I recently installed Hooker headers and find excessive exhaust backfiring/popping when switching gears or downshifting .

I currently have 43M rods and size 74 jets installed and purchased new Primary rods 48B and size 75 jets from Carbs Unlimited which I intend to install next weekend.

Question#1: is there a difference between the "M" and "B" designation on the rods?

Question#2: can I mix and match rods & jets?

Question#3: does anyone have experience with these "B" rods?

(see) http://www.carburetion.com/rodshang.asp

Any suggestions would be appreciated.

 

Old style rods are usually stamped with a letter like B, C or D behind the size number. They go in carbs that have a part number beginning with 70xxxxx.

New style rods are usually stamped with a letter like J, K, L or M behind the size number. They go in carbs that have a part number beginning with 170xxxxx.

You can't use the old style rods in the 170xxxxx carbs. The "M" rods are used in trucks and have a fatter tip. Lars has excellent documentation of this forum that explains this in detail..

 

Here is a link to Lars' tuning paper. He can also give you some good advice. His email is V8FastCars@msn.com

http://www.corvettefaq.com/c3/QJetTuningPaper.doc

You can mix and match rods/jets. The M rods suck for WOT due to larger tips. They are designed to run lean for truck applications. Here is Lars' note on them.

(NOTE: Some post-1975 Q-Jets for truck applications have metering rods stamped with an “M” designation following the number size. The “M” rods have .036” diameter power tips, and are not suitable for performance tuning unless the tuner takes this larger power tip diameter into consideration when calculating resultant metering area at WOT.)

Edit: Those B rods may not even fit you carb. Need to check the length.

 

The backfiring and popping could be caused by a very small exhaust leak. Try going over all the fasteners and snugging them up once again. This may solve your problem.

 

 

You are popping in the exhaust because your engine runs leaner with the headers - you have a lean condition, and this makes the exhaust pop on decel. If you put the 75/48 combo in the carb, you will make the lean condition much worse - you need to richen the carb up, not lean it out more.

The "M" rods are truck rods used in 1975+ carbs. They are not suitable for a passenger car carb application. Also, keep in mind that you cannot install 1975+ rods in a pre-75 carb - the length is different. "B" rods are pre-75 rods. You cannot mix and match the two different length rods in the two different series carbs.

Lars

 

...you also might look for a vacuum leak which wil cause popping on deceleration.

 

Thanks everyone for your responses.
Sadly my computer is not functioning (I am using my wife's) and I won't be near my Vette until Saturday so I can't follow-up on any suggestions until then.
I will be back next week.

 

I ordered and installed new Rods & Jets but the backfiring/popping when shifting gears or downshifting still exists.
I found some improvement but the problem is not totally solved, otherwise the car runs fine.

I tried the following combinations:
Jet=0.077, Rod=0.039 Total Area=0.00346204
Jet=0.079, Rod=0.039 Total Area=0.00370708
Jet=0.079, Rod=0.035 (Power Tip) Total Area=0.00393956(+)
(power tip - defined as no taper at bottom)

"Carbs Unlimited" doesn't sell any smaller Rods or larger Jets so I am not sure where to go from here.
Lars' tuning paper lists Rods as small as 0.026 and a Jet one size larger = 0.080 but I don't know where to get them or if this would really help.

Any other suggestions?

 

Assming that your carb is in good shape and that you set the timing properly, I would start from scratch. What is your carb number? Go to Lars paper and find the original combo for that number carb.

I don't think that you should have to richen it up more than 10% due to the new headers. You have tried combos in excess of 15% richer. Your last combo is 25% richer than your starting combo of 74/43. My guess is that you might be seeing puffs of black smoke out the tailpipes if you blip the throttle.

If it is still happening, after these changes, I'd make sure that you cover the previous suggestions of exhaust/vacuum leak. JMHO.

 

I purchased the Q-Jet on eBay and rebuilt it - Model #7042211 which I now see is listed in Lars' paper for a "CHEV 72 350 TRUCK SERIES 10 FED M/T"

I still have the original Q-Jet Model #7043213 FV (in pieces) and think I will reconstruct it and switch back to it.
I misdiagnosed some problems that I attributed to the original Q-Jet :o and replaced it with the eBay one.

How different is a Q-Jet designed for a Truck vs. a Vette? They both use the same Rods & Jets and the Rochester Quadrajet model number decoding is practically identical?

 

Don't frett, many guys have truck Qjets running on their vette's. I think that Lars retuned a truck carb on a vette in one of his tuning for beer events. If your original is rebuildable, go for it.

My first thought...probably not your carb since you significantly richened it up and problem remained. Go back and make sure all vacuum lines are tight and there are no exhaust leaks as per previous suggestions.

JMHO

 

The .026 deminsion IS the power tip. Primary metering rods have three fuel metering serfaces. They are the major diameter, the minor diameter and the taper between the two. The major diameter is for cruise mixture control, the minor controls WOT. All performance rods (B's) have .026" minor diameter or "power tip". All "M" rods have .036" tips. These are also called truck rods. They are not for performance work. Primary metering rods are identified by their major diameter (a 39 rod having a .039" major diameter).

The first combo you have listed is WAY rich. You should be using a 47 or 48 rod with that .077" jet to begin your tuning. The second is even worse. What you're looking for is a cruise metering erea of around .0028to .0030 to start your tuning.

If all you have is "M" rods you can sand the power tips down to .026". Go easy.

BigBlockk

Later.....

 

You need to get back to a known good baseline.

First, your carb was initially all screwed up - your 7042211 carb is an early-model Q-Jet, and it does not matter that it's a truck carb. Only the post-75 truck carbs use the .036" power tips on the rods. The 43M rods that you pulled out are post-75 truck rods and do not fit your '72 carb - they are the wrong series rods (wrong length) for that carb and will not work. "M" and "K" series rods can only be used in post-75 integral choke Q-Jets and will not work in a 1972 divorced choke carb unless you compensate for the rod length change by altering your rod hangers.

Set up the carb with a 75/43B jet/rod combo. You cannot use a 39 rod in that carb - it's way too rich. Make sure the pin sticking out of the bottom of your power piston has not been cut off - it should be sticking out of the piston about 1/4". Verify power piston seated height by pushing the piston down and verifying that the inner brass sleeve sticks up above the inner lip of the plastic retaining collar by about .020" - .030". If it is lower than this, you need to raise it by bending the tang in the throttle plate or installing a new piston with an un-cut bottom pin. Verify that you do not have lead plugs installed in the idle air bleed holes in the outboard wells of the float bowl. Make sure float level is set to .375" and you'll be set to go.

 

Getting very frustrated……I did everything suggested but no real improvement.

1. Set up the carb with a 75/43B jet/rod combo. You cannot use a 39 rod in that carb - it's way too rich. <DONE>
2. Make sure the pin sticking out of the bottom of your power piston has not been cut off - it should be sticking out of the piston about 1/4". <OK>
3. Verify power piston seated height by pushing the piston down and verifying that the inner brass sleeve sticks up above the inner lip of the plastic retaining collar by about .020" - .030
<DONE – approximately .020”>
4. Verify that you do not have lead plugs installed in the idle air bleed holes in the outboard wells of the float bowl. <DONE – also blew air through the holes to confirm that they were not plugged>
5. Make sure float level is set to .375" and you'll be set to go. <DONE>
6. Checked Exhaust for leaks – found one loose bolt at rear of header.
7. Checked vacuum lines and connections – found one loose hose connection at EGR.
8. <Additionally> I adjusted the ignition timing as described in Lars’ How to Set Your Timing for Peak Performance technical tip.

Question: I have been running with the Spark Control Solenoid (TSC) bypassed & disconnected and the EGR vacuum line is routed to it – effectively shut off. Could this setup be the cause of my problems?

Any other thoughts?

 

So what are the symptoms and problems now?

 

Same thing - popping sounds from exhaust when shifting between gears or downshifting.

 

Thinking slightly sideways - if you've confirmed you have the correct carb and timing settings, no leak in the mufflers and no vacuum leaks - then what mufflers do you have? When I put CORSA mufflers on my C5 it did the same thing - that was a muffler design. Without hearing what you hear, that popping may be a feature of a performance muffler. Some folks like that sound.

 

Are you sure you don't have an exhaust leak possibly where the pipes bolt to the headers? If you're sucking air when downshifting and slowing down it'll lean out and pop like you're describing.

 

With the carb jetted to its stock specs now, try backing the idle mixture screws out 2 turns each (rich) and see if that affects the symptoms. If it does, increase jet size 2 sizes bigger than stock.
Lars