Thread: Looking for another opinion. What is causing my backfireing.

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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.

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/

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.
Adjusting hydraulic valves .pdf

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.

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.

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.

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