| Author | Message | ||
| Steve (Steve)
Member Username: Steve Post Number: 421 Registered: 2-2001 |
OK just came in and I am at the best I can do. I'll give you my readings from the picture below. Front carbs are the nearest to you in the picture. FR 3.5 HG , 3.5 FL 7 , 7 , RR 3.5 , 3.5 , RL 4 , 4 . Runs very good at 1000rpm it seems the FL carb with the brake vacuum hose will not settle down to the 3-4 range. At this point I am ready to button it up and drive it. The trick is setting the throttle openings (screw A) also I the LR carb has both air balanced screws closed.  | ||
| Bill Sebestyen (Bill308)
Member Username: Bill308 Post Number: 473 Registered: 5-2001 |
Hans, I just purchased a new set of 40DCNF45/46/47/48 carbs, which I believe are correct for early US GTB/GTS cars throurgh 77. All bodies have the necessary tab and all are tapped for the adjustment screw. However, the 46/48 carbs, for the right side, are not fitted with the adjustment screws and and springs, nor are they fitted with the necessary linkage levers that would contact the end of the screws. One would have to fabricate and retrofit workable levers along with screws and springs to make it all work. | ||
| Steve (Steve)
Member Username: Steve Post Number: 416 Registered: 2-2001 |
I guess the trick goes back to setting up the idle screws. So I will back out the F&R left carb idle screws and then adjust the spring loaded throttle adjustment screws on the right side F&R carbs. Once done I should be able to get them all sync'ed to the 3-4 range on the meter. Here in lies the trick to starting this.I thought it was interesting that the left side F&R carbs were flowing higher then the Right sides. It must be because I had idle screws to play with but not a similar setup for the other 2 carbs.I assume once this is done the mixture screws are no different than holley or rochester carbs. | ||
| Hans E. Hansen (4re_gt4)
Intermediate Member Username: 4re_gt4 Post Number: 1509 Registered: 4-2002 |
I don't think that there is a place to put idle screws in the right carbs. | ||
| Bill Sebestyen (Bill308)
Member Username: Bill308 Post Number: 472 Registered: 5-2001 |
Rich, No great harm will come if you install throttle stop screws in the 2-right side carbs. One down side though, off idle balance now becomes more of an issue, IMHO. There is likely to be some play in the linkage that takes a different set when of the throttle stops and all carb throttle shafts are opened slightly, at say 1200-2000 rpm. As long as you maintain a good off idle balance, you should have good driveablility. | ||
| rich (Dino2400)
Member Username: Dino2400 Post Number: 262 Registered: 10-2001 |
Is it possible to just add "A" screws to each carb?(I assume these are what I call "idle speed screws") I'm sure Pierce Manifold would be happy to sell them to you. Then you could set the carbs up with the linkage off. Then adjust the linkage so that it slides on without moving anything. Then you'd be all set... This just seems like a better set up to me than having a linkage in constant tension even at idle. | ||
| Steve Magnusson (91tr)
Intermediate Member Username: 91tr Post Number: 1883 Registered: 1-2001 |
Steve -- Don't be too overly concerned about the absolute number shown on the Synchrometer. It's just my particular ex-308 experience that at 1000 RPM and 7 deg BTDC ignition timing my Synchrometer would read about 3.5 Kg/Hr (my bad for mis-stating the units earlier) for each barrel. It's much more important that each barrel have the same idle airflow and that when you disable a cylinder (either by removing spark or fuel) the idle RPM seems to drop about the same amount (i.e., each cylinder is doing quasi-equal work at idle). IMO with the 2-to-1 airflow difference you've reported there's no downside to having a go (and leaving "as-is" really isn't the best option). | ||
| '75 308 GT4 (Peter)
Advanced Member Username: Peter Post Number: 2856 Registered: 12-2000 |
Steve, basically the "B" screw counter-acts the action of the "A" screw depending on which direction you turn it. This isolates adjustments of one carb from the opposite one (and will allow you to bring down the high readings of cyl's 3,4,5,6, while maintaing the ideal readings of cyl's 7 & 8 [1 & 4 should be reading 3.5 KgH too]). | ||
| Steve (Steve)
Member Username: Steve Post Number: 415 Registered: 2-2001 |
I'm starting to see this whole operation.I need to figure out the "B" adjustment screw and how it operates. I'm just not seeing it yet but I'll get it.I think it would have been better if they had the "A" throttle screws on all 4 carbs. I remember doing the sync with my 3 SU's on the XKE was easier because they had a coupling on the throttle shafts and throttle screws on each carb. Thanks Steve I'm seeing the light here. I'll get to it this week. Work has got me tied down lately so don't have much time to work this .I'm also looking at the WSM . Also where did you get the 3-3.5 reading on the syncometer. | ||
| Steve Magnusson (91tr)
Intermediate Member Username: 91tr Post Number: 1877 Registered: 1-2001 |
Steve -- The "funny spring type connection device that links carb 1-2 with 3-4 and 5-6 to 7-8" are the "B" screw mechanisms. The engine speed that corresponds to 7 Kg-hr is around 2000 RPM. And yes, the Synchrometer measures the "total" airflow going into the cylinder (i.e., the air going past the throttle plates + the air going thru the air bleed circuit). | ||
| Steve (Steve)
Member Username: Steve Post Number: 413 Registered: 2-2001 |
OK Steve got your point. It appears that the throttle opening affects the sync meter readings. So I guess I need to close down both the air bal. screws as well as the idle adjustment screws.My carb set up has this funny spring type connection device that links carb 1-2 with 3-4 and 5-6 to 7-8. In my readings you can see my 3-4 and 5-6 read high (5-7) where the other 2 are lower. Also you mention when you get someone to step on the gas to get the flow at 7 and check all carbs. Do you know what rpm range this is at? I'll give it a go tonight and see but I think we are all getting a carb education here. Also I'm running electronic ign. at 7BTD. 1977 308GTB Thanks SK | ||
| Steve Magnusson (91tr)
Intermediate Member Username: 91tr Post Number: 1876 Registered: 1-2001 |
Steve -- First, IMO you shouldn't have "all of the bleed screws have been adjusted out somewhat" per my previous message (i.e., start with them both fully closed and then only open the bleed screw on the lower flowing barrel to match the higher flowing barrel if necessary). I too would just also say what Peter said about making them all more "equal" (you should be able to get them equal within ~1/4 KG-hr or less IMO). See Fig 19 (page C20) in the 308GT4 WSM: 1. Disconnect one end of the linkage rod running between the 3-4 and 5-6 carbs. 2. By adjusting screws "B", you should be able to match the 7-8 flow to the 5-6 flow and the 1-2 flow to the 3-4 flow. 3. Then adjust screws "A" to bring the RPM back down to 1000 RPM and to match 1-2-3-4 flow to 5-6-7-8 flow (if you're running about 7 deg BTDC at 1000 RPM idle timing the flow per barrel will be in the 3 ~ 3.5 KG-hr range; if you've got the R2 stuff working and are at 3 deg ATDC at 1000 RPM idle timing the flow per barrel will be in the 3.5 ~ 4 Kg-hr). 4. Reconnect the linkage rod (and adjust it's length, if necessary) making sure that it doesn't lift either linkage set off it's "A" setscrew stop. 5. Readjust mixtures when all flows are equal. (You may need to make several loops thru steps 2-5 if you're coming in from a long way out). 6. Once you've got them all equal and idling well, have an assistant open the throttle using the accelerator pedal until one bank is flowing at ~7 Kg-hr per barrel and then check that the other bank has about the same flow too (if not tweak the length of the linkage rod). Good luck... | ||
| Steve (Steve)
Member Username: Steve Post Number: 412 Registered: 2-2001 |
What else effects the flow. Is it the throttle plates? If I shut down the air balance screws I still get a 5 flow on the 7 carbs and you can feel the power loss. What is the reason that the 2 left side carbs are running higher then the 2 right side carbs? Is the linkage the trick here?The way its running now I am tempted to close it up and drive it. Thanks | ||
| PeterS (Peters)
Member Username: Peters Post Number: 784 Registered: 1-2003 |
To you guys that are giving the fix here, you are all awesome. I am not a mechanic and I know that this is one tricky job (that my mechanic just finished). I believe the $1100 I paid him to rebuild the carbs on my '77 308 was worth every penny!..Carry on! I just poked my nose in to read the thread...Long live FChat! | ||
| '75 308 GT4 (Peter)
Advanced Member Username: Peter Post Number: 2849 Registered: 12-2000 |
Based on your diagram, cylinder #'s 5 & 6 are flowing way too much at 7 KgH. Cylinders 4 & 3 are a tad too much as well at 5 KgH. You have to have them all equal. | ||
| Steve (Steve)
Member Username: Steve Post Number: 411 Registered: 2-2001 |
OK Steve let me tell you where I am. Engine at Idle (1000rpm) water temp 165 and here are my Syncometer readings. 7 7 3.5 3.5 5 5 4 4 Left 1 -4 Now I am at between 4 -5 turns on the mixture screws and all of the bleed screws have been adjusted out somewhat. Runs good with good power so what am I doing wrong here? | ||
| Steve Magnusson (91tr)
Intermediate Member Username: 91tr Post Number: 1871 Registered: 1-2001 |
Steve -- With regard to the air balancing screws on each 40DCNF carb, you are correct -- start with both fully closed and then open only the lower flowing barrel, if necessary, to match the higher flowing barrel (i.e., on every carb at least one of the air balancing screws should be fully closed and sometimes both are fully closed). The linkages between the carbs are then tweaked to match carb-to-carb and bank-to-bank. On the point about how many approximate turns on the mixture screws should be open, IME stock (5-bolt top cover) F 40DCNF are in the 4~5 turn range while the aftermarket 40DCNF (4-bolt top cover) are in the ~2 turn range -- so this gets confused in the Haynes book, etc.. Bottom line (if you don't have a gas analyzer) is once the engine is idling quasi-decently you should close the mixture screw on a cylinder (that cylinder will miss and the RPM will drop slightly) and then slowly open the mixture screw until that cylinder starts firing again and then open 3/8~1/2 turn more. Do this one cylinder at a time. Do a search on "Synchrometer" and another on "Syncrometer" to get to previous discussions. | ||
| Steve (Steve)
Member Username: Steve Post Number: 410 Registered: 2-2001 |
OK here's where I am. By the way Rich if you turn thr mixture screws out 2 turns my car won't run. So I started with air bal. screws closed. Mixture screws 4 turns out and idle screws 1 turn in. Started car ran until warm , was rich and fouling plugs. Shut down engine replaced sparkplugs with NGK 5 ES. Re started , set idle at 1000 , looked at timing (7 degrees OK)and started from there. Went to each carb and started working air bal. screws. I found I needed to adjust both per carb to smooth out and this also increased RPM. Did all 4 carbs and check with Mano for air flow.Seems left 2 carbs F/R have more flow then right 2. Started to adjust mixture screws and needed to lean them out (5-5 1/2 turns). Started sounding like it ran before.I do have a periodic pop in cyl. 6 carb at idle maybe carbon on the seat or weak valve spring (cold comp was 160). Now runs dead solid at 1000 and strong power. Before I put the air cleaner on I need to try it again when I have more time to verify its in.I found that the air trim has the most effect on the adjustment but what has me confused is that the Haynes book says you only need to balance 1 barrel with the other which means 1 screw is closed and the weaker barrel is adjusted? Am I reading this right? So thats where I am today. I'm heading out to a wedding so I'll be in no shape to get to this today. Thanks for the info. | ||
| '75 308 GT4 (Peter)
Advanced Member Username: Peter Post Number: 2841 Registered: 12-2000 |
Gotcha Hans... Good point John... Rich, yes, our engines are transverse and the carbs are side-by-side... | ||
| John_Miles (John_miles)
Junior Member Username: John_miles Post Number: 88 Registered: 7-2001 |
Also check to see that when you activate the throttle that they all start moving at the same time. ... making sure that when you activate the throttle by hand, you do it by manipulating on the actual throttle cable, not the microswitch actuation tab or any other convenient pivot point. | ||
| rich (Dino2400)
Member Username: Dino2400 Post Number: 257 Registered: 10-2001 |
Oops. Sorry about saying to disconnect all linkages. I didn't realize that you don't have individual screws for each carb like I did on the Dino V6. You can use the adjustments on the linkage then to balance carbs to one another. Also check to see that when you activate the throttle that they all start moving at the same time. | ||
| Hans E. Hansen (4re_gt4)
Intermediate Member Username: 4re_gt4 Post Number: 1495 Registered: 4-2002 |
Yeah, agree, Peter. It's just that people are saying to 'disconnect all linkage'. If you do that, you don't have idle adjustment on the right carbs. | ||
| '75 308 GT4 (Peter)
Advanced Member Username: Peter Post Number: 2836 Registered: 12-2000 |
Hans: "...The right side carbs get their idle setting from the linkage attached to the left carbs..." Yes and no. Yes, the throttle stop on one carb shares its duty of setting idle speed with the other carb because of that joining link. But no, the joining link has an ultra-fine threaded screw and an opposing spring-loaded pin, which is used to adjust throttle shaft/plate position, to vary airflow of that carb. Basically, that screw on the joining link acts in the same manner as the throttle-stop screw on the other carb body... | ||
| John_Miles (John_miles)
Junior Member Username: John_miles Post Number: 87 Registered: 7-2001 |
One problem with "set the idle" screw is that, at least on the older cars, there is no idle adjustment screw on 2 of the carbs. Nor is there a place for one. That's normal. The idle speed adjustment affects both the left and right carbs simultaneously. For that reason, you don't want to uncouple the left-right linkages to adjust the carbs; just the front-back ones. (I didn't even do that much when I went through mine). | ||
| Hans E. Hansen (4re_gt4)
Intermediate Member Username: 4re_gt4 Post Number: 1483 Registered: 4-2002 |
One problem with "set the idle" screw is that, at least on the older cars, there is no idle adjustment screw on 2 of the carbs. Nor is there a place for one. The right side carbs get their idle setting from the linkage attached to the left carbs. Or is my car just plain screwed? | ||
| rich (Dino2400)
Member Username: Dino2400 Post Number: 254 Registered: 10-2001 |
I'm assuming you have 40DCNFs? There are three adjustments you can make. Idle-speed screw, air-bypass screw, and idle-mixture screw. Start by making sure that everything in your ignition system is perfect: wires, coils, and fresh or known good spark plugs. Then run the car until it is completely warmed up. 1. Screw all the idle-mixture screws in all the way and then back out 2 turns. 2. Disconnect all throttle linkages. If you have cable activated chokes, make sure none are stuck, which sometimes happens. 3. Back off each idle-speed screw and then screw them back in until they begin to touch the throttle lever and then go 1/2 turn. 4. Start the car and it should idle now. Add or subtract a bit of idle-speed screw evenly on all carbs to get the car idling at the speed you would like. Again, the car must be fully warm to do this so no cold-start mechanisms are in play. Base-setting has been achived. Now balance each carb to itself so that both barrels on one carb are drawing an equal amount of air. A synchrometer is best for this but you can use a piece of 5/16" fuel hose held to your ear and get remarkably close. 1. pick a carb, any carb 2. loosen the locknuts on the air-bypass screws and screw both of them in until they lightly begin to hit the seats. Put the hose in the top of one barrel and listen to the hiss. Check the other one. The one that is passing the greatest amount of air is your target so adjust the air bypass screw on the other side till the hissing is equal. tighten the locknuts. 3. Repeat for all the other carbs, until all carbs are balanced to themselves. 4. Now balance the carbs as a group. Start by readjusting the idle speed if you need to. 5. Listen to all the carbs and pick on in the middle range to be your target. Then adjust the idle-speed screws on the other carbs until they all are making the same sound. You should be pretty much done now! You may need to tweak the idle-mixture screws a bit. You can use a tool like a color-tune for this or just let the car idle for a while and then look at the plugs and make adjustments that way. It's challenging but I love it. | ||
| James Selevan (Jselevan)
Member Username: Jselevan Post Number: 581 Registered: 6-2002 |
If you can get the car to idle, start there. There are two things that must be accomplished. 1) Synchronize airflow with a manometer. 2) Adjust idle mixture. The order in which you do these is subject to debate, but I attack mixture first, then adjust volume of flow, and then readjust mixture, then flow, etc., until all are synchronized. First, loosen or disconnect the throttle linkage to each carburetor so that they can be adjusted independently. There are many opinions on how to do this. I would start by, one at a time, turning each idle mixture screw in all the way and then opening each screw by 1.5 turns. This gives you a starting point. The metric you use is your ear. When the mixture is correct, the cylinder that you are working on will contribute more force and raise the RPM slightly. Thus, if too lean or too rich, the mixture is not optimum, and the RPM will drop slightly. Slowly turn the idle mixture screw out for one cylinder and see if the RPM rises, if after slowly turning a full turn of the mixture screw there is no change, then go back to the starting point and slowly turn it in to lean the mixture. DO NOT LEAVE THIS CYLINDER until you find an optimum point for that mixture screw. Once you convince yourself that you can raise and lower the RPM by adjusting this screw for this cylinder, and you have found the optimum mixture, then move on to the next cylinder. Once you have optimized the mixture for all cylinders, then use one of the various airflow measuring tools to check flow. Adjust the idle screw for each cylinder to achieve synchrony. Once you have airflow equal, then you must go back and recheck the mixture. This process is laborious but fun. It is easy. As is the education process in neurosurgery, ��watch one, do one, teach one.� Jim S. | ||
| Steve (Steve)
Member Username: Steve Post Number: 409 Registered: 2-2001 |
OK so I got the timing thing right now after playing with the carbs thinking that was the problem I now managed to screw them up. So is there a proceedure some where that gives you tips on how to adjust these webers without a CO set up. I got the weber manual but starting setings are way out so I need some help from the experts here on the site. Thanks |