help with vac advance, fuel economy, EGT and all things related to distributors on modified, low vacuum engines

[Unclejake]

Yeah, do what Spencer says. The important bit is to know how much mechanical advance the dizzy gives you. It will all be in by 3,000RPM I expect. It's probably something like 22 degrees. The vacuum advance can be blocked off and forgotten about

 

The most useful thing I ever bought for stz like this was an adjustable timing light. They are da bomb (but not so good for MSD systems)

 

I assume you're not running a brake booster either then?

[Rookie]

Brake booster is only really a problem if you want to be braking while doorting, no?

[Valiant]

Have you fixed the manifold vacuum leaks you were having trouble with?

[BlownCorona]

Yup no manifold vac leaks, I ended up building a 2nd manifold with lessons I learned from the first. It idles with about as much vac as factory. And seems I'm still running a Booster which seems to behave, but as noted, I don't use it unless I'm off the gas, and if I do brake under throttle it won't be heaps.

I think my brake Booster does have a leak as when I press the brakes at idle, I can hear a quiet hiss and vac drops by around 5 and sometimes stalls the car out, but it purrs when brakes are off

[tim13]

22 degrees seems weak for a 4 cyl with carbs , cam , upped compression ? maybe advance till it pings or fights starter then remove 2-5 degrees . has always worked for me (often about 12-15 at idle) you could mess with spring and weights in your mech advance too

[Unclejake]

Nah, I meant the mechanical advance will be something like 22 degrees. 22, plus a static 10 = 32 degrees @ 3,000RPM (approx.)

 

The last jap dizzy I measured (I built a benchtop dizzy tester) increased from 0 (at 0RPM)  to 22 degrees (at 2,100RPM)

[Valiant]

Good work.

It seems a bit odd that you have no vacuum at cruise speed is all. I'd be curious to know why.

I wouldn't worry about your booster it's doing what it should, they use a bit of vacuum per brake application.

As people have said above disconnect your vacuum advance and set your total timing for around 34 deg.

[ta63-1uzze]

as above question : vacuum is created by the cylinders trying to draw air through the closed throttle body, since there is no connection to the 4 throttle bodies of each carb, there is no manifold vacuum.

this is made worse by the fact you have to wait a few strokes for the vacuum to be drawn again , so you will get vacuum but it will be pulses of it rather than a entire manifold worth. and it will be substantially less because of the volume in each runner verses the size of a factory plenum is somewhat smaller.

 

 

why has no one asked about the cams!!!     how hot are they? you can loose all vacuum by having hot cams , to much overlap . . .

 

 

 

as for bridging each port and making one vacuum signal line I am unsure of how you would go about this , even with the aid of one way valves you will end up with a very up and down vacuum signal, that is only going to make the advance indecisive . I cant think  of a simple way to get that idea to work . either way you will end up with continuous  vacuum or steady low vacuum, or extremely pulsated signal .

 

in my mind the only answer is do ditch the dizzy for timing control, and get a crank trigger, throttle position sensor, and some igniters and use some aftermarket programmable ignition module. and use the dizzy for just for the high tension circuit . that way the module will control the advance because it will see when your of the gas and advance accordingly

 

 

[ajg193]

Just put a vacuum pump on your motor, problem solved.

[AllTorque]

So you have your vacuum advance all in all the time?

or were you being funny?

[BlownCorona]

you can see the daisy chained vacuum line in this photo. 

 

but after seeing how it runs with it all blanked off, im of the opinion is doesn't work, and was putting the shits up the timing 

 

 

as for the cam, its not too hot, just something that rob/voldo put together to help open up the engine

basic specs are 

220degrees duration @0.050", 0.010" more lobe lift, ground on around 105degrees lobe separation (thats copypaste from what rob said via pm, actual spec may vary slightly, but i dont have the spec sheet with me)

[Valiant]

ta63-1uzze, The thread says earlier that the cam is mild and that it has just slightly less vacuum at idle than a stock one, suggesting again the cam is fairly mild.

That's a good point about the cruise manifold vacuum. I've always found that a mechanical gauge will give a reading at engine speeds above idle on side draught style manifolds.

Took too long writing that one.

[Spencer]

Yea lots of individual throttle plate setups still give decent vacuum at cruise, some don't. Its very engine/setup dependent.

[BlownCorona]

no vac seems to be pretty common on bike carbs

[Testament]

Sometimes using a small vessel in the vacuum line helps damp the pulses, some three daihatsus had these standard. But really just blank it and doort

[BlownCorona]

agreed, will need to research/test to find things like amount of vac advance, and just carefully set it all up. but shouldnt be too tricky! also im gonna biff one of those little digital units in that do away with points and stuff.

 

cheers for everyones help! 

 

might aswell leave this thread going cause im sure others have questions or similar issues

[BlownCorona]

situation same as previous in this thread, at about 3500rpm at cruise (100kmh) at roughly 1/8th throttle, i had issues with the exhaust glowing hot. i suspected it was lean, and the newly installed  wideband has confirmed this, as in this condition it can spike to around 16.5 maybe worse, but if i feather the throttle a bit, it comes back to 14.5ish. this makes me think its on a transition point, does anyone know what part of the carb i need to be looking at to fix this up? the internet is not being very helpful.

 

also i dont know how to edit the title, since it wasnt much related to vac advance and static timing (thought i did improve the engine greatly by adjusting that)

 

[downtrail]

You sure it's not too retarded causing your glowing issue and weird fuel readings

[kpr]

as above, sounds like timing is retarded.  definitely  isn't the fuel or lack of, that is making the exhaust heat up.    just under stoich is hottest burn, everywhere else the temps will drop off.    

 

Don't know how carbs work/your carbs.  but are you sure those ports you have your vac hoses connected to, are on the correct side of the throttles?     you should get at least some vac. 

[BlownCorona]

i was under the impression that a lean mixture burns hotter, and can cause exhuast to glow, its entirely possible that that the exhuast is just a peice of shit and its not flowing properly since its around the collector that it glows. 

 

i dont think the timing is the issue, vac advance is removed as it doesn't function properly with these carbs, and ive added slightly more initial timing to help compensate. i do get vacuum, at idle its as much vacuum as my manual states it should have from factory, maybe a touch less, but it dropps off alot quicker and at about 3500rpm theres non left, its just how these types of CV carbs work. 

 

currently the entire rev range is slightly too rich, except for that one very precise spot where it leans way out, and then another mm movement on the pedal returns it to slightly rich, 

 

i havnt tried yet, but i expect i can sort out the richness by dropping a needle clip position and a half turn on the idle air adjust screw. but this will possible also make the lean spot worse.