I am considering going from vacuum to mechanical control on a Nikki carb. My tuner says it is a disadvantage on a street car giving flat spots at low revs.

What are the advantages/disadvantages of going mechanical linkage? What is the best for a vehicle used mainly on the open highway? I normally drive in the 3,000-5,000 rpm range.

Thanks

 

The disadvantage should be obvious , Paul. Instead of the engine determining when it needs the secondaries tipped in based on vacuum signal, you determine when the secondaries will come in with your foot. How do you know when you have sufficient vacuum signal from your engine to effectively use the extra air and fuel? Most of the times, you won't and bbaawwgg is what you get plus a marked decrease in fuel efficiency...In a race vehicle that is always hovering well past idle, mechanicals are great. If you're idling on the street and you want crisp, precise acceleration, they're not...

 

From what I've heard from people who have done this, Mar3 is exactly right. From what you said, going to mechanical secondaries sounds like a bad idea for you. Best performance for you will probably be achieved by leaving things just how they are.

 

Holley makes a spring kit for theirs. Anyone know if they work in the Nikki ??

 

After I switched my car over to mechanical, I noticed a HUGE difference. Way more throttle response. I think it may make a difference that I have headers, etc.. but I get no bogging or anything.

 

Just out of curiousity, do you have before and after dyno stuff for us to see? Butt dynos don't count!

 

Just remove the spring, no need to go mechanical.

It all depends on your setup though... sometimes it'll work great, other times it'll bog if you whomp it below 4000.

 

It's just a matter of learning how to drive with it. Sure it will bog if you floor quickly it at really low revs but you have to kind of ease the throttle on. I definately prefer mechanical myself.

 

Paul, If you look in the archives, I have a write up on precisely this topic, as well as an accelerator pump modification that will help with any flat spot.

REVHED is correct. You will simply need to relearn how to drive your rotary car. Simple as that. However, that being said, if you spend most of your time in the 3K-5K RPM band on the highway, then you're not experiencing the full potential of your engine as it is.
To convert the absolute stock set-up to mechanical may not yeild great results. Fuel mileage will surely suffer. And you won't gain the full benifits of the conversion without headers, more fuel volume, and larger secondary jets and airbleeds.

As far as being able to tell what your engine needs/wants through your foot by controlling the secondaries, that's not a problem. You just learn where the threshold is. You never stomp the pedal...You can stomp it half way to the point where the seconds are beginning to open, and then you ease into it, feeding it as much as it will take.

When all is set up correctly, it feels really good on the ole "***-o-meter"! And when you keep pushing the pedal down just fast enough so that it's on the cusp of feeding the engine the max at which it can accelerate, it just seems like a bottomless pit-o-power!!!

Tuning the accelerator pump is KEY!

 

I used to floor it at 2000 and it wouldn't bog

I still see no way to adjust the accelerator pump, by the way.

 

After reading Sterling's excellent archive I now understand the concept a little better. The vacuum approach is really like having an automatic, easy but not with optimal air/fuel settings .

I am uncertain why fuel consumption is thought to be higher with mechanicals, is it because drivers with modded cars have heavy feet? At the moment I get between 28-31 mpg while averaging 70 mph on a 780 mile run over 2 lane rural roads. I do the journey every two weeks so can get a good sample of the impact of tuning changes.

Having to keep relatively close to the legal limit, most of the time engine speed is typically 3,250rpm. However, the occasional traffic is mainly big rigs including road trains whose legal speed limit is only 6mph slower. What I am aiming for is great acceleration from 60 to 80mph. Thus I will fit a carby with mechanicals; out of interest it was on Revhed's car until he upgraded to a Webbere, so it must be good.

 

peejay- the threaded arm that sticks out at you beside the AP cover...It has a little nylon nut on it. Turn it in for a bigger shot, out for less. Only do 1/4 - 1/2 turn increments.


Paul, the mechanical seconds and the AP mod will tremendously help your 60-90 time!

3K is fine for cruising the highway, and if you could time your mechanicals correctly, you could have the best of both worlds. Unfortunately it would mean that they would have to come on slightly later relative to the primary shaft position, and there would be a bog that no silly AP mod could ever bridge when you floored it. Basically you would need another pair of barrels in there!
It would also require the fabbing of a different length linkage arm for the secondary shaft, blah blah blah.

But when you do the secondary conversion and ditch that ridiculous "back-up spring" that caused peejay so much trouble, then you'll be able to feel when the secondaries are opening thru the pedal!


I have a Yaw modded. I run big jets, and my mods. I get exactly 16 MPG.

 

hehehe... I can feel the econdaries opening... but my throttle sticks So I have to blip it to get it back down..... and .... since I took all that crap off the carb.... it wont idle..... and when I cruise it sounds like I have a miss... but under acceleration.. no miss..... I dunno.... but hell yeah... the mechanical secondaries are great... I have yet to do the AP mod.... but I soon will....


LATERZ, Andrew

 

Okay, looking at the carb, I see how that works. By altering the length of the threaded arm, you alter the geometry (simple trigonometry). By shortening the rod it causes the throttle shaft's crank to be closer to a right angle. That is somewhat analogous to changing the pump cam on a Holley. However it still does nothing for the size of the squirters. You could do more tuning with different size squirters, IMO.

 

Sterling. thanks for your expert advice. Your fuel consumption is way beyond my current experience. I have just checked my last months use and I averaged marginally over 9 lires per 100 km [Australian method of consumption] which equals 26 mpg using US gals. That is with minimum engine mods- good header and exhaust, K&N filter, removal of air pump. Most of my mods are to improve handling- shocks, springs, sway bars, brakes, wheels and tires which improve average speed on winding rural roads.

 

It's your highway miles that are getting your high MPG figures. When I took my '79 to El Paso which is a 650 mile trip, my MPG jumped from the 17's to the 25's. If you really are spending that much time in 3000+ RPM territory, then go with the conversion. You'll feel the diff for sure and since you won't use it often at RPMs below 2000, your MPG might not suffer that much...it's definitely the lead feet that kill the MPG efficiencies, not the conversion...

 

peejay-
The mod I discuss in the write-up is basically a way around changing nozzels. Since there are no replacement nozzels that I'm aware of, controlling the volume and distribution via duration is the next best thing. Perhaps it is necessary to make those changes anyway, because the nozzels are really quite large. If you went to smaller ones, you would have the duration (might need to change to a weaker rod spring), but you would'nt have sufficiant volume for the whole stroke.
The amount of fuel it (the pump) squirts is all set for those nozzels and is expelled just when the primaries open
There is no need for AP help when the secondaries are vacuum operated.
I don't think there's much to be gained by playing with nozzel size but not doing both other mods as well.
Now if you really wanted %100 tune-ability, then sure - you could go thru making different nozzels. But @ 300-400 something feet per second, the intake flow is sucking that fuel in fast!, So IMO spiffy nozzels are'nt gonna do much.

People change nozzels, cams and diaphragms in the Holley 4500 series to time the pump shot...Example, they can make it come on weak at first, then strong, or vice versa.
Very nice feature if you are totally plugged into your car like Johnny Nuemonic, and know absolutely every need for your racing event.
But it's NOT magic, and with some head scratching and a little linkage fiddeling, anyone can come up with a way to make our Nikki APs have the same feature.

Paul-
mar3s right...If you're running stock jetting, then the primaries will retain your MPG, and the secondaries will open the cans of beans you need when you stomp it. (The secondary jets are pretty big.)
You will need get a good sense of the pedal...where the seconds come on.
Doing the AP mod will help with that, as well.
Don't throw out anything. If you don't like it, then you can always put it all back the way it was. I stress that alot. (Don't want to get blamed for permanent changes to someones carb that made them unhappy!)

You guys have propted me to drive my car at under 5500 RPMs for a tank of fuel just to see what kind of good mileage I can get without swapping jets - something I would do if I were goin g on a trip.

...Nah. I would'nt!

 

Paul - wow, that's nice mileage. I drove my '79 to Canada last year on a 3500 mile road trip, and never got over 20.7~.8. Of course, that's with an SA tranny that put me in the 4200 RPM range on the interstate. I too have a stock carb (not mech. yet), headers, and K&N.

Sterling - I've got a '79 manifold (my car has all '85 stuff in it now) and am looking forward to modding it to fit and putting it on when I convert my carb to mechanical secondaries. Need a clutch first, though. But I'm excited to try the carb mods!

Armen

 

I think it would be easier to modify a Holley for tuneable air bleeds, than it would be to modify a Nikki for adjustable accelerator pumps Plus you can find Holley parts ANYWHERE.