Datsun Roadster Forum-311s.org

I rebuilt the carbs a couple months ago, and had it running great. I went to start it a few days ago and now it has a throttle response issue. The floats are floating, the banjo screens are clean, the bowls have fuel, the fuel filter is full. So I don't think it's a fuel delivery problem. If I cover the front carb by hand the engine dies quickly, if I cover the rear carb by hand it still runs. So probably a vacuum leak? I have verified that the vacuum line is connected and the manifold bolts are tight. I have only driven it around the block. Also, the dash pot oil is ATF, I added more to it with no change. Any help is appreciated.



Thanks again

Did the throttle cables slip or get hung up due to a frayed strand? I recently replaced my cable due to that and the cable , which is lubed along its entire length, works well. Can also check accelerator pedal and linkeage at firewall to be sure they are lubed also.
Maybe sticking fuel valves due to sitting. You can take the lids off and spray carb cleaner up into the valves, might work. Be sure you have good bowl gaskets..
Good luck
Linda

Well, we know that the front is doing OK and the rear not so much. I would look for the piston in the rear carb being partially hung up so that the needle is lifting slowly. It happened to me, after giving them a thorough cleaning. I took the cover off and piston out and sprayed them good with carb cleaner and it straightened out.

You have to have a leak if the engine will continue to run with your hand over the throat. It HAS TO draw air from somewhere to keep running.

Is fuel line hooked correct? Inlet/outlet. Had my son in-law truck with same symptom and the fuel was being sucked from return side. It would idle but when giving gas it starved like that.

From the sounds of it, the car pistons are fighting against the the dash pot oil. A few years back another members carbs were acting the same as yours but he was running the marvel oil in the dash pots.
I supplied him with some straight 20W oil that I keep around for my the dash pots and he found that the carbs immediately ran proper again after replacing with the 20W. It was cooler weather when this occurred, so possibly the original oil weight was affected by the lower temperature.

Hope that works for you.

Cheers
Ray B.

I didn't get much time to look at it over the weekend. But I can confirm that both pistons are moving freely, all carb and manifold nuts are tight, it has a new intake gasket. I topped off the oil in the dashpots, and it seems like it's not staying full, fill it, run it, and it's low again? Will have to take a second look at this.

I kind of think that since it's running when I cover the throat on the rear carb, it has to be a vacuum leak? Wouldn't you agree? I'll have to start it up and spray some carb cleaner or starter fluid and see if the idle richens up.

The funny thing, is the last time I worked on it, only a month or so ago, it was running sooooo good. Quick throttle, no hesitation, wanted to rev. etc.

Thanks for the reply's.

I don't know SUs, but on a set of Mikuni-Solex 44PHH-2s, this would evidence a stuck choke.

It might be leaking at the throttle plate, or nozzles. Both are areas that are rebuilt with Z Therapy and the like rebuilds.
Linda

Under most circumstances, for the engine to run with one carb completely covered suggests you MUST have an intake leak where that carb is drawing air from somewhere else to feed those cylinders. I have seen an engine idle surprisingly well with the carbs way out of balance where one was barely working. So, once you check for intake leaks with the the carb cleaner and if not finding any, disconnect your linkage and check the flow in each carb.
As I recall, SUs have a screw which adjusts the throttle plates individually. Screw them in a bit and start the car (may take some effort with the linkage disconnected). Once those are set so the carbs are flowing the same at idle at about 900rpm, inspect and observe your linkage for what happens when you hook each part back up. You may find the linkage itself is messing things up and needs adjustment, most likely causing one carb to open a lot more if it's a linkage issue.

Overall, think about how an SU carb works... engine vacuum is controlled by the throttle plate, which then influences the carb piston to raise up which then exposes the thinner parts of the fuel needle allowing the airflow to pull more fuel past the seat and feed the carb. Go through them step by step searching for what is influencing or changing this system. You'll eventually find it!

bossbob wrote: ↑Mon Nov 30, 2020 12:19 pm I didn't get much time to look at it over the weekend. But I can confirm that both pistons are moving freely, all carb and manifold nuts are tight, it has a new intake gasket. I topped off the oil in the dashpots, and it seems like it's not staying full, fill it, run it, and it's low again? Will have to take a second look at this.

I kind of think that since it's running when I cover the throat on the rear carb, it has to be a vacuum leak? Wouldn't you agree? I'll have to start it up and spray some carb cleaner or starter fluid and see if the idle richens up.

The funny thing, is the last time I worked on it, only a month or so ago, it was running sooooo good. Quick throttle, no hesitation, wanted to rev. etc.

Thanks for the reply's.

I've almost always found that if something was great - then I made some change/repair/assembly/etc - and then have an issue -> I'd go back back to the "ok what was the last thing I changed - or even the last thing I might have touched/disconnected/etc

So I'd say start here -> what was the last thing you did to the car when it was running great.

Edit: So I watched it again and noticed a few things that may/may not be related..
- your dog bone lock nuts are moving (screwing/unscrewing) while you are testing. Remove the dog bones and make sure they are correct length and nuts are locked
- was car full warmed up when you tested the rev
- it seems you are pulling up at the dog bone and not the throttle - I'd pull at throttle only.
- throttle seems to stick at a point (bent or sticky needle or sleeve that needs cleaning? dashpot oil? sticking piston(s)?)
- the throttle cable doesn't move when you pull up throttle but actually bends -> make sure the throttle cable has no resistance when pulling back and forth on the cable