# carburetor modification



## nissanwagon (Jun 23, 2012)

Hi to everyone

My name is charlie and im from Guatemala. here in Guatemala we have a LOT of nissan sentras and mostly B11, B12 and B13 because they are economic cars. i have a daily driver 1987 sentra wagon since i bought it from the dealer a long time ago. its my baby and you guys have no idea the things that i have done to take care of her. ive been recently thinking on doing some mods to keep her in good condition. now i´m interested on modifying the engine to get decent power increase from the E16S and i have run into some questions hoping any of you guys can help me.

i have the hopeless chokeless carb and its a pain in the back because it wont start in the morning and as soon as i press the gas it dies. i live near a lot of yunk yards and wreck yards and i found a honda CBR 954 RR carb assy and i took it home since i saw a couple of threads showing how to make this conversion. how to attach the brake vacuum hose to the carb since they are individual runners? would the new carbs need any other modification? do i need a fuel pressure regulator? 

I know i need to swap the distributor and get one from an 83 sentra. how do i identify the new dizzy? is there any series # that i need to look for?

I will also will be building my own set of headers. is there any tube diameter, runner length, or any special instructions i should fallow?

i appreciate in advance all your help. anything will be much of a help.


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## LvR. (May 26, 2010)

Brake vacuum hose doesnt get connected to the carb at all - its supposed to be connected to manifold vacuum directly so imo this is a non-issue.

Chokeless carb? - OEM or after market? - I cannot remember ever seeing a OEM motor sans choke.

Bike carb assembly on a Sentra? ................ everything is possible. The bike carbs will then feed each of your Sentra cylinders individually no?. Problem with that is vacuum advance for the distributor ................ where are you going to pick that up? Even if you mod the bike carbs to somehow create a port next to the butterfly you will have another bigger problem : Vacuum advance is a function of air speed past the throttle plate and that bike carbs will most certainly have a different throat diameter resulting in a funky vacuum signal for the timing - chances are good negating the whole purpose of the mod.

I would suggest going with another actual 1600cc engine's carb - something like a Weber/SU/Pierburg that is already properly calibrated with the correct flow characteristics and that will already make a vacuum advance port available for use


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## jdg (Aug 27, 2009)

LvR. said:


> Brake vacuum hose doesnt get connected to the carb at all


Old school V8's with big ol' Q-Jets sometimes had the brake vacuum connected to the base of the carb...but, as you say, non-issue.



> Chokeless carb? - OEM or after market? - I cannot remember ever seeing a OEM motor sans choke.


Again, really old school...back in the day when a person had to know how to do the 'big block boogie' on the gas pedal when starting it on a cold day.



> Bike carb assembly on a Sentra? ................ everything is possible. The bike carbs will then feed each of your Sentra cylinders individually no?.


I dunno...some of those bike's have pretty big engines, fairly good sized carb's.



> Problem with that is vacuum advance for the distributor ................ where are you going to pick that up?


Straight off the manifold vacuum...if there's a manifold left after adding 4 small carb's!
Wow...talk about synchronizing hell...


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## LvR. (May 26, 2010)

I doubt the vacuum advance I am talking about can be tapped off the manifold vacuum directly - manifold vacuum is directly proportional to load sure, but hanging the distributor ported vacuum advance off that is going to royally screw the pooch ito timing (carb'ed Sentras run with a dual vacuum advance dissy) - you will be idling with around 30 degrees timing (not really that big a problem as long as you are aware of that and compensate for it at the time of setting timing and idle mixture on the carb) but you are completely losing the ability to compensate properly for lean mixtures at higher engine speed with partial throttle settings (high flow rate across throttle plate) and you will see it in both performance and economy.


> some of those bike's have pretty big engines, fairly good sized carb's


Sure ito of fuel and air flow rates and mixing its absolutely no problem, but ito feedback vacuum signal calibration and compatibility with the Sentra's distributor's required calibration its going to be a crap-shoot if not plain crap.

Yeah ............ and as far as the Q-jets and Holleys on V8s are concerned - I really doubt most of the Sentra drivers here even know what we are talking about. I still run a Pierburg carb on a Ford V6 that also don't need a choke if the operator is awake and aware of the situation. Perhaps a very good mod for the OP is to rather fit an old 650 or a 750 cfm Q-jet on the Sentra - imo those things had tremendously accurate metering systems due to their miniscule primary throats (read very nice economy and driveability) and you could extract every ounce of power from the motor once those secondaries open up


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## smj999smj (Jan 1, 2006)

LvR.: The "hopeless chokeless" he is talking about is the VCM (vacuum control module) system that Nissan used on the 84 through 87 Sentras. It had no choke, but did have a ton of vacuum hoses, a couple of thermo switches, a vacuum switch or two (depending on year), a vacuum control module on the right side of the engine compartment and a throttle actuator assy. on the left side of the carb that people would mess with the adjusment screw when they should have just left it alone. A lot of times, the tiny filter in the VCM would get dirty and keep the system from working until you hit it a few times with a hammer. The service manual never really did a good job of explaining the system, but there was a very good Tech Talk magazine article on the system a long time ago. The choke system used in 83 and earlier models was a much simpler and for the DIY'er, a better system. The VCM system, when it work was a good system. The biggest problem was when it didn't work right, most of the people who tried to fix it didn't have a clue on how it worked and didn't take the time to find out.


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## LvR. (May 26, 2010)

Ah - thanks for the clarification - probably the same sort of "choke-less" carb system the smaller Opels of the same era employed with an equally perplexing set of plumbing


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## smj999smj (Jan 1, 2006)

Could be but I can't say... I didn't work on too many Opels!


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## nissanwagon (Jun 23, 2012)

im sorry but im kind of slow for this. i was just trying to get some info about how to add this bike carb assy to my E16S bu it seems that i will run into many problems with the distributor. how about a ford EDIS with a crank triger? is that something possible to do?


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## smj999smj (Jan 1, 2006)

nissanwagon said:


> im sorry but im kind of slow for this. i was just trying to get some info about how to add this bike carb assy to my E16S bu it seems that i will run into many problems with the distributor. how about a ford EDIS with a crank triger? is that something possible to do?


It'd be the first that I heard of!


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## LvR. (May 26, 2010)

Anything is "possible" if you have the know-how and the motivation but tbh to me it sounds like you have neither


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## smj999smj (Jan 1, 2006)

LvR.---You forgot to mention an unlimited wallet!


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## fairlanephantom (Feb 3, 2010)

I guess it's really more of a question of what you're willing to get into with a modification on the car. I would say go with the Weber conversion as it's already been tried and proved to work well on the engine. I did it on mine about two years ago with my daily driver Sentra, and never had any trouble with the carburetor or ignition. I went with a vacuum advance distributor off an 84 just because there's only one vacuum line unlike the original design. Multi carb setup would be cool, but in my opinion not worth the hassle of synchronizing if it's a car you drive to work.


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## smj999smj (Jan 1, 2006)

84 Sentras in the US had a VCM, so no vacuum advance on the distributor; 83 was the last year for the vacuum advance on US models. IIRC, I think Canadian models were still using the vacuum advance in 84, but I'm not 100% sure.


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## fairlanephantom (Feb 3, 2010)

Depended on the emissions package. Canada/California had vacuum advance and Federal went to electronic advance. My B12 is an 87 with Federal, so it did have the electronic setup and VCM originally. As far as the rest of the ignition, I have the original canister type coil from a B11 and a new ballast and condenser off a 50s Ford just because they're easy to get.


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## nissanwagon (Jun 23, 2012)

*re*

thanks for all of your answers guys...you´ve helped me a lot to make a decision. i went in search for the parts for that conversion but there was no place here in guatemala that could sell either a 36-1 ring neither a megasquirt ecu so i just dropped that as an option but i still want to do the carb conversion either with individual carbs or a weber unit. 

i guess i didnt understood the info provided by previous posts you guys made so here it goes again:

brake vacuum hose: on factory set up, the hose is connected at the intake manifold but if i have individual runners...do i have to get vaccum signal from only one or do i have to get it from all individual runners?

ignition: i found a crane cam fireball XR700 optical ignition system that i had no clue i had it since my dad brought some stuff from USA a couple of months ago. is that something i can use in my car? is there something else i need to do? in regards to coil...can i use the existing coil or do i have to use a different one?

thanks in advance.


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## LvR. (May 26, 2010)

nissanwagon said:


> ...........
> 
> brake vacuum hose: on factory set up, the hose is connected at the intake manifold but if i have individual runners...do i have to get vaccum signal from only one or do i have to get it from all individual runners?


The intake only shows "usable" vacuum at near closed throttle position - iow your vacuum assisted brake booster will show a marked tendency to only work as designed at close to idle speed on any engine - as such its basically irrelevant where you pick up the vacuum signal as long as its as close to the valves as possible. Its basically impossible to pick up a vacuum signal from all individual runners simultaneously - that defeats the whole purpose of individual runners ie to have individual runners that is totally disconnected/isolated from the others. If the engine is in good condition the vacuum signal (meter reading anyway) per runner will be exactly the same as in a combined intake manifold - there will be a difference in the volume of air that can be sucked though and that may manifest as a delayed effective operation of the booster but your eventual brake assistance ito power will be exactly as before.

ITO the ignition - if you currently already have electronic ignition on the car I doubt that there is anything to gain by replacing the ignition system.


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## fairlanephantom (Feb 3, 2010)

With the ignition, it depends on what coil you have in there already. If you have the conventional type coil like the B11 had or any classic car, you'll be all set. If you have the later coils they had when they went into Hitachi and Mitsubishi coils, you'll have to retrofit the older style in. It's simple though, just get a universal coil bracket, and it'll bolt right into the same location with no drilling. Like we mentioned before, you'll still have to get the vacuum advance distributor if you don't already have it.

You'll also need a ballast and condenser to connect to the distributor. Without the condenser in this setup, you'll never get spark. Also without a ballast, you be killing coils every few weeks.


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## nissanwagon (Jun 23, 2012)

it is actually a the second option you mentioned...i mean, the hitachi and mitsubishi coil system that is in now. no condenser or ballast is on now, however with this CRANE XR 700 it tells me that there is no need to use them. i just want to make sure that by the time i finish the installation would i have any problem with the computer reseting the timing? would i be able to ditch the computer at last? would i need a different dizzy from an 83 or 84 model? 

thanks in advance


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## fairlanephantom (Feb 3, 2010)

There's other things that are routed through the computer and it also controls your fan. So you still have to keep the computer, but it won't be doing nearly as much as it is doing now. Simply put, it won't have any impact on the engine performance anymore after the change over. 
No matter what carburetor setup you decide to go with, you'll still need to get that vacuum advance distributor. The original works with the computer and the original feedback carburetor to determine timing. So you'll have to locate one, and if you're lucky it may come with the original harness if you find a salvage one. I made a harness myself because I went with a new distributor.


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