#$%#@R# Backfiring STILL!

#$%#@R# Backfiring STILL!

odie
odie

June 12th, 2008, 2:58 pm #1

So I replaced all the Jets with the forum recommended ones, as well as peirce manifolds agreed with the recommendation. I put on a new intake/exhaust gasket and tested, no leaks. Absolutely no change in the back fire. It still only does it after it is warm. This is driving me crazy! if it were a burbly pop, I could live with it. but it is more of a sawed off shotgun BANG!!!

here are the stats again

Dual 42 DCNF Idle Jets 125 Main 40
Stock cam
4-1 header no Cat (or dog for that matter)

Odie
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Joined: December 21st, 2004, 3:04 am

June 12th, 2008, 6:42 pm #2


Stock cam straight up... and timing?  Confirmed?

BTW...  if its an occasional BANG... it would still appear to be a rich mix, or possibly a drip or leak that accumulates...

Could you try a set of tube/rods/jets that are EXTREMELY lean to prove or disprove the "jets" as being the cause?


My best,



Tony
Black Tooth and me... since 1983!
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odie
odie

June 12th, 2008, 8:24 pm #3

I dont have any other jets that are smaller, and ~$50 is kinda up there for a test. I will double check this weekend if it is coming from the carbs, I will run it with the filters off and with the rain tray mod I should be able to see


Odie
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Joined: December 21st, 2004, 3:04 am

June 12th, 2008, 8:41 pm #4



My best,



Tony
Black Tooth and me... since 1983!
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<font face_"Times Bew Roman" size=1>Disclaimer: This message does not necessarily reflect the views of the sender, or, for that matter, any other carbon-based life form; Any resemblance to actual persons, living or dead, is unintentional and coincidental; All models are 18 years of age or older; Dry clean only, do not use bleach, Tumble dry and remove immediately; Do not bend, fold, staple or mutilate; Anchovies, cheese or jalapenos may be added to orders upon request for an additional charge; Professional driver on a closed course, do not attempt to duplicate; Your actual mileage may vary; No substitutions are allowed; Available for a limited time only while supplies last; Offers are void where prohibited by law; Humor is provided "as is" without any warranties expressed or implied; The user assumes full liabilities and originator is not liable for damages due to use or misuse; We are an equal opportunity employer, with no regard to race, religion, sex, or sexual preference; No shoes, no shirt, no service; Read at your own risk; This post may contain material some may find to be objectionable; Parental advisory: Explicit lyrics; keep away from pets and small children; limit one-per-family please; No money down; No purchase necessary; Ask us about our guns-for-dope trade-in plan; You need not be present to win; Some assembly required; Batteries not included; Action figures sold separately; Contents were packed full at the factory, but may have settled during shipping; Peanuts and peanut products are produced in this establishment and also consumed by the author; Sanitized and sealed for your protection; Do not use if safety seal is broken; Do not use while operating a motor vehicle or heavy equipment; Safety goggles may be required during use; Use gloves when handling; Call before you dig; Use only with proper ventilation; For external use only, if a swelling, redness, rash, or irritation develops, discontinue use; Do not place near a flammable or magnetic source; Keep away from open flames; Avoid inhaling fumes or contact with mucous membranes; Contents under pressure, may explode if incinerated; Smoking may be hazardous to your health; No smoking allowed, except in designated areas; The best safeguard, second only to abstinence, is the use of a good laugh; Text is made from 100% recycled electrons and magnetic particles; No animals were harmed while writing this post; No salt, MSG, preservatives, artificial color or flavor added; If ingested, do not induce vomiting, If symptoms persist, consult a comedian; Ribbed for your pleasure, slippery when wet; Must be 18 to enter; No one under 21 allowed; We check IDs to age 40; Seniors Welcome; Ask for our Senior Menu; Present discount coupons to cashier before totaling; An 18% gratuity will be added to the total for parties of 8 or more; We reserve the right to refuse service to anyone. Return all carts to the store; No dogs allowed, except for the visually impaired; Penalties for early withdrawal, one size fits all; Offer is valid only at participating sites; Prices slightly higher west of the Rockies; Allow four to six weeks for delivery; If defects are found, do not attempt a repair, return to an authorized service center; Please remain seated until the vehicle has come to a complete stop; Objects in the mirror may be closer than they appear; Fasten seat belts; In the unlikely event of a loss of cabin pressure, oxygen masks will fall from above, cover your face first, then assist those around you. Your seat cushion may be used as a floatation device. This disclaimer is invalid in the event of hurricanes, floods, earthquakes, sonic booms, vibration, electromagnetic radiation from nuclear blasts or any other act of war, unauthorized repair, improper installation, misuse, typos, misspelled words, missing or altered signatures, and incidents owing to computer or disk failure, accidental file deletions, and Acts of God. Other restrictions may, or may not apply.</font>
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86X
Joined: April 19th, 2006, 3:46 pm

June 12th, 2008, 11:32 pm #5

So I replaced all the Jets with the forum recommended ones, as well as peirce manifolds agreed with the recommendation. I put on a new intake/exhaust gasket and tested, no leaks. Absolutely no change in the back fire. It still only does it after it is warm. This is driving me crazy! if it were a burbly pop, I could live with it. but it is more of a sawed off shotgun BANG!!!

here are the stats again

Dual 42 DCNF Idle Jets 125 Main 40
Stock cam
4-1 header no Cat (or dog for that matter)

Odie
A couple of thoughts, since it is only occurring after the engine is warm it is likely due to thermal expansion changing a relationship somewhere. Having said that, I would look at the exhaust for leaks at the joints. An additional thought is that your mechanical advance may be sticking when hot. The pins can wear (or just cruddy) causing the weights to stick. Could be vacuum advance sticking as well but less likely. Of course the obvious is the static advance is within reason. Lastly, carbon build up or worn guides causing sticking valves may cause an occasional backfire. OK I’m out for now. Aside from the backfiring did the jet change help overall?
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Colin Duckworth
Colin Duckworth

June 12th, 2008, 11:46 pm #6

So I replaced all the Jets with the forum recommended ones, as well as peirce manifolds agreed with the recommendation. I put on a new intake/exhaust gasket and tested, no leaks. Absolutely no change in the back fire. It still only does it after it is warm. This is driving me crazy! if it were a burbly pop, I could live with it. but it is more of a sawed off shotgun BANG!!!

here are the stats again

Dual 42 DCNF Idle Jets 125 Main 40
Stock cam
4-1 header no Cat (or dog for that matter)

Odie
Did I read right? 125 idles and 40 mains? Are you sure? Mate . . . that's gotta be the other way around! 40 idles and 125 mains (and probably with a 185 or 190 air correct. Sorry . . . I know bugger all about DCNF emulsion tubes so can't assist you there).

If indeed it is as read / written, then do not drive that car very far with that set up (if at all . . . it'll be far, far too lean . . . which would also explain your 'shotgun' popping as distinct from a 'rich' burble / pop).
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Mike Richmond
Mike Richmond

June 13th, 2008, 12:06 am #7

So I replaced all the Jets with the forum recommended ones, as well as peirce manifolds agreed with the recommendation. I put on a new intake/exhaust gasket and tested, no leaks. Absolutely no change in the back fire. It still only does it after it is warm. This is driving me crazy! if it were a burbly pop, I could live with it. but it is more of a sawed off shotgun BANG!!!

here are the stats again

Dual 42 DCNF Idle Jets 125 Main 40
Stock cam
4-1 header no Cat (or dog for that matter)

Odie
I fear that you bought way too much carb for your little 1500.

But some of this depends on the venturi size. You should be able to peer down the throats and see the stamped number.

I would also like to see what number (Fxx) is stamped on your emulsion tubes (the main jets are on the bottom of these).

With too big venturis, you don't draw air at a high enough speed to mix up the fuel into droplets. So you idle badly and fuel can just slosh around. Then eventually it gets into the exhaust unburned and BANG.

I had a sweet 1500 with DCNFs and stock cams. Purred and roared. But I had 36DCNFs with *26*mm venturis.

MikeR ('86X)
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Colin Duckworth
Colin Duckworth

June 13th, 2008, 12:13 am #8

I am running 40 IDFs with 36 mm chokes no problem. OK, so they can be a little 'stuttery' around town on light throttle, but they run fine (on F11 emulsion; 185 airs and 130 mains). Fair enough . . . I have the (relatively standard for Australia) cam knocked back 4 or so degrees; I run an MSD 6; I have a set of IPAs on it, but it works fine.

My concern for our mate here is with the quoted jetting. Even if he had 36 IDFs (as fitted to Alfa Sud Sprint GCLs for instance) with 26mm vents, he'd still need mains around the 100 / 110 mark at least . . . certainly not 40.
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86X
Joined: April 19th, 2006, 3:46 pm

June 13th, 2008, 12:18 am #9

Did I read right? 125 idles and 40 mains? Are you sure? Mate . . . that's gotta be the other way around! 40 idles and 125 mains (and probably with a 185 or 190 air correct. Sorry . . . I know bugger all about DCNF emulsion tubes so can't assist you there).

If indeed it is as read / written, then do not drive that car very far with that set up (if at all . . . it'll be far, far too lean . . . which would also explain your 'shotgun' popping as distinct from a 'rich' burble / pop).
For sure this would be a problem. I assumed a typo. However I don't think it is possible to have them reversed since (IIRC) jet sizes for the DCNF's don't go that small for the main or that large for the idle. I suppose they could be soldered/drilled.....
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Mike Richmond
Mike Richmond

June 13th, 2008, 12:23 am #10

I am running 40 IDFs with 36 mm chokes no problem. OK, so they can be a little 'stuttery' around town on light throttle, but they run fine (on F11 emulsion; 185 airs and 130 mains). Fair enough . . . I have the (relatively standard for Australia) cam knocked back 4 or so degrees; I run an MSD 6; I have a set of IPAs on it, but it works fine.

My concern for our mate here is with the quoted jetting. Even if he had 36 IDFs (as fitted to Alfa Sud Sprint GCLs for instance) with 26mm vents, he'd still need mains around the 100 / 110 mark at least . . . certainly not 40.
I'm not going to argue with anyone who has a setup that works.

What I do know is that I've seen many, many, people spend a lot of money on dual carbs and never get them to run right, and many others who get them to run and then are disappointed that they get 20MPG in a car that should get a lot more.

Unless you are a dedicated tuner, and doing a bunch of wide open racing, you want a carb on the street that behaves nicely like a single carb but let's you have some fun when you put your foot in it.

If it were me, I'd get these:

http://www.geneberg.com/product_info.ph ... cts_id=897

Then I'd go with F36 emulsion tubes (personally recommended to me by PBS years ago).

Based on formulas (really rules of thumb) communicated to me by someone at Pierce Manifolds (also years ago), I'd be using 110 or 115 main jets.

With stock cams, a setup like this will purr around town and get 30MPG on the highway.

MikeR
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