Rochester 2 jet tripower problems.

HELP!...I have been doing some serious hours trying to get my model A pickup ready for the VHRA Pendine sands race in a few weeks. I've run into a problem with the motor and am at a loss as to what's wrong. The motor is a 283 running a solid lifter Isky cam and an edelbrock tripower intake with 3 Rochester 2 jet carbs. With what's available here in the UK I have refreshed the carbs to the best of my ability. Throttle shafts are tight and it would appear there are no air leaks. Idle circuits have been taken out on the outside two carbs and on tickover they do not draw any air.........Here's the problem, the motor won't tick over at low rpm. It fires up, stumbles at low revs, I hit the throttle and it dumps fuel into the middle carb and the revs pick up to about three and a half grand...and stay there?!...I wind out the idle screw to lower the rpm and very suddenly it goes from making no difference to making the motor die....there's nothing in between...it's all or nothing? My first thought is the motors not getting enough gas on idle so I wound the mixture screws out to 3 turns....no difference?

I have checked online for similar problems but have drawn a blank...Any help would be hugely appreciated... I missed last year with a wrecked motor...top end let go! Determined to make this year but desperately need some advice!
Thanks everyone


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Are the center carb throttle blades hanging openin the bore? Can you look down the carb and see them? Accelerator pump plunger spring in place? Does it happen with the outer carbs disconnected from the center? When the solo center carb is operated manually at the throttle shaft? Any place where the linkage could be binding, like going through the firewall?

 

Not an expert, just a guess. Plugged idle circuit holes in the base plate, incorrect base gasket, over tightened air fuel screws with the tips broke off in the holes?

 

Block off the two end carbs first. It's a lot easier to deal with one carb at a time, that's what I did and it works and old Pontiac guy told me that and He's right. The Float leavel seams to be very critical on the 2G carbs and make sure the butterflys are completely sealed. When the middle carb is right start on the end carb, then the last one. I really like mine it works well. Good Luck hope you get done in time.

 

Thanks guys....yeah the carbs have been all gone through to the best of what I have...gaskets are all good and idle circuit is present in the centre carb but not in the outer two. It makes sense to block off the outer two, however I should've said I'm running a progressive linkage and I have carefully filed the butterfly's on the outer two carbs so they seal in the bores well and do not start to draw any air or gas until about 3 quarter throttle. I have played with the float level in that centre carb as I had a feeling it was over filling the float chamber...maybe I've adjusted this too much? I'll check it out... Thanks


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Like @iddragracer stated, it's a lot easier to diagnose if you take the end carbs out of the equation. I would take the time to make some block off plates for the end carbs and reset your float level on the center carb to spec. Reset your idle screws (I like to start at about 2.5 turns out and then adjust from there). Make sure no linkage/butterflies are binding and keep your fuel pressure below 3.5 lbs.

Check for vacuum leaks at the carb base and where the manifold mates to the heads. Keep in mind too that three deuces require one heck of a return spring (at least mine do) or it will not close the butterflies on the secondary carbs completely when you release the accelerator.

 

And add each end carb one at a time. The temp. end seal plates will not allow the end carbs to open with the linkage because the butterflies will hit the block off plate. but after they are proven to not leak then the linkage can be added and adjusted.

 

Ok, thanks everyone, that sounds like a plan,...thanks for all the helpful advice...I'll get on it and keep you posted on how it goes. All being well, I'll be running down the beach in no time!


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I completely remove the end carbs and block off the manifold with the plates. Once everything is running right on the middle carb, I remove one plate and add one end carb at a time then check it without the linkage and the butterflies closed, then add the linkage and adjust it. I think that's probably what your saying though, right? I don't put the block off plate under the carb, just the plate to block off the manifold.

 

I found when hopping up engines they never idled too well until I added a small air bleed hole in the butterfly (or notch on slide-valve carbs) close to the idle air bleed inside the bore. This would allow me to close the plates further yet still idle, destroyed an annoying hic just off-idle plus delayed the transition from idle circuit to the higher circuit.

I start small & usually wind up at .o94" or .125" dia. Too big? wrong position? Just solder it up & redrill.

 

That's interesting...I have actually seen that before and always wondered what it was about...ok thanks...I'll move onto that once I've tried blocking the end two off.


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Couple Q's, have these carbs ran for you before? Have you actually seen them run below on someone else's engine? What did they come off of?

Now my experience with them on my 348. The setup was a swap meet unit that I didn't know anything about. Turns out that they were just "normal" 2bbl carbs, not tri-power ones and yea there is a difference. Well after putting them on it wouldn't idle and seemed like it had a HUGE vacuum leak so I pulled the intake....3 times thinking it was leaking in the valley since there was no sign of leaking outside. Finally I go ahold of an old school carb guy and he said drill the air bleeds out because a 250 six cyl would have a different vacuum pull than a 348. Never thought of that and actually didn't know they were different. I had an olds J2 intake and carbs (original) that I had just bought for the carbs to rebuild and was sitting on the floor. I pulled the air bleed out of the center carb and cleaned the dirt dobbers out of it and bolted (actually screwed) it on and it fired right up and idled perfectly! Of course after yanking the intake 3 times and the gaskets, time and cursing during the entire process!! LoL!

That all being said, I would suggest the air bleeds! LoL!
Let us know!!


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Proof!! LoL!


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This has nothing to do with your problem....but I'm curious as to how you mounted those frogmouth air-cleaners to the 2gc's....the 2gc's don't have much of a lip to attach to.

 

Lots of great info here, thanks! The set up was from the swap meet and not an original tripower setup....no idea what they were all off originally. When you say the air bleed...is this the air horn that runs down into the carb? This is something I definitely need to look at... Never paid it any attention before!
The setup has run on this engine before, but never very well....better than this though! :/ I always put the lumpy idle down to the cam, maybe that was never the cause all along....
In answer to how the frog mouths are on,....just a small amount of silicone.....you can't see it and holds them well.


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I have to do this to my tripower. Mine is not as bad as yours. It's rich at idle and I have to turn my idle mixture screws almost a full turn before I get any change. That's because it's getting fuel at idle from the transfer circuit, which is not adjustable, and is not supposed to happen. Here's what happens.

Big cams don't like to idle at low rpm's so you have to open the throttle plates more to get it to run. There are two holes in the throttle bore, an idle circuit hole, and up above it the transfer circuit hole. As the throttle opens from idle toward the time the nozzles start to flow fuel, the accelerator pump and the transfer circuit add fuel so the engine doesn't stumble.

When you turn the idle speed screw up to keep the engine with the big cam running you expose the transfer hole and it starts to flow fuel, when you don't need it. It makes yor idle mixture screws unresponsive, fouls plugs etc. Pull the center carb, without doing anything to it. Flip it over and look at the throttle body. I bet you will see both holes exposed.

When you drill a hole in the throttle plates you are creating a passage for air (rpm) so the engine can idle at the rpm it needs to, yet you can now close the throttle plates to cover the transfer hole, preventing it from prematurely flowing fuel. This will get your mixture screw adjustment back.

Make sure the carbs are right on all other levels before you do this of course.

 

Rochester 2g carbs can't handle much more than 3psi using an electric fuel pump. Mechanical pumps have a bypass but electric just overpowers the needle and seat. If this is the problem put in a pressure regulator and a gauge.

 

Well, depending on how brave you are. I've plugged, re-drilled, slotted and otherwise modified transfer port(s) but I always seem to end up drilling that hole too.

 

Yeah me too. My first "real" 40 hour a week job after high school was at Kirks Carburetor here in Detroit. Worked there for 6 years then went on to another rebuilder shop for 3 years. Drilling the hole is for sure the easiest. Carburetors are a big mystery for a lot of people once you get past float level, mixture screw and idle speed screw adjustment. Rightfully so unless you've played with them a lot.

 

This sounds very much like what I need to do...the cam is quite a big one...up on the revs it's fine it's just low down it's bad...and I've always had trouble with fouled plugs....There's a wealth of knowledge here...thanks everyone


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Yep. Start small with the holes. I normally locate them sort of in the middle of the plate between the forward edge and the throttle shaft. It will take a few times of pulling the carb and looking at the idle holes and the transfer slot but you'll see your getting closer each time. These pics will help explain better.

First pic showing both the idle and transfer circuit holes.
Where your throttle plate should be at idle (only idle circuit hole exposed

And probably close to how it is now, both holes exposed

 

That's great, thanks, those photos are really useful!


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Yeah, but F6 didn't mark where to drill the hole.

Don't drill so close to the edge that you can't drill the hole out to 3/16" without breakout.



I start with about a 0.50" or 1/16" drill. & drill both butterflys the same. The size isn't super critical, so that'll help; but that usually won't be quite large enough.

Still it's best to start small & work up. If it gets too big the engine won't idle down & you'll have to reduce it (or compensate in more complicated ways.)

 

If you go back and read, and I'll quote myself:
"I normally locate them sort of in the middle of the plate between the forward edge and the throttle shaft."

I think where you marked on my pic is a bit too close to the edge. It doesn't need to be on the edge of the plate, it just need to be there. It could be between the rear edge of the plate and the throttle shaft for that matter.

But here's a pic of the way I do it. Not my pic but same execution.

 

Thanks, well that's a good job for the weekend, I'll keep you all posted as to how it goes


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HehHehHeh...I did read the description. I just thought a photo was called for.Yeah that works. I tend to go closer to the edge of the butterfly, because I think the extra turbulence helps break up flow at the transfer slot/port a little longer.

But that's just a theory.

When I do slide-valve carbs, I put a notch right on the edge, & I have seen butterfly carbs done that way too.

I don't, because if you overshoot on the slot size, it's hard to solder up the slot neatly. Holes are easy to solder up.

 

It sounds to me like you have a vacuum leak. It could be the end carb butterflies or something else. The mixture adjustment screws won't work well with a vacuum leak. When set up right, fuel and air can only enter through the center carb until you floor it. The center carb is not modified. it operates just like the engineers expected it to operate. I modified my own end carbs using parts from Vintage speed and picking his brain. It runs fine with just the center carb. due to illness I have not been able to work on it and add the end carbs into the working system. My end butterflies are sealed off pretty good so I don't expect any problems.

3 deuces are more for decoration than sheer performance. You won't find many old pictures of drag racers running trips except in the stock classes with the factory units.

 

The thing about carbs is that transfer port size and location is adjusted to the vacuum map of the engine it's intended to serve. If you put it on an engine with less low RPM vacuum but a strong top end, it'll wind out, yet respond poorly to idle adjustments just as if you had a vacuum leak.

Of course it's not at all unlikely that the engine in question has both a vacuum leak, and an un-corrected transfer port, and fixing neither alone will make it right.

Impossible to tell from here though.

Drilling that hole "corrects" the fuel delivery at light throttle, or at least allows the idle speed & mix screws to operate normally, right at the RPM/load where a hopped up engine is often a bit ragged. An immediate reduction in light-throttle bogging or hick-up (due to ragged fuel delivery) is almost always noticed as soon as a hole is drilled & the screws twiddled.

A car where you fussed the screws for hours without satisfaction will suddenly tune up so easily you'd think little hot-rod elves had swapped the engine while you were asleep.

 

Ok, so drilled the holes and hey presto....the motor idles! I still dot think it's quite there yet and will go up another size on the hole, but wow what a difference!



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Congratulations!

Now you should be able to detect any small vacuum leaks much easier & really get this baby tuned.