Datsun Roadster Forum-311s.org

I'm having a strange issue, I can't get the engine to rev past 3500ish rpm. I have checked the electrical issues, changed the fuel filter, checked the screens in the carbs, checked the fuel level, the jets, and everything else that I can think of.
I pulled the engine last week to replace the starter, waterpump and transmission, and upon reinstallation, I can't get it to rev past 3000 rpm. At 4000, it stumbles and dies. However it will idle fine all day long. I didn't ajust or change anything from when I drove it to my friends garage. On the drive over it was fine, I removed the rear carb and now it is acting up. If I open the chokes, it will rev beyond 4k no problem. Any ideas, I'm stumped.

Thankls,
Willl

Add a temporary fuel pressure gauge at the carburetor (tape it to the windshield or something like that) Watch for a dropoff in pressure.
Do you have an electric or mechanical fuel pump?.
Last time I had something like this it was a clogged screen in the fuel tank, the engine would just choke out and die. Blew some low pressure air back down the fuel line from the pump to the tank with the gas cap off. Never happened again after that.
The part with the closing choke sounds like it may be only one carb starving for fuel and the choke adds enough fuel to go the extra RPM.

Will

Sounds like you are running out of fuel, the reason this makes sense to me is when you put the choke on it causes a rich mixture and your engine speed increases you can check this by starting the car and partially cover the carb openings. I agree with checking the fuel pressure our engines like arond two to four lbs of fuel pressure is their anyway you can temporarily use a seperate fuel source, a friend holding a gas can with a line insetred in it will tell you if something is wrong with your tank if that doesn't work check the fuel flow from the float bowl needle.

Good luck

Russell

I have a mechanical fuel pump and the plugs are fouled. I'm almost thinking its just bad gas and I got lucky driving it across town. The gas has been in there for over 3 months. I checked the pilot ajusments, screwed them closed, then opened them 3 turns and still the same thing. The float bowls appear to stay full, they arn't drained after I rev it and it quits.

thanks,
Will

Have you done anything with the points?

Sid, that was the first thing I checked, they were right where they should be. I checked the wires, the plugs, the coil, fuel filter, cap, rotor, fuel lines, screens in the carbs, jets, and jetblocks. It ran fine on the way over, I removed the rear carb, set it in a box out of the way, removent the engine, and re-installed everything. The timing is right where it should be as well. The only thing that seems to make a difference is when I open the chokes, then it will rev fine. Would old gas make this sort of a problem? Especially on a higher compression engine such as mine?

Thanks,
Will

Will

What about a leak between the carbs and manifold or between the manifold and head? Take a carb cleaner and spray both the phenolic(spelling?) blocks and the manifold where it is between the head and manifold...if it revs higher then you may have a leak where air is coming in. If so this woul cause a lean condition, and would explain the reving.

Russell

I think Russell is on to something... It's simple and fits the sequence of events. You can also listen through a tube or hose , like a stethescope for the hiss.

Look for the vacuum leak for sure. If none - How long was the carb off the engine? Gas will turn to varnish and gum up passages in the carbs. If you haven't already, I'd take the jet blocks out and throughly clean them - disassemble the jets from the blocks and pull out the bleed pipe if you're running one. Make sure they're spotless. If that doesn't help, take the aux. venturi out and make sure it's not plugged up at all. I use guitar string clippings to push through the passages. Obviously, don't force something too big through.
The choke just dumps more fuel in through a port after the throttle plate.

The rear carb was off for 48hrs. I didn't touch the front carb, just the rear so it would clear the brake master when I pulled the engine. I did pull all the jets and jet blocks, all look good. The insulators look good, and I don't think its going lean, the plugs are fouling. I'll clean them off, start it let it run and when I rev it and it dies, the plugs are fouled. After driving the car over, I pulled all 4 plugs and found them all a nice light brown color. I looked at my Mikuni manual and that was pretty much worthless.

If I had a vacuum leak, wouldn't it show on a uni-syn?

Thanks,
Will

SLOroadster wrote: I'll clean them off, start it let it run and when I rev it and it dies, the plugs are fouled.

Without having put the choke on to get it to rev?
If that's the case then they must be fouling at idle. If you choking it to get it to rev and then checking the plugs, they're likely fouling when the choke is on. the symptoms sound like lean.

I'm thinking it's going to be a very basic simple thing
Correct me if I'm wrong...
1. It's stalling out when you try to rev it past 3500 and at that point you have full fuel bowls and 4 wet plugs.

2. If you choke the motor, it will rev higher and not stall out, or will it stall anyway?

3. Have you tried choking only 1 carburetor? Other thought, either pinch off or remove and plug the fuel line to the rear carb, start it up and see what happens. Switch to the other carb if necessarry and compare.

4. You checked and found no vacuum leak?

5. Nice strong blue spark at plugs, no damage to coil or plug wires or plugs

Now the stupid and maybe obvious stuff, most from personal experience.

Could anything have gotten into the intake when the carb was off, rag paper towel?

Wherever you attached the chain to pull the motor is there anything that could have been damaged?

Are both throttles opening basically equally, linkage and such looks normal? (did that on a motorcycle, ran really funny on three carbs, ran much better after I connected the 4th throttle cable)

If the gas was that bad, you would smell it, but top off the tank if you can, or substitute another source of fuel, gas can, hose to fuel pump, or gravity feed to carburetor with large fire extinguisher nearby. (I set the backyard on fire doing exactly this)

Was anyone beside you working on the car and may have made a change or adjustment you are not aware of?

A timing light pointed into the carburetor acts like a strobe light, you can actually see the fuel droplets in the venturi. All 4 throats should look the same. Probably need a mirror. While you have it hooked up, check the timing.

If the engine is running lean, exhaust will be really hot, plugs will be on the whiter side of tan, rich will smell gas at tailpipe.

Have someone else look at it. Start from the beginning, don't tell him what you already checked. A different set of eyes can make all the difference. (I wasted half an hour diagnosing a no start, only to have someone point out the car was in drive. 20 years later my Sister still won't let that one go )

Ok, I tried the different chokes, opening just one had about the same results between the two. There was nothing that could have gotten into the intake when I reinstalled the engine, it is not running lean, hence the fouled plugs, the spark is huge, and the gas could be bad, however it started and drove over just fine before I took it all apart. The carbs are ballanced. I was the only one working on the car, except when I pulled the engine and put it back in.

I'll try the fuel can trick, that will tell me if its just the fuel.

thanks,
Will

Ok, I tried different fuel, no change. I replaced the points, no change, regapped the plugs, no change. Replaced the fuel filter, no change. Pulled all the jets out, they were all clean, and that resulted in no change. I pulled all the fuel lines, cranked the car over, the fuel pump pumps fuel, all the lines are clear of obstruction. I checked the fuel inlet screens into the carbs, they were clean. I verified that there was nothing blocking the intake with a Uni syn, all 4 pull exactly the same. I checked the cam timing and the ignition timing, both are spot on.

The car starts immediatly, and idles perfectly. If I roll into the throttle, it hesitates at 2500ish rmp , then falls on its face about 3000ish rpm, and cuts out and dies at 3400.

I can't figure out what its issue is. 3 different people have looked at it, and they are all stumped as well.

Will

Will have you checked under the point breaker plate to see if one of your springs are broken... Bill