(2)450 DP's vs (2)750 DP's

-
I wouldn’t worry too much about the tilt. While you’re driving the fuel in the bowls is far from static in level and always moving around. When you’re on the gas hard (when all of this tuning really matters) all of the fuel will be crammed one way in the primary and one way in the secondary. Make sure the jets are always in liquid and forget about it. Don’t overthink it.
 
TT5.9mag said:
I wouldn’t worry too much about the tilt. While you’re driving the fuel in the bowls is far from static in level and always moving around. When you’re on the gas hard (when all of this tuning really matters) all of the fuel will be crammed one way in the primary and one way in the secondary. Make sure the jets are always in liquid and forget about it. Don’t overthink it.
Click to expand...
Thanks for the input, you could probably smell the gear oil burning in my head from thinking and rethinking this notion! LOL
 
I’ll say it again. A .033 MAB is bigger than any Holley built before they crapped the bed.

As that gets bigger you will be richer sooner and leaner later.

And it doesn’t take much to make a move. You can see this easily on the dyno.

What happens is once the MAB is that big, it causes you to do other things to get the fuel curve back in shape.

As Mattax said above .028 emulsion holes is as big as I ever go. Tunnel ram stuff with small carbs OR a quality annular booster may only want one emulsion hole of .024 size.

Tuning this stuff in the car verses a dyno is two different worlds. I would never try to do another TR without it going on the dyno.

It’s so much quicker and easier to sort it out. The relationship between the booster, MAB, the T slot restricters and throttle opening and load is near impossible to control in the car.

I know a dyno is out of the question (talking engine dyno) but even a chassis dyno would make things easier.

I can’t emphasize how critical it is to get the MAB correct and then sort out the fuel curve than to just drop a MAB in there and tune around it.

I believe that most of the horrendous tune ups I see coming in some of these carbs is directly related to MAB sizing.

The other thing is guys get hung up on numbers. Like saying a .026 MAB seems too small to me.

Or, one of my favorites from my early teens trying to explain to full grown men that your dirt bike doesn’t need to and shouldn’t be pumping black goo out of the pipe and why mine didn’t.

I ran smaller jets than they’d ever heard of so they were sure I was going to stick a piston. Their fear of a number made them do bad tuning.

To that end, main jet sizing is the same. If you get the MAB sized correctly you probably will need to reduce main jet sizing by 4-6 numbers.

It looks like my carbs are going to be in the mid to low 60’s for a primary main jet. That’s on a 1.410 X 1.750 carb that SHOULD be in the low to mid 70’s.

That’s because the MAB is .026 and that’s probably a bit big. For now that’s where I’m starting.

Add to that Mark Whitener’s boosters and I can run jets that small and not be lean.

Fuel distribution is generally better with smaller jets IF YOU DONT NEED THE FUEL.

And don’t forget to get the PV open sooner when reducing MAB sizing if you need to cover a home in the fuel curve.
 
Ok @Newbomb Turk ,I’m thinking out loud here. Suppose a guy was to take the swing and put .026 MAB’s in his carbs. And he jetted down say 4 numbers. And suppose he decided to decrease his emulsion down to say one .024 bleed. Where would you suggest he start? Above, at, or below float level. I was thinking he may want to start in the middle, right about float level. As I understand it that would likely help start the flow earlier than above float level. What are your thoughts?

Edit: when I say middle I’m referring to the top emulsion hole, as the kill bleed would be higher.

And I don’t want to exclude any other folks from their input. All thoughts and ideas are welcomed..
 
Last edited:
Have you tried more main jet as a test to see if the area that you suspect is too lean really wants more fuel? I'm not saying this will be your permanent fix but it might help determine what the motor wants first, then you can come up with a strategy to fix it.
 
92b said:
Have you tried more main jet as a test to see if the area that you suspect is too lean really wants more fuel? I'm not saying this will be your permanent fix but it might help determine what the motor wants first, then you can come up with a strategy to fix it.
Click to expand...
Haven’t tried anything yet, just asking questions.
 
10-4. I love a good mystery. Waiting for the next clue to see how you solve this.
 
Update: Went ahead and swapped the primary mains from 55 to 60 and here are the results. About the same until 40mph where she started trending the polar opposite direction from before. Also shows fat 12afr under light acceleration above 11” of vacuum, before the PV opens. When the PV opens she gets pig fat at less than 10 on the AFR. Did a short WOT and she went really rich 10 AFR and trended leaner as the rpm increased. Got as high as 13 but didn’t notice the RPM just let off because of traffic.

So one would say she wanted more fuel fore sure. Now to figure out the best way to get all of it to work. The .026 MAB might be a legitimate way to approach the solution, maybe try a smaller jet. I believe I have some 57’s. Although that won’t help the trending leaner at WOT. One thing at a time Mark! :BangHead: LOL Here is a pic of my notes.

image.jpg
 
Last edited:
Ok last update of the day, too hot in garage to enjoy going any further.
I went ahead and put the 57 jets in. I also put the .070 IAB’s back in. Idle just seemed a little “muddy” if that makes sense. Grabbed my notebook and went back out. Only one problem, I left my pen on the bench. Anyway seems to be better. Tried to take a video while driving but have yet to look at it for any possible data from it. Not real thrilled with the idle and steady low speed, idle to say 25-30mph but it’s not terrible. I may have a sync problem with the carbs. I’ll look into that in the morning. Bottom line, the jury is still out. It’s 100% drivable, reliable, strong in acceleration, just doesn’t seem right for some reason.
 
After reviewing video.
Idle still in the 12’s
40 mph 14.5 afr
45 mph 14.2-14.5 afr
65 mph 14.5-15 afr
78 mph 13-14 afr not consistent
Moderate acceleration w/PV open 11-12 afr
WOT 13

Again it feels strong under acceleration but everything else……meh
 
MomsDuster said:
Ok last update of the day, too hot in garage to enjoy going any further.
I went ahead and put the 57 jets in. I also put the .070 IAB’s back in. Idle just seemed a little “muddy” if that makes sense. Grabbed my notebook and went back out. Only one problem, I left my pen on the bench. Anyway seems to be better. Tried to take a video while driving but have yet to look at it for any possible data from it. Not real thrilled with the idle and steady low speed, idle to say 25-30mph but it’s not terrible. I may have a sync problem with the carbs. I’ll look into that in the morning. Bottom line, the jury is still out. It’s 100% drivable, reliable, strong in acceleration, just doesn’t seem right for some reason.
Click to expand...


I think you might be surprised how little the carbs can be out of time and have it affect the tune up.

I thin before I spent more time chasing this relatively low speed drivability issue I would get the unisync and carb hat and get the carbs timed up.

Once you do that it may change the way it drives.

I wish I knew the science behind what happens when the carbs are out of time but I haven't a clue. At my age I'm probably not going to bother learning it.

I'm just know how important it is.
 
Newbomb Turk said:
I think you might be surprised how little the carbs can be out of time and have it affect the tune up.

I thin before I spent more time chasing this relatively low speed drivability issue I would get the unisync and carb hat and get the carbs timed up.

Once you do that it may change the way it drives.

I wish I knew the science behind what happens when the carbs are out of time but I haven't a clue. At my age I'm probably not going to bother learning it.

I'm just know how important it is.
Click to expand...
I’ll take care of that in the morning. Thanks for the input.
 
Ok, not going to go through the 4 tunes I’ve been working through the last week or so, but here’s the latest.
.032 IFR
.067 IAB
56/66 mains
.026 MAB
8.5 PV
.033 PVR
.024/blocked/.024 Emulsion

Running really well. The AFR’s are fairly steady to 40mph (14-14.5) At about 45-50 or so she’s peaking at 15 but she doesn’t seam to complain. After that she mellows out in the 13-13.5 range up to 80 or so mph where she starts going a bit richer. Overall pretty happy. Haven’t done any major data gathering yet. However our club did a 3hr cruise through 2 different cities visiting some retirement facilities and a fallen police officer’s home yesterday afternoon/evening. Car ran flawlessly.

I tried a few other tunes with smaller mains and may go back there with a .024 MAB just to see the difference. The logical side of my brain says to cover up the AFR gauge and drive the crap out it though! Been an interesting journey visiting the smaller emulsion and MAB thought. I’m not hating it for sure! Big shout out to @Newbomb Turk for the input and encouragement throughout the process.

And for those that know me, you all know this ain’t over!
NOT BY A LONGSHOT!
:lol:
 
Another little thing I’ve noticed. On some left hand turns it has a slight stumble as I accelerate out of the turn. My thought is that the jets are uncovered as the fuel is pushed away from the jets in the turn. My initial reaction was to put jet extensions and notched floats in the primary side. But then thought that would likely cause the same problem, just in right hand turns. So that leaves me a few options.

1) Leave it alone. Just be mindful of the possibility it could stumble.

2) I could up the fuel pressure a little and see how it responds.

3) Raise the float level. If I do this I’ll jack up the *** end till the bowls are level and adjust to mid sight glass. I realize this will likely require re-jetting and such.

Whatever I do, it won’t likely happen till next weekend as I have multiple club and family functions this weekend. That should give me plenty of time to over think the whole thing!
:lol:
As always, feel free to throw out your thoughts, ideas and opinions.
 
MomsDuster said:
Another little thing I’ve noticed. On some left hand turns it has a slight stumble as I accelerate out of the turn. My thought is that the jets are uncovered as the fuel is pushed away from the jets in the turn. My initial reaction was to put jet extensions and notched floats in the primary side. But then thought that would likely cause the same problem, just in right hand turns. So that leaves me a few options.

1) Leave it alone. Just be mindful of the possibility it could stumble.

2) I could up the fuel pressure a little and see how it responds.

3) Raise the float level. If I do this I’ll jack up the *** end till the bowls are level and adjust to mid sight glass. I realize this will likely require re-jetting and such.

Whatever I do, it won’t likely happen till next weekend as I have multiple club and family functions this weekend. That should give me plenty of time to over think the whole thing!
:lol:
As always, feel free to throw out your thoughts, ideas and opinions.
Click to expand...


I can’t remember if your carbs are inline or sideways.

If they are inline you may have enough fuel in the bowls that Turing one way will lift the float and shut off the fuel. But it would have to be an aggressive long turn for that.

The other thing I’ve seen is too much fuel in the bowl and when you turn it will dump fuel out one of the boosters.

At that point I lower the float level a touch and drop the fuel pressure.

I have learned that 5 PSI is plenty of fuel pressure. I’ve run some as low as 4 without issue.
 
Newbomb Turk said:
I can’t remember if your carbs are inline or sideways.

If they are inline you may have enough fuel in the bowls that Turing one way will lift the float and shut off the fuel. But it would have to be an aggressive long turn for that.

The other thing I’ve seen is too much fuel in the bowl and when you turn it will dump fuel out one of the boosters.

At that point I lower the float level a touch and drop the fuel pressure.

I have learned that 5 PSI is plenty of fuel pressure. I’ve run some as low as 4 without issue.
Click to expand...
They’re 4150’s so they’re mounted sideways. There is no fuel coming out of the vents as I’ve run it without air cleaners during some of the runs and no sign of that. Fuel pressure is 5.5 so that’s probably not an issue. I’ll level the car out and see where the floats sit.
 
MomsDuster said:
They’re 4150’s so they’re mounted sideways. There is no fuel coming out of the vents as I’ve run it without air cleaners during some of the runs and no sign of that. Fuel pressure is 5.5 so that’s probably not an issue. I’ll level the car out and see where the floats sit.
Click to expand...


Do your carbs have center hung floats or side hung floats.

Pretty sure you posted pictures of your carb but I forgot.
 
You must log in or register to reply here.
-

Back
Top