hesitates at higher rpm when warm

[davids]

As a recent Datsun owner I'm not too familiar with the quirks, so any help is appreciated. I have a 68 1600 that starts fine, has good acceleration and idle when its cold, but once its warmed up, if i try to accelerate the rpm above 3500 it hesitates and even backfires a bit. I have changed the cap, rotor, fuel filter and most of the fuel lines, plugs and such. Any suggestions?? Thx

[SLOroadster]

did you check your points for the correct gap? Are your carbs balaced? Is your mixture correct? I have a high RPM (about 4700) miss but I think it is due to my points being out of ajustment. I need to fix that.

Will

[davids]

I had the rotor and dist changed by Carl Jeager, so I'm pretty sure he would have done the proper adjustments there. As for the carbs would they be that different when hot or cold? The other thing I've noticed is that there doesn't seem to be alot of fuel in the fuel filter (its clear) as it gets hotter, so I don't know if there could be some back pressure caused by the heat or a vacume problem??

Dave

[SLOroadster]

Carl knows his stuff and won't steer you wrong. I wonder if your fuel pump is on its way out. Now that you mention that your fuel filter is low, you might not be getting enough fuel. The quick way to tell is to warm it up till it starts to misfire then pull a plug to see if the mixture is way lean (Plug will be white). From there, open up the carbs and check to see if there is sediment in the bottom of the float bowls, then remlove the screen filters from the fuel inlet T and blow those out with compressed air. The screens fit around the bolt that holds the assembly together. The other thing is you could be vapor locking for some reason. Do you have a fuel return going from your rear carb back into the tank?

Will

[Tycorcr1]

also, check your engine block ground strap. The conections can and do rust-out. You can never have enough ground and the ground that is stock is usually just enough to begin with. The amount of current the ground has to carry increeses with rpm because of the more frequent firings of the ingintion. ground ground ground!
That's this Electrician's view point.

Keith

[davids]

Hey, I had Carl replace the fuel pump with a original, as the previous owner had put in a electric pump, so we hoped that might be the easy fix, wasn't, so I guess I need to go into the carbs and check the ground, as per your suggestions. One question for the experienced guys, how full is the fuel bowl on a proper running roadster once its warm, so I have a base line to know if I'm getting fuel.

Thanks for the help
Dave

[S Allen]

The magic number is 23 MM or 7/8" from the top of the float bowl cover. Mark a line on the sides of your float bowls that is 23mm(7/8") from the top lip of the bowls. Attach a length of clear plastic tubing to the outlet on the bottom of the float bowl. With a mechanical fuel pump, disconnect the coil. Electric fuel pump do nothing. To make cranking the motor easier remove all the spark plugs. Ensure the car is not in gear and apply the parking brake. Turn the motor over until the fuel in the plastic line reaches a level and stays there. Compare this level to the mark you made on the float bowl. Adjust the float lever on the lid until the fuel in the line matches up with the mark on the float bowl. Re-set and recheck until you get it spot on. That will have your floats adjusted properly.

Another thing you can check fuel level by pulling off the domes, springs and damper with needle of the SUs and peer down inside. You should see fuel level with the hole the needle slips into. If no fuel=too low--fuel running over=too high. Again, you should see fuel even with the top if they are adjusted properly.

Backfiring usually indicates a lean condition-lack of fuel. Good luck.


Steve

[davids]

Now I know what I'll be doing this weekend.

[davids]

Hi, thanks for all of the advise so far. I have one question, the car has a clear fuel filter and I've noticed that the fuel flow into it is very intermitent, and not at all at certain points, is this suppose to be a steady flow or is it based on something I'm technically not aware of yet. I have replaced the fuel pump and checked the floats in the carb, as well as replacing the fuel lines from the pump up, I am wondering if I have a plugged up tank or line between the tank and pump, the car did sit for atleast a year, but started right up. Any ideas??????????? P.S. I've been putting my interior back together and driving the car, and its a hoot, I managed to get my hands on some used panasports, so it's looking good, just need to paint. Makes all the work worth while.
Dave

[S Allen]

Sounds like you are making real progress. The fuel filter will fluctuate some but it should always have a bit of fuel in it when the car is running. Maybe you have a bit of crapola floating around in the gas tank that is causing your problems. Junk in the tank has caused many a headache/sleepless night. You could go through the sender access panel in the trunk, pull the fuel sender and carefully with no sparky devices peer down in to see what you can see. You could have a piece of debri floating around in there that only gets hung up when you are driving the snot out of the car. Any way, something else to look at. Ain't these cars fun?

Steve

[davids]

Thanks Steve, I'll give that a shot, I just have a feeling its a fuel delivery problem. How much would the gas cap affect it? The one on there is just a standard cap with a small hole drilled in it for venting, I've ordered a proper one .
Dave

[S Allen]

I did not realize your project was a 1969 for some reason I thought it was an early year. On the later cars there is line I beleive that runs back to the gas tank. It has a little check valve in it. If this valve malfunctions it can reduce the flow fuel to the carbs. You could try taking the valve completely out of the circuit to see if that makes a difference. The early cars non-smog have a vented gas cap but the later smog laden cars had a non-vented gas cap. I am not sure what year the smog stuff kicked in but look for the valve on top of the manifold by the rear carb and putting a new-correct gas cap on cannot hurt. Maybe it was '70 for the non-vented cap. Carl should know for sure.

Steve

[spl310]

Only some of the 70 models had the flat unvented cap. To retrofit, you can buy the brushed one that Rallye sells and not braze up the vent holes, or replace the filler neck and hose.

[davids]

Hi, I'll try the valve to see if thats it, as for the year of my car, going by what ralley says on there site it sounds like a 68 registered as a 69, which I'm told was common, so it should be a vented cap, as per ralley once again, I'll let you know if I have any luck with the clog, I'm hoping its something that simple $$$$ and not a carb problem.

Dave

here is the gas cap info page for your fyi

http://www.datsunroadster.com/PIC_PAGES ... 172-10.htm

[browne]

i had the same problem. are you feeding the carbs first to the front and then passing to the rear? if so run your fuel line to a 't' between the carbs to feed both at the same time. this should help at high rpms.