Power loss at full throttle

[Pierre]

Hello everyone,

I have some issues with my 68 2000.

It starts without a problem and It idles fine at around 900rpm.
When I drive at first everything is "fine" but with less power then usual.
While driving, when I push the gas peddle fully down there is a deep "baaaaaa" sound (like if I would pull the choke while driving" and I loose all the power. Then I have to go one gear back to prevent the engine to stop.
When I Stop the car the engine does not stop it idles fine.
When I push the accelerator peddle very slightly when driving it accelerates slowly but "normally".
But all in all I also find the sound deeper then usual an less power.

In the last couple of weeks I did some changes to the roadster.

I pulled the head - It was resurfaced (0,18 mm) and had new valves.
Changed the intake and the Carbs (SU) - bought from sincitydatsuns.
Changed the headers - mine where a mess
New exhaust
New Radiator
new fuel filter

So I mounted all this during the last couple of weeks.

Adjusted the vales
the carbs where adjusted by a professional mechanic (who mostly works on British classic cars).

Some observations :

The fuel filter is sometimes almost empty some times almost full.
I checked the fuel pump by pulling the line behind the pump and letting the card run - there was a good amount of fuel coming out in 10 sec. (should I measure it to know the exact flow?).
How much fuel passes through the return valve? I pulled the line behind the carbs and not much fuel was coming through that line .
My gas cap is not vented, does this matter ? There is no vacuum sound after the car was running and I open it.


So I am kind of lost with thin one.

Clogged fuel lines? but the filter is clean. (the old one was a bit dirty)
Just badly adjusted carbs?
Ignition?

I would be thankful for any thoughts!

Thanks,
Pierre

[spl310]

Check the vacuum advance tubing to make sure that it is connected. Was the timing set? Is the distributor smog or non smog curve?

[redroadster]

68 2L
I'd check into the fuel flow 1st
Doubt the cap would make a difference as gas build pressure In summer heat , try the cap loose
Likely you have calcium build up in the lines by the description , can check the centrifical advance too with a timing lite and just twisting it .
Could temp dis connect vac. Advance
Could be a carb piston sticking or bowl dry
Not overheating ?

[Gregs672000]

Congrats on all the work you've Done! Don't worry, we'll get it sorted...
Based on your description, it could be a couple of things... likely either fuel or spark, but could be a cam timing issue given that the head has been off, though that's much less likely and we won't go there unless everything else checks out.

Ignition timing has more to do with actual power output than air/fuel ratio. As suggested, check timing with a timing light and make sure it is operating as designed: 1) correct initial timing for whichever distributor or igniton curve you have (non-smog or smog, 17 initial or 0). Check the manual or consult with others whether the vacuum advance line should be connected (I don't recall) to check initial idle timing, but regardless check and confirm the timing advances smoothly to approximately 36 degrees total advance above 3000 or so rpms (you can estimate by looking at how far past the final 20 degree timing mark the timing advances, just confirm that it does about what you might expect), then connect the vacuum advance line and confirm it HOLDS timing as you hold the engine at say 3000 rpms. The vacuum advance has a diaphragm that can leak down or not allow vac advance to operate (vac advance is more related to fuel economy than power however so I don't suspect a problem there unless the whole distributor is gummed up or not working/advancing mechanically, or the vacuum advance line is not connected to the carb but is instead connected to the manifold). Vacuum advance occurs under low load conditions, like when you're cruising in order to advance the timing a few degrees to increase burn time and use all the fuel. It drops off when you hit the throttle and put a load on the engine. If timing is working correctly, we can look at fuel...

An engine that runs lean will typically surge in how it feels as you drive it. If you try to accelerate, it will initially do so but then pull back and hesitate as it leans out, like it's starving. A lean carb can backfire out the throat if reved quickly. Conversely, a rich condition will just go flat. It won't accelerate and will sound fluttery. Someone driving behind you will likely see blackish smoke from the tailpipe. Obviously here is where I preach about using a wideband O2 meter to know, but you can do the old method of reading your plugs. I suggest a brand new set of NGK BPR6ES or BR6ES plugs for testing. You may want to take them with you to your testing road and find a place to swap them out in the field (bring gloves, they're hot!). Then, go drive the car and make the problem happen over and over, but try to not let the car idle for long if at all... we're trying to get an idea of what's happening by reading the deposits being placed on the plug to understand if you're lean or rich. So, drive it while it's happening, select a safe place to pull over ahead of time, then as you approach your spot take it out of gear, shut the throttle and turn off the ignition and coast to your spot (make sure you don't lock your steering wheel when you turn off the ignition!). Don't idle the engine if you can avoid it. Remove at least plug 2 and 3 (I suggest all four to check for individual cylinder problems, keep them in order, but 2 and 3 will tell you what each carb is doing) and see what they look like... white is lean, black is rich (what I suspect), tan just right. It may take some time before you have enough deposits to know, especially if its lean, rich will be much clearer. Differences between 2 and 3 (really 1 and 2 vs 3 and 4) will be due to some issue with that carb, an individual difference in any plug not seen on the others suggests a problem with that cylinder that is not carb related.
Once we figure out what it's doing (rich/lean or timing) and where (which carb, both etc?) then we can direct the next move (balance or adjustment issue, fuel needle change, corrected timing, etc).
Be systematic and do one thing/change at a time.
You can also check for vacuum leaks around your intake gaskets by spraying carb cleaner on them etc but they usually show up more at high vacuum like when your idling.
Hope this helps, report back my friend!

[autoairman]

Pierre
I have been reading of your problem,and what has been done,and your replies.One thing not mentioned,and easy to diagnose,is an exhaust system that is partially plugged.Your notice of a "baaaa" sounds like some plugged exhaust systems that were diagnosed in our shop back in the day.Since all of that is new,perhaps a quick visual might show some damage.
When those cars were checked,a quick vacuum guage check was done .Gauge to full manifold vac,pay attention to idle vac.What you describe under idle conditions,guage should be steady,somewhere 18 or so inches.A quick rap of the throttle on a good running motor will drop and quickly rebound to its idle reading.A car with plugged exhaust will drop and have a long rebound time.The second verification was to drop exhaust and run the car.A NORMAL,but loud running car was verification of the vac gauge reading.With everything new down there,dropping exhaust at the flange might be a good option for you.All of this of course after cam timing has been verified
I must say,all of those cars thru our shop with plugged exhaust,were catalitic converters that were not given correct cat food .
just my 2 cents
Rick

[redroadster]

Why not make a video of the conditions
It revs normal?

[Pierre]

Wow Thanks for the detailed responses!

@spl310,

the vacuum advanced tubing is set I checked it!
I think it is non-smog but I am not sure.
But I also bought a new EI distributor and a new coil from datsun restoration products.
I will install it this week and set the timing to 17 right?

@Greg,
Thx for your very detailed message!!
After installing the new EI distributor I will make sure the timing right and stays as it should!
The vacuum advance line is correctly connected to the carbs.

I just changed the plugs as well do you thing I should buy an other set to make the lean/rich test while driving?
But I will definitely do the driving and changing plugs test this week!

I noticed something else, while pushing the peddle all the way down the car makes that deep "baaaa" sound from 2000 to 3000/3500 rpm but past the 3500 it accelerates again. weird no?

@autoairmen

I really doubt it is a clogged exhaust system.
The exhaust is brand new and is really good quality, so I changed everything from header to end oh the exhaust system.

@redroadster

My fuel lines are 3 years old do you think calcium build up can be possible in such a short time?
But I better check the fuel tank, it was repainted when I bought it but how does it look inside
The temperature looks good there is no sign of overheating.

you are right, I will definitely do a video this might better explain the condition.

I will report back as soon I changed the distributor and did the rich/lean test and this time with a video.

Thank you very much for your advises!

Pierre

[Gregs672000]

We all are always happy to help if we can Pierre! You said that the engine is slow to respond from 2000-3000/3500 rpms... well, depending on what camshaft it has in it, a U20 is different from what you might experience or expect from a more modern engine, even those of similar size or displacement. They are not an engine that creates a lot of torque at lower rpms. While they won't be terrible or slow at lower rpms, they typically begin to make decent power above 3000rpms, and that should increase until 6000+rpms, with peak power somewhere around there. Since I'm not there to drive your car it's hard to know what's going on or how normal/abnormal it is acting. I gather that it's different from before, so it may be helpful for you to compare or describe the "before and after." Or if you can post a video that captures sound and shows us rpm/tachometer we might get a better idea. Keep working!

[Gregs672000]

By the way, you don't HAVE TO get new plugs (in fact, I'd pull them now and see what they look like) but it does make it easier to read a specific rpm where you're testing as it does take some time for deposits to develop as well as to change from what's on there now. You'll use the extra plugs someday anyway...

[rwmann]

I don't know SUs, only Solexes, but from your description of the flat spot, it 'sounds' like you are driving into, through, and beyond an RPM range where an overly rich mixture is being supplied. From what I understand of SUs, that could be from an over-full float bowl, dashpot oil that is too thick, or a binding mechanical choke.

[Pierre]

I think i have solved the problem.

rwmann had the right intuition.

The problem seemed to be the carb oil in the dash-pot.
It was to thick, that's at least what I think.
I used "carb oil" that was ISO 68, the one I tried now is ISO 32 ( more like SAE 15 I think).
Since I changed it the roadster runs way better.
I think what happened is that the thicker oil forces the piston to open slower at first ( that's why the car had no power / stuttered / hesitated between 2000 - 3000 rpm) . But past the 3000 rpm it accelerated more or less normally.
Did someone else experienced such a condition what thicker oil?

@Greg, the mixture is still to rich for the moment, I check the plugs and thy are really black.
I will try to adjust them in the next days.
Does anyone use those see through spark plugs ? the ones where you can see the color of the flames?
Is this method good/relatable to tune carbs?

I also upgraded to an EI distributor and the coil ( after I solved the oil issue) and it runs fine ass well.

here is a small video where you can see / hear the acceleration. ( sorry for the black border - it was my cellphone case )




Thanks for all your thoughts, tipps, recommendations and explanations.
Really great and helpful community!!!

Thx
Pierre

[spl310]

Oil is often used to tune the response. Good news that you found the solution!

[Gregs672000]

Ah Ha! That was much easier than we thought... well done to all. Regarding the plugs, give them some time to clean up or try a new set to see how it is really running now that the oil issue is sorted out. If it still needs adjustment then you can proceed. There are SU experts on this list who will be better able than I to help you there!

[redroadster]

I wonder how the motorcycle type slide carb would work on them ?
Probably a vibration issue so the vacuum type
& A fuel economy situation