Datsun Roadster Forum-311s.org

Installed a header, did a full tune up adj carbs and linkage, use a color tune for mixture. Car sound like a raspee mess at high way speed, like it's missing???. back fires every time it's shut down. Runs good, stats good, idles good, accelerates good, just when acceleration is held at 3500 to 4000 rpm. sound like hell.

How's your temperature at speed?
Could be lean....What needles are you running? Carbs set up well (float level etc.)? May need adjustment if the exhaust works better than the old one...
Could be timing....Both vacuum and centrifugal advance working?

You might need to retorque.

Linda

Actually the stock exhaust manifold should breath better

Roman wrote:Actually the stock exhaust manifold should breath better

starting to wish I would have never made the swap.

Daryl Smith wrote:How's your temperature at speed?
Could be lean....What needles are you running? Carbs set up well (float level etc.)? May need adjustment if the exhaust works better than the old one...
Could be timing....Both vacuum and centrifugal advance working?

will I still get back fire when lean? new non smog needles, vacuum and centrifugal advance are working

Linda wrote:You might need to retorque.

Linda

I think Linda's onto something...could be air leaking at the manifold gasket. Did you use sealant, and what type of gasket was installed?

"Actually the stock exhaust manifold should breath better"

I think this is a common misconception from way back. Pretty much all these two particular manifolds do is provide a path for the exhaust to the back of the car. That's it, no more.
Although the stock manifold is closer to the preferred 4-2-1 design it's not really... the 2-3 cylinders are dumped into a single pipe right outside the head....Both manifolds have way too large pipe to really do any good, but, that's partly because the ports are way too big. Proper size primaries for the stock engine would be 1 1/4" to collector for a 4-1 header or 1 3/8" to 1 1/2" to collector for a 4-2-1 header. both would be compromised at the head because the ports are too large, but the reduction from the head to 1 3/8" is workable.

Still, I would put the ceramic coated header on over a stock manifold, for the heat reduction benefits and durability. If any breathing advantages, it would likely go to the header. If you decide to build a 2L stroker later, the 1 1/2" header is closer to the proper size needed... and a good start to modify to a 4-2-1 which would have the proper size pipes.

Non-Smog needles? or the ADQ needles recommended by the group?
Wouldn't a manifold leak typically show worse at idle/low rpm?
Points or electronic dizzy?

I'm still leaning to the carb adjustment/needles or timing.....Sounds like everything works well except at cruising speed....
Easy to adjust the carbs a bit rich and see how it runs in your 3500 - 4000 range...

I may have spoken from a place of second hand knowledge about the stock manifold breathing better. I have however heard it from many a more knowing guy than myself. just looking at both side by side, the stock seems to have bigger "tubes".

Still, I prefer the header. A good header with a flex pipe, a 2 inch exhaust pipe, good muffler, and a nice resonator tip is the ticket for me.

@ Guy. I think when you get it all dialed in you'll love it!

Daryl Smith wrote:"Actually the stock exhaust manifold should breath better"

I think this is a common misconception from way back. Pretty much all these two particular manifolds do is provide a path for the exhaust to the back of the car. That's it, no more.
Although the stock manifold is closer to the preferred 4-2-1 design it's not really... the 2-3 cylinders are dumped into a single pipe right outside the head....Both manifolds have way too large pipe to really do any good, but, that's partly because the ports are way too big. Proper size primaries for the stock engine would be 1 1/4" to collector for a 4-1 header or 1 3/8" to 1 1/2" to collector for a 4-2-1 header. both would be compromised at the head because the ports are too large, but the reduction from the head to 1 3/8" is workable.

Still, I would put the ceramic coated header on over a stock manifold, for the heat reduction benefits and durability. If any breathing advantages, it would likely go to the header. If you decide to build a 2L stroker later, the 1 1/2" header is closer to the proper size needed... and a good start to modify to a 4-2-1 which would have the proper size pipes.

Non-Smog needles? or the ADQ needles recommended by the group?
Wouldn't a manifold leak typically show worse at idle/low rpm?
Points or electronic dizzy?

I'm still leaning to the carb adjustment/needles or timing.....Sounds like everything works well except at cruising speed....
Easy to adjust the carbs a bit rich and see how it runs in your 3500 - 4000 range...

I have tried everything I'm stuck.
full tune up adj carbs and linkage, use a color tune for mixture
starts good
runs good,
accelerates well,
runs cool
try cruising at speed 3500 rmp and above will not purrrr. best I can discribe it is a flooder rumble raspee -sounds like hell.
Back fires ever time its shout down

So I did not see if you mentioned that you retorqued? Heat cycling can require retorquing. Try tightening everything.

Linda

Linda, are you suggesting a leak at the exhaust/intake gasket on the head?

Yes, exhaust or intake could be loose causing noise.
I believe you might get some hissing from carb spacer/ gasket being loose too.
Not supposed to have any vacuum leaks anywhere
Linda

Besides the heat cycling, those gaskets compress like a mother.

Backfires out a carb or out the tailpipe?