My just completed 32 Ford hot rod has a 65 Pontiac GTO motor installed using a repro 66 OEM tripower manifold. When it was time to purchase carbs, I ended up buying small base Rochester’s. I did not know there were small base and large base carbs at the time. I ended up resorting to adapters to overcome the problem. I’ve been having poor running, fouled plugs and gas seeping from the top gasket which only has few hundred miles on it. I have tried different float levels with no luck. The engine has a Crower Thumpr cam hydraulic roller cam with a quite choppy idle and 12 “ of vacuum at idle. I changed the power valve for one with a weaker spring. No improvement. I installed a fuel pressure regulator and adjusted the idle pressure to 3.5 psi. Still not improvement. I finally measured the angle of the carb flanges and found them to be at -7.5 degrees from horizontal (tilted towards the front of the engine). The valve covers measure at -2.5 degrees so the manifold apparently has a cast in 5 degrees of downwards slant. I checked with the vendor who I bought the carbs from and he says that 7.5 degrees is too much for these carbs to cope with. So I am racking my brain to come up with a solution, I would like to stay with 3 carbs as it fits the traditional style of the car. I have considered the following solutions: 1. Machining the adapters to reduce the downward angle to 3 degrees. 2. Finding another manifold that will fit my heads (OE Pontiac 65 GTO 77 heads) that does not have a cast in slant 3. Using Holly 2300 2 bbl carbs with adapters on my current manifold. 4. Modify the motor and or the transmission mounts to tilt the front of the motor upwards. Number 1 is tricky, it appears I will have to remove at least 1/4” at the back edge of the adapters which are two piece totaling 1” in height. I checked with the vendor for the adapters and he was quite helpful and suggested a 1” riser that could be milled without interfering with the countersunk bolts used in the adapters. But when I acquired the risers and mocked up the setup, I felt it looked like too much of a bodge and not the look I wanted. Number 2 is a possible approach but there are not many aftermarket manifolds for 65-66 Pontiac heads other than the KRE Tradewinds manifold which is more of a high performance manifold. I have yet to investigate whether the KRE manifold has any downward slant cast in. Number 3 is also possible as there are adapters to use the 2300’s on my manifold. I’m not sure if Holly 2300 2 bbl carbs are any more tolerant of the mounting angle I have than the Rochesters however. If anyone has knowledge in this regard, that would be very welcome as I would not want to purchase another set of carbs only to have the same problem. Number 4 is also possible but probably only a modest amount of angle correction is likely achievable before interference with the firewall or any number of other obstacles or pinion angle considerations limit the repositioning of the engine. So I asking the collective wisdom of the hot rodders for help as I see no easy solution. I have attached pictures of the car and the engine. Thanks in advance for any help.
Factory Oem Tri Power was designed specifically for match heads & cam shaft combo. Out side of OEM spec engine If difficult to run correct with a aggressive cam shaft , Allso If not Oem using carbs in 1 carb & equal numbers in better choice the Odd numbers of carbs .
This is properly outside the box, separate the water crossover from the intake and turn the intake round. No idea if it’d actually work but in theory I can’t see why everything wouldn’t line up.
Option 5 - (3) large base Rochesters. However - exactly what carbs do you have ? A small base for a 283 Chevrolet are going to need m***ive recalibration before working on a 389 Pontiac. MAYBE you can get it work with m***ive recalibration (no, I no longer do carbs). Do you have an idle byp*** in the center carb ? ***uming you changed the spring on the power valve actuating valve, not the power valve ? Did someone ever tell the inboard marine manufacturers they could not use Rochesters on a greater than 7 1/2 degree manifold ? The adapters are not helping either. Check the second line in my signature block !!! Jon
James - very nice looking deuce - especially with a Pontiac V8! Attached photo is a factory ‘65 Tri-Power on a 400 block. Center carb is small base - end carbs are large base - same end carbs as ‘66 (if I’m remembering correctly). As you no doubt learned, the end carbs are tri-power specific - no chokes and no idle adjustment circuits. The factory manifold had these carbs at a pretty good rake and end carbs are progressive - either mechanical or vacuum linkage starts opening throttles when center carb throttle opens something like 60%. It might be worth fabbing block-off plates for end carb bores - see if things straighten out with just center carb functioning. Good luck!☘️
This was needed to correct the angle of the engine in the engine bay When installed the carbs were level. @James Halsey A car can easily drive on a 7° gradient, so it isn't a major cause/ issue Also are your small 2G's rebuilt? An often overlooked problem on 2G's is the check-ball for the accelerator pumps [they leak bad and cause it to run too rich] The ball needs to be seated by using a small br*** punch [use a piece of brazing wire] You give the ball a gentle "twack" with a screwdriver handle onto the br*** punch. The stainless steel round ball seats against the "shoulders" of the hole. It is under the boosters [you pull out the T retainer that locates the spring]
I worked in Pontiac dealerships as a front end mechanic in the 70's and the engines always sit in Pontiacs at that angle at least from early 60's on for transmisison clearance. That is the reason that the manfold has the carb mount surfaces at that angle. Lower the back of the engine to where the intake manifold carb mount surfaces are level and problem solved except for totally wrong carbs. Who ever installed that engine in that car installed it wrong for what it is and set the car up like a flathead or some earlier V8's with the intake on the same plane as the valve covers call for sitting level.Early nailhead Buicks come to mind there. I'm rather thinking that with GM you see a difference between up through 57 intakes and 58 or maybe 60 and later on most V8 engines.
Thanks for all the replies, answers below to questions asked: Oneball: that IS outside the box, cutting off the water crossover is a pretty common modification for racers that remove the intake frequently, the KRE Tradewinds manifold is sold with no water crossover. What is less obvious is whether the manifold ports are absolutely symmetrical. I have an extra manifold, I will take photos of the ports and study them. Carbking: Option 5: three large base Rochesters, if I could find them, they are hideously expensive ($5k) and would they be any better than the small base 2G’s operating at -7.5 degrees ? I don’t see why, the float bowl design is the same, yes? The carbs came from Vintage Speed, completely reconditioned and designed for tripower use. The center carb has idle circuits and a power valve while the end carbs do not. I have the end carbs blocked off now for trouble shooting purposes. Idle byp*** in center carb, yes. Spring changed was in the power valve actuating rod, not the power valve itself. The engine was run on the dyno ( I presume level carbs) and made 367 HP at 5000 rpm and 433 ft lbs of torque at 3900 rpm. Joeblow: yes, Keith Cornell built this car Ratmother: I do have the end carbs blocked off at the moment and I am using the Vintage Speed mechanical progressive linkage. Mimilan: I did change the accelerator pump check valve but just dropped it in the hole without giving it a gentle “twack”. I will do this, thanks. Mr48chev: I understand what you are saying about the engine angle in the ch***is of original Pontiacs, I have a 65 GTO Tripower 4 psd that I have owned for 40 years and the carbs sit close to level. I can raise the engine and lower the trans maybe 1/4” without major issues, not sure that will be enough to cure my problem. I will pursue the idea Oneball had of turning the manifold around and also call KRE to ask about their Tradewinds manifold. Thanks again for all the help.
Just spoke with KRE and they say their Tradewinds manifold has a cast in angle similar to OEM. But they say there is plenty of material there to mill it flat. They also said OEM manifold ports are symmetrical and it will fit turned around. So this is what I will attempt, cut the water crossover off with a bandsaw and reverse the manifold. That should give me a carb flange angle of +2.5 degrees. That could be reduced, if needed, by milling the manifold directly but I doubt it will be necessary. Thanks again Oneball and everyone who responded. Another issue I have been working on is the suspension has a loud banging noise over bumps. Sounds like its coming from the back. I have inspected the suspension thoroughly and see nothing obvious. The car employs torsion bar springs so its somewhat unusual. It uses heim joints with no rubber anywhere. When I get the engine sorted, I will post pictures of the suspension and see if anyone has ideas as to the cause of the noise.
I guess with my daft idea the critical bit is going to be leaving enough of the flange to close this off. And distributor clearance
It seems to me an easier option is to weld the manifold bolt holes shut and re-drill to fit your current carburetors, then try to match the ports best you can. A good aluminum welder could also close up part of the ports so you would have room to open back up to match the bases. Turning the manifold around sounds like a bad idea and sealing nightmare.
Had the large base on an edlebrock on a 355 chevy. Flow rates on those are a bunch-like 350cfm for the smallest one if I remember correct. I'm cheap so I'd shop the meets and find dogs that I could overhaul. Got a few for like 5 bucks. Use progressive linkage, run off the center only (here its nice if you can find a stock one and just rebuild it). Level angle on the center is what I used, ends were for dumps only-didn't matter. Original tri power probably had set angles, etc. but running off just the center one sure seems to work.
Why not start with this? Then see where you are at before cutting up a perfectly good intake manifold. Step 2, as @hepme suggested, search out a large base carb to use in the center. If rod still runs badly, some additional research and troubleshooting may be required.
I don't know what carbs you have, but they do have Chevrolet throttle arms, so..... In the "for what its worth" category: Small base 283 Chevrolet Rochester was 1 3/32 inch venturi (area 1.88 square inches) 1965 Pontiac 3x2 center small base Rochester was 1 1/4 inch venturi (area 2.45 square inches) The 389 Pontiac center is a bit more than 30 percent larger than the 283 Chevrolet carb. Jon
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