Datsun Roadster Forum-311s.org

Any thoughts?
I'm just driving along at about 60 in 5th gear and the car sputters and starts losing power. It's done it before, but only for a moment or two, usually in 4th (same RPM range) up long hills. This time it kept sputtering. Just as a test, pulled out the choke and it cleared up. Drove home with about 1/3 choke and it drove perfectly normally. I can open the choke at idle and lower speeds, but need it over about 3000 or under load.
Leads me to believe it's not a lack of fuel to the carburetors, as the choke on uses more. Haven't added or changed anything. Fuel is 6 months old, non-ethanol with stabilizer. Full tank.

I'm leaning toward sticky carb piston maybe?

Any input appreciated

Could it be the summer fuel formula?

You have fuel restriction. The choke richens the MIXTURE by "choking" down the air intake rather than by increasing the fuel.

I thought only my roadster stumbled . A few weeks back, it was suggested I pull my choke to see if
it helped stumbling at high rpm ... it was the tank clogging as the choke did nothing. This is likely nothing you
don't already know ...

The choke indicates the mixture is too lean at high rpm. You said it has happened before, maybe
it was always a little lean at higher rpm and now some other change exacerbates this?
I think you were asking if Joe (Zappa) would set your high speed mixture for you LOL ...

Maybe a weak fuel pump? After I changed my out, all intermittent fuel issues disappeared. I checked the diaphram on the old pump and could not see a tear, yet the pump would occasionally drip gas, and the engine would occasionally sputter over 3500rpm. Played with carb set up before changing the pump, but still had intermittent stumble issues. New pump, no issues.

It's a fuel restriction. Check the simple stuff first, float bowl screens, piston oil, fuel filter. It's most likely one of them.

If you have the old glass fuel filter the seal can leak. Mine only did that when I got on her hard for a time, or a long hill climb at speed. Did it enough that I just got rid of the glass filter (even though they are cool looking!).

Are you running SUs? If so, I'd check the needles. They might have slipped down causing a lean condition. By pulling the choke on, it lowers the nozzle in relation to the needle allowing more fuel in. The fuel filter seal is a good thought, but I'd suspect its fine. Its a fuel restriction from either the screens in the carbs, or perhaps from the car itself.

The fact that it runs ok with the choke pulled means you are getting the fuel to the carb. The fuel isn't getting from the float bowl into the carb well however. The chokes DO allow more fuel in because of how they work. Its not a choking down of the air, its an enriching of the the fuel since by lowering the nozzle down, it acts like lifting the piston up. There is less needle in the nozzle to limit the fuel flow.

If running Mikunis, its possible that the fuel jet fell out of the bottom of one of the jet blocks. I've had it happen.

Will

I'm closer to Will's thinking. Talking to Gary on the phone, had a sudden thought it might be the fuel hose getting kinked, choke moves it, kink gone, fuel flows. I'll look this weekend. Rebuilding the fuel pump is a good idea, I don't always get ethanol free fuel, and it's been in the car since Bob built it.

Did you resolve this? I'm having the same loss of power under load and if I pull over and let it idle for a minute it fixes itself and diving with the choke out also helps. I've blown out all the lines checked screens, filters, float chamber needle seat and nothing seems to make a difference.

This is all on a car that hasn't been driven for 2 and a half years and most stuff is new or changed since it last drove so anything can be the culprit. There was old gas in the tank but less than 1/8 of the tank, I topped it off with new gas so I don't think the gas should be the problem.

Hey

Mine was doing the same thing.
Double checked float levels...
Changed the fuel filter.

All good now.

It's a new fuel filter, I put a glass one in and used it while getting the car running. There was a little debris in it so I cleaned it out but it didn't seal up good so I just put a new metal one in.

I kept thinking it was a restriction, the banjo filter was dark and kind of looked clogged so I cleaned it and it ran good for 2 drives then back to sputtering . So I took out the banjo filter, changed back to my old needle seat valve but still act starved. The rear bowl was low so I disconnected the coil and left the rear float bowl top off and put it in a plastic bowl and cranked it over a few times and it did pump fuel. Maybe just not enough? I guess I'll get a fuel pressure gauge and see if I have enough pressure at the carb and see if my pump is maybe going out. My fuel filter is after my pump, how easy is it to clog the pump and would it be worth opening it up? or will that just ruin the gaskets, I heard the later pumps weren't rebuild-able.

another thought, what is the different between the 67.5-68 and the 69-70 fuel pump? If my engine is from a 68 and my pump is from a 70 would it make a difference?

Take your Air filter off and go for a drive..

My guess is the s bend in the line just off the tank or pickup tube

May be more than one problem