back fire through the exhaust on deceleration

[zdude1967]

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

[Bruce Palmer]

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....

[zdude1967]

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

[Gary in NJ]

A good place to start is 2.5 turns. From there you can fine-tune as required. Do you own a unisync?

[zdude1967]

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

Edited October 14, 2011 by zdude1967

[d240zx2]

Adjust your valves. Then go to the carb adjustment.

[zdude1967]

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

[5thhorsemann]

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.

[zdude1967]

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

[Bruce Palmer]

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.

[soundmasterg]

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

[AWAUDIO]

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