Tuning a big roller cammed motor - how to do it?

[HEMI-ITIS]

Go larger on the primary air bleeds 2 or 3 numbers and see if that improves idle quality.

 

[68 HEMI GTS]

They also need some gas to idle, about the leanest I can drive mine around happily is 13.5A/F 1050rpm in neutral and it's around 14.1A/F 900 rpm in gear. It'll idle leaner but it's not real happy and wants to stall. It's even better if I fatten it up but the cruise a/f goes to ****, kind of a compromise.

Also you don't need 7.5 psi fuel pressure. I run both of my cars at 5#. You need volume, not pressure.

 

[Mike Gaines]

Sounds like something OTHER THAN ignition advance, valve lash, fuel pressure or idle settings on the carb. I don't think just a little off on any of those 4 things could cause the problem you have....just an opinion....

 

[dieseldazzle]

Check the motor grounds as well. Maybe **** spark since you say you hear a miss somewhere.

 

[Brewzer67]

I wouldn't get too excited then until you get your new converter in. By the way, the pv shouldnt effect the idle unless it's blown. I run a similar duration 260/266 and run a 6.5 with the six pack. 5.5" @ idle.

I find that odd as the way the power valve works is to add extra fuel under low vacuum conditions such as full throttle. That is why I was taught you always adjust baseline timing and idle conditions on a new combination with a plug in instead of a power valve. That way the power valve is removed as a variable.

 

[68 HEMI GTS]

Power valve is part of your main circuit, that shouldn't come into play at idle unless something is very wrong.

 

[Brewzer67]

Check the motor grounds as well. Maybe **** spark since you say you hear a miss somewhere.

Spark is good on 1 and 2 (all that I spot checked) as checked with one of those variable gap spark testers. Both tested good to over .100 gap so I didn't check the rest. I only run .040 so I should be good.

 

[Brewzer67]

Power valve is part of your main circuit, that shouldn't come into play at idle unless something is very wrong.

I was always taught that if the power valve is too big it will bring some contribution from the main circuit in too soon. Is this not so?

 

[Brewzer67]

Sounds like something OTHER THAN ignition advance, valve lash, fuel pressure or idle settings on the carb. I don't think just a little off on any of those 4 things could cause the problem you have....just an opinion....

I am thinking this is just what the problem is...off by a little in several areas. That's why I am hoping someone runs a similar set up and can share exactly where their baseline is.

 

[68 HEMI GTS]

True kind of, (it changes when the enrichment of the mains begins) if the main circuit is in use. you shouldn't see anything coming out of the boosters at idle though. Idle and main circuit are separate and that's how they should be viewed. I run the 6.5 to help cover up the lean spot when my vacuum carbs open. I ran a 4.5 in my DP. Ive ran a 4.5 in the 6 pack and seen no change at idle when I went to 6.5 on the a/f meter

All I'm saying is the pv isn't the problem unless it was blown.

What size air bleeds do you have?
what's the ifr size?
Also you said t/s are equal, how much is exposed?

 

[Brewzer67]

True kind of, (it changes when the enrichment of the mains begins) if the main circuit is in use. you shouldn't see anything coming out of the boosters at idle though. Idle and main circuit are separate and that's how they should be viewed. I run the 6.5 to help cover up the lean spot when my vacuum carbs open. I ran a 4.5 in my DP. Ive ran a 4.5 in the 6 pack and seen no change at idle when I went to 6.5 on the a/f meter

All I'm saying is the pv isn't the problem unless it was blown.

What size air bleeds do you have?
what's the ifr size?
Also you said t/s are equal, how much is exposed?

I am not sure what size bleeds are in it as it was put together 10 years ago by Don at FBO. I'll have to check. Transfer slots were set so that they were just a square showing below the plate. Since trying to figure out the tune they are open a tiny bit more than that. I try not to go past this point as you start to tip into the main circuit. That is usually where I can tune it to behave using the "idle" valve the demons have built in to the baseplate and proper idle screw adjustment coupled with playing with timing to find what it is looking for at idle. That just isn't working this time. Normally my vacuum is a little steadier and my timing more stable. Both are moving around a bit more than I have experienced when I was running the old solid cam before. Before timing was rock steady at 20° at idle and had a steady 15" of vacuum. I confirmed tonight no vacuum leaks, no header leaks, and I changed to some fresh plugs. All looked like they were firing but looked like it was running pig rich. Tomorrow I'll swap out for a new 3.5 PV to make sure I didn't grab a bad one from my box. Then I'm going to run it until hot, set the timing back to 20° and redo the lash to make sure I'm not too tight on any cylinder. I know I tend to like a firmer feeling lash adjustment than some of my friends so I could be a little tight. I am also going to put the transfer slot back to a prefect square and bring the fuel pressure back to 5.5 lbs. This should give me a better starting position to tune from.

 

[Brewzer67]

I also switched to the non-clipped electrode plugs and gapped them at .045. I did have problems with the advance mechanism on the pro billet distributor and fixed that this weekend as well. The advance was hanging a little so I cleaned up the advance bushing and everything else that moves under the cap.

 

[IQ52]

You don't have to pay any attention to me if I'm all screwed up. I would run a dead minimum of 20 degrees and 24 seems better to me.

Your carburetor tune really has me baffled. The 850 Mighty Demon comes with .070 idle air bleeds, .031 H.S. air bleeds, 6.5 power valve, #85 primary jets and #93 secondary jets. I've always found that the primary to secondary 6-8 jets size stagger is required when using a power valve. Maybe your carburetor has some super trick modifications to change the stagger that I've never seen before.

Oh well, good luck.

 

[Brewzer67]

You don't have to pay any attention to me if I'm all screwed up. I would run a dead minimum of 20 degrees and 24 seems better to me.

Your carburetor tune really has me baffled. The 850 Mighty Demon comes with .070 idle air bleeds, .031 H.S. air bleeds, 6.5 power valve, #85 primary jets and #93 secondary jets. I've always found that the primary to secondary 6-8 jets size stagger is required when using a power valve. Maybe your carburetor has some super trick modifications to change the stagger that I've never seen before.

Oh well, good luck.

I don't know the bleed sizes but will check tomorrow. I think you and I are on the same page. I initially set the initial at 24 because that is where the motor seemed to want to be. As I added initial it kept speeding up until I hit 26 and then didn't anymore so I backed it back to 24 and set the distributor to throw 10. I downsized the 65 to a 25 as I had a few backfires through the carb so I wasn't sure the 65 was still good. Once I get everything sorted out I am sure my idle vacuum will be near 10" so I will go back to the 65.
I don't recall what I have for jets on the secondary side but I believe they are either 93's or 94's. So where do you think my problem is?

 

[HEMI-ITIS]

Power valve is part of your main circuit, that shouldn't come into play at idle unless something is very wrong.

If the value of your PV is lower than idle vacuum it will open causing a very pig rich idle.

 

[malex]

If the value of your PV is lower than idle vacuum it will open causing a very pig rich idle.

That's my thought too. With that big cam your vacuum could be lower than your PV causing it to open and run rich. I couldn't see your idle vacuum posted.

 

[68 HEMI GTS]

 

[68 HEMI GTS]

If the value of your PV is lower than idle vacuum it will open causing a very pig rich idle.

If this were a true statement my 511 would run pig rich at idle with the 6.5. It does not its very clean at idle. Again the pv system is part of the main circuit. You dont use the main circuit at idle. I've been spending a lot of time playing with different things on my rr since I bought an A/F meter. I can tell you when my vacuum drops to 5" the carb does not richen up from the pv opening, it actually goes the other way do to the fact there is not as much pull on the idle circuit. Not going to continue to argue this, believe what you like.

Though If someone can explain to me how the pv system enrichins the idle circuit and bypasses the IFR I will surely listen. I'll even include a picture of a metering block so someone can explain it. By the way this is the metering block that's currently in the rr's center carb. Anyone spot what i've changed since it's out of the box configuration?

 

[HEMI-ITIS]

Thank you for setting me right! I always thought the PV was supposed to be 2 or 3 inches lower then the idle vacuum.
Good to learn some thing new

 

[68 HEMI GTS]

Thank you for setting me right! I always thought the PV was supposed to be 2 or 3 inches lower then the idle vacuum.
Good to learn some thing new

Think of it this way, if you plug the power valve and go up on jetting your doing the same exact thing as having the pv open all the time. Your idle doesn't go to hell, just your cruise where the mains are active. But a large cammed motor comes right up on vacuum once it's off idle so the pv circuit keeps the cruise in check. If you look at a metering block the pvcr ties into the main well right above your main jets. Every combo is gonna be a bit different but should be viewed as what vacuum point do I need the extra enrichment.