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Dual SU cold start issues and running rich when hot


mojojojo78

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I agree the total mechanical advance for an L series is 32-34 degrees? Base timing and mechanical should not exceed this. If you add to one you have to take away from the other. The '79 620 has 12 base and 22 total mechanical @ 3,900 for 34 total degrees. 

 

It seems that bringing the advance to maximum sooner helps with power and torque yet car makers delay this to  3,000 or more RPM so they must have a reason. Seems counter productive. I doubt it's pollution control because they did this in the 60s also.

 

 

Vacuum advance is strictly a part throttle event to add even more timing to slower burning less than full cylinders. At full throttle there is virtually no vacuum signal (and it's not needed) and the engine runs safely at maximum 12 + 22 = 34 degrees. At the same RPM and part throttle the mixture takes longer to burn and needs to be lit sooner to reach maximum pressure at that 'sweet spot' at or near 17 degrees after TDC maximizing the efficiency.

 

For racing or continuous full throttle this has no part in a discussion but for street cars that spend 99.999% or their time at part throttle it is a big deal. It's all about getting the most out of your gas.

 

 

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But street cars, especially 510s aren't just driven at partial throttle. How many Datsun owners do you know that never mash the pedal to the floor?

 

I've said this to you before Mike, and I'll say it again, vacuum advance allows for one damaging variable. Let's say you're at part throttle, cruising along at 3000 rpms. Then you decide to floor it. What happens is, since the vacuum advance has pulled more timing in for economy reasons, the timing advance is already more than the engine can handle at full throttle, and since it is a mechanical piece that is spinning wildly, the centrifugal forces don't allow it to react quickly enough to bring the timing back down to where the engine needs it to be, and you get detonation. Sometimes that detonation can be harsh enough to crack piston rings, or worse.

 

Eliminating the vacuum driven advance mechanism from the equation allows for a more stable timing curve.

 

As far as the timing curve itself, as I have mentioned many times before, the centrifugal advance can be adjusted with spring tension. And the total timing can be limited by the width of the slots in which the spring weights move. Since the SSS parts are no longer available, one can simply weld up the slots in the distributor cam to shorten the throw of the weights. You then need to file them to get the welding burrs out and to set the final total advance movement. In general, most L motors like 12-15 degrees of base timing and about 32 degrees of total timing. If you want to get it just right, make a protractor out of paper and cut a hole in the middle of it so you can slip it over the distributor cam while adjusting the slots in the distributor cam. As for the rate of advance, ie- the spring tension, one can simply remove one of the two springs. This is generally close enough.

 

I don't like being forced to defend a concept that works. If you don't believe in it, that's fine for you, but forcing your ideas on advice seekers is harmful to the growth of the sport. I feel like you're beating them over the head with theoretical science, when you haven't tried it yourself. Maybe you could have someone re-curve a distributor for one of your cars/trucks and find out for yourself.

 

 

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1 hour ago, Stoffregen Motorsports said:

But street cars, especially 510s aren't just driven at partial throttle. How many Datsun owners do you know that never mash the pedal to the floor?

 

I've said this to you before Mike, and I'll say it again, vacuum advance allows for one damaging variable. Let's say you're at part throttle, cruising along at 3000 rpms. Then you decide to floor it. What happens is, since the vacuum advance has pulled more timing in for economy reasons, the timing advance is already more than the engine can handle at full throttle, and since it is a mechanical piece that is spinning wildly, the centrifugal forces don't allow it to react quickly enough to bring the timing back down to where the engine needs it to be, and you get detonation. Sometimes that detonation can be harsh enough to crack piston rings, or worse.

 

 

 

Doesn't matter what you drive very little time is spent at WOT. (well maybe a Civic) You still have to drive to where you WOT and then drive back.

 

 

Except that there have been millions of street cars made and you can floor them all day long as you describe and nothing happens. They don't ping.

 

What centrifugal forces are involved with the vacuum advance? It's just a vacuum servo and lever that pull on the breaker plate

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I'd be a horrible defense witness or defendant in a criminal case because I get very passionate about a cause. I would be yelling "YOU CAN"T HANDLE THE TRUTH!" to the whole jury. But in this case, there's no amount of trolling that will make your theory or your clever arguments the right one.
 
Again, I invite you to try driving an early Datsun that has been equipped with a SSS style, re-curved distributor. It seems that may be the only way I can convince you.
 
 
ac·qui·esce
/ˌakwēˈes/
 
verb
verb: acquiesce; 3rd person present: acquiesces; past tense: acquiesced; past participle: acquiesced; gerund or present participle: acquiescing
  1. accept something reluctantly but without protest.
    "Mike acquiesced in this descussion"
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 Somewhere we parted ways on distributors. I'm talking about about every distributor ever made and what far and away most people run in their cars. I guess you are talking some esoteric modified distributor. I guess we should have seen we are talking apples and oranges here.  

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12 hours ago, datzenmike said:

 Somewhere we parted ways on distributors. I'm talking about about every distributor ever made and what far and away most people run in their cars. I guess you are talking some esoteric modified distributor. I guess we should have seen we are talking apples and oranges here.  

Jesus. No. Not esoteric. Stock Datsun parts modified to get better performance.

 

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Just an update and a couple more questions.

It is still running weird. I have tried running it without the vacuum advance hooked up and with it hooked up. Both run about the same. With it hooked up is slightly smoother when accelerating. The first time I had tried this the PCV hose had come loose and it ran really bad.

So what I have noticed is that in the morning, about 60F outside, it runs fine. Traffic is smooth and I am usually cruising most of the way to work.

On my way home it has been in the low to mid 70's. I hit more stop and go traffic. The temp stays about 170 or so. But once it starts to get warmer it starts to act up.

I added the vent hose as suggested by Datzenmike. When I got home today I looked to make sure nothing was loose.

So when I looked under the hood today I saw some gas was coming out of the vent hoses. The front was worse than the rear.

Would this be causing extra fuel to be getting into the carb? This would explain the gas smell I occasionally notice. I thought it was running rough because the timing was getting messed up from a bad vacuum advance canister. But could it be getting flooded? It stumbles on acceleration in 1st and 2nd gear, unless I really give it gas. At higher RPM it smooths out. Could the heat from the exhaust be causing the float to expand and stick in the float bowl? Is it simply removing the 4 screws on the top of the float bowl to remove the float? I am guessing there is no pressure because of the vent tube.

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First of all, heat may be boiling the fuel in the float chamber and it bubbles over. This 'heat soak' happens shortly after shut down on all engines as there is latent heat in the engine and the circulation stops. It may not even overflow when driving. A heat shield for the carburetors and perhaps some insulation wrapped around the fuel lines to slow heat absorption.

 

Like most carburetors the fuel level in the float chambers is critical. If over flowing then that is a problem. The float(s) may be set too high or the fuel pump over powering them and pushing in. Dirt may be preventing the needle valve from closing properly. 

 

A fuel pressure regulator set lower than 2.5 PSI might help but only as a last resort. This should work as is with the stock pump pressure. 

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Just a little update. I haven't really had time to work on this.

I took the front carbs float bowl top off and the float moves freely. The valve seems to work. The float is an aftermarket one made of that plastic. I didn't see any pinholes or anything in it. But I didn't test it to see if there were.

I was going to test out some heat shielding to see if too much heat on the carbs is what is causing the issue and instead I burned my starter.

I had a spare but it for some reason doesn't want to engage the flywheel. It has the same amount of teeth as the one that was on there. It looks almost identical, but it just won't engage. I can hear the click from the relay/magnetic pull. I have 12.5V at the battery and at the starter connection. I am stumped on this one.

I also was able to test the vacuum advance. I had a vacuum gun and hooked it up. It won't hold any vacuum. So I am guessing the diaphragm is torn or something.

Is there a source for these? From what I could gather is that the distributor is off a 79 or so 620 pickup. I am sure there are other models it was used on, but that was the one that I actually found something on. But I cannot find the vacuum advance canister mechanism any where. Does anyone have suggestions? Any other distributors that are more readily available that might fit?

On the starter I ordered another one. I bought a high-torque starter for a late 70's 280zx I think it was. It fit, has the same gear and works off the car. When I have it all bolted up, nothing works. Its the same problem as the other replacement I had. I don't know what I am missing here.

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Can you purchase a new/rebuilt distributor? If so, that may be the only place you'll find the vacuum advance. Buy one and rob it for the parts.

 

Does the starter work when not bolted to the engine? Try bench testing the starter. If it works on the bench, but not in the car, it could be bad ground, or bad power cable or connection. It could also be something wrong with the housing that makes it bind up when the bolts tighten.

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Swapping parts hoping to fix something gets expensive. Probably nothing wrong with the original and it's a weak start voltage from the ignition switch. With it out of gear and the small wire off the starter solenoid hold an old wrench across the battery lug and that terminal. This gives a full 12v signal so if it turns over every time then the ignition signal is weak. Don't be surprised if it sparks and makes a noise. 12v won't hurt you or wear gloves.

 

If so you'll have to make and install a 'hot start relay'.

 

 

 

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I tried the first starter out of the car and it worked. So I figured it was binding somewhere, like the gear was hitting the flywheel and not engaging.

I will try both starters again off the car and then try and take some measurements to see if there are any discernable differences.

As for the distributor, I had thought of buying a rebuilt one but it's just too costly right now. I am glad I have a second vehicle to drive in the meantime. It's just not as fun.

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blklighthotstartrelay.JPG.eefdb65c790f308cc190e77593005321.JPGblklighthotstartrelay.JPG.eefdb65c790f308cc190e77593005321.JPGyou have to make sure its a 79-91 280zx starter . Just a smaller motor part and the gears do the rest to give more torq.

a 240/260/280. I had order one and it just locked up on me the back is wrong on those the 280zx doesnt have the cup housing.

 

Be hones the regular starters are just fine .

 

some one on here said these Wilson starters are good(rebuilds) 91-25-1012 280zx and the 510 one is 91-25-1003.  I never tried these myself . said rebuild in Canada at the time the ones painted Black.  but said Wilson is owed by BBB so I dont know if later ones are chinese clones.  I just look at the price I guess.

 

my 521 was having starter issues and I took the old one in and said it was good. I put another spare and worked for a weak and same thing happen. swapped cables . then I put a hot start relay in there and has been good. I used a Ford relay but most people use the black light box relay.

 

as for the distributor what makes you think its bad?  can you adjust the timing on the car?  turn the dist and go say between 0-25dbtdc?  SU carbs I think want 12 in the old 1600 dual SU motors..  You could pull it out if you want and spin by hand just in case the spring blow up inside there.  I had a worn out dist that I woud loose power in the high end. Find out the bushing was worn out and causing the dist shaft to wobble. As for the vacuum is is locked up? car should still be able to run.

 

as for SU I dont know shit about them. If anything thats more a issue. but make sure motor timed valve lash good intake tight ec..... gas spilling over???

 

you still have pcv valve in the SU intake system?

Edited by banzai510(hainz)
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