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I have a Rotary Engineering twin 36DCD setup on a stock port 12A with a Racing Beat exhaust system. I received the kit used, in somewhat rough condition, so I made a lot of updates, including adding a vacuum port on one of the primary runners. I later found the original RE installation manual, and it says to leave the vacuum advance actuators on the distributor disconnected.
Why? Is it because the vacuum advance mechanism inside the distributor is unreliable?
The distributor vacuum advance pods were originally hooked to a ported vacuum source on the original Nikki carb which means at idle there is no vacuum advance because the throttle is closed. As throttle is opened, vacuum is induced in the port the vac advance hose is hooked to and advances the ignition timing in conjunction with the centrifugal weight system in the distributor to give total ignition timing advance. Were you to hook up vac advance hose to primary runner port you made,ignition timing would jump ahead at least 20 degrees at idle,maybe more. In other words your timing will be overadvanced at low rpm and you ignition curve will be screwy as a result. Leave vac hoses off nipples on dist. so they're open to atmosphere and set your ignition timing with a timing light and engine revved to about 3k rpm for a total timing figure. I'm on my phone so don't have access to specs at the moment,it's found in fsm. You will need to mark your pulley for timing degrees,been discussed here several times,you can search for recommended method of marking.
Elwood, can you locate a ported advance nipple on your weber?
Everything GSLSEforme said is excellent but I can add to it. In my experience, if you set the dizzy at 0 degrees at idle, that is correct and more accurate than reving to 3000 due to differences in dizzies and their curves over the years, how old and stiff the grease is etc.
Just think of it like this. The mechanical advance will do what it's going to do as RPMs increase and the weights will move out the way they do. As for how much actual advance you will get at 3000 RPM vs 4000 RPM, I really couldn't tell you as there are several factors involved. But I can tell you is you don't want to aim for a specific amount of advance at a high RPM if it screws up your timing at idle. The engine will be harder to start and make less power down low as a result. Do you really want that?
Thanks, gentlemen. It's now mostly clear. I'm good with setting the timing, and I appreciate the perspective on idle timing. I'll check idle and high RPM.
No, I don't have a ported advance nipple on my Webers, so I'll leave them disconnected as the instructions suggested.
I thought that vacuum advances were typically used to advance the timing at light throttle cruise for improved fuel mileage. Is the function the same here?
I thought that vacuum advances were typically used to advance the timing at light throttle cruise for improved fuel mileage. Is the function the same here?
yep, Mazda used either ported vacuum, which works like that, or manifold vacuum with a solenoid to turn it off at zero throttle. the later ones have two solenoids.
i think the aftermarket stuff assumes that simpler is better, with a side of improved VE canceling out the need for more timing.
I ran my Dellorto with and without vac advance over the years. It made the idle kind of lumpy but it really breathed some life into the low rpm acceleration when hooked up. I did set my final timing at 4k to around 27 degrees if I remember right.
I have the 36 DCDs and had to deal with this very issue. I have collected several of these carbs over the years and noticed that the 36 DCD carbs made in Spain come with a ported vacuum port while the original ones made in Italy , while much higher quality, did not have the ported vacuum outlets.
I experimented with both setups and have found that using the ported advance greatly improved cruise performance and increased my mileage from the high teens to about 23 or 24 mph. (This is on a street ported 4 port 13b,) so that is what I am running now. Take note that I am not as knowledgeable about the theory of vacuum advance and the ins and outs of how and why it works as some of the smarter folks , but my experience led me to use the ported vacuum,
I even modified my Italian carb bodies by adding the vacuum ports , copying the set-up I found on the Spanish bodied ones.
That having been said, drilling a hole into a carb as rare as an Italian 36 DCD is not for the faint of heart!
Also, I would encourage you to make use of a wideband O2 to help tune if you are not already and capitalize on the fact that you can tune the primaries and secondary's separately, giving the lean mixture needed at cruise for economy, and then a slightly richer mixture at WOT for power.
I have the 36 DCDs and had to deal with this very issue. I have collected several of these carbs over the years and noticed that the 36 DCD carbs made in Spain come with a ported vacuum port while the original ones made in Italy , while much higher quality, did not have the ported vacuum outlets.
I experimented with both setups and have found that using the ported advance greatly improved cruise performance and increased my mileage from the high teens to about 23 or 24 mph. (This is on a street ported 4 port 13b,) so that is what I am running now. Take note that I am not as knowledgeable about the theory of vacuum advance and the ins and outs of how and why it works as some of the smarter folks , but my experience led me to use the ported vacuum,
I even modified my Italian carb bodies by adding the vacuum ports , copying the set-up I found on the Spanish bodied ones.
That having been said, drilling a hole into a carb as rare as an Italian 36 DCD is not for the faint of heart!
I did this exact thing on a pair of Weber carbs on a VW GTI with 4 valve Calloway cyl. head.
While power increase was substantial, fuel economy was miserable. These carbs had no provision for ported vacuum,use of manifold vacuum made for a very lumpy idle and less than ideal part throttle operation. I looked at carb bodies and saw an undrilled casting boss just above throttle plate area. There was already a small hole machined into venturi. I measured carefully and slowly drilled boss on carb til it met the machined opening.
Opened hole to about 3/16",used a piece of brake line tubing to insert into opening,hooked vac. advance and set timing for smoother operation,better torque curve and jump in fuel economy to about 26 mpg,compared to the 13 mpg average previously. Part throttle driveabiity was most noticeable aspect. Possible to post pics of your spanish weber with ported vacuum nipple to see if op could compare to his carbs to see if an untapped boss possibly on his?
Bumping this thread as I found it to be some great info.
I'm running a RB Dellorto Dhla on my street port 12a and have so far been running with vacuum advance disconnected. Running stock FB distributor.
I ordered the proper vacuum nipple adaptors for my carb and will try it out soon.
I have read that the large two bbl carbs can have a weaker vacuum signal which would make sense, at least compared to what a 4bbl Nikki can generate.
I'll post back here with how it works out. I might run a gauge to the cabin temporarily so I can see what the vacuum is at various loads.
Lots of varying opinions on this forum with vacuum advance. The piston engine guys swear by it for a proper streetcar.
You're looking at 2 different vacuum signals.
Large 2 bbl carbs have a lower venturi vacuum, this is the vacuum generated at the narrowest part of the venturi, where the fuel is sucked from the ports. The vacuum there is dependent on the size and shape of the venturi, and the velocity of the air passing through it.
Ported vacuum is engine vacuum drawn from just above the butterflies of the carburetor. Essentially it is the same as manifold vacuum at all throttle positions except for 0% throttle (where the butterflies block it from seeing vacuum). You may get a lower vacuum signal than stock because of the street port, but it shouldn't be significantly effected by the carburetor you have.
The vacuum source I am describing is the factory location on my carb. It seems like all the various versions of DHLA and presume DCOE or IDA may or may not have these taps. See photo...
I have had my carb apart and did not trace the vacuum port signal location at that time.
Here is an image I found on Lotus forums that describes the carb perfectly. There is a post on these forums where someone places NPT nipples threaded into the progression hole covers in an erroneous attempt to add vacuum adapters...!
My vacuum adapters are taking a while to arrive but once they do I will add to this thread the vacuum signal I get on my ported 12a!
What is interesting, though, is that at least on my '81, the vacuum advance gets its vacuum source from MANIFOLD vacuum. And the emissions computer controls a solenoid via TPS position. So it is kind of like an electronically controlled ported vacuum.
Which is kind of curious because the Nikki had ported vacuum sources.... Buuuuuut anyway
My plan when I get around to putting the Dell on my 12A is to fudge a TPS setup that makes the emissions computer happy, so it controls the vacuum advance solenoid, because that looks way easier than modifying the carb to have a ported vacuum source.
This might only work with a Lake Cities style intake manifold, since the wraparound type requires that you remove the solenoid rack.
I got hung up on how to modify the carb to accept the evap/engine breather hose. I keep thinking to just put a fitting in the bowl cover, but not sure how good/bad an idea that is.
Dellorto dhla,s do have ports for vacuum advance. Just above the throttle plates. This is a ported vacuum. It will greatly improve drivability. Easier starts and nearly eliminates the surge at cruise. Find the nipples with the smaller opening 0.8mm.
Dellorto dhla,s do have ports for vacuum advance. Just above the throttle plates. This is a ported vacuum. It will greatly improve drivability. Easier starts and nearly eliminates the surge at cruise. Find the nipples with the smaller opening 0.8mm.
Thanks LW.. a trusted Dellorto user!!
That is what my research as led met to... the DHLA-48, at least in the factory Racing Beat modified version I have, does have vacuum ports. Half of the posts say it is ported vac, the other half say it isn't.. is manifold vac... and won't work.
I ordered the nipples from dellorto.co.uk and they have not yet arrived. Once they do I will post real data here!
Peejay, the blank slotted screws, just inboard of your mixture screws and adjacent to your progression hole covers.. should be the vacuum source!
Maybe I need to go hunt down some factory Lotus or Alfa Romeo documentation to figure more out.
Last edited by tommyeflight89; 04-16-23 at 03:49 PM.
Got a stock port 12a w/ a Street Warrior 600cfm, RB intake, full exhaust. I know the carb is big, I know, but I'm working with it as best as possible.
Recently replaced everything on the ignition system from coil to plug (igniters included, woohoo!) and after that noticed that the vacuum advance was disconnected and one of the vacuum ports on the carb was plugged.
I connected everything again, reset timing to stock specifications (both leading and trailing) and noticed that my car has a lumpy idle and, from what I can tell, the exhaust is turning a light brown hue suggesting it may be running lean and heating up the exhaust.
I question this setup not because of it's pros, which are a REALLY good low end response out of the car while still pulling through the rev range, way better than before but because of it's erratic idle, weird fuel consumption and exhaust discoloration.
I have a FPR, gauge, and AEM wideband that should get here momentarily to help me fine tune the carb as best as possible.
Thoughts on the current setup as to w/ or with out???? I'm wondering if I should let go of the vac operated advance and run the car as is?
Update on my side, for the Dellorto and vacuum gauge going!
Background on setup:
-Streetport (large?) 12A
-RB Full Streetport Exhaust
-RB DHLA48 carb
- I think I'm, running an 82 distributor but not certain.
I hooked both L+T vacuum advance up to the DHLA ports and tee'd in a vacuum gauge.
Once i got the car warmed up I noticed a very big difference in the idle I had setup (without vac advance)
I was able to lean out the idle screws and also reduce the butterfly opening quite a bit. The sound is different and seems to be smoother with the advance.
-->Vacuum reading at idle, approx 13inHG or 330mmHg.
This leads me to believe that the Dellorto ports are not ported vacuum. I have advanced timing at idle, based on distributor curve( they seem to vary by year and I can't find all of them) and verified by timing light :
--> Leading +4deg or nearly maximum vacuum advance
--> Trailing +15deg or nearly maximum vacuum advance
I have yet to actually go and drive this setup. When revving the engine by hand I can see vacuum approaching zero and as I let go of the throttle plate it spikes to ~20inHg.
Question as I am still trying to wrap my head around the vacuum and centrifugal advance. If I go and try this on the street, is there a a chance I can be running too much advance under load? Do the advance values add or come to some sort of limit?!
The idle and off throttle response seem better so I think its worth taking for a drive. I might run the gauge into the cabin or tape to windshield so I can see vacuum at cruise and WOT...
Another thought, the nipples that the OMP feed, what purpose did they have in the non rotary application?!
Your description is correct. Vacuum at idle and part/light throttle applications. The centrifugal takes it from there. Vacuum advance helps starts from a srop and smoother part throttle applications only. Maybe better MPG, but I haven't noticed this on my car.
The idle and off throttle response seem better so I think its worth taking for a drive. I might run the gauge into the cabin or tape to windshield so I can see vacuum at cruise and WOT...
do that, i've got an old boost gauge i use for stuff like this
distributor should have the part number on it, i'm lazy to look but 81-82 should be like 1480/1481/1482, or maybe N201/N202. GSL-SE is N304. 84-85 should be something like N249
there is a pad with the Mitsubishi number, and the Mazda number is above it
- I plumbed the vacuum gauge temporarily into the cabin
- Got the car warmed up, the advance L+T timing at idle has certainly had me change the mixture+butterfly screw.
- Still playing with the idle. I have a decent idle setup at 1k RPM with a pretty nice streetport brap brap sound. The wideband AFR reads off the charts lean or occasionally pulses to 16:1. I think the sensor response time can't pick up the pulses.
- The difference in off idle response and the behavior of the engine while cruising is huge. Seems much snappier on throttle application and smoother while cruising. Holding second gear with low load at 4k RPM the engine seems happier and sounds better.
- I am trying to wrap my head around a AFR change I think has happened. My wideband gauge is showing around 1 while point richer while cruising. Into the high 12's or low 13's. Wondering if the addition of advance has caused this? Might try to lower float height or go down one on the Idle jets. I think i have 82's in there now.
The DHLA vacuum ports are not ported vacuum and seem like manifold vacuum (?). Cruising and part throttle vacuum is 300mmHg or so. When I let off the throttle on decel the vacuum maximizes to much more than this, 400mmHg +. WOT quickly goes to ~atmospheric pressure.
- I am trying to wrap my head around a AFR change I think has happened. My wideband gauge is showing around 1 while point richer while cruising. Into the high 12's or low 13's. Wondering if the addition of advance has caused this? Might try to lower float height or go down one on the Idle jets. I think i have 82's in there now.
I'm fighting the same thing. Running 14-15 sporadically at idle. Glad to know you have 82's in it, it'll give me a good reference point.
05-21-17, 02:54 PM
