Popping through carb at 4000 rpm

[jststrd1]

I posted before about this. I just replaced motor. Everything is new except fuel pump and intake manifold. It is a mercruiser 5.7 vortec carbed with a rochester 4 barrel. I transfered old carb replaced it. I transferred distributor with Hei and replaced that. No difference. The old motor popped through carb at the same 4000 rpm range. If everything has been changed what am I missing. 2 different techs can't find the issue. 2 different motors same issue. Timing has been doubled checked. The only thing I am left with intake is stamped Volvo Penta but it is for a vortec carbed motor. At wits end

 

[Scott Danforth]

Check fuel pressure. Could be stuck anti-siphon valve

 

[kenny nunez]

Is the popping sound with the same delay between events or does it do it randomly? Have you ever tried a separate small gas tank? Also with the engine off push the throttle all the way in forward then look at the roller in the ”V” notch on the shift linkage. It must be in the middle of the “V”.

 

[jststrd1]

Is the popping sound with the same delay between events or does it do it randomly? Have you ever tried a separate small gas tank? Also with the engine off push the throttle all the way in forward then look at the roller in the ”V” notch on the shift linkage. It must be in the middle of the “V”.

It reacts the same way right around 4000 to 4200 rpm increases as rpms get to 4500

 

[jststrd1]

It reacts the same way right around 4000 to 4200 rpm increases as rpms get to 4500

I would think fuel but ..m

It reacts the same way right around 4000 to 4200 rpm increases as rpms get to 4500

Both motors old and new dis it around the same rpms.

 

[jststrd1]

I would think if fuel would happen across the rpm range. It runs great until the 4000 rpms range

 

[nola mike]

I would think if fuel would happen across the rpm range. It runs great until the 4000 rpms range

you need more fuel with higher rpms, so not really. secondaries opening? fwiw, I absolutely love my a/f gauge that I installed. takes all of the guess work out of the equation as to whether/when you run rich/lean

 

[jststrd1]

you need more fuel with higher rpms, so not really. secondaries opening? fwiw, I absolutely love my a/f gauge that I installed. takes all of the guess work out of the equation as to whether/when you run rich/lean

Where did you install it and. Het it?

 

[stresspoint]

try disconnecting the tach wire , use a hand held digital tach to get your reading .

the reason i mention to do this is i have seen tachometers with internal issues causing high RPM flutter.

put a volt meter on the + wire to the HEI and confirm no less than 12v across the RPM range using the hand held.

 

[dubs283]

Lean condition, pre ignition, or mis adjusted/worn valves

Would be neat to know full advance number but you've checked timing twice. So, it's okay.....

New long block, hope valves adjusted proper

I'd focus on fuel delivery - pressure/vacuum first

 

[nola mike]

Where did you install it and. Het it?

they make a spacer that sits on top of the exhaust manifold. dunno what the rest of the question was

 

[Scott Danforth]

Check fuel pressure. If fuel pressure drops at higher RPMs under load. you have an inlet restriction such as anti-siphon valve or fuel pickup tube filter.

If fuel pressure is low all the time, it's the pump

 

[jststrd1]

Where did you install it and. Het it?

Meant to ask where did you install the air fuel guage and where did you get it?

 

[nola mike]

Meant to ask where did you install the air fuel guage and where did you get it?

AEM 30-0300 X-Series Wideband UEGO AFR Sensor Controller Gauge https://a.co/d/cS98SEW

Hardin Marine has the spacer