Idle Mixture Screw (1 Viewer)
- Thread starter N919HJ
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inkpot
SILVER Star
I would replace it. It should not have the step ring on it where someone has been forcing it into the seat. The seat in the carb body could also be damaged from this screw being forced in way too far. Check the outlet in the bore of the carb. If it is all flared out, then it may be the body is damaged. John
Looks pretty ringed-someone screwed this in way too tight. You could chuck this up in a drill press and use some coarse emory cloth to smooth out the groves. I have not found any specs on the exact taper, but regardless if you got it smooth, it just may take different turns to get the right idle mix(Not the normal 2-1/2 turns out)
I just got a replacement screw from "914RUNNER" It was in great shape for $20. Maybe want to contact him to see if he has more.]
I just got a replacement screw from "914RUNNER" It was in great shape for $20. Maybe want to contact him to see if he has more.]
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No, it doesn't need replaced. That small indent makes no functional difference. They almost all look like that (or much worse). The steel carb body can not be damaged by the brass screw.
Does turning the mix screw in make the idle drop? Then the screw works.
FWIW, there are no new 2F mixture screws available in USA. There are still some F carb screws available, but they are too short.
Does turning the mix screw in make the idle drop? Then the screw works.
FWIW, there are no new 2F mixture screws available in USA. There are still some F carb screws available, but they are too short.
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You're welcome.
If it really bugs you, you can chuck the screw in the lathe and repoint the tip.
Actually, that would be an interesting experiment, to test the effect of different tip angles on idle quality.
If it really bugs you, you can chuck the screw in the lathe and repoint the tip.
Actually, that would be an interesting experiment, to test the effect of different tip angles on idle quality.
- Thread starter
- #10
Ha... This only my second carb rebuild, just trying to get it to run right, nothing crazy. After this is back on the truck and working properly, you and I can talk about a hybrid 79/mid 80's carb that makes a little power and has a fancy idle mixture screw.
Just my .02 here, but if the screw works just fine with the damage in the photos, why did the mfg bother to make them so smooth?
My opinion is the screw is junk as it is, and needs the drill press tapering or a polished replacement.
The carb hole/seat(receiver) prob. will not be damaged as it is much harder material than the brass screw.(as described)
My opinion is the screw is junk as it is, and needs the drill press tapering or a polished replacement.
The carb hole/seat(receiver) prob. will not be damaged as it is much harder material than the brass screw.(as described)
No, it doesn't need replaced. That small indent makes no functional difference. They almost all look like that (or much worse). The steel carb body can not be damaged by the brass screw.
Does turning the mix screw in make the idle drop? Then the screw works.
FWIW, there are no new 2F mixture screws available in USA. There are still some F carb screws available, but they are too short.
The original taper in those mixture screws is what gives the manufacturer a baseline setting (number of turns ) to aid in initial setup . The screw will work just fine , I would smooth it out a bit but by no means make it any sharper - material needs to be removed from behind the groove , not at the tip .
With that screw grooved it does make a difference in sensitivity when adjusting the mixture . As long as the mechanic knows what he's doing - no issues . Most of the factory carbs have a pretty wide adjustment range in number of turns - look at the thread count (extra fine) . Aftermarket carbs are generally much more sensitive since they use more common heavier threads - but they are not being metered for emissions -hence the low/long taper and extra fine threads .
So , confused enough yet ?
Sarge
With that screw grooved it does make a difference in sensitivity when adjusting the mixture . As long as the mechanic knows what he's doing - no issues . Most of the factory carbs have a pretty wide adjustment range in number of turns - look at the thread count (extra fine) . Aftermarket carbs are generally much more sensitive since they use more common heavier threads - but they are not being metered for emissions -hence the low/long taper and extra fine threads .
So , confused enough yet ?
Sarge
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Bottom line is: does the screw still have enough authority to change the AFR from 12.5 up to 15+?
Yes: screw works, no need for replacement.
No: repair or replace screw.
Yes: screw works, no need for replacement.
No: repair or replace screw.
- Thread starter
- #14
I used the screw. Carb is installed and still idles like sh*t. Vacuum is at 8-9 off the brake booster. Everyone will say "vac leak off the manifold". I know, but here is where I got stuck. I used almost a whole can of carb cleaner looking for a vac leak with know change in idle. All vac lines are new, Jim did a big cap dizzy for me, timing set to 10 degrees per Jim, new cap, rotor, wires, and PCV with gromit. So the engine is running, full temp, I pull the first plug wire and it runs worse. Same thing with 2, 3, 5 and 6. When I pull number 4 I get no change in the already poor idle. All wires are providing spark. Plugs are pretty black with only a few hundred miles on them. So I replace the plug, but nothing gets better. Now I'm standing on the drivers side where all the emissions crap is and all of sudden I hear this sucking sound followed by what sounds like whatever was being sucked is released. Like trying to suck a lemon seed through straw and having it shoot in to your mouth. Once this happens the idle is perfect, I can set it to 650-660 and vacuum is now 17-18 inches. The choke is now functioning, it didn't really do anything before this event. I take it for a test drive and get down to the end of the street and it will barely idle, to the point where I almost don't get it back to the garage. When slowing down in gear it is real bouncy as a posed to a constant deceleration. I'm thinking it may be a stuck EGR valve, I haven't tested it yet. By that time it was getting dark and I needed a beer. You guys have any other ideas? Could the EGR even cause that much trouble? I've read so many threads now I'm kind of lost on what to do next. I do have a 2f FSM and emissions FSM, but the emissions manual is for an 81. Hope I'm not too hard to follow.
It sounds like a hot mess. Having a dead #4 cylinder can't help. Do a compression test.
8-9 inches of vacuum is not good either. The fact that it climbs to 18 inches randomly indicates that you have an intermittent vacuum leak. This does not imply that it has to be a leak to the outside where carb spray could have access. The brake booster, or the EGR valve (it it is '79 and up) and a cracked manifold bottom could also be the source. A crack would not be intermittent.
8-9 inches of vacuum is not good either. The fact that it climbs to 18 inches randomly indicates that you have an intermittent vacuum leak. This does not imply that it has to be a leak to the outside where carb spray could have access. The brake booster, or the EGR valve (it it is '79 and up) and a cracked manifold bottom could also be the source. A crack would not be intermittent.
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Pull all the plugs, see what they look like.
While plugs are out, do a compression test.
If compression is good, install 6 new W14EXR-U plugs and proceed w/ diagnosis with the knowledge that the engine is solid (no burned valve, burned piston, blown HG....)
If compression is bad or some plugs are very oily, continue engine teardown & inspection to find and correct problem.
While plugs are out, do a compression test.
If compression is good, install 6 new W14EXR-U plugs and proceed w/ diagnosis with the knowledge that the engine is solid (no burned valve, burned piston, blown HG....)
If compression is bad or some plugs are very oily, continue engine teardown & inspection to find and correct problem.
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- #17
Correct me if I'm wrong, but I don't think it's the brake booster. I'm taking the vacuum reading from the brake booster line. If it were the brake booster, taking it out of the loop would give me a good reading. Right? Only one vacuum line to the booster?
Sounds like one of the emission valves. I've dealt with this once before and couldn't believe all the weird stuff it caused the engine to do. But definitely check #4 compression first.
...via IH8MUD app
...via IH8MUD app
If you disconnected the vacuum hose to make a reading then yes, it is not the booster. If you used a T, then the booster could be bad.
The EGR can be a source of vacuum leak, but only for 79 and up where the EGR connects below the throttle plate.
Edit: I noticed that you have a '79 in your sig line, but this does not show in my iPad app. If it starts idling rough, rap the top of the EGR valve with the handle of a screwdriver to try to make it seat. If it is leaking and seats, it will idle better right away.
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^What he says^
Plus, on the 79 if the EGRVM or the EGR VSV are failing, they can leak vacuum to the EGR valve at the wrong time. So in addition to tapping the EGR valve, remove the vac hose and put a BB in the end of the hose to plug it. Continue w/ testing.
Plus, on the 79 if the EGRVM or the EGR VSV are failing, they can leak vacuum to the EGR valve at the wrong time. So in addition to tapping the EGR valve, remove the vac hose and put a BB in the end of the hose to plug it. Continue w/ testing.
