Carb/manifold question

[Don Jordan]

A little back ground. I've got dual carbs on an otherwise stock P15. The carb in the back will suck your hand in and stall the engine. The one in front barely sucks at all and if you cover it it doesn't change the idle. I rebuilt both. Adjusted again and again. Today I was talking to an old boy who said it sounds like a blockage in the manifold. Is there a way to check that without taking the manifold off? The car runs perfect - but I'm sure it's running on just one carb.

I can't imagine what would plog it up.

Final question: Will it hurt anything if I keep driving?

Thanks.

[Plymouthy Adams]

Don, does this condition exist at ALL throttle positions and for sure is the carb throttle plate opening accordingly...if so then without a doubt there is some restriction...was your manifold "split" or just modified with extra carb base ..

[Don Jordan]

It was split I believe. I think George Asche did it. The linkage is fine. Everything I can see I've checked. I guess I may have to remove the manifold.

I have a manufactured manifold but it's for 2 barrel carbs and only a single exhaust. I was thinking of selling it but have no idea what it's worth.

[claybill]

hey...SAME THING HAPPENED TO ME LAST YEAR.

i thought i had both carbs adjusted for VACUUM. car ran well etc jjust as you describe.

on the forum someone said ...double check your vacuum EQUAL ON both CARBS using A TOOL. i didnt believe it. true as could be..when i fiddle with adjustments i actually got the rear carb to suck when the front one wouldnt...now they are equal....

no it wont hurt the engine. it cant.

hardly something blocking the manifold.

valves are obviously ok.

do a super vac adjusting on the balance of vacuum on carbs....it is very very slight difference but it works!

bill

[P-12 Tommy]

Don Coatney has some tool and some tips on adjusting both carbs. You can ask him.

Tom

[Don Coatney]

I also believe you have a linkage adjustment issue as your carburetors are not syncronized. Pictured is the flow meter tool required to adjust your linkage to insure both throttle plates move at the same time.

[claybill]

first...make sure that your linkage to the carbs is shutting the butterfly closed..EXACTLY......one might be just a hair open..make 'em slam shut.!

set vac from there.

bill

[Don Jordan]

I feel really silly asking this but how do you set the vacuum? I have one line from the vacuum advance and a Y to the carbs.

Don - I have that tool. On one carb the red ball pegs at the top. The other one it barely moves.

I guess it's time to double check the linkage. I assume you just look down the throat how else would you measure?

This is all new ground for me. I only put the 2 carbs on because I thought it looked neat. And no one sees them but me.

This week end it's back to the linkage.

thank you all.

[Don Coatney]

I feel really silly asking this but how do you set the vacuum?

Don - I have that tool. On one carb the red ball pegs at the top. The other one it barely moves.

I also want to know how to set the vacuum?

You need to adjust your linkage until the tool shows the ball close to equal on both carburetors. You will be surprised at how much better your engine will run.

[Don Jordan]

While we're on the subject:

I bought this manifold a long time ago. Can anyone tell what kind of carbs go on it? Right now I have 2 single barrel B&B on the car and an extra manifold.

Just thought I'd pass on a picture.

I will check the linkage Saturday now that the temp has dropped to a more comfortable 90°

[james curl]

Ford Flathead carbs. Stromberg 97s, or Ford Holley 94s.

[cole18cars]

I also tried to use the Uni-Sync tool, but when I put it over one carb, the engine almost died. I have dual one barrels on a Offy intake. I opened the tool as much as possible, but still no luck, lack of air? Spent good money on this tool, but can't seem to get it to work. Any help? Thanks

[rustyzman]

If your car stalls with a unisyn over one carb and the tool is all the way open, you have an adjustment problem on the carb. The idea is to adjust airflow of each carb so the ball floats at the same level on each carb. Chances are one carb is sucking way too much air at idle when it should be distributed equally between the two. Try adjusting the throttle stop open more on the other carb (the one that does not stall the engine) and close the one on the carb that stalls the engine a bit. See if that makes a difference. Of course do this at curb idle to start with. With some work, you will get them perfect.

I would recommend the use of a unisyn or its equivalent rather than using ported vacuum or even manometers by the air horns. This is the most accurate and user friendly method I have used.

My 240Z had dual SU's and later on the race car we had three 2 barrel Mikunis. With a unisyn we could adjust all 6 barrels to be equal at idle and 2500 RPM, so stick with it and you will make it happen.

I would expect this to improve your throttle response and power output a bit as well.

Just my opinion, someone else may have a better way.

[james curl]

I keep my Unisyn in my glove box, always handy and you can show people what you mean when you tell them to use a unisyn.

[greg g]

The synching tool is used not to set the idle adjustment screw. It is used to assure the throttle plates open equaly and together (synchronized). To start you need ot make sure your carbs are fully closed. then set each one at the idle adjustment to 1 1/2 turns open and the idle speed adjustment screws out till they are off the stops. then turn each one till it just contacts the pad on the throttle plate linkage. If you screw it down to tight it will immedialty screw up the syncronization, you need to start at zero with bothe carbs and then adjust from there. but the adjustment is to the linkage, not the carb screws.

To measure intake vacuum, attach the vacuum gauge to the windshield wiper port. not the carb vacuum. Those signals are opposite to each other.

I seriously doubt there is any blockage in your intake manifold unless you have a serious mouse infestation in you garage.

Fluid dynamics will dictate that the air will flow through the path of least resistance, so if one throttle plate is even slightly more open than the other. You need ot address that before any other adjustments will have any other effect.

[Don Jordan]

Thank you very much. Just because I thought I had done everything right I'm going to switch out the weak carb with a spare and then try adjusting all over again.

I did rebuild both carbs and with my luck I may have screwed something up. It's like when you're looking for something you always find something else you were looking for. If I keep fiddling I'm bound to get it right.

That's one of the reasons I like these old cars - I couldn't do this with my Toyota.

thanks again.