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PostPosted: Sat Oct 12, 2019 2:40 pm 
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Joined: Thu Oct 10, 2019 7:07 am
Posts: 7
Car Model: 1972 Plymouth Scamp
Hi everyone

Wanted to provide an update on the 350cfm carb tuning.

I put in 58 jets (smallest I have at the moment), put in a 28 squirter and pink cam.

Confirmed timing and then started to adjust the idle mixture with a vacuum gauge. 19" vacuum was where I dialed in the best value. In drive I had 15". So I put in a 7.5 power valve.

I did some quick tests in the driveway and noticed an off idle bog. I started to increase the squirter nozzle and went to 31. The bog was there but less. Moved to an orange cam and bog disappeared in the driveway in park.

Engine at idle still a bit rich 12.5-13.0 - so initially I am guessing I need to move to smaller jets.

Driving around noticed a significant improvement, however still having a bog at cruise when I accelerate. My AFR gauge went up to 19 from the 14.9-15 at cruise.

I moved to a blue cam with no real improvement. So I moved to a 35 squirter and an orange cam.

I noticed somewhat of an improvement here. Is it normal to need this much pump shot? FYI the intake I am using is an Aussie speed Hurricane.

I think I also need to move up to an 8.5 power valve.

At cruise, I still observe a bog from 1100-1600 rpm before the engine rebounds. Above these rpm's it is much less although still there.

At highway speeds, AFR is 14.9-16.0 (around 2400 rpm) and I run lean when all in on accelerator (17.0 - 18.0 which is why I think I need an 8.5 power valve).

All told I know I have moved to better. An 8.5 power valve will certainly help on my top end, but I don't know that I will be able to improve my transition circuit any more on this carb.

But Dusteridiot, I have to say you are right, so thanks for your help.

I think what I am seeing is the limitations of this carb on my setup. I am having low-velocity issues at lower rpm's and while I seem to be able to find a better tune it is still far short of where I need to be.

A Holley 390 cfm 4bbl will be forthcoming this winter.


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PostPosted: Sat Oct 12, 2019 3:24 pm 
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Joined: Tue Oct 29, 2002 8:27 pm
Posts: 9760
Location: Salem, OR
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Quote:
Engine at idle still a bit rich 12.5-13.0 - so initially I am guessing I need to move to smaller jets.


Now... I told you to get the book, right... on a Modular Holley, the Jets only meter fuel for the mains... which come in
after about 2000 rpm on a stock carb.... Idle screws, and the idle bleeds meter the amount of fuel or air in the idle
circuit and transition circuit from the throttle plates at rest until they sweep past the top of the transistion slot in the
base plate... any more fooling around and you will have to ruin a metering block by pulling the well plugs and drilling
the PVCR passages....it's better to get a carb that is more in the engine's comfort zone than blow $150 in parts to
make the one you have kinda work).

Acc Pump colors are strictly on pump shot size, they are also how long the acc pump will pump gas when the throttle
is opened (and if already opened, it is to bandaid that dip in fuel feed when going from idle to "go" mode....)


Quote:
Is it normal to need this much pump shot? FYI the intake I am using is an Aussie speed Hurricane.


Uh yeah...street hyperpak settings needed as much as 20% more fueling than standard setup for an offy or clifford shorty...
usually you run the pink cam and the 28 for a longer stronger shot....the bigger the shooter, the sooner your shot runs out...

Move up to a 10.5:PV and leave it...(you'll run it on the 390 holley...)...the economizer valve only blankets the dip when the
vacc drops off abruptly...

Quote:
I still observe a bog from 1100-1600 rpm before the engine rebounds.


This is where the pump shot and PV need to provide the bandaid... even if you plug the high speed bleeds that will only
bring the mains on at about 1800 rpm instead of 2000...

Like I said all this is in the book...


Quote:
A Holley 390 cfm 4bbl will be forthcoming this winter.


Unless you have somekind of outside heat provision, long ram manifolds aren't happy under 45 degrees ambient temp...and may be a project for the spring time...FYI...

Quote:
But Dusteridiot, I have to say you are right, so thanks for your help.


I did write a bunch of posts and data on making a hyperpak conversion to a street engine just like this build...pictures are gone, but posts are still
around... I think the search term is under Hpak.


Another issue you will encounter...once you dial in the carb, your distributor curve will need to be improved... that being said carb and timing have a relationship...
if you set the carb, then goof with the timing, you will be back to tweaking the carb again...


Been there did a lot of that...


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