heard this one yet????

[stevenfromtexas]

Originally Posted by Steeveedee

.055" plug gap is pretty big if you are still using the stock coil. I'd close them down to no more than .040" as VWNate suggested. Too big a gap with the stock coil can also cause performance issues. There is no amplification with the Pertronix ignition; it is just a more reliable switch than breaker points.

Make these changes one at a time so that you can trace down the actual culprit! If you don't, the next time something crops up, you'll be looking at what you did and wondering which one actually was the solution...and doing them all again until it's fixed.

yes sir, it is a stock coil. i was a cheapskate (was reading a post earlier and saw that word used....thought he was talking about me) and did not buy the coil at the same time as the ignition stuff....so yes, to answer your statement, it is the stock coil.

[Steeveedee]

Originally Posted by VWNate1

The distributor vacuum advance port is on the inside of the carby facing the rocker box where you'll often burn your fingers trying to re attach the rotten and loose fitting hose .

It gets NO vacuum until the throttle is opened, even a tiny bit .

GM made millions and millions of these carbys and some had three + vacuum ports .

Maybe a picture of yours would help ? .

No, DO NOT close those spark plug gaps ! .

When HEI was first introduced the recommended gaps were .065" ! .

My '69's engine burns a LOT of oil so I'm running the 1977 250 i6 HEI GM dist. and .070" plug gags, it runs so sweetly and strongly it's amazing ~ no oil smoke either as it gets fully burned ~ you can smell burned oil when standing behind my truck but not see any smoke .

Leave the plugs be for now, it sounds like you're closing in on any problems .

An HEI can fire that gap, but if he has a stock coil (he's only added a Pertronix), it really isn't up to the task. As I posted above, he should be making the changes one at a time, anyway. He can wait on the plugs.

[stevenfromtexas]

Originally Posted by Steeveedee

That port is above the throttle plate, so should have no vacuum at idle- it is "ported spark"- the correct place for your vacuum advance to be hooked into.

i wonder if my butterfly is screwed in on the wrong side of the throttle shaft., is that possible??...currently the butterfly closes just above the hole where i'm attached.
i defiantly have vacuum at idle...hmmmmmm and when i throttle up the advance canister arm thingy at the distributer losses it vacuum

[stevenfromtexas]

i just stumbled on a very interesting article from "stove bolt forums". i think the guy has the same issue i do. i sounds like his rochester B has the vacuum hole drilled in the wrong spot. his is below the butterfly also and not above. he said he has vacuum at idle and it drops off when he opens the throttle. same dang thing I've got going.
.................................................................................................... ..
"ported vacuum" ( and other myths) #34913 Mon Apr 29 2002 07:16 PM
F
Frank54 Offline OP
Shop Shark Joined: Aug 2001
Posts: 539
Scranton, PA. USA
An update:

A while back the topic of "ported vacuum" for the distributor advance was being kicked around.

My own difficulty was with a 1972 Chevy 250 six which had had it's original "Transmission Controlled Spark" system & Monojet carb replaced with a Rochester "BV".

The problem was that the distributor was getting a "full vacuum signal" (and thus full advance) at idle, and as the throttle was opened, the vacuum dropped and the timing retarded. Essentially, the stuff was working backwards from the way it should; I had "vacuum retard" instead of vacuum advance.

I looked at the base on a spare BV carb and noticed that the vacuum port into the carb throat from the 1/8" pipe tapping was drilled on the "manifold side" of the throttle butterfly, so naturally, anything connnected to this port would have full vacuum @ idle, and less than that as the throttle opened.

I studied the operation of the vacuum advance on the 235 on my '55.1, and the vacuum advance doesn't operate until the throttle is opened.
So, reasoning that the advance should function the same way for any engine using the B-type carb, I determined that the 1/8" hole through the carb throat was on the wrong place!

Experimenting on the spare carb, I filled the existing hole with J-B Weld, and when that had cured, I drilled a new 1/8" hole through the throat, at the top side of the pipe tapping, which comes out above ("air cleaner side")of the throttle butterfly. wink

After smoothing the inside of the throttle bore w/ emery paper, I put the "improved throttle body" on the truck, and hooked a vacuum gauge into a tee on this line.
After re-timing the engine (sticker says "4 BTDC")
I got it running, and at idle pulled no vacuum, no advance. : smile
As I opened the throttle, the vacuum began to rise, and the ditributor advance began to do its thing. smile smile
So far, so good. So I opened the throttle quickly, and the vacuum jumped to 10 or 15 inches, and the engine responded smartly. grin

So, I've been driving it this way for about two weeks, and the performance is greatly improved, although not yet perfect: if you floor the accelerator while under load, the engine begins to "break-up and stumble" as the revs get higher ; so I'm going to go through and tune up the whole engine and see how we're doing.

Prior to making this "improvement", the truck had been a real "dog", even for a six; no power, heating-up on a pull, using way too much gas (my rear bumper is black from excess richness in the mixture!).

I'm planning on doing an "exhaustive" vacuum gauge analysis on the 235 vacuum advance system when I get the brakes done on the '55.1; the weird thing is , my inspection mechanic has a '65 Chevelle with a 230 six, and the Rochester BV, and his vacuum system is just a steel line from the brass fitting on the carb throat to the distributor, and his works just fine.

Did GM vary the position of the distributor vacuum port relative to the closed throttle plate on the Rochester "B" carbs and their variants?

Frank McMullen
1941 Chevy dump truck
1949 GMC 100 pick-up
1954 Chevy 3600 pick-up
1955.1 4400 dump truck
1941 De Soto & other MoPars
1928 Model A F**D Special Coupe
Re: "ported vacuum" ( and other myths) #34914 Mon Apr 29 2002 10:43 PM

4ontheFloor Offline
Extreme Gabster Joined: Oct 2001
Posts: 5,487
Forest Grove, OR
Yes, GM had differant versions of the Rochester B carb with ported and manifold vacuums. The B on my stovebolt is ported. It doesn't matter which you use as long as the distributor curve matches. Some are meant for ported and some are meant for manifold. Ported vacuum only is in effect when your foot is not on the gas pedal. Once the throttle is opened slightly it becomes manifold vacuum.

Paint & Body Shop moderator
A lone amateur built the Ark. A large group of professionals built the Titanic.
Re: "ported vacuum" ( and other myths) #34915 Mon Apr 29 2002 10:46 PM

4ontheFloor Offline
Extreme Gabster Joined: Oct 2001
Posts: 5,487
Forest Grove, OR
PS- timing always advances with vacuum and retards with less vacuum. In other words, as the load on the engine increases, it drops the vacuum and the timing needs to retard. Your problem is probably the transmission controlled advance distributor curve. You need a distributor from a manual transmission truck to match your carb and you should be set.

Paint & Body Shop moderator
A lone amateur built the Ark. A large group of professionals built the Titanic.
Re: "ported vacuum" ( and other myths) #34916 Tue Apr 30 2002 01:57 PM
F
Frank54 Offline OP
Shop Shark Joined: Aug 2001
Posts: 539
Scranton, PA. USA
Josiah,

Thanks, I think I know where I can get such a distributor for cheap!(Free, that is! grin )

I'm really resisting the idea of putting that TCS system back on the vehicle- too many gizmos to get out of whack!

Will let you know what a different distributor does for things!

Frank McMullen
1941 Chevy dump truck
1949 GMC 100 pick-up
1954 Chevy 3600 pick-up
1955.1 4400 dump truck
1941 De Soto & other MoPars
1928 Model A F**D Special Coupe
Re: "ported vacuum" ( and other myths) #34917 Tue Apr 30 2002 10:30 PM
B
banjo Offline
Member Joined: Jul 2001
Posts: 93
Florida
If an ignition system is designed to operate with ported vacuum, you can get some bad results using straight manifold vacuum.

Like 4onthefloor said, ported vacuum becomes manifold vacuum after the throttle is opened slightly. When the throttle is closed, there is, essentially, no vacuum.

At idle, there is no need for additional advance. Piston speed is slow, but the fuel mixture still burns at nearly the same rate. Using additional advance may make the egine idle somewhat rough and the typical response is to adjust idle mixture rich to smooth it out. This shortens the life of spark plugs, carbons up the combustion chamber, and can contribute to poor off-idle response.

Upon opening the throttle, manifold vacuum is allowed in and vacuum advance begins to be phased in. If the throttle is opened a lot, manifold vacuum will remain low and, consequently, vacuum advance will remain low (or none). As RPM rises, mechanical advance will phase in, regardless of vacuum. As RPM comes up and the throttle is reduced, vacuum and vacuum advance will increase.

At cruise condition, you will have maximum manifold vacuum and maximum vacuum advance.

If you hold the throttle wide open and wind up the engine to several thosand RPM, vacuum will be coming up with RPM. Guess what would happen if you suddenly release the throttle to idle position? a HUGE vacuum spike. This is hard on Vacuum advance cans (if you tend to replace them often, check your vacuum connection). However, the ported vacuum saves the day by blocking the spike from the advance can.

Now, all this is for systemd DESIGNED to work with ported vacuum.

YMMV

1954 Chevy 3100
454, T-400, GM 14-Bolt Disc brake
Monte Carlo Front Clip
Re: "ported vacuum" ( and other myths) #34918 Thu May 02 2002 01:12 AM
F
Frank54 Offline OP
Shop Shark Joined: Aug 2001
Posts: 539
Scranton, PA. USA
Banjo & Josiah,

Well, what you fellas are tellin' me makes sense; the situation I "inherited" when I purchased the truck sounds like it was a case of a "ported vacuum (TCS) distributor" & advance hooked up to an older carb that furnished manifold vacuum to the brass distributor fitting.(Previous owner probably thought:"A vacuum line is a vacuum line.")

So, I think I've created some sort of "ported vacuum" by moving the location of that hole into the throttle body to the upper("atmosphere") side of the throttle plate; it mightn't be perfect (which is why I "operated" on the spare carb), but I'm gettin' closer! wink

Soon's I get a little "mad-money", I'm going to get a distributor & vacuum advance for the earliest 250 application I can find, and hook that up to a Rochester BV that I personally removed from a 230/250, and see how I'm doin'.

The way it was before my "elective surgery" to the throttle body, she'd spike to almost 25" of vacuum if I closed the throttle while going down the highway; that'd turn lots of things inside out!

Frank McMullen

[Steeveedee]

I'm going to disagree with him on that "vacuum retard" business. That's not a feature on our trucks. He doesn't have the same type of carburetor as you do, so I'm not going to rely on that information.

You set the base timing with the distributor vacuum disconnected, right?

What do you have for a vacuum gauge?

[stevenfromtexas]

we are in bid-ness!!!!

took the carburetor off again today for yet another look-see. i want to apologize for the misinformation i gave you guys yesterday. after looking at my butterflies, i was wrong in saying they close above the vacuum port. i was looking at the air/fuel screw hole

i was thoroughly confused when y'all said ZERO vacuum at idle. i had 17'' and the advance would drop off when i accelerated. so, i started over with all of the settings and made sure the butterfly was closed completely and started from there this morning.

i also subscribe to Mikes carburetors on youtube. very good and informative. i pulled up his "vacuum advance" video on a single barrel. he mentioned a particular gasket to block off the hole at the base of the throttle body. that got me to thinking.....

soooooo, guess who found the most bestest, most perfectly drilled and shaped gasket in their rebuild kit.......
it seemed like this hole was the turd vacuum leak/sucking i was hearing.

cleaned everything this morning and put that gasket on and started from the beginning.

don't laugh at me.....well you can i guess 'cause i'll never know.....
i have a HF brake suction/vacuum pump i used to assist with bleeding my brakes. once i got the pick up started with fuel in the bowl, i put the gauge on the port at the carb. it still has 5'' of vacuum because i needed to turning he idle screw slightly to keep it running. i assume this is still an issue i'll tackle with y'alls suggestions.

anyway, i set the timing at 10* disconnected. and when i open the throttle slightly the gauge actually runs up 20''. i then attached the hose to the canister and advance actually does what its supposed to .....advance
ALSO........NO MORE SUCKING SOUND.....yeeeeyaaaaaa

so buttoned everything up and made my 5 mile loop and no more buckin' and jerkin' with 1/4 throttle.....the throttle response was awesome too!!!

thats when i went and found the guy up the street with a drag car and i raced him!!! that part is not the truth.

i want to thank all of you guys for ALL of your help and suggestions

[Steeveedee]

Great! You also ironed out that accelerator pump squirt issue. Feels great, don't it!? Next person who posts questions about a problem with a Rochester Monojet, you'll be one of the experts.

[stevenfromtexas]

Originally Posted by Steeveedee

Great! You also ironed out that accelerator pump squirt issue. Feels great, don't it!? Next person who posts questions about a problem with a Rochester Monojet, you'll be one of the experts.

hahaha no expert here. YALL however are the experts!! thank you brother for your help!!!

[Missourimark]

Fantastic glad you stayed with it , no doubt a new educational situation. Head scratchers like these can be such a challenge for the best of mechanics and are easy to give up on.

[stevenfromtexas]

Originally Posted by Missourimark

Fantastic glad you stayed with it , no doubt a new educational situation. Head scratchers like these can be such a challenge for the best of mechanics and are easy to give up on.

yes sir!! i was at that point of sending it to a pro technician. i don't have an ego with 99% of things, BUT with this little pickup i want to say i did EVERYTHING.....(except the tint on the windows and machine work on the block).......

"educational situation" for sure!!!! the whole project has been. this has been a BLAST!!!

[VWNate1]

Don't you just love the satisfaction of taking the time to work through a problem your own self ? .

This is why I always push folks to try the DIY, you'll usually do a better job because YOU CARE and won't rush the job .

Now, figure out why it needs so much idle screw to keep it running and eventually you'll get to zero vacuum @ idle .

Peppy little thing, ain't it ? .

Unless you have problems with missing or stumbling, leave the spark plug gaps where they are ~ the ideal thing is : as much gap as they'll take without causing running issues .

We can help you get a better coil *if* you want, what the Pertronix does is : drives the stock coil to it's maximum output all the time ~ points don't really do a good job of switching the current on and off .

[stevenfromtexas]

Originally Posted by VWNate1

Don't you just love the satisfaction of taking the time to work through a problem your own self ? .

This is why I always push folks to try the DIY, you'll usually do a better job because YOU CARE and won't rush the job .

Now, figure out why it needs so much idle screw to keep it running and eventually you'll get to zero vacuum @ idle .

Peppy little thing, ain't it ? .

Unless you have problems with missing or stumbling, leave the spark plug gaps where they are ~ the ideal thing is : as much gap as they'll take without causing running issues .

We can help you get a better coil *if* you want, what the Pertronix does is : drives the stock coil to it's maximum output all the time ~ points don't really do a good job of switching the current on and off .

yes sir!!!!!.....thank YOU so much for your help.....you guys are the best!!!
this was Dads pickup and the one i learned to drive a standard in, so i will have it forever, and do small tweaks here-and-there but i do have the fever to do another one.


took my daughter ( 9 ) to the tennis court today to hit-a-round.....good friggin' feeling!!!!!

[VWNate1]

Very nice indeed .

I hope your Daughter likes the rig and decides to keep it .

I was worried there'd be some Hot Rodders who don't understand how ported vacuum works, some really big heavy duty V8's use manifold vacuum, it's very strange .

The collective knowledge here is amazing .

[stevenfromtexas]

Originally Posted by VWNate1

Very nice indeed .

I hope your Daughter likes the rig and decides to keep it .

I was worried there'd be some Hot Rodders who don't understand how ported vacuum works, some really big heavy duty V8's use manifold vacuum, it's very strange .

The collective knowledge here is amazing .

little legs aren't quit long enough yet to teach how to dive it, but she is defiantly trying.

she's so sentimental, i am very certain she will never let it go...hahaha

[VWNate1]

Nice ! .

My Son is a racer (a real one, not some dufus with fart cans) so he didn't want my old 1949 Chevy 3100 shop truck, I now have this 1969 C/10 I doubt he'd want for free as it has a 250CID i6 and TH350 slush box, PS & PB etc. .



As you can see I was working to - day on a vintage Motocycle, closely supervised by Gizmo (foreground) and B*tch (under the truck) .

[stevenfromtexas]

Originally Posted by VWNate1

Nice ! .

My Son is a racer (a real one, not some dufus with fart cans) so he didn't want my old 1949 Chevy 3100 shop truck, I now have this 1969 C/10 I doubt he'd want for free as it has a 250CID i6 and TH350 slush box, PS & PB etc. .

Attachment 1808428

As you can see I was working to - day on a vintage Motocycle, closely supervised by Gizmo (forground) and ***** (under the truck) .

little lady is asleep here.....in the morning ill show her this post. she's gonna go nuts over your pup passed out on the shop floor...... that is her and my dream to have a dog to hang out in the shop/garage floor.....

[VWNate1]

Be sure to look at Pound Puppies first and always go for the mutts ~ mixed breeding makes for healthier pets (NOT inbreeding !) .

My Sire Gizmo here, was horribly abused and didn't like anyone ~ he wasn't vicious, he simply didn't like people, they were going to put him to sleep so I gave him a chance and am ever so glad I did .

B*tch simply refused to allow me to leave the pound without her .

Dogs give unconditional love and they're really smart too ! .

All of mine have been able to train me to give them belly rubs on demand .

[VWNate1]

"question....

the "C" shaped linkage at the bottom of the carburetor, connecting the accelerator pump to the throttle body, can that be adjusted/tweaked/bent slightly to help with anything I've got going? " .

Yes, that is why it's 'C' shaped .

There's quite a lot of good info on these old Rochester 'B' series carbys in the factory shop manual, it's still available brandy new for about $30, used ones with dog eared and greasy pages for far less, you really should buy one

GM also printed a nifty 194/230/250/292 engine manual......

[stevenfromtexas]

Originally Posted by VWNate1

"question....

the "C" shaped linkage at the bottom of the carburetor, connecting the accelerator pump to the throttle body, can that be adjusted/tweaked/bent slightly to help with anything I've got going? " .

Yes, that is why it's 'C' shaped .

There's quite a lot of good info on these old Rochester 'B' series carbys in the factory shop manual, it's still available brandy new for about $30, used ones with dog eared and greasy pages for far less, you really should buy one

GM also printed a nifty 194/230/250/292 engine manual......

thank you so much, ill look this up, i like having books on hand to reference random stuff....

[57rookie]

My first thought was fuel filter. Since that has been addressed it has to be one of the two following: needs a carb rebuild or reman carb - if you don’t want to do this I would adjust the idle screw(s). When engin warmed up turn the idle screws out one at a time until you feel the engine running ruff then turn back a half turn. Do this for both the left then right side for an v8 engine carb and it will give you good air/fuel mixture. If this is not it then it needs either plugs or wires or both.

[stevenfromtexas]

Originally Posted by 57rookie

My first thought was fuel filter. Since that has been addressed it has to be one of the two following: needs a carb rebuild or reman carb - if you don’t want to do this I would adjust the idle screw(s). When engin warmed up turn the idle screws out one at a time until you feel the engine running ruff then turn back a half turn. Do this for both the left then right side for an v8 engine carb and it will give you good air/fuel mixture. If this is not it then it needs either plugs or wires or both.

thank you very much for your reply.... i have a straight 6 with single barrell rochester (250). i've actually got everything running really well now.

thank you so much for your reply.

[Benjamin]

Great thread! Feel-good story of the year.