Datsun Roadster Forum-311s.org

I have a '69 Roadster with a 2L engine and SU carbs. It runs and revs up, but there is no airflow thru the rear carb. I can completely cover the rear carb with zero effect on the engine. This car is new to me and was purchased as a running project so I am learning all the things wrong with it as I go along. Thoughts on what to look at for no airflow thru the rear carb?

Sounds like the carbs are not synced. Go to www.311s.org & search for synching the carbs. Since the car is new to you, you might wasn’t to go through a full valve adjustment & tune up so you’ll know the condition of the engine. Checking compression wouldn’t hurt either, as well as topping up the carb damper oil.

Changing all fluids would be good too.

Peace, Pat

+1 to everything Pat just said. If you want to balance them yourself, do yourself a favor and get one of the "snail" type carburetor synchronizes. There are some cheap knockoffs but the original is made by SK. They are SO much better than the Unisyn type because they don't cut off the airflow to the carb so you can leave it on while tuning. Here is an example:

https://www.carburetion.com/Products/Pr ... art=STE-SK

For my Mikunis, I also use the STE6 adapter shown on that page. It has a 90 degree bend that makes it much easier to read the dial on the synchronizer. Not sure if that fits the SU's or if a different adapter is needed but there are a bunch listed on that page.

For starters you can use your finger or a small mirror to feel / look down the throat of each carb and see if the piston level is equal. My guess is that they are way off.

Here is a photo of what I'm talking about regarding pistons being equal.



Also, this is required reading for an SU owner:

https://datsunroadsterranch.com/welcome.html

If you opt to purchase a Carb synchronizer, here is the STE SK linked above at a very good price.

https://www.ebay.com/itm/282321951276

I'm confused by the various models of the "snail" style synchrometer (BK, SK, etc.), and adaptors. Can someone tell us which specific model, and adaptor (if needed), will work with the R16 38mm SU's? ...
TIA,
Mike

Ok, I made some progress. I adjusted the rear carb idle and got some significant airflow into the rear carb. Now I noticed when I put my hand over the front carb to chock it off, I get gas on my hand but that does not occur on the rear carb. Now I am chasing why no fuel into the rear carb maybe the float is stuck?

Hard to believe you have a car that runs (idles) without all 4 cylinders participating to some degree. No way it's running on two cylinders (smoothly)

Make sure the linkage on the carbs is installed properly, good luck

69 roadster driver, post photos of the carbs as they installed on your engine, sounds like something is not installed correctly.
Dave

69RoadsterDriver wrote: Sun Apr 28, 2024 6:09 pm Ok, I made some progress. I adjusted the rear carb idle and got some significant airflow into the rear carb. Now I noticed when I put my hand over the front carb to chock it off, I get gas on my hand but that does not occur on the rear carb. Now I am chasing why no fuel into the rear carb maybe the float is stuck?

First, welcome to our site! We're happy you're here and are excited to help you with your car. Ok, first things first, ya gotta get the airflow equal, and that really does require a synchronizer as discussed. You may get close, but carb balance (or should I say "equal airflow through the engine") is critical to how this engine functions. These cars are not complicated once you gain some basic understanding of how things work. It will be helpful for us to know what your general level of knowledge is about cars/engines, and what experience you have so as to tailor our responses to you.

As mentioned, it is also important to know that the valves are adjusted properly, so checking these is a part of making sure you're adjusting the carbs correctly as the valves are directly associated with airflow. If they are not opening properly or equally then its possible that the carb(s) will not draw as much air. So, to do this correctly check and adjust the valves first (this can be done in a cold engine. It requires a 19mm and 14mm open end wrench and the right sized feel guages for tools). You can choose to assume they are ok and go ahead with carb adjustment, but they should be checked sometime and you may end up adjusting the carbs some again later. I see that you have some understanding of the linkage, and it is important to know what does what and what you can adjust. Fuel delivery is not tied to linkage (except for the choke/warm up system via a cable), so read up on SU carbs and how they work. Once the airflow is the same you can check/adjust idle mixture and determine if you have a fuel delivery issue as you described, or not... could be due to a difference in how well it's able to suck in air...

We can help you with any of this process. I recommend a read through of our online manual in the tech wiki to get familiar with the engine, and review other articles that some of our resident "experts" like Keith have posted on SU carbs and other things. Don't worry about asking questions... nobody gets flamed here for asking what might be a "newbie" question... we just want you to enjoy your car!

Welcome to the family!