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Thread: back fire through the exhaust on deceleration

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    Default back fire through the exhaust on deceleration

    My duel SU round top 3 screw runs crappy and is backfiring through the exhaust when you pull your foot off the pedal(deceleration) what does this mean, does it mean it is running rich? Any help would be appreciated. It is also running rough and seems to want to stall. I just adjusted the carbs and float.I set the floats at 9/16 they do not suck full at that point but begin when the float starts to drop, I used the FSM for the carb sync.

    any ideas
    ZDUDE1967

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    Where are your nozzles set in relation to the full up position? I suggest taking them down a half turn and see if the situation improves. If so that would be an indication you are lean which is what I suspect anytime backfiring under deceleration or spitting back through the carbs occur.

    You might also check for vacuum leaks....
    Bruce Palmer
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    Thanks Bruce, I think my nozzles are somewhere in the 1.25. to 1.5 turns from the top range. I will try your suggestion and re-post with the results

    zdude1967

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    Registered User Gary in NJ's Avatar
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    A good place to start is 2.5 turns. From there you can fine-tune as required. Do you own a unisync?
    Gary
    Guardian of HLS30-91415
    Previous Owner of a 10/70 240Z ('83-'85)

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    I own a synchometer, I think it is an SK brand from Germany. I may have gotten it from motorsports. Would having 2.5 inch exhaust with headers and a glass pack make it pop on deceleration or is popping strickly carb / lean fuel / vacuum leak related? I turned it back to 2.5 and went from there I actually redid the whole carb sync thing. It still pops. To be honest it was hard to hear when the idle smoothed out and increased when turning the mixyure nuts because my valves are so noisy. Which is concerning as the engine has 500 miles on a fresh rebuild. When I got it back from the rebuild the valves were to tight???,so the car ran on 5 cylinders. Perhaps needless to say but my faith in the engine rebuilder may be fading, that is unless the valve adjustment is common on a fresh rebuild-which I assume it is not, or is it? In any case I will be trying to find a vacuum leak and/or readjust the carbs then maybe bring it back for the valve adjustment as I only have a month left on warranty.....ah fun!!!! any additional suggestions or comments/ insight into the valve noise or popping would be greatly appreciated
    zdude1967
    Last edited by zdude1967; 10-14-2011 at 02:41 AM.

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    Adjust your valves. Then go to the carb adjustment.
    First & Third owner of HLS30-00721
    B. 01/70 D. 03/12

    New owner of HLS30-15653
    B. 12/70 D.

    Frank in Houston, Texas

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    I am bring the car back to the engine rebuilder for the valve adjustment. I just got off the phone with them and they claim a valve adjustment is not included if you install the engine they rebuilt yourself. I know this is off from the carb issue but how would installing the engine yourself hsave abvything to do with the valves being done corectly on an engine rebuild

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    Quote Originally Posted by zdude1967 View Post
    I am bring the car back to the engine rebuilder for the valve adjustment. I just got off the phone with them and they claim a valve adjustment is not included if you install the engine they rebuilt yourself. I know this is off from the carb issue but how would installing the engine yourself hsave abvything to do with the valves being done corectly on an engine rebuild
    Soooo......................They want to charge you to adjust the valves once in the car, as apposed to on the engine stand in their shop? I think you need to find a mechanic that isnt going to beat the crap out of you on price and charge you for doing things they MISSED the first time around.

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    I agree 5th horsemann...They ended up not charging me but I am not impressed with the whole experience

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    I find it hard to follow you guys thinking it's the engine rebuilders responsibility to adjust your valves after install. They do the machine work, and assemble it, YOU roll it out the door, put it in AND get it running. Adjusting the valves HOT is part of that "get it running" deal. YOUR PART.....

    Your builder did it just to get rid of you.

    That would be like buying a set of carbs from any rebuilder and expecting the carb builder to fly to your location and set 'em up for you.
    Bruce Palmer
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    I agree with Bruce.

    I had a local and well-respected builder put together an engine for my Volvo 1800 years ago. He did not adjust the valves either. Since they are supposed to be adjusted hot, unless they start the engine at the shop, how could they even consider adjusting the valves for you? They would need your carbs and gas and an engine stand where they could hook a starter and battery power up.

    Learn to adjust the valves yourself....it isn't hard and takes less than an hour. Then you can do them yourself when they are due the next time too...

    Greg
    1972 Datsun 240Z
    1969 Volvo 1800S
    1985 Audi 4000 Quattro
    1989 Volkswagen Vanagon

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    FWIW, have you checked your dwell and timing advance? My 1972 240Z was doing something similar when I got it...
    dwell and timing were way off. Put in new points, set the dwell, set the timing (was off by 5 degrees) and backfire went away. Didn't touch the carbs till later (after buying Bruce's video BTW). Also, when my engine was rebuilt, valves
    were set "loose" to avoid burning, then they were adjusted to spec after 1500 miles or so. They were just clicking a
    little and after adjustment were nice and quiet. Still are. Hope this helps.
    Andy

    1972 240Z

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    Thanks for the advice. This is my first rebuild that I have installed myself, so I am not sure of what should be included. If the valve adjustment should not be included then that's cool even better since it indicates I am wrong and not the engine rebuilder. My pride has no problem being wrong, I rather be wrong then question the rebuilders integrity or skill. So with that said the engine rebuilder said the car is running rich as the exhaust smells like gas. Can popping occur with a too rich mixture or does it just happen when you are to lean. I adjusted the floats with the ZT video but the video is showing ball valves I have the old school needle valves and adjusted the float height to 9/16. The fuel flow is off at this height and begins to flow fuel once the float drop from the 9/16 position. Does this sound right?

    Thanks
    zdude1967

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    Shoot me a phone number off list and I'll call and walk you through float level setting without the Grose Jets. I keep telling steve and Pam we need to re-do that part of thew video but it just doesn't seem to get done.
    Bruce Palmer
    Salem Or
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    Quote Originally Posted by Bruce Palmer View Post
    I find it hard to follow you guys thinking it's the engine rebuilders responsibility to adjust your valves after install. They do the machine work, and assemble it, YOU roll it out the door, put it in AND get it running. Adjusting the valves HOT is part of that "get it running" deal. YOUR PART.....

    Your builder did it just to get rid of you.

    That would be like buying a set of carbs from any rebuilder and expecting the carb builder to fly to your location and set 'em up for you.
    If the bench adjustment was so far off that the valves weren't closing and allowing unspent fuel to explode into the exhaust system, yea, theres a problem. The cold adjustment of the valves should be loose (noisy) but the motor should run well enough to get through the break in period at which point you take it to him and he does the hot adjustment. It has no relation to a carb setup, the carbs are flow bench adjusted to a meanial standard, whereas the valves are adjusted to a known set of mechanical measurements. Worst case, the builder should have told Zdude to do the brake in regiment and then come in for a hot adjustment, after all, it's not a track car with a pit crew, it's a classic car with an old fangled in line 6 and a shade tree mechanic/ owner. (No offence Zdude)

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    No offense taken 5th horsemann.

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    For any of you out there who may be feeling uneasy about adjusting valves, our Tune Up DVD does a fair country job of showing how it's done plus other goodies that may help in your Z enjoyment.
    Bruce Palmer
    Salem Or
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    www.ztherapy.com
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