at-jefft Posted June 1, 2009 Share Posted June 1, 2009 Hey Everyone, My early 260z spent the winter in the garage. Now its 3 screw SU's are out of tune I think. I say that because the front 3 cylinders are firing normally, while the back 3 don't fire at all. I took off the air filters and started the engine. I noticed a hissing noise which seems to be coming from the rear carb's throat. Also I noticed the rear carb's piston is much higher than the front. The Engine surges from a near stall to and what I estimate from sound to be 3-4k. Are my carbs that badly out of tune or is there a vaccum leak( i checked most hoses with propane torch and didn't find anything) or what? Quote Link to comment Share on other sites More sharing options...
at-jefft Posted June 1, 2009 Author Share Posted June 1, 2009 O and it idles at like 2-3k depending on how I adjust the rear carb. Also I can push up the piston on the rear carb and the rpms will change. That means that there are cylinders firing on that carb right? But when I pull the plug wires I don't hear rpm drops. Partially covering the rear carb with my hand makes the rpms climb. Quote Link to comment Share on other sites More sharing options...
SypherSlayer Posted June 2, 2009 Share Posted June 2, 2009 im no expert on su's but first is the car running way rich? if the piston is up then im pretty sure the rear half of the motor is going to be running extreamly rich in comparison to the front half. make sure the piston falls all the way to the bottom and isn't being hung up. good luck Quote Link to comment Share on other sites More sharing options...
jtl260z Posted June 2, 2009 Share Posted June 2, 2009 Hey Everyone, My early 260z spent the winter in the garage. Now its 3 screw SU's are out of tune I think. I say that because the front 3 cylinders are firing normally, while the back 3 don't fire at all. I took off the air filters and started the engine. I noticed a hissing noise which seems to be coming from the rear carb's throat. Also I noticed the rear carb's piston is much higher than the front. The Engine surges from a near stall to and what I estimate from sound to be 3-4k. Are my carbs that badly out of tune or is there a vaccum leak( i checked most hoses with propane torch and didn't find anything) or what? Sounds like your carbs need a good cleaning. If they have been sitting for a while they will gum up a little. With your rear carb piston in the up position you might have flooded the back 3 cylinders. You should be able to lift each carb piston to the top, let go, and watch it drop. You can do a basic clean by first removing the dashpot then the piston. Clean the piston with carb cleaner and reinstall. Make sure you apply the proper amount and weight of oil in the top of the carbs 20 W is recommended (I like ATF). Don't forget to check you plugs. You may need to clean or replace them. Good luck. 1 Quote Link to comment Share on other sites More sharing options...
at-jefft Posted June 2, 2009 Author Share Posted June 2, 2009 im no expert on su's but first is the car running way rich? if the piston is up then im pretty sure the rear half of the motor is going to be running extreamly rich in comparison to the front half. make sure the piston falls all the way to the bottom and isn't being hung up. good luck Actually I think that the front is kinda lean. I push up the piston very slightly (to test mixture) and it sounds like it is gonna stall. Quote Link to comment Share on other sites More sharing options...
at-jefft Posted June 2, 2009 Author Share Posted June 2, 2009 With your rear carb piston in the up position you might have flooded the back 3 cylinders. You should be able to lift each carb piston to the top, let go, and watch it drop. You can do a basic clean by first removing the dashpot then the piston. Clean the piston with carb cleaner and reinstall. Make sure you apply the proper amount and weight of oil in the top of the carbs 20 W is recommended (I like ATF). Don't forget to check you plugs. You may need to clean or replace them. Good luck. I put oil in the dampers and the rear piston falls smoothly when I put it to the top. I have since started from scratch to tune the carbs. I turned both idle adjusters to 1.5 turns from closed and the front still fires but not the back. I have loostened the balance screw and the fast idle screw as per the fsm. My new hypothesis is that the rear carb is not getting fuel at all. Although when I removed the float cover there was fuel in the bowl. Quote Link to comment Share on other sites More sharing options...
at-jefft Posted June 2, 2009 Author Share Posted June 2, 2009 Further info: I replaced the exhaust manifold with a header recently and obvioulsy had to take the intake off to replace the gasket. Could I have screwed anything up durring that process? No matter what I do to either carb, The front still fires and the back still doesn't. I'm thinking there could be something preventing fuel from going to the rear cylinders. I'm considering just rebuilding the carbs. but I wanna know for sure if it is a fuel delivery issue not something else. I did a little experiment and I think It tells me that the issue is fuel delivery. I sprayed carb cleaner in the throat of the bad carb, the rpms rose rapidly. Next, I unplugged the wires on the cylinders fed by the bad carb. sprayed carb cleaner again, and only a slight increase in rpms. Does that evidence support a fuel delivery problem? Thanks, Jeff Quote Link to comment Share on other sites More sharing options...
SypherSlayer Posted June 3, 2009 Share Posted June 3, 2009 like jtl260z said, pull your plugs and inspect them. if the car was running grossly rich the 3 rear plugs could be foul and wont fire regardless of what adjustments you make to the carb. so check your booster for a leak, inspect your plugs and replace if needed. there are a number of things you should check/adjust before you start messing with or tearing down your carbs 1 Quote Link to comment Share on other sites More sharing options...
at-jefft Posted June 3, 2009 Author Share Posted June 3, 2009 like jtl260z said, pull your plugs and inspect them. if the car was running grossly rich the 3 rear plugs could be foul and wont fire regardless of what adjustments you make to the carb. so check your booster for a leak, inspect your plugs and replace if needed. there are a number of things you should check/adjust before you start messing with or tearing down your carbs I put an old plug that was firing fine before it was replace in the #5 cylinder and still didn't get any combustion. I'll check for more vacuum leaks today. Quote Link to comment Share on other sites More sharing options...
jtl260z Posted June 3, 2009 Share Posted June 3, 2009 Check your metering valve in the float bowl and also check your jets to make sure their not clogged. Quote Link to comment Share on other sites More sharing options...
at-jefft Posted June 3, 2009 Author Share Posted June 3, 2009 Check your metering valve in the float bowl and also check your jets to make sure their not clogged. I pulled the float cover off and the needle valve seemed to be able to be pushed in smoothly. Is that what you mean by check the metering valve? Thanks again, I really appreciate the help. Quote Link to comment Share on other sites More sharing options...
at-jefft Posted June 3, 2009 Author Share Posted June 3, 2009 Pics: Jet (it is distorted could that be a major issue?) Spring (distorted as well but the piston still comes down) Needle (was pretty bent but I straightend it some and is less bent than the front needle) With the engine off the piston sticks probably 1 out of 5 times I lift it. but I haven't had it stick when the engine is on. Quote Link to comment Share on other sites More sharing options...
at-jefft Posted June 4, 2009 Author Share Posted June 4, 2009 Can anybody tell me if the jet, spiring, or needle are a problem? Quote Link to comment Share on other sites More sharing options...
Dayz Posted June 4, 2009 Share Posted June 4, 2009 1- make sure you have spark - Dismantle the spark plugs (rear) and test them against the block. 2- Make sure you have fuel going in the carb - Dismantle the inlet line put it in a bucket and see if there is fuel coming out. 3- Check your float height, dismantle it and clean it. Make sure there is no dust blocking it. 4- Dismantle the fuel hose from the bowl to the carb and see if you have fuel coming there. - Also, measure your intake as no leaks...if you dismantled it, it might have a gap somewhere between the head and the intake, or the carb and the intake, or the Balance tube and the intake. ... If it was working before and you didn't do anything to the carb, and moves very easily, it should still be OK. The needle is straight- the suction tube is well adjusted, your spring is OK...it should be OK. Check these vids on Youtube - It will help Quote Link to comment Share on other sites More sharing options...
at-jefft Posted June 4, 2009 Author Share Posted June 4, 2009 1- make sure you have spark - Dismantle the spark plugs (rear) and test them against the block. 2- Make sure you have fuel going in the carb - Dismantle the inlet line put it in a bucket and see if there is fuel coming out. 3- Check your float height, dismantle it and clean it. Make sure there is no dust blocking it. 4- Dismantle the fuel hose from the bowl to the carb and see if you have fuel coming there. - Also, measure your intake as no leaks...if you dismantled it, it might have a gap somewhere between the head and the intake, or the carb and the intake, or the Balance tube and the intake. Also is there supposed to be a gasket between the bell and carb body? ... If it was working before and you didn't do anything to the carb, and moves very easily, it should still be OK. The needle is straight- the suction tube is well adjusted, your spring is OK...it should be OK. Check these vids on Youtube - It will help ya, I have watched those vids on youtube. I put the float bowl cover on in a pan. cranked it and fuel came out the valve so I think that is good. I have adjusted the height of both floats. Also is there supposed to be a gasket between the bell and the carb body? if there is my carbs don't have it. Quote Link to comment Share on other sites More sharing options...
at-jefft Posted June 5, 2009 Author Share Posted June 5, 2009 Tightened the intake bolts. Some were kinda loose and engine runs smoother now but still not getting the rear cylinders to fire. Quote Link to comment Share on other sites More sharing options...
Dayz Posted June 5, 2009 Share Posted June 5, 2009 You are sure you have spark?...plugs are a the right place? Quote Link to comment Share on other sites More sharing options...
at-jefft Posted June 5, 2009 Author Share Posted June 5, 2009 Spark is what I am starting to question at this point. although, when I spray carb cleaner down the throat the rpms rise rapidly so there's atleast something. Quote Link to comment Share on other sites More sharing options...
Dayz Posted June 5, 2009 Share Posted June 5, 2009 Make sure your spark happen at the right time in the right cylinder. Post back when you confirm this, we will go through the problem. Quote Link to comment Share on other sites More sharing options...
at-jefft Posted June 5, 2009 Author Share Posted June 5, 2009 Alright I appreciate the help. How do I make sure the spark is happening at the right time? the only think i can think of is a timing gun. I really haven't diagnosed ignition problems before. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.