Hi guys, I finaly fixed all (hopefully!) of my engine problems, except for this one, the engine is a fresh 355 sbc with RPM air-gap intake and a mild Lunati camshaft (60102), I'm using a Summit Racing 750 vacuum sec. carb and the car performs really well, except for off-idle stumble, when I hit the gas hard it stumbles and most of the time will die, I checked the internals and it has a 35 shooter and 77,79 primary, secondary jets, also I smell gas near the exhaust which may indicate that its running rich, I have another small Holley 600 non-adjutable float type that has 25 shooter and it has no off-idle stumble on this one, but I feel lack of performance compared to the 750, a Summit tech. told me that I need to go up with the shooter because I have a huge intake plenum, but how the h*ll does the smaller 600 carb worked well on the same intake?? , where should I look into first? , I feel I'm lost with sooooo many variables to think about!
off idle stumble (and sometimes stall) with summit racing 750 VS carb
- Thread starter Kamaro
- Start date
This site may earn a commission from merchant affiliate links like Ebay, Amazon, and others.
ProStreet383
Veteran Member
Install a wideband and that will tell you exactly how rich or lean you are running. Helps make tuning very easy. Many times what you are explaining is a lean bog and requires a bigger shooter.
GM72_SuperSport
Veteran Member
In my opinion 77 primary is rich for a mild 355. I would Say a 70 primary is more in the ballpark.
Last edited:
This site may earn a commission from merchant affiliate links like Ebay, Amazon, and others.
falloutboy
Veteran Member
as ProStreet said, but sometimes it can be a too rich condition that would kill the engine.
One way could be to kill the car this way on purpose and then look at the carb, throttle blades and all the spark plug right after you killed it.
But a wideband would be an excellent choice for this and all other tuning / maintaining issues.
One way could be to kill the car this way on purpose and then look at the carb, throttle blades and all the spark plug right after you killed it.
But a wideband would be an excellent choice for this and all other tuning / maintaining issues.
I had the same issue with my nova when I had a edelbrock 750 on it changed the jets and meatering rods and cleared up the issue a wideband is a good idea
DesmoEd
Veteran Member
I run a 35 primary shooter on both cars - its plenty of pump shot for a 355 and any bigger will require a 50cc pump.
77 primary jets are huge - 68-71's up front should be more realistic. 75-76's out back
Then check:
Float levels
Transition slots
Idle mixture screws using a vacuum gauge
i bet that gets you close
77 primary jets are huge - 68-71's up front should be more realistic. 75-76's out back
Then check:
Float levels
Transition slots
Idle mixture screws using a vacuum gauge
i bet that gets you close
Last edited:
ProStreet383
Veteran Member
DesmoEd said:I run a 35 primary shooter on both cars - its plenty of pump shot for a 355 and any bigger will require a 50cc pump.
77 primary jets are huge - 68-71's up front should be more realistic. 75-76's out back
Then check:
Float levels
Transition slots
Idle mixture screws using a vacuum gauge
i bet that gets you close
Yup I agree with everything what Desmo said above.
DesmoEd said:I run a 35 primary shooter on both cars - its plenty of pump shot for a 355 and any bigger will require a 50cc pump.
77 primary jets are huge - 68-71's up front should be more realistic. 75-76's out back
Then check:
Float levels
Transition slots
Idle mixture screws using a vacuum gauge
i bet that gets you close
Guys, I bought one of the first 4010 when Holley brought this design back out in the early 80's. IIRC the jets were 79 squared. You can tinker and tinker and tinker with this carb - it will do good until the next tinkering session is required. Ford quit this carb, Holley quit this carb, and now Summit has it as a house brand - go figure...
To add a positive note for the OP, in my experience with this piece of equipment, go through the basics like Desmo pointed out, then take a hefty swing upward at the power valve. If you're running anything upward of 210* on a cam, you're going to have a lot of headaches.
This particular carb has been discussed several times on NastyZ, so much it's about #3 on Google hits. http://www.nastyz28.com/forum/showthread.php?t=34200
Pfrederiksen
Veteran Member
I would agree that the jets (primary) are off.
You may also want to make sure you havent got to much of the transfer ports exposed at idle, you may not even be on your idle circuit, which will make it idle rich like crazy.
Also sounds like your secondaries could be opening too soon. Try a tighter spring (or stretch the one you have in a pinch).
Just some more thoughts to mess your tune up.
Good luck.
p
You may also want to make sure you havent got to much of the transfer ports exposed at idle, you may not even be on your idle circuit, which will make it idle rich like crazy.
Also sounds like your secondaries could be opening too soon. Try a tighter spring (or stretch the one you have in a pinch).
Just some more thoughts to mess your tune up.
Good luck.
p
Get a wideband so you dont have to guess who bad off your tune is
