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post #1 of 11 (permalink) Old 07-22-2008, 04:58 PM Thread Starter
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Replacing throttle plates

I have a pair of Dellorto DHLA's (I assume that Webers are similar enough that the same guidance would be applicable) I'm trying to resolve some issues with. I've struggled with getting them to run well long enough to realize that the problem is in the carbs themselves and not the tuning (see this thread: http://www.alfabb.com/bb/forums/carb...-question.html). So, I've switched to a pair of Solex carbs that run really well, but I still have these Dellortos I'm interested to see if I can make right. Plus, I have a spare pair of Dellortos that are incomplete, but can be used as parts donors.

My basic problem is that air is getting by the throttle plates when they are fully closed at idle (three people on two continents have tried unsuccessfully at tuning them over many hours so there is no question it's not a simple "turn this screw" type solution). I can see what I consider too much stray light getting by the throttle plate edges looking through them like binoculars at the sun. Note that I'm doing this with the linkage completely removed as I found that also impacts how closed the throttle plates can be and I'll be addressing that separately. When I hold up the other pair, I don't see nearly the amount of light coming through. One plate in particular seems to have a defect (might have hit against something?) where it has a flat spot along the bottom edge that could explain why it is more impervious to tuning than the other three.

So, is it possible to just swap throttle plates between the two pairs of carbs without any more disassembly being required? There are two screws where the throttle plates appear to be attached to the spindles/bars. If I unscrew these can the plates just be slid out and the ones from the spares slid in and screwed down? Seems like an easy procedure, but I don't see any guidance on operations like this compared to all the posts about re-jetting and other "plug-and-play" operations. Because I have the Solexes running well, I'm pretty brave about it. I did try these screws on the throttle plate but right away noticed that some of them seem to turn easily and others I'm not able to "unfreeze" at all with the screwdriver I'm using at least. So, I just need to know if I need to be more determined about it (finding a better screwdriver or screwdriver operator) or turn back now as this is a carb-expert-only procedure?

Thanks,
Gary

73 Giulia Super 2.0 (Prugna)

Last edited by Gary73Super; 07-22-2008 at 05:02 PM. Reason: provided more detail
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post #2 of 11 (permalink) Old 07-23-2008, 06:14 AM
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one way for air to get around the throttle plates is for the bushings the shaft for the plates can wear, letting in air. another way is the bypass circuit these use instead of chokes.
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post #3 of 11 (permalink) Old 07-23-2008, 07:19 AM
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I once replaced a throttle plate in an old set of SU carbs I have...I managed to do a fair bit of damage to the bore. They're really snug in there, and I guess I didn't know 'the trick'. Go very slowly, and be sure to threadlock the screws afterwards. I've never done it on my DHLA40Hs, thankfully they don't need it.
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post #4 of 11 (permalink) Old 07-24-2008, 12:29 AM
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Question Butterfly adjustments

I can only do this job with Webers because I have the patience, and know some of the tricks. The throttle plate alignment job can be a real pain, particularly if someone has messed with it and the body has then had some use. You can start off with simple stuff, like bad end bearings, worn center bushings (egg shaped) or worn or bowed butterfly shafts. Once the bores are grooved by misaligned butterfly plates, you have trouble. Of course, if they are 40's they can be bored and made into 45's, if you want to spend the money! If the bores are grooved by the plates, the plates are history. While you are at it, replace the butterfly shaft, both end bearings, and rebush the center. V e r y lightly, polish the grooves. Don't try to remove them, just smooth them. Then with new plates, NO return spring tension, align one plate, shining a bright light from the plate side as it is adjusted with the plate screws as tight as possible, while still allowing plate movement. When it looks perfect, tighten the plate screws and check again with light. If it moved, readjust, and try again. I usually do the plate with the most bore damage first. Then repeat this procedure with the second plate. When no light shows through, add the return spring. If light shows, the center bushings are too loose. Start over. I can continue like this for pages. At some point you, as the owner, or the rebuilder will admit the cost in time, machine work, and parts may make this job non cost effective. I have saved some really nasty ones, but this can be very labor intensive. Is it worth it? What will another carb body, in better shape cost? The money spent with new plates, a new butterfly bar, four screws, two bearings and the re bushing job, might be better spent investing in a carb in better condition. Ever seen a twisted butterfly bar? Think about it, a .002 twist, end to end, will allow one butterfly to be completely closed, while the other is cracked enough to idle! You can really get into deep trouble with these things quite easily. My best advice is to understand your own limitations before you begin.


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post #5 of 11 (permalink) Old 07-24-2008, 08:33 AM
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When it looks perfect, tighten the plate screws and check again with light. If it moved, readjust, and try again. I usually do the plate with the most bore damage first. Then repeat this procedure with the second plate. When no light shows through, add the return spring. If light shows, the center bushings are too loose. Start over
Gordon, there can't be a more perfect example of a misspent youth than this. What a lunch time conversation this would be!

Jim . . . '72 Super 1300, '70, 1750GTV, 2nd series,
'62, Lancia Flaminia Zagato3c, 2nd series

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post #6 of 11 (permalink) Old 07-24-2008, 10:56 AM
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I can't agree more Jim. The amount I charge for this nonsense is stupid. I do it BECAUSE I CAN and most can not. It was learned in a misspent youth. I guess it keeps me out of bars and away from loose women. When I really get a bad one, it makes me sweat, and my wife asks " Why are you sweating when it doesn't look like you are doing anything?"
Good question! The owner only gets to see the finished result, and seldom has any idea what happened along the way. I just did a photographic commentary to a customer who is also an aerodynamic engineer. Jon Burningham, a BB member. If you ask him, perhaps he will write up a commentary with pictures. His had no bore grooving, but serious other problems. Remember, nothing is achieved without effort!


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post #7 of 11 (permalink) Old 08-13-2008, 04:28 PM Thread Starter
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Quick update

Since I've got a nice pair of Solex carbs running on my Super now (described elsewhere), this throttle plate project was backburnered. But, I handed over the carbs to my mechanic friend today and he quickly noticed by flipping the plates over that someone (Dellorto upon original manufacture?) spread out the end of exactly one of the two screws for each of the 4 throttle plates so that it covers the spindles a bit, apparently by drilling out the screw ends. So he's going to try to grind down that bit where the screws are splayed out and see if he can unscrew them. Obviously there was a strong intention not to make it easy to remove the throttle plate screws and thus the throttle plates. I'll update with any further progress.

Gary

73 Giulia Super 2.0 (Prugna)
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post #8 of 11 (permalink) Old 08-13-2008, 06:12 PM
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Originally Posted by Gary73Super View Post
Obviously there was a strong intention not to make it easy to remove the throttle plate screws and thus the throttle plates.
Yes, but not necessarily for service reasons. Ever see what errant throttle plate screws/throttle plates can do to the inside of an engine?
It ain't pretty.

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post #9 of 11 (permalink) Old 08-15-2008, 10:58 AM Thread Starter
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Originally Posted by papajam View Post
Ever see what errant throttle plate screws/throttle plates can do to the inside of an engine?
My mechanic friend said that newer Toyotas sometimes are having a related catastrophic failure. Not with carbs obviously, but some other valve that's in the intake path that is breaking free and lunching the engine.

We were able to free the throttle plate screws. It's amazing what having the right professional grade tools will do. I've already swapped in two throttle plates from my spare Dellortos for the two worst ones and it's clear that there is significantly less light getting by, but I can still see a bit at the moment (haven't tightened down any screws).

Now I'm just going to take my time and finesse them and see if I can get better than that. I don't think there are any grooves in the throttle bodies from misaligned plates, but it does appear to be cleaner/brighter in color where the throttle plates come into contact with the bores. Interestingly, the area where it must have been coming to rest (the brightest area) was exposing the first progression hole. That seems like a very bad situation to me and it's not that way currently now that I've swapped the plates.

I also have a hypothesis that the linkage does not allow the plates to close 100% as well even when the idle lever adjustment screw is turned completely out (meaning the linkage is hitting up against the stop on the carb body before it really should be--maybe that's an intentional design precisely to prevent grooves in the bores?). So, a metal file should resolve that situation and give just a bit more range of adjustment. When I substitute the linkage of my spare H series Dellortos on the primary F series carbs, it seems to partially but not fully resolve this issue.

As it stands, if I want to have the linkage on there and fully close the plates by hand, I have to reverse the linkage so that the lever that contacts the carb body can't do so. When I do that and then close the throttle plates a bit tighter than they do with the springs (although my springs seem fine to me as far as the amount of force), that's when the air around the throttle plate edges completely disappears when I hold them up to the sun like binoculars. Maybe that means I can get a bit better alignment on the plates, although it probably also means that the spindle could be twisted a bit as mentioned above and only by forcing it does it straighten and the plates completely close. Is it imperative that no light at all be visible with this "binocular test" (even with my spare pair before I started swapping the plates a couple of the plates let in a bit of light so I'm guessing that's typical for all used carbs)?

I'll probably tighten down throttle plates when I can't make any further progress on alignment and give them a shot. Then, I'll make the decision what to run the next couple months until driving season is over. If it's the Dellortos, I'll take them out and use loctite on the throttle plate screws. If it's back to the Solexes, I'll make the Dellortos a winter project and probably switch to my spare H series pair to use as the bodies swapping over everything else as practical from the F series.

Gary

73 Giulia Super 2.0 (Prugna)

Last edited by Gary73Super; 08-15-2008 at 11:01 AM.
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post #10 of 11 (permalink) Old 08-15-2008, 12:20 PM
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Gary,
Bore wear from twisted or bowed plate rod, or misaligned plates will often cause some light to show past even new rods and plates. An unfortunate fact, however if the light shows evenly, in the same places past both butterflies, they should work fine. It is possible both the balancing adjustment and air correction may require a little more attention than usual, but generally they should work fine. Getting the used ones as good as possible requires patience and time. One of the most frustrating areas with used ones is when both plates are right, screws tightened firmly, but when you tighten that final little bit, it removes or adds just enough bow in the shaft to show a crack of light in the LAST one tightened. Typical.


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post #11 of 11 (permalink) Old 08-15-2008, 01:12 PM Thread Starter
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Thanks Gordon. That's the kind of advice I need to know to help answer whether it is futile (meaning stick with the Solexes) or whether I should give it a shot. I'll at least work towards balancing any light (meaning air) leakage between the two plates of each carb (and hopefully pretty close between all 4). It's mostly from the bottom of the plate/bore where there is a bit of light, which I suppose is also typical. Also, I'm not seeing anything obvious like a twist/distortion of the spindle where one plate is completely closed and the other lets in anything more than a bit of light (compared to before I swapped the plates out and that one cylinder just seemed to leak significantly more light than any other).

I had pretty severe issues before with not being able to set a steady idle below about 1600 rpm because under that speed the one really bad cylinder (which matches with the one really bad throttle plate for light leakage) started missing regardless of air bypass and idle mixture--just didn't want to respond to settings at all compared to the other 3 cylinders. Shorting or pulling the spark plug for that cylinder made hardly any difference compared to the other 3 cylinders as well. That's with the throttle plates closed as far as possible (idle lever adjustment completely out), and idle mixture screws turned out like 7 complete turns just to give it enough fuel to match (meaning steady idle) what should have been closed throttle plates! Then I never got it to come down to idle from higher rpms in anywhere near the time it should (5 to 10 second is what it took). Plus, there was a brief hiccup/stumble/hesitation when I hit the throttle in gear before it would take off. Between the throttle plates not closing completely and progression holes being exposed when they shouldn't be (I've checked all the jetting and it's correct for a 2.0 even though these carbs originally came from a 1.3 GT Junior based on the id tags), my assumption is that that can explain all those symptoms.

I'll be interested to see what the throttle plates look like on the Solexes when I get those off. I first installed them before I figured out that a big part of my issue with the Dellortos could be the throttle plates. It is also pleasant that the Solexes tune exactly like they should as far as how far the mixture screws need to be out (rather than wildly rich) and that the idle lever adjustment screw does need to be cracked just a bit to let a bit of air by. This exercise isn't out of necessity at the moment since the Solexes are good, but I'd like to compare the Solexes and Dellortos when both are running well to know what to use long term. Then, given that they are both somewhat "off-brand" here in the U.S. (Solex especially) as far as obtaining replacement parts, I figure having a good set of backup carbs is a wise idea.

Thanks again,
Gary

73 Giulia Super 2.0 (Prugna)
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