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

[BlownCorona]

As the title may suggest im after some help with my distributor and vacuum advance related stuff. 

 

my situation is,

 

toyota 18r 2000cc mild can regrind, bike carbs, raised compression and extractors. 

 

the engine ran well, however when i drove it 3km up the road cruising at 100, it seemed to drive fine. but when i got back i checked the engine (as i have after every drive while i tune and run this engine in) and this time it was night, i noticed the extractors glowing red hot at the collector (4-1) but not on the actual headers. 

 

now ive done a fair bit of reading up on this, and came to a possible conclusion that it was timing related, and possibly vac advance related. since i dont have any vacuum what so ever when cruising at 100 (about 3500 rpm) it also felt a bit weird. 

 

today i disconnected the vac advance hose, and the engine runs amazing, it delivers the power very smoothly and feels like its making more power, though it was wet and wheel spinning all over the place. 

 

now its my understanding, with all the stuff ive read, that at idle and other low throttle open situations, the mixture is overly lean, and produces a slower burn, and thats when vac advance is supposed to kick in and dial in ~20 more degrees so the max cyl pressure is still achieved just after TDC. but my engine makes no vac at 100kmh cruise and wont be dialing in extra advance, and this is probably causing the high egt, and slightly weird running. though i dont know what will happen with extended 100kmh cruise with vac advance disconnected (in theory it will still run hot etc? yeah)

 

now to my questions

 

how do i remedy this

and

is fuel economy as greatly impacted by vac advance as the internet makes out, as id really like to be able to drive this often and not worry too much about it drinking fuel on the open road

[Unclejake]

It depends on your budget. I'd be tempted to:

 

1. Mechanically lock the distributor
2. Blank off the vacuum advance
3. Fit an adjustable MSD retard module and a cheap programable timing curve device (MSD sell a few)

[tim13]

low buck option is ; blank off vacuum advance , set total advance at about 34 btdc (ie about 34 @ 3000rpm) and drive it like that , not sure on economy , but performance and drivability should be ok

[Beaver]

Just blank of vacuum advance. You will still get the mech advance at rpm. I had vac all disconnected when I was running bike carbs.

[BlownCorona]

any concerns with running the engine at around 18btdc before mech advance kicks in then? factory seting is 10btdc

[Testament]

its the amount of advance it tops out at higher rpm that matters. too much and you can blow things up.

[ajg193]

High static advance will make the engine harder to start.

[BlownCorona]

Testament - am I correct in thinking that if I adjust initial advance, added to mechanical advance, so that it totals the amount of advance it would have had, had back adv been working, I'll be safe?

And the car starts very easy so I should be all good on that front

[Beaver]

I would just ignore the vac advance as I said earlier. Set initial advance as per spec (10 degrees at idle or whatever) and let the mech advance take care of the rest.

 

Spencer will tell you vac advance is just for increased economy. Aint nobody got time for that.

[Spencer]

Yea if you are pulling no vacuum at cruise then vacuum advance is a waste for time for you. As others have said set the base timing at 10ish, mark up your pulley with degree markings, then when you rev it take note of when the timing is all in and how high it goes and work from there.

[tortron]

Perhaps you would like to peruse thus

//oldschool.co.nz/index.php?/topic/45631-modifying-distributors-to-your-modified-engine/

cheers ears

[BlownCorona]

that was the first thing i read, very usefull, albeit in depth! ill probably read it a few more times aswell haha

[yetchh]

If you don't care about economy as above I'd blank the vacuum hose and leave it.. not just economy, my xc also uses it to increase engine speed when hot.. 

[Testament]

Testament - am I correct in thinking that if I adjust initial advance, added to mechanical advance, so that it totals the amount of advance it would have had, had back adv been working, I'll be safe?

And the car starts very easy so I should be all good on that front

 

 

Yea if you are pulling no vacuum at cruise then vacuum advance is a waste for time for you. As others have said set the base timing at 10ish, mark up your pulley with degree markings, then when you rev it take note of when the timing is all in and how high it goes and work from there.

 

 

As Above but for clarity

 

• If you can, check Crank and block markings for TDC to ensure they are correct by using engine stop and pointer method (google how to do this)
• Disconnect and block off any vacuum advance
• Set advance at idle to 10deg BTDC, or whatever the factory setting is. you can adjust a couple of degrees more or less to get it to idle better if it doesn't want to idle at that advance but 8 is probably too much (or your makrings on the crank are wrong, so go back to step 1).
• with engine in neutral rev engine to a higher RPM (and hold it steady at that rpm) than the "all in" advance rpm for the mechanical advance weights and note the amount of advance
• adjust timing such that the "all in" advance is correct per  the manual/whatever max advance is correct for your engine setup
• Rev engine again to check you have got it on the money
• reconnect vacuum advance
• Doaskid

[BlownCorona]

Sounds pretty straight forward, except that I make no vacuum off idle, so vac advance is pretty much useless Yeah? Fuel economy would be nice, but I didn't put 4 carbs on an old car to save the planet

[Spencer]

Yes its useless, try tapping the runners together and see if you can get some more vac at cruise maybe?

[Rookie]

Dude if you tune your car good then you won't notice the fuel economy. Like it might be 1l/100km or something difference.

[BlownCorona]

ive got vac advance coming from ports on each of the carbs. so should  be as much as i can get, which is still 0. not too fussed over it, as long as i can tune it properly without it, which i can. 

[Rookie]

Good man.

[Spencer]

Yup you can't magic any vacuum back so its useless.