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

[BlazerBill]

It seems to me that it’s not a timing issue that’s preventing it from starting and running although it still needs to be properly timed once I get the engine to idle consistently. I say this because late yesterday, I decided to pull the plugs for inspection. They were all normal black and dry however I when checked the gap on them I found they were all .045”. I looked at the instructions that came with the crate engine and the spec is .040”. Not sure how or why I originally gapped them at .045”. So, I re-gapped them to the .040” spec. I re-installed the plugs/attempted another start and it fired right up and was running great for the first time. Since I had previously adjusted the idle screw for troubleshooting purposes, it was running at a high rpm so I jumped out of the cab and lowered the rpm to idle speed and it idled smoothly for several minutes with enough time to spray the carb base for vac leaks/none. Thinking my troubles were over I jumped for joy but since it was late, I turned the engine off and planned to restart it this morning assuming it would start/idle again so I could formally adjust the timing and better tune the carb. It failed to start again. Now, I’m thinking that there’s something intermittent that’s causing it to start/run/run good/good throttle response/not start/run bad/backfire/no throttle response/ fail to start/run . When it’s struggling to run, it does not respond to throttle input so I’m now in the process of going through the entire fuel system to see if there’s contaminants/trash that may be intermittently plugging the fuel system. Keep in mind, that I have restored nearly everything with new hoses, filters, pump, etc. I inspected the RobbMC inline 40 micron fuel filter b/w the pump and carb- clean. I pulled/inspected the Holley carb filters-clean. I pulled the float bowls-clean. Interestingly, the primary has a brass float, the secondary has a plastic (phenolic) float. I did notice orange silicone (or rubber plug?) in one of the accelerator pump orifices in the primary float bowl (see pics). Is this normal? If not, could that explain why the engine doesn’t respond the throttle input. I think the orange plug is supposed to be there because its shape is perfect. Any other recommendations before I put the carb back together and continue my fuel system inspection all the way to the tank/filter sock? The tank sender unit isn’t working either so I’ll have to check it out. Thanks again for sticking with me through this!

 

[BlazerBill]

Additionally, for T/S purposes, I removed the RobbMC 40 micron filter element between the RobbMC mechanical fuel pump to make sure that it wasn’t causing a fuel flow restriction but still have the Holley carb filters in place. See pic.

 

[AuroraGirl]

I re-installed the plugs/attempted another start and it fired right up and was running great for the first time. Since I had previously adjusted the idle screw for troubleshooting purposes, it was running at a high rpm so I jumped out of the cab and lowered the rpm to idle speed and it idled smoothly for several minutes with enough time to spray the carb base for vac leaks/none. Thinking my troubles were over I jumped for joy but since it was late, I turned the engine off and planned to restart it this morning assuming it would start/idle again so I could formally adjust the timing and better tune the carb. It failed to start again.

key thing here warm vs cold, how its primed, and did you set the choke or did you just try cranking.

The fuel needs to stay primed , see through filter would help see this, if this is issue, youd need to crank a bit to get fuel to the carb, then two pedal pushes but not to the floor, then set the choke, then crank


If your fast idle setting is off youre not gonna idle well either but if its untouched it should be at worse high

but you may have adjusted the high idle low if you were in the high idle cam when you were idling the day before

 

[BlazerBill]

Update, without the engine being able to idle long enough, I had no idea where the timing was set at but like I said before it seemed like it needed more timing but I was hesitant to keep giving it more and more. After rechecking my electrical and re-inspecting my entire fuel system I decided to give it more timing. I advanced the timing two more times and the engine fired right up easily and ran great and responded to throttle inputs without further backfiring. Now, I can put a timing light on it, tune the carb properly and break in the hydraulic cam. Thanks for all your recommendations that enabled me to get to this point!

 

[JBswth]

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.

Sounds like some of your intake valves are not closing completely, or your camshaft is off by one tooth.

J. B.

 

[Ricko1966]

Update, without the engine being able to idle long enough, I had no idea where the timing was set at but like I said before it seemed like it needed more timing but I was hesitant to keep giving it more and more. After rechecking my electrical and re-inspecting my entire fuel system I decided to give it more timing. I advanced the timing two more times and the engine fired right up easily and ran great and responded to throttle inputs without further backfiring. Now, I can put a timing light on it, tune the carb properly and break in the hydraulic cam. Thanks for all your recommendations that enabled me to get to this point!

Did you remember to fix your pcv,valve cover snafu?

 

[BlazerBill]

Yes sir, I corrected the PCV with a PCV specific hose and moved it to the drivers side. I also removed the breather from the front passenger side and installed an oil filler cap and installed the breather in the rear passenger side cover just like the diagram you provided. I also adjusted the timing by using a vacuum gauge to reach its highest 16” of vacuum by advancing the distributor, then retarded it down to about 14” at 800 rpm however, it is still backfiring (although much less). It still has an off-idle stumble and dieseling (run on) after I shut the engine off so in thinking it is running lean and hot, I rechecked and raised the float bowl fuel level by turning the screw CCW to raise the fuel level a little higher in the bowl to where it just spills out with gentle rocking. The fuel level was a little too low before. But, when I attempted another engine run is when I noticed the upper radiator hose was severely swelling up from what I think was a stuck closed T-stat and is why I made a separate post for that issue that you saw and commented on. Hopefully, the new Robert Shaw FlowKooler T-stat that I ordered but won’t get until next week will stop the overheating and dieseling. Then, I’ll have to keep troubleshooting the off-idle stumble and backfiring. Thanks again for your help and the help from everyone else.

 

[Ricko1966]

Yes sir, I corrected the PCV with a PCV specific hose and moved it to the drivers side. I also removed the breather from the front passenger side and installed an oil filler cap and installed the breather in the rear passenger side cover just like the diagram you provided. I also adjusted the timing by using a vacuum gauge to reach its highest 16” of vacuum by advancing the distributor, then retarded it down to about 14” at 800 rpm however, it is still backfiring (although much less). It still has an off-idle stumble and dieseling (run on) after I shut the engine off so in thinking it is running lean and hot, I rechecked and raised the float bowl fuel level by turning the screw CCW to raise the fuel level a little higher in the bowl to where it just spills out with gentle rocking. The fuel level was a little too low before. But, when I attempted another engine run is when I noticed the upper radiator hose was severely swelling up from what I think was a stuck closed T-stat and is why I made a separate post for that issue that you saw and commented on. Hopefully, the new Robert Shaw FlowKooler T-stat that I ordered but won’t get until next week will stop the overheating and dieseling. Then, I’ll have to keep troubleshooting the off-idle stumble and backfiring. Thanks again for your help and the help from everyone else.

If your throttle stop screw is turned in too far it will stumble on acceleration,and run on when you turn it off. Not the only thing that can cause it,but it is something that could cause it so could a vaccum leak.

 

[Rustisbest]

Always works as a good starting point for me:

12 degrees initial timing usually winds up at 32-34 total with an HEI

Vacuum advance on manifold vacuum adds another 12-16 degrees at idle and at part throttle depending on the can. Make or buy a limiter or if needed after road testing

Square up the transfer slots. 2 corner idle carb mixture screws 1.5 turns out. Verify the accelerator pump is working and shoots with slightest movement. If its got the pink pump cam (slow initial movement) take it off and use a green or orange

 

[83Stepper]

Still sounds like timing is still off tbh. Once you get past that stumble the mechanical vacuum advance will kick in moving the timing to a more acceptable and running timing advance that won't show the signs like before when it was backfiring etc.

Air mixture screws at a starting point like Rustisbest (edited to give credit where it's due) said, 1.5 turns out, or 5x half turns, will get you to a good starting point.

Make sure that your timing is right where it needs to be and the engine is running at normal operating temp before even starting to make any air/fuel screw adjustments with a vacuum gauge. A lot of people think they've got an issue with the air fuel mixture, but they never take the time to make sure that the timing is right beforehand and get into a mess that never seems to be able to get resolved.

 

[Rustisbest]

Air mixture screws at a starting point like Ricko said, 1.5 turns out

i think we're both offended

 

[Ricko1966]

i think we're both offended

I'm not offended,you gave great advice,but you deserve the credit. I think some one got mixed up because I told him a few posts back if he's got his idle stop screw in too far it will stumble and run on. I just didn't tell him about squaring the transition ports. Something I show people when I'm in person and there is a carb on hand. I don't trust my ability to explain it in a way the average guy,that 90% have no idea what a transition circuit is will understand. I just had to show a friend how-to set up a Brawler on his Vortec 350,yesterday.

 

[AuroraGirl]

I cant tell for certain but your hose to the PCV still looks like its Fuel line, the EPA compliant with green text you had used before. While this isnt something that will affect your operation like your current woes, its a long term thing that needs correction, you need something that has SAE J30R7 on thew hose printing , if it says J30R7 youre okay, usually it will be labled PCV/Emission Control or Fuel Return hose

 

[xm20k]

Transfer slot adjustment and secondary idle stop adjustment. Since you have a bigger than factory cam square both primary and secondary.

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Once you get it idling at reasonable RPM if it still backfires out the top when you stab the throttle than you need to adjust (fatten up) the accelerator pump shot.

 

[83Stepper]

i think we're both offended

Lol sorry bud, no ill intent there. Edited original post to update it correctly.

Have seen many people think it's a timing issue when it's air/fuel related and vice versa. Seen many people try to adjust afr when it's been running for like 3 or 4 mins from a cold start, definitely not even remotely close to even opening the thermostat.

Had an issue with my 4160 Holley where it was doing the same thing. Off idle stumble and would diesel when shutting off after it was at temp. Timing was out by a degree and a half or so, bumped it up to 8° btdc from 9-10ish and it resolved the issue.

Hopefully OP can get that baby running great