Backfires

[dwleo]

Hi guys,
Almost finished my T-bucket.
One problem I have is the engine. Its a 350 sbc and when I start it, it fires immediately and runs strong for about 30 - 45 seconds then starts to misfire and backfire through the pipes and ends with (sometimes) a carb backfire then dies. It will startup right away and repeat itself.
I am running a slightly mild cam and a fuel slayer 4 barrel. Changing the timing seems to make it worse so I am leaning towards incorrect valve lash.
Is this a reasonable assumption?
Thanks.

- Daryl

 

[HenrysT]

Try checking your exhaust gaskets and/or exhaust leaks.

 

[dwleo]

Try checking your exhaust gaskets and/or exhaust leaks.

Interesting thought.

 

[fletcherson]

Did it ever run right, or is this since cam change, etc?

 

[RPM]

Do you have a vacuum gauge to put on it?

 

[dwleo]

Do you have a vacuum gauge to put on it?

Yeah, that's amongst my jobs for tomorrow

 

[fletcherson]

It could be any number of things, vacuum leak, bad fuel, moisture in fuel, timing, firing order, fouled spark plug, cracked or dirty distributor cap, bad plug wire, improper ballast resistor to coil, carb issue, valve adjustment. A vacuum gage will help, but if you have a misfire, it can fool you. I would recommend starting fresh, check all of the basics. Plugs would be first. Determine if it's firing on all, if not, why? Timing, set it to factory specs, use a timing light and be sure the timing remains constant and doesn't jump around. If it's not electrical, look for a vacuum leak, then carburetor, is the float level right, are the needle valves sealing, is any fuel dripping from the annulars, is the power valve working and correct for the tune, then a compression check to see if the valves are closing, etc... if it's a mild hydraulic cam, the lash is pretty simple and forgiving, so unless you are unsure if the initial adjustment was ok, I wouldn't suspect it as my first choice. Good luck!

 

[dwleo]

It could be any number of things, vacuum leak, bad fuel, moisture in fuel, timing, firing order, fouled spark plug, cracked or dirty distributor cap, bad plug wire, improper ballast resistor to coil, carb issue, valve adjustment. A vacuum gage will help, but if you have a misfire, it can fool you. I would recommend starting fresh, check all of the basics. Plugs would be first. Determine if it's firing on all, if not, why? Timing, set it to factory specs, use a timing light and be sure the timing remains constant and doesn't jump around. If it's not electrical, look for a vacuum leak, then carburetor, is the float level right, are the needle valves sealing, is any fuel dripping from the annulars, is the power valve working and correct for the tune, then a compression check to see if the valves are closing, etc... if it's a mild hydraulic cam, the lash is pretty simple and forgiving, so unless you are unsure if the initial adjustment was ok, I wouldn't suspect it as my first choice. Good luck!

Thanks

 

[ellis8500]

If you can, try a different carb. If it starts and runs strong for a short time then that indicates that the all is good for a short time. Something changes to cause a very lean / almost no fuel condition. It's probably an internal problem in the carb. I had a similar problem with one. Would start good, run for a while then fuel would start dripping from left annular. Checked float levels, needle seats, etc. all was good. Bought new carb; problem solved.

 

[dwleo]

Do not have a second carb to try.

 

[Intrepid]

fuel pressure gauge?
another possibility is the fuel pump is over powering the carb and flooding the engine out.

 

[dwleo]

Is that possible with a manual pump?

 

[Intrepid]

Its possible.
Unlikely if its a stock factory pump and carb.
But if its an aftermarket pump from summit or the like then its more of a possibility.
I do know from experience that Holleys are a bit sensitive to pressure.
No experience with the Slayer though.

 

[ellis8500]

What CFM do you have. I have Ford 302 with mild cam. A 600 CFM was too much; could never get a good idle. Put on a 500 CFM and it ran great from idle to top end. Looks like the Slayer is based on Holley design. Backfire through carb is usually an indication of too lean. Too rich you would be seeing black smoke and smell gas.

 

[ellis8500]

Read this, might help.

Why does my engine spit or backfire?

 

[dwleo]

I think it's best to give the engine details first then some history

Intake: ProComp 22000
Heads: Aluminium
Cam: Elgin e921p
Lift: intake .450 Exhaust .460
Duration @.050 224/224 lobe center 114
Lifters: Hydraulic
Carb: Quickfuel Slayer 600
Primary Main Jet: 68
Secondary Jet: 74
Idle Air Bleed: 70 / 39
High Speed Bleed: 31 / 31
Pri Nozzle: 31
Sec Nozzle: X
Needle & Seat: 100
Power Valve: 6.5

I assembled this engine and had it running smoothly at 2000 rpm for 20 mins or so to break in the cam as required about 6 months ago. I then changed the oil and then worked on other things. I had reason to pull the dizzy and relash the valves as a looseness occurred. Nothing has been right since. I would crank the engine until the marks would show on the damper and both valves would be closed and the piston is at the top of the cylinder. I set the distributer to hit plug #1 at that point. When I finally got the setup almost right, the engine started right up albeit now with the aforementioned backfiring that would happen after about 20 - 45 seconds of running. The engine would either die or I would turn it off. Hit the gas once and crank it and it would start. But again for the limited time then backfire would start. Over the past two days I have checked the valve lash and readjusted the mixture screws to 1 full turn out. I regapped the plugs (disgustingly fuel dirty with soot) to .040 and rechecked the vacuum lines. I hooked up a vacuum gauge a short while ago to the full vacuum line on the carb bottom and started it up. I kept it running (through the backfires) by upping the rpm's a bit but the vacuum showed nothing. I popped off the cap for the PVC line (don't have one) and the gauge read 6 immediately and drop to zero when I put my finger on it.
So, because it was running ok during break-in period and prior to when I pulled it, the distributer is naturally suspect however that would not explain the backfiring. There is a fuel smell to the exhaust, plugs are sooty and it almost seems (to me) that the carb is getting overcome with fuel and is flooding into the cylinders leading to misfiring and backfires. If I play with the timing setting, it doesn't get better but does get worse.
I'm flummoxed.

 

[fletcherson]

If you are only pulling 6 inches vacuum at idle, you have an issue. The black, sooty plugs are timing and too rich mixture. You have a not far from stock engine, not a radical cam. You should have near normal vacuum readings. Heres what I would do if I were to try to address this issue. I would first re adjust the valve lash on a cold engine, lifters bled, so the springs were felt compressing. To do this, you bring each lifter to the opposite side of the lobe, look at a chiltons and it will tell you which valves can be set at tdc, 90 degrees past, etc. you simply loosen each rocker untill you have free play in the push rod, then tighten until it just stops having play, or snugs when spinning it, then tighten 1/2-3/4 turns to preload the lifter, and your done. Next, with clean plugs and verified correct firing order, start it, set the timing to factory or cam card reccomended specs with the advance locked out. Then I would adjust the carb. To do this, connect the vacuum gauge and adjust each air, fuel mixure untill you achieve the highest vacuum reading you can get, do this several times. Idle it down to say 600, 650 rpm, turn each mixture screw in untill it starts to stumble, or the rpm reduces, them open it back up just before that point, repeat each mixture screw. After, open each up about 1/2 turn, no more. If at that time it's not running right, you have a problem other than basic tune and it's time to go through each system until it's resolved. Hope this helps! I don't know what your level of knowledge or experience is, I'm not trying to infer or offend, simply explaining the basic tune procedure, if you know this, please don't be offended!

 

[dwleo]

If you are only pulling 6 inches vacuum at idle, you have an issue. The black, sooty plugs are timing and too rich mixture. You have a not far from stock engine, not a radical cam. You should have near normal vacuum readings. Heres what I would do if I were to try to address this issue. I would first re adjust the valve lash on a cold engine, lifters bled, so the springs were felt compressing. To do this, you bring each lifter to the opposite side of the lobe, look at a chiltons and it will tell you which valves can be set at tdc, 90 degrees past, etc. you simply loosen each rocker untill you have free play in the push rod, then tighten until it just stops having play, or snugs when spinning it, then tighten 1/2-3/4 turns to preload the lifter, and your done. Next, with clean plugs and verified correct firing order, start it, set the timing to factory or cam card reccomended specs with the advance locked out. Then I would adjust the carb. To do this, connect the vacuum gauge and adjust each air, fuel mixure untill you achieve the highest vacuum reading you can get, do this several times. Idle it down to say 600, 650 rpm, turn each mixture screw in untill it starts to stumble, or the rpm reduces, them open it back up just before that point, repeat each mixture screw. After, open each up about 1/2 turn, no more. If at that time it's not running right, you have a problem other than basic tune and it's time to go through each system until it's resolved. Hope this helps! I don't know what your level of knowledge or experience is, I'm not trying to infer or offend, simply explaining the basic tune procedure, if you know this, please don't be offended!

Thank you Fletcherson. If I was easily offended, I would not let out my laundry here to dry. I appreciate all queries and not easily offended over my lack of some understanding.

 

[fletcherson]

Another thought came to mind. You said that you had to tighten the lash because it loosened up? That doesn't sound good. Did you use break in lube and follow the break in procedure? Not to be an alarmist, but the new material that they grind the flat tappet cams on isn't what it used to be. Hopefully a lobe didn't disintegrate and pump metal partials inside. I had that happen on a Ford engine not long ago, and I've done this for years, never had one do it before.

 

[dwleo]

Break in lube and oil and ran for the prescribed break-in time. Drained and replaced. Loosened up 'cause I'm not too clever (obviously) and set the lash without regard for loading the lifters.