Travel'n Man

Backfires in Exhaust Note: It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm. The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers. Why This (normally) Happens: 1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system. 2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat. 3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there. 4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires. .

Just in case you are following this and wondering where all this jetting is located and how do you get to it and how difficult it is - here is a quick snapshot of how easy it is to change a jet...............

Anybody keeping up with this thread make sure you have the right tools to keep your SU's or triples balanced - this is the best on by far.

185 kept me on the rich side - put back in the 190's. Out of town for a couple days on business. This weekend I'll change the pilot jets with the 190's. I'll record that session with all the edits I did to that series of jetting, pick the best recorded AFR's and spark plug condition. I plan on doing this same systematic series with 3 or 4 setting using different Main Jets and get getting my favorite 3 setting (with 3 different Main Jets) and off to the dyno's to see the best one.............

I'm dropping in 185 airs today and I'll let you know what that does to my AFR. I would like to lean out my cruise just a little. I'll post the results this evening. If the smaller airs don't lean me out in cruse a little - I'll put the 190 airs back in and drop my pilot to a 62.5. I do like to make one change at a time so I can keep everything recorded to see exactly what happens per 1 item change to my AFR's and plugs................stay tuned.

Don't forget that your pump's should have a very small filter on them - make sure the jet holes can release the fuel and the holes are not blocked by the ribs in the filters.

Everything I have read tells me to keep my idle at 1000 RPM's. That is where I am set. If I go lower than that my car want to bounce a little while at a redlight. New setting as of last night (as you see this lower air jet made me a little leaner for my total mix) Lowered my Air to 190 and below are my AFR's 1000rpm idle - 14.4 WOT @ 5000 RPM - 13.1 35mph @ 3rd - 13.0 60 mph @ 4th - 12.9 60 mph @ 5th - 13.1 I did change to a hotter plug - BRP5ES-11 Backfire on decel did decrease with this setting Still a little rich at cruise - going to decrease the air one more notch to a 185 this evening - they do say the magic spread is 40 points from the main fuel to the main air.............. Want to get my AFR cruise in the 13.5 - 15 range

I went on a long drive as well to a local brewery - I only had 2 - 4oz pours (don't drink and drive). But pleasantly when I pulled in the garage I looked at my AFR gauge and she was reading 14.2............Now need to go back to the brewery with my wife driving!

What the heck is another $24.00 + shipping. I'll shoot Todd an email - I'll grab a 67.5 and a 70 pilot jet. That will give me 57.5 / 60 / 62.5 / 67.5 / 70....................hell, you can't take it with you! Thanks for the direction. It's worth a shot. Timing is about 35 degree's @ 3000 RPM RPM backfire is any decel downshift or when the engine is holding you back..............no backfire on cruising or acceleration

Here are three charts that I like to refer to: [ATTACH=CONFIG]72111

What bleed pipe are you running?

Currently I'm at: Main Air - 200 Main Fuel - 145 Pilot - 65 Screws - 2.25 Pump Nozzle - 40 Bleed Pipe - T AFR@1000rpm - 13.5 AFR @ WOT - 13.5 AFR @ 60MPH - even pull - 13 Spark Plug - BPR5ES-11 Good pulling power on low accel and high accel Slight backfire on decel - 15 to 16 AFR I've got some 195 airs coming from Italy (just grabbed a set of 185's from Todd also). Trying to reduce the slight backfire on decel first - then raise my AFR a little leaner next. My plugs look dry and gray.......I would like them on the more tan side though. I bought a set of .45 pump nozzle but my car did not like that at all. What plugs are you running - I have tried 3 different series so far. Keeping notes on the plug reading along with my jet setting so I can review my best 3 setting - then go to a dyno with my best 3 and lock and load for best performance.

This may help with the tank side

Enjoy the ride!

General Information Last Activity 01-30-2007 Join Date 02-09-2005 Referrals 0

Nice!

Chris- Did Whitehead dyno it?

^^Agreed^^

Edan has been nothing but professional and helpful any and every time I have dealt with him. I would highly recommend him to anybody on this site. I have called him on his cell phone with no issues and recieved mulitiple deliveries with no issues. I'm sure if you call him or email him your issues will be discussed and resolved.

Just watched the movie last night - great flick - started off a little strange but zone'd right in. Highly recommended. Jerry - how did you make the base plate to attach? I have two fully restored chairs in my storage closet - I could put them to use in my office as well.

Nice!! She's a beauty!

Arne I'm sure most of us are still gaining knowledge from your past posts. I know I did this past winter as I was working on my venting hoses and vent tank. Hope all is well and always ENJOY THE RIDE!

This is what I buy: Brake line repair made easy. Where to buy brake line, fuel line, brake line flaring tools, brake line nuts and brake line fittings This is how to flare - dead perfect every time with the flaring press! 1st pic is with flaring press 2nd pic is with hand flaring tool - and flaring press

A person could go "one click hotter" to a 5 instead of a 6. If you do be sure to pull your plugs and make sure they are not too hot - blistered- your porcelain cooked and so forth. Don't want that plug too hot! If you are tracking and pushing your engine up in the 6500 - 7000 RPM you may want to go colder to a 7. I would think anybody with electronic ignition would want to go with a larger gaping than stock.

For any of you that do not know how do decode a NGK plug.......... http://www.ngksparkplugs.com/techinfo/spark_plugs/partnumberkey.pdf