Paging Lars or any Ignition guru

Entropy

Entropy

Active member
Joined
Dec 30, 2008
Messages
27
Location
IA
I had my distributor tuned by Lars about 4 years ago, but since then I've changed exhaust and the gas is different in the area I'm in currently. I've noticed on initial throttle tip-in I'm getting some slight knock. More aggressive application seems to cause it to disappear(either that or my exhaust is too loud for me to hear it). I'm thinking that the springs I'm using in my distributor are too "soft" and I should run something a little stiffer to keep the timing from coming in so quick.

Am I thinking correctly?

Base timing is 12 BTDC. I'm thinking (although I haven't checked in a bit) that the 36 comes in around 2800.
 
I'll defer this to Lars, as he keeps records of all his rebuilds. BUT, I would keep the curve the same and reduce your mech total to 34*.;)
 
big2bird said:
I'll defer this to Lars, as he keeps records of all his rebuilds. BUT, I would keep the curve the same and reduce your mech total to 34*.;)
Click to expand...
I suppose changing the curve would affect throttle response. I really should have paid more attention when he taught this stuff.
 
Entropy said:
big2bird said:
I'll defer this to Lars, as he keeps records of all his rebuilds. BUT, I would keep the curve the same and reduce your mech total to 34*.;)
Click to expand...
I suppose changing the curve would affect throttle response. I really should have paid more attention when he taught this stuff.
Click to expand...

I just know some basics, but I agree with bird....relax the initial by two degrees and test it....

I hate modern 'gasoline' also....

gimme Sunoco 260.....fyne tipe jiggle juice....

:shocking::1st:
 
I'm assuming the exhaust change was for the better. A lower restriction exhaust system will usually have two (if not more) immediate changes to the combustion process. The lower restriction causes a change to the carb vacuum signal (at intake opening) so that the carb usually ends up leaning out the mixture a touch. This obviously increases the likelihood of detonation. The lower restriction exhaust also reduces the amount of residual exhaust gas left in the cylinder. I won't bother explaining what residual EGR does to the combustion process ('cuz I'm just a EE, and I believe you're way ahead of me in that chemistry stuff), but the reduction of EGR also increases the likelihood of detonation. So, you could be a touch lean, or just a bit overadvanced with the present setup you have.
Just my thoughts. (Go Hawkeyes!)
 
69427 said:
I'm assuming the exhaust change was for the better. A lower restriction exhaust system will usually have two (if not more) immediate changes to the combustion process. The lower restriction causes a change to the carb vacuum signal (at intake opening) so that the carb usually ends up leaning out the mixture a touch. This obviously increases the likelihood of detonation. The lower restriction exhaust also reduces the amount of residual exhaust gas left in the cylinder. I won't bother explaining what residual EGR does to the combustion process ('cuz I'm just a EE, and I believe you're way ahead of me in that chemistry stuff), but the reduction of EGR also increases the likelihood of detonation. So, you could be a touch lean, or just a bit overadvanced with the present setup you have.
Just my thoughts. (Go Hawkeyes!)
Click to expand...
It was a fairly substantial improvement. I went from ramhorns and 2" to Hooker Super comps and 2.5". The gas in the area was around 93 octane, and I didn't have any issues.

Now I live in N. Iowa and 91 is the best I can find.

I think my mixture is okay, because the color on my plugs is the nice tan. I'm thinking it's a timing issue (that and Lars states that 90% of running issues are timing issues:lol:)

I can only hope my cyclones find redemption this year with a new coach.
 
Last edited:
If you are getting slight detonation upon light throttle tip-in only after having changed the exhaust, you are running lean. Richen the carb up 2 jet sizes and see if the problem persists. If it does, and it only occurs at modest acceleration (not at WOT), you can shorten your vacuum advance curve by 5-6 degrees by installing a limit bushing on the vacuum advance rod or by welding a little bit of material into the end of the slot in the vacuum advance. This will retain your throttle response and peak power by not altering the 36 degrees total, yet it will reduce and limit the amount of timing you're running at light throttle when the problem occurs.

If I built the distributor for you, you can return it at any time for additional tuning at no cost other than return shipping. If you detonate after richening up the carb, send the distrubutor back and I will custom tailor the curve a little for you.

Lars
 
lars said:
If you are getting slight detonation upon light throttle tip-in only after having changed the exhaust, you are running lean. Richen the carb up 2 jet sizes and see if the problem persists. If it does, and it only occurs at modest acceleration (not at WOT), you can shorten your vacuum advance curve by 5-6 degrees by installing a limit bushing on the vacuum advance rod or by welding a little bit of material into the end of the slot in the vacuum advance. This will retain your throttle response and peak power by not altering the 36 degrees total, yet it will reduce and limit the amount of timing you're running at light throttle when the problem occurs.

If I built the distributor for you, you can return it at any time for additional tuning at no cost other than return shipping. If you detonate after richening up the carb, send the distrubutor back and I will custom tailor the curve a little for you.

Lars
Click to expand...
Thanks Lars.

You tuned it at TFB Chicago.
TFBinChicago.jpg
I'm in the green polo.

I drove it again yesterday, and it's only happening when the car is all warmed up. When it's cold, it doesn't happen. I think it was also a factor of old gas from last fall that had STABIL in it. I'm driving it again today so I'll keep an eye on it. I filled up with some better gas and it almost went completely away.

Now I have to get my hands on some jets and remember how to take the top off my carb. :lol:
 
Since I did both the carb and distruibutor for you, both items are eligible for return and re-tune & test at no cost other than shipping.
Lars
 
You must log in or register to reply here.
Top