flow meter, set the idle speed while your doing it, and make sure the throttle cable is slightly loose, when setting the 1600 Roadster carbs I believe your supposed to remove the linkage connected to the carbs, what a pain, I just made sure they both moved at the same time when I pushed on the throttle cable and called it good, there are a lot of little things, that is where a manual is a good thing to have, I believe I used a Chiltons manual to learn to adjust the carbs and rebuild the 4spd transmission, but that was 25 years ago so I don't remember for sure it was that manual.

Edited by wayno
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