hi guys, i have a set of brand new SU's carbs on my car, they ran good for a while, but now they have started acting up.

The problem i have is that the car will run great until operating temperature or a bit above. I notice it the most when I get on the highway, or sustain rpm above 2000 for any length of time. as long as my revs are up, this problem persists, if I stop and let the car idle rough for a couple minutes it smartens up and runs nice again.

at first i thought it was the stock mechanical pump not being able to keep up with the 2.9 stroker. I saw a lot of air inside the lines, it was boiling the fuel in the lines so I decided to install a carter pusher pump at the tank. this resolved the fuel boiling issue but my car still isn't running right. I've thrown a good bit of money at it now and nothing has changed.

I have checked and rechecked the carbs for dirt, engine bay fuel filter was just replaced, and last chance filters are spotless, fuel pressure is right, bowls are staying full, and float levels are correct (I checked them right after it started acting strangely the last time)

2 things that i have not had the ability to check are carb synch and timing. But I can't see those being intermittent problems. It runs so happily when it decides to run right. I did back off the timing a bit to see if that would help with the backfires (lean) but it didn't do much.

I'm contemplating selling these things and just getting a holley manifold and 390 carb. I can't stand being so frustrated with this beautiful machine.

Edited June 5, 201410 yr by azriel_strife

Burping out of the carbs is usually timing and if your timing is set properly then that means you are too lean. I dont know how much the "2.5 turns" applies to you since youre running a 2.9L stroker but IIRC 2.5 turns was a baseline setting that should get your car started so you can tune it further. I wouldnt get so hung up on the # of turns but keep turning out until the car runs properly and make sure you have no vacuum leaks ( which can lean out your mixture)

Edited June 6, 201410 yr by porkbun

Stupid question......Oil in the carbs? 3 in One or a light 20 weight. Also make sure you're nozzles aren't stuck in the open position after choking the engine....put your hands directly under each carb and push up on the nozzle.

Edited June 6, 201410 yr by Diseazd

Have you reset the needle height in the dashpot piston per the Z Therapy video? This wouldn't necessarily change how it runs, but if the needles are too low they would require the mixture nuts to be lower to compensate.

Stupid question......Oil in the carbs? 3 in One or a light 20 weight. Also make sure you're nozzles aren't stuck in the open position after choking the engine....put your hands directly under each carb and push up on the nozzle.

Not so stupid!

I recently bought a '72 that wasn't running for cheap. Set the valves still no go, bought 3-in-one oil blue bottle for $3. the plungers wouldn't even get wet with oil they were so low. It started then. That $3 saved me thousands off the asking price.

carbs are working sweet on the mechanical aspect of things, not getting stuck, dropping at the same rate, nozzles are at correct height and not binding, no vacuum leaks, needle height in pistons is set.

topped up the atf yesterday, i'll advance the timing a bit at a time and listen for knock, I'll also try unplugging the vac advance, it might be doing some funny things.

Should you be using SM needles on a stroker?

I've brought this up before with the SU's and that's to check the hoses from the float bowls to the carb. I had hoses that would collapse with heat and thus make the engine run lean. Probably check while engine is warm and feel the hose for flat spots.

Timing check is a must.

Battery condition and charge

Inspite of being sure the float level is right, 4 1/2 turns down would be neccesary if the fuel level was too low. You said the bowls were full. The level has to be near perfect. Just a thought that hadn't been brought up.

Have you tried pulling on the choke to see if anything changes. It's a handy way to check the mixture especially if it's lean.

Call me and I'll be happy to give you the number at the shop and you can talk to Steve Epperly the owner of ZTherapy. Steve is a very talented mechanic and diagnostician. My number is 503-587-9800.

Our ZTV01 Tune up dvd covers a multitude of these engine related issues that can drive you nuts.....

I have the "just SU" dvd. it was very informative, 4 hours of carbs is quite the long watch!

I got my sych tool and synched the carbs, they were out a bit, about half an inch on the beads on idle, and about the same on high idle sych.

it seems to be running much better now, doesn't fire through the carbs when I blip the throttle anymore, it's sitting pretty happy at 3 turns out for mixture, I have yet to drive it and see if my crappy drivability still persists.

Well, the car seems much better after the synch, it does stumble here or there if i stab the throttle, i can get around that by being smooth with my throttle application.

I believe there is still some mixture fine tuning, i'll try 1/4 turn richer, and if that doesn't improve it, i'll try a bit leaner than it is now.

Glad I grabbed that uni-syn tool.

Try a thicker oil in the carbs. ATF is about 7 weight. Thin oil will allow it to lean out on throttle opening. Try 10W30 engine oil. It works better for me that ATF.