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Dank_Mochi

carb tuning

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hey guys, I have a 1971 521. This is the first ever car i have taken apart and put together. Had the engine rebuilt, replaced a lot of parts as well. One thing i didn't keep in mind when building was selling the stock exhaust header and replacing it with an aftermarket one i bought from Datsun Parts in Rancho Cucamonga. I have the stock Hitachi carburetor also had that rebuilt. Truck is running pretty lean now, had someone tell me to drill out the main jet in the carb one size bigger to compensate for the extra air flow? Any advice would help, I broke down the other day just started to bog out. Afterwards could turn the truck back on but as soon as i hit the gas it would just bog and shut down. Thanks for the help 

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Don't listen to others. A header is not going to work any better than the exhaust manifold it replaced, so...

 

How do you know it might be running lean???

 

 

Carb may have been rebuilt but it sounds like the primary jet is part blocked which will produce a lean condition.

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What makes you think it's running lean ?

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Someone who worked on the truck for me prior to this happening had said it seemed to be running lean. I know very little about carbs. He told me the aftermarket header was pushing way more air than the stock header and said could possibly need to size up the primary jet because of that. If you guys have any solutions or ideas thank you. Would love to find another stock header for an L16 just havent had any luck searching for the past few weeks. stock header back on it would probably solve my problem

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New to me, I've never heard of extractors causing an engine to run lean...

 

Sure they can improve the air flow OUT of your engine but that has no real direct effect to your fuel or airflow into the engine. The entire "back pressure" myth is just that.

 

Now like Mike said, it could be your jets clogged up but when was the last time you checked out the fuel filter? Start with the simple things and go from there. You're lean, okay so you don't have enough fuel... Start with that. 

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IF your exhaust manifold was restrictive and you removed any restriction it can sometimes alter the exchange of exhaust gasses for intake gasses. This won't be the case on the L16 manifold as you can see....
 
hearervsstockL16-18-1.jpg

When the primary jet is blocked or partly blocked it restricts the amount of fuel available for the air drawn in by the engine. Power drops off and the engine will 'fall on it's face' when you step on the gas. If part blocked, you may be able to rev the engine sitting still but it isn't driveable under load. If badly blocked it won't even rev up in neutral.

 

The porcelain insulators on a lean running engine will have a very light or white color, normally they are tan or light brown. The lighter the color the leaner the mixture.

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read Olddatsuns.com the tech section.  read it!!!!

 

If truck was running fine and then it changed then you you have to trouble shoot the carb or the distributor.

 A header isnt going to change it all that much like a 2 stroke  when the exhaust is swapped.

 

buy a timming light and just the timming.

 

does the truck run better when the choke on?(manual choke pull the cable. Electric just unplugg the wire)

 

 

you see gas squirted in the carb when you cycle linkage?

 

I would get a weber DGV myself as most carbs are just worn out by now. But if the stock carb runs fine I would still use it as a daily driver. As Im old and dont care anymore but Webers are ezer to maintain.

 

 

521s are a labor of LOVE. Mine breaks down at least once ever 2/3 months. Its just old. Now if you have limited skills it maybe a purdon on some. These are simple trucks  but hard to trouble shoot on side of road soemtimes for a new guy. Most is preventative main.

 

You could have a rocker arm tight or dist out of time or points ignition out of adjustment.   Most like a simple fix to use oldtimers.

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Have you looked at your spark plugs to make sure they are running lean?  Could be something else that makes it bog down.  If the carb is good why spend money to buy a 32/36 weber.  Better to keep your stock exhaust manifold.  Your lucky to get 5hp gain with header.  Just trying to save you money, time and resources.

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Alright so you guys were right I was wrong. Checked the spark plugs and its telling me carb is running rich, the plugs were dark brown. So what would be my next step here? I checked the fuel filter to see if it was dirty, looks good to me. so what would you guys suggest I do next? I completely stumped and don't know enough but any suggestions will help

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Alright so you guys were right I was wrong.

Who would have thunk that hey?

 

1. Unbolt the carburettor and throw it over your left shoulder.

 

2. What Ko said (its one of the better long term solutions you can implement).

 

3. Check that you are using the right heat range of plugs.

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haha damn, i have been seeing that a lot of people on here are all recommending the weber. will a 32/36 bolt right onto the stock manifold for an L16? Might just have to get rid of the hitachi sick of the headache im having trying to figure it out, i just want to drive

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will a 32/36 bolt right onto the stock manifold for an L16?

No, needs adapter plate.

 

Try using the search function as this topic has been coverd a gazillion times.

 

You will only get spoon fed for a little while!

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Contact piercemanifold for 32/36 weber. Lots of fake webers being sold. Their tech people is very good at giving tech assistance and unlike most businesses you can actually talk to a live person if u have questions.

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Plugs could be from driving short distances with choke on. Maybe the idle mixture set too rich, dirty air filter, worn rings or bad valve seals.

 

If you didn't rebuild it or the carb you are at the mercy of who did!

 

 

 .  

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Not getting any fuel squirting from accelerator pump so I'm gonna pull off the carb and see if cleaning out the primary jet and pump will help. I also noticed that the secondary throttle plate was closed and stayed closed when throttled. Does this sound right? Does it open up at specific rpm?

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see if fuel pump is pumping gas first.  then see if float bowl is correct if sight glass.  Weber don't have that.

 

wack where the gas line goes in the needle valve may not let gas in.

 

accell pump diaphragm maybe bad but most times thay leak so bad that you can smellit and drips on the exhaust manifold

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Fuel pump seems to be working just fine. float bowl is about a little less or close to 3/4 full. If theres plenty of fuel in the bowl would this mean there is a linkage problem with accelerator pump?

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174_4d9fd4894d535.jpg

 

Take a good look at yours. Does the valve cover side look like this one?

 

The accelerator pump is under that black rubber thing. When you open the throttle that horizontal arm goes up on the left and down on the right pushing the piston down. If it's moving then the problem is inside.

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Gotcha, all of what you just stated was true for me. Arm was working and being pushed down, I'm taking the carb back to where i got it rebuilt and will have those guys figure it out. I'll post some pics of my truck soon when it's running again. Thanks for all of the help 

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