engine missing after sitting at signals

[wayno]

As discribed in the title, after I sit at a signal light for a minute or two, I start out and the engine starts to miss unevenly, by the time I get to third it stops missing, but at the next signal it does it all over again. I have dual SU type carbs that were purchased new from nissan in a late 90s clearance sale, wish I had bought another set as it was $400.00 for a set, the engine is my fresh LZ23 with a fresh head also, it seems that the hotter it is outside, the more noticable it is. If I don't have to sit at the signal idling very long, it doesn't miss at all. What do you guys think the problem is, the engine is not running hot, but I do run regular gas.

[Jester]

Sounds like the carbs are loading up on you at idle then clearing out after some RPM's.

[I'm BLUE]

Hi Wayno !

 

Thanks for sharing ,,,

 

Did you pull out the spark plugs to see what they tell you ?

 

How often do you have to adjust the carbs mixture ?

 

Size of SU Carbs ?

 

Type and Model of Spark Plugs ?

 

Ignition System ? (electronic or points condensor)

 

Stock or Performance Coil ?

 

Compression Ratio ?

 

Are you Carb Floats ok ? ( stupid question )

 

Sorry for loading up with questions !!!

[wayno]

Sounds like the carbs are loading up on you at idle then clearing out after some RPM's.

 

 

So your saying that gas is pooling between the needle and butterflys? Do they load up from being wore out? Never had this issue before, can the SUs be ajusted wrong/unevenly, I have not tuned them since the engine install/startup when new/fresh.

[wayno]

Hi Wayno !

 

Thanks for sharing ,,,

 

Did you pull out the spark plugs to see what they tell you ?

 

How often do you have to adjust the carbs mixture ?

 

Size of SU Carbs ?

 

Type and Model of Spark Plugs ?

 

Ignition System ? (electronic or points condensor)

 

Stock or Performance Coil ?

 

Compression Ratio ?

 

Are you Carb Floats ok ? ( stupid question )

 

Sorry for loading up with questions !!!

 

In the past, once I had the SUs ajusted, I never had to ajust them again, it's the reason I like them so much, low maintaince. I have not checked anything yet, as it is such a weird symptom that goes away, that I thought I would ask you guys before I dig in, then I might not have to check everything by narrowing it down. The only thing close to this in the past has been when the wire connections were loose on my electronic dizzy, that was a weird day. I will admit these SUs have been around a long time now, but they are servicable ajustment wise, but they are not ajusted evenly anymore, but all the plugs are the same color when pulled in the past(old engine), I have not pulled the new plugs yet, as the engine may have 3000 miles on it so far. The carbs are 38mm? Whatever nissan sold for the L blocks, NGK plugs, EI distributer and coil, and I have never messed with the floats since I bought them new.

[dvboth]

check the chokes one of them may be sticking! when it does it pull over open your hood (duh) and push up on the pins on the bottom side of the carbs if they pop up when you push up they are getting dirty! a small speck of dirty will cause them to stick open! speaking from experience!!!

[Z chopper]

38mm hitachi su carbs on the l16, I think slightly larger for the l18 sss engine. are you Hitachi SU carbs round top or flat top?

[Duke]

Are you running the heat shield that goes between the carbs and the exhaust manifold? If not my guess is that you are boiling the fuel in the float bowls. When you sit at a light there is no air flow through the engine bay and the heat radiating off of the exhaust can get the float bowls seriously hot. Once you start moving the float bowls cool down and the stumbling goes away.

 

I had the same problem when I had my SU's on the l-series in my car. Once I installed a heat shield between the carbs and the header the problem went away.

[datzenmike]

Word!

[wayno]

check the chokes one of them may be sticking! when it does it pull over open your hood (duh) and push up on the pins on the bottom side of the carbs if they pop up when you push up they are getting dirty! a small speck of dirty will cause them to stick open! speaking from experience!!!

 

 

My chokes are sticking every once in a while, but that is not the issue here, when they stick the idle gets all screwed up, I have no idle issues with the problem I am experiancing.

[wayno]

38mm hitachi su carbs on the l16, I think slightly larger for the l18 sss engine. are you Hitachi SU carbs round top or flat top?

 

 

They are round tops.

 

[wayno]

Are you running the heat shield that goes between the carbs and the exhaust manifold? If not my guess is that you are boiling the fuel in the float bowls. When you sit at a light there is no air flow through the engine bay and the heat radiating off of the exhaust can get the float bowls seriously hot. Once you start moving the float bowls cool down and the stumbling goes away.

 

I had the same problem when I had my SU's on the l-series in my car. Once I installed a heat shield between the carbs and the header the problem went away.

 

As you can see in the photo, I do have a heat shield in place, there are a few reasons why I would never run dual SU types without a heat shield, the biggest one is I don't want any fires in the engine bay.

 

 

[datzenmike]

Wayno, love the PS set up. I had almost the same thing on my L20B. Even had the slight link in the inlet to the fluid reservoir.

 

[Cameron.]

sounds like to me you need to take your carbs apart and clean them, they probably have gunk all in them.

[wayno]

I tore into the engine today, basicly I did not find anything to tell me why it is doing what it is doing, all the spark plugs are burning the same(see photos), all 4 look the same, so I have the fuel mixture the same on the two SUs, even though you can see the differance between the two fuel mixture ajustment screws(see photos), they are old now but are servicable, what I mean by servicable is that if I close the fuel ajustment on one carb, and disable the other carb, the engine will die, and viceversa, so I am able to control the fuel mixture. My roadster carbs got to the point where one was not servicable, I could close the fuel mixture screw, disable the other carb, and it would continue to run, so I had them rebuilt. I also took a oic. of what plugs I am using. I do not know if the color of the plugs is the color you want them to be, but they are all the same.

[Duke]

Is it more of a hesitation like the motor is not getting fuel for a second? Or an actual miss?

 

My vote is still for boiling fuel in the float bowls. It's amazing how much that area can heat soak, even with the heat shield still installed. You might try using some heat shielding material attached to the heat shield and even wrapping the float bowls in it.

[Guest 510kamikazifreak]

Do I see bare wires on the matchbox dizzy connection :unsure:

[wayno]

Do I see bare wires on the matchbox dizzy connection :unsure:

 

I do understand what you are saying 510freak, but they have been like that for over 10 years. I did have an issue maybe 5 years ago when I would let off the gas and the engine would die, but if I gave it the gas it would start again before it came to a stop, that turned out to be a loose connection exactly where the two wires your talking about are, I crimped them a little with a pair of pliers, no trouble since. The miss I am having an issue with now is an uneven miss, the longer I sit at the intersection waiting for the light to change, the worse the miss, and only when the truck is warmed up on a hot day, but it just started recently, like in the last week, it was a lot hotter a month ago, but no issues then.

[Guest 510kamikazifreak]

Run it at night idling, pop the hood see if there is a little light show arching somewhere.

seems electrical. :unsure:

[wayno]

Wayno, love the PS set up. I had almost the same thing on my L20B. Even had the slight link in the inlet to the fluid reservoir.

 

 

:lol: I don't remember how that kink happened anymore, but it doesn't create any problems, so I leave it be. I put the PS in there about 5 years ago, a couple years after the disc brakes I fabricated, and then fought with for 2 years trying to get the issues it created worked out, when I finnaly figured it out, the truck drove great. I had a 521 frame(just the front) in my driveway that I mocked up the PS package before I installed it, that way I didn't have to fight that for 2 years. The biggest problem with the whole PS setup is the center link, it is a custom piece, and as I have said before, "custom parts suck", you can't go to the auto parts store and buy them. Here is a photo of the custom centerlink, I used to have a better oic. of the part by itself, but am unable to find it anymore.

[datzenmike]

Is it more of a hesitation like the motor is not getting fuel for a second? Or an actual miss?

 

My vote is still for boiling fuel in the float bowls. It's amazing how much that area can heat soak, even with the heat shield still installed. You might try using some heat shielding material attached to the heat shield and even wrapping the float bowls in it.

 

Also a header which are way worse than the cast iron manifolds for throwing heat.

[datzenmike]

:lol: I don't remember how that kink happened anymore, but it doesn't create any problems, so I leave it be. I put the PS in there about 5 years ago, a couple years after the disc brakes I fabricated, and then fought with for 2 years trying to get the issues it created worked out, when I finnaly figured it out, the truck drove great. I had a 521 frame(just the front) in my driveway that I mocked up the PS package before I installed it, that way I didn't have to fight that for 2 years. The biggest problem with the whole PS setup is the center link, it is a custom piece, and as I have said before, "custom parts suck", you can't go to the auto parts store and buy them. Here is a photo of the custom centerlink, I used to have a better oic. of the part by itself, but am unable to find it anymore.

 

The reservoir was meant for use on the passenger side and when mounted on the left side didn't point the inlet properly so I pulled it over and it started to collapse. I used a D-21 pump I believe and made a mount down where the air pump would have been. For a PS box I used a '79zx. It worked right from day one and for about 2-3 years before a major change and had to switch to a D21 box mounted to the outside of the frame rail. It would turn 10.5 x 31s no problem and even 12.5 x 33s on a solid axle later.

[wayno]

Also a header which are way worse than the cast iron manifolds for throwing heat.

 

I have had a header on this truck since I put the first L20b engine together with the super head back in the 90s, but now that you mention the header, I had to change out the transmission a week or two ago, last time I had the head off(to fix the head gasket) I had to weld a crack in the down pipe after the collecter, when I pulled the tranny, the engine rested on the exhaust setup, when I put it back together, it seems that I have cracked the down pipe again, it is not nearly as bad as it was before, but a leak all the same, this issue started after that tranny swap now that I thing about it. I have a new header waiting to be installed, I also have a L16 stock exhaust manifold, the header will need modded as the Z block ID surface is an issue with the header flange, and the L16 ex. manifold has an issue with the case breather tube, that is why I welded the header back together the last time the head was off, the truck is a daily driver, and having it down has to be planned.

[banzai510(hainz)]

My 38/38 seems to load up also at the stop light trying to take off. I just live with it. a 32/36 would be better but I like the 38/38 as it seems to have more Power over the 32/36.

 

 

Try a carb swap when you have time.

 

Possible worn distributor Possible but maybe very unlikely

[wayno]

My 38/38 seems to load up also at the stop light trying to take off. I just live with it. a 32/36 would be better but I like the 38/38 as it seems to have more Power over the 32/36.

 

 

Try a carb swap when you have time.

 

Possible worn distributor Possible but maybe very unlikely

 

I have another set of round top SUs that were supposedly rebuilt, but I wonder now as when I bought the setup, it was supposed to be an A block dual SU package, but when I got it home I realized that they had dropped a set of L block carbs and linkage, which was zip tied on the side, as it was to big to fit between the carbs, I just thought it was to keep everything together, till I got it home and tried to put it together, that's craigslist for you, and me not paying enough attention, but maybe they were rebuilt, as the person said they worked for Dan Hart in the past. When I change out the header, if nothing changes, then I will try the carbs.