| Author | Message | ||
| magoo (Magoo)
Advanced Member Username: Magoo Post Number: 3397 Registered: 2-2001 |
Hans, This guy seems to know what he is talking about doesn't he. He should be invauluble to us guys who own the carbed 308s. These carbs are better understood by dealing with them on a professional basis as obviously he does. So J.R. with only 14 posts it appears you have a lot to contribute to the F.C.. Glad to have you aboard. | ||
| Hans E. Hansen (4re_gt4)
Member Username: 4re_gt4 Post Number: 380 Registered: 4-2002 |
Go, J.R., go. Good stuff. I'm dating myself, but I used to do rebuilds on old Rochesters and Holleys. Some OEM applications had phenolic (sp?) insulators. Most didn't. Some cars had very thick soft gaskets, but with hard bushings in the mounting bolt holes to avoid any problems with overtightening. Interesting stuff. Somehow I find this old technology fascinating. However, I'm the kind of guy that likes to work on vacuum tube radios as well. | ||
| magoo (Magoo)
Advanced Member Username: Magoo Post Number: 3396 Registered: 2-2001 |
J.R. You are not userping my authority of which I have none here. I did not want this to turn into a federal case but let me say I agree with you in your correction of the problem. At the same time let me say that this guys problem was solved as I said it was. My reply certainly was not a criticism of your, the, technique it merely was a statment of how I saw it done before and UNBELIEVABLE, is still holding. Now with a professional explanation like you just gave how can one argue with that. Regards, | ||
| J. R. Vallandingham (Jrvall)
New member Username: Jrvall Post Number: 14 Registered: 11-2002 |
Magoo, I understand what some can get away with, and I'm not trying to userp your authority. If you look at the original phenalic blocs (spacers w/gaskits) one can notice that the gaskits are very thin and permanently glued to the insulator block. The design serves 2 very important purposes. 1) to insulate the carb from the direct heat transfer from the manifolds and 2) very thin gaskits to avoid compression on the base corners. By the time the 308 series had come along, carburators (and their problems) had been around for a long, long time. Weber was well aware of the problems that occur with casting in zinc (pot metal) and then subjecting that metal to heat and stress. The carb acted as a heat sink, sucking the heat from the surrounding metals, weakening the metal and boiling the fuel in the bowls (causing vapor lock). Rather than cast the carbs out of better metal (although improvents were made along the way), insulators were used to minimize the heat soak problems. But, they still had to use very thin, rather hard paper to keep the gaskit compression at the corners to a bare minimunn in order to prevent/slow the corners of the carbs from turning down, leaving a gap in the center. The problem gets exasterbated over the years by the overtightening of the carbs when re-installed after repair work or the adding of mulitple gaskits that will compress. In extreme cases even the tops can warp leaving a visibil gap between the top and carb body. These same problems have been common on all the carburated Ferrari's all the way back to the 50's. The guys that have done their share of the V12's have been dealing with these same problems for a long time. | ||
| magoo (Magoo)
Advanced Member Username: Magoo Post Number: 3393 Registered: 2-2001 |
J.R., The one I saw was corrected. However I guess it depends on how much it is warped. Peter and you are correct to do it correctly planeing is the way to go. This one just happened to work and the guy has not had any problems since installing new gaskets. If it were bad enough it probably would not have worked. About a year now. | ||
| Hans E. Hansen (4re_gt4)
Member Username: 4re_gt4 Post Number: 373 Registered: 4-2002 |
This is part of the reason that I'll be doing a carb tear-down. I suspect some air leak from *somewhere*. My idle mixture screws are opened up a bit more than they probably should be, and I have some subtle driveablilty/idle probs. Mid-rpm on up works great, tho. Also, the damned things are 27 years old. | ||
| J. R. Vallandingham (Jrvall)
New member Username: Jrvall Post Number: 8 Registered: 11-2002 |
Sorry guys, but thicker gaskets makes the problem worse not better. The leak is the symtom, not the problem. The 'problem' is that the pot metal is 'bent' from the 'compression of the gaskets on the corners where the tightening force is applied, caused by the heating up and cooling of the metal under stress. The carb base becomes cured into a warped shape. Thicker gaskits only cures the symtom (air leaks) temporarily. Believe me, as I've done a lot of them, repairing things right is very rewarding and lasts Soooooo much longer than 'rigging' them. Any good machine shop can 'surface' the bases for about $40 bucks each. Giving the carb a second lease on life. While simply pileing on gaskets causes the base to warp even further as time goes on, leading to the reasons most shade trees start opening up the idle jets to overcome the air leaks. Which is why I suggested 'checking' (measureing with jet gauges) all the jets to insure they haven't been tampered with. Usually the first thing a rookie does is to install bigger idle jets to mask the air leaks. | ||
| magoo (Magoo)
Advanced Member Username: Magoo Post Number: 3390 Registered: 2-2001 |
Also, It is not a band aid if it corrects the problem and the problem does not re-occur. | ||
| magoo (Magoo)
Advanced Member Username: Magoo Post Number: 3389 Registered: 2-2001 |
Peter, Machineing the plates is of course the way to get the plate straight, but if the gasket takes up the couple of thousands of unlevel surface then why won't it work? If the plates are a tight seal and are not disturbed from their position on the thicker gasket what can be the problem. If you are saying the plate will warp more then a shaved plate will warp more also. I see it as a way to correct the problem using new gaskets next time they are removed. JMO | ||
| '75 308 GT4 (Peter)
Intermediate Member Username: Peter Post Number: 2149 Registered: 12-2000 |
Magoo, thicker or doubling-up base gaskets are a band-aid. Machining them true and flat is the correct way. The only critical part is those screws. You should grind the exposed, staked threads first before removing them from the shaft. If you don't, it'll chew up the shaft threads when removing. I'd still go with staking them, not Loctite. Following the methods given in the Haynes and Braden books you won't bend the shaft. As for centering, with the screws lightly seated (but still allowing some movement of the plate in the shaft slot), gently open and close several times the throttle then check centering by holding them up to a light. It'll be bang-on. | ||
| magoo (Magoo)
Advanced Member Username: Magoo Post Number: 3383 Registered: 2-2001 |
I have seen this unlevel problem cured by using a thicker gasket material. | ||
| J. R. Vallandingham (Jrvall)
New member Username: Jrvall Post Number: 5 Registered: 11-2002 |
A tip, the bearings almost never wear out, (certianly not immpossible but extremly rare and unlikely a problem), the grease they are originaly packed with disappears/gets hard, causing the leaks, as well as the carb bases warping causing even more leaks. The proper fix for 308 carbs that refuse to adjust to 2.5-3.5 % CO at idle is to machine all bases flat, repack bearings with thick grease (old type Bosch Distrib. points grease), check very carefully all jet sizes, replace all jets not standard factory issue, set float levels exactly, no matter how many tries it takes (always use new Needles & Seats) and use new base gaskets being very careful not to overtighten upon reinstalation, then insure to exacting standards that ALL linkages are set properly and pull both carb lvers at exactly the same time/distance. Of critical impotance is that 'both' distribs are timed exactly the same and that their advance mechanisms are in perfect working order, as well as condition and dwell of points. Both systems, Ignition and Carburators must be in top working order and work harmoniously to achieve perfect running results. | ||
| Bill Sebestyen (Bill308)
Member Username: Bill308 Post Number: 401 Registered: 5-2001 |
Hans, To replace the bearings you do have to remove the throttle shaft. DCNF series carbs use ball bearings on the throttle shafts so there shouldn't be any slop or air leakage. Make sure you get the chamfered edges of the throttle plates oriented correctly and don't cross thread the throttle plate screws when refitting the throttle plates. You should use new screws and make sure they are clean for reassembly. Put a drop of Loctite thread locker on the clean screws and loosely assemble. The throttle plates are recentered by snapping the throttle closed a couple of times, with the screws slightly loose, then tighten the screws with the Loctite. Thread locker should be sufficient as you just need to protect the screws from vibrating loose. Stud-n-bearing Loctite is probably overkill. | ||
| magoo (Magoo)
Advanced Member Username: Magoo Post Number: 3379 Registered: 2-2001 |
Right on Ed. I think the blue Loctite is the strongest. | ||
| Hans E. Hansen (4re_gt4)
Member Username: 4re_gt4 Post Number: 369 Registered: 4-2002 |
Carbs are obsolete?????? I must have been sleeping. Kidding aside, which Loctite? Seems there are a zillion types, I'd want to be sure I got the correct one. | ||
| Edward Gault (Irfgt)
Intermediate Member Username: Irfgt Post Number: 2202 Registered: 2-2001 |
I used to do this all the time before carbs became obsolete. They are easily recentered and you can use Locktite on the screws which will hold them forever. No Problem! | ||
| magoo (Magoo)
Advanced Member Username: Magoo Post Number: 3370 Registered: 2-2001 |
Peter GT4 did his I think. When replacing the throttle shaft screws be sure to use brass again. | ||
| Hans E. Hansen (4re_gt4)
Member Username: 4re_gt4 Post Number: 365 Registered: 4-2002 |
I am contemplating a carb rebuild. One problem area is the throttle shaft bearings - they get sloppy and/or leak air. However, to replace them, it's my understanding that the throttle plates must come off, as the throttle shaft needs to be removed. Concern: An old carb specialist told me years ago to never remove throttle plates, as they are very difficult to reinstall accurately centered - which is critical for idle and part throttle operation. Also, you risk bending the shaft when 're-staking' the screws. Additionally, if the screws aren't adequately staked, they could fall out.  So.... Comments? |