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How to fix hi idle?


mars23z

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I just had my L26 swapped out for an L28. The car runs great except the idle is around 1100 I like it around 750. I have tryed to adjust the idle but it has no effect. I can turn the idle screw all the way out and it still stays around 1100. I have never adjusted my SU carbs before but they worked perfect on my L26. Is there a bigger jet or something when using the L28 with roundtops? Also It looks like my hood is a little crooked now and was wondering how to line it up. Thanks Chris

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Do you have a uni-sun or a similar devise to balance the air flow in the carbs? You need to back out the air balance screws and the idle will come down . Once the air flow is balanced Then you will need to reset the fuel mixture adjustments. Now if the throttle shafts are badly worn and are leaking , you may have a problem with the fast idle issue. They need to be really loose though. I tried various richer Needles from the 54 stock needles and found them to be too rich. I am also running a 280 engine with a cam and headers as well as 2 1/2'' exhaust. The 54s work the best in my engine . I live near sea level . Gary

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Does the high idle eventually go down. With SUs when the car is cold the procedure as most of the folks here know is to press the accelerator pedal a few times and then turn the ignition without pressing anymore on the pedal. At least that's how I do it. So when the car is cold it should high idle until warm then it should go down to 750 or a little higher. Even though Datsun called for an idle of 750 it's ok for it to be a little higher depending on how the car feels. Some like the idle at 900 or more. There may be one other issue and that is choke adjustment. You may want to check that.

As for the hood. It's interesting you should ask that. After the exhaustive restoration of my series one car I asked the shop to adjust the hood because there was more space on one side then the other where it meets the fenders.

He looks at it and BTW I trust the fellow immensely. He tells me there is no adjustment point and this is how it's done. He takes his hand and pushes on the hood. There now it's straight he says and sure enough it was perfect. Evidentaly in his opinion the two hinges are the key. They get how should I say just a tad altered forcibly.

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i'm with mckrack. it's more than likely the choke is the culprit. take out your nozzle, make sure that it's not sticking. if it's sticking, clean it out with some carb cleaner. also, lube your choke linkage and make sure your choke is returning to fully closed when you let off the handle. could also be a bent needle keeping it from closing.

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The answer to first question is no. It actually seems ok when I start the engine at any temp it will idle around 750, and then if I drive it it will idle around 1100. Tomorrow Iam going to atempt to adjust them using my books and the pages I have copied from this site that I think Arne posted. I think I can figure it out. As far as the hood dont you just loosen the four screws and then adjust accordingly? Thanks Chris.

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I wish I knew about that adjustment. Oh well the way the shop did it worked but maybe I shouldn't be recommending that. If the idle goes up after it's warm Chris it may very well be a needle issue. I screwed around with the screws once on my car and I completley effed it up and I had to get a friend of mine to reset the carbs for me because I had no idea what the hell I was doing. Anyway. It wouldn't hurt to check and lube the choke. Maybe for some reason it's binding however it would normaly do that when starting the car.

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Hood adjustment:

http://www.classiczcars.com/forums/showthread.php?t=15175&highlight=hood

SU carbs do not have a pump mechanism like Weber, Holley or other Detroit downdraft carbs. The pump mechanism in those carbs (accelerator pump_) actually squirts a measure of gas into the intake on heavy acceleration. It's common for those vehicles to need a squirt of gas into the intake to start properly. Since SU's do NOT have that, pumping the acc. pedal does nothing but lift and lower the gateway.

Additionally, Automatic chokes are also present in those carbs and their engines would exhibit the higher rpm's at cold start with an eventual decrease in rpm's as the temperature of the engine released the choke. The SU's choke mechanism is strictly manual. You either pull on the cable and actuate it or not. If it's stuck ON in one or both carburators it more than likely would be stuck ALL the time and not just suddenly come on.

Nobody has asked whether your car is an automatic transmission or not. The automatic cars had a second set of points that were actuated on by a temperature relay. Not sure if this changed the timing, dwell or just added a second spark (mine were never connected). As a result that may be a part of the problem.

2¢

E

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I've also just swapped my L26 for an L28 and converted to manual. After driving it last weekend had the exact same symptoms you've described. I have the round top 240 su's fitted. When I got home left engine running and dropped the idle screws on front and rear carbs and used my balancer to make sure still in balance. Haven't taken it out since so don't know how my changes will affect it cold. I've also just fitted extractors and a 2.5 system.

I was assuming I would have to change needles in the carbies to handle the extra capacity. Is the best bet here to take to someone with dyno who knows a bit about su's?

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I've also just swapped my L26 for an L28 and converted to manual. After driving it last weekend had the exact same symptoms you've described. I have the round top 240 su's fitted. When I got home left engine running and dropped the idle screws on front and rear carbs and used my balancer to make sure still in balance. Haven't taken it out since so don't know how my changes will affect it cold. I've also just fitted extractors and a 2.5 system.

I was assuming I would have to change needles in the carbies to handle the extra capacity. Is the best bet here to take to someone with dyno who knows a bit about su's?

I replaced my L-24 with a with a F-54 from a '82ZX and kept the round top SUs. Just as a information note. I have a Street cam milled head , headers and 2 1/2 exhaust. Not a big deal but I thought like you that because of these mods coupled with the added inches I would require different needles. I tried SMs, 47s , both of which were so rich I diluted the oil in the engine with unburned fuel. The engine ran strong though. I went back to the 54 needles that I was running with the old 240 engine and the engine runs just as strong. No more foul smelling oil and the mileage is decent. The plugs look great and I now have additional 9000+ miles on the engine since the swap and so far no problems. I did check the mixture with a color tune and all looked good. Gary what are extractors?

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E I read the link you provided and due to my attention deficit disorder I am glad the shop took the hood and gently pushed laterally on it and it fixed the problem. As mentioned I suppose recommending that procedure might not be the most professional approach. I wish I had that link. I would have copied it and given it to the guy and come back the next day.LOL

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My car is a manual the only mods are a 6 into 2 header and exhaust and I use roundtops. Now as I said they worked great on the L26 but when they were on my L26 I removed the dashpot could that effect how they perform on the L28 in the way I am describing? Other then that Iam very happy the L28 doesnt smoke and it has lots of power, but I dont like the idle at 1100. I am planning to get rebuild kits however I would like to find the problem first and fix it before I do.

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You can eliminate the dash pot entirely. I haven't had one in 7 yrs. Have you lubed the linkage and ALL the pivot points? I have found that when the idle changes when I drive my Z and if I just tap the gas peddle and the idle drops to where it was set. The linkage is at fault. Now having said this , I am not running with a dash pot , witch function is to retard the linkage from dropping to idle quickly. Gary

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