Backfire thru muffler

[jbcollier]

The four speed tranny gives high revs at speed.  High revs and light throttle can lead to mix issues.  Too rich may lead to running issues such plug fouling.  Too lean and you may run hot and/or burn a valve.

[Stevensonjr]

5 hours ago, jbcollier said:

The four speed tranny gives high revs at speed.  High revs and light throttle can lead to mix issues.  Too rich may lead to running issues such plug fouling.  Too lean and you may run hot and/or burn a valve.

4th gear and 70mph shouldn’t give overly high revs because I’m running a cut down TR7 rear end with a 3.63 gear. I seem to remember about 4000 rpm or a little less. What should I look for on my plugs besides a tan color on the center insulator and very little soot?   Could it be just that a plug failed  and my fuel air ratio is ok? I would hate to go chasing a problem that’s not there. 
Thanks, Bill

 

[jbcollier]

In your case, the plug failed indeed.  The failure is not mixture related.

 

Please do not set your air/fuel ratio using spark plug colour.  Get an A/F-R gauge!

[Stevensonjr]

1 hour ago, jbcollier said:

In your case, the plug failed indeed.  The failure is not mixture related.

 

Please do not set your air/fuel ratio using spark plug colour.  Get an A/F-R gauge!

 

 OK JB, if you have the time I’ll need a bit of where, what and how on a A/F-R gauge. I am completely in the dark on this, and yes, it sounds like I need one. 
Thanks, Bill

[jbcollier]

Get a O2 sensor bung welded into the exhaust right after all the pipes come together but before the muffler.

 

Then buy an Air/Fuel Ratio gauge with a wide-band 02 sensor.  I haven’t bought one in a while so I can’t give you any current advice there.  I use an Innovate LM1 which is out of production now.  I’m sure someone will chime in with more current recommendations.  There are bluetooth gauges you can run off your phone.  No need for a dash gauge.  You’re going to dial in your carb and then unplug everything so no need for anything fancy, just functional.

 

Warm up the car thoroughly.  Then fit the O2 sensor and hook up your gauge per its instructions.  Now go for a drive.  Here are the critical modes to test:

 

- part throttle, low rpm, steady state, between 25 and 35: in other words just moseying along.

- part throttle, steady state around 50.

- part throttle, steady state at hwy speeds

 

In all these cases you want mid to low 13s.  Anything exceeding 14 is too lean.  This is mostly adjusted using the idle jets

 

- medium acceleration from a rest to 35: low 13s to high 12s. Watch for faltering and lean/flat spots which may indicate too small an accel jet.

 

- WOT, hard acceleration 3000 to 5000 rpm: low 12s to high 11s.  This is all on your main jets.

 

Once the mains are set, then:

 

- WOT, hard acceleration 5500 and up: low 12s to high 11s.  This is where you set your air correctors.  A larger air corrector leans the mix.

 

I rough set my idle jets and idle mix so it is ok.  Then I dial in the mains.  Next the air correctors.  Finally I go back and fine tune the idle jets.  Time spent here can find valuable fuel efficency.

 

Side draft carbs can have a seemingly infinite variety of jets, chokes and nozzles but really you find the recommended settings and are just using the AFR to fine tune things for your local climate and driving style.

 

Downdraft carbs are easier and harder in equal measure.  Easier because there just isn’t as many options available, and harder, you guessed it, because there aren’t as many options available.  You’ll have to compromise as you can’t nail it like you can with a sidedraft.  Just remember, always err on the RICH side, never lean!

[fastg]

I totally agree with JB, you will learn more is a 15 minute drive that you will in many hours of guessing. When you do make changes you can repeat the drive and see the real effect of the changes. Innovate and AEM are well respected suppliers 

 

https://www.innovatemotorsports.com/

 

https://www.aemelectronics.com/

 

Graham 

[speedwagon]

JB, I printed off your tuning guide. It made good sense to me and I shall profit from your experience. Got a wide-band when I first started messing with rotaries and discovered how easy it was to tune a carburetor with one!  Still have some webers and dellortos and still hate them far to complicated especially the 48.     But thanks a lot for the info.     john                                               Ironically my favorite for draw thru is a 2" SU.

 

[jbcollier]

There are better and more detailed guides to be found online.  Just wanted to provide a basic introduction and emphasize that the universally quoted 14.7:1 “ideal” setting is way too lean.

 

My perfect carb are modern Mikuni flat-slides.  They easily outflow Weber and Dellorto sidedrafts.  At WOT, there is almost literally nothing impeding airflow.

 

Just one more thing, carbs transition from idle to progression to mains.  It all blurs from one to the other.  In those transitions, you can get fleeting lean spots.  Watch carefully for them as we commonly drive with the engine in those transition zones.  Lean = heat = burnt valves and holed pistons.  While you certainly will toast an engine quickly being lean at WOT, you can do it just as well being lean at part throttle.  Just takes longer.

[Stevensonjr]

The Champion plug that failed was part number RN9YC, anyone have a heat range and part number for a NGK or Bosch replacement? Thanks, Bill

[toldfield]

You might look at Burton and see what they recommend.

 

https://www.burtonpower.com/spark-plug-ngkbp7es.html

 

Or my fallback, call Ken at Dave Bean.

[jbcollier]

Bosch WR7DC or WR7DC+ for mild to medium street driving.

 

Bosch WR6DC or WR6DC+ for medium to hard driving, or if you climb steep, long hills.