| Author | Message | ||
| Steve Magnusson (91tr)
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John M. -- I just wanted to note that the best reference for carb data (i.e., float levels, jet sizes, etc.) will be the corresponding OM for your particular model/year/version (which gets updated every year while the WSM does not -- e.g., in '78, the US 308 GTB/S float height is given as 50 mm in its OM). Another option for F DCNF specs is that there is some tabular data in the Haynes Weber book appendix, but it can really only be matched up if you know the WW/XX/YY/ZZ series for your particular DCNF set (which I believe is stamped on the forward facing outside of each float chamber). If you don't have the Haynes reference, I'd be glad to crosscheck if you provide the series. With regard to the # of turns there are differences in the DCNF series. My experience was that my-ex stock set (72/73/74/75) was ~4T open while my replacement 4-bolt top cover DCNFs were/are ~2T open. Probably related to the cone angle of the screw/seat and/or the thread pitch, but I never looked at the details. PS If you can't confirm any other data, I'd start with the 48mm float height too. | ||
| Bill Sebestyen (Bill308)
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John, To calibrate your Synchrometers, you might want to do it on the engine, if the car will maintain a steady idle in the 1000 rpm range. Simply select a reliably firing barrell and insert one of the Synchrometers and note its reading. Then substitute the second Synchrometer and adjust it to read the same air flow. As long as the air flow doesn't change during this process, you should end up pretty close. I should have mentioned that before you start adjusting the carbs, you need to ensure that other engine related systems are up to snuff. This includes the ignition system, plugs, timing, and dwell, plug wires and extensions. Cam clearances and timing should be to factory specs. Emissions equipment, if fitted, should be operational. Potentional sources of vacuum leaks should be rectified. When all this is up to snuff and the engine is hot, then proceed to adjusting the carbs. Bill | ||
| John_Miles (John_Miles)
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Yeah, my previous '77 GTB had a persistent exhaust-popping habit for years. That can be a frustrating problem... nothing helped until the car had a valve job. In this case, though, the only popping I get is through the intake. The car leans out severely on entrance/exit ramps under even moderate acceleration, accompanied by "putt-putt-poof" noises from the intake below the passenger window. My '77 never exhibited that symptom under any conditions. Sure, it would lean out under hard, sustained cornering -- they all do that -- but I could generally solve the problem by putting my foot down further. In my '76 that doesn't work at all. Given the choice, though, I'd have to say I prefer having the intake-popping problem.  Having the '77 emit a loud exhaust backfire every other time I downshifted got well and truly old. | ||
| Richard Stephens (Dino2400)
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Popping in the exhaust can also be caused by an ignition problem. I spent quite a lot of time tinkering with the three DCNFs on my 246 dino engine (with syncrometer, colortune, reading the plugs...) and managed to tune out the popping to some extent, but it was still there. Then I replaced my points and had the distributor set up on the bench (to properly align the point sets a perfect 180 degrees from each other). Bingo! All popping was gone and car ran much smoother. Exhaust leaks can also cause the popping when you abrubtly let off the throttle since oxygen will flow in and allow any unburnt fuel to ignite in the exhaust. -Rich | ||
| John_Miles (John_Miles)
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Hmm, I must've hit cancel by mistake -- my reply isn't showing up. Just wanted to say thanks for the hints, especially with regard to the mixture-screw settings. I do have a pair of synchrometers from Pierce Manifolds, although they're nowhere near each other in calibration (as determined by holding them up to the kitchen range vent hood). I pried the side covers off of one of them, and it appears that it can be easily calibrated against the other. Will definitely do that first. | ||
| Bill Sebestyen (Bill308)
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John, I've been there and done that. It's an iterative, but straight forward process. Be patient, it'll be worth while. After the float levels are set, balance the air flow, barrel-to-barrel and then carb-to-carb. You must use a Synchrometer, or better yet, a pair of matched Synchrometers, to get accurate air flow readings. If you have two Synchrometers, one can be left in a barrell and becomes a reference for all others. A barrel that isn't flowing at idle, will not respond to mixture adjustments. The normal process is to lightly, but fully seat (closed) the barrel-to-barrel depression screws. Take a set of air flow measurements. Adjust the low flow depression screw, within a carb, to equal the high flow barrel of that carb. When all pairs flow equally, adjust the linkage to equalize the carb-to-carb flow. During this process, ensure idle speed remains between 800-1000 rpm. Adjust idle speed as necessary, after each adjustment. With the engine good and hot, you can begin to adjust the mixture screws, one at a time. If all cylinders are firing, begin to close one mixture screw at a time, 1/6 (1-flat) of a turn and wait 10 seconds for the engine to respond. Continue this process until misfiring in this cylinder occurs. Once you've reached this point, back out the adjustment screw, 1-flat at a time, until reliable firing occurs. Repeat this process with each succeeding barrell. Now repeat the process on all barrels again. With a hot engine , there should be no popping. Popping after a cold start is common with Webers. There should be no popping with a fully warmed up engine. I've found that popping is usually a lean condition. I like to set my mixtures somewhat lean for idling purposes, but the engine must be hot, to properly accomplish this. If popping continues, try richening the affected barrel, 1-flat at a time until it stops. 3-turns out on the mixture screws is the normal start-to-tune setting after a rebuild. Your final setting will be dependant on many factors including the fuel you use. Many fuels are oxygenated, which may require a slightly more rich setting. If you wind up more than about 4-turns open on the mixture screws, you may want to consider the next size larger idle jet. The turns to open setting assumes that the carbs have not been damaged in any way. When the mixture screws are fully closed, the tip of the needle comes in contact with an aluminum wall, at a 45 degree angle. If this needle has been overtightened in the past, it will distort this wall and will give you a false initial setting or reference point. If you follow the above instructions, you will arrive at an acceptable setting. Bill | ||
| magoo (Magoo)
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John, Peter GT4 may not be tuned in. He did his Webers. Give him a email shout at forza a barth @ mybc .com For sure he can help out . | ||
| John_Miles (John_Miles)
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This weekend, I'm going to pull the tops off my Webers and check/adjust the float levels. My objective is to get rid of an annoying tendency to "pop" or backfire through the intakes in turns. My previous carburetor-servicing experience has been confined to changing the main and idle jets, so I've never gotten my fingernails quite this dirty before. It looks like the top removal procedure is straightforward enough. Once inside, I plan to replace the existing quarter-century-old floats and needle valves, and then set the float levels with a caliper according to the GT4 shop manual directions (48 mm). Any pitfalls I should know about before tackling this job? Also, any good tips for adjusting the mixture screws without a CO meter? Some sources maintain that the screws should be no more than 3/4 turn out, while others say 3/4 to 1 1/2 turns. The GT4 workshop manual says start at 3 turns, if I remember correctly. Not as precise as I'd like. Would like to hear from anyone on the board who has "been there and done that." |