help quick! vacuum problems!

[ZeroGravity58]

ok guys i have my 350 installed and running but i have run into some problems with the timing. first off heres the info on my motor. 75 350 with slight head porting. custom cam with 220 duration, .460 lift, and 110 degree lobe seperation. edelbrock intake with a holley 670 street avanger carb and a inch spacer. Here is my problem. the distributor is a vacuum advanced distributor, when the motor idles everything is perfectly fine and the timing is around 12 degrees advanced with the timing light on it. when i hit the gas alittle the timing jumps back to 4 degrees before advancing up. when the timing jumps back the motor is between 1500-2000 RPM\'s and it backfires through the carb. My uncle and I have figured that when i hit the gas i lose all vacuum and it retards the timing then as it builds vacuum back up it starts advancing the timing. The motor runs great above 2000 RPM\'s with no problems. When i set the timing i disconnected the vacuum advance like you are supposed to. Should i go out and buy a vacuum canister or should i buy a mechanical advance distributor or does anyone have any other ideas? thank you, Jonathan

[spacecadet]

Where you do have the distrubtors vac line hooked to. I believe it is suposed to be hooked to a small vac port in the middle of the carbs body.

The lines on the bottom are full vac all the time. The ones in the middle are timed port vac. Hook a vac gauge to the port your using. See if it is actully operating correctly. - At idle it should have a little vac, but once you twist the trottle the vac reading should jump up.

Other thoughts/ideas:

Also did you check for leaks. Is it posible that if there is a leak than it could hide the vac siginal from that port.

Maybe clean that port - spray some carb cleaner in to it, maybe it has a blockage.

[mzoomora]

Most motors will not have vacuum at full throttle. You should be setting you base timing without your vac advance hooked up. Try using 8-10 degrees without the vacuum advance, then hook it up, it should be a noticeable improvement. I like to hook up the advance to a full vacuum port, some people like timed ports, its mostly preference. The only difference would be at idle and very little throttle. If you are going to use a timed port give it more base timing.

About the losing timing- when you lose vacuum it should only go back to your base setting, which is why it sounded to me like you were setting it with the vacuum hooked up, after rereading it I see you werent. The full vac ports are at the very bottom of the carb, timed ports are further up, just in case.

[ZeroGravity58]

i set the timing without the vacuum advanced hooked up. the base timing is set at 12 degrees. and with the vac* advance it was up around 14 or 15. when you open it up slowly everything is fine but as soon as you stomp it to rev it up the timing drops to 4 degrees and it backfires through the carb. i tried both ports with the same results. the truck also does not have any exhaust on it right now except the headers. could something be wrong with the distributor? or should i buy mechanical advanced one? i thought about buying a mechanical advanced distributor anyways.

[ZeroGravity58]

after just messing with it i was wrong. when get into the trottle slightly the timing advances alittle. when you get on the trottle quickly the timing does not go up but stays set at 12 degrees which causes the popping thtough the carb. after 2000 RPMs the motor pulls strong and runs well. i was driving it around abit today. as soon as you step on the gas and the RPM\'s jump up into the 1500 RPM range the motor has a bad hesitation and has no power. all it wants to do is stall. after you get past that into the upper RPM\'s it runs excellent. Should i go and get exhaust put on it, buy a mechanical advanced distributor or what? i tried a vacuum canister and that did not work. it seems like when i hit the gas quick the timing doesnt go up until the mechanical advance grabs. thanks for all the help guys becuace im really stumped.

[Scotty_S-15]

What happens to the timing when you slowly bring up the revs? When you set the initial advance without the vacuum, is the engine idling as low as it can go? That\'s the best way. You can leave the vacuum disconnected until you get this sorted out. You only need the vacuum for economy and maybe helps a little with driveability, but for performance, it\'s not needed.

......... What you are saying, that the spark actually retards, is darned near impossible. Some might say a sloppy timing chain, etc. could do that, but I doubt that, and you\'ve probably got a new one anyway.

........... So that leaves the harmonic balancer. Maybe the outer ring is floating around some. I\'ve never seen that, only seen them move and stay, or fall clean off like mine did once! That might explain why the engine is running crappy, because you don\'t even know for sure where TDC is.

[ZeroGravity58]

the problem me and my uncle have come up with is that the distributor is losing its vacuum and is not advancing. I dont have a vacuum guage but im going to get one tomorrow.

[Scotty_S-15]

Geez, I was writing my reply, and Zero\'s writing too, beats me, and makes my info irrelevant! http://www.s10v8.com/images/smilies/icon_eek.gif http://www.s10v8.com/images/smilies/icon_biggrin.gif Well, somewhat anyway.

.............. It still wouldn\'t be a bad idea to check TDC to make sure it\'s OK. But for sure I\'d take a look at the advance mechanism. Does it all work OK, not rusty and gunked up? Again, I\'d leave the vacuum disconnected until I got this thing working OK. Another thing I might try (temporarily) is to advance it to like, 20 degrees initial or so and take it for a spin.

.............. You might have a bad acelerator pump, or maybe the power valve is stuck shut. (don\'t think I\'ve ever seen THAT though)

[ZeroGravity58]

we double checked the balancer and it is right. when you jump in the gas wuick it doesnt lose timing. the timing just doesnt advance and the motor backfires through the carb. if i get into the trottle slow the timing goes out. im thinking i have a vacuum problem.

[ZeroGravity58]

both the distributor and carb are brand new.

[Scotty_S-15]

On 2004-08-17 21:03, ZeroGravity58 wrote:

the problem me and my uncle have come up with is that the distributor is losing its vacuum and is not advancing. I dont have a vacuum guage but im going to get one tomorrow.



That is NOT the problem. When you are accelerating with WOT (wide open throtte) the is NO vacuum, and no vacuum advance. Like I said earlier, the vacuum advance is mainly for economy and drivability in the cruising mode.

........ But the vacuum gauge isn\'t a bad idea. Hook it directly to manifold vacuum, and watch it while you drive, you\'ll see what I\'m talking about.

[ZeroGravity58]

i knwo i will lose vacuum. but why would it backfire through the carb?

[mzoomora]

Give it 14 degrees initial and check you accelerator pump setting. What carb are you using? Try stepping up the accelerator pump. You may just be losing carb signal from the quick stab and the accelerator pump isnt giving enough fuel.

[gdmclnh]

I tend to agree with mzoomora, the backfire is probably the carb allowing a lean condition when the throttle is opened quickly and the accelerator pump isn\'t putting out enough. The rpms start to come up, the distributor starts to advance the timing but there isn\'t enough fuel to satisfy the engine and it backfires through the intake because the timing is somewhat advanced. It is not plausible for the timing to retard below what you set it at with the vacuum advance disconnected.

[ZeroGravity58]

it doesnt retard below what i set it at. but it will drop back a few degrees to the initial timing then it moved up. would that cause my back fire at around 1500 RPM\'s? i was driving the truck around today abit and it seems like it had no power between 1500-2000 RPMS. above that it ran fine. in that RPM range it wanted to bog down and hesitated really bad. Im running a 670 holley street avenger carb. should i just tun the nut down for the excellorator pump on the front of the carb?

[ZeroGravity58]

i turned the nut on the accelorator pump down and it stopped the backfiring. the timing is way far advanced according to the balancer so i think the balancer is wrong. right now the timing is over 20 degrees advanced which i know isnt right. other then that it still has a hesitation when you first get onto the gas. is there anyway to fix this?

[mzoomora]

You probably need more accelerator pump cam, or change the setting of the current cam. That nut isnt supposed to give it more fuel, you are supposed to set it so that at full throttle there is .020\" clearance between the nut and lever. Any amount of fuel is changed with a different pump cam or different setting of the cam.

[Scotty_S-15]

On 2004-08-19 19:16, ZeroGravity58 wrote:

i turned the nut on the accelorator pump down and it stopped the backfiring. the timing is way far advanced according to the balancer so i think the balancer is wrong. right now the timing is over 20 degrees advanced which i know isnt right. other then that it still has a hesitation when you first get onto the gas. is there anyway to fix this?



That\'s 20 deg. initial, with vacuum disconnected? Hell, try even more! If it runs better still, then yeah, either your mechanical advance isn\'t working at all, or the balancer is NG, and the outer ring has moved. If the engine is hot and easily spins over and starts with the 20+ degrees, chances are it\'s the balancer, because most engines won\'t spin over with that much advance when hot. In other words, you don\'t really have 20+ degrees, the balance just says that.

..............You can check TDC easily, just takes a little time, and a special $10 or $15 tool. Just ask if you want to try that, we\'ll walk you thru the procedure.

............ Maybe I can add to what Mzoomora said about the accel. pump. The nut you turned eliminated some \"play\" in the linkage. There should be no play. What Mzoomora mentioned is the proper adjustment. That is, at full throttle, if you depress the pump lever (underneath) there should be the clearance specified. In other words, no slop at idle, and not binding at WOT.

[ZeroGravity58]

how do i change the setting of the cam?

[mzoomora]

There is a pont where the accelerator pump lever contacts the throttle lever. Right there is a plastic cam. There are a few different colors, and each has two settings. I have a list of each cam and gow much fuel they deliver, or you can find it on Holley web site in the tech articles. If you are on the lower setting, you can try changing that.