When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
CarburetorsCarb discussion and questions. Upgrading your Third Gen's carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.
I rebuilt the M4ME carb on my ‘85 T/A, LG4 305, 700R4.
I sourced my rebuild kit, throttle bushing install kit, and a new ethanol friendly float from carbjunkies,
After completing the repairs and reinstalling and dialing in the carb (vacuum gauge), i was quite pleased with the overall performance.
The car sat for a week, I went to restart it. I observed the accel pump spray while cycling the pump (checking to see if there was still fuel in the bowl) - and noted no spray for the first two pumps, then on the third pump, fuel sprayed.
The rebuild kit included a new check ball and complete accel pump assembly. I lubricated the accel pump and bore with silicon lube.
What would cause the accel pump to loose its prime?
Are you using the filter inside the carb? That filter should have a check valve at the inlet end, which keeps the fuel in the bowl and not siphoning backwards.
Are you using the filter inside the carb? That filter should have a check valve at the inlet end, which keeps the fuel in the bowl and not siphoning backwards.
Yes, I replaced it with a new delco filter. Spring in first, then filter, filter seal against line adapter.
there must still be fuel in the bowl, as I was able to get fuel through the squirters after a couple of tries?
Last edited by theraymondguy; 11-20-2020 at 05:26 PM.
Yes, I replaced it with a new delco filter. Spring in first, then filter, filter seal against line adapter.
Well, you got that part right. Fuel evaporating?
The only other thing I can think of is the ball bearing. Did you use a punch and small hammer on it to form a new seat, or did you just drop the new BB in?
Well, you got that part right. Fuel evaporating?
The only other thing I can think of is the ball bearing. Did you use a punch and small hammer on it to form a new seat, or did you just drop the new BB in?
I did form the new seat. 👆However, I went pretty light. That’s gotta be it.
There is one thing I still don’t get on this carburetor, it has no metered vacuum port fie the distributor. I keep reading how its necessary to have a metered port for the distributor including In the “please read” sticky.
The port for the distributor (according to the vacuum map on the hood of my car) always has vacuum on it - as it should, it seems it’s ported to the transfer slots.
According to the emissions decal this car should be setup for 4* initial advance - is it plausible that this carb I this application is meant to have vacuum on the distributor port at idle, and that’s why so little initial timing?
Last edited by theraymondguy; 11-20-2020 at 06:10 PM.
There is one thing I still don’t get on this carburetor, it has no metered vacuum port fie the distributor. I keep reading how its necessary to have a metered port for the distributor including In the “please read” sticky.
The port for the distributor (according to the vacuum map on the hood of my car) always has vacuum on it - as it should, it seems it’s ported to the transfer slots.
According to the emissions decal this car should be setup for 4* initial advance - is it plausible that this carb I this application is meant to have vacuum on the distributor port at idle, and that’s why so little initial timing?
Oooo, M4ME, is this car Canadian, or emissions removed now with M4ME carb? If so, I have something for you.......
BTW, port H on the carb is ported vacuum advance.
Last edited by NoEmissions84TA; 11-20-2020 at 06:28 PM.
Oooo, M4ME, is this car Canadian, or emissions removed now with M4ME carb? If so, I have something for you.......
BTW, port H on the carb is ported vacuum advance.
Canadian car
Last edited by theraymondguy; 11-20-2020 at 06:44 PM.
Great, that makes things easier.
My car will have emissions removed, and this is the modified diagram I came up with. Hopefully this diagram makes things clearer. https://www.thirdgen.org/forums/tech...ml#post6139987
Great, that makes things easier.
My car will have emissions removed, and this is the modified diagram I came up with. Hopefully this diagram makes things clearer. https://www.thirdgen.org/forums/tech...ml#post6139987
That agrees with the decal on my car, but the h port has vacuum on it at all times. Confirmed port is marked “h” on carb.
That’s what makes me wonder about whether or not it’s true (no vacuum at ported) for this carb.
How could ther not be vacuum on the port when it goes right to the transition slots?
The transfer slots are at and above the throttle plates at idle.
They should be nearly closed and only about 1/4 of the transfer slot should be showing. Ideally, "squares" is what you are looking for.
As the plates are slowly opened, the transfer slots get more exposed to manifold vacuum - aka ported.
If you have vacuum at port H all the time, then your throttle plates are open too far at idle.
What is your timing at idle with the vacuum advance disconnected? It should be 4 degrees.
Last edited by NoEmissions84TA; 11-20-2020 at 09:52 PM.
The transfer slots are at and above the throttle plates at idle.
They should be nearly closed and only about 1/4 of the transfer slot should be showing. Ideally, "squares" is what you are looking for.
As the plates are slowly opened, the transfer slots get more exposed to manifold vacuum - aka ported.
If you have vacuum at port H all the time, then your throttle plates are open too far at idle.
What is your timing at idle with the vacuum advance disconnected? It should be 4 degrees.
With the throttle base plate assembly removed from the carb, primary throttle plates fully seated - I can spray brake cleaner through the port I can’t pull vacuum but there is a restriction from the throttle plates (brake clean tastes much better than gas IMO). With the plates closed, I see the “squared” port you speak of, it blocks about the top 2/3 of the transition slot.
I’ve got the base timing set at ~ 4 degrees at idle (500 rpm in gear). Adding timing has a notable effect on idle speed (with the vacuum removed and capped).
Curb idle screw has less than 3/4 of a turn on it when contacting throttle linkage, might even be less. I confirmed the sealing of the primary plates after installing the bushings.
The engine runs clean when the vacuum is removed from distributor, makes good vacuum (18” Hg) but sounds far better with the vacuum
line connected (20” Hg, idle bumps to 650).
This carb is probably as close to a “virgin” core as a guy will get in 2020 - the mixture screws had been exposed but I believe that its never had any other work aside from maybe a kit put in it in the past.
Either something is physically damaged in the area of the port, or this is the way Rochester built it, intended to have a vacuum signal on distributor at idle. It’s definitely odd either way.
Thanks for sticking with me on this, I appreciate the input.
Last edited by theraymondguy; 11-21-2020 at 06:51 AM.
if the dist was set up for ported advance and you want to change to manifold advance you'll likely need to re-tune the dist with an HEI tune kit that includes weights and springs (some even include a new vac can that can be adjusted. Either way a good dist tune can't hurt.
converting from ported to vacuum advance and re-tuning the dist-and removing the tamper caps on the idle mixture screws- was one of the first mods we did back in the early '80s. if I may digress...along with the fuel filler neck mod (that allowed the use of then still available leaded fuel) once the dual exhaust had eliminated the cat
i don't know why your acc pump may not be squirting on first try. if you're not experiencing any symptoms while driving, the acc pump well may just be emptying some while sitting.
if the dist was set up for ported advance and you want to change to manifold advance you'll likely need to re-tune the dist with an HEI tune kit that includes weights and springs (some even include a new vac can that can be adjusted. Either way a good dist tune can't hurt.
converting from ported to vacuum advance and re-tuning the dist-and removing the tamper caps on the idle mixture screws- was one of the first mods we did back in the early '80s. if I may digress...along with the fuel filler neck mod (that allowed the use of then still available leaded fuel) once the dual exhaust had eliminated the cat
i don't know why your acc pump may not be squirting on first try. if you're not experiencing any symptoms while driving, the acc pump well may just be emptying some while sitting.
I believe noemissions84 has correctly diagnosed the accel pump issue as me being overly cautious when seating the new check ball. I’ll resolve this tomorrow.
Noteworthy - when I bought the car the idle adjustment screws had been exposed, there was a bb installed into the EGR vacuum line. Someone’s has been here before. Now what else they did to the carb?
As far as the ported vs manifold I’ve resigned myself into the fact that manifold vacuum is present at idle on the distributor port: now the only question is to how to take best advantage
The only other thing that I can think of is that the new accelerator pump in the kit was different than the original to the carb. But if the carb has been overhauled in the past, then there is no way to compare. I see that you are very conscientious, so I am betting that you actually did a side-by-side comparison of the pumps.
The only other thing that I can think of is that the new accelerator pump in the kit was different than the original to the carb. But if the carb has been overhauled in the past, then there is no way to compare. I see that you are very conscientious, so I am betting that you actually did a side-by-side comparison of the pumps.
Yes, I did give the new parts a thorough once over.
I’ve been fortunate enough to come across another Rochester of the same vintage.
Not exactly the same, but a 17082283 vs the original (I believe, now suspect otherwise) 17085580.
Lo and behold, if I block the metered port above the throttle plate on the “new” carburetor, it will NOT allow me to breathe through the attached tube!
Also note the caps over the idle adjustment screws don’t look like they were removed with a hatchet, this one is nice and tidy. Also note the throttle shafts aren’t completely whipped, and the base plate is nice and straight.
It would seem this carb id a “virgin core” aside from an updated vacuum solenoid fit the choke.
I bought it as a parts carb (the coil died on my electric choke) It needs a good cleaning, and a kit but that’s old hat by now. I think I can source a gently used carb kit from somewhere...
Last edited by theraymondguy; 11-26-2020 at 06:23 PM.
It’s been awhile, but I’m still working through carburetor issues.
I built the Cutlass carb, ran that for almost a year until it started to leak from the welsh plug at the inlet. This carb had the same symptoms as the Firebird carb, extended crank after sitting for a few days. I don’t think there’s anything “wrong” at this point, I believe that part is more in my head.
My setup has changed some in the past year. 1.5” headers, Accel distributor, Edelbrock 2101, and just over Christmas break a 420/443 cam.
I purchased Cliff Ruffles book on Qjets and learned a fair bit, even been lucky enough to get some direct feedback from the man himself. Most notably - the timed (distributor) port is meant to have restricted manifold vacuum at idle. There is a coolant temperature switch that interrupts vacuum when the engine is cold. This is purely an emissions effort.
Because I’ve replaced the distributor (vacuum advance hanging up) I’ll be moving it to manifold vacuum, running like an apt equipped pre 75 carb. Lots of fussing left I’m sure, but I’m hoping the changes I’ve made will eliminate fuel dribbling at idle. I intend to retain the catalyst, pcv and canister, but that’ll be extent of emissions on the car.
I’m working through calibrating the 17085580 Firebird carb. I’ve opened up the idle tubes, down idle ports, primary screw ports and a significant change to the float height. It’s been cold enough that I haven’t fired it up yet after the last go through. That changes this Tuesday with an expected temperature nearly tropical 40*.
Last edited by theraymondguy; 01-30-2022 at 05:40 PM.
It’s been awhile, but I’m still working through carburetor issues.
I built the Cutlass carb, ran that for almost a year until it started to leak from the welsh plug at the inlet. This carb had the same symptoms as the Firebird carb, extended crank after sitting for a few days. I don’t think there’s anything “wrong” at this point, I believe that part is more in my head.
My setup has changed some in the past year. 1.5” headers, Accel distributor, Edelbrock 2101, and just over Christmas break a 420/443 cam.
I purchased Cliff Ruffles book on Qjets and learned a fair bit, even been lucky enough to get some direct feedback from the man himself. Most notably - the timed (distributor) port is meant to have restricted manifold vacuum at idle. There is a coolant temperature switch that interrupts vacuum when the engine is cold. This is purely an emissions effort.
Because I’ve replaced the distributor (vacuum advance hanging up) I’ll be moving it to manifold vacuum, running like an apt equipped pre 75 carb. Lots of fussing left I’m sure, but I’m hoping the changes I’ve made will eliminate fuel dribbling at idle. I intend to retain the catalyst, pcv and canister, but that’ll be extent of emissions on the car.
I’m working through calibrating the 17085580 Firebird carb. I’ve opened up the idle tubes, down idle ports, primary screw ports and a significant change to the float height. It’s been cold enough that I haven’t fired it up yet after the last go through. That changes this Tuesday with an expected temperature nearly tropical 40*.
If it is drizzling fuel at idle and the float level is correct, needs more bypass air.
If it is drizzling fuel at idle and the float level is correct, needs more bypass air.
Thanks Fast,
I'm at "trial and error" stage for sure with a rather sweeping group of changes in my last effort - including a big swing at float height. My last kit purchased recommended 13/32 a stones throw from where I had it - probably because I wasn't checking the height at the correct location.
The car smelled like a gas station the last I had it running, I'm hopeful for better tomorrow - maybe today if the sun stays out.
Idling at 700 rpm, a little more than 19”Hg and steady as a rock. Vacuum advance still connected to the emissions port. Not bad for 37* Fahrenheit. It will idle down lower, but the vacuum drops with it - I’ll leave that until spring, go hunting for that ‘mean’ idle that’ll still let me drop it into gear without shinnanigans.
Quadrajet owners, seek out Cliff Ruggles and his quadrajet book.
11-20-2020, 03:28 PM
