Total timing done, small stummble/backfire
Thread Starter
Racer
Joined: Jul 2007
Posts: 256
Likes: 0
From: Glasgow, Scotland
Total timing done, small stummble/backfire
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
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
Thread Starter
Racer
Joined: Jul 2007
Posts: 256
Likes: 0
From: Glasgow, Scotland
but it's set to 35 max for total, so cant do any harm??
dave
Last edited by agent kronus; Oct 14, 2007 at 01:57 PM.
Thread Starter
Racer
Joined: Jul 2007
Posts: 256
Likes: 0
From: Glasgow, Scotland
Thread Starter
Racer
Joined: Jul 2007
Posts: 256
Likes: 0
From: Glasgow, Scotland
Your timing and setup method is correct - good job! The initial timing you end up with depends on the length of your advance curve and on whether or not you're getting a little bit of centrifugal advance coming in at your idle speed. It's not uncommon to have a 16-degree advance curve, although most are in the 20-24 degree range. Initial timing anywhere from 12 to 24 degrees will run fine as long at total is limited to 36 (or less, depending on the engine's needs). Good job with the setup - it should run well like that.
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
Last edited by agent kronus; Oct 14, 2007 at 02:27 PM.
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Tech Contributor
Joined: Aug 1999
Posts: 14,380
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From: At my Bar drinking and wrenching in Lafayette Colorado
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.
Thread Starter
Racer
Joined: Jul 2007
Posts: 256
Likes: 0
From: Glasgow, Scotland
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
Racer
Joined: May 2006
Posts: 264
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From: Fort Myers, Fl / St. Paul, Mn
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.
If the engine does not get that richer mixture BEFORE it starts to accelerate it will stumble and pop through the carb.
Thread Starter
Racer
Joined: Jul 2007
Posts: 256
Likes: 0
From: Glasgow, Scotland
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.
If the engine does not get that richer mixture BEFORE it starts to accelerate it will stumble and pop through the carb.
cheers
dave
Thread Starter
Racer
Joined: Jul 2007
Posts: 256
Likes: 0
From: Glasgow, Scotland
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
cheers
dave
Last edited by agent kronus; Oct 15, 2007 at 01:55 PM.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
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 12 degrees initial and 36 degrees total.
cheers
dave
cheers
dave
Thread Starter
Racer
Joined: Jul 2007
Posts: 256
Likes: 0
From: Glasgow, Scotland
Advanced
Joined: Sep 2006
Posts: 78
Likes: 0
From: Nashville NC
Sounds like your accellerator pump needs adjusting. 35 deg. total timing is a plenty. More than that is a waste.
Melting Slicks
Joined: Sep 2004
Posts: 2,412
Likes: 3
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.
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.
Last edited by enkeivette; Oct 16, 2007 at 06:43 AM.
