Diagnosing Intake backfire at speed - R16 w/ SUs

[datsun65]

Been working on my R16 1600 and trying to diagnose a long standing issue.

Issue: In 4th gear on the highway at speed the car starts to backfire through the intake/carbs. Starts at about 60-65mph. Feels like the car is struggling to go fatser. However, If I pull the choke all the way out the cars comes back to life with a burst of power and can immeditely start going 70-75 with minimal intake backfires. However, after 75, the carb again feels like it is struggling to go any faster. If I approach an overpass/hill, the car struggles to make it up the hill at speed.

Revving up through 1st, 2nd and third all goes very smooth, well into he 4K range. At the very top end of 3rd gear occasionally will have the intake backfire

Background: Car has been restored and in good conditional overall. Currently running both a Roadster Dizzy w/Petronics and also have the same issue with an EI roadster dizzy as well. Engine has strong compression between 190-200 across the cylinders. Car has new cap, wires, coil.

Diagnosis: My understanding is that the backfiring through the intake is due to running too lean. However, I believe the car is running pretty rich right now. Tomorrow, I'll richen up the carbs again and give it a spin. It doesn't seem to be a fuel clog issue, as the issue is much improved (but not fixed) once the choke is fully opened.

I'd appreciated any ideas of what to check next.

Thanks.

[redroadster]

Retard the timing 3 to 5 degrees , whats it do ?
Using premium gas ? fresh ,? check valve adjustment , check for intake manifold leak- carb spray
Carb slides free ? If it is too rich it may have a carbon section glowing to cause pre ign. With the high compression
how much you loading it in 4th ? No brake drag ?

[keith0alan]

Sounds like a classic case of running lean. Try pulling out the choke a bit to see if it clears up. Stock Nissan needles all seem to do this, too rich at idle and lean at top end. If this is your problem then the ADQ needles will work better for you.

[Gregs672000]

Definitely a lean condition. I have limited experience with SUs, however richening the carbs with the dial on the bottom is going to richen it everywhere. Assuming no intake leaks (which should be more prominent at high vacuum, i.e. with the throttle plates closed), as the resident SU expert said, I think you're into trying new, more appropriate needles and maybe the nozzles as well.

[Linda]

Nozzles can be checked with a guage pin, .092 as I recall from Keith’s Tech Wiki write up on SUs
Linda

[Daryl Smith]

"Engine has strong compression between 190-200 across the cylinders."

How long have you been running the engine?
That is pretty high for a 'stock' rebuild. If this wasn't intentional your cam may be advanced 1 tooth....
If your engine was built with the intention of cranking compression that high you should consider(?) running a longer duration cam and at least premium 94 octane fuel.
For a street engine I target 185-190 psi cranking pressure to be safe... with a good quench clearance. Zero decked the pistons?
Not sure what a good stock rebuild should be for cranking compression, but, I would suspect in the 165 to 175 psi range.

I agree with previous on the ADQ needle for richer mixtures, especially with the cranking compression you have, you'll want richer mixtures at higher rpms so you don't burn a valve or piston.....

Timing....advance mehanisms working properly? If not you will can also lose power in higher RPMs....
Spark - strong spark always helps, required with higher compression.

[Gregs672000]

Cam timing was not something I considered Daryl... interesting idea!
As an example, the hot cranking pressure of my 11.7 to 1 compression U20 engine (all be it with a B cam duration, higher lift cam) with throttle plates wide open is 210-215... so you're right his may be high, especially considering an R16 is 9 to 1 and a stock U20 is 9.5. It would be good to confirm that cranking pressure with another guage. Normally high compression would be cheered, but it could indicate something else!
Given all that Curtis went through with the different metric vs SAE gears, let alone the idea of being off a tooth, cam timing certainly is a thought. More info is needed!
What do you know about this engine Dan?

[datsun65]

Thank you everyone for the feedback. Some great suggestions to work through over the next few days. Thank you.

The compession numbers shared were cold. I'll check the numbers when warm, but expect it will be in the 180s. It appears the car was restored many years ago. I've owned the car for several years, though I don't have details about the engine build.



As mentioned, the drivability is better (still can't get past 75-80) and the backfire via the intake lessens when the choke is fully pulled out. The car has no pinging occurring.

I think it is back to all the basics: Timing, Valve adjustment and Carb Tunning, and maybe new needles as pointed out.

I'll update as the issues are sorted out. Again, thanks for all the feedback

[notoptoy]

Don't underestimate the importance of the float levels too, it may be just starved for fuel because there isn't enough in the bowls to keep up with higher RPM fuel demands. That is one fine looking Roadster, though it could use some center caps!

[datsun65]

100% Agree on the caps. I actually have the center caps for the front, but unfortunately they are too narrow & don't fit over the dust caps. I'll need to figure out a solution. Thanks for the suggestion on the floats.

[Gregs672000]

I see, it's an engine you're familiar with. Beautiful car by the Way! I have the same air dam and I think they're great.

[C.Costine]

My pretty much stock 1600 runs real close to 200 psi. It was expected due to minimal head thickness and domed pistons.

[redroadster]

My pretty much stock 1600 runs real close to 200 psi. It was expected due to minimal head thickness and domed pistons.

Yeah but it ran on 98 octane fuel, back then try some octane booster its a cheap / quick fix
You can only hear a bit of detonation driving

[OU812]

I have the very same issue.. 1966 R16 stock in good working order. Starts right up every time. Back fire out the intake, choke out 1/2 inch goes away. Adjust valves, and moved the timing all over the place, with no success. Made small adjustments to SU also. Checked for intake leaks...no leaks. The car is a little cold blooded...but after a short time... just fine.. Have oil in carburetors. Use 95 octane fuel. Just can't seam to finger the back firing out. Any suggestions would be appreciated. Hot or cold the car starts right up, idles 600 RPM. I don't know..???? Thank you OU812

[datsun65]

Found the factory fuel filter had disintegrated. Additionally, the cork gasket was rock hard. Replaced both, but unfortunately no improvement.

I think the next step will be to take off the carbs, check the needles and clean throughout.