Technical CHEVY TRI POWER QUESTION

I have a beautiful 32 hiboy roadster that I found on the H.A.M.B. classifieds. It has a 327 , 1.94 heads, a touch more cam than the old 30-30 chevy cam. As mentioned it has three Rochester 2 barrel carbs. I cant get the engine to idle. Everyone says I probably have a vacuum leak. My gauge says between 13 and 14 inches. Is that enough? I have tried just about everything. Had all three carbs gone thru by the original supplier, checked the timing, sprayed starting fluid thru a tube all around the manifold, blocked off the end carbs. All to no avail. I get no real response from the low speed needles, in or out
, seems to run pretty much the same. When I stop, it feels like it is running out of gas, [too lean?] and under 1800 rpm's going down the road it surges. I love the way this setup looks, but am preparing an old edelbrock 3cb 4 barrel manifold, and looking at holley 600's as an easy and simple solution to my delima. Any suggestions will be greatly appreciated, Rich

 

The front and rear carbs should not have any idle screws. That is all done from the centre carb. If you want the best advice contact Larry http://hotrodcarbs.com/

 

To me what your describing is way lean and Vac leak. That could come from a bad power booster or Vac advance can on the dist. First thing I'd do is Check the fit of butterfly,s on both end carbs. That means removing them and shining a light to check fit One good back fire can bend the plates and make a Vac leak at idle. Also bad T shaft bushings = vac leak. Both end Carbs should be totally out of play below 1k rpm. Hudson48 is correct, end carbs don't have idle adj screws. Those adjusters fall out of play at about 850 rpm, some sooner. If you can't get it to idle below 1k your not going to get anything out of them. Process of elimination my friend. Start in the front and block things off till it comes around.
The Wizzard

 

Had similar problem with multiple Rochester set up. Like Wizzard said, in my case butterflies needed to be finessed to close completely. Once they did , carbs were tunable. Remove carbs, look for light leaks around butterflies. Good 1st step anyway.

 

Block off the outside carbs with a gasket or beer can and work on the center carb then one at a time add the others into the mix and you should find your problem.

 

The throttle plates on the correct end carbs are thicker and will fit the bores tight, but you may have to loosen the throttle plate screws and let the find their own location. Then retighten the screws.As clunker said... no light. I had to use extra springs to completely close the end carbs.

 

Thanks for the response. As I said in my initial post, I tried blocking off the end carbs, no change. The end carbs have the idle screws blocked, and the brass thing in the bottom of the bowl plugged also. I unplugged the vacuum line from my distributor, and plugged it off, still no difference. If there is a vacuum leak, it has to be on the underside of the manifold. Has anyone ever sprayed starting fluid down the breather pipe to check for such a leak? How about my 13 to 14 inches of vacuum? is that acceptable? Thanks Rich

 

To check for an intake leak on the bottom side...remove your pcv valve and block that hole then start it up and with all the crankcase breathers blocked off put your vacuum gauge on the dipstick tube and see if you get vacuum on your gauge if so the leak is in the lower intake gaskets.

 

Might have to start from the beginning, this helped me.

http://www.jalopyjournal.com/forum/threads/tri-power-problems.465986/

 

Before I pull the manifold off, looking for the vacuum leak that I don't think exists, does anyone with a similar setup know what vacuum they are pulling?

 

I will add some info from my set up that might help. The cam is a solid roller with 300 deg. advertised duration and a single 4 bbl. I couldn't get more than 12" vac @ 1000 rpm and I was swapping power valves to clean up the idle. I put new seals on the secondary throttle shaft and was able to get 13.5 inches of vac. at 900 rpm and went back to a 6.5 power valve. The point is that the vac loss with the throttle shaft had to be small, but was significant and if I increase rpm to 1100 I get 15-16 in manifold vac.

 

I recall a similar problem years ago on a 283. The heads had been "cleaned up' on the seating surface when the engine was rebuilt. This caused the intake manifold to head alignment to be just enough off that it was sucking air between the intake and heads. Spray some light oil along those two surfaces. If it gets sucked in or the exhaust smokes, there's your leak. Not saying this is what it is on your setup, just my experience.

 

From what you are describing I'd be looking at the idle circuit, you say you can run the richness screw in or out without effect? That is a problem. You took the carbs back to the original builder? I'd have somebody else look as he might be overlooking the same weird thing.

 

With the cam you describe, your vacuum is probably OK

 

Make sure the end carbs are closed, as stated above, take off the progressive linkage too.
Also try advancing the distributer, sound like it's running a little retarded. Rich

 

Lots of good advise already given:

Block the end carbs with solid gaskets. Work only with the center carb till you give up or fix the problem. Then add one end carb at time.

Engine vacuum has nothing to do with carburetors, camshaft YES, carbs NO.

End Tri-power carbs are special, if they have idle adjustments, they are incorrect carbs. If they have thin throttle blades, they are incorrect. If they have soft return springs, they are incorrect.

Be sure the ignition system is functioning correctly and set correctly.

Be sure the power supply to ignition is correct.

Check for vacuum leaks, proper working PVC valve, distributor advance, wiper motors if vacuum, transmission modulator if it has one, and brake booster.

Plug all holes that let air into the engine block, install vacuum gauge on dipstick, and look for signs of vacuum, it should have a slight pressure if rings are not 100%.

 

Are you running an Edelbrock C355 intake by any chance?

 

Thanks guys,I really appreciate the comments. Most of the suggestions were already tried. As mentioned in my earlier posts,I have tried blocking off the end carbs, sprayed starting fluid around , looking for vacuum leaks, Bumped timing up to where it rattled like crazy under load. checked fuel pressure, [6lbs.] and just about everything that has been so generously suggested. I have an old stock rebuilt Rochester carb coming in the mail, that I bought from the H.A.M.B. classifieds. I am going to give it a try as soon as I get it. I have felt that it is a carb problem from the beginning. My carb guy has said that he was going to get me another primary carb to try, but fell off his air boat, and broke his neck! Needless to say, my lousy idle is probably the last thing on his list of priorities. These Rochesters are mostly in the 50 year old range, while gone thru by the various carb vendors there is still a great probability of some major flaw in the basic parts being assembled. I will report on the results as soon as I get to try the new carb. Rich

 

Oh by the way Saxman, it is an Offenhauser manifold.

 

I got no response from my adjusters until I went up in main jet size. I kept going up until I could get adjustment out of the adjusters. Mine was acting similar to yours. How do your plugs look? I took off the end carbs, got the center dialed in - rejetted and adjusted to where it ran good - then put the ends back on and went from there. I went with new bases for the ends, even though my original ones had the idle circuit blocked and the power valves out, they weren't sealing good enough.

 

rjones, thanks for the suggestion, what size jets did you end up with? Had the plugs out Saturday for a compression check as suggested by a buddy, 145 to 150 across the board, plugs were perfect, look like I just put them in! Rich

 

If the plugs look like you just put them in, it's too lean. To be honest I drilled my jets, and I don't know where I ended up. Not 100% sure where I started!! I had the drill bits, didn't have any more jets, so I went with it. It was a lot of taking apart and putting back together, test driving.... BUT once I started to see color on the plugs, I started to get adjustment too!!

 

I did a little thinking on the jets. Seems like they were stamped 53 or 55 maybe??..and I know I went about 10-11 drill bit sizes past that.

 

IT IS ALIVE! Just got my new old stock rebuilt Rochester 2 barrel that I purchased from a fellow Hamber. This thing probably sat around on someones shelf for the last 40 years. Gaskets are dried out, leaks gas from multiple spots, but best of all, it idles like a dream. now comes the easy, fun part, I bought one of Hotrod carbs premium rebuild kits a while back, cant wait to go thru my new found jewel. Thanks guys. Rich .

 

Right on! Definitely a step in the right direction! I got my center running and going good, then added an end, made sure it was still good, then the other end, then all three. Got pretty good at taking those things off and on. Have fun!!

 

While you are freshening up the new carb do an "autopsy" on the old one. See if there is something different between them that may have been the problem. It will help you in the future and help us too if you find something to share, like a plugged air bleed, missing gasket, casting flaw, etc. Like you said; 50 year old carbs, they have had a lot of hands messing with them over the years, maybe some parts swapping too. Glad you found the cure.

 

"I love the smell of blowby in the morning", that's what I used to say when I drove my flathead powered avatar car to work in the morning. My now sweet running chevy powered roadster has an open crankcase breather system, like my flathead. I would like to seal it up and plumb in a PCV system. The openings in the base of the carbs are all plugged with pipe plugs. I would like to use the opening in the primary carb, will that ruin my nice new idle? Thanks Rich

 

Probably. Sucking in more air, just have to richen it up I would think.

 

i assume you have a small journal block with the road draft tube. In '62 or63 Chevy went to the PCV system and pitched the road draft tube; you can buy some if not all the the parts you need from restoration places like Ecklers or Corvette Central. you can also canibolize the draft tube by cutting the tube off and welding a washer and hose nipple on to the piece that bolts to the block. You can use a 1/4 in. pipe elbow to get room between the center carb and rear carb for installing the pcv valve.

 

Sorry to say this and others will chime in but the Offenhauser 3-2 manifolds were not very good performers when compared to other manufacturers. There is a guy I found on the internet that made them work for the nail head Buicks but not known if anyone else played with them. When I looked in one I thought I should use a holesaw in the outside 2 carb holes to let them feed the air/fuel mixture better but never did it. The stock passages looked pretty restricted. Good Luck.