suspected lean fuel condition

[fleathatsme]

Hi there folks.
Background:
1968 2000 with stock SU carbs (roller shaft throttle bodies recently installed/carbs rebuilt)
Followed directions as best I could and got the car running much better than it ever has before.
I ended up leaving the mixture screws at 2 1/2 turns because when I played with these it seemed to only make it run worse.
Now has nice balance at idle 600-700 rpm. Unfortunately, I seem to have a fuel issue under load. (Suspect lean)
If I go full throttle, and start to run through the gears, it will crack and pop and then seems to starve out completely.
Take it out of gear and pump the gas a bit until it seems to "come back" and I can continue on but at 1/8 or 1/16 throttle.
It will run smooth like this everywhere and through every gear. If I get greedy and want to stick my foot in it especially in top gear on the hwy,
it will crack and pop and then begins to feel completely fuel starved until I take it our of gear and just easy pump the gas until it responds.
It can actually feel like it's loosing cylinders. If I just hold steady at "barely" throttle, everything is happy.
I suspect that this is a lean condition but I'm wondering whether the pump is weak or the carbs aren't set right, or both?
Would love to hear that I'm ignorant and missed something obvious after the carbs were rebuilt with the roller shaft conversion.

[funkaholik]

That does sound like a very lean condition. It could also be timing advance? If you have it in neutral and rev it, will it rev up without the problems? There was a recent thread about this on another car, and it turned out that the pressure regulating device after the carbs that returns extra fuel to the tank was in the wrong place, and had no internals. He replaced it with a good part, in the correct location, and his problem was solved.

Check it out here: viewtopic.php?t=38035

Here is a photo of the offending part. It should have been after the rear carb, between it and the return line to the fuel tank.



If the problem happens in neutral with no load on the engine, then I'd use a timing light to make sure your timing is advancing properly. Remember that if you have the "smog" advance still in your distributor, you'll have to set your base timing at 0.

[fleathatsme]

Thanks for the quick reply. I did put an electronic ignition distributor built by east coast roadsters on it before the new throttle bodies. Does that take care of the timing issue? Again thanks for the tip and I'll look at that return fuel line device ASAP.

[funkaholik]

Yes, the EI distributor solves the "smog" timing issue. Your new distributor has 15 degrees total advance, and with SU carbs you should set the idle timing to 16 degrees BTDC at about 700 RPM idle. Since you've added a new EI distributor, as long as it's timed right, I don't think it is your problem. You can still double-check with a timing light that it's advancing properly.

I added a small inline fuel pressure gauge that allows me to see if the pressure drops. The caveat is that by simply revving it in neutral with no load, it will probably not show a drop. You'd have to be on a chassis dyno or have another way of putting a load on the engine to see how the gauge reads then. An electric sender with a remote gauge could possibly be wired up in the cabin to check it while driving.

[Gregs672000]

Quickest way to see if it's lean to pull the spark plugs and look at them. Do you have a timing light? If so, confirm that the spark is advancing well past the last mark to the right when rpms are above 3500. Check for vacuum leaks with carbs cleaner (not as likely given its idling well). Check for any fuel line or fuel filter leaks that allow air in (can be sneaky) as it can lead to fuel starvation. Make sure the float bowls are getting fuel and no lines are plugged (there inline fuel screens where they connect to the float bowls).

It does sound lean... backfiring, popping, no power... not good for the engine so be nice to it. Again, check the plugs or better yet throw in a new set and drive it where it's bad for a few miles, then pull the plugs and you'll know.

[Gregs672000]

BTW, "pumping the gas" does not help an SU carburetor like it might one that has an accelerator pump jet system, etc. SUs are vacuum only operation, so it will likely recover faster if you hold the throttle near closed and steady.

[Habitat.pat]

I was fighting a low fuel flow problem & added a tee in the fuel line by the carbs& ran some 1/4” fuel line from the tee, out through the grille & up to a pressure gauge on the windshield. Worked well & didn’t cost much.

Good luck.

Peace, Pat

[redroadster]

That type air filter ( looks like the type that clogs with powder , & did that on my Truck , shudders ,on load ,might beat it against a tree 1st
SU have a power valve function ?
Or a arcing wire /cap rotor
What variant & yr of 311 ?
Old gas ? Compression too low too high? Exhaust plugged? A mis aligned intake will cause a stumble on accel
Valves too tight ?

[funkaholik]

Pat - nice fuel pressure gauge hack! That is an easy, cheap, reversible way to see what's going on with the fuel while driving under load.

[SLOroadster]

If you think its lean, what happens if you pull the chokes on? Those should fatten up the mixture. If there is little or no change, you don't have a lean condition (or your chokes are disconnected.)

Will

[fleathatsme]

First things first, THANK YOU to everybody!
I woke up today sick as a dog and spent the morning, Happy Easter! Urgh
I fully plan to check things out in this order once I feel human again: (again open to suggestions)
1. Check timing at idle and at 3500 to make sure I'm set and advancing correctly.
2. I'll do the carb spray test to eliminate vacuum leaks on the spacers and intake. (low and mid rpms)
2. I'm going to check the fuel regulator (I attached a pic of the position on the rear SU carb) I'll probably have to take pics and show and tell as I don't know what is correct unless I see something stupid obvious.

PXL_20240327_235425993.jpg

3. I will put in a fresh set of plugs and go for a test drive (when she acts up I promise to be gentle, I had a scary backfire yesterday)

I hope this sounds like a good plan and order of steps; I'll try to get it done after work this week Wednesday and report back.

[fleathatsme]

Sorry I forgot to answer the question that this problem is non-existent out of gear in neutral (sitting still). I can rev it normally even to high rpms like a teenager at a red light.

[Gregs672000]

Sorry to hear you're sick! That sucks. I've read recently that there is a stomach flu going around that is VERY contagious and pretty hardy (able to stay in the environment for a long time). Best defense is regular and thorough hand washing, especially if you're out and about in public areas a lot.

So, when you're better, I'd go ahead and pull the plugs now before restarting and just see what they look like. Could be one carb is lean, (1 and 2 or 3 and 4) or one plug may be very different... that would tell you a lot, and it never hurts to look now. Then move on to your list... no worries, we'll get this!

Be well!

[fleathatsme]

Oh thank goodness I'm human again!
After the stomach deal I got to enjoy the full on flu with head cold!
That's what I get for bragging that I never get sick...
Back to it...
1. Checked timing at idle and 3000 rpm, everything good.
2. Checked for vacuum leaks at idle, no leaks.
3. Pulled plugs for a read and to replace for testing. #1 lean, #2 milk chocolate, #3 milk chocolate, #4 milk chocolate
hmmm...right?!
I pulled the tops off the bowls to get at the fuel pressure regulator, looked down and felt immediate shame. The front bowl is set too LEAN!
I really thought I was thorough when I set the float levels. I took a good look at the floats and sure enough the front carb float is set too lean.
I am ashamed...
I set the floats again until I have them at the correct level and exactly the same. (I may be a tad rich now but time will tell with some plug reads after some long drives)
I put it back together, reset mixture screws to 2 1/2 turns, fire it up and check the balance. Everything looks and sounds good.
Test drive goes very well. No starvation under load at higher rpms. No popping, no cracking up, nothing acting wrong. She's happy.
Now I notice that the Tachometer doesn't seem to be reading right at higher rpms...put that on the to-do list.
Thank you to all the good advice and things to check! I may even take her to caffeine and octane tomorrow!

[Gregs672000]

Woo hoo! Welcome back to both of you! It's always a learning process eh, all of us included. Glad you got it sorted. Do check plugs for a bit, especially since you saw some difference in #1... could be it will become normal, but there may be a lash adjustment that's off, or some other unknown issue like an intake leak that didn't show itself yet... something specific to that cylinder. Compression check wouldn't hurt.
Enjoy the coffee!