Another carb help...

[tim_mc]

The mention that it will idle, but drop RPM when put in gear AND aftermarket GM HEI caught my attention. I had a very similar issue which was an aftermarket GM HEI that wound up causing my problem: Truck had an occasional backfire or load up when driving. It progressively got worse to where it would start OK, but would die when put into gear. It would idle and rev OK, but ran badly under a load, or when stopping at a stoplight. I had to put it in neutral a few times to keep it from dying at intersections. Later it would lean out and backfire after it warmed up and eventually would not restart when warm.

Replacing the GM HEI coil and module with Davis Unified Ignition parts did the trick. No issues since June 2021. Coil and module are relatively cheap & easy to replace if you at least want to eliminate them as a possible culprit.

Also, I compared a stock HEI module out with a Davis module - what a difference in performance, especially acceleration.

[72SB]

Originally Posted by Kalums

So... I should disconnect from the carb and connect the VA to the manifold?

I went back and read you post again... And literally walked out to the truck to try this... When I have the VA plugged into the carb (timed port?) it runs rough (like I need new motor mounts... Which I have in the garage) at 12 dgrs advanced (manually set by me using a digital timing light) but if I turn my distributor ever so slightly til it is about 20 dgrs advanced it speeds up my rpms AND runs smoother. I can then turn down my idle to 800 rpm and it only drops to 650-675 when I put it in gear, still a drop but no stalling ...... My light bulb just came on... Was I manually retarding my timing? Too much?? Was I the "malfunction" *face palm*

No, you use the manifold vac port on carb, lower one on DS. The one slightly higher on the front, PS, is timed. The fact you advanced timing manually shows you want VA on manifold, so you get the additional timing ON TOP of your base (8-10 degrees)

Set your base back to around 8-10 without VA and the vac port on carb plugged. Then with VA connected to the manifold vac port on carb adjust idle RPM as needed

if you don't have it you can download owners manual for the 1406. and yes again it will say to use the timed vac port....its an emission thing, don't do it. It will however have all the tune and trouble shooting steps if you need them.

I am assuming your timing marks are right and the carb otherwise is in good working condition, To me, your issue was and is no VA at idle because you used the timed vac port.

[Dashman]

Timed port vacuum is NOT a failed emission thing. That's a myth, fallacy, propaganda... The fact is that manifold vacuum drops when the throttle is increased, and timed port does not. It was an improvement to simply fill in the drop in timing, just for a moment, until the mechanical timing caught up. The only time a person needs to recurve a distributor's timing advance is when it's a performance engine or race vehicle.

I believe that Geezer#99 probably has it right. There's a mismatch in the balancer and timing tab. Find top dead center of the #1 cylinder, and verify where the timing mark on the balancer is with respect to the timing tab.

I experienced a similar situation when the distributor gear bushing was wearing out, so that's something else to inspect. You may also want to verify the fuel inlet needle and seat, the fuel pressure, and the fuel filter. I had fuel leak at my fuel filter and temporarily ran my engine without the fuel filter, forgot about it, and dealt with near stalling while in gear. When I installed a new fuel filter to cure the leak and when the engine dropped RPM while in gear, the engine idle stayed smooth. It could also be a float level too high. Good luck!

[Rust_never_sleeps]

Thunderhead289 on Youtube has good videos on timing, carb tuning, and how they work together. I like this one, but it feels like drinking from a firehose at times:

https://www.youtube.com/watch?v=Ort2Gk7Lvuk

His stuff is usually about Holey, but I think Edelbrick works comparably

[Kalums]

Originally Posted by geezer#99

That there tells you one thing!
Your timing marks aren’t accurate.
Either the mark on the balancer is wrong or the timing tab is wrong or a combination of both.
You need to find true top dead centre on #1 cylinder to see where your timing tab/balancer marks are at.

Found TDC by pulling plug on #1 and manually turning crank( the balancer is dead on the little plate is off by 12 dgrs). With the VA disconnected (and plug# 1 reattached) I timed it to 8 dgrs advanced... It didn't really like it so I went to 12 dgrs... Then my fuel line from the pump to the carb started leaking AND the carb backfired.... (see photo) now I've melted the cable on the bottom of the carb (not the throttle cable connected to the gas pedal, the one on the bottom next to the return spring). Today... I'm done fiddling with it and am very glad nothing else happened. Unfortunately not my first gas fire (first in this vehicle) but it still sucks. On a good note it was running fine before the leak and subsequent fire. I think I'm going to eliminate the fuel filter by the carb and just run a line from the hard line to carb.

[geezer#99]

What tool did you use to find tdc #1.

[Dashman]

If you found that TDC was 12 degrees on the tab, then you were actually setting the timing to 0 degrees. You had a backfire through the intake because your timing is off, either early or late. I believe your timing is still too late. You could find the correct balancer or timing tab, or you could use a white marker pen to mark a new zero line on the balancer. Set it TDC at 12 degrees like you did before, and mark the balancer at 0 degrees. If you set the new zero mark to 12 degrees on the timing tab, you'll probably be just right.

Like Geezer#99 wrote, you need to physically determine TDC through the spark plug hole. I have used one of these:
https://www.summitracing.com/parts/c...iABEgI6ePD_BwE

You can find a video or instructions on how to do it.

In regards to your fuel filter, sometimes they are made for 5/16" hose and they will leak with 3/8" hose.

Transmission kick down cable. Cut the burnt plastic off of it, and see if it is still funcitonal.

[Rust_never_sleeps]

Originally Posted by geezer#99

What tool did you use to find tdc #1.

The smoke detector ;-)

[72SB]

OP. Edelbrock makes a hard line that attaches to carb, specific to the 1406, and a AN fuel filter to that hard line and also an AN steel braided line to FP. You would need adapter fitting to AN of FP which several (Earls, Russell, etc) all make so you get rid of that soft hose and clamp on fuel filter...fire starter.

Regarding ported vs manifold vac...while some have their opinions on which to use, I and so many others have found manifold is better which allows VA to be "part" of idle applying advance. Ideally a combined sum of base + VA = 20-24. Yes VA goes away quick but mechanical takes over as you apply throttle. Yes it does involve setting up the VA itself (stop plate) and can involve adjusting advance curve or how soon mechanical comes in with lighter springs. Doing so though makes for a EXCELLANT running motor across the RPM band, idle, part throttle as well as WOT use.

If its just about putting around, use ported but MPG suffers and rich condition occurs especially in part throttle driving....which is at least 90% of street use driving.

Pics: Edelbrock fuel line setup I mentioned

[leftybass209]

If you're going to run vac advance on manifold vac, the vac canister will most likely need to be modified with a stop limiter; many vac cans add too much advance; this is especially true if you're running a higher initial timing.

You can use a hand vac gauge and customize a vac advance with a screw and a metal stopper to make sure it pulls exactly what you need. I slotted the stopper to make it slightly adjustable.

[72SB]

^^^^, yes. Unfortunately Crane does not make the adjustable VA and detent stop plate anymore. Homemade versions will work. I will say, doing so to really nail when and how much VA the can provides along with base timing and dizzy curve adjustments really make for a perfect tune on carb motors

[MikeB]

Originally Posted by 72SB

^^^^, yes. Unfortunately Crane does not make the adjustable VA and detent stop plate anymore. Homemade versions will work. I will say, doing so to really nail when and how much VA the can provides along with base timing and dizzy curve adjustments really make for a perfect tune on carb motors

Do you typically limit vacuum advance to 10-12 degrees?

On my last build, I used an off-the-shelf canister with about that much. I'll have to search for the number, but I got it at O-Reilly Auto Parts. Wish I would have bought a couple more!

Ah, just found these notes: My 82 C10 GM HEI has a Borg Warner canister, but I think it's the same part to fit a 75 Buick 350:
Part #: ECH VC1838 Line: Echlin Ignition
Also known by GM as AR12

Can't remember the vacuum specs though.

Turns out NAPA still has the Echlin P/N.

[Steeveedee]

I have a document published by Lars Grimsrud and Duke Williams that shows the specs for (I think) all GM distributor vacuum advance units. One could pick and choose an advance unit based on that data.

Here you go- Lars' email address is in the first post. You can email Lars for that doc.

https://www.corvetteforum.com/forums...-and-duke.html

[72SB]

Originally Posted by MikeB

Do you typically limit vacuum advance to 10-12 degrees?.

Yes.

Here is list of VA part #'s and spec authored by Lars, a 10-12 can be found

[MikeB]

Here's the article by Lars. HEI canister listings and specs starting on page 5.

http://outintheshop.com/faq/Vac%20Adv%20Spec.pdf

[Steeveedee]

Originally Posted by MikeB

Here's the article by Lars. HEI canister listings and specs starting on page 5.

http://outintheshop.com/faq/Vac%20Adv%20Spec.pdf

Faster and better than bugging Lars, for sure!

[Rust_never_sleeps]

A bunch of other options for adjustable VA cans if it doesn't need to be a Crane

[72SB]

I believe "adjustable" VA canisters are still available as Crane, now Holley, does not make theirs anymore. These are the ones that use an alan wrench to adjust when and how much VA is applied. Summit, etc sell thse adjustable VA's cans

But the "stop plate" detent is not made so a homemade version is the option

Pretty good article on the "why and how" VA is beneficial, especially using manifold vs ported vacuum for it and an adjustable VA canister

https://www.motortrend.com/how-to/16...r-drivability/

[tim_mc]

The can that came with my aftermarket GM HEI was pulling too much vacuum (20 degrees at 7.5" Hg per the spec) putting the total timing around 48 or so and causing some surging at highway cruising, so I needed something a bit more mild. I'm currently using a Standard Motor VC204 with 10-12 max advance, starts at 7-9 Hg, which is a match for an AR12/VC1838 for 1975 350 Buick per the attached article. Works well with my setup, which is around 42 degrees total timing (12 initial+16 mechanical@2300rpm+14 vacuum), manifold vacuum source (not ported/timed). Edit: I see others have already posted Lars' article!

[MikeB]

Originally Posted by tim_mc

I'm currently using a Standard Motor VC204 with 10-12 max advance, starts at 7-9 Hg, which is a match for an AR12/VC1838 for 1975 350 Buick per the attached article.

Thanks, man. That's another P/N to add to my list. I see where Amazon and Summit have it.

[Dashman]

The guys timing tab and balancer marks were mismatched coupled with a fuel leak, which caused an engine fire that could have resulted in a total loss. Did you all get that? It had nothing to do with timing advance curve or the vacuum advance canister. ...and what's the point of showing him fancy fuel lines, when his truck is very much original? How about addressing the real issue of what caused his fuel leak? It seems like some of you need another truck to build, hahaha.

Kalums, everybody has a near miss from time to time. You've already done a lot to your truck. You'll figure out the rest.

[MikeB]

Originally Posted by Dashman

The guys timing tab and balancer marks were mismatched coupled with a fuel leak, which caused an engine fire that could have resulted in a total loss. Did you all get that? It had nothing to do with timing advance curve or the vacuum advance canister. ...and what's the point of showing him fancy fuel lines, when his truck is very much original? How about addressing the real issue of what caused his fuel leak? It seems like some of you need another truck to build, hahaha..

In the immortal words of Sgt Hulka: https://www.youtube.com/watch?v=syV2LkGpQB0

[72SB]

aside from mismatched timing marks and pointer, OP had a not stock, but perfectly fine, Edelbrock carb with non-stock, cut, fuel line feeding it that had a soft fuel line to filter than to carb that was sketchy and leaked.

A more secure fuel line routing and part specific to OP's carb was shown to eliminate his fire starter setup

OP did not have a "stock" setup where a pre-bent hard line went from FP to carb.....so a better fuel line and filter was suggested to OP for "his" current setup.

[Kalums]

Originally Posted by Dashman

The guys timing tab and balancer marks were mismatched coupled with a fuel leak, which caused an engine fire that could have resulted in a total loss. Did you all get that? It had nothing to do with timing advance curve or the vacuum advance canister. ...and what's the point of showing him fancy fuel lines, when his truck is very much original? How about addressing the real issue of what caused his fuel leak? It seems like some of you need another truck to build, hahaha.

Kalums, everybody has a near miss from time to time. You've already done a lot to your truck. You'll figure out the rest.

Thank you for bringing it back around. I did appreciate all the information though it seemed like I opened a carb/timing "medical" encyclopedia with no prior knowledge of healthcare 😝

[Kalums]

So I finally got a chance to start repairing my faux pa's, I got a new detent cable... What a pain in the rear, no one around here had one... Mine was only 36" long(roughly), had to make one that I hope is the right length... Old one got hot enough to melt the plastic onto the spun cable making it non-movable. Replaced the throttle return spring... It un sprung(never saw that before, but it was the original) replaced the ICM wiring from the "nose" of the HEI. ( off subject but my coil ohm(ed) out properly, but for the life of me I can't figure out what the ohms should be on my ICM (original, GM stamped into the plastic, it read 19,180 ohms between "B" and the "G" posts, couldn't get a reading on any other posts. Bought a new (delco) one the ohms out at 140k between the same posts))Also replaced the original temp sensor wire... It was crispy too...solderedand heat shrank all my connections. Changed out my VA weights and springs for stock ones I found at my local yard ( the rest of the HEI was pretty toast and owner let me have them cause I bought some other parts. I'm completely rebuilding my HEI, the thermal paste was more like chalk dust. I've gotta figure out my TDC... When I pulled the cap my rotor was point almost directly at the carb, NOT towards cylinder #1. When I check with my protractor app it was roughly 20 dgrs off from where I personally would say cylinder 1 is. I did the "pull plug #1, finger over hole, wait for compression to move finger off." Then I put a Boro-scope camera in there and had my son turn the engine back and forth slightly and it appeared to be TDC but I'm not convinced. I didn't even remember to look where the marks ended up after" the wiggle". Even tho this engine has "GM GOODWRENCH 350" on The valve covers I'm not sure it isn't the original block it has the original temperature sensor with the funny little knob connection head... I haven't seen one in years. The ones I see all have spade connections. *shrugs* I got distracted and pulled my dash apart to replace all the burnt out bulbs (only 2 worked, top right speedo and passenger side blinker indicator) and figured out why my voltage gauge wasn't working. One of the connections from the main harness on the PCB had peeled off a little to the right, not broken or missing just pushed over...what should the voltage meter and or temp meter ohm out to? Finally ended up pulling out the speedo cable as my speedometer bounces pretty bad after 45 mph. Sorry for the rambling it was a busy day, thanks for all the help so far. Tomorrow I will be elbow deep once more... Man I love this stuff! As a hobby... I'd commit Hari Kari if I did it for what mechanic's have to put up with these days.
Edit: there is also a hydraulic/brake line from my proportioning valve to a brake box under my dash... Votes on if I should leave it or swap for a Prodigy P2 I already have.