right I set the total timing today, made it 35 degrees, which then gave me a base timing of 25 degrees, then I checked with vac can connected again, seems to be about 45 degrees total, is that ok my base being so high, any reasons for it??

my vac can has this number on it "62610", not sure what will give me??

adjusted the carb today, float levels good, sorted out the highest vac with the idle mixture screws, which gave me about 13 hg, the screws were set at 1 and 1/2 turns out at that point, no vac leaks.

it still seems to stumble/sometimes backfire up into carb, It used to do this before, since then Ive changed the intake manifold and carb, new spark plugs, put some blue springs into dizzy, which brings all timing by about 3000rpm.

only thing Ive not changed yet is the dizzy leads, maybe a problem there?

could there be anything else deeper in engine, that could cause my problems??

Heads and cam - unknown
intake - rpm air gap
Carb - holley 4160 vac sec, 750cfm

cheers

dave

 

Backfire back thru the carb is from Ignition occuring before the intake valve closes. I'd say you have too much timing. I cant believe its not pinging with that much advance

 

but thats the thing, if go back down to 8 base timing, it still backfire's up through carb, or any where inbetween, it still does it.

but it's set to 35 max for total, so cant do any harm??

dave

 

Something wrong here. With 35 total, shouldn't your base should fall somewhere around 14ish....Are your advance springs not bringing the advance weights back?

 

they seem to be working fine

 

I found this old message on a thread, so looks like it shouldn,t be a problem??

the rpm was at about 750/800rpm when I checked the initial timing, it said on my springs info, that it starts bringing in adv at 800rpm - 3200rpm, so maybe there was a small amount of adv coming in at that point??

my low vac has alot do with I think.

cheers

dave

 

pop sounds like a tight rocker

or

worse, a bad sealing valve

 

can both these probs be fixed with new heads and valves?

 

Popping up through the carb on initial acceleration is a sign of a lean condition. Make sure you have a solid pump shot and that the carb is set up rich enough for the mods you've made. Don't be afraid to err on the rich side - it's better than running lean.

 

hopefully lars is right. richen it up(set up pump shot )

 

how much by would you suggest?, 1/2 a turn out or more??

so would you say that my 25 initial is ok?, is possible to only 10 degrees for my adv curve?

what cause this kind of thing?, cams or heads?

and what exactly is the pump shot??(is this done via the idle mixture screws?)

cheers

dave

 

The pump shot they're talking about is the accelerator pump. Make sure that the accelerator pump starts feeding a shot of fuel the instant that you move the throttle. With the engine off look down the carburetor and make sure you see the shot as soon as you touch the throttle. If not adjust it until it does.
If the engine does not get that richer mixture BEFORE it starts to accelerate it will stumble and pop through the carb.

 

thanks for that info

cheers

dave

 

Im just trying to get one thing sorted, when your doing total timing, 36 degrees, etc, do you do it with the vac can unplugged from a vac source or plugged into vac??, some peeps in the Uk, say do it with vac connected, if I do it connected I get 36 degrees total which leads to 12 degrees initial timing, with vac unpluugged from vac can for initial timing check

cheers

dave

 

You do it with the vacuum hose unplugged. 36 degrees total timing is with no vacuum advance. With the vacuum advance hooked up, the total combined maximum timing will come out to be about 50 to 54 degrees.

 

Thanks

dave

 

Sounds like your accellerator pump needs adjusting. 35 deg. total timing is a plenty. More than that is a waste.

 

25 base sounds way too high to me. I run 18 and I was told that that is high. My understanding is that more timing requires more fuel. If this is a Holley style carb, the idle mixture screws need to be set wherever they cause the motor to make the most vacuum at idle, then set the base timing wherever it needs to be to get your motor to idle properly. Make sure the engine is up to operating temp when you do this.

If this is a Holley or Demon style carb, you need larger squirter nozzles. Bump it up 5 sizes. I just fixed this problem on my Mighty Demon 750. I went from 31s to 35s, and the stumble/ lean spit is almost gone. Should have gone with 37s or larger.

And of course, make sure that the acc pump arm is adjusted correctly, the slightest movement should push fuel through the squirter nozzle.

If all of this sounds like French to you, you need to call your carb company and speak with tech support, they will tell you how to properly setup the carb. Odds are, you'll probably have to change every fixture in your carb to get it to run nice. You need to make sure you have the right number PV, and if you're lean spitting at idle odds are you need not only larger squirter nozzles but larger jets too.

 

Check your spark plug routing. You may have one swapped on the wrong plug or terminal.

Don't ask me how I know this.

 

Thanks for all the info guys, check some things over at the weekend

cheers

dave