Adding a Vacuum Manifold
I'm considering an aftermarket programmable ignition module. I'm wanting a timing increase above 3.5k (where stock timing no longer increases). More importantly, I want to add a vacuum advance to improve performance, throttle response and MPG below WOT. This will require adding a MAP sensor.
I was thinking of running a hose from both carb vacuum ports to a manifold, then running from the manifold to:
The air bypass system (yes I left it in after the ear shave)
and the MAP sensor.
Will this sort of cross-connection cause some issues with the carbs?
The rational is this would give a more steady signal to the MAP sensor, since it's reading both carbs. I would also expect the petcock to open sooner on start up, maybe giving a quicker start to fuel flow. In any case, I plan to only use the MAP sensor from 3K-8.5K, to avoid the pulsing vacuum at lower RPMs. I would simply program in stock tuning from idle to 3K at all MAP sensor readings.
It sounds like there's little to gain below 3.5k without risking knock. The same linked thread indicates that there's some gain above 5k though. It looks like dariv is now banned, so I never found out the results of his re-tune. But his last post showed 37 at 5.5k instead of the stock 25! He seemed to be active for another 2-3 years after that, so I'm guessing he didn't blow his engine!
Experiments can always be fun, but it seems like possibly quite an expense (for the aftermarket ignition) and a lot exercise for potentially very little if any gain, with the risk of catastrophic failure. Ignition timing can be really tricky. Sometimes there's no warning of going too far before pistons are turned into ashtrays. I only say this because I've done exactly that, and it only took about 60 seconds, granted the engine was one that's ultra sensitive to timing advance.
This engine is possibly right near the edge with ignition timing, since it can use regular gas with a 10.5:1 compression ratio. Lean mixture and a lot of timing combined will put you right into the ashtray business, so plan on re-jetting, it's going to need more fuel and octane to cool off the combustion.
Don't believe there would be any interference by having the carb vacuum lines linked together. The vacuum there is only made available to operate other components, independent of carb function.
Curious about what you did with the air system hose that ran into the airbox?
If you dive in, maybe you can also find out what the mystery wire to the CDI does.
Always wondered why that poster was banned, think it happened the day after I registered.
"We can offer programmable unit TCIP4 standard (129 USD) with connector adapter (5 USD). This is plug and play connection. You can to add pressure sensor with this unit. We can offer pressure sensor 82051 (16 USD). Unit will be pre-programmed for work on your bike. Recommended accessories is USB/COM cable for connection with PC (12 USD). Freight DHL is 36 USD."
$200 all in.
From what I understand, partial load causes partial filling of the cylinder, which burns slower than a full cylinder.
Adding vacuum advance compensates for that.
Since it's less than WOT, I don't need to be crazy with it, as I'm not trying to push max HP.
Just trying to get a bit more out of each spark.
Since it's dual plug, this engine already doesn't need a lot of advance.
Tune Your Vacuum Advance for Better Drivability
I plan to ride at 60, 70 and 80 and note the RPM and vacuum at those speeds.
Then I was going to put it on a dyno at those RPM and vacuums, and bump up the timing 1 degree at a time until torque no longer increases (or until I hear bad noises).
Then I'll back off 1-2 degrees.
For WOT tuning, I'd similarly bump timing.
Lastly, I want to retard the timing at high vacuum/high rpm, to see if I can spit fire on downshifts. :D
This unit can hold two different timing maps, switchable on the fly.
I plan to keep the factory timing on one map, so I can switch over with bad gas or if I notice issues while riding.
The original crankcase hose goes to one on the left side of the bike.
The air system hose goes to one on the right side of the bike.
Good question. After you first mentioned it, I wanted to connect a variable power supply to it to see if it affects ignition timing.
I never got around to it.
I gotta get my bike in top shape before I mess with the ignition.
The TCIP4 has a starting mode that requires a minimum amount of rotations for starting.
But given how many cranks it takes to get fuel flowing again (especially when the carb is dry), that might not be enough.
Mystery wire is the red/white branching (mid harness) off the black that runs from the start button. Shows as 4th pin from the left on the CDI in the diagram. edit: Have wondered if varying voltage on the red/white would change timing, and if it retards or advances.
All black/yellow are grounds. So don't apply any power there.
Not as pricey as I thought it could be. Does this replace the CDI or piggyback onto it? If it replaces, could be the way to go for a fried CDI.
edit: :D If you want to make a big cannon shot, plumb in a valve on a vacuum line, open the valve then go into a long decel. Not responsible for muffler damage! :)
If you want the decel pop, disable the coast enrichers. Might help MPG too?
just a side note about crank time needed to start the 750... one in good running condition with a good tune on the carbs will fire within 2-5 revolutions of the crank..
also, I may be wrong here, but the vacuum ports on the sides of the carbs may be venturi vacuum, not vacuum on the downstream side of the throttle plate
Venturi vacuum should remain near constant and wouldn't be useful to sense engine load. Otherwise, the petcock would close when you hit full throttle.
You would need to tap into the carb boot between the carb and the head in order to use a MAP for the ignition control.
I wouldn't mess with the timing advance. There is a good chance of seriously damaging your engine.
Well, that changes everything! I might have to rethink this plan...
But wait, doesn't the air bypass system use downstream vacuum to enrich the air on closed throttle?
Also, when synching the carbs, the instructions are to use these vacuum ports.
If it is venturi vacuum, this might still be usable.
I read that "the venturi vacuum is approximately proportional to the total mass flow through the engine". If this is the case, I can use a MAP sensor as a mass flow sensor.
It would just work inversely, with more vacuum equaling more load.
It could still be used to create an ignition timing profile.
This sort of cross talk between the two carbs could cause some issues with running though.
Two of my Vulcan 750s were bought new. Lots of money. Couldn't afford to risk damage. Last one cost $2700 used. And I spent over $700 on it after buying it getting it in tip top shape. A really good condition VN750 engine is valuable. Probably more than most Japanese motorcycle engines. Increase the ignition advance, and you'll likely wind up with detonation and holes in the pistons. But it's your engine.
This is my first bike, and I have yet to find another bike that I enjoy more all around (though the Triumph Legend TT is more fun, but not as comfortable living with). I don't want to replace it. After all the time and money I'm investing in this bike, I have no plans to risk damage! The bike may be worth $1000 or so on the market, but it's worth a whole lot more to me!
I understand the risks, and I appreciate the concerns. I plan to proceed with great caution.
The more I think about it, the more I think the vacuum ports are manifold rather than venturi.
It's a bit foggy, but during synching, I recall vacuum around 15 inHg at idle, that would fall when cracking the throttle.
This seems way to high to be venturi vacuum at idle.
The air bypass system is designed to open with vacuum (correct?), which would be strongest at throttle closed.
The external location of the port looks like it is downstream of the throttle plate.
I can verify this in person, when I get home and can look closely at the carbs.
Additionally, it sounds like a CV type carb always has a small amount of vacuum, even at WOT, due to the restriction of the butterfly itself being in the air stream.
This might be enough to keep the petcock open at WOT?!?
That's the only part I'm not sure of.
I hope in the process, that many can learn from my experiments.
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