HIGH rpm

[JohnMc]

Yes, you can do that. The PCV system on thse is pretty simple, practically no moving parts, none of it is very essential to the way the motor runs.

One hose runs to the fitting on the intake manifold. This fitting has a very small hole in it, to sharply limit the flow. And the intake manifold only has significant vacuum in it when at or near idle, the pressure rises as the throttle goes down, to close to ambient at WOT.

So at lower throttle/idle, there's more vacuum, and the little hole will suck the small amount of crankcase vapors through into the intake manifold, pulling fresh air into the block from the air cleaner via the other hose.

Step on the gas, the vac goes away, and the draw through that tiny hole goes mostly away. Then the flow reverses, and the larger amount of crankcase fumes (blowby from the pistons, mostly) will go out the hose into the air cleaner, where it will get sucked into the carb.

So you can certainly run it with that PCV hose plugged, it will just passively vent to the air cleaner like it would under throttle while driving. See if it makes much of a difference with the hose leading tot he intake plugged or not plugged. The hole is small, it's not supposed to make much of a difference. It's possible someone replaced it with something else at some point which is letting too much air in (which would make it run lean).

 

[JohnMc]

I had a man by the name of Thomas Bryant rebuild my carbs... he seems to have a good reputation and to be confident in his work. Have you heard of this problem when he rebuilds carburetors? How can I check to see if they are off. Is it something I can see or is their an other way to conclude that it is off

Don't know him, or his reputation. I haven't had SU's on my PV for a couple of decades (more or less). But when I had them I had the slightly worn throttle bushing issue, and occasionally the throttles would sag and hang up on the side of the bore, not closing completely. I just learned to 'tickle' the pedal a little bit to get them to 'unhang' and close properly.

Then I stuck DCOE's on it instead of trying to have the SU's rebuilt.

You might be able to see it by looking down the bores, look carefully at the top and the bottom of the throttle valve, see if one side seems to be sitting flush with the carb throat, and one is sitting even a tiny sliver away from it? But on an SU, they sit way back behind that constant vacuum regulating piston up front, you'd need to lift that all the way up and peer behind it with a light, might be hard to see.

The thing is, though, an air leak can only keep the idle speed up so much on a carbureted vehicle. Air leaks are not going through the carb, so they're not getting fuel added, they're lean air leaks. So a modest air leak can make the mixture leaner, and raise the idle. But you shuold be able to drop the idle down by closing the throttles more and more, even if the air leak is still letting air in, closing the throttles ALL the way will cut off the flow of fuel since no air will be going through the carbs.

So if you've really closed those idle adjusters all the way down, and it's still running (and running too fast), then it rally means that air is still going though the carbs, which means the throttles aren't rally closing all the way for some reason.

 

[spock345]

I don't know why I didn't remember this earlier but I recall having a problem with the fast idle screws keeping my throttle plates open a bit. If screwed in too far they will contact the fast idle cam and cause a high idle even when the carbs aren't choked. The end result is that I could unscrew the regular idle screws all the way and the idle would hover around 1500 rpm. Also make sure the jets are fully seated when the carbs aren't choked.

Also have you tried looking at the fuel pressure? Maybe too much fuel? I know that modern aftermarket mechanical fuel pumps can be a bit overeager. My weber would just run like crap and far too rich on 5psi. The SUs would be guzzling fuel and causing all sorts of problems.

Emailing Tom is probably the right way to go. He also has a guide on how he tunes these carbs on his site.

 

[1968 volvo]

I know my car isn't choked. I had a bit of problems earlier with my choke that I had to sort out... That said and done I think I can eliminate the choke from being a factor. Ill check the idle screws and the email Tom about the fuel pressure. Someone else told me to check with him to see that the butterfly valves are seated properly.

I plan to go out to my shop today and get greasy today so Ill update you guys on my situation either tonight or tomorrow. Ive got a bit to check.

 

[alschnertz]

Since then I cleaned the carb and manifold mating surfaces and I replaced all the gaskets.

Did you check that the intake manifold was not warped?
With mine, I taped emery cloth to a known flat surface and then slid the manifold across it.
It was easy to see the imperfections (noted by the shiny surfaces) after that.
Just eyeballing it was not good enough.

 

[1968 volvo]

Did you check that the intake manifold was not warped?
With mine, I taped emery cloth to a known flat surface and then slid the manifold across it.
It was easy to see the imperfections (noted by the shiny surfaces) after that.
Just eyeballing it was not good enough.

I checked it by seeing if it rocked in any directions when placed against a flat surface. I also held a flashlight on the opposite side of the manifold and saw if there was light poking through. are those tests conclusive? My manifold seemed to pass the test. I suppose its also possible for the head to be warped? I didn't check that however.

 

[alschnertz]

I doubt the head is warped.

Is it still ok on start-up but the rpms go up as it warms up?
Did you spray carb cleaner directly at the base of the shared stud where the intake and exhaust manifold meet?

 

[1968 volvo]

A little update. I checked the choke idle speed screws and with them completely backed out there was no change in idle speed. I did notice one interesting thing about my carburetors. While the engine was running, I was able to reach into the carburetor and manually pull the round thing that has the nettle on the bottom, down. This dropped the rpm. And then when I released it the rpm rose. Is this normal behavior. I tried to replicate the effect on the other carburetor but I failed to do so. I also now realize that that was stupid to do because it could backfire on to my hand or something but nothing bad happened so what can I say.
I haven't tried to inspect the pcv yet. The sentence about the carburetor thing sounds confusing to me but I don't know what the part is called and I cant seem to explain it better. If it does not make sense let me know and Ill try to describe it better.

 

[1968 volvo]

I doubt the head is warped.

Is it still ok on start-up but the rpms go up as it warms up?
Did you spray carb cleaner directly at the base of the shared stud where the intake and exhaust manifold meet?

Thats a relief.

The idle is ok upon start up. And then the rpms don't go up until the car is warm. Ill double check it but I'm also pretty sure that the rpms don't go up unless the car is reved. Not like make some noise revs but just normal driving revs.

I haven't tried to spray carb cleaner directly in that location. Also my intake/ exhaust manifold are one piece... is that a factor in the test?

 

[spock345]

A little update. I checked the choke idle speed screws and with them completely backed out there was no change in idle speed. I did notice one interesting thing about my carburetors. While the engine was running, I was able to reach into the carburetor and manually pull the round thing that has the nettle on the bottom, down. This dropped the rpm. And then when I released it the rpm rose. Is this normal behavior. I tried to replicate the effect on the other carburetor but I failed to do so. I also now realize that that was stupid to do because it could backfire on to my hand or something but nothing bad happened so what can I say.
I haven't tried to inspect the pcv yet. The sentence about the carburetor thing sounds confusing to me but I don't know what the part is called and I cant seem to explain it better. If it does not make sense let me know and Ill try to describe it better.

Have you got dashpot oil in them? The oil is supposed to provide a dampening effect on the pistons rising. The dashpot cover works as a one way valve that resists the upward movement of the pistons. The pistons rising too far due to too little or too light of an oil may cause this condition. The factory stuff is supposedly SAE 30. Tom's blog posts talk about 15w50 having similar consistency at the SU's temperature when warmed up.

I use this stuff and it works pretty well.

 

[1968 volvo]

Have you got dashpot oil in them? The oil is supposed to provide a dampening effect on the pistons rising. The dashpot cover works as a one way valve that resists the upward movement of the pistons. The pistons rising too far due to too little or too light of an oil may cause this condition. The factory stuff is supposedly SAE 30. Tom's blog posts talk about 15w50 having similar consistency at the SU's temperature when warmed up.

I use this stuff and it works pretty well.

My dashpots are full of oil. A man by the name of Nat Kramer helped me tune the carbs to get my car started a number of months ago. He filled them for me but said that the oil didn't matter that much as to which kind to use as long as the viscosity wasn't like water thin. Ill double check to see if there is any oil left. I don't know where it could have gone.