Tune up/adjustment questions: carb, timing, mixture, etc

[Anonymous]

Hey gents, I know this has been covered before, but I'm still a bit foggy (whats new right? :roll: )

I want to dial in my timing and carb more/better, so what's the right order to go about things: timing, idle speed, mixture, valves

first, I want to fine adjust the valves s'more. I followed Mike's directions for adjusting them at two stops on the crank at 180degrees first. That seemed OK, but I think I was making them a bit too tight and/or my TDC isn't quite balls-on my mark. Would it be best for me to hand-turn it to TDC and set them or would it be better to try to set them while it's running? It's pretty tough to adjust them while there moving! It was recommeded that they be adjusted while moving so that the push rods still spin just a tad. True or should they NOT be able to be spun while rocking?

Second, in setting the timing, idle, and mixture, what comes first, the chicken or the egg? Should I set the mixture first with my vac gauge (and on which port on the carb or manifold)? Should I try to set the timing first? Idle? It has been going round in circles a bit because the idle effects the advance on the timing and the mixture effects the idle. Also, should the car be in gear (automatic tranny) for these? You can withold the BE CAREFUL messages...I'll make sure it's snug in place! :roll:

Third, I'm planning to downsize the jets on the carb soon based on recommendations here. Will this effect the adjustments above? Do the jets effect the idle circuit flow or just the throttle?

Lastly, what are you feelings on Holley PowerValve selection? Shoud I just take half of the @ idle manifold vac as has been suggested or should I calc on some other method?

Thanks all!

 

[Anonymous]

i guess this one is just too easy for anyone to reply to, huh? :roll:

c'mon fellers...going to go at it tommorrow evening and I'd like to at least get close!

 

[CobraSix]

This one is so easy that even I can answer it.

I don't know about the 180* valve adjustment stuff, but here are the instructions from Comp Cam that I've found to be able the best set of instructions. I've used them a few times.

http://www.compcams.com/Technical/Curre ... 08-412.asp

I would change the jets first. No point tuning the carb up and then tearing it apart. You'll have to tune it again anyways. I would tune it up, but realize you'll have to adjust it again when you put the new jets in. Whether or not because of the new jets, I don't know, but tearing the carb apart will usually neccesitate the re-adjustment of the carb.

For the timing stuff, the chicken and egg come at the same time. It's kind of a circular thing so running in circles is correct. Personally, I do a base idle set with the old rule of thumb, close the idle mix screws, then back out 2 complete turns. This usually gives a good starting point. Then set the initial timing by starting the car, disconnect the vacuum advance and adjust the initial timing. Your RPM and idle quality will change. Next, set the idle mix by using either your ear, vacuum gauge, or tach. If by ear, adjust the idle mix 1/8 of a turn at a time until the idle RPM starts to go down. Now reverse direction on the idle mix and turn the idle mix 1/8 turn until the RPM goes down again. Count the amount of 1/8 turns between the two. Half that number of 1/8 turns the way you went the first time. That will usually give you the idle with the max Vac and smoothest idle. You can do the same basic thing with a vacuum gauge or tach.

Now...check your timing again and make sure it's still right. If not set it again and start over above.

For the PV, I like the trial by fire method. Go out and drive it. If you feel the PV is coming in too early or too late, change the PV accordingly.

I'm not a huge numbers fan because what works on one engine won't work on the other. The right answer is what makes your car run the best. The rest is just best guesses to get you close.

Slade

 

[Anonymous]

Thanks Slade. Great link. I'll go through that stuff. I'm not experienced enough to know what the "right" PV would feel like, but...

from Holley:

A stock engine, or one that is only mildly built for street use, will have high manifold vacuum (17" to 21" Hg) at idle speeds. To determine the correct power valve, the vehicle should be driven at various steady speeds and vacuum readings taken. The power valve selected should have an opening point about 2" Hg below the lowest steady speed engine vacuum observed.

I get between 10 and 15ish Hg at idle and varying cruise speeds. Based on this, I should be using a PV rated at around 8 then, right? Maybe that's why I just don't seem to get the rip off the line as of recently. I put a PV #5 in when I redid the carb!

 

[mugsy]

I ran a Holley List 3310 on my '68 Catalina 400. I changed the PV from #8 to a #5 and there was a huge bog. I would think the #8 is right, and if you are so inclinded, buy a #7 or 6.5 and test that. Obviously the lower number you use will save some gas. You just have to figure out the compromise between power and economy.

As for the carb, I typically use a vac guage. My '65 Shop Manual sez to use a tach. I found on my car when I did each method that the tach was about 1/4 turn out/open more than the vac method.

As for the order, I think Cobra6 has it right: Set the mixture screws and then get the car to run by setting the idle and timing. Then go back to the mixtures and adjust 1/8 to 1/4 turn to find highest vac. Then set the idle, and then re-check the timing. And do it all over again starting at the mixture screws. Personally I do three "rounds" or iterations and call it good. The timing should not move at idle; depending on which carb/dizzy on the car, there should be zero advance at idle (generally)

tanx,
Mugsy

 

[CobraSix]

What is your idle vacuum? Hopefully 15psi. If you are getting less then that, even with your cam and 1.6:1 rockers, you may have a vacuum leak.

Right now, with my 390CFM 4V Holley, I'm running a 6.5. I'm thinking about dropping to a 5.5 as the 6.5 comes in a hair early at low RPMs when I give it gas.

Slade

 

[wsa111]

Bryce, First set your timing to where you want it-36-38 total advance with initial in the 12- 16 area.
With the 500 holley try 71 main jet size & a 6.5 power valve.
When you set the idle speed set it 25-50 rpms higher than your desired setting. Then adjust the idle mixture screws to achieve a 25 to 50 rpm drop in idle speed, this is called the lean drop method. Anyone with professional experience with the proper air fuel ratio measuring will benifit with an idle mixture not to rich, the highest vacuum setting is usually very rich, resulting in poor fuel ecomomy & just a too rich fuel mixture at idle.
Im not trying to discredit other posts, but i did this testing for a living with the latest electronic testing equipment.
The back yard method does not give you the proper settings for maximum fuel economy. Any questions ??? William

 

[CobraSix]

I debate the benefit lowering 25-50 RPM from max vacuum will give you. Maybe it's since I do most of my driving on the highway, I doubt I'll see any improvement in mileage. Even around the city...how much of a difference is it going to make?

I have to run my explorer at 1000 RPM because of oil pump issues, and my mileage may have decreased by 1MPG vice the normal setting of 750RPM.

I understand what you are saying, and I know you are right. I just wonder how big the benefit really is. I tune for smoothest idle. To me that's more important then gaining 1MPG. Also, with the temperature ranges in New England, general good tuning is better then fine tuning to max.

Slade

 

[Anonymous]

William has been my "carb-mentor"...I was waiting for him to chime in!

Thanks guys for the suggestions...I'll let you know how things go.

 

[wsa111]

Slade, when you do the lean drop method of setting idle speed & mixture, just drop the rpm's enough & still maintain a fairly smooth idle. On some engines you may only be able to drop the rpm's 15. on some others you may be able to drop up to 50.
When you do this lean drop it lets you keep the idle air fuel close to 13 to 1, or above. this helps fuel miliage,rich idle smell & emissions. Remember the idle mixture also keeps the transistion from idle to the main metering leaner. William

 

[wsa111]

Bryce, this an old post on quick change jet kits for holley carbs.
http://fordsix.com/forum/viewtopic.php?t=16497
This will save you much time & money for fine tuning the holley carbs.
Hope this helps. William