427 SBC TUNNEL RAM

[Steve K]

I just installed my 427 SBC with a Tunnel Ram in my 79 Camaro with a faceplated TKO 600. It's a solidd roller with this cam
http://www.compcams.com/Company/CC/cam- ... d=375&sb=0
11.0 compression, AFR210's and other fun stuff. Goes like there's no tomorrow. BUT...when I hit 2nd, 3rd and 4th gears at full pop (lifting just a itty bit) I get a bit of a stumble. I'm running Edelbrock 1405's (600 cfm). It dyno'd at over 600 hp. Feels like a weak pump shot. Is there a bigger squirter available? I have the carbs set at the top of the three pump shot holes. Would 750's work better?

[hondo383]

If you are not lifting how could it be the accelerator pumps?

[Steve K]

ok, I might be lifting a bit, but not much. I changed that part.

[F-BIRD'88]

The 750 eddys are ALWAYS BETTER if its power you want.
Sounds more like a ignition issue.
Check for correct diz cap rotor phasing. Check for excessive spark plug gap.
vacuum advance limit issue causing timing flair ( when "lifting" on shifts)
Connect vac advance to the ported vac source on a carb.
limit its max travel to 12deg 15deg MAX.
The mechanical advance curve may need some work too.

Which tunnel ram is this? What is the current 1405 carbs jetting and rods?

Yes there are 3 or 4 other shooters. You can also drill yours a bit with #'d pin drills. I believe stock is .025" a 1/32" drill bit is .031"

[Geoff2]

Just drill the squirters out if you suspect they are too small. Done it many times.

[Steve K]

The 750 eddys are ALWAYS BETTER if its power you want.
Sounds more like a ignition issue.
Check for correct diz cap rotor phasing. Check for excessive spark plug gap.
vacuum advance limit issue causing timing flair ( when "lifting" on shifts)
Connect vac advance to the ported vac source on a carb.
limit its max travel to 12deg 15deg MAX.
The mechanical advance curve may need some work too.

Which tunnel ram is this? What is the current 1405 carbs jetting and rods?

Yes there are 3 or 4 other shooters. You can also drill yours a bit with #'d pin drills. I believe stock is .025" a 1/32" drill bit is .031"

-Edelbrock 7110 Tunnel ram
-I don't have vacuum advance hooked up
-34 degrees total timing
-I've got the thinnest rods,(can't read the numbers with these tired old eyes), lightest springs and stock jets
-Spark plug gap is 35 thou
It feels like a fuel issues more that an ignition issue. Same deal when it was on my 406 but a little more pronounced with the 427.

[cjperformance]

Have you checked float height?
Stumble on a slight lift when hitting next gear could indicate either a rich OR lean area right on shift recovery. Real simple way to know which is start by raising the float level by .040/.060", this will richen it.(i prefer to richen before leaning with this sort of test). Test drive and asses the change then you will know where you are and from there its an easier route to a cure.

[Steve K]

Have you checked float height?
Stumble on a slight lift when hitting next gear could indicate either a rich OR lean area right on shift recovery. Real simple way to know which is start by raising the float level by .040/.060", this will richen it.(i prefer to richen before leaning with this sort of test). Test drive and asses the change then you will know where you are and from there its an easier route to a cure.

I'll give that a try. Thanks

[jeff swisher]

Top of the 3 pump shot holes brings the acc pump up higher making you think you will get more of a pump shot or at least more fuel longer duration of it.

Well lets think for a moment.
You are accelerating and the fuel is sloshing to the back of the carb leaving the front of the carb with a little less fuel height.
HHMMM you have that acc pump raised up probably in the air now.

The 1470 acc pump has a longer rod that makes the hole taller or farther from the diaphragm.
Now you can use that one and use the 3rd hole and still have the pump in the fuel.

Or like was stated raise the fuel level.
I turn my carb air horn upside down so floats are facing up and adjust floats until level with gasket.

Now you can go a little more and have the far end of the float away from the needle closer to the gasket.
I used to raise just the acc pump side for drag/road racing.

Yes you can drill the squirters also like stated.
But think about where that acc pump rubber is at under full throttle acceleration.

I will agree the 750's should pull harder on the big end.
The fuel curve on the 750's comes in to the mains much later than the 600's though.
Just a couple hundred rpm off idle the 600's start streaming fuel from the booster nozzles.
the 750's are usually 400 or more rpm behind that.

I use that to my advantage running on the idle circuit on the 750's gives me a bit more MPG than the 600's
Not that you worry about MPG.

[F-BIRD'88]

The lifting on the shift may be causing the metering rods
to osculate up and down on the shift. Change the power pistons springs to
a stiffer spring and test.
The secondary air door may also be osculating on the shift Close/open
causing a brief but annoying up set of the fuel flow .

[redliner]

DO NOT LIFT ON SHIFT!! problem solved....

[Steve K]

Have you checked float height?
Stumble on a slight lift when hitting next gear could indicate either a rich OR lean area right on shift recovery. Real simple way to know which is start by raising the float level by .040/.060", this will richen it.(i prefer to richen before leaning with this sort of test). Test drive and asses the change then you will know where you are and from there its an easier route to a cure.

Would the stiffer springs cause it to run richer in low load or cruise conditions? This engine makes maybe 6" of vacuum.

[cjperformance]

Have you checked float height?
Stumble on a slight lift when hitting next gear could indicate either a rich OR lean area right on shift recovery. Real simple way to know which is start by raising the float level by .040/.060", this will richen it.(i prefer to richen before leaning with this sort of test). Test drive and asses the change then you will know where you are and from there its an easier route to a cure.

Would the stiffer springs cause it to run richer in low load or cruise conditions? This engine makes maybe 6" of vacuum.

Stiffer step up springs require more vac to pull the rods down so stiffer springs can richen it at light throttle cruise when on the mains.
Is that 6"hg at light cruise or idle?

[Steve K]

Have you checked float height?
Stumble on a slight lift when hitting next gear could indicate either a rich OR lean area right on shift recovery. Real simple way to know which is start by raising the float level by .040/.060", this will richen it.(i prefer to richen before leaning with this sort of test). Test drive and asses the change then you will know where you are and from there its an easier route to a cure.

Would the stiffer springs cause it to run richer in low load or cruise conditions? This engine makes maybe 6" of vacuum.

Stiffer step up springs require more vac to pull the rods down so stiffer springs can richen it at light throttle cruise when on the mains.
Is that 6"hg at light cruise or idle?

6" at idle. I'll double check the cruise vacuum.

[Steve K]

Lots of good advice here. Thank you. I've ordered the 1470 pump (4...two engine to do) and once I get those I'll check the float levels.