[mwritter]

I finally got my 421 running and now I can’t get it to idle at a reasonaly RPM without burying the idle speed screw (reasonable being about 1300, unfortunately). I’ve read every forum thread on the subject, along with Jim Hand’s book, a book on tuning Rochesters from 1973, and Carb King’s tuning directions, and I’m still unsure where to start to address the problem. I’ve checked thoroughly for vacuum leaks and found none. I think I’m going to end up having to drill the throttle plates (as Paul Spotts suggested to me yesterday), but before I do anything irreversible, I thought I’d post my combo and get some feedback.

Engine is a ’64 421 bored to 4.15 to accept a .030 over 428 piston (433). Heads are mildly ported 6X, with 1.5 roller rockers. Compression is just under 9:1. Tripower is a ‘65 Mike Wasson restored unit. He did the best he could to “tune” the tripower to my proposed setup, but the exact jetting escapes me at the moment. I know he trimmed the power piston spring for me, but I understand that this is guesswork. Cam is a Comp hydraulic roller, custom grind. Specs are 281 int/287 exh, with 230 int/236 exh at .050. Lsa is 110, and valve lift is .510/.520. Cam was degree when installed. Ignition system is a Dave’s small body HEI hooked up for break-in, but will eventually be fired by a 6AL. Exhaust is RA manifolds to Pypes x-change.

So as I stated, any advice on where to begin to get this thing to idle like one of my 389s would be great.

[Tripower455]

What's it doing if you bring it below 1300 rpm?

I hate to say this, but it sounds like a vacuum leak. Make a set of blocking plates and put them under the end carbs to remove them from the equation. If you don't have any, you can borrow mine.

Don't drill the plates just yet. It should idle lower than that with your setup. I had to do it on mine because i am running a lot more CID than the unit was ever designed to run. I was getting an off idle hesitation, that was due to not enough idle fuel volume. I drilled the idle tubes in the cluster until it didn't stumble off idle any more, but then it was running really rich (sooty plugs and exhaust). The engine needed more air than the little venturi could supply it with the throttle plates in the proper position over the transfer slots, so I drilled the plates. It still wasn't enough, and I got a carb with a larger venturi AND holes in the plates. Now I have an acceptable idle mixture, but again, I am running 30 CID more than you are, so I'll bet you can make yours work.

I'd lend you a set of throttle plates to try, but the '65 has a smaller throttle body (I think) than the '66 ones.

That HR should make decent vacuum and you're not too far from factory displacement on the engine, It should be pretty easy to get it set up right!

We'll get you running!

[BVR421]

make sure the end carbs are absolutely totally closed at idle. cant even see daylight. If Mikes worked on them they should be sealed tightly at idle.
what do you mean when you say cant idle below 1300? does it die or is that as slow as it will go with the idle speed screw backed out? You arent on the chokes fast idle cam are you?

[mj64car]

This topic is of great interest to me. I also have a 421 (now 433) with e-heads and a 236/244 roller cam - lift is .520/.525, 10 to 1 compression. Also got my carbs from mike - great guy , very helpful.

Just to get it to idle on the breakin stand stand the builder opened up the idle tube bottom to .040 and .058 on the top. The engine builder also added a 3/16" spacer between the intake and heads to line up the intakes for better flow.

It now idles at 800 (as smooth as the cam will allow). I havn't driven it yet - have to install the exhaust this weekend. The color of the exhaust at the head pipes is a nice smooth light gray.

[mwritter]

I guess I should have been more descriptive of the problem. Part of the problem is that I've been in the driver's seat while my uncle has been manning the carbs... The car will fire right up and run on the choke at about 1800rpms, but if I touch the pedal, it wants to stall. The two times I've had it running, I've let it warm up until the choke is off and at that point, it needs the idle screw buried in order to stay running (at about 1300rpms). And it's not idling particularly smoothly either.

I'm almost certain that the end carbs are closed at idle. I've treated the problem as a vacuum leak until I read Tripower455s thread which mentioned drilling the throttle plates. Maybe I' mgiving it too much fuel which is masking the lack of air??

I'm willing to fab the block-offs for the end carbs, but how do I go about dealing with the fuel lines?

[Tripower455]

Originally Posted by mwritter

I guess I should have been more descriptive of the problem. Part of the problem is that I've been in the driver's seat while my uncle has been manning the carbs... The car will fire right up and run on the choke at about 1800rpms, but if I touch the pedal, it wants to stall. The two times I've had it running, I've let it warm up until the choke is off and at that point, it needs the idle screw buried in order to stay running (at about 1300rpms). And it's not idling particularly smoothly either.

I'm almost certain that the end carbs are closed at idle. I've treated the problem as a vacuum leak until I read Tripower455s thread which mentioned drilling the throttle plates. Maybe I' mgiving it too much fuel which is masking the lack of air??

I'm willing to fab the block-offs for the end carbs, but how do I go about dealing with the fuel lines?

Are any of the carbs overflowing from the vent or cover gasket when it's running? My front carb inlet valve came apart last week and I had the exact same symptons (wouldn't idle at all and ran rough) due to all the fuel pouring into the front carb. It will also happen to a lesser degree if the float height is too high. I'd take the center carb apart and go through it. Make sure the inlet valve is actually sealing.

I made my plates out if 1/8" steel, and just sandwiched them between the carb bottoms and the intakewith the gasket on the intake side and the linkage removed. It was easier than fabbing a new fuel line to run the single carb.

FWIW, my car idled fine down to 700 rpm with no holes in the plates, it just wouldn't transition well at all.

[mwritter]

I haven't had any carb overflow issues with this tripower, though my two '64s have been known to do it. Float height was set by Mike, so I hesitate to mess with it. I've had all three carbs apart at least once, the center one two or three times, and I can't find a problem. I was mostly looking for vacuum leaks, however.

Before I fab the block off plates, I think I'll temporarily swap the center carb from one of my other GTOs, just to eliminate the center carb itself as the problem.

Someone emailed and asked me where the idle mixture needles were set and to be honest, I haven't touched them from where Mike set them. I was hesitant to make the problem worse, but I will seat them and then back them off 1.5 turns and see where I am. I'm also going to try a different PCV valve, as I found a crack in the F-connector when searching for vacuum leaks. There was no change when I fixed that.

I'm off on Friday so that's probably the next time I'll get to play with it.

[DiamondJim]

Be careful with the idle screws, there are 2 tapers, one short and one long. Short tapers are 1-1/2 to 2 and the longs take 3-1/2 to 4 turns. Basically what you need at idle is enough air so you can keep the butterflies in the proper postion over the transfer slots. With the throttle open too far, you loose idle tunabilty(is that a word?-LOL) I was going to take pictures of the holes but my daughter took my camera on spring break. -Jim

[Jim Doran]

Matt, Here is a pic to my block off plates along with temporary fuel routing using a fuel barb in carb. Good Luck and many of us are following along as you trouble shoot and achieve success.

[tom s]

I had the same issue with my tripower.I found when looking at the base of the carbs I could see light around the throttle blades.I called Jon and he was too busy to do them for me so I sent them to Mike and after he did them it was like a new engine.To prove your problem,make some blockoff plates for the end carbs and run the fuel line direct to the center carb and see if it will idle.If it still wont idle your problem is somewhere else.Tom

[Tripower455]

Originally Posted by DiamondJim

Be careful with the idle screws, there are 2 tapers, one short and one long. Short tapers are 1-1/2 to 2 and the longs take 3-1/2 to 4 turns. Basically what you need at idle is enough air so you can keep the butterflies in the proper postion over the transfer slots. With the throttle open too far, you loose idle tunabilty(is that a word?-LOL) I was going to take pictures of the holes but my daughter took my camera on spring break. -Jim

I had the long taper screws on the old (original tripower) carb, and it took around 4 turns to get it right.

As for the holes in the throttle plates......

Your engine should idle acceptably below 1300 without any holes, and I'll bet it'll even run acceptably if not perfectly without them.

The problem I had was that with the original calibration, there was not enough fuel coming through the idle circuit to feed the additional 73 cubes I was running. In order to give the engine the fuel it required to transition and idle correctly, I had to open up the idle tubes in the cluster. This made the engine happy, as it now had the right amount of fuel for the idle and transition circuits to feed the 462, but by adding more fuel to the same amount of incoming air, I made the idle mixture too rich. It needed more air to lean it out, which is why the holes in the throttle plates were necessary for my engine. In fact, just the holse weren't enough, and I changed my bowl to a 1 1/4" then 1 3/8" venturi with holes to get mine right!

I would bet that the volume of incoming air is not your problem.

The first thing to figure out is if it's way too rich or way too lean when you back it off. The problem, if it's not a vacuum leak or fuel spilling in through the top is most likely in the idle circuit, and I'd bet that in this case that it is lean.

Try opening the idle mixture screws. Mike can't set them properly unless they were on your engine, and he probably just ball parked it. If you don't already have one, go buy a vacuum gauge.

[mwritter]

Great advice everyone. I'll dig out the vacuum gauge and spend all day Friday on it.

[66GTO Jim]

Diamond Jim,
I have the idle screws that are steel with the elongated hex shaft with a screw slot in the end. These can be adjusted from the side with a 1/4" open end wrench. The rebuild kit Jon Hardgrove sent me had (I think) brass idle screws and only had the screw slot at the end. I didn't install Jon's because they would be nearly impossible to adjust on the car. Are one of these style the long style and the other style do you know? Jim

[DiamondJim]

Both the long and short taper have the same scew slot/hex head, so it is hard to tell unless you back them all the way out. I wish I had my camera, I could take a photo of the 2 types side by side and you could compare. Maybe someone else here could do that? -Jim

[mwritter]

This thing is going to drive me to drink. I started by installing the center carb from my '64 and the car ran and idled pretty well. I think with a little tweaking, it could have ran really well, but I did not want to mess with it, given how well my '64 runs.

Anyway, I'm pretty convinced that the center carb has issues, something wrong that I'm not going to "tune" out of it. For the first time, it started leaking gas out of the throttle shaft, which I am thinking might be the problem. I'm going to give Mike Wasson a call and see what he has to say.

[Tripower455]

Originally Posted by mwritter

This thing is going to drive me to drink. I started by installing the center carb from my '64 and the car ran and idled pretty well. I think with a little tweaking, it could have ran really well, but I did not want to mess with it, given how well my '64 runs.

Anyway, I'm pretty convinced that the center carb has issues, something wrong that I'm not going to "tune" out of it. For the first time, it started leaking gas out of the throttle shaft, which I am thinking might be the problem. I'm going to give Mike Wasson a call and see what he has to say.

Your inlet valve isn't sealing and you're overfilling the bowl....... Either too much fuel pressure, float height too high or faulty valve would be the first things I'd look for..... Set the float a little lower and see if it seals. Also make sure your cluster is screwed on tight and that you have the right gasket.

[mwritter]

Originally Posted by Tripower455

Your inlet valve isn't sealing and you're overfilling the bowl....... Either too much fuel pressure, float height too high or faulty valve would be the first things I'd look for..... Set the float a little lower and see if it seals. Also make sure your cluster is screwed on tight and that you have the right gasket.

Thanks for the response Tripower455. I'm not running a traditional inlet valve, it's the checkball system that Mike Wasson sold for a while. He claims he's never had that issue with the checkball system. I've also measured the float height and that doesn't appear to be the issue.

Since for the first time I'm getting gas out of the throttleshaft, I'm thinking there might be a vacuum leak in that area?

Maybe this is putting it too simply, but it seems to me (a tripower amateur) that the carb isn't providing enough gas during the idle circuit. That's why it will idle with the choke on, and then needs the idle screw turned all the way in once the choke is off, which means it isn't running on the idle circuit anyway. It doesn't seem to want to run in the idle circuit at all.

As an aside, I was amazed to see the amount of fuel the carb from my '64 was pumping at idle when I had that installed. Visually much more than the '65 unit, which leads me to the diagnosis above.

[Tripower455]

Originally Posted by mwritter

Thanks for the response Tripower455. I'm not running a traditional inlet valve, it's the checkball system that Mike Wasson sold for a while. He claims he's never had that issue with the checkball system. I've also measured the float height and that doesn't appear to be the issue.

Since for the first time I'm getting gas out of the throttleshaft, I'm thinking there might be a vacuum leak in that area?

Maybe this is putting it too simply, but it seems to me (a tripower amateur) that the carb isn't providing enough gas during the idle circuit. That's why it will idle with the choke on, and then needs the idle screw turned all the way in once the choke is off, which means it isn't running on the idle circuit anyway. It doesn't seem to want to run in the idle circuit at all.

As an aside, I was amazed to see the amount of fuel the carb from my '64 was pumping at idle when I had that installed. Visually much more than the '65 unit, which leads me to the diagnosis above.

Did you try opening up the mixture screws a bit?


If fuel was dripping from the throttle shaft, it must be coming from somewhere, and i'd bet it was the bowl. I use the disk type check valves on my end carbs, and I had one come apart last week and fuel was puoring out of the vent and into the bore, making it run horribly.....

But, if it runs with the choke on.....hmmmmmmm..... that screams lean....

Maybe there is something clogging the idle circuit?

We'll get it figured out....

[mwritter]

I've adjusted the mixture screws to no avail. Mike Wasson thinks I may need to enlarge the idle circuits due to my increased displacement (this tripower was supposed to go on the 400 that ended up in my other GTO).

A clog in the idle circuit could also be an issue. I have a call in to Mike and I'll report back with his diagnosis.

This whole ordeal brought me back to 1999 when I bought my first tripower GTO - the seller included a huge box of spare carbs, carb parts and at least three rebuild kits. He said "you're going to need these." He was right.

[Tripower455]

You might need to enlarge the idle tubes, but I found with mine that it would idle OK with the tubes too small, but just wouldn't transition well........

They can be frustrating, especially for such a simple carb...... Clean the cluster and base really well, and make sure you can blow air through all the orifices.