Inliners International Forums General Discussion Engines Vacuum Questions - 292 with Multi-Carbs

Hey Folks, this is my first time posting a question and I really hope you can help. I recently finished building up a beautiful 292 that I put into my '58 GMC pickup. I'm using an Offy 3 Carb setup with three Rochester Model B carbs (the center one has a cable choke), Langdon Headers, Mallory dual point dist, Clifford performance Street/Strip cam, Stock Valves with Z28 springs. I recently had the deck resurfaced and the head rebuilt. The engine is from a '66 Chev pickup and only had 43k miles on it was a very sound engine when it was pulled. The problem that I'm having is this: When I first start it up it runs pretty well but after it warms up it seems as though the fuel mix gets richer and when I adjust the idle screws it seems to have no effect on the idle speed. I pulled the first carb off yesterday and there was a pool of fuel on the manifold floor. I'm pretty certain that I'm not getting a good seal at the intake manifold because there's about a 1/16" difference in the thickness of the flanges on the Langdon headers and the Offy intake. I've installed studs and fine thread nuts w/lock washers in place of the manifold bolts and I had it running GREAT before I did this recent rebuild so I know that everything works well. Also, I should say that this time I completely rebuilt all three carbs. Is it OK to use the red hi-temp RTV sealant on the Manifold gasket to insure a good seal or is there some other way to achieve this. I have the stock type FelPro metal gasket on it right now (with no sealant) but I'm not opposed to trying another type of gasket and sealant. If anyone has some advice I would greatly appreciate it. Here's a couple of photos of the engine if anyone's interested:
http://pedaldoctor.com/images/Finished_292_Engine.JPG
http://pedaldoctor.com/images/Finished_292_Engine2.JPG

You should not need any sealant on the flanges. First, make sure the intake and the exhaust flanges are flat. For the difference in thickness, use the oem clamps or the ones from Stovebolt and grind which side will be on the thick manifold to compensate. The modified clamp should put equal force on both manifolds. Also make sure you retighten the manifolds as they loosen up after a couple heat cycles.
Since you installed studs this is probably not needed, but make sure the end guide pins are placed with studs also.

Dear PD;

Bruce is correct, sealant shouldn't be needed, anywhere on the intake system.

Most '3-pot' systems only idle on the center Carb., with the mixture screws all the way in to seal the fuel ports.

One of your floats (needle/seat) may have a leak/set to high; so give that a "look see" too.

Good luck.

I think you are afflicted with a classic problem with using multiple carburetion - too much carburetion at idle. When you have multi carbs the idle speed races - to slow the idle speed you close down all the throttle plates - doing this richens up the idle airflow, since the idle circuit tends to richen up as the plates are shut the overall A/F ratio at idle is too fat - OK for a cold engine but at operating temp it will be too rich.

You must find a way to lean out the idle circuits. On Holley 2Vs this is done by idle transfer slot modifications and idle air bleed changes (usually necessitating changing to removable air bleeds ala racing carbs). Kind of tricky but a good carb blueprinting shop should be able to perform the needed mods. I'm not real familiar with Model Bs but this is what I have learned playing with (3) Holley 2Vs on my junk.

Also, check out the Tech Tips page. Pat Smith has a good article on setting up the B's and Tom Langdon writes about manifold heat. You might also consider disabling the idle circuits in the end carbs, GM Tri-Power and J2 setups went so far as to seal the butterflies at idle on the secondary carbs with Glyptol or the like.

French is correct also.

This is why the mixture screws are both "closed", (end carbs) to compensate for that problem.

Happy trails.

"Most '3-pot' systems only idle on the center Carb., with the mixture screws all the way in to seal the fuel ports."


Thanks Guys. So should I turn in the idle screw on the two outside carbs and only set the idle on the center one? Then, if I set the linkage up in progression will that work nearly the same as the stock single carb setup at lower speeds when the outside two carbs haven't kicked in yet? I'm trying to get the both of best worlds here (economy and performance) I'm even running a 3-Speed w/overdrive.

Oh I forgot to mention that I am running a water heated intake. I spoke with Tom Langdon a couple years back and ended up buying one of his plates. That DEFIINATELY made a difference with the off-idle performance.

You don't need the linkage "progressive", all should work together. Less fuel at idle is all.

There's a choke on the center one, right??

Thanks John, Yes there's a cable choke only on the center carb. I've never had to use it though. Most likely because I've got all carbs running with the idle screw out approx. 1 1/2 turns and it's rich in the first place. I'm going to try closing the idle screws on the two outside carbs tonight and see where that gets me. It makes sense to me that one carb set at 1 1/2 turns out is enough for a stable idle so the other two are only used to squirt more fuel when you lay into it. Thanks guys, I'll keep you posted.

Did you synchronize the carburetors? Are the carburetors identical (other than the choke)?

Jon.

Tim,
Could you go into more detail about how the Glyptal was used?
Do you mean they used it to seal the idle fuel and air passages? Don't understand how you could seal the butterflies.
Since Glyptal is expensive, is there a suitable epoxy that would hold up to gasoline?

I used to think it'd be cool to split a Quadrajet, one primary and one secondary.
(and still have it work right!) Lots of passages to seal off.
Thanks

Hi Pete, With the butterflies closed there is still a small gap in the venturi, from the factory they painted the clearance with Glyptol (or equivalent) of course putting an antisieze on the butterfly itself, ensuring a leakproof seal, and eliminating the extra air to maintain a proper A/F ratio from the center (primary) carb. If you look at a stock, untouched J2 or other tri-power from the '50s or '60s you will see this. The end (secondary) carbs didn't have a idle system at all. It seems like a lot of trouble for such a small clearance, but hey, they have an extensive engineering budget, so I will go with them.

Ok I have to ask Or add a 2cent comment. which i have seen done many times when idel was a issue or to much carb for idel. What was the thing with drilling a hole in the butterflys. I was always under the impression it was to help at idel??
I have one or two holley 500 2b with them in them.
Ones with out a choke. The one i have here with a choke does not.

The late 1950's early 1960's GM factory tripowers had the edges of the throttle plates on the end carburetors only coated with a product called DAG-213.

This is NASTY stuff, and probably should not be used by the general hobbyist (we WON'T use it, and have been rebuilding tripowers quite successfully for more than 40 years!). The DAG-213 was used as a time saver; as the individual centering the throttle plates in the throttle body could be less precise (time saving). This stuff really isn't necessary.

One other issue those who would attempt to use non-tripower carbs for ends should at least be aware of. A normal (not tripower end) carburetor is never asked to completely seal; there is always some clearance (maybe .015~.020 inch) at idle. Thus thin throttle plates are used. The factory tripower end carbs, which must completely seal, used MUCH thicker throttle plates to prevent binding in the throttle bore due to heat expansion when they are closed cold.

As to the holes in the throttle plates, these are used for two totally different issues in different applications.

The most common applications are holes from about 0.060 to 0.090 used on primary carbs only; generally either large engines, or engines utilizing cams with lots of overlap. The purpose is an inexpensive idle air bleed system. The holes allow a certain volume of air to bypass the normal path around the edge of the throttle plate. This allows the throttle plate to be closed sufficiently that the air velocity past the throttle plate is increased; causing a better signal at the idle and off-idle discharge ports.

Some factory GM tripower end carbs (generally those with auto transmissions) had much smaller holes (about 0.020 from memory). These acted as a bleed to bleed off pressure in the throttle bores of the end carbs due to heat. This pressure would be in the form of fuel vapor, and would create an instantaneous "rich" hesitation when the end carbs were opened. The holes minimize or eliminate this condition.

Jon.

Using a oxygen sensor in the exhaust, what range of air to fuel would you like to see for mild street engine like the doctor is running? What would you expect the cruise ratio to be? Joe

Well, after much tweaking and head scratching I discovered why my 292 began running so badly (and quit running altogether). It wasn't anything directly to do with the carburetion. I began to think that the vacuum was weak because the cam was too radical (even though it is the cam that is recommended for my setup) so I decided to pull the cam and reinstall the stock one until I could procure some more information. That's where I found the trouble! The machine shop that pressed on the timing gear DIDN'T INSTALL THE KEYWAY! The gear had slipped on the cam and the valves were opening late. It's so rare that this should happen on an inline six that I didn't even consider it. (I've had the timing chain jump a tooth back when I was running a BBC) I'm relieved to find the trouble and after the initial frustration, I'm glad to say it's now running incredibly well again! I'm now joining the Inliners Int. as a member this month. Thanks for all your emails and priceless advice.

nice lookin engine friend. sure would like to go for a ride.
bob

Glad to hear it worked out okay. I had forgotten a similar problem I had. Back when I was 15 or so a friend helped me overhaul a 235. He was on leave from the Air Force and had to go back before we finished. I never could get the thing to run right. All the mechanics I talked to said it was probably cam timing (since that’s a common thing for newbies to screw up). But I was sure I had lined up the marks right. Finally on an impulse, I dropped by the local Chevy dealer parts department and had them let me look at a 235 cam gear. (That’s how long ago this was) I checked the location of the timing dot relative to the keyway. The one on my 235 was a few teeth off!! I reset the cam based on the dealer’s gear and that was it. It ran great. It must have been 4 months getting that guy running.

Regards,
Leon