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240z runs very rough hot, intermittently


whamo

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I did some searching and couldn't find a solution to this problem. I have a 1971 240Z that I have recently restored. We have had an intermittent problem that has popped up occasionaly since we purchased the car 3 years ago and it continues to this day. It will intermitently run extreamely rough and virtually stall when hot. Its is like its dropping one carb. I dissassembled and cleaned the SU carbs per the Z therapy video (several years ago). The fuel tank has been cleaned and treated with POR. The fuel lines have been cleaned, flushed and all of the hoses replaced. I added an electric fuel pump from an RX7 in the back and replaced the mechanical fuel pump up front with a new MSA replacement pump. I recently replaced the fuel inlet valves and reset the float levels. The car has a 280zx distributor with new plug wires, cap and rotor and we have replaced the module once trying to resolve this issue when it completely quit one time. I am begining to think that one of the carb slides/pistons may be sticking when the engine gets hot. I don't remember this problem occuring right after I cleaned and adjusted the carbs. I'm looking for any suggestions.

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I see you are running a 280zx distributor. Do you have the matching 280zx or similar electronic coil? The coil could break down when hot. Have you bypassed the ballast resistor?

Actually I am running the stock 240z coil. I previously had a Mallory or MSD coil (can't remeber which) but that was removed. I had the same type problem with that coil.

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It will run with the stock coil but not properly. Have you bypassed the ballast resistor? You will need to do this even with the stock coil.

I did bypass/remove the ballast resistor. Are you telling me that I should use a 280zx coil instead of the original 240 coil? I haven't tried that.

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Vapor Lock ? The car is famous for this....

Jay

Not just a Z - zdisease

Yeah, I know. I installed an electric pump in line with the mechanical, I believe that was nissans solution for the problem. Are there any other "solutions" to the vaporlock problem?

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If you are running the stock SU carbs. Have you checked or cleaned where the coolant runs through the carbs? I pulled apart where the line enters the rear carb on mine and found that it was plugged up bad.

Follow the hardline that runs around the back of the block/head and you will see a soft line that hooks that line into the carbs. Remove that soft line and unscrew the nipple where the soft line hooks into the carbs. You will see there are 4 small holes inside that fitting that restrict the flow of coolant. I have found these small holes will get plugged up over time. If thats the case it could cause you headaches much like what your talking about.

It's a very easy fix that gets overlooked.

Hope it helps.

Edited by carguyinok
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Jeff;

Thanks for that picture of the restricter on the carbs. I have never seen one installed and know my 70 never has had one. I have had this car in every southern state in the us in the summer and have not had the vapor lock problems. The flat tops were known for vapor locking.

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Interesting. My '71 does not have that restrictor either. I wonder what years it normally came on?

Edit: Looks like it was available as a dealer installed option or fix of some kind. Perhaps part of the '73 "fixes" for hot running. The parts list shows it as 14100-E8850, and calls it a "ASSY - THERMO MANIF WATER CONTROL".

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I know this car came from Texas. So I would rule out that its being used for a cold weather fix of any kind. Maybe if you slow down the coolant flow in the intake that would let the coolant pull more of the heat from the intake and carbs?

Atleast thats my guess. Then again I have been wrong before......

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The coolant flow through the intake has been bypassed on my car. I didn't think that restoring that flow would reduce intake heat issues but I suppose it might. Also, I was reminded by Gary (beandip) that we installed a 280zx coil on the car in place of the MSD blaster coil I had been using previously.

I dont know the fuel pressure delivered by the RX7 pump but I'm sure its not more than 5. Of course mine is a used one so it might be weak.

Edited by whamo
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The coolant flow through the intake has been bypassed on my car. I didn't think that restoring that flow would reduce intake heat issues but I suppose it might. Also, I was reminded by Gary (beandip) that we installed a 280zx coil on the car in place of the MSD blaster coil I had been using previously.

I dont know the fuel pressure delivered by the RX7 pump but I'm sure its not more than 5. Of course mine is a used one so it might be weak.

I take it the electric pump isn't off an injected RX7. If it is it will have way too much pressure. What year was it off? I agree the intake coolant flow is not your problem.

Edited by Nigel1943
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I take it the electric pump isn't off an injected RX7. If it is it will have way too much pressure. What year was it off? I agree the intake coolant flow is not your problem.

The pump came off a carburated car.

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I have a thought here, when you say it feels like it is dropping one carb, is it then dropping one or does it just feel like it do ?.

have your engine been getting new pistons/rings recently ?.

It might sound stupid to ask this.

Chris

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Hi Chris

Why are you asking that? is there any case related to that... so here goes what`s happen with me.

I did a big restoration on my car , and engine was one of the parts that i restore as weel.

I changed the piston rings, timing chain kit, gas, water and oil pumps, new hoses, etc...

At beninig I run it and felt like the carbs were stop working and the engine stop working too and i was forced to pull side of the road.

I changed the correct termostat degrees for 76.5 C, not 81C, because i saw a little high tempetarure on gauge them normal. I change it and keep it running and he did it again for a couple of times (only)...

Now i put new carbs and everything works fine... but still don`t know what it was...

Now i have a friend that has this problem as well, and i want to help him, the engine was redone, and car works fine at idle readlly good engine sound, but at road he can not put the throttle down as car might stop working. The carbs or engine start to fail and he has to pull over and wait a little of time, them it starts again and run a little better...

Any solutions?

Filipe Azevedo

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post-14342-14150806398681_thumb.jpg

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