Backfire under loads
 

My son just got a new to him 82 K10 with a 305 and 700r4 tranny. Definitely has been messed with and trying to straighten out some things to get it on the road.

It originally had the ESC installed that was disconnected before we got it. It has TCC that is disconnected too. Different HEI was installed. A/C has been disconnected. Motor has been apart but have no idea what was done to it.

Truck ran but not great at all. After checking timing it was way off. Pulled dist. and reset. I had a way hard time setting the timing up. Not my first rodeo but this thing just didn't make sense. Eventually got it but was back at a previous spot where it wouldn't run good. Whatever...

Pulled intake and carb off and put an Edlebrock intake with a Holley carb. (I'm not looking to get into Holley vs others) Runs much much better but not 100%. Timed right up with initial at 12*. Installed new vac advance, plugs and wires.

Though the truck runs way better it still seems to be held back to me. Doesn't rev up like I think it should. When holding foot on brake and giving gas it will backfire out carb and quit.

So in all this I keep coming back to the torque converter lockup circuit. I have no experience with them so I have no idea how they work. But my gut says that if I swap it out to a manual trans the problems will go away. Though I do think that the timing chain is too loose but still.

Does the TCC stuff have to hooked up? How does that system work? This stuf is too new for me. I'm used to older trucks.

So to recap with specs:
Initial timing is 12*
Total is around 32* give or take
Idles pretty good in park. No tach so I don't know the exact RPM
Vac advance is hooked up to manifold vacuum
305 sbc
700r4 tranny
I'm guessing 3.08 gears??

I'm sure I'm missing some things but ask away.

Thanks!! Brian

Re: Backfire under loads
 

Have you checked and run the valves?
Tried more timing?
Checked the balancer/timing tab are accurate?
Done a compression or leak down test?

Re: Backfire under loads
 

I'm guessing you have already checked the plugs? Have you checked the gap?

Is it good and warmed up when you get the backfire?

Re: Backfire under loads
 

Possible exhaust restriction? Does it still have the riser valve? Is it stuck shut?

Since you replaced the intake and carb, the source of a lot of hidden vacuum leaks went away but might check all the hoses anyway.

What are the plugs telling you? With the new intake, carb and now decent timing set, the plugs should tell you if any of the cylinders are out of sorts with the rest.

2nd on trying more timing. Since the engine was modified by a PO, possible the timing marks are lined up off.

Re: Backfire under loads
 

Yes I did check the valves and all were ok. Was really surprised they were.
I haven't tried more timing.
Balancer is questionable. There are 2 marks on balancer and I tried both of them without much difference. Which makes wonder.
I haven't done a compression check.

Yes it's good and warm.

Yes I will try more timing and see what happens. I thought the same thing as there as been some crazy things the PO has done. It also was acting like the timing chain had a lot of slack in it when trying to set distributor in and set timing. I never had 1 give me so much problems before.

I will try these and let you guys know. Thanks!!

Re: Backfire under loads
 

TCC on 700R4 does nothing in 1st gear. If you have the problem in 1st then it's not the TCC.
TCC is fed by 2nd gear oil. If there is a problem where it locks on in 2nd the motor will lug horribly at the1-2 shift.
Yes, the TCC is wired. Power thru the brake switch so touching brake pedal will dump it.

My guess is that the cam is off a tooth on the chain, or my favorite - someone put a high RPM cam in a stock motor and converter won't flash enough to let it run right. Neighbor had that on his Ferd, put in a junkyard motor that turn out to have been built to run 4500-7000. Explained why they couldn't get it to idle at 800. Set the idle at 1500 and added a ton o timng and it made scary power.

Re: Backfire under loads
 

Double check your firing order.

Re: Backfire under loads
 

So a little update. I added a ton more initial timing. It's close to 30* now and runs much better. Not great but WAY better. Doesn't backfire under load. Still feels like it's lugging. In park it idles and pretty much dies when dropped in gear. Wound up it sounds pretty good. I'm thinking the same that cam is off a tooth or super loose chain based on previous timing events with this.

Double checked and it's good!!

Re: Backfire under loads
 

IIRC, a lot of 305s from that time came with the nylon ring timing gear. If so, could be enough wear to cause the cam timing to be off.

Maybe a PO fixed the timing gear and at the same time added the type cam D13 suspects. The PO might also have indexed the cam for extra fun.

Re: Backfire under loads
 

That kind of timing tells you that your balancer/timing tab is really inaccurate or you’re reading your timing in the wrong spot on your 305.
A 305 should have a timing tube at the 12 o’clock position that you sight down to the balancer. Right behind the water pump.
Are you timing it with this or is there a timing tab at the 2 o’clock position?

Some info here.
https://www.crankshaftcoalition.com/..._TDC_Lines_SBC

Re: Backfire under loads
 

The only tab on the cover is in the 2 o'clock position. This is what I am timing it to. There are 2 marks on the balancer and I am using the 1st line that comes up to the tab. However if I use the 2nd line it doesn't run much different really. So it having the tab on 12 o'clock would make a world of difference. I'm trying to time to incorrect marks. This would make a lot of sense.

So which timing mark on the balancer would be the correct one to use?

Re: Backfire under loads
 

So after looking at the link Geezer posted I have what appears to be the balancer in the pic that shows 2 marks. Now if the balancer I have has been modified and the line is in the correct location then I should be ok right?? Am I missing something?

Re: Backfire under loads
 

Use the timing tab and mark on the balancer that correspondes to true tdc #1.
True tdc is found using a piston stop.
Like here.
https://www.crankshaftcoalition.com/...op_dead_center

Re: Backfire under loads
 

Timing chain jumped?

Re: Backfire under loads
 

I did a quick check of the play on the timing chain and it seems I have about 8* of play when moving crank before rotor moves. That seems like a LOT to me. Each time I do something I get a little closer. I'll pull it apart this week and replace the timing chain. You never know maybe something else will be uncovered while in there!!

Re: Backfire under loads
 

I had an 81 C10 and the timing chain got so loose it wore a hole in the cover but never jumped a tooth. It's always nice to have anew one though. Did you ever get TDC figured out?

Re: Backfire under loads
 

I didn't find TDC yest. I ordered the right tool to find it and in the mean time I checked the play in timing chain. I'll pull it off and hopefully find something real positive out. By the time I get it all back together the tool will have come in.

Re: Backfire under loads
 

So more of an update. I pulled the timing chain cover off and the chain was loose. Not as loose as I had hoped but loose enough. Turned it all over to see if the timing was correct between cam and crank and it was dead nuts on. I took the tool to find TDC and located it and found the timing tab was correct with the mark on balancer. Curious now I took the dist cap off to see where rotor was and rotor was 180* out. well that will make it run like crap!!! So at least I found a definitive problem and will correct it while the motor is all apart so I know it will be installed at TDC. Now the problem is with myself. I know I had brought up the #1 piston and felt compression many times on this motor but man I missed something. This isn't like me and it will eat me up for a week now trying to figure out where I went wrong.

Re: Backfire under loads
 

I personally have never seen a "305" with anything but the timing chain cover pictures below. If you are timing off the other style timing tab configuration, I'm surprised it would run at all.

With this cover you can only shine a timing light down at twelve o'clock, directly behind the water pump.

Attachment 1815488

http://67-72chevytrucks.com/vboard/s...d.php?t=498127

Re: Backfire under loads
 

Interesting. Balancer must have been changed at some point.

Re: Backfire under loads
 

It won’t run like crap 180 degrees out. Because It just won’t run. It can’t.
When the rotor was pointing at #6 it’s likely #1 plug wire was right there on the cap.
You can put #1 anywhere on the cap as long as #1 is on tdc compression.
And then wire the rest of the wires correctly.
Sticking your thumb over the plug hole works good if you understand there’s pressure twice in the cycle. Once on compression, once on exhaust.
What tool did you use for finding tdc?

Re: Backfire under loads
 

I would say you might have a flat cam lobe(s). You could take valve covers off and see if any rockers are loose and check for bent push rod as well

Re: Backfire under loads
 

Ok so I'm getting somewhere now.

First off is I jumped the gun on the TDC and 180* out. It's been a long time since I have built a motor and the 20 years have been aircraft so my mind is in that mode. Any how no it wasn't 180* out. It was correct all along. Then I confirmed my no 1 timing mark locations on balancer. Someone must have installed a different one at some point as it has both marks for 12 and 2 o'clock position.

Secondly - remember I mentioned my son bought a truck. Well that means he's helping me. With that said I pulled the plugs to see what they looked like since they had some run time on them. Found many were black. So me thinks "hmmm, Hey Evan didn't you say you changed the plugs?" Evan-"I thought you did??" Me-"Well lets put the new ones in and try it" I had given him a list while I was at work and just figured he replaced them because he said he did everything. well I was wrong.

Popped a new set in and man it's like a different truck. Much smoother you can actually drive it around without any issues.

However I'm not out of the woods yet though. I can't time it. Moving the dist in either direction and it will drop RPM and die. Disconnecting vacuum advance line and sometimes RPM will rise. Sometimes not.

I have never connected to ported vacuum on any of my vehicles and never had any issues. Not saying that this no different but just would have never have tried other wise. I will try connecting it to ported and see what happens.

The timing is confusing me at the moment. I have never seen a motor have the same affect when turning it in either direction. Any thoughts on that one??

Not sure who said it but yes I have no clue as to what was done to this motor and everything I touch has been touched and usually not in a good way.

It always amazes me on how much effort goes into hacking something when they just could have fixed it the right quicker right out of the gate. Especially electrical things. Sadly this happens on airplanes too!!

Re: Backfire under loads
 

Check the weights in the distributor aren’t sticking.

Re: Backfire under loads
 

Yes I noticed the counter weights were extremely worn out so I swapped it out with a known good distributor off my other truck. Same results.

So the thing that me wondering is this: Before when I first started this whole thing I could move dist. and it would respond normally. That's when I noticed that when advanced it to around 29* it ran much better. Now after some time it has no affect. So what did we change. Other than wires, plugs, cap, rotor and other bolt on things. The biggest thing was the timing chain. After all back together turning dist. has no affect. I did check timing and initial is at 29* it will go up maybe a couple degrees before starting to stumble. Retard it and it stumbles right away.

Very frustrating and my son is losing hope and talking about selling it.

Re: Backfire under loads
 

When you’re turning the distributor is the vac advance hooked up?

Re: Backfire under loads
 

No it is not hooked up.

Re: Backfire under loads
 

I’ve had motors that wouldn’t start unless you had the distributor in exactly the right spot.
How does it run when you’re at the sweet spot of about 29 degrees?
Does it run good. No backfiring. No pinging.

Re: Backfire under loads
 

It doesn't run that good at all. Don't know about the pinging yet. Definitely not smooth and never like any other motor I have ever got running. But yet when I put the new plugs in and started it up and got it all warm it ran pretty good. Started it the next day and was a total different animal. Won't idle in gear. Starts right up each time but just runs like crap.

I just remembered that the vacuum advance is adjustable so maybe I'll try to adjust so it comes in at idle. Not sure why though as it all worked before I started this all.

Re: Backfire under loads
 

Have you looked in the carb while it’s idling?
Could be something in the needle/seat that makes it flood a bit.

Which Holley carb?

Re: Backfire under loads
 

Yes I have looked in while running and it's dry until accelerator pumps squirts.

Holley 1850

Re: Backfire under loads
 

Likely time for a compression or leak down test.

And what intake?

Re: Backfire under loads
 

I did a quick compression check and all were 120lbs and above. I haven't done a differential check yet.

Intake is Edelbrock Performer.

Re: Backfire under loads
 

Is it an airgap type intake?
Compression is a bit low.
I think you mentioned checking the lifter pre-load. Did you reset them and if so how far?

Re: Backfire under loads
 

Me thinks you might have to go back and put about 2 tablespoons of oil in the spark plug hole of the cylinder you are testing to see what a wet compression test indicates. Assuming that you did your initial test with the throttle open while cranking, 120 psi on a dry test indicates worn out rings, or the possibility of compression gas leaking past a valve. If your readings jump up dramatically with a wet test, it isn't the valves.

The other thing I would suggest is putting a vacuum gauge on a manifold source and using what it tells you to help sort out if you have a hidden problem. Watch the gauge to see whether the numbers jump around, climb, or stay stable.

https://www.motor.com/magazine-summa...-vacuum-gauge/

I didn't see it mentioned in the thread, but do you have any idea how many miles are on the "82 K10 with a 305"?

Re: Backfire under loads
 

Good stuff Mr. Cruiser!
Might I add a visual chart to go with your link.
https://www.google.com/search?tbm=is...mc3ymmivPmJ9M:

Re: Backfire under loads
 

I went back through the thread and didn't see that an oil change has been done.

You could pull the dip stick and see if the oil smells like gas.

I seem to recall that if the fuel pump develops a pinhole in the diaphragm, it can allow raw gas into the oil sump. There should be a small hole on the bottom of the pump you can look at to see if there is evidence of leakage, or wrap a rag around it and let the motor run for awhile, and take the rag off and smell for gas.

Diluted gas ruins oil sealing at the rings, and might account for your low compression readings.

ps, Thanks, geezer#99 :)

Re: Backfire under loads
 

Whaere's all the Holly experts? Wouldn't this be a symptom of the classic Holly blown power valve?

Re: Backfire under loads
 

It is not an airgap style intake.
When checking pre-load I just did the quick rocker arm check while running. I know it's not the correct way but it was a quick check that I have done over the years. However maybe you can tell how to properly adjust them after the lifters have been filled with oil? On aircraft engines we hold the lifter under pressure of some stroke to bleed off the oil then go from there. Same way here??

I didn't have the throttle wide open when doing comp check. Didn't know you had to.

I will get a vacuum gauge and see what it tells me.

I have no idea how many miles on it. I do know it has been apart from tell tale signs. Block and heads are different colors, chrome timing chain cover. Intake had been off before and just over all inside of motor is pretty clean. Doesn't show the age for what the odometer says. Odometer says 8xxxx.

My son drained the oil and changed it. I'll ask him if it smelled. The fuel pump is new on it now. Previously who knows what happened????

Power valve is new but I also swapped it out with same size known good carb with same results. Trying to eliminate things in my head.


I really appreciate the help with this and love the input from all!!

Re: Backfire under loads
 

I reset the valves with the motor at idle. Use something to deflect the oil splash. Strips of cardboard work well.
Back off the nut until it clicks, tighten a touch until it’s quiet. Then tighten again to 1/2 turn. Go slow. Too fast and the motor might quit.
Manual likely says 1 turn. I do 1/2 turn.
Most important thing to watch for is how many turns out before it clicks.
If they’re set up too far you’ll lose compression and idle like crap.