Gday Bewy! Thanks a bunch! One thing I don’t have a good understanding of is “how does timing affect vacuum?” Especially at idle.
Base Timing Questions
- Thread starter 340inabbody
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Good God. I explained it pretty well in post #17 and you even thanked me for it. Go read it again. And again.
Thanks Rusty I got a better handle on it now that I have reread it. Looks like tomorrow I can see what the engine really wants in the initial timing as I may have artificially stopped advancing thinking and not listening and watching the vacuum gauge.
mderoy340
Well-Known Member
Since you have an MSD adjustable timing box set the timing to 25* initial timing with VA disconnected. +-7* adjustment will allow you to figure out the best idle timing. Run the engine up to full operating temp with the hood closed and turn the engine off. Wait 5 minutes and allow the engine to heat soak. Adjust timing knob to the point the starter does not kickback and the engine instantly starts. Check timing with a timing light, this is the initial timing your engine wants. Set up the distributor for max mechanical timing 34* is all I've ever needed on my 340. As an example my 340 10.5:1 with [email protected] SFT cam has run for years at 28*/34*/48*with ported VA starting to move at 8" .
VA also allows you to add timing when the engine is under light load, low throttle opening conditions (cruising). It also keeps the engine coolant temps lower and improves mpg.
VA also allows you to add timing when the engine is under light load, low throttle opening conditions (cruising). It also keeps the engine coolant temps lower and improves mpg.
Thanks for the info on your motor! I am running close to what you suggested. I am at 34° all in and at 24° base. My springs are set up for a slow advance curve to try and tame any detonation. I will play with the remote adjustment after I get a chance to see if and how much more advance she wants at Idle. Then Il try what you suggest since I have the remote box set in the center allowing me to add or take out some timing.
I believe the remote box was envisioned for fine tune/change timing ~ +/- like 5° to account for fuel type and or altitude etc.
My initial may end up being higher as I hope to revisit that today if possible.
I probably will buy a VA distributor if after I have had enough time to tune the motor and assess from there.
Thanks agin for your input is helpful onowing how you set yours up.
Ok I was able to play in the garage for a little bit. My engine indeed wanted more base timing. A lot more. Like 42° which tipped me up just past 50 all in at 3000 rpm. My vacuum made 15hg.
I know thats too high…..
It takes some cranking to start up however so I started to back off 10° which helped.
One thing I noticed was that trying to adjust my A/F on the 4150 the vacuum never really changed until the darn screws were almost all the way in. Currently they are set to a default of 1.5 turns out. What am I doing wrong?
I know thats too high…..
It takes some cranking to start up however so I started to back off 10° which helped.
One thing I noticed was that trying to adjust my A/F on the 4150 the vacuum never really changed until the darn screws were almost all the way in. Currently they are set to a default of 1.5 turns out. What am I doing wrong?
Sounds like you might benefit from manifold vacuum on the advance can.
Newbomb Turk
Well-Known Member
It’s a 650 double pumper with only two A/F screws. Part number 4777 with a MSD mechanical dizzy and a 6AL.
....and worn out throttle shaft holes from FOUR return springs. Dammit man.
Thanks I guess I needed to re-visit my throttle adjustment. One spring now. Always good to have a second pair of eyes from experienced guys. Thats why I am here. I don’t have the benefit of working with anyone here as nobody in mt neighborhood seems to do anything …. I mean anything. I am the only one in my neighborhood who works on cars cut his lawn and work on my house. Seems like everyone else is either rich or can’t do anything themselves or both…..
I have my suspicions that this engine has a 12:1 CR and needs 50* at idle. The 15" of vacuum & needing that much idle timing is also odd. At what rpm was the vac measured & was the trans in gear if auto?
If the mixture screws are backed out a long way, but still adjust the mixture ok, it suggests the engine is getting extra air at idle [ vac leak?? ] from somewhere. One place can be the PCV, but with 15" of vac, I would expect the pintle to pull in & close off the air supply [ which would be normal operation ]. With poor vacuum, the vac is insufficient to overcome the pintle spring & a LOT of extra air comes through the PCV port on the carb, into the engine. If you put your hand on the PCV, engine idling, & you can feel it 'rattling', it is not working.
As I said, some of the info quoted seems contradictory.
If the mixture screws are backed out a long way, but still adjust the mixture ok, it suggests the engine is getting extra air at idle [ vac leak?? ] from somewhere. One place can be the PCV, but with 15" of vac, I would expect the pintle to pull in & close off the air supply [ which would be normal operation ]. With poor vacuum, the vac is insufficient to overcome the pintle spring & a LOT of extra air comes through the PCV port on the carb, into the engine. If you put your hand on the PCV, engine idling, & you can feel it 'rattling', it is not working.
As I said, some of the info quoted seems contradictory.
RPM @1200 in park not in gear. Drops about 400 rpm in gear.
The vacuum is not smooth on either of my gauges +/- 1/2 hg bounce….
12:1 TRW pistons seen with bore scope may or may not be exactly 12:1 depending on decking gaskets etc. but that’s what I am communicating bc thats all I can go by.
Did I say she wanted 50°? Sorry. I believe she wants about 42° to get the highest vacuum and “smoothest “ possible idle.
Note my oil pressure gauge is fluctuating 50psi +/- 4 psi normal? Again it’s a radical cam BUT I am not ruling out a vacuum leak……
PS the PCV is a new mopar unit. But I’ll check that again. I did blow through it before I installed it.
Thanks,
Ron
Woodys_Cuda
Ontario, Canada
Ron, again not an expert but just some observations. That engine should idle when warmed up at about 900- 1000 max rpm when set in gear, I think 1200 is too high.. The RPM drop from neutral or park should be around 100 - 200 rpm so you still have timing and carb adjustments needed. If you have a stock torque converter that comes into play as well. What is the stall of your converter? You're getting lots of great advice from the others.
Hi Woody yup you’re right about the stall converter etc. Unfortunately I am dealing with a combination that’s not very good for my application. I don’t have any information on my cam or torque converter. But the converter is to tight with a low stall for a higher revving 340. The cam is way too aggressive and set up I believe for 4 speed strip application.
I have slowed the distributor curve as much as I can which has helped with detonation. And I just got off the phone with Ray ( @halifaxhops ). We will try one of his vacuum distributors and see how that works. I learned also to be weary of my MSD box as if I needed another bad variable in the mix. Lots against me here but the good news is I have the guys here to help me try and work as best I can through it….
Ron
Woodys_Cuda
Ontario, Canada
Ron the MSD box is the least of your worries right now. The main issue with them is if they die you're stranded. There is no easy swap out like the original mopar boxes. I've had my MSD box going on 15 years now and it's fine.
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