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SU Carburetor Problems

C

chrisjbrooks

New member
Joined
Aug 20, 2008
Location
Alameda, California
I just added dual SU's to my 145. The issue I am having is that it seems there is no fuel at all getting to the front carb. If I cover it up the engine runs a little better. If I cover the back one it dies.
Any ideas what might be causing something like this? I think I might just rip the carb apart and see if I can figure it out, but I figured I would just ask to see if anyone here might have had a similar problem.
 
Id pull the top of the float bowls off to see if anything is in there and if the floats are ok. Next check the jets to see if they are at the same depth on both carbs. This is the nut at the bottom of the carb. You can turn them all the way up- clockwise direction until they stop then look into the carb and raise the piston to see if the jet is even with the housing. If its higher then turn back the nuts until its even. Then go about 1-2 turns counterclockwise or 8 to 16 flats to lower the jet but make sure its the same on both. This should get you close for tuning to see if thats it. You have oil in the dampers too right?
 
122power said:
Id pull the top of the float bowls off to see if anything is in there and if the floats are ok. Next check the jets to see if they are at the same depth on both carbs. This is the nut at the bottom of the carb. You can turn them all the way up- clockwise direction until they stop then look into the carb and raise the piston to see if the jet is even with the housing. If its higher then turn back the nuts until its even. Then go about 1-2 turns counterclockwise or 8 to 16 flats to lower the jet but make sure its the same on both. This should get you close for tuning to see if thats it. You have oil in the dampers too right?
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Pretty much exactly, but I usually do 2-2.5 turns out on the nozzle. But I would definitely start by opening up the float bowls. Make sure you use penetrating lube on the screws first because the metals like to corrode together.
 
Are these HIF6's or HS-6's?

The HIF6 has an integral float bowl:
fzx1280reb.jpg


The HS6 has a separate float bowl with a small line leading to the jet:
Hs6.jpg


It can be a little easier to deal with the HS6 because it's far easier to take the lid off the float bowl and inspect the float and needle valve without taking the whole thing off.

First off, how do you know fuel isn't getting to the front carb? Just because it doesn't seem to be adding anything to the idle speed? Does it run on two cylinder when you drive it around?

I'm suspecting it's merely the idle balance between the two, the front carb has the throttle closed all the way, the back one is open and supplying all the air for the motor (via the balance tubes to the front 2 cylinders). If you don't have an air meter (I got an old used Uni-syn cheap on ebay years ago for balancing SU's, works for the DCOE's as well) it can be a bir tricky, but basically just find the idle adjusters, screw them in and out in small increments, until both have a similar effect on the idle speed when screwed in a half turn. Tighten them a half turn (opening the throttle slightly), note the effect on idle, then return. If one carb has less of an effect, then keep it screwed in slightly. Do this tighten/note/return iteratively until they are both balanced front and rear *and* the overall idle speed is where you want it to be.

Of course, this gets blended in with finely adjusting the mixture front and rear as well. You can't properly adjust the mixture until the air flow is balanced, and then without the mixture properly adjusted, the affect on idle isnt what it will be once it is. So it's iterative again, you have to go back and forth. It's a bit of a black art.

FWIW I used to have to lay hands upon the PV's SU's every few months. Just seemed to need periodic tweaking, like adjusting the valves. Probably keeps a lot of 'classic car' mechanics in beer money. I spent a while getting the Weber DCOE's set up on it initially, but since then it's probably been a good 5, 6 years and I simply haven't had to touch them a single solitary time since.
 
Ahhhh to be at the steep end of the Skinner's Union learning curve!

They are easy once you know how to adjust them.

Takes practice and a good ear.

A length of hose can be used if you've not access to a Uni-Syn. The hose gets placed at the bottom of the slide as the engine idles. You will listen to the other end of it. You are shooting for the same amount of noise from both carbs at idle and at 2500.

Throttle shafts wear out and make for vacuum leaks and headaches with linkage.
Floats sink...... Needle valves stick open or closed.
Screws fall out for all manner of disaster.

It is all fixable.

DCOEs or programmable engine management are both wonderful ways to keep your car from burning to the ground!
 
JohnLane said:
DCOEs or programmable engine management are both wonderful ways to keep your car from burning to the ground!
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One of those cute little fuel lines on the bottom of the HS6 popped off once on the PV. On the highway. I about passed out from the massive influx of gas fumes as it gushed fuel all over the hot exhaust manifold. And alternator.

Luck was on my side, though, it didn't catch fire.
 
JohnMc said:
One of those cute little fuel lines on the bottom of the HS6 popped off once on the PV. On the highway. I about passed out from the massive influx of gas fumes as it gushed fuel all over the hot exhaust manifold. And alternator.

Luck was on my side, though, it didn't catch fire.
Click to expand...

Mine caught fire. Looked like a bird took a tremendous poop on the blue hood.

Used my firehose to put the fire out. Worked. :roll: Good thing that I had a very full bladder!!
 
Hey, thanks for all the advice guys. I have the HIF6 carbs with the integral float bowl. I pulled the front one off and took the bottom off. The needle valve was clogged. I unclogged it and cleaned out the inside and put it all back together.
I put it back on and now it runs, Fuel is clearly coming out of both carbs, but the issue is that I can't get them to idle down. I have the idle screw adjusted all the way out so that the butterflies are as closed as possible. It still is revving pretty high and all I do with the mixture settings on both doesn't change it. What could this be? Leaking throttle shafts? It seemed like they were ok...

Also, anyone know who has rebuild kits for these?

Oh and I would love to have webers or, even better, a fuel injection setup but I have these and don't have a whole lot of money seeing as I am graduating from college in a couple weeks :)
 
The intake manifold has a vacuum leak where crankcase vapors get pulled into it. You are in Kalifornia. Don't get caught making use of a road draft tube like the old cars got. :roll:

The Brake booster will often make for a vacuum leak.

As it idles...... spray some carb cleaner around the intake...... A change in idle will tell you where it leaks.

Throttle shafts are common for vacuum leaks. When they are worn you can grab the throttle shaft and pull it toward you from the passenger side. It will have noticeable slop and will change idle speed and quality as you move it.
 
Redwood Chair said:
No smog inspection on his car.
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Well it would be illegal to suggest that one do something that may keep crankcase vapors which add nothing positive to the intake charge and in fact do an amazing job of making carburetors utterly disgusting and effectively reduce the octane of the fuel being burned.

Older cars had a road draft tube. It will bolt in place of the junk that is there.

But that would be naughty. :roll:
 
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