Thread: Tri Power and Vaccuum Reading

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Just to give everyone an idea of what I am working with I have a 1955 pontiac 287. I installed an offenhauser tri power intake, new intake gaskets and rebuild 3 rochester 2gc. I bought the rebuild kits for the carbs from vintage speed also bought the bases for them. Got everything installed and the car runs fine. My only complaint/concern is I can only get 15 psi of vaccum out of the carb when adjusting the mixture, by the way I have #50 jets in the primary carb. Now this may not be a big deal but I can usually adjust a carb and tune the motor to get 16, 17 psi maybe even more so the fact that I am only getting 15 - 16 psi makes me wonder if I have an intake leak. I tried using carb spray to find the leak but no luck so far. Anybody have an idea or am I worrying about nothing.

I'll assume the carb your setting is the center one? And the front and rear carbs have the butterflies shut tight? Are you certain?? Is the linkage progressive to the outer carbs? Is there play in the linkage so it isn't pulling the outer carbs open?

What's the engine timing at idle? What's engine rpm at idle?

Yes I am tuning the center carb. Butterflys are completely shut. Like I said I bought new bases for all the carbs. The end carbs are solid aluminum. The only linkage that is installed right now in the center carb. Timing is 12 btdc, idle is 900rpm.

There are a few 'things' that will affect the vacuum. First of all, are the end carb throttle plates closing completely? While you purchased new or rebuilt bases from a, hopefully, reputable shop, that still doesn't mean that those butterflies are set right. Then there is initial timing - is it correct. can it be jacked up a degree or 2 without ping? Also, are you sure that the harmonic balancer is telling you where TDC really is? That's a 57 year old engine, and those balancers are seldom considered in an overhaul. Next would be the camshaft - what is it? Stock? Mild? Beyond mild? Next would be the actual valves themselves - while they may not be 'burned to crisp', the seats, unless someone has changed them are not hardened for unleaded or even very low lead gas and may be starting to erode. I have to assume (dangerous) that you have checked those very fey vacuum lines, carb base gaskets and air horn gaskets.

Now, with all that said, 15 in Hg, if steady, no blip, slow drop, and with a triple carb set up isn't really bad - though I would be expecting 17-18 if everything is ok

I would imagine the manifold is used so it may have been milled to fit go on a eng with milled heads or maybe your heads are milled so go to home ripoff and get a cheap propane torch take the tip off and put about 18 in of vacuum line on let the eng idle at operating temp turn the valve on (don't light it LOL) move the hose around the and see if the idle goes up or down that's the best way I have found to find a vacuum leak.....ted

Just another thought... Are you using a PCV Valve? If so, see if the vacuum reading improves when you remove and plug the hose from the vacuum source.

Have you tried to stick a screwdriver or something down inside the front and rear carbs to push against the throttle blades to see if they are seated? Or disconnect the linkage so that you are on the primary only?
Something to consider, seems to me taht motor idled about 450rpmish and the original carbs' internal passages are sized so that a lower depression will flow fuel for the idle circuit. Are you using a carb from say the mid 60's for your primary? That would be more consistant with the higher idle than the original rochester. I am not sure how that would play out (using the original) but it might be messing with you in odd ways.