Backfiring through carb/won’t idle/runs well at higher rpm (new GM crate SP350 w/new Holley 4150). Pls help!

[BlazerBill]

I installed a new GM crate SP350 (385hp/405tq) into my ‘83 K5 4WD and am trying to get it to run and tuned properly for the first time.

Configuration: all new GM turn key crate SP350 engine/HEI/Holley 4150/vacuum advance plugged/spark plug wires routing verified/TDC #1 cylinder on compression stroke verified multiple times/fuel pressure between 4-7lbs depending on rpm/idle mixture screws 2.5 turns out/oil psi good/RobbMc mechanical fuel pump with in line 30-50 micron filter/stock in tank fuel sending unit with new sock/mid-grade octane gas/95% restoration of Blazer.

Issue: the engine backfires through the carb and shuts down and will not idle. It does however run well (fairly smooth and without misfires) at higher rpm (around 2500rpm/new tach not working).

Actions taken: I lowered the floats in an attempt to prevent flooding/backfire/flame by turning the float level adjustment screw a whole turn clockwise but didn’t stop the backfire and flame. I raised the idle with the idle adjustment screw to try to get it to idle but still won’t idle. I advanced and retarded the ignition timing to see what would happen. Advancing (seems) to reduce the backfiring and the engine will run with higher rpm. It (seems) like retarding the timing makes the backfiring worse and the engine won’t start or run. The distributor was already installed in the new engine as being a turn key engine. As a side note, I didn’t have to pull the distributor to pre-lube the engine by spinning the oil pump shaft and used my pressurized pre-luber tank instead. So, the distributor has never been out since it was installed by GM. Again, the engine runs well at higher rpm but starts to die and backfire when I let my foot off the gas pedal. I can’t get it to idle so I can set the timing and adjust the carb for optimum performance. Should I keep advancing the timing? Any other recommendations would be highly appreciated. I did make an effort to search for similar subject matter. Photos for reference purposes. Thank you!

 

[Ricko1966]

Is the choke open or closed? Have you checked for vacuum leaks. Your PCV is done wrong. I'd pull and plug all vacuum sources including the pcv. Check for vacuum leaks with carb cleaner. If it runs okay then add your vacuum hoses back in 1 at a time.

 

[xm20k]

4 corner idle air should start out at about 3/4 turn out on all four. These carbs idle off of all four barrels. Also back the idle screw out until it's just touching the lever than give it one full turn in. What is your initial timing set at? Just noticed you have dual grommets in one cover and a single in the other put PCV in the side with the single grommet.

 

[Robert Bare]

Sounds like plug wires are on wrong

 

[BlazerBill]

Ricko, the choke seems to be functioning properly as is closed when cold and gradually opened as the engine and ambient temps increased. There are only 3 hoses, the PCV hose, the vacuum advance canister hose which is plugged at the canister and at the carb and the brake booster. What other vacuum sources are you referring to? I’ll check for leaks at the carb base and intake manifold. How should the PCV hose be routed?
xm20k, the GM instructions state to install the provided “chrome dome” breather into the drivers side valve cover into the single grommet? Is this incorrect? If I move the PCV to the drivers side cover, what do I put into the grommet hole where it currently is? I will admit that I installed the aftermarket breather on the passenger side cover since GM didn’t provide an oil filler cap. Should I remove that breather and install a filler cap instead? If so, why? If I move the PCV to the drivers side single grommet in place of the breather, then there will be no breather on that side. Is that okay? Also, this 4150 carb only has 2 idle mixture screws on the primary side and none on the secondary side of the carb. Regarding the timing, I can’t check or set the initial timing since I can’t keep it running unless I keep it around 2500 rpm. I adjusted the idle screw per your instructions.
Robert, I have verified more than once the spark plug wiring 1,8,4,3,6,5,7,2 clockwise on the distributor and routed each wire to their respective cylinders. Thanks everyone. Please provide anymore recommendations. I’ll try another engine start in the morning? See additional pics showing idle mixture screws in the primary side and none in secondary side of carb. The holes are blocked off inside with no accommodation for adjusting screws.

 

[Ricko1966]

You only want a breather on one side you the pcv on the opposite side so all air and vapor drawn through the pcv valve was drawn through the entire crankcase. The way you are setup right now your pcv is just drawing from one end of the passenger side cover to the other end of the valve cover. If you only have the 3 vacuum sources temporarily plug them,and check for vacuum leaks with carb cleaner.. I found you a pic of how pcv is suppose to work look at the arrows,see how airflow is supposed to be.

 

[AuroraGirl]

I installed a new GM crate SP350 (385hp/405tq) into my ‘83 K5 4WD and am trying to get it to run and tuned properly for the first time.

Configuration: all new GM turn key crate SP350 engine/HEI/Holley 4150/vacuum advance plugged/spark plug wires routing verified/TDC #1 cylinder on compression stroke verified multiple times/fuel pressure between 4-7lbs depending on rpm/idle mixture screws 2.5 turns out/oil psi good/RobbMc mechanical fuel pump with in line 30-50 micron filter/stock in tank fuel sending unit with new sock/mid-grade octane gas/95% restoration of Blazer.

Issue: the engine backfires through the carb and shuts down and will not idle. It does however run well (fairly smooth and without misfires) at higher rpm (around 2500rpm/new tach not working).

Actions taken: I lowered the floats in an attempt to prevent flooding/backfire/flame by turning the float level adjustment screw a whole turn clockwise but didn’t stop the backfire and flame. I raised the idle with the idle adjustment screw to try to get it to idle but still won’t idle. I advanced and retarded the ignition timing to see what would happen. Advancing (seems) to reduce the backfiring and the engine will run with higher rpm. It (seems) like retarding the timing makes the backfiring worse and the engine won’t start or run. The distributor was already installed in the new engine as being a turn key engine. As a side note, I didn’t have to pull the distributor to pre-lube the engine by spinning the oil pump shaft and used my pressurized pre-luber tank instead. So, the distributor has never been out since it was installed by GM. Again, the engine runs well at higher rpm but starts to die and backfire when I let my foot off the gas pedal. I can’t get it to idle so I can set the timing and adjust the carb for optimum performance. Should I keep advancing the timing? Any other recommendations would be highly appreciated. I did make an effort to search for similar subject matter. Photos for reference purposes. Thank you!

your PCV is done wrong
You have no PCV valve and 2 breathers, your engine is going to live a short life if you dont flip a valve cover (either of them) and put a PCV valve in

 

[AuroraGirl]

You must be registered for see images attach

I see you have a PCV. one of those is supposed to be the oil fill, probably the one on your passenger cover with the push in filter thingy

That needs to be a cap, not a breather. Consider a late 60s corvette oil cap

Youre also using a fuel line for the PCV, that is not the correct hose but it will work for a while but it will degrade and start to collapse at some point.
youre also missing the charcoal canister for the brake booster hose. you need that to prevent your brake booster from degrading. must be installed vertically
That also appears to be fuel hose or not booster hose. Thickwalled hose is different appearing

 

[xm20k]

If the rear two holes are not drilled and tapped for screws in the rear metering block than the two front screws should be set at 1 1/4 to 1 3/4 turns out.

 

[BlazerBill]

I incorporated all of your recommendations and it still won’t idle. It will start and run sometimes but only at high rpm with the idle screw and timing adjustments. I’ve also raised and lowered the floats in an attempt to stop the backfire and flame. It does seem to run better with more timing but still backfires and shuts down. Once, it stayed running with a high rpm for about 30 seconds on its own before it shut down. Should I continue to advance the timing to see if it will run and idle enough to put a timing light on it? Since it won’t run long enough, I can’t check for vacuum leaks, either but I’ve pulled off all hoses and capped the openings. Thanks again!

 

[xm20k]

If no vacuum leak at carb base
1 1/4-1 3/4 turns out on idle air and idle turned all the way out with one turn in it should start and idle with only a bit of timing adjustment
Sounds like a timing issue not advanced enough, distributor off by one tooth, wires not in correct order.

 

[nvrenuf]

Backfiring through the carb is typically related to just a handful of things -

* Timing (to include distributor position relative to the #1 plug wire OR the distributor installed correctly, and timing chain installed correctly)
* Firing order and/or plug wire routing
* Valve adjustment

 

[Sad Sack]

And it is really easy to get a distributor 180 degrees off using the old school way of dead timing and letting the piston hit the beginning of its decompression stroke and thinking it is at TDC.

 

[xm20k]

How I set initial timing ie drop in distributor.

Find TDC on number one
Advance to initial timing let's say 10 degrees BTDC (this makes it less likely to need to crank in a ton of advance when first fired)
Drop distributor in with rotor pointing lined right up at number one terminal on the cap and cylinder number one.

Will fire immediately. and with the carb settings I gave earlier will sit and idle with minimal adjustment. Timing first than carb tuning.

Even on race motors I've built with 34-40 total a locked out no mechanical or vacuum advance distributor whether an MSD billet or a modded HEI the same process applies.

 

[AuroraGirl]

Backfiring through the carb is typically related to just a handful of things -

* Timing (to include distributor position relative to the #1 plug wire OR the distributor installed correctly, and timing chain installed correctly)
* Firing order and/or plug wire routing
* Valve adjustment

Ive found(not this situation) forgetting to put throttle springs on can mimic it for a short while lol