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240z surges when hot


Montana Z

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Bare with me here... first post. I need some help figuring out what's wrong. I have a 1972 240z with SU carbs. I have recently replaced the plugs, wires, coil, points and condensers. My problem is that after about 20 minutes of driving the car begins to "surge". I usually first notice it coming off of a stop, but once it starts I can't drives out of it (meaning that it continues to surge regardless of speed). I know my SU's are worn because in the summer they run very rich regardless of what I do :angry:so my plan is to replace them. Is there anything else I'm missing?

Thanks for any advice,

Scot

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I'll take a stab at it. I think the first place I would look is the carbs. Check the throttle return springs. 1 on each carb, They attach to the heat shield on the lower end. Clean everything, Oil level, Drop test on pistons, Choke operating correctly & Float level. Check out the 1st Gen S30 forum, SU Carb Central. There you can find the info you will need to clean and adjust the carbs. Virtually all of the work can be done without removing the carbs & with a little info, easy to do. Before you start adjusting them, finish the tune-up with a valve adj, timing & dwell adj. too.

Mark in Portland

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Mark, thanks for the advice. I've been through the carbs several times with the help of Z- therapy's video but have not replaced the return springs. The springs on my car are different lengths, do you know if they are supposed to be? I plan to get the remanned carbs from z-therapy before the weather gets nice so if they are the culprit that should fix it.

Olzed, vaporising fuel was what I thought it might be so last summer I insulated the fuel rail. Unfortunately there was no change in performance. Thats why I'm leaning towards over fueling ,carbon build, or erratic fuel/air mixture due to worn carbs. The last time I pulled the plugs they looked good so it might not be a fuel problem.

I've had this car for 16 years and it is running better than it ever has... for about 20 minutes.

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I haven't worked on the L24 for a long time, but this sounds like the intake is trying to compensate for an air/vacuum leak. Check for loose fitting, hardened, softened, cracked vacuum hoses large and small and one-way check-valves. I'd say just replace all hoses due to age, but checking them while the engine is running will identify or eliminate where the problem is, if it is. Hose to mechanical advance in the distributor would be a good place to start. Also, I'm pretty sure those throttle return springs should be equal. Good luck!

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Olzed, I have thought about that but I don't think air temp is my problem. It was may daily drives when I was in California and ran fin in temps over 100 degrees. Since it seems to be having issues even in cool temps I think something is failing such as vacuum lines or carbs. So... with that in mind I have another question for you Gurus. Do you think it would be possible to have a vacuum leak in one of the brake booster lines without noticing degraded brake performance? The brakes are initially weak (maybe 1 minute) from there on they grab very well.

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Pick up a few feet of good quality small diameter vacuum hose at the parts store and cut to lengths. Larger moulded stock pieces you can get from your dealer or catalog. It's good maintenance to check or replace them due to age and operating conditions. They may appear decent, but soft hoses can collapse, and brittle hoses may leak at connections and at minute cracks, especially in cold weather.

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Thanks alternativez, I think I actually have a host of issues. The vacuum lines to the brake booster have looked ratty for years, I checked the vacuum advance on the dizzy today by sucking on the line and it appears to be bad (I can rotate the plate by hand). So right now my plan is to replace the brake booster lines and the vacuum module on the distributor and go from there. I will post results as I go and again thanks to all for the help.

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  • 2 months later...

I know it's been a while, but I thought I would post an update. Yesterday I received new Carbs from Bruce at Ztherapy. So far they appear to have solved my problem which leads me to believe my old ones had air leakage. I did notice my old carbs had side to side play in the butterfly and the news one do not. Anyway, thanks for all the input and hope to see some of you on the road.

Cheers,

Scot

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