Build The Clustertruck Rides Again - Refurbishing a 1975 Chevota

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Ok, I found the leak...Part of the hissing is simply air sucking through the primaries. The second, however, is not one of the gaskets...it's the electric choke?! That makes zero sense to me...Any clue if this is normal? Does it pull air to cool the coil inside or something along those lines? In either case, I sprayed carb cleaner around every gasket, surface, the choke pull-off, etc. and the idle never changes. However, if I spray the cap directly, the truck tries (or succeeds) at stalling immediately.

Thoughts?

Video below:



Also, for @78fj40mg - below is a short video of the new carb in action.

 
Cdaniel said:
Was your qjet converted to elect choke? Might be missing a plug
Click to expand...

I was kind of wondering the same thing...I can't think of any other reason an electric choke would be pulling vacuum. However I didn't see anything immediately telling last night that would indicate it had previously been a hot air choke - i.e.: no plugged inlet tube hole or anything of that sort.
 
Cdaniel said:
I don't think it's visually obvious if missing. Click the link
Click to expand...

I see what you're saying, and I'll definitely have another look at it. However, I believe if this had originally been a hot-air choke, there should be a second plugged "port" on the choke housing that would have received a tube running to the intake manifold, to provide manifold heat to the coil. Maybe SMI just did such a good job of plugging it I missed it entirely.
 
RWBeringer4x4 said:
:

THE BAD:

1. Monster vacuum leak - Very loud hissing on the passenger side of the carburetor. Hit the area with carb cleaner, and the idle stumbles pretty significantly. As a result, my mixture screws are pretty unresponsive. It seems like it's one of the 3 primary gaskets (baseplate, air horn, etc.) but I essentially just doused the whole area with my last bit of carb cleaner. Seems like it's coming from right behind the choke...The silver lining is the truck seems to run OK with the leak, but it will need to be addressed. I just ran out of carb cleaner. I'll have to pick up some more to continue the investigation.......
Click to expand...

Are you sure you have a leak?
If you're not seeing a performance problem what indicates the leak?

Carbs suck air and make noise, right?

Is it possible to tell from a vac gage reading if there's a leak in the carb? Seems like you'd see the same flutter as a manifold or brake booster leak.

Different sounds aren't always bad ones. It's easy to treat cars like the bad kid in class and think they're up to no good, even when they've turned a corner...
 
71FJnick said:
Are you sure you have a leak?
If you're not seeing a performance problem what indicates the leak?

Carbs suck air and make noise, right?

Is it possible to tell from a vac gage reading if there's a leak in the carb? Seems like you'd see the same flutter as a manifold or brake booster leak.

Different sounds aren't always bad ones. It's easy to treat cars like the bad kid in class and think they're up to no good, even when they've turned a corner...
Click to expand...

The carb performs pretty well. Most of the Noise I'm hearing is the carb sucking air. However, the idle mixture screws are fairly unresponsive. I can also stall the truck by spraying the choke - per the video above. I'm guessing @Cdaniel might be right - I could potentially have an open vacuum passage in the choke. It very well could be an old hot air choke.

I'll probably try to RTV the passage in the choke housing, and also unhook the electrical to see if maybe spraying the choke with carb cleaner is just shorting out the engine (doubtful). If neither works, I may just live with it. It holds a VERY steady 650RPM idle and throttle response is good. If it's burning a bit of extra gas at idle it certainly doesn't seem to be complaining.
 
I wouldn't rtv anything yet, call the carb rebuilder first. RTV can and will get sucked into the carb/motor.
 
Robs76fj40 said:
I wouldn't rtv anything yet, call the carb rebuilder first. RTV can and will get sucked into the carb/motor.
Click to expand...

That was actually the rebuilder's recommendation.
 
don't RTV, there is a passage that allows vacuum to pull the choke open when you get into the throttle.

I wish I could see it, but I think you need to pull the carb and verify the base gasket is correct. I saw your gaskets, but what I need to see is the base of the carb and, if you think about it, the intake manifold. Simply I'm not convinced that your spraying around the choke didn't also get around the base gasket as well.

And you have a stumble off idle, that's either a vacuum leak or issues with the vacuum advance. you have verified your vacuum advance is working, right?
 
Isn't there supposed to be a small cork or rubber gasket/o-ring for the old style choke? I would think you would still need that in place to seal off the vacuum port even though it's no longer used.
 
I too was having an issue with my q-jet. It had a whistling issue and ran rich, even after a rebuild it just wasn't right, I'm convinced the whole thing was bent from being torqued down too much over the years. I bought a new one from national carburetor, the base gasket they sent is a lot different then the one I had. It runs perfect now. I also upgraded from the hot air to electric Choke, no air moves in that area. Here is the type of base gasket I am using now.
ImageUploadedByTapatalk1444923842.156733.webp
 
Robs76fj40 said:
Isn't there supposed to be a small cork or rubber gasket/o-ring for the old style choke? I would think you would still need that in place to seal off the vacuum port even though it's no longer used.
Click to expand...

It is upgraded to an electric choke. It's no longer hot air activated. The deal is removed so that the electric choke can ground through the housing. However, the internal vacuum port into the carb should be sealed as well. I suspect this inner passage might have been overlooked and is thus still drawing vacuum through the now unsealed choke cover.

laveycrafter said:
I too was having an issue with my q-jet. It had a whistling issue and ran rich, even after a rebuild it just wasn't right, I'm convinced the whole thing was bent from being torqued down too much over the years. I bought a new one from national carburetor, the base gasket they sent is a lot different then the one I had. It runs perfect now. I also upgraded from the hot air to electric Choke, no air moves in that area. Here is the type of base gasket I am using now. View attachment 1147536
Click to expand...

As mentioned above I've converted to electric choke - which shouldn't be drawing vacuum at all. Hence my confusion.

My previous qjet was also badly warped. This one, at the base at least, is level. It might have the wrong gasket though. It seems like should work but I'll let @SuperBuickGuy be the judge.
 
SuperBuickGuy said:
don't RTV, there is a passage that allows vacuum to pull the choke open when you get into the throttle.

I wish I could see it, but I think you need to pull the carb and verify the base gasket is correct. I saw your gaskets, but what I need to see is the base of the carb and, if you think about it, the intake manifold. Simply I'm not convinced that your spraying around the choke didn't also get around the base gasket as well.

And you have a stumble off idle, that's either a vacuum leak or issues with the vacuum advance. you have verified your vacuum advance is working, right?
Click to expand...

I'm pretty convinced the base gasket is sealed. I rand the carb cleaner around the bottom using one of those red straws, and didn't get a noticeable change in idle. What's odd is, I'd expect the idle to INCREASE with the addition of carb cleaner, but the engine is stalling. Idle is also very steady, and holds well, not a lot of fluctuation. However, the mixture screw response is pretty weak (engine will run with the screws only about 1/2 turn out) which leans back toward a leak...

I haven't ruled out the possibility that the carb cleaner is somehow causing a short in the electric choke, which is in turn shutting down the engine...I'll probably repeat the "spray the choke" experiment with the carb unplugged at some point as well.

I thought the throttle response was pretty good off idle - Didn't detect much of a stumble...not sure if it's just the video quality or if you're seeing something I don't.
 
I think I have a couple carbs that I can demonstrate the issue (the leak could be internal).... too be continued tonight.

In the meantime, be sure your brake booster holds vacuum and that your vacuum canister on the distributor holds vacuum as well. Spraying carb-cleaner down the carb should make it run faster unless it's already running rich and/or there's not enough advance dialed in.
 
SuperBuickGuy said:
I think I have a couple carbs that I can demonstrate the issue (the leak could be internal).... too be continued tonight.

In the meantime, be sure your brake booster holds vacuum and that your vacuum canister on the distributor holds vacuum as well. Spraying carb-cleaner down the carb should make it run faster unless it's already running rich and/or there's not enough advance dialed in.
Click to expand...

I can confirm that my brake booster and vacuum advance are working. I'm pulling a steady 15lbs of vacuum at idle, which is about the best I've been able to get through carb adjustment. A little low but not unexpectedly so...

Advance is hooked up to ported vacuum. It seems to be good for about 10 degrees. I get a total of about 32 degrees advance at 2500RPM between mechanical and vacuum. I know you mentioned a lot more advance than I am currently running (12 degrees) but from what I've read, the 283 only had 4 degrees advance from the factory. Even a "worn in" one like mine probably wouldn't want much more than 12, maybe 16...It will certainly ACCEPT a lot more, but I also don't want to self destruct my starter.
 
on cars that we race and street drive - we've been known to pull this "trick". On a race car, you rarely run vacuum advance - but if you have vacuum advance you can build yourself a cheap spark-retard by hooking the vacuum canister up to manifold vacuum.

I know some of my questions are redundant or even a bit annoying, but as I think I've mistakenly not done all of those that I've asked about - have patience with me :)
 
SuperBuickGuy said:
on cars that we race and street drive - we've been known to pull this "trick". On a race car, you rarely run vacuum advance - but if you have vacuum advance you can build yourself a cheap spark-retard by hooking the vacuum canister up to manifold vacuum.

I know some of my questions are redundant or even a bit annoying, but as I think I've mistakenly not done all of those that I've asked about - have patience with me :)
Click to expand...

No worries at all! I'm learning a lot - sometimes it takes a couple attempts to connect the dots! I'm always happy to take any advice I can get.
 
alrighty
you'll see where the carbs are circled - one has additional vacuum channels compared to the other 2


and actually, that's not the one that I was looking for - I had a cadillac one that had a vacuum channel by the brake booster that ONLY a cadillac gasket would cover (it had the other half of the channel in the stock, cadillac manifold).... point is, be sure you got the right gasket.

oh, and note there is no difference between the chokes - on the right is an electric choke, and on the right a heat-riser choke
 
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