u20 w/solex that goes - pu-pu-pop-put

[kogi]

hello roadster gurus...
well, my 66 spl311 has a u20, solex w/ b cam, and elect dizzy (msd al6 soon to come in play)... anyway, i synched the carbs the best it can be and the throttle response is amazing ... i played with the timing (20-15 btdc) but during down shifting or using the engine in low gear to slow down a hill... the motor goes "pu-pu-pop-put"... basic serious of back fire through the exhaust... can that be normal? it sound kinda cool... like a harley...
but is it healthy? is their something that i need to adjust?

thanks in advance for any suggestions.

ken
(having hard time keeping the car under 90mph
)

[Tycorcr1]

This is common to a lot of carburated cars. Basically you are running a little too rich and when you back off the gas, or downshift, the unburnt fuel in the exhaust system ingnites, causing your backfire on downshifting. Aslong as it is not excessive, it's fine to be a bit rich, the engine will run a little cooler. But this is also a bit tough on your muffler's internal baffling.
The second thing that can cause popping on downshifts is an exhaust leak at the header flange which allows cool outside air into the exhaust track and also ingnites unburnt fuel in the exhaust. You can check for this by listening to your engine for ticking noise (louder than mis-adjusted valves) coming from the exhaust flange area. Again, these are both very common problems with just about any carburated car.
Hope this help, Keith 67 spl #8001 still in parts on my garage floor

[SLOroadster]

If you have ever watched a 24 hour race such as Le Monds or Daytona (not nascar) all the cars shoot flame upon downshift. I think its kinda cool to see. To much is definetly a bad thing. I was working on a 68 solex that was way rich, I had been tuning it all afternoon (new engine) and it was well after dark. Another friend came over about the time I was ready to test the ajustments and all of us jumped in roadsters (back when mine was in one piece, slipping clutch and all) and took off. It was the first time I had seen the other car at night. Upon downshifting it was shooting a 2 foot blue flame out the tail pipe. Mine never did that even with a set of worn out SUs and the big cam. So it is common to see with a tuned slightly rich carburated engine of Nissan origin. (We got some strange looks from other drivers)