GT6 GT6 Fueling Issues

[Richter12x2]

So this has never run really well since we've owned it, and I blame my ignorance of mechanical fuel injection, not the car, but I'm going nuts trying to figure this out, so any advice would be great.

I rebuilt the carburetors, set the float level to 18mm (ZS CD150s), tightened the mixture screws all the way in, then out 3 full turns and bought a new mechanical fuel pump that seems to have good suction, but I can't get enough fuel to keep the car running. If I pump the fuel pump by hand, I can get it to start for a second or two and then die. Or die immediately if you open the throttle. Then if I give it another pump by hand, I can get it to do the same thing again. Lather rinse, repeat, but it'll never catch without pumping by hand.

This has been ongoing for a while, so one thing I attempted was an electric fuel pump with a regulator, but scrapped it because it overflowed the carburetors (I thought). After putting the new mechanical pump on, it turns out it did the same thing, and I found a bad o-ring on the bottom of the front carb. Now that it's changed, and since the mechanical pump still can't keep up, I went back and tried the electric again. Now there's no leaking

But for a car that's supposed to run between 1.5 and 3psi, I can't get it to even START until I get to 3psi, and if you open the throttle, it wants to die. At 4psi, with some tweaking on the distributor, I managed to get it to actually allow you to rev it a little, but it smells REALLY rich at idle, and under load (like putting it in gear and attempting to actually get it to move under its own power,) you have to slip the clutch a lot or it'll die again (starvation again?)

Anyone have any ideas? We're moving in a few weeks and it'd be an INCREDIBLE success to drive it the 5 miles we're moving. Heck right now, I'd settle for being able to drive it onto a trailer!

Oh, when I replaced the o-ring on one of the needle seats, I noticed that it was missing the little washer that goes on top. I couldn't find anything to work locally, so I ordered 2 new washers from Victoria British (.95 cents each plus like $10 shipping). That shouldn't account for what I'm experiencing here, should it?

Also, there actually was a brief period of time when it drove great to the end of the street and had power and torque, but definitely not anymore!

 

[ArcticOne]

After I rebuilt my carbs (same CD150) I attempted to follow the instructions about adjusting the metering screw by so many turns, etc. and then playing with the piston rise / fall.

After several attempts where, like you, I couldn't get the car to run I decided to only follow the instructions as far as getting the carbs installed.

Then I simply adjusted the metering screw based on what made the car run the best. Found that in my case the 3 turns was either too much or too little (I forget).

Perhaps play with the metering screw, records the adjustments and record the results.

I also have a mechanicla pump.

jb

 

[TR3driver]

18mm seems awfully high (which would make the fuel level way too low). I don't have a GT6 manual handy, but for the CD175s on a TR4, the float setting is only 7/16" (about 11mm).

By pushing the fuel pressure up too high, you are raising the fuel level inside the bowl.

 

[Richter12x2]

18mm seems awfully high (which would make the fuel level way too low). I don't have a GT6 manual handy, but for the CD175s on a TR4, the float setting is only 7/16" (about 11mm).

By pushing the fuel pressure up too high, you are raising the fuel level inside the bowl.

Well at the very least, that seems like it would help, so I may try that next - I ordered the Haynes Stromberg carb book from eBay, so hopefully I'll have more info on the settings soon.

 

[tdskip]

Good to have that manual but not likely to offer any shocking revelations.

One tip in the manual - after fixing the float levels - is to raise the jet bridge up to the piston just touches them and then lower the specified turns (three I think)m that will get you close on mixture.

Also make sure your venting / advance lines are correct (ask how I leaned to doublee check that).

 

[Richter12x2]

Yeah, I did the 3 turns bit, but it hasn't done much for me. Can you tell me more about venting/advance lines? I have the early CD150s, and as far as I know, only one vacuum nipple on the front one that connects to the advance on the distributor?

 

[poolboy]

You have the air valve diaphram oriented correctly so that the holes in the bottom rim of the air valve are facing the throttle disc ?

 

[Richter12x2]

You have the air valve diaphram oriented correctly so that the holes in the bottom rim of the air valve are facing the throttle disc ?

The rubber diaphragm in the top? It had little slots and tabs so that it should only go one way, I think, as long as you've got them lined up (although I guess you could do it wrong if you weren't paying attention to those). I didn't notice holes or anything though, so I'm not sure. I'm sure I'll end up pulling them both back apart if I don't magically solve it before the book comes in, though.

 

[dklawson]

If your car still has the original CD carbs, they will be the early type with the jets that adjust from below the float bowl. I'm sure you have gone through the carbs but did you completely disassemble the jet and jet bushing components (the brass bits) inserted through the bottom of the float bowl? There are several small fuel passages in those parts and if fuel sits in them long enough the brass will develop a coating of corrosion and varnish that plugs the small holes. Until those are all opened up the engine will stumble and backfire through the carb(s) when revved.

These jets also have to be centered (like SUs). If you don't center the jets the piston can "hang" as the needle scrubs against the jet. This can make the engine impossible to start or at a minimum... unresponsive to the throttle.

Once centered, it is generally helpful to "feel" for when the jet is fully up. Just don't crank the jet nut up all the way. Remove the dashpot and either feel (or use a dial indicator) to tell you when the jet contacts the piston and starts to lift it. From the point where the jet first contacts the piston, lower the jet the 2-3 full turns you have tried before.

If in doubt about the vacuum advance, plug it for now. Vacuum advance is for improved fuel economy during part-throttle cruising. Plugging it may make starting easier and will not hurt anything while you sort this out.

It has been a while since I have set the float valves on our GT6 and I do not remember the measurement. While you have ordered the Stromberg book, the Haynes manual may/should have a good discussion about all of the settings. I believe a lot of the Haynes was taken from the factory service manual.

Unrelated to the fuel system, did you check that the bob weights in the distributor are free to swing, that their springs are not broken, and that the 6-lobe cam on the dizzy shaft is not seized in place?

 

[Richter12x2]

Yep, they are the original carbs - I disassembled the jets and piped out the passages with an engine brush set when I rebuilt them, but that was a while ago - it's easy enough to do it again.
I did have a problem with the jet on one carb that wanted to bind up, but removing the screws from the top (piston side) and removing and replacing the piston with the jet screwed in seems to have helped a lot - is there a better way to do that?
I really haven't done anything with the distributor - I tested vacuum advance with a mityvac, and adding suction causes it to move, but it's not light and easy, and that's pretty much all I've done other than setting the points. The hex piece at the top does rotate though to open and break the points - is that the dizzy cam you're talking about?

 

[dklawson]

Yes, the hex piece is a 6-lobe cam that opens the points as it rotates. That cam is separate from the dizzy shaft proper. The simple test method is to remove the dizzy cap and grab the rotor. You should be able to twist the rotor clockwise and counterclockwise to and feel several degrees of motion. When you let go of the rotor, the rotor should return (approximately) to where it was before you started.

The vacuum advance is only effective at part throttle cruising. I'm glad yours works but that's not what I was referring to. Under the breaker plate in the dizzy are two bob weights and their associated springs. As engine speed increases the weights swing outward against tension from the springs. As the weights move outward, they move that 6-lobe cam on the dizzy shaft (mimicking what I told you to do by twisting the rotor). That action mechanically advances the ignition timing. The mechanical advance is much more important for driveability than the vacuum advance is.

I'm not sure I follow you about what you adjusted on the carb and how. The early carbs have the adjustable jets mounted up through the float bowls. The needles on those carbs are "fixed" in the piston by a single set screw. You make no adjustments of the needle apart from positioning it so the "shoulder" of the needle (where the taper starts) is flush with the bottom of the piston. The jet is a whole series of parts mounted in o-rings and sandwiched to the bottom of the carb body with a crush washer. To center the jet you must loosen the jet bushing (this is NOT the same as lowering the jet... and in fact the jet must be adjusted fully UP during this procedure. With the jet bushing just a tiny bit loose you remove the damper from the top of the carb, lift the piston and allow it to drop freely. The needle will push the jet wherever it needs to be to "center" around the needle. You gently snug the jet bushing and repeat the piston drop test to make sure the piston goes all the way down. You keep adjusting the jet position until the piston always falls freely. From that point on you make your mixture adjustments.

All of this will be in your Stromberg manual. However, the basic procedures for these early Strombergs are nearly identical to what is done for SU carbs. (The number of turns and the fact that the SU does not use a diaphragm are obvious differences).

 

[Richter12x2]

That's awesome info, I'm going to start following up on these things and checking the distributor tonight!

 

[Richter12x2]

Sorry it took a while to get back - On the distributor, twisting the rotor, I get a few degrees counterclockwise I believe, but nothing clockwise, and it does return when I let off.
Now I've never changed the condensor which might be worth a try - I know they're not expensive - any idea if there's an option locally? Or is this another $1.50 part that costs $10 after shipping?

 

[dklawson]

I don't know what parts interchange with the Delco condenser. In the old days places like NAPA could easily cross reference parts. It is still a possibility.

As a quick check, you COULD buy a Bosch condenser for an air cooled VW. (I don't remember ever seeing more than one electrical rating for condensers... something like 23 micro-Farrads). Find a place to attach the condenser where its case is grounded, then attach the wire from the condenser to the coil (-) terminal. That is electrically the same as mounting it under the dizzy cap. Of course... if you do that, you need to remove the original condenser from under the dizzy cap.

The motion you found on the rotor is OK.

 

[vagt6]

Doug, you're the man.

I don't know what we'd do here without your expert input. How'd you learn so much about these cars?

 

[Richter12x2]

I'm sensing he's sworn, cursed and beat on them quite a bit more than I have in my few years of ownership. :laugh:

 

[Richter12x2]

Disassembled and cleaned the distributor bits today - everything looks in good order. With the bottom plate removed,the weights and springs are clean and good as new. The vacuum advance plate was a little stiff so I gave it a shot of cleaner and worked it back and forth some. I had the car set to 10btdc by the mark on the crank pulley, so when I put everything back together I put a timing light on #1 and then twisted the distributor slowly until it just fired, then locked it down, so I should be spot on.

 

[dklawson]

How'd you learn so much about these cars?

I learn from others here and on other forums. In the case of the external condenser, I take no credit... that was from board member Sarastro (Steve Maas). Steve even has identified capacitors you can buy from DigiKey that will be more reliable than the condensers we hide under the dizzy caps.

Richter, how did the engine run after you went through the dizzy? I assume you are still going to go through the carb again when the book arrives.

 

[Richter12x2]

Alright, I think I've figured it out - a test run later today will prove it, but here's what I found:

I took the air cleaner off to adjust the carbs again, and discovered my rear carb was binding about halfway up (never closing unless you jiggled it.) and the front carb was dragging a bit. I loosened the four screws on the top and it fell right down into place. Raising the piston by finger it moved freely now and always fell right back down to the bottom.

I had also ordered from VB the missing washer that goes on top of that jet at the bottom, so I installed that, and did the same. Loosened the four screws on the top - heard the thump when the piston fell, then jiggled it to confirm it was loose and lightly tightened the screws back down (may have overtorqued the first time, I tend to do that, and I swear every bolt on this car is measured in inch/lbs, including suspension stuff which just makes me pucker.)

So last night distributor was set close, but thinking I finally got it, I adjusted the screws until I could see that tightening them pushed the piston up, then backed off 3 full turns to get them close (friend has a Colortune, so once I get the car running consistently for any length of time, I can really nail it down. Likewise I'll order the flowmeter to balance the carbs.)

So I give it a shot and it fires up and idles kinda nice. So I reached over and took the FPR down to 3 PSI, the top of range, and it's still going good, and still revs up, doesn't fall on its face (again, no load, so we'll see how the road test goes.)

So I put the air cleaner back on and put all the tools away, and figure "You know what, just to see" tried to start it? It caught and ran really rough. Tried to add throttle and it fell on its face and died. Hmm. Fiddled with the choke, no change.

So I took the air cleaner back off. After a second or two, runs and drives strong.

Stupid aircleaner isn't flowing enough air. Engine demand is the same, so suction draws that piston up, overriches due to lack of air and kills it. Gives the same symptoms as not enough gas though, because as soon as you add throttle it bogs and dies.

Not sure if I got bad ones/wrong ones, or if the carb vomiting fuel out the vent like it did at first ruined them, but these things feel like they weigh 3 lbs and look like they're made out of construction paper. I'll see what O'Reilly has after my wife leaves for work, but I feel pretty good about it now.

edit:
...although I can't figure how 4psi would kinda work before? Unless... I guess the suction opened the jet for more fuel, but without venturi pressure from the air coming in, it really WAS starving, and 4psi just got enough juice in it to keep it chugging along?

 

[dklawson]

You are on the right track but....

When you loosen the 4 screws on the top of the carbs you are leaving the diaphragm loose a bit. That can result in a vacuum leak that will not pull the piston "high" as it should be under throttle. The sticking and dragging you described are symptoms of jets not being centered. When you have that tuning book it will discuss how to center the jets.

I assume the air filters are new. If not, at least blow them off to remove as much dirt as possible. Your O'Rileys isn't likely to have anything close to "correct". However, this will give you an excuse to build that order you were talking about adding a condenser to.

If the filters do have a lot of resistance to flow you will have to adjust the mixture to get the engine to idle well again. However, if the resistance is really bad, you might be able to get it to idle well but run poorly everywhere else.

Oddly enough, Paul Tegler found on one of his cars that the low-restriction K&N filters he wanted to use had such a low pressure drop that he could never get that particular engine to run well. Eventually he put the OEM type filters back on and the car ran great. In short, don't automatically assume pressure drop through the filters is always bad.