Emergency help needed. Anybody willing to lend me SU's?

[Jeff G 78]

Makes sense Jim.

While tuning the carbs, when I crack the pistons, both carbs bog indicating a lean condition. Do I open the mix knobs a bit to adjust that? I was watching some MG tuning videos and on real SU's, they have another screw on the side which is how he said to adjust the rich/lean when the pistons are raised slightly.

EDIT: After rewatching the video, I see that MG SU's don't have the knob on the bottom, so I guess the mix knob the right place to adjust for rich/lean, right?

Edited July 10, 2013 by Jeff G 78

[zKars]

The raise the piston test is a very subtle effect. If it bogs totally, its too lean. Richen it up a tad. Don't know much about MG carbs... The knob on the bottom is the only spot for our SU's.

Sorry I'm out of touch for the rest of the day. Play!

Edited July 10, 2013 by zKars

[Jeff G 78]

Thanks Jim! I did the dynamic level test on the rear carb only so far and it appears to be fine. The level stays pretty close to the proper level with the floats adjusted to the FSM float-to-lid measurement. The fuel pressure stays at 7 psi on my crappy little gauge. I think I'll be okay at least until I get to the track and put it under a load.

[240260280z]

I assume you have bent the fuel float tabs "down" so that the fuel level raises "up"? ... I did the opposite once or twice including recently in Austin...DOH.....Hardway is such a patient guy

You could always screw and re-seal a weight to the float to raise the fuel level a little.

[Jeff G 78]

Yep Blue, I was going the right way with the tabs. I tried going both ways and nothing seemed to help. Right now though, the carbs appear to be working okay dynamically even though statically they show low.

[240260280z]

Well if you do run out of fuel on the long stretches during testing, pit and do the trick by replacing the short 4" hose from each jet nipple to fuel bowl with 2' hoses to hold more fuel. Just be sure to secure them so that they are lower than the jet bottom and no air traps are formed.

[Jeff G 78]

Will do. Thanks Blue.

[Wade Nelson]

Both bowls are 5mm low no matter what I do to the floats, valves, or pressure

I'd submerge the floats in a bowl of gasoline and make sure no bubbles are seen coming out.

Are the needle valves and seats replaceable?

It's been 25 years since I've been inside an SU, but carb floats typically operate the same way a toilet does, the float goes up and shoves the needle into a seat blocking further flow. Unless it's unnecessarily high the fuel pressure shouldn't change the float level at all.

Is there slack on the "tang" on the float that surrounds the needle? Could the needle seat be moving up and down?

[Captain_Zeros]

Not sure if it's relevant, but with stock needle style float valves it's important that the tab hits the needle squarely, if it hits at an angle (easy to do when you're bending stuff around) it can **** the valve sideways and prevent it from operating correctly. If you're running ball style grose jets it doesn't matter though.

[zKars]

Back from a day of "fun in the sun" as they say... I'm done, take me out of the oven...

I've reviewed the day's progress. Glad the float level whie running is now correct.

You mentioned your fuel pressure is 7 psi? That's too high! You are lucky the needle valve holds that, the things are designed for 3-4 I'm pretty sure. They may be sporatically letting too much in. Need a reg to control that. It's not about pressure, its about having enough volume to keep the bowls full when the needles do open.

The 2 feet of hose trick is cool. Coil it up in a tight little pack to keep it out of the way. Hey submerge it in a can of cool water or ice prior to the race to get cold dense fuel.

[Jeff G 78]

I'm back home after the race weekend. Friday practice went fine. The car ran great for the few 30 minute sessions we ran in the morning, but when they broke for lunch, we rolled the car into the tech line for the race. We failed tech due to non-operating brake lights. WTF? The lights have always worked and they were working in the morning practice. After some diagnostics, we found that the brake switch plunger was stuck in. No matter what we tried, it wouldn't pop out. Luckily, my dad lives about an hour from Nelson Ledges and I have a huge stash of parts stored there. I knew I had a few peddle assemblies, so he met me 1/2 way and gave me the switch I needed. We got through tech fine, but we didn't get to run the car as long or hard as we wanted.

8am Saturday and the green flag flew for the 14 hour endurance race. The car ran great for the first 40 minutes or so and then our old problems came back. The car started missing and running like hell at high RPM. It continued to get worse as we came up with a plan to fix the car. We brought the driver in and we cracked open the box with the carbs we borrowed. With the car smoking hot, we pulled the stock airbox, choke cables, hoses and carbs off the manifolds and installed the borrowed set. Of course, they weren't tuned for our car, so I also had to tune and sync them before we could leave pit lane. They were way off when we first fired it up, so I reset everything to base and started from scratch. The front and rear had to be quite a bit different to balance, but I got it tuned. From the time the car came to a stop until the car left pit lane only 35 minutes had passed. It took about 31 minutes to do the R&R and then I tuned a set of SU's in less than 4 minutes!

The driver went out and within 5 laps reported that the car was once again stumbling badly. @$#(& We brought him back in and swapped the coil just in case it was getting hot and causing the problem. Nope, same issue. We then brought him in and put Vice Grips on the fuel return line which had worked in the past. Unfortunately, this time it didn't work. We were running out of ideas, but the one thing we suspected was vapor lock due to heat coming through the steel fuel rail. I told the driver to stay out while we did some prep work. We cut some fuel hose and added a T fitting to eliminate the rail and go straight from the supply line to the carbs with no return. When we were ready, we called him in and within about 2 minutes, had the new fuel supply on and tight. He went out and after 5 laps reported that it was running great. We had heard that line before, so we sat and waited for the bad news. It never came! We FINALLY fixed the car and it was running really fast. We had dropped to nearly 30th place in the standings with 9.5 hours left in the race. From that point on, the engine ran flawlessly and we were fast. We clawed our way up to 10th place with 3 hours to go. Just when we thought we were in cruise control, or driver called on the radio that he had spun and had a vibration. He thought he might have mud in the wheels causing the vibration so he was coming in. When he got to our pit stall, we found that it wasn't mud, but massive tire blistering on both left side tires. We did a tire swap and sent him back out in 11th place, 6 laps down to 10th. He once again started turning fast laps and made up the time getting us close to the top 10 with 2 hours to go. I got in to finish the race and by the time the checkers flew at 10pm, we had finished in 9th place! It was a huge victory for us considering the issue we had early in the race.

Thanks to all who help out!

Here are a few pics.

[Captain Obvious]

Awesome!!

Hard to believe that your fuel rail is picking up enough heat to boil the fuel, but can't contest the results of getting rid of it!