Dual Weber 51.5 IDA

I recently installed 51.5 Weber IDA's which were 48 IDA's souped up, in my in my Speedster with a 1835cc motor. I had installed before Dual 48 IDF Webers in the car prior the car ran great. I am having a problem with the idle now. The Speedster runs great but when hot it has a hard time idling. I got the jetting perfect and the timing and the Linkage almost perfect. Do any of you Weber experts no what could be the problem, why the motor does not hold the idle when the motor is hot?

Thanks in advance, Kawika55

The 51.5 Weber IDA's are too big for a mild 1835

So were the 48s. Way to big!

Even 44s are oversize unless you choke down to at least 34mmm vents.

Is it April Fool's Day? Really, an 1835 would run well on dual 40idf, and I am sure you could get it to run on bigger, but why would you want to use such big carbs? Are you a drag racer? Do you need bragging rights?

I got the Carbs at a unreal deal and I sold my dual 48 Weber IDF's a friend really wanted them. I am going to try looking at the Floats. I know I can make the carbs work. Nothing is impossible. Alot of VW owners with 1835cc motors run dual 48 Ida Webers. I will update you guys. Thank you for all your comments.

Aloha, Kawika

There is a big difference between running well at WOT when you're ringing it's little neck, and actually having a driveable car.

Driveable is your friend.

Your most driveable engine is the smallest carb that doesn't choke the engine out. It idles, has low end, mid-range and acceptable (though not 100%) top end. It provides zippy throttle response. Fun and DRIVEABLE.

Over-carbed is is truly dreadful. I've done it, regretted it, and would really hate to see a buddy do the same thing...

angela

I've got 48 Dellortos with 42 mm vents on a 200 h/p 2332 with 44 mm intakes, and it's too much. I'm choking my vents down to at least 40 mm this winter.

The commonly accepted rule of thumb is that the carburetor venturi size should be about 4 mm less than intake valve size (for a race application), and usually less than that for a nice, crisp street engine. If your 1835 has 40 mm intakes (which would be a big head for that engine), then your vents should be about 36 mm and no more. A stock 44 IDF comes with 36mm vents. A 48 IDA comes with 37mm vents, which could be made to work with a 40 mm intake valve, but which would present some real tuning challenges. The 51 IDAs (and the EMPI clones) generally have 48 mm vents. That is nuts.

A venturi only works if it is the "choke point" for air-flow. When you have a venturi that is 8 mm LARGER than the intake valve, it's going to be very, very difficult to meter fuel at all. It's going to be soggy everywhere.

I'm not telling you you can't do what you want-- everybody can do what they want. I'm telling you your big-'ol-51s belong on a 2.3L+ full-on drag motor. Your 48 IDAs were a LOT closer to optimal. A 44 IDF would be better. 40 IDFs with 34mm vents (an aftermarket replacement) would be better yet. 40 Dellorto DRLAs would be perfect.

So I think the consensus is to sell the big boys on ebay and buy some 40 mm Webers or Dells......

But, hey, it's a big world, and you can do what you want. But I think the experience and knowledge here is saying somrthing else.

Giday, Stan. I'm always trying to learn more about the relationships between carb sizes and intake runner length / flow. It seems that the longer the intake runners are, the smaller the carb can be. For instance, a modern motorcycle has virtually no intake runner length yet my center mount Weber 32/36 has a runner length of about 12", then doubled runs just fine on my 2.2 Soob. Is it that the volume of the manifold acts as a "reservoir" for the fuel/air mixture and the intake valves sip on that for as much as needed thus making it less sensitive to carb size? Is there a document or rules of thumb that can be referred to on this ? Thanks.

I agree with the comments regarding carb sizing but I think he should be able to get his carbs to idle. Check/ clean the idle jets and adjust the idle mixture and linkage. The rest of the ranges may be a challenge. A Wide Band meter is a plus.

"I recently installed 51.5 Weber IDA's which were 48 IDA's souped up, in my in my Speedster with a 1835cc motor. I had installed before Dual 48 IDF Webers in the car prior the car ran great. I am having a problem with the idle now. The Speedster runs great but when hot it has a hard time idling. I got the jetting perfect and the timing and the Linkage almost perfect. Do any of you Weber experts no what could be the problem, why the motor does not hold the idle when the motor is hot?"


David, carb size aside, what do you mean by "linkage almost perfect"? Sort of like "partially pregnant" is it not? Either the linkage is on the money or it is not, and the linkage on any dual carb installation can make or break it. What sort of linkage are you using? Have you gone through a complete idle sync with the linkage disconnected using a snail or similar reasonably accurate device? Some say they can do that by ear, I can't.

Many of the hex bar set ups for linkage are very poor after just a bit of use. Wear occurs very quickly, adjustments change, and the geometry was not good as originally designed, and certainly gets much worse with use.

There are several threads on how to improve hexbar linkage if that is what you have. Personally I prefer center pull linkage as sold by CSP, and others.

What happens to your idle when hot? On a center pull idle speed goes up when the engine is hot, due to expansion of the engine case itself. On hexbar, about anything can happen as it heats, depends on how worn it is, how it is set, and how good or bad the design is as installed....

And as others have suggested, unless your engine is a full out race engine you don't need 51mm carbs.....I have a friend with a 1720 that runs 48's very successfully, canks out about +150hp, but it is a 912 based engine that has a tuned exhaust, very hi tech everything, and he is a Weber expert....and he spends a lot of time keeping the tune perfect...and at about 7500-8000 rpm the engine really cranks it, but a VW based engine should not attempt thoses sorts of rpm, unless you want to rebuild it often, or pick up a lot of loose road debris....that was once an engine

All of us tend to suffer from a "bigger is better" complex. I have proven to myself more than once that isn't true, but then I do it again....

Even if you get them tuned, you're also going to have major throttle sensitivity issues. They're so big that the intake will reach max flow when the throttle is only partially open. It'll feel like your only choices are idle or WOT.

Stan -- I also have 2332 with 44mm intake valves, and Dell 48 tri-jets. Like you and others here I sucked into the bigger is better myth. It is miserable!! What a pig to drive.

I dropped from the stock 40mm vents to 36mm. Trust me, you WILL like it!! Don't be timid about making the bigger step from 40mm to 36mm. You can tromp on the gas at 2500 rpm and your head hits the helmet fairing instead of your face hitting the steering wheel. WAY more fun!

Still, I'm grossly over-carbureted. (For you Weber guys, Dell 44s are kind of like Webber 48s, Dell 48s belong on a GTO).

Idle wanders around 1100 - 1300, and I've learned to expect that is good enough. It is pointless to try for 800/900 idle. The butterflies are too big. To get that kind of rpm the butterflies have to be shut practically tight -- just open enough for the air leaking around them to run a cylinder. In that state the difference between practically tight and actually tight (dead cylinder) is less than .001"; which is realistically unmeasurable and functionally impossible to maintain. So the trick is to set the butterflies wider open for a faster idle; then the slight settings variations through temperature, etc. have a less significant impact than the difference between running and dead.

And, forget it altogether if you don't have Perfect linkage and Perfect balance (Right Danny?).

David -- I don't think you will ever be happy with those carbs, or else you'll just never know better (hey -- it took me a long time to figure out things weren't the way they should be). But if you want to mess around with what you have, make sure you have perfect linkage and balance; then set your idle for 2000 or so. Drive around like that for a while. If your idle hangs-in pretty reliable then drop it to 1800 for a while. If that works then try 1600, etc. But you can forget about 800/900.

Well said, Mark. I kind of take my linkage for granted now, it's been bulletproof for 20,000 miles, car has 23K on it now. It's funny how much of an improvement it is, and how few people actually have good, precise, and reliable linkage. But what's even funnier to me is the ostritch mentality, people sticking their heads in the sand and overlooking why their cars run like crap, even after you show them how much play is in their linkage....

luftig came with 48s. Was told that they were 44s. Didn't discover this until removal for gear changes. Talked to Pat at cb. He gave us all the different jet sizes. Runs smooth and fairly quick with no hiccoughs. Not good gas mileage though. Have mild 2110. Pat knows carburation, I believe.

Bob

Bob -- I do believe Pat / CB Performance knows what they are talking about. That's why I use the book they wrote as my technical reference when I gave the advice above.

As a practical reference: "been there, done that". It is not good.

But don't let me discourage you or anyone. I used to like standing in the parking lot saying "yep -- them there are dual Dellorto 48 Trijets!" just to make guys with relatively whimpy Webers shrink in embarrassment. It was fun for a while. But now I know there might be someone in the audience that actually knows what he is talking about and knows I've got rocks for brains. So now I don't brag so loud, in fact I'm a little embarrassed myself for having rocks for brains.

Mark-

You seem to know what you are doing-- but it sounds like your float level is too high, or fuel pressure is off. Do you have the "Dellorto drip"? You ought to be able to get those carbs to idle like a purring kitten.

In my experience, aside from some diagrams, the much vaunted "Dellorto bible" is borderline worthless. I'd never recommend anybody buy it.

My 48s (non tri-jet) with the big vents actually work really well as I've got them set up-- I have no flat-spots, and pull hard from idle (which is set for about 800 RPM cold, and 1000 RPM hot). I get 28-30 MPG or so on the highway, and 22-25 MPG standing all over it.

I've got a set of tri-jets coming (Samba purchase), and have vent sets all the way down to 36 mm in my spares. I plan on starting with 40 mm vents. I'm looking for more "snap" and am willing to give up some top end. The smaller vents should bring me off the idles quicker, and help in the transition to the mains (which I plan to set back a couple of sizes). The tri-jet should richen it up at WOT.

I've got an LM1 wide-band and a tray full of jets and ma making some changes that will dramatically mess up my state of tune (new headers, etc.). I love this stuff.

David-

If you really want to run those carbs, perhaps you can jack them up and drive one of Jeff Denham's 3L Type 1s underneath it.

Mark, not disputing or agreeing with any of your post. I'm not mechanical. Just stating what I was unknowingly left with and how it worked out. Hope you find success.

Stan - I don't have any of the issues you mentioned (floats, leaks, fuel pressure). I can make it purr like a kitten at 900, but the fact is it is so touchy I don't want to mess with it.

I think of it this way: A butterfly opening of .003 at idle is a good target to be around (according to Tomlinson, who I do place credence in). Naturally .003 is going to pass more air on big throated 48s then on smaller 45s or 36s (Dellortos). More air = faster idle

So to cut the idle speed on the big throated carbs means cutting down that .003. Think about it ... what are you going to cut it to, .00234? Or .00177? I don't believe the system has the mechanical repeatability to fine tune to that kind of precision and maintain it for practical periods of usage.

So yeah, as you and I both know we can force big carbs to work somewhat manageable. In other words we have learned to work around a problem. The problem is the carbs are too big.

But I'll keep my Dells because they do have that "mine's bigger than yours" parking lot zipper-check effect. And it is still kind of fun to whip 'um out once in a while. But I would have no regrets if I had chosen the correct carbs in the first place.

On the Trijets -- I'm not convinced 2332 is big enough to pull enough air even in sporty driving to bring in the Trijet circuits. I'd like to know what others think.

Mark-

I certainly didn't mean to offend you, but I thought you indicated that you had a lot of issues tuning your 48s. You seem to know what you are doing, but my results couldn't be more different than yours.

At idle, my butterflies are completely closed. That's what the hole in the throttle plates are for, to bypass enough air to keep the engine running at idle. If I was trying to do it "by the book" (CB's worthless manual), I'd be idling at about 2500 RPM. You are correct-- trying to set them up this way would be very, very inaccurate. Actually, with the mixture screws set for best lean idle and the plates completely closed (and bypass screws fully in), I idle at about 1300 RPM. I actually back some initial timing out of the ignition (I have a Mallory Uni-Lite with an adjustable curve) to bring the idle down to under 1000 RPM. Glen Ring over on teh Samba put me onto this, so there must be something to it.

As for 48s being too big for a 200 HP motor-- everybody's got an opinion here. Mine is this: at 99% of operating conditions, nobody will ever need that much carb. However, as long as the venturis remain the choke-point of the intake-- then at WOT, the engine will flow everything the carb can give it (assuming the vents are appropriately sized). I couldn't care less what size the throttle plates are, it's the venturis that matter.

I've got vents in every size from 34 to 42, and every idle and main jet that Delloroto ever made for this carb. I've spent many, many hours getting it right.... and now I'm going to mess it all up-- not because it's bad, but because this is my hobby.

The "commonly accepted" rule of thumb is to hold the venturis 4mm less than intake valve size-- but in operation, the true maximum size can better be determined by the cross sectional area of the smallest point of the induction system. In most heads, that's going to be the intake port. However, I've got some pretty good sized intake ports (the heads are welded).

I'm running really short connecting rods to keep piston speed up, which creates a big draw on the intake, even at slower engine speeds (RPM). It works really, really well-- although everything on the bottom end must be seriously over-built.

As far as "braggin' rights", I couldn't care less. Nobody here knows what the heck this thing even is, let alone that 48 tri-jets are cooler than 40 IDFs, but not as cool as a set of IDAs. I just want what works, and the transition circuit of a Dellorto DLRA is second to no other carburetor, and darned near as nice as some homemade EFI systems. I want to try smaller vents not because I'm having issues with tunability (ask somebody who's ridden in the car), but because I'd like to transition into the mains a bit more quickly.

That's my experience. Your mileage may vary.

LOL Stan -- no offense was ever taken. We are talking about carbs here, hardly anything I'd consider personal.

You must have bigger holes in your butterflies than I do. So you are saying that you can back your idle screws all the way off (butterflies snapped shut) and you still have idle rpms at 1300? That doesn't seem a little odd? And you work around it by jacking with the timing until it lopes at 900? This is a good thing?

I guess I'm just pulling your chain a little Stan, so laugh along with me. I do have to agree with you on one point: Tuning methods you've described are not in the Tomlinson book, or anything else I've read.

More seriously,what do you think: At what point will a 2332 suck hard enough to pull in the Trijets?

Great, Mark-- we're on the same page. I never know who's on the other side of the screen, and some people want to fight a holy war over really silly things, so I wanted to make sure.

Heck yes the way I'm advocating isn't in the Tomlinson book, but in my experience precious little of any value is. It's got great exploded views of a bunch of carbs that have nothing to do with our engines, however.

I've gleaned a lot from spending a fair amount of time over on the Samba. If Glenn Ring or Mark Harney are talking about it, I'm listening. John Connely has been a wealth of information, and has some really unconventional opinions about a bunch of tuning issues (he advocates for A/F ratios in the 16:1 range at cruise, a point at which I get a lot of lean misfire). Generally, I will say that almost everybody runs their car pig-rich, and that leaning out to something approaching stoich at cruise and holding things in the 12:1- 13:1 range at WOT brings the engine alive.

I test everything I read against what I observe with my pile 'o jets and wideband meter. I know almost nothing about what any carb besides a DLRA does and likes.

My hope is that I can drop the vent down one or two sizes, get off the idles a bit faster, run a slightly leaner main, and use the tri-jets to richen up the mix at WOT. I'm hoping to be able to observe the tri-jets coming on with the A/F meter, and suspect that they will only come into play at WOT with some pretty significant air-flow through the engine.

Anyhow-- it's fun talking about it. To get back to the point of this thread: I feel 100% confident in saying that (even though I'm an IDA ludite) a 48mm vent on a 1835 is too big, no matter how hopped up the engine.

I don't know how anyone else feels about the conversation between Stan and Mark.
But, like Chris Matthews said, I'm getting a tingle up my leg...:-0
Good stuff, guys!
And, yes, 51s are totally ridiculous on an 1835.

...or, you could replicate the set-up on this notch-back...

...Holley 750cfm spooled through a turbo...

There's a daily driver for you...
Tell that alien to get in his car seat!

Holy smokes!

angela

Hi gang I went back with my 48 IDF Webers the car runs smooth again. I know for a fact the 51.5 Weber IDA's would have worked but I gave up I got the motor to run awesome but I could not get the car to idle right with those big carbs I needed to spend more time on the jetting. I am going to run the 51.5 IDA Webers on the new motor I am having being built for my Dune Buggy a 2500cc motor. Right now I have a Stoker Motor a 2074 with 48 IDA Webers the car runs awesome. Thank you for all the help and comments.

Aloha, Kawika

Nice buggy David. I love it!

Nothin' cooler than street legal and wheelie bars!

DIG IT baby!

angela

Nice!