I am at my wit's end on my racecar. I cannot get my SU fluel levels to set right. Both bowls are 5mm low no matter what I do to the floats, valves, or pressure. I've tried everything I can think of, but I am now out of time. I leave for a race on Thursday afternoon and I have no confidence that I will get it to run right.

I'm looking to borrow a set of known good SU carbs that I can have overnighted to me right away. I'd have them for about 1 week. After my race, I plan to send mine to ZT if I can't solve the problem, but for now, I need something that works.

I just built a 2.8" 9.5:1 motor with a mild cam that should make about 175 RWHP, but it won't do me any good if I can't get the carbs to work right. The first time I will know for sure is on the track as the car is not street legal, so all I can do is tune mine in my driveway and try to compensate for the low fuel levels.

I know it's a lot to ask, but I'm out of spit.

Gotta ask two questions. Are you measuring the float height with the car running? How are you measuring it?

Changing pressure will not affect fuel levels, until its high enough to overcome the valve, or so paltry low that its barely there. Both carbs behaving the same way? Only thing in common is the fuel supply.

wish I was home to send you some carbs....

That sucks. Wish I could help in some way. Weird deal about the carbs

Gotta ask two questions. Are you measuring the float height with the car running? How are you measuring it?

Changing pressure will not affect fuel levels, until its high enough to overcome the valve, or so paltry low that its barely there. Both carbs behaving the same way? Only thing in common is the fuel supply.

wish I was home to send you some carbs....

Checking with engine off and electric fuel pump. I can check them in the bowls or in a plastic test bowl with the same result. Both carbs do the same thing. I can watch the float rise and when it stops, the float to lid is way closer than the spec - yet it's still low.

call Z Therapy they can help anybody

Yeah, I will call them tomorrow. I just spent an hour chatting with Tony D and he couldn't come up with any good explanation. Hopefully Steve at ZT can offer some advice.

I have a set of rebuilt SU's being Fed-Ex'd to me tomorrow to borrow for the race, so if ZT can't help, I'll do a swap on Thursday before I leave town.

Wow- Tony D didn't have an answer after an hour. Best get another set

Exactly Steve. Tony had some questions and thoughts, but nothing that was an "ah ha" thought.

I will try some stuff today, but I'll probably have to swap them with the 4 screw carbs I'm borrowing.

Send some more pics.

The intake vacuum will draw the fuel ....not the gravity difference in stock level and 5mm below stock.... it is the top of the jet orifice hole (0.100") and the point in the needle that really matters for fuel metering.

The problem with 5mm down is that you can run out of fuel on a long straight stretch.

The only ways i can think of to reduce running out of fuel is to raise the fuel pump pressure (but keep it safe)

or

make the fuel bowls "effectively larger" If you have fuel bowls with the front drain you can connect a large hose or another bowl in parallel as a "dish/reservoir" the levels will be 5mm lower but more fuel will be pooled in that reservoir.

Likewise for a quck and dirty you can make crazy long 2' hose to connect fuel bowl nipple to jet nipple to hold more fuel .... just secure it carefully and keep away from the manifold... or use a 1' hose with some sort of sealed can in the middle to be the reservoir.

Here is how an SU turbo gets more fuel.... this is sort of the idea but you only need one inlet and float and the second bowl is just a reservoir.

For your 5mm down problem it don't know... is the small orifice on the end of the hard return line of the fuel rail missing? Are the fuel bowl tops somehow sealed so that pressure builds up...this is a guess as it should cause fuel to flow out the jet normally?

i hope this gets you in the race

I have several sets of spare SUs but they all should be cleaned and rebuilt. I'd be happy to send you a set if you end up needing them. Good luck!

Chuck

Eureka?.... you could have the weird lids with the long casting arms holding the hinge deeper in the bowl..... or you could have the corresponding extra long fuel needles causing the same problem.

thanks to ztherapy as mentioned above

Edited July 10, 201311 yr by Blue

Yes, I do have the '72 carbs with the long arms in the front carb. Both carbs have the same issue though. The return does have the orifice in place. I have also pinched off the return with no change.

Hey Chuck, thanks for the offer. I do have a set being shipped from a guy in VA. Want to come to the race? Make my car right and I'll give you a 90 minute race stint. The race is at Nelson Ledges this Saturday. We'll be there late Thursday night for a Friday practice.

Edited July 10, 201311 yr by Jeff G 78