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240z carb sync ignition timing disaster NEED HELP :(


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#21 greylandimports

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Posted 07 April 2011 - 03:43 AM

Hello Gents!
My first day back in the garage from a long winter and i had a lot of success! I went to a junk yard and found the spacer and adjustment plate underneath the distributor from a 280z. I put those on and reread my manual on carb tuning. I figured out one of the things i was doing wrong was adjusting the throttle speed screws while they were almost all the way screwed in. I saw in the manual that it says to unscrew them for a lower idle (among many other things). Now i have my idle at a little under 1000 rpms! Now to the problem...

I have been messing with all the adjustment screws and nobs and here is where I'm at. The idle sounds semi-smooth but when I rev up the engine the car 'tops out' at 3000 rpms. It starts to break up real bad and does not rev much higher. I'm trying to adjust the distributor to find the sweet spot but I'm having no luck. My assumption is that the timing is off because it idles at low speeds o.k. but at higher rpms where the timing becomes more critical- it cant do it. Am I correct? Or might I have the air/fuel mixtures off in the carbs and that's causing the problem?

-Thanks guys!


#22 greylandimports

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Posted 07 April 2011 - 06:03 AM

Hello Gents!
My first day back in the garage from a long winter and i had a lot of success! I went to a junk yard and found the spacer and adjustment plate underneath the distributor from a 280z. I put those on and reread my manual on carb tuning. I figured out one of the things i was doing wrong was adjusting the throttle speed screws while they were almost all the way screwed in. I saw in the manual that it says to unscrew them for a lower idle (among many other things). Now i have my idle at a little under 1000 rpms! Now to the problem...

I have been messing with all the adjustment screws and nobs and here is where I'm at. The idle sounds semi-smooth but when I rev up the engine the car 'tops out' at 3000 rpms. It starts to break up real bad and does not rev much higher. I'm trying to adjust the distributor to find the sweet spot but I'm having no luck. My assumption is that the timing is off because it idles at low speeds o.k. but at higher rpms where the timing becomes more critical- it cant do it. Am I correct? Or might I have the air/fuel mixtures off in the carbs and that's causing the problem?

-Thanks guys!

#23 d3c0y

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Posted 07 April 2011 - 07:17 PM

OK you arent making life very easy on yourself here.
Set everything to what the factory settings are and see if it runs ok like that first. 10 - 15 degrees of advance, mixture knobs 2 turns out, idle air 1.5 turns out. I cant double check those settings because im still at work but they are pretty close. Car should run fine like that and should rev out. I did this when i bought my car and it ran  really rich but it drove fine. If you can still only rev it to 3000rpm i would maybe look at pulling the rocker cover off and checking the cam timing - it might be a tooth or two retarded.

After it runs properly like that - then set up the idle etc. You need to make sure the two carbs are flowing the same amount of air though when you adjust the idle air screws.

#24 zeds4ever

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Posted 07 April 2011 - 09:58 PM

??? You don't mention bout changing the COIL to a 280z unit ,260Z not suitable. Would suggest you check the vacuum unit in your DIZZY if this is not functioning properly it will cause similar problem. Suggest you check these out before doing anything. Check your carb pistons are in sync, to do this remove air filters units. Raise pistons to max with fingers, release pistons & note if they  fall in unison. If they don't this can also be part of your problem. By the way I note that a lot of guy's use trnsmsn oil etc for carbie tops. The recommended oil is 20 grade engine oil or if you wish the oil they use on chain saw blades fill about 1/2" above piston reservoirs. Finally are you using an electric fuel pump or a mechanical unit. 

                                                      Regards: Alan.  :D

#25 greylandimports

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Posted 13 May 2011 - 02:38 AM

This Z really knows how to kick me when I'm down!!!!!!! I opened up the two knobs under the carbs and it ran rich but it was running real smooth! When I reved it up it still wouldnt go above 3000 RPMs so I bought a timing light and a week later when it came in the mail I went down to set the timing. When I started the car it was only running on three cylinders!!! I checked all the connections, pulled all the plugs, cleaned everything up.. but no luck. The spark plugs are sparking and they are wet with fuel when I pulled them out. What could have happened???? The cylinders that are NOT working are 2,3,5. I bought a compression tester online and I'm waiting for it to come in. I really hope the problem isnt loss of compression. The motor was rebuilt not even 1500 miles ago (about). The reason we rebuilt it in the first place was that it was only running on three cylinders.. so I'm not hopeful. How could this have happened? Could running the car out of timing and carbs out of tune do this??? I don't have the experience to rebuild the motor. I did it five years ago with a mechanic and I don't think I can do it by myself. Any ideas??  :'(

#26 Zedman240®

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Posted 13 May 2011 - 10:56 AM

When plugs are soaked with fuel, they basically don't do anything... while its running, place the timing light sensor on each lead and that will tell you which one is / isn't firing. Or you can just carefully remove each lead off each plug (without getting electrocuted!!!) and listen to the engine and if you don't notice a change, that plug isn't firing. Reset your carby fuel settings back to "default" and start again.. Go back to the basics.. Make sure there is fuel, spark and air! and triple check the timing..

#27 neRok

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Posted 13 May 2011 - 03:39 PM

maybe you would be better off reading the factory service manuals, available on this site: http://carfiche.com/manuals023/cars/ the instructions they give to tune the carbs are much the same as the youtube video posted before.

timing is dead easy to get in the ballpark. take the rocker cover and spin the crank pulley over clockwise. pay attention to cylinder 1, the one at the front. when the exhaust rocker is almost closed and the intake is just opening, this is near enough to TDC. pop the top off the dizzy, take note of where the rotor is pointing. undo the dizzy retaining bolt so you can turn it. hold the cap loosly over for reference and turn the dizzy so that one of the 'towers' lines up with the rotor. you have now set the dizzy to ~TDC, good enough to get it running. do the bolt back up so you cant move the dizzy, lock the cap on, and stick spark plug 1 to the tower you lined up with the rotor. now put the rest of the plugs on in the correct order. i believe for the L engines, this is anticlockwise direction!

that is it for timing, easy as. next step would be get the carby running sweet, but not worry about the idle so much. just fire it up and let it run whereever. focus on adjusting the flow through the carbies and getting the mixtures good. then go back and set the timing. when that is good, adjust the idle on the carbies, and you should be done.

one thing that occured to me before is when you checked the timing, did you disconnect the vacuum advance. vac advance moves the spark 20+ degrees advance, ie the timing marker would move anti-clockwise. when setting the inital timing, you set it with the vac disconnected, because you set it ~10-15 advance (not sure on the datsun specifics here, read the service manual). also, my engine has 2 lots of timing markers on it, i presume the ones to use are located on the same side as the dizzy.

#28 greylandimports

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Posted 19 May 2011 - 05:55 AM

Well, I got a compression tester in the mail and my compression is good. When I hook up the timing light, the light doesn't flash on the cylinders that are not firing. I know that 2,3,5 are not firing. When I pull the plugs on these cylinders nothing happens to the idle of the engine but when if I pull 1,4,6 the engine dies. The plugs on 2,3,5 are wet when I pull them, and 1,4,6 are dry. There is some black discharge leaking out of the plug holes too but I think I didnt have them tight enough. I made sure all the plugs had good connections and I put the plugged in spark plug next to a body ground and it sparked. What else can I check?

#29 greylandimports

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Posted 19 May 2011 - 06:04 AM

Also,
I was reading other posts on here and I was putting a lot of fuel through the carbs. I turned the knobs under the carbs two full turns out. Could this cause a problem?? On only three cylinders??

#30 greylandimports

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Posted 20 May 2011 - 12:09 AM

God damn it. The gas is bad. I was rereading the post and someone suggested it before. Draining it now. Thank you for the great tips!!!! Does this make sense to everyone else??? I think its my big problem. Now how do I go about flushing the system?

#31 neRok

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Posted 20 May 2011 - 10:38 AM

i would pop the fuel line off the pump and using compressed air blow the fuel back down to the tank. do this with the fuel cap off the tank, so the air can get out.

#32 salty

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Posted 20 May 2011 - 11:46 AM

i would pop the fuel line off the pump and using compressed air blow the fuel back down to the tank. do this with the fuel cap off the tank, so the air can get out.


u would be putting any crap back in the tank, better to blow out fuel lines from the tank forward

The gas is bad. I was rereading the post and someone suggested it before. Draining it now. Thank you for the great tips!!!! Does this make sense to everyone else??? I think its my big problem. Now how do I go about flushing the system?

if the fuel is bad, wouldnt both carbies be effected and all plugs would be the same?




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