Thread: Need to pick your brains about headers

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What is the vacuum reading and what rpm does the motor idle at in Park or neutral. How low can you get it to idle without stalling?

Could you explain EXACTLY what the people you have talked to mean by the vacuum being off and what vacume sourse you are using to get your vacume reading. A vacuum gauge can be a very good tool for diagnosing engine problems but different symptoms give different gauge readings and knowing what those readings are is the key to answering your question.

As mentioned the vacuum problem may be an external problem such as a mis-sized PVC valve, leaking manifold or the end carbs on your Tri-Power. Unless they are completely blocked off (i.e. a solid plate between the carb and manifold) you may have a vacuum leak originating from there.

From what you indicate it seems the intake worked well on the 283, but not so well on the 350......not a real big surprise considering it was now being asked to feed 67 additional cubic inches. That big of a jump would require major re-jetting and there is a good possibility the center carb should be changed out for a large base version of the Rochester.

The plates are simple to make and good tool for isolating a carb problem/vacuum leaks. Get a roll of roof flashing (any hardware store) which is simply thin aluminum sheet. Use a carb base gasket as a pattern and cut the plates with heavy scissors (bolt hole can be punched with a hollow gasket punch as a drill will tend to catch and twist the sheeting up. Install the block-off sandwiched between 2 carb gaskets (Car Quest part number G7327 are a good gasket to use) and then bolt the carb down. This will eliminate the end carbs as a potential problem, and either give you a place to look for a vacuum leak to move on to someplace else.

I can't tell for sure but it looks like your may be using an Offenhauser intake and these can be a little touchy to get dialed in correctly especially on an engine bigger than a 327. The manifold was originally designed in the early 60's when SBC engine displacement was only 283 and 327 cubic inches. Early literature I've seen from Offenhauser actually show them configured 2 ways....for "Economy" with a single 2 BBL carb in the center location and the end sealed with block off plates (in effect what you are doing) and for "Power" with a block off plate in the center and 2 end carbs installed. Based on this litterature it appears that Offenhauser never really intended to have these set up for 3 carbs with progressive linkage.

Configuring them for 3 carbs with progressive linkage is not a big deal and they do work well that way (personnal experience of having used the same manifold on a 307/327/383 and 350 (currently on a 72 LT1) they can definatly be made to work).

Anyway for now the best bet is to provide what the specific vacume readings are are concerning people to give people a better idea what is occuring.

Originally Posted by poncho62

As the Irish guy said..........vacuum leak. If you are not using the other 2 carbs, it would be best to seal off those carbs. A simple sheet metal plate under the carb between 2 gaskets would do it.

Yes they have been plated )sealed off) already by last owner, who reverted to that to solve the problem. He tried to make the progressive linkage work, but the car would not idle right, it ran OK once going faster. He ran out of money and gave up LOL. He did not have money really and that is why he sold the car when he moved to Northern CA. he couldn't keep it financially. Anywhoo, this is WAY down on my list of "to do" things but I like to start investigating ahead of time so i am ready when the funds are.

Engine runs good, never stalls. It does seem to idel a bit rough (possibly - but not sure) and may be eating up too much gas (about 10 to 12 miles per gallon). Otherwise powerful and pretty fast.

Ok tell me why you dont like Rams horns?
Sorry i will make a post on it

Originally Posted by daveyboy1956

Ok tell me why you dont like Rams horns?
Sorry i will make a post on it

They have a distinct sound that I dont like.... almost like marbles in a can at high RPM and I dont think they are very attractive to look at when showing your engine bay

Originally Posted by HiboyGal

Yes they have been plated )sealed off) already by last owner, who reverted to that to solve the problem. He tried to make the progressive linkage work, but the car would not idle right, it ran OK once going faster. He ran out of money and gave up LOL. He did not have money really and that is why he sold the car when he moved to Northern CA. he couldn't keep it financially. Anywhoo, this is WAY down on my list of "to do" things but I like to start investigating ahead of time so i am ready when the funds are.

Engine runs good, never stalls. It does seem to idel a bit rough (possibly - but not sure) and may be eating up too much gas (about 10 to 12 miles per gallon). Otherwise powerful and pretty fast.

I'm with Mike P, what is the vacuum reading, the idle speed and where are you sourcing the gauge from?


One experiment you could do would be to try each carb in the center position with the outer two carb mounts blocked off by a plate.
Sounds like you already have plates that would work.

If all the carbs function ok why not run them with straight linkage?
I've had many different multi-carb setups and ran them all with straight linkage.

Progressive linkage is ok . . . if you get a good one . . . there's a lot of junky stuff out there posing as progressive, but some of it is downright dangerous.
One setup I see now and then is nothing more than bent wire for the sliding part.

You can make a high quality straight linkage for about $30. - $35. for parts and a little time with a hacksaw, grinder and die.

Depending on what you're running for a throttle control (cable - ok, rod- best) you could make up a nice throttle rod setup to match.

HBG,
What you need to find is a greybeard that can tune a real motor.
Three two's should run fine on your 350 with straight or progressive linkage. If it has a stock cam (which I suspect) that would explain why it runs so well with end carbs. blocked off. Vacuum leaks and the way the pvc and breather are installed is something that should be checked. I have used a vacuum gauge to tune for years and it is how you adjust a carb. and I use it to set timing (although a race motor is another story). Some folks will probably not agee with this method but there's more than one to skin a cat.
I adjust carb. for higest vacuum reading (adjusting mixture and idle speed) and adjust dist. untill I get the highest reading. I usually have to back off timing a hair if I have trouble starting, but that is usually not a prob.
So much for my rambling. I hope you figure it out soon.
Keep on Truckin' (Roadsterin' )
Jim
P.S. 12 MPG sounds awfull low, 18-20 seems more in line with your combo.

There have been numerous dyno tests on the SBC with 2 fours and with 3 twos ... a single 4 barrel carb on a aluminum performance intake always made more power ... gave better mileage and was a LOT easier to live with ...

That's why most folks use them ... over the old outdated multiple carb systems ...

They ( single 4 barrel setups ) also have less vacuum leak issues ... and are easier to tune
I would buy a new 600 CFM Edelbrock and a Edelbrock aluminum intake for it ... seal up the intake well and you would be amazed at the power gained and the mileage improvement ...

Yeah that is all well....but..... Those 3 x 2 barrels on that manifold blend in well with the rest of the roadster. They make it look "complete". 1950's style, it would be shame to wreck that look.

The other thiing is vacume leaks, try and locate the sorce of the leak manifold vacume lines, carbs.

Manifold - It has been around for a while it could of been decked to suit the other chevy, that is a new crate motor in there, so you could have a leak between the manifold and the heads from different angles and not lining up properly.

vacume lines probably the easiest fix, if they ar hard cracked or broken just replace them. Also while your at it check to see that the vacume advance canister on the distributor is not ruptured.

Carbs old worn linkages leak vacume from the slop caused by wear, see if you get much radial movement. If you want all 3 set up and going again, might pay to get them rebuilt and balanced, it is expensive for this plus getting the right linkage set up, you probably want with those carbs, using amechanical progressive linkage something like the 2 end carbs gradually coming in when the primary middle carb is around a bout half open. The whole idea being that the transition happens smoothly.

No, no, no I am not going to a 4 barrel i LOOOOVE my 3 deuces the only thing i like more than 3 deuces is SIX DEUCES !!

Pretty sure those are 6 Stromberg singles, but we get the point...........

isn't it something? really beautiful....