Chevy 250 ci (4.1L) straight six Carburetor setup

[Racer-X-]

The engine is a 197_ (ish) Chevy Straight 6, 250 cubic inches (4.1L). 

 

The carburetor is an Amazon Chineseum special. Here's a few pix from the listing on Amazon:

 

 

 

 

 

The full listing is here:

 

https://www.amazon.com/gp/product/B07B3PMJ45

 

 

First, the disclaimer, I'm mainly a fuel injection guy. I haven't owned anything with a carb in decades, and before this one, I hadn't touched a carb in about 30 years. So I'm basically clueless about this thing.

 

I took this one out of the box and installed it.  It came with a thicker gasket than the one I was replacing, so I had to replace the carb mounting studs with longer ones.  Also, the throttle linkages from the pedal were hanging the throttle plate open a bit and causing it to idle at around 1500 RPMs.  This was because of the thicker gasket.  So I made some throttle linkage adjustments.

 

Now, it seems to be idling a bit low, especially when cold.  But once it warms up, the idle gets higher than optimal.  It's also surging a bit, both at idle and at constant throttle cruising. It doesn't hold a constant engine speed or power output.  It varies up and down some.

 

The only adjustments I see are on the second picture.  There are two big silver colored straight slot screws that are under he choke actuator shown in the firs pic.  You can just see the knurled screw under that choke actuator in the first pic.  There's also a small brass straight slot screw right at the base of the carb.

 

I'm open to any suggestions for setting this thing up and getting it to run better.  YouTube links would be awesome.  A "how to" page that covers this particular carb would be good as well.

 

I know enough to know that I don't know how to tune this thing, or which screw adjusts what.  But I think I'm probably smart enough to learn.

 

And if it matters, there's what looks like a PowerGlide automatic behind it. The vehicle is a 1961 Rolls Royce (Austin) Princess.  I'm told the Chevy 250 straight six was a common swap for these when the Austin inline six wore out, died, or suffered from typical British reliability issues. 

 

We're already discussing and exploring possible updated engine swaps for this one (Possibly a LV3 4.3L V6, or maybe a Gen IV or Gen V 5.3L V8). But until that happens, this one has to drive reliably every Thursday, Friday and Saturday.

[joe_padavano]

The first question that comes to mind is why would you buy a Chinesium carb? This is a Rochester Monojet. It's probably the simplest carb GM ever made. It's trivially easy to rebuild, and rebuild originals will always run better than a one-size-fits-none replacement. And if you want to learn how to tune the carb, start with a factory Chassis Service Manual. Get a real original paper manual, not a free PDF download that is missing pages and has fuzzy scans of the photos.

[Racer-X-]

23 minutes ago, joe_padavano said:

The first question that comes to mind is why would you buy a Chinesium carb? This is a Rochester Monojet. It's probably the simplest carb GM ever made. It's trivially easy to rebuild, and rebuild originals will always run better than a one-size-fits-none replacement.

 

Valid points.  Ordinarily I'd be in complete agreement with you.

 

Let me address this particular case.  First, it's not my car.  I didn't buy the carb.  This carb was handed to me and I was told to install it on the engine in this car.  I was also told that the carb that was on the car now came from the same place (Amazon, I reasonably believe the same listing) and lasted only a year or so before it started leaking badly.

 

If we can't get it running smoothly and reliably with this carb, I will definitely recommend getting a genuine original Rochester Monojet for this engine. I'm not entirely sure where to source one that's serviceable, or at least a good rebuildable core.

 

23 minutes ago, joe_padavano said:

And if you want to learn how to tune the carb, start with a factory Chassis Service Manual. Get a real original paper manual, not a free PDF download that is missing pages and has fuzzy scans of the photos.

 

Also something I generally agree with.  However, in this particular case, I'll need to "start with a factory Chassis Service Manual" for what exactly? 

 

Honestly, the current owner of the car doesn't even know what this particular engine came from originally. The engine swap was done before he bought the car.  I'm told (by the current car owner) that the engine is a Chevrolet 250CID Straight Six.  It's definitely a Chevrolet Straight Six.  I haven't really looked closely at it to determine for certain that it's a 250.  They made those in other displacements. The intake manifold is definitely for a one barrel carb, not a two barrel.

 

The "Chassis Service Manual" for the Rolls Royce/Austin Princess that it's installed in won't help at all for this particular part. 

Edited November 10, 2021 by Racer-X- (see edit history)

[joe_padavano]

18 minutes ago, Racer-X- said:

Honestly, the current owner of the car doesn't even know what this particular engine came from originally. The engine swap was done before he bought the car.  I'm told (by the current car owner) that the engine is a Chevrolet 250CID Straight Six.  It's definitely a Chevrolet Straight Six.  I haven't really looked closely at it to determine for certain that it's a 250.  They made those in other displacements. The intake manifold is definitely for a one barrel carb, not a two barrel.

 

Fair enough on the manual. I admit that I didn't read far enough.

 

These should get you started.

 

https://www.carburetor-parts.com/assets/manuals/rochester_monojet_service_manual.pdf

 

http://docplayer.net/138263289-Contents.html

[Racer-X-]

Thanks @joe_padavano  !!!

 

Those are excellent links.  On my own research, I also found this page which covers the adjustment of the carb after it's installed.

 

https://itstillruns.com/adjust-monojet-carb-air-fuel-8061119.html

 

I'm probably going to start with idle adjustments first and see if that clears things up.

 

I went to work on these cars (this one and the other late 40's Mopar I posted about the brakes) this afternoon.  Unfortunately, "Bubba's Brakes and Mufflers" has also done some brake work on this one.  He made some "custom brake caliper adapters" for it and mounted some kind of Chevy truck brakes on it (6 lug rotors, ).  Unfortunately, he drilled the holes so close to the edges of the adapter plates that the edges of the holes cracked and broke.  One caliper pin is missing, and that caliper randomly flops around and sometimes catches the inside of the wheel and locks that wheel.  On the other side, the hole for one of the center bolts that holds it to the suspension upright broke on the other plate, and it's flopping around loose with the caliper, but the movement is much more restricted. It's mainly an annoying noise issue with some weird shakes from that side.

 

Now I have to find a caliper pin, which means I need to identify the mystery calipers. It's always something.

Edited November 11, 2021 by Racer-X- (see edit history)