All,

This may be slightly off the path here, but it is about a 429. <g> I've got a '96 "F-series". They weren't using F-600/700/800, etc then, I don't believe, just different GVWRs, but this one is the equivalent of an F-700/750 class.

Now, you may be wondering how I'm talking carbs on a '96 -- shouldn't that be EFI? Well, this baby was originally a propane burner, which was converted over for sale, and I bought it, to use as a good Sunday-go-to-meeting farm truck. <g>

It has this big "riser block" thing on top of the intake manifold, which part of the original propane carb system, and they just mounted a Holley 4160 class carb, which by the model number is a 650CFM. They also put a dinky little electric fuel pump, which looked like it came off some little Honda at a junkyard. Empty, it ran fine, but loaded, it couldn't pump enough fuel by a long shot. <g> It took me a while to figure out what was going on, but when I did, I got a decent electric pump.

Tuning the carb for the thing has been a quest. I've got it running as best as I can manage and haven't messed with it in several months, but I stumbled on this forum and thought I'd post about it.

Originally, it was running worth a cuss. The timing was jacked way up -- propane, I'm sure -- I mean way up, and I backed it down to a comfortable 14 degrees -- the sticker said 10 degrees, but 14 seemed fine.

The thing would "hiccup" big time on flooring the accelerator. I figured it was all the extra volume of the big "riser block" under the carb, and so I got an accelerator pump kit and began playing.

Second, the vaccum secondaries would flutter open on goosing it. I had some springs and put the strongest black spring in it, and she still fluttered. I ended up playing with *two springs*. It's got two plain colored springs in it now, which was the weakest combination that stopped the flutter. However, what it is doing to the secondary opening curve and airflow, I don't know. <g>

After all that, I finally got it running decently. Remember this is a big truck, low geared and made to pull a big load up a hill, not do a 1/4 mile. It runs great "screaming" (to me) between 3000 - 3800RPM. I have another F-700 with a 370 in it, and this 429 will walk it going up a hill when it's in the "zone" of 3000 - 3800RPM.

However, she's not so good between 2000 - 3000. There is flat spot at WOT somewhere around 2500RPM. If I take the springs out and go weaker (and let it flutter on goosing) that flat spot seems *worse*, and if I go with even stiffer double-spring combos, I can't really tell much difference in the flat spot, although the 3000-3800RPM performance seems poorer.

Driving behind it and watching, I think I can see a little black smoke coming out during the flat spot, but I wouldn't swear to it. I'm running number 63 jets on the front, and I forget what the number the rear plate is stamped.

All of this just seems crazy to me, but that's the way she's acting. Any thoughts would be appreciated.

-Richard

 

First thing, I can't picture in my mind what kind of riser block you are talking about. Can you take it off and bolt the carb. to the intake? And how high or thick is it?

The secondary should never open on this carb when in park and reving it up unless it has been modified to due this. take the top of the vaccum pod off where your spring is and look and see if the small ball bearing is still in the hole in the back left hand corner.(looking at the side of the carb) If it has been removed or lost this will cause the secondary to open in the way you have discribed.
When tuning this carb. swap your discharge nozzles also to get a better response.

 

Thanks for the response. To picture this "riser block", just imagine a carburetor sitting on a little tower on top of the intake, which raises the carb about 8" off the top of the manifold. It would be a PITA to remove this "tower" because of the air plumbing. I imagine that's why they left it on when they converted it over. It uses a heavy duty air cleaner, like on tractors and diesel trucks mounted up front, using large stiff hoses that will not fit right if the carb is any lower. To remove the riser block and mount the carb normally on top of the manifold, I'd have to fabricate or otherwise rig some air horn extension to go on top of the carb.

Eyeballing the volume of the block, I would figure it adds about 1 to 2 quarts of volume to the intake under the carb. My guess about the fluttering was the additional volume caused enough additional rush of air when the throttles are thrown open to start activating the secondary.

This carb does NOT use the check ball, but uses a pressed-in brass thing with a small restrictor orifice where the check ball would normally be. When I took it apart, I thought I'd found the problem of a missing check ball, then saw the restrictor orifice.

The orifice is pretty tiny as it is, but I wonder if a smaller hole would solve the problem. Are there any other tricks to damp secondary opening? If I had access to some carbs I could try without buying :-), I'd like to see if a larger carb had the same fluttering problem.

-Richard

 

Try disconecting the vac. secondary by disengaging the diaghram rod from the linkage, and see how it works. This will just eliminate the secondary opening to look at what happens, you could go with a 50cc accelerater pump also to see if you can fill the dead space with more fuel at take off.

Having an 8 inch spacer under the carb. on a stock engine can be heck to get lined out. I think I would try to get rid of it some how, it seems like it would kill your low end.

 

...carb jet. Try going up two numbers at a time and see if it gets better.

The black smoke you see during seconday opening is not due to richness but the combo falling on its face as the secondarys open too fast and then catch.

That spacer is causing most of the issues and may even be causing mixture distribution issues.

Scott Johnston / Re in"Car"nation High Performance.
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Scott and Deerhunter,

Thanks. Sounds like the consensus is the 8" tower/riser block needs to go. And now in wintertime when I'm not using the thing is perfect time to do it..... Anyone have any ideas on how to rig about 8" worth of some air horn on top of the carb? :-) I'll rig something somehow given enough time and effort. :-)

I wonder how this 8" spacer block behaved with the original propane setup. When I first bought this truck, the parts guy at a local truck dealer let me look at a parts diagram blow-up of the intake system. The original propan carburetor sat up on top of this block. There was a vaporizer on it that used the heater hose for the heat exchanger. It was a pretty large assembly.

-Richard

 

I do not know anything about a propane set up, but why can't you do away with the stock air plumbing and tower, mount the carb to the manifold and use a after market 14" breather assembly. You may even have to use another intake, but that isn't a hard job.

Also could there be a computer issue?

Let us know what you do to get it lined out.