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post #1 of (permalink) Old 03-13-2001, 04:14 PM Thread Starter
 
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engine vacuum question

I just installed the Rochester 2bbl carb on my 78 CJ with the 258 and it runs fine, starts good, and idles good. My qestion has to do with ported and manifold vacuum. I wanted to hook up a Cagle fuel pressue regulator and first needed to know which one of these to hook it to. Second I was just outside messing with the Jeep and measuring the vacuum at idle at both sources. At idle the ported vacuum was 18, when you step on the gas it went down to 10 briefly and then back to 18 if you held the throttle at a steady rpm. When you let off it briefly jumped to 22 and then back to 18 again at idle. Manifold vacuum did the exact same thing at the exact same vacuum readings! I measured manifold vacuum at at a port on the #6 intake runner where there was a plug, and the ported vacuum was read at the front of the carb(dist advance) and the rear choke pull off port. These numbers seem high compared to what it is supposed to be at idle. Are they even close to what it should be? I thought ported and manifold vacuum should be different also? There were no other ports on the carb to test either. This was all checked after I reset the timing to 8 btdc at 700 rpm idle with the vacuum advance disabled. These numbers were from a chiltons manual for my 78 CJ-5 with 258 I-6 and at altitude(mile high). Thanks for any help.

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post #2 of (permalink) Old 03-13-2001, 05:19 PM
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Re: engine vacuum question

The front ported vacuum should be pretty low until you open the throttle. The port for the ported vacuum is located right above the throttle plate, so when the throttle is closed it sees the air above the throttle and when it is open the port sees manifold vacuum. You may need to keep the advance connected to the carb and tee off it to get the right reading, since advance directly affects idle vacuum. The curb idle may be set improperly. How did you set the idle speed and mixture? The curb idle should be set with the vacuum advance CONNECTED and the engine completely warm. If your chilton's shows the procedure for a '68-71 V6 CJ, use that. That Jeep used a 225 cid V6 with the Rochester 2bbl.
The idle speed screw should be set for 650 rpm (7 is ok too), then the left mixture screw set for maximum speed, then the idle speed screw set again, then the right mixture screw adjusted for maximum rpm, then the idle speed screw set again, then the mixture screws turned in (leaned) by 1/2 turn, then the idle speed screw set again. This procedure gives the optimum idle mixture. (I think it's called peak optimum idle method, or something. )Yes, there are two mixture screws. The engine may seem like its idling ok with way too rich a mixture, so it's easy to be fooled if you just turn the screws until it seems right. This is not only bad for fuel economy but also bad for your piston rings due to washdown of the oil.

The manifold vacuum numbers sound just fine, maybe even on the low side for some engines. The idle manifold vacuum on my 1.8L 4cyl Miata is 26 and on my 2.6L Straight-6 BMW is 23. The brief jump is just what it should do. The brief jump number should be 5-10 inHg higher than the low number on let off, and lower on blip, depending on altitude and atmospheric conditions.

The idea behind vacuum advance is to provide better fuel economy at low rpms and part throttle. At idle, the advance is slight, maybe a few degrees. Off idle, as the throttle plate opens, the port sees manifold vacuum and advances the timing to add low rpm power and improve fuel economy. As the engine speed increases, the centrifugal advance adds advance to the vacuum value, because more advance is needed at higher speeds. As the throttle is opened more, the vacuum drops and the timing is retarded slighly by the vacuum advance to prevent knocking from the extra air/fuel mixture. As road speeds are reached and the gas is let off and the throttle closes some, the vacuum builds back up in the manifold, and the timing is advanced more. If the throttle is closed at high rpms, the timing is advanced fully due to the high vacuum and full centrifugal advance and the early firing of the pistons helps compression braking. Vacuum advance makes no differance in full throttle engine power, it is just there for better fuel economy and knock prevention.

I don't know that you need the complexity of the Cagle regulator. A simple pressure regulator set at 4 psi should do the trick just fine. The 2G carb has a good float/needle valve setup that will hold fine as long as the float is not bad. (I ran a 2GC on my jeep with a 225, I just got a Qjet and an Offy manifold) I had one go bad due to a backfire and the carb flooded (literally) and I almost had a fire. I have a mechanical fuel pump. I replaced the foam float with a brass float when I rebuilt the carb.
Regards,
Josiah

Hillsboro, OR
1955 Willys CJ5 Buick 225 V6 160HP 270ft-lbs, T90 trans, Warn OD, PTO winch, Spicer 18 T Case, RS9000's, Dana 25F/ 44R,
5.38:1 gears, 11" brakes, Bestop Supertop, Hurculiner
post #3 of (permalink) Old 03-13-2001, 08:11 PM Thread Starter
 
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Re: engine vacuum question

I set the idle speed after the engine was warmed up and with the vacuum advance connected. To check the vacuum I t'd off the vacuum advance line to my gauge. For the vacuum advance I used the port on the front of the carb just below the bowl. That was the only port on the front of the carb. The carb has a larger port on the back side that is threaded, this I used for the PCV. Just above this and to the right side was another port which came already connected to the choke. To set the idle mixture, after the engine was warm I turned both of them in 1/4 turn at time together until the engine began to stal and then back out 1/4 turn each. I'll go back and use the method you suggested and see if that helps. If I am seeing the same vacuum at ported and manifold what could be the problem? The carb was a new Holley rebuild so is in good condition. I only disconnected the vacuum advance to set the timimg. If it was running rich could that cause the high ported vacuum reading? I took it out for a drive and under load the manifold vacuum would drop to about 3" Hg under load going uphill and when lugging the engine off road. During straight level highway driving it would settle at about 18. Thanks again, and I'll post back once I readjust the idle mixture.

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post #4 of (permalink) Old 03-13-2001, 09:59 PM Thread Starter
 
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Re: engine vacuum question

I just reset the timing with the vacuum advance plugged and rpm @700. I then readjusted the idle mixture according to your instructions. The left screw is set at 2 turns out and the right one at 1 turn out and the idle at 700 rpm. Is it ok that the two screws are at different settings? The vacuum at the ported site only went down to 16" Hg which is still what the manifold vacuum is also. Could this high vacuum reading be because of a bad rebuild of the carb by Holley? The port was above the throttle plates so it is the right one, maybe just a bad carb? It runs and idles great it's just that high vacuum reading that makes me wonder. Thanks for your in depth help in your last post, I'm gonna print it out and keep that with the shop manual!

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post #5 of (permalink) Old 03-14-2001, 02:24 PM
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Re: engine vacuum question

Hi, again,
One thing I forgot to ask, are you running with the air cleaner off or on? How restrictive is the air cleaner? I think a restrictive air cleaner could cause some funky readings. As far as I know, you have the thing hooked up right. If it is running and accelerating well, and not knocking, I would be tempted to just leave it as-is. It is really unlikely that Holley screwed up your carb, since the 2 bbl Rochester is about the easiest carb in the world to rebuild. The idle mixture should always be set with the cleaner on.

As far as I know, having the two screws in differant settings is ok. One thing I forgot to have you do was start both screws 2 turns out, but it sounds like you got it ok. It depends on how your intake manifold is constructed. I think yours is probably a single plane, so both barrels of the carb flow into the same place. On my engine I have a dual plane, so half of the carb flows to the left cylinder bank and half goes to the right, and the screws end up being close to each other. In either case, the lean best idle method (there, I rememberd it) I described will give you the best mixture.

I don't have my Jeep running right now. I pulled the gas tank and radiator to get them cleaned up and re-cored over the winter. I am also in the process of putting in new wheel cylinders and a master cylinder, so it will be a few weeks before I can check the vacuum readings on my carb, but I will do it before I take the 2bbl manifold off and put the Offenhauser and Qjet I just got on it.

Regards,
Josiah

Hillsboro, OR
1955 Willys CJ5 Buick 225 V6 160HP 270ft-lbs, T90 trans, Warn OD, PTO winch, Spicer 18 T Case, RS9000's, Dana 25F/ 44R,
5.38:1 gears, 11" brakes, Bestop Supertop, Hurculiner
post #6 of (permalink) Old 03-14-2001, 02:56 PM Thread Starter
 
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Re: engine vacuum question

For an aircleaner I'm using a 12" round open element cleaner, so it should be less restrictive than stock. I did set the timing idle etc with the cleaner off, just cause it was easier to access this way. I'm happy with the peformance so I'll just run it as is. There are others on this board that are having the same "problem" with the high ported vacuum, maybe it is just a quirk of running the Rochester on a 258. Oh well, good luck with your project and thanks again. Jason

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post #7 of (permalink) Old 03-14-2001, 03:13 PM
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Re: engine vacuum question

I went home at lunch and re-connected the carb and put my vacuum gauge on both ports.....They both look to be manifold vacuum.[img]/wwwthreads_images/icons/crazy.gif[/img] I am reading a steady 20hG on the small nipple just above the throttle plate and about the same at the base of the carb. On close inspection of the carb I also noticed several areas that looked to be places for vacuum connects that had been lead filled by the manufaturer. I am sure there are many incarnations of this '2G' series carb.
I do however find it hard to believe that there is no spot on this carb "other than the one feeding the choke" that is ported vacuum.
I picked up a book from a buddy simply called "Rochester Carburetors" by Doug Roe. I will give it a going over and see what else I can pick up.
I do have to say though....this is still the best my rig has idled in ALONG time. So I am curious to get it out and run it around to see how it behaves.

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post #8 of (permalink) Old 03-14-2001, 07:24 PM
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Re: engine vacuum question

Hmm, could be. Let us know what the book says. Did you check to see how the one feeding the choke behaves?
Josiah

Hillsboro, OR
1955 Willys CJ5 Buick 225 V6 160HP 270ft-lbs, T90 trans, Warn OD, PTO winch, Spicer 18 T Case, RS9000's, Dana 25F/ 44R,
5.38:1 gears, 11" brakes, Bestop Supertop, Hurculiner
post #9 of (permalink) Old 03-14-2001, 08:38 PM Thread Starter
 
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Re: engine vacuum question

I checked the port feeding the choke at the same time I did the others and it read the same 18" Hg at idle, and behaved the same at load etc.

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