Looking for another opinion. What is causing my backfireing.

Ok here goes I have a mild built sbc in an 1981 Chev 1 ton chassis and a custom 1950 cab mounted to it as a work truck. Been a few year build constantly upgrading but here is where I am at now.
1981 sbc stock bore stock crank 16000original miles rebuilt it because as everyone knows bad year for camshaft.
Put new rings bearings honed cylinders
comp cam110 degree normal lift longer duration cam
Double roller chain stock pin on cam gear
New jegs stock length pushrods
New roller rocker stock ratio valve train
New jegs 2.02 aluminum heads
New champion plugs
New spark wires
new hei distributor
New680 quick fuel vacuum second
Jegs duel plain high rise intake
All brand new gaskets
Custom 1 5/8 right to 3 inch no restrictions stack mufflers
Turbo 400 trans
4.10 rear gear

Timing is set to 12 degrees
Carb has 11 inches of murcury max at 600rpm idel
Jetted the carb from quick fuel had it at 70 78 I have it down to 68 72 and the plugs are still black and some wet. Number 5 cylinder is just starting to turn dark brown.
Vacuum ported advance
6psi fuel pressure
Fuel is in middle of the sight glass on the bowls

Now here is the problem the truck is a total dog has no get up and go. And I have a dead spot from 2000-2500rpm where it backfires out the exhaust almost constantly and even will ocationaly stall and refire under light accelerations at slow speeds. It seams to run fine before 2000rpm and after 2500 it stops back fireing but feels as if it had a miss in a random cylinder every once in a while.

Should I be looking at the weights in the distributor or fuel delivery system. At a loss everything I try dose not seam to remidy the problems.

What intake manifold?

Total guessing from here... but yes, I think you need to document when/where your timing is during the rpm change.

I'd also set the fuel level to the bottom of the site glass.

OH, and welcome to the site. Nice to have you here. And we really like to see some pictures of the project we're discussing.

Quote:

---

Originally Posted by HDSBC

Timing is set to 12 degrees

---

What is TOTAL timing (timing at 3500-4000rpm, with vacuum advance disconnected)

Quote:

---

Originally Posted by HDSBC

Carb has 11 inches of murcury max at 600rpm idel

---

JMHO, vacuum is low

Quote:

---

Originally Posted by HDSBC

the plugs are still black and some wet. And I have a dead spot from 2000-2500rpm where it backfires out the exhaust almost constantly and even will ocationaly stall and refire under light accelerations at slow speeds. It seams to run fine before 2000rpm and after 2500 it stops back fireing but feels as if it had a miss in a random cylinder every once in a while.

---

Classic signs of too rich. Check the power valve

JMHO, 1981 SBC truck motor had 8-1 compression which may not match well with your cam and head combo, requiring some serious tuning.

Quote:

---

Originally Posted by HDSBC

1981 sbc stock bore stock crank 16000original miles rebuilt it because as everyone knows bad year for camshaft.

---

JMHO, the camshaft (flat tappet) problem was (is) caused by oil formula changes. If you went back with a flat tappet cam, BE SURE you break it in correctly and use the correct oil (and/or additives).

to me it sounds like it could be a lifter problem or cam. could be #5 but I would start there. I think that after 2500 the other 7 cylinders are carrying it. what is your compression on all 8?

Quote:

---

Originally Posted by HDSBC

comp cam110 degree normal lift longer duration cam

---

JMHO, 110 LSA is a little tight for the street, is this a THUMPER cam?

To answer everyone s questions it has a jegs duel plain aluminum intake I checked the torque on all the bolts has been gaskets and did the old propane torch wave to look for any leaks no signs of leaks.

I will try turning down the float levels that might help with the richness.

I can't document the timing threw the rpm range the only tool I am missing is an adjustable timing light I only have an old school light hoping to get one or borrow one soon.

Yes 8-1 stock but I put 64 cc heads raise the compression ratio up to more like 9-1 I didn't want to go to wild remember this is a heavy hauler not a drag strip racer.

Yes much agree signs of too rich senario but carb is brand new has less then 200 miles on it how could the power valve be gone already and if so what dose it look like when it's broken.

I agree vacuum is low but I can't pull any more then 11 inches no mater what I do and when I adjust it minimum is 10 inches of murcury. Everything I read it should be 12-14 but it just can't get there and I am not even running a vac brake booster. My brake booster is run off the power steering pump because my chassis was a g30 coe design didn't have enough room for a big booster under the hood.

Yes I did a royal purple brake in oil and did the proper brake in procedure when I rebuild the engine.

Compression tested all cylinders at wot lowest cylinder was 122 highest was 130 psi all checks out.

Cam is comp cam high energy line if I remember correctly.

I don't know how to post pictures from my photo gallery if someone tell me how I will most definitely post some pics of the old girl

Quote:

---

Originally Posted by HDSBC

To answer everyone s questions it has a jegs duel plain aluminum intake

I will try turning down the float levels that might help with the richness.

I can't document the timing threw the rpm range the only tool I am missing is an adjustable timing light I only have an old school light hoping to get one or borrow one soon..

---

You can check the timing - use you timing light and by changing the idle screw and watching the tach, come up through the rpms by 100 and see where the timing is. Not the fastest way, but it does work. Generally most engines like total timing of 30 or 32 degrees by 2K to 2200. And don't forget, the vacuum chamber isn't working as the throttle is being opened and dead at full throttle, so don't confuse that operation by just disconnecting and plugging the hose. Once you know where the timing is at, you could maybe add some initial timing or start playing with weights and springs. There was a thread about posting pics. I'll see if I can find it.

Quote:

---

Originally Posted by HDSBC

TI can't document the timing threw the rpm range the only tool I am missing is an adjustable timing light I only have an old school light hoping to get one or borrow one soon.

---

You can use timing tape with your light to see the timing curve.

Quote:

---

Originally Posted by HDSBC

Yes much agree signs of too rich senario but carb is brand new has less then 200 miles on it how could the power valve be gone already and if so what dose it look like when it's broken.

---

I’m sorry, I didn’t mean to confuse you, I wasn’t suggesting the PV was blown, but rather the power valve with your low vacuum may be coming on too soon. Also, all carburetors need to be adjusted to match the motor they are installed on, no carburetor can come out of the box perfect. While idle mixture and main jets will usually suffice, some need more advanced tuning.

Quote:

---

Originally Posted by HDSBC

I agree vacuum is low but I can't pull any more then 11 inches no mater what I do and when I adjust it minimum is 10 inches of murcury. Everything I read it should be 12-14 but it just can't get there

---

Check your gauge, it may be out of calibration.
JMHO, 600rpm idle is a little low for your cam, raise the idle to 700-800, should improve vacuum. Also too rich will lower vacuum, as will incorrect ignition timing.

Quote:

---

Originally Posted by HDSBC

Yes I did a royal purple brake in oil and did the proper brake in procedure when I rebuild the engine.

---

Break-in oil and correct cam break-in procedure with a flat tappet cam is essential, as is high pressure lube (cam grease) massaged into /onto the cam lobes at installation, also confirming lifter rotation (push rods rotating). Unfortunately, still no guarantee the cam will not fail with todays oil formulas, especially a more aggressive cam.

Quote:

---

Originally Posted by HDSBC

Compression tested all cylinders at wot lowest cylinder was 122 highest was 130 psi all checks out.

---

Check and make sure your valves are not adjusted too tight.
Did you break-in your rings?
While your rings will probably take a little more than 200 miles to fully seat (and raise compression test readings), they do require a seating process to assure it.
DENNY wrote an excellent post on the procedure and I’m sure if prompted he will link you to it.

As "34-40" stated, a little more initial timing will help, BUT you must be careful with a stock distributor, advance stops will be needed. Usually a stock distributor adds 20-25° of mechanical timing through its curve, 12° with the added high side of 25° is over the edge of detonation at TOTAL 37° for a SBC (34-36 max).

Without knowing what the total timing is at 4000 RPM, you're guessing and gambling. So without knowing, I'd say 12° initial would be max. Having said that, you can add vacuum advance to the idle by putting its source to manifold vacuum (vacuum at idle), which will probably help too.

IMPORTANT: what is your fuel pressure??

So just a little clairification the motor is fully broken I. I rebuilt engine the first time 3 seasons ago and have about 2000 miles on it and redid the top end this winter with the proformance top end kit or heads intake new carb distributor that kinda stuff. And have only drove about 200 miles this year testing and tuning.so all the major components s like rings and what not have had time to seat by now.

How do I set up the power valve please give me some insight into that never have played with one before.

Vac guage is brand new right out of the box bought it for this project to set up the carb

Valves were set to snug then 1/2 turn

I have been conversing with an old stock car racer he said same as you need to know advanced timing but with the cam and heads combo he suggested going to the 12 degrees initial timing.and the motor is not nocking

Fule pressure is 6 psi right what the carb manufacturer recommend for street application

Quote:

---

Originally Posted by HDSBC

Valves were set to snug then 1/2 turn

---

Tell me more about you did this process, I think your valves may be a little tight.

Rule of thumb with a power valve you take your idle vacuum and cut it in half, however that is not always the case. Do you know what number PV you have? And it's always good to check the gasket at the PV when there's a rich issue, occasionally the manufacturer makes mistakes.

By your description, the problem comes on after 2000rpm, which is just coming out of the transition circuit into the cruise (primary jets). With Holley type carbs, I jet the primary for cruise first and don't touch the secondaries until the cruise is right. Remember, WOT is the accumulative of the primary, secondary jets and the POWER VALVE. The Power Valve is HUGH in tuning these carbs for the street.

But first, we need to get the idle vacuum up or nothing is going to tune right. And, this is why I'm asking about the valve lash setting, it is REAL easy to set valves a little tight if you're not experienced with setting valves (no disrespect meant). Tight valves will cause all of the troubles you are experiencing. It may not be actually running rich, but rather not burning the fuel because the valves are not completely sealing.

Just to confirm, you could loosen all rockers 1/4 turn (really doesn't take much) and see if the vacuum comes up. If it does, we'll go over the process of setting valve lash in more detail.

I don't know the power valve size off by hand but I will take apart the carb and see.
I will most definitely reset the lash on the valvetrane see if that is my low vacuum problem. Most likely going to be on the weekend I will be able to do that task. Thank you for the information fingers crossed I can get this stumble out of the engine.

So a little update tonight went through the valve train. Brought #1 to Top Dead Center

Exh.- 1 – 3 – 4 -8​
Int.- 1- 2 – 5 - 7 ​
​
Get #6 to Top Dead Center

Exh.- 2 – 5 – 6 – 7​

Int.- 3 – 4- 6 - 8

Brought it to finger tight on the pushrods and turned past 1/4turn and set the set screw on the roller rockers.
Still same vacuum inches of murcury
600rpm =10.5inches of murcury
800rpm=13 inches of murcury
And the higher I went on the rpms the inches of murcury increased
I have all 4coners of the carb idel screws set to 3/4turn out from bottomed
Even though it is an auto trans it seams to really like 600rpm idel.

Went for a ride down the highway it's still backfire ing out the exhaust probably not as bad but still doing it in the 2000-2500rpm range.

So a little update tonight went through the valve train. Brought #1 to Top Dead Center

Exh.- 1 – 3 – 4 -8​
Int.- 1- 2 – 5 - 7 ​
​
Get #6 to Top Dead Center

Exh.- 2 – 5 – 6 – 7​

Int.- 3 – 4- 6 - 8

Brought it to finger tight on the pushrods and turned past 1/4turn and set the set screw on the roller rockers.
Still same vacuum inches of murcury
600rpm =10.5inches of murcury
800rpm=13 inches of murcury
And the higher I went on the rpms the inches of murcury increased
I have all 4coners of the carb idel screws set to 3/4turn out from bottomed
Even though it is an auto trans it seams to really like 600rpm idel.

Went for a ride down the highway it's still backfire ing out the exhaust probably not as bad but still doing it in the 2000-2500rpm range.

Quote:

---

Originally Posted by HDSBC

I have all 4coners of the carb idel screws set to 3/4turn out from bottomed
Even though it is an auto trans it seams to really like 600rpm idel..

---

There is only one idle screw, the screws you are turning I believe are the mixture screws, would you have a picture of this carb? Or can you tell us if it is a holley? Or??

Does your carb look like this one? see link and scroll down.
https://www.dragzine.com/news/how-to...y-carburetors/

Quote:

---

Originally Posted by HDSBC

So a little update tonight went through the valve train. Brought #1 to Top Dead Center

Exh.- 1 – 3 – 4 -8​
Int.- 1- 2 – 5 - 7 ​
​
Get #6 to Top Dead Center

Exh.- 2 – 5 – 6 – 7​

Int.- 3 – 4- 6 - 8

Brought it to finger tight on the pushrods and turned past 1/4turn and set the set screw on the roller rockers.

---

This method chances tight valves, especially with a cam upgrade from stock. Here is a better way, I've used it for years, it works.
Attachment 72456

Yes your correct there is only one idel screw poor choice of wording on my part the mixture screws on all 4coners is at 3/4turn out from bottomed. It's a quick fuel technologies essentially a Holly with a fancy paint job.

I had done that method of setting up the valves 1once before at this point I have tried every way all my books and the internet has sujested including the old school running the engine listen for the clicking and turn down that method I found incrediblely hard the roller rockers just do not click like the old style stamped steal rockers. That motor will not pull more inches of vacuum that I can achieve. it's at the point now where it's running the best that it has but is still not acceptable. I have to have a mid range timing curve in the mechanical weights of the distributor off. Or the power valve in the carb is causing a rich senario. Or lastly I have an intermittent dead cylinder that is doing whatever it feels like it want to do.

It is possible your mechanical advance is stuck and not advancing, total timing check would tell. With the cap off of the distributor, can you rotate the rotor (about 1/4") and it springs back?

JMHO, sorry don't mean to beat a dead horse, but low vacuum with other symptoms points to valves.

The most common mistake with setting valves is HURRYING. You must take your time. Get the valves positioned correctly for the cylinder, loosen the rockers completely and let it set a few minutes to allow the plunger to completely rise.
Second most common mistake is FORCE. This is not a strong arm procedure, you do not want to use force. It's delicate, you just want to take the slack out without compressing the plunger spring. If you compress the plunger spring even a little your valves will be too tight.
Third most common mistake is ALIGNMENT. Get the positions correct, close doesn't work here, especially with a upgraded cam (even more so with tighter LSA).

Not trying to be mean or disrespectful, but, IF the valves are too tight (I suspect they may be) and you continue down this present road, you will cause an expensive repair (if it's not to late already).

No i value your opinion obviously you know more about the subject then I do it's just frustrating. I have tried all these different ways to set the valves spending hours of time and no improvement I just wanna take my kid for a ride rather then wrench the same thing over and over again. Would be nice if there was a set in stone way to do it not 15 different ways to achieve the same thing.

Surely I don't want to ruin the new engine just blew thousands of dollars don't need to do that again.

I will check the distributor Tomorrow see about that and redo the valves one last time on the weekend with your proposed method of each cylinder threw the firing order. And if I can't get anything more then 11inches of murcury then that's it with valves I know at that point the engine just can not achieve any more.

Quote:

---

Originally Posted by HDSBC

Would be nice if there was a set in stone way to do it not 15 different ways to achieve the same thing.

---

The method I gave you is the most foolproof method out there, it is practiced successfully by thousands. Most of the other methods assume the user knows not to apply too much pressure, which gets most into trouble.

Quote:

---

Originally Posted by HDSBC

I will check the distributor Tomorrow see about that and redo the valves one last time on the weekend with your proposed method of each cylinder threw the firing order.

---

Notice on the method I gave you it instructs you to hold the rocker tip firmly against the valve stem, this assures there nothing pushing down on the lifter. Your trying to just take the slack out, not compress the lifter. You should be moving the push rod up and down quite a bit at the beginning with the rockers loosened, then tighten a little (remove the wrench each time so it doesn't put down), slowly gently with the up and down movement taking out the slack. This takes time, it's not a race, do it right and you'll never have to look back.

Quote:

---

Originally Posted by HDSBC

And if I can't get anything more then 11inches of murcury then that's it with valves I know at that point the engine just can not achieve any more.

---

Low vacuum is just one symptom, by itself it may not be too concerning, however, combined with your other symptoms it certainly points towards the valves being slightly tight, and why I suggested backing off the rockers slightly to see if the vacuum comes up a little.

The best advice I can give you about hot rods, is learn to enjoy working on them, chances are you'll spend more time working on em than driving em.

OK so I had some time tonight with the young lad to do some valvetrain setup. I did exactly like what your pdf said even printed it off taped the indicator to the balancer and followed the instructions to a T.

I didn't start the engine ran out of time tonight before I could get the valve covers back on.

But what I did uncover was interesting spark plug colors
#1 black and dry
#3 Dark Brown Dry tip but wet threads
#5 Dark Brown Dry tip but wet threads
#7 Dark Brown completely wet tip and threads
#2 Dark Tan wet threads
#4 Black Dry tip but wet threads
#6 Dark tan dry
#8 Black completely wet tip and threads

I checked all plugs all seam to be firing fine and blue arcs.

I popped the distributor cap off and spun the rotor it can defiantly spin more then a 1/4inch with light pressure and springs right back to where it was. I would estimate more like 3/8ths of rotation. again with light pressure I was not forcing it.

Distributor mechanical advance is working.
See how it runs now, pictures of the plugs would help.

I think I figured out how to post a picture if it works I will send pics of the plugs. Mind you it's not the best picture truck is apart at the moment.

so I got her running like a top now.
It could have been 1 of 3 things or a combination but hears what's going on now.

put the motor back together tonight valve train all set up and rocking with no ticking. got the vac up to 11.5 inches of mercury at 600rpm and 13.5 inches of mercury at 800 Rpm. air fuel mixture screws all backed out to 3/4 turn from bottomed.

took apart the carb and checked the power valve its a 65 by the way but if it was stuck I might have solved it by disassembly and reassembly.

but the real gremlin that was hiding I think was the distributor, turns out that the clamp that bolts to the intake was not putting proper pressure to hold the distributor from retarding it self back. the little fingers were not bent enough to put proper pressure on the distributor I think or the bolt was turning it self loose so a pair of pliers and some Loctite solved that.

Head out on the highway and she purrs like a kitten no stumble, no hesitation, no backfire, at all. Threw the old heavy Chevy in first and let her go and that was the first time that dully truck has ever squawked the tires before. Plus it sounded like a drag car roaring down the road. put a smile on my boy's face so it was all worth it hahah.


Thank you everyone for the input and information. and a special thank you too 36 seadan.

HDSBC, thanks for taking time to tell the story of getting it right! Not everyone does, and that kind of explanation may be a big help to someone else down the road. Glad you got it running, and that your boy got a kick out of the ride!

Yup - what Roger said! Hope you enjoy this rig for a lot of years and keep making memories with you youngster!
Glenn