Choke And Puke!!!

[toofun]

Probably been covered a dozen times but here it goes. I have a built up 350 with a 650 carb on it. The car seemed to run pretty good but seemed to bog slightly while driving it over the summer. It has gotten alot colder here in New England (40 degrees) and I took my car out for a spin. Before I did I rechecked the carb and noticed that the butterflies were not quite opening all the way up. Just slightly but still, so I readjusted it so that it would open up all the way. I also readjusted the detent cable so that it pulled all the way open at WOT. Let it warm up fully and took it for a spin. Everytime I mashed on the throttle the engine choked and made a god aweful spitting sound like the engine was going to shut down all together. Thought maybe the gear I was in was too high so I manually downshifted and tried it again. Mashed the throttle and again it choked and spit like the motor was going to die again and then the motor cam alive. I thought if I gradually put the throttle to the floor it would make a difference but the motor really sounded like it was struggling and skipping!! Any ideas? I have a stock 350 4 bolt main xe 268 cam, air gap manifold, 1 5/8 headers, trick flow 23 degree aluminum heads and 1.6 ratio full roller rockers

Any help would be greatly appreciated!!!

Mark
TOOFUN

[OHCbird]

Mark- is that a vac secondary 650, or a DP?

Do you have a guage to watch your fuel pressure? What is it set at?

Have you checked where you're vacuum adv is hooked up to? It should be to ported vacuum, not direct vac from the carb base.

Is it 'popping'?

It could be simple stuff like float level, accelerator pump shot (the plastic cam on the side), etc.

[toofun]

It is a 650 double pumper with vacuum secondaries. I dont know if it is hooked up to the ported source or direct source. What is the difference? What would be an example of a ported source?

[OHCbird]

Ported vacuum comes from a port just above the blade, vice below it like the powerbrake fitting, etc. If you hook it up backwards, it'll be at full advance at idle, then actually back off timing as you get in it (until the mechanical advance climbs). To check for it, put a simple gauge on the various ports at idle- the correct port shouldn't have 'vacuum' until you give it a bit of throttle (it'll show better under load).

The vac adv is hooked up there so that it runs static timing until it needs a little bump (under load, accelerating)- so you give it a little throttle, it exposes the 'timed' port to the vacuum, then this pulls in more timing.

What distributor are you running?

DP w Vac sec? That's considered a vacuum secondary carb. Double pumpers by design have a direct linkage to the secondaries, where the vac sec uses a diaphragm on the side. It sounds like the Holley Qjet replacement; Is it a spreadbore, (are the primaries are smaller than the secondaries)?

[andrewb70]

It is a 650 double pumper with vacuum secondaries. I dont know if it is hooked up to the ported source or direct source. What is the difference? What would be an example of a ported source?

Take a picture if you can. A Holley is either a double pumper or vac. Sec. not both.

Andrew

[toofun]

Hey Guys,

Thanks again for answering. Here is a link showing pictures of my carb. From what I have learned so far, it is a 650 DOUBLE PUMPER with MECHANICAL SECONDARIES!!! Guess that would have been helpful to know ay? Also, the vacuum advance is hooked up to a port ABOVE the throttle blades. You can see the braid covered vacuum advance hose in the front picture. Thanks for any info you may have.

Mark
TOOFUN

http://pg.photos.yahoo.com/ph/draggingmedowntoo/album?.dir=25de&.src=ph&store=&prodid=&.done=http%3a//photos.yahoo.com/ph//my_photos

[OHCbird]

Do you know the jet sizes?

Is it popping or surging?

[toofun]

Dont know if this makes sense but it makes a "cha" sound almost like a muffled cough.NOT!! a "pop" sound like a backfire. I know this sounds silly to describe it like that but that is what it sounds like. The car will make that sound then almost die , then if I let off the throttle it will come back alive again. If I give it a three quarter throttle stab while going 35 miles an hour it will make that "CHA" sound then sound like a rev limiter on a motorcycle at a drag light. Hope this helps. If there was a way to explain it via audio I would do it in a heart beat. Thanks again for the help it is appreciated.

Mark
TOOFUN

[andrewb70]

You need to check your timing and move the vacuum line to the distributor so that it is connected to a port below the throttle blades. For a quick test, disconnect that hose completely, plug the port on the carb and take a drive. Report back what it does. Check the static timing as well and let us know what it is.

Andrew

[toofun]

You need to check your timing and move the vacuum line to the distributor so that it is connected to a port below the throttle blades. For a quick test, disconnect that hose completely, plug the port on the carb and take a drive. Report back what it does. Check the static timing as well and let us know what it is.

Andrew

Just want to get this clear. So I should be hooking up the vacuum advance line BELOW the throttle blades???? I thought OHC BIRD said that if I do that then it will pull in full advance at idle? Unless I misunderstood??? Also what is static timing?? and how would I measure that??? Thanks for the advice just want to be clear on this.

Mark

[OHCbird]

Static timing is the base, non-mechanical or vacuum advance you set at idle. Normal street dizzys have both mechanical and vacuum advance- the mechanical is done with flyweights under the rotor. As the shaft spins, the weights 'expand' out against spring pressure. when they expand out, this rotates the shaft forward, giving you mechanical advance. For most distributors, this is a linear curve that increases with engine speed.

Vacuum advance is for light / part throttle timing assist. It gives you more advance when you need it- off idle, and part throttle cruise conditions.

This is a good, basic article on timing that deals with a Vac advance HEI like yours: CHP timing article (http://www.chevyhiperformance.com/howto/97438/)

The Cha sound could be several things; one time it turned out to be a burned valve. Just to make sure- have you gone over the simple stuff like plug wires (routed to the correct cylinder), plug gaps, voltage to your HEI, rocker lash settings, etc? Before you take a stab at tuning the carb, all these need to be checked and set properly.

It sounds like you might be a bit fat (rich) on the secondary side (no pop). This could be from a secondary float level being off, a clogged air bleed, or just jetted wrong.

What do the plugs tell ya? paper bag brown is what I shoot for (now with the aid of a wideband O2, I might add).