My engine sure seems to want more jetting

[Lil Demon]

So I built this 340 years ago and have messed around with it over the years and it's never really ran that great. My carburetor tuning skills are good but I guess I have been underestimating the engine so that cancels out my claim to being a decent carb tuner LOL. Any way it's a 10.3 compression aluminum headed .040 over 340 with a SFT cam with 238/242 or so duration and mid .500's lift or so. I can't remember exactly sitting on a 108 CL, Engine makes about 9 inches of vacuum. 1-3/4 TTI headers and backed by a ZF6 and 3.91 Dana. My current configuration for sure is a 650 dbl pumper and timing is set at 24 initial and 36 all in by 2400. I've usually always ran it using 73 primaries and 76 secondaries and a 4.5 PV. It's always had a pretty significant low speed cruise lugging condition just off idle I've never figure out. So I decided to start jetting up these last few days and it's making a very nice improvement so far but it's still asking for more. I am currently sitting at 78/80 on the jetting and the lugging is almost gone but if I push it it's still there. I'm about to just skip up to a 82/82 setup to see how it likes it. It has been improving the cruising and of course it's making a lot more power all across the board and it's even idling smoother yet I haven't messed with the idle circuit. I have the transition slot barely visible and it idles almost smoothly now at 1k rpm. I guess I am just surprised it's asking for that much fuel and liking it.

 

[TT5.9mag]

Post pictures of your plugs. You can cover up a lot of bad tuning by throwing jet at it. And it sounds like you have the balance, front to back on the carb, already pretty far out. Do you have a wideband?

 

[Lil Demon]

Post pictures of your plugs. You can cover up a lot of bad tuning by throwing jet at it. And it sounds like you have the balance, front to back on the carb, already pretty far out. Do you have a wideband?

You know I have always relied on the wideband and ended up with that problem. So I just went back to "feel". I just checked the plug and it is a little dark, but then again they always have been so I am going to drive it for a 20 mile highway trip I have today and will check them after the run. Hmm I didn't realize fattening out the mix may actually be covering other problems.

 

[Newbomb Turk]

Post a picture of your plugs.

I can already say you are way too fat and you have far too much timing too soon. A stick car will buck and trailer hitch far sooner because you don’t have a converter dampening the bucking.

Turn off the O2 sensor and read the plugs.

Which 650 carb is it?

 

[Lil Demon]

Carburetor is a QFT 650 mech sec. It’s the billet blocks so I can custom tailor lots of circuits I just not too familiar with that depth of tuning

 

[TT5.9mag]

You know I have always relied on the wideband and ended up with that problem. So I just went back to "feel". I just checked the plug and it is a little dark, but then again they always have been so I am going to drive it for a 20 mile highway trip I have today and will check them after the run. Hmm I didn't realize fattening out the mix may actually be covering other problems.

It can. Specifically your timing curve. Yours comes in pretty quick for a street car.
Take some jets with you on your drive, only focus on the main jetting. Keep stripping fuel out of it until it surges lightly on nice flat level ground cruising at a pretty good clip (on the main circuit not the transfers). Then add 1-2 jet sizes and leave the main jetting alone. After that all of the fuel you add for load needs to be from either the secondary or the power valve channel restriction. Then you can tune the transfers slot and idle stuff to get the low load cruise where you want it.

 

[TT5.9mag]

View attachment 1716482386

Pig rich everywhere.

 

[Illahe]

Perhaps she's begging for a 750cfm. To me it sounds like the transition is being compromised by the air bleed diameter on the primary side and the jetting is making up for the emulsion issues from the air bleed diameter. The transfer slot sensitivity is controlled by the air bleed and the larger primary jet may just be introducing fuel from the main circuit sooner (crutching the tune).

Have you given the tech at Allstate Carburetors a ring...My bet is the air bleeds are your issue. 3.91:1 shouldn't be lugging.

One combination we use is a 242/242 110lsa .510 lift SFT with 3.91:1 and a Holley 750 with a four speed and its very streetable.

 

[TT5.9mag]

Perhaps she's begging for a 750cfm

You can only know that by watching vacuum at a wot. And he didn’t give any of that info. A 650dp is likely plenty of carb at this point.

 

[Lil Demon]

I can definitely feel the PV kick in when I am trying to throttle past the bucking. I was thinking it just needed more fuel to not have to depend on the PV at that point. The bucking is basic neighborhood cruising, or just off idle . So I have to keep it a higher rpm (above 2k) to avoid the bucking. Since I started adding fuel it’s began to drive normal in those scenarios

 

[Super-Cuda]

So I built this 340 years ago and have messed around with it over the years and it's never really ran that great. My carburetor tuning skills are good but I guess I have been underestimating the engine so that cancels out my claim to being a decent carb tuner LOL. Any way it's a 10.3 compression aluminum headed .040 over 340 with a SFT cam with 238/242 or so duration and mid .500's lift or so. I can't remember exactly sitting on a 108 CL, Engine makes about 9 inches of vacuum. 1-3/4 TTI headers and backed by a ZF6 and 3.91 Dana. My current configuration for sure is a 650 dbl pumper and timing is set at 24 initial and 36 all in by 2400. I've usually always ran it using 73 primaries and 76 secondaries and a 4.5 PV. It's always had a pretty significant low speed cruise lugging condition just off idle I've never figure out. So I decided to start jetting up these last few days and it's making a very nice improvement so far but it's still asking for more. I am currently sitting at 78/80 on the jetting and the lugging is almost gone but if I push it it's still there. I'm about to just skip up to a 82/82 setup to see how it likes it. It has been improving the cruising and of course it's making a lot more power all across the board and it's even idling smoother yet I haven't messed with the idle circuit. I have the transition slot barely visible and it idles almost smoothly now at 1k rpm. I guess I am just surprised it's asking for that much fuel and liking it.

Just my opinion but if you really want to maximize the performance of your combination you need a dyno.

Measuring and calibrating a/f ratio over the rpm range of your combination is paramount in gaining max performance.

 

[TT5.9mag]

I can definitely feel the PV kick in when I am trying to throttle past the bucking. I was thinking it just needed more fuel to not have to depend on the PV at that point. The bucking is basic neighborhood cruising, or just off idle . So I have to keep it a higher rpm (above 2k) to avoid the bucking. Since I started adding fuel it’s began to drive normal in those scenarios

If you can feel the power valve it comes in way too late. Put an 8.5 in it. Or better yet what’s your vacuum reading while cruising? I’d bet you have 16-17 inches.

 

[TT5.9mag]

Just my opinion but if you really want to maximize the performance of your combination you need a dyno.

Measuring and calibrating a/f ratio over the rpm range of your combination is paramount in gaining max performance.

“Need” is a strong word. Sorting out a carb can easily be done by driving it. It does take a little time and some patience but if you follow a few very basic steps the initial tune can go quickly.

 

[Newbomb Turk]

I can definitely feel the PV kick in when I am trying to throttle past the bucking. I was thinking it just needed more fuel to not have to depend on the PV at that point. The bucking is basic neighborhood cruising, or just off idle . So I have to keep it a higher rpm (above 2k) to avoid the bucking. Since I started adding fuel it’s began to drive normal in those scenarios

What’s in the carb now for air bleeds and emulsion?

I suspect you have a lot of issues running together.

 

[Super-Cuda]

“Need” is a strong word. Sorting out a carb can easily be done by driving it. It does take a little time and some patience but if you follow a few very basic steps the initial tune can go quickly.

Agree that it can be done by driving it . Also agree it takes a lot of time and patience to do it that way.

Just saying that using a dyno, you can evaluate the performance and get a starting baseline , then , with the engine under load, using the rear end gears and the transmission ratios in the car fine tune from there in a static environment.

Hard to buzz the engine to 5000-5500 on the street without attracting a lot of attention LOL !

 

[TT5.9mag]

Agree that it can be done by driving it . Also agree it takes a lot of time and patience to do it that way.

Just saying that using a dyno, you can evaluate the performance and get a starting baseline , then , with the engine under load, using the rear end gears and the transmission ratios in the car fine tune from there in a static environment.

Hard to buzz the engine to 5000-5500 on the street without attracting a lot of attention LOL !

Dyno time is very expensive. My Modi operandi is to get the baseline tune done long before it goes on a dyno. Do the final trimming and wot power stuff on a dyno. Doing drivability tuning on the rollers is definitely easier but my money is spent elsewhere. And that’s coming from someone who has friends that own and operate a dyno.

 

[Illahe]

You can only know that by watching vacuum at a wot. And he didn’t give any of that info. A 650dp is likely plenty of carb at this point.

I agree to a point..."A 650dp is likely plenty of carb at this point."

His camshaft is large enough to warrant some CFM with those 3.91:1's. IMHO. Unless he has a 28-9" tire...

 

[TT5.9mag]

I agree to a point..."A 650dp is likely plenty of carb at this point."

His camshaft is large enough to warrant some CFM with those 3.91:1's. IMHO. Unless he has a 28-9" tire...

The general use of the vehicle is a large determining factor of what I would use as far as carb size for that engine. A small block, stick shift, with 3.91s could use a 1050 if it was a race only deal. Mostly street car? A 650dp is just fine.

 

[Just Send It]

OP...What are your Float levels set to?

Also, does that carb come with a 30cc Accelerator pump? I'm guessing it has a pink cam?

I agree with the others...That's A LOT of jet for that motor. And the plugs tell the story.

Once you clean her up, your SOTP dyno will definitely notice a difference. She's drowning in fuel.

 

[Kendog 170]

 

[MobileCustoms]

I'd say your power valve number is way too low, for starters. You could probably run 68 primary jets with with a [edit] 8.5 to 10.5 power valve [end edit] and get great improvements, all around, depending on what your vacuum readings are while cruising around 50-55mph. If you don't have a vacuum gauge routed inside the car yet, I'd do that first, even if it's just a temporary setup with a long vacuum line and some duct tape.

 

[Illahe]

The general use of the vehicle is a large determining factor of what I would use as far as carb size for that engine. A small block, stick shift, with 3.91s could use a 1050 if it was a race only deal. Mostly street car? A 650dp is just fine.

I failed to catch that he is not running an 833 gear box.

How, or does the ZF6 gear ratio spread play into his combination.

 

[TT5.9mag]

I failed to catch that he is not running an 833 gear box.

How, or does the ZF6 gear ratio spread play into his combination.

I dont think the difference between the ZF6 and an 833 is enough to concern anything with regards to tuning. Might have to be a little more picky on cruise afr for overdrive but timing plays a big role there also. Either way the intended usage will be most important.

 

[Newbomb Turk]

I failed to catch that he is not running an 833 gear box.

How, or does the ZF6 gear ratio spread play into his combination.

I just did a quick search and if I found the same box it says that transmission has a 5 something first gear!!

5.79 to be exact.

That’s a 22.6 first gear. That’s a lot.

 

[Lil Demon]

Ok finally back. You are all rockstars , great feedback. I was hosting a company luncheon and was sneaking in everyone’s comments during the event. Let me try and answer all I saw.

Transmission is a T56 sorry so it’s got a 2.66 1st gear

Carb has 30cc pumps, pink cam and a 35 front nozzle

Floats set at just below sight glass

My problem occurs in low RPM, between 1k to 2500 roughly

Street use mainly

I had a vacuum gauge so I hooked it up before heading back.

Vacuum readings:

Cruise 7-9

Street take off bucking 4-8

Idle 7-8

Plugs after this run

Yup, rich!! I even kept it in 5th gear @80mph to stay at 3k rpm. Nasty!!