l16 hitachi cuts out

[danfiveten]

So i just got the rebuilt carb, idles fine, starts fine, but once she warms up, when you go to get on the gas, she slighly cuts out. If you pump she doesnt, otherwise theirs a chance she dies.  

 

Now the 2nd time i took her out, i had turned in the idle screw all the way in, and then went about 3 turns out, it took about 5 turns to turn it in, so the idle was to far out, she was also ideling high and fast.  Still slightly fast but not as bad, and the cut out is not as noticable, just wondering if anyone thinks im on the right track. I also removed the needle from the fuel cut off solenoid.  Shes spraying properly as well, its just the idle and her trying to cut out when you get on the gas, has no problems when under load otherwise. Just once you get on the gas for a split secound. 

 

Not one thing its another 

[ggzilla]

Did you adjust for best air-fuel mixture, or just set it so many turns out?

 

The "turns" setting is just to get it running, then you need to adjust by ear, or by gauge.

 

I'd also verify the fuel level is at the line in the glass.

[danfiveten]

I just went so many turns out since i suck at adjusting carbs, since it was like 5-7 turns out and ive been told to get runing is more like 2-3, so thats what i went with and shes running way better. What im mainly trying to find otu is if the idle being to far off on a automatic 510, could cause it to cut out when you get on the gas for a split secound.  Nat carb had it way to far off.  

[bananahamuck]

Even 2-3 turns out seems way to many.

 

Dirty thumb method i use is hook up a vacuum gauge to manifold vacuum and shoot for the highest reading with RPMs adjusted to spec  .... I will have to say also i have a oldschool Sun tune machine and tail pipe sniffer so that would just be a start but it would be closer.

 

 

(edit) i don't know if this has anything to do with the stumbling at all just thought i would add my .02

[ggzilla]

A rebuilt carb MUST be adjusted after getting it running. The float level should already be pre-set but verify it anyways. You can just look.

[danfiveten]

Yah i suck with stock carbs, su's im fine, but even my old man doesnt know about stock carbs, he never ran stock. So far im just playing it by ear with adjusting it.   How many turns would you normally say, i had read on one page on here mike had commented on that said around 2-3 turns out, 2 for economy, 3 for more performance..?  Really been working out my ass trying to adjust her and just go off what sounds good. 

 

 

I figure worst case take her up to dan hart, or out to Nelson and have them adjusted. I just want to be sure the cut out isnt something else, never had that before. 

[ggzilla]

No no no. You turn the mixture screw to where it runs fastest. It is not a fixed amount.

 

Or take it to an old guy. Do it right.

[67_1600]

It's easy enough to set the idle mixture by ear:

• Start your car and screw it in until it starts to stumble.
• Turn the screw out 1/2 turn at a time and the engine speed will increase.
• Keep turning the screw out until the engine doesn't speed up.
• Turn the screw back in by 1/2 turn.

It is important that everything else is good (valve lash, timing) otherwise you're just wasting your time.

[Fat510]

Did you make sure the screws holding the carb together are tight?

[danfiveten]

Very tight, i double checked everything before it got bolted down. 

 

and thanks 67 ill try that tomorrow night. Definatly better than some of the stuff i read. 

[Labdork]

I can't help but want to jump in here and clarify some things. The mixture screw you are talking about is only for the idle circuit. It is what keeps your engine running when your foot is off the gas. The FIRST thing you have to check, is that you don't have the idle speed screw set so high as to be opening the main venturi. If this is the case, the idle mixture screw won't do anything. If your idle speed setting is ok, and the mixture screw DOES affect the RPM, then you do want to set it for max RPM. If it is too high for your liking at this point, set it back down with the IDLE SPEED adjusting screw. If it is cutting out "when you go to get on the gas" that sounds like it could be accellerator pump. With the engine off, you should see a little stream of raw fuel being squirted into the carb if you 're looking into it from above, as you "pump" the gas. This compensates for drop in manifold vacuum as you open the throttle plates (get on the gas). You see, just as you mash on the gas, opening the throttle plates to allow all of the perfectly atomized air/fuel mixture into your motor, you also play a cruel trick on it. You drop the manifold vacuum to NOTHING and without manifold vacuum, the carb is really just a smelly aluminum paperweight. The accelerator pump mechanically helps deliver some much needed fuel to your motor, in it's most urgent time of need. So... check the accellerator pump. If it's working, I seem to recall a way to adjust the plunger travel to possibly increase the amount of fuel squirted and may want to try to get it to squirt more. If it aint squirting, THAT is probably your problem. I apologize for the length of the post. It's just WAY more fun talking carbs than running an automated blood cell analyzer...... :thumbup:

[danfiveten]

Its squriting good, fresh rebuild on the carb, everything is working great i showed a video to a few other members on facebook, otherwise i got to take it in to one of my friends to adjust that knows something about stock carbs. Im a su guy, i know dual su's, even my old man is clueless with stock hitachis and hes been building 510's since the late 70's.  

[ggzilla]

Hitachi downdrafts are adjusted just like Webers and most other carbs.

 

Stock Hitachi sidedrafts are adjusted just like SUs. But the flattops are adjusted like Webers unless converted to old-style.

[danfiveten]

Problem is ive not had to adjust my su's in 10 years.  I feel like when i try to listen i cant tell if speeding up or not, starting to think i need to get my hearing tested....just another thing to add to the shit screwed up with me.   I just need her running right so i feel comfortable driving her to one of the guys i like to work on my stuff, could be accelerator pump but all seemed good there, could just be i need more fuel coming from it. Its just when you go from idle, to getting on the gas that she wants to cut for that split secound. Sure its something i fudged up, didnt adjust right or something, i start to hurt so bad bending oer to install stuff i tend to overlook the most simple of a thing.   Thanks for all the help guys, has definatly pushed me more in the right direction. 

[ggzilla]

Make sure the float level is perfect before doing anything else.

 

SU types are adjusted similarly to downdrafts, but look different. Still has an idle speed screw. Instead of mixture screw has the Idle Adjusting Nut. With both types of carbs turn them for "best idle" (fastest smooth idle).

 

But yeah takes a good ear.

[danfiveten]

So with all this talk on the pump, i got to thinking what position it was on on the old carb,  The new carb is on the lower hole, old carb was on the higher hole for the pump. So that may be one of the issues right there then, i wont know untill i can go out tomorrow and then take her for a drive.

 

[DanielC]

Post 204 in this thread shows me messing around with my stock Carb.  You need a vacuum gauge and a tach that shows low RPM to set idle mixture.

http://community.ratsun.net/topic/30606-my-ratsun-datsun-521/page-11

 

First, get it warm, or better hot.  At normal operating temp.   You need the valves adjusted properly.  You need the ignition timing set.  you need the engine to idle at around 700 RPM, because if the idle is much faster, centrufugal advance starts kicking in, and it makes it hard to set the proper timing.

Hook up the vacuum gauge, and a dwell tach, or watch the car's tachometer.   Turn the idle mixture screw for best vacuum.  Readjust the idle speed,  Make sure the timing is right.   Everything interacts with everything else.  If you change the idle speed, you need to reset the idle mixture.  Change the mixture, it changes the idle speed.  Advancing or retarding the timing changes the idle speed, and the mixture needs to be reset.

 

Try to make idle stop screw adjustments end on screwing the screw in, or faster.  To do this, turn the idle too slow, blip the throttle, and then turn the screw in, that eliminates the possibility fo slack in the throttle linkage.

When adjusting the idle mixture, make a small change, and wait for the change to affect the engine.  It helps if you go foo far lean, and rich, back and forth, trying to find the "center"  As you go back and forth, do not go as far each time, until you get to the best point, and close in on the best point.

 

If the car has an automatic transmission you need to set the idle in gear.  Make sure the emergency brake can hold the car.  Block the wheels.  Ideally, have somebody in the car, with thier foot firmly on the brakes.

[smoke]

I take it the car ran fine before switching to the new carb? I ask because retarded timing will cause what you are describing. Make sure when you blip the throttle that you are getting a vacuum signal to your vacuum advance can.

[danfiveten]

Found my timing light...its in vancouver with a friend that borrowed it almost a year ago >.>  

 

I pm'd Adam, see if he has time saturday for me to come by and get his help, i just hate bothering people, but id rather give him a few bucks to help than waste gas going to vancouver to get my light, or spend 30 bucks for a semi decent one at harbor freight.

 

I figure its these 3 things, timing off slightly, the accelerator pump slot in a different location than last carb, and needing adjustment. Hope that takes care of ever think.

[danfiveten]

After timing, and adjusting its better, timing was past 15, still cuts out a little bit, but didnt kill it like it did before, and happens way...way...WAY less, so i just need to keep working it i guess. 

[DanielC]

If you are used to a fuel injected engine, a carb engine is not going to run quite as well, most of the time.  Sure, it can be tuned to improve throttle response, but in most cases, a FI engine is going to run better.

 

A modern fuel injected engine before you even get the engine started measures barometric pressure, coolant temperature, air temperature, and other factors I am leaving out.  When the engine is running, it checks the throttle position, if the throttle position is changing, and how fast the throttle position is changing.  It is measuring the air flow, and temperature into the engine,  It is measuring the coolant temperature, listening for any knock, checking the air fuel ratio in the exhaust, checking the absolute pressure in the intake manifold, and again, a lot of other factors I left out.  It uses all that information, and more to figure out how much fuel to instantly push in to the engine, usually around 40 to 50 PSI, right on top of the intake valve.

 

This is how a carb works.  Air going through a carb creates a low pressure area, and fuel is sucked into that area, based on the air speed only.  The carb does not know how cold or hot it is, how far the throttle is opened, or how fast, or the barometric pressure, or the humidity.  Air weights a lot less than gasoline.  It can respond to throttle changes very fast.  On the otherhand, since the gas flow starts to change AFTER the air speed has changed, through the carb, carbs have a pump that squirts a shot of gas into the manifold, to cover the period of time the air flow has increased, but the gasoline flow has not caught up yet.  But the accelrator pump is just a simple positive displacement pump.  If the throttle plate moves 10 degees, it pumps X amount of gas.  It does not matter how fast the throttle was moved, just how far.  In fact, the accelerator pump only pumps for part of the throttle plate movement.

 

In the winter, our Datsuns are not going to run as well as they will during the summer.  You can help off throttle hesatation, by learning to drive the carb.  Sure if it has a huge hole, or flat spot off idle, it needs some more tuning.  but if it has only a minor hesatation, mostly when the enfine is cold, I think I would try to live with it, unless it really bugs you.

 

One more thing.  If a carb enigine is running good when it is cold, it is probably too rich.  it might be a minor problem, or it could be so bad you are washing oil off the top of the cylinder walls on a cold engine.

[danfiveten]

I started out driving on dual su's, so im more used to carbs than injection, never had thsi cutting out issue before, but then again i never ran stock carb. but my old stock carb even when it was pissing out fuel i never had this kind of issue. 

[DanielC]

One of the problems with a butterfly throttle carb is you can open the throttle too much, especially at low rpms.  When youn open the throttle too much, the air speed in the throat of the carb really drops off, and it is air speed that pulls the gasoline out of the float bowl.

 

On the SU carbs, the piston is not controlled by the throttle directly, and it opens only enough for the engines needs.  As the engine speeds up, the piston, or slider oens more, to meet engine demands.

 

So at low speed, the venturi stays small on the SU carbs, and the air speed stays higher, and the carb works better.  I am nort sure if SU carbs even have an accelerator pump.

[danfiveten]

It just sucks that the bad carb, even though it pissed fuel, wasnt doing this, i hate the chance of her wanting to die if i dont pump it a bit, its just having to learn to re drive it a different way than im used to.  I just need to get it adjusted professionaly to and have my guy look at it, just he lives about a hour away from me so...i really hate driving to him.

[datzenmike]

Correct timing is about 12 BTDC. There are two other forma of ignition advance to consider, one is RPM dependent (mechanical) and the other load dependent (vacuum).

 

Obviously mechanical advance isn't really an issue at or near idle.

 

Motors need more advance at light throttle where the cylinder is not filled completely and the throttle is close to closed. (high manifold vacuum) even compressed 8 to 1 the flame takes longer to jump from widely spaced molecules than if the cylinder is fully crammed with them.  Weak cylinder filling needs more time to burn and so needs to be lit sooner (advance) 

 

Keep in mind the zero vacuum advance at idle is strictly an emissions thing imposed on all motors since the late 60s. ALL motors will idle better with more advance. To prove this, set your idle timing to stock. Now twist the distributor clockwise slightly to advance it several degrees. What happens? Well the idle increases and improves, why?? Because it's.getting the advance it craves and needs. Retarded idle timing reduces NO_x emissions... that's all.

 

The vacuum advance is OFF at idle but comes ON just above idle. So check the vacuum advance hose for leaks or a sticking vacuum advance on the distributor. Some vacuum advance lines have a restriction in the line to slow the advance.