Poor fuel economy

[charbly]

Howdy everyone! 

  While the title might suggest a poorly running 620, I have the opposite. My 75 620 is bone stock and I love it! I’ve spent a bunch of time tuning and getting my little truck to run properly and I have to say for being 45 years old it runs pretty dang well!!! But I’m still yielding around 15mpg, which according to a lot of people on here I should be yielding around low 20 if it’s in shape.  I’ve rebuilt the carb, installed new coil, plugs, points. Everything seems to be adjusted properly, my valves are set, points gap set, timing adjusted, carb idle adjusted, 5 speed installed to lower rpms on highway. The whole nine yards have been accounted for. In my search for better fuel economy in the stock form I put a wideband 02 sensor to monitor my fuel usage. I have tuned the carb for a steady 14.7afr at idle and it holds that great, there seems to be no vacuum leakage and I have total power, no lags or bogs thru the whole powerband. The curious part of this is that it’s running rich the minute I open the throttle. As I monitor my o2 while I drive, I read 11.3-4 afr as enter the slow circuit (~1000-1500rpm) all the way up to plenty-enough rpms. Now I know that you wants relatively rich mixture when you getup in your throttle towards wot, but it seems like the slow circuit is way too rich to yield good fuel economy. Does anyone know what to expect in terms of afr range for a stock hitatchi dch340-45 if the system is running correctly? Should I look to rejet my slow circuit? Does anyone know where to acquire jets for a hitatchi dch340 if I want to lean my mixtures the primary system? Or am I missing something completely thatall you datsunoholics know about this? Thanks in advance for all the help!!! Yeehaw!

[datzenmike]

Any chance you have a gas leak? Neighbors siphoning your gas?

 

You should get more like 25 or even more out on the highway. High 12s at WOT... 13s even better on the O₂. It should go rich as soon as you step on the gas. NOT 11s but richer than 14.7

 

 

 

 

Do you make short trips into town, say under 5 miles? The engine spends most of it's time with the choke on. Have you checked that the choke does turn fully off after 8-10 minutes?

 

When you swapped the transmission did you put the 4 speed speedometer drive in or use what ever came on the 5 speed? Possibly the odometer is out. 

 

Very generally around 100 for the primary and around 160 for the secondary jets. They are interchangeable (by mistake during a rebuild) and a 160 primary would run rich indeed. Where do you live? as the altitude can affect jet sizes. Higher altitude... smaller jet.

 

 

[charbly]

Those are great thanks for those numbers mike.   Choke is working fine and opens normally as the engine warms. I did have that instance of switching the primary and secondary jets when I first rebuilt the carb, I couldn’t get it to idle correctly and finally re checked the jets and swapped them to their correct spot. Tranny drive from 4 speed so not to get incorrect readings. I just can’t seem to think of another reason for such a rich mixture thru the entire throttle. I guess I’ll pull the carb and look at the slow jets to see if they are set for an  altitude situation. All I can find for jet information is in the 1977 FSM which notes a hitatchi dch 340-45b carb for California. Is that the same specs (jetting and Venturi sizing) as the 1975 dch340-45?   Thanks!

[datzenmike]

Close enough I would imagine.

 

 

You're in California? At what altitude?

 

 

 

You have the intake and exhaust that is bolted together. A 'heat riser' is used to warm the under side of the intake using hot exhaust. Is it rusted and stuck in the open position. The valve is on the firewall side of the exhaust manifold just under the intake.

[charbly]

Altitude is just above sea level as I live a couple miles from the ocean. No guarantee the car was distributed nor purchased here but most likely it is a California car. I had no idea about the heat riser, years ago when I got the car I removed the air galley emission tubing as it had rotted thru and I sealed the exhaust ports with bolts so no leaking would occur.  I’ll look tomorrow to find the valve. Thanks!

[banzai510(hainz)]

I get about 20mpg with 4 speed and drive normal with 521

[wayno]

I never did get much better than 17/18mpg myself with an L20b as I drive too fast, I then built my LZ23, I did get 23mpg going 75mph with that engine when I first built it, don't know what it gets now as I haul a trailer around and I don't want to know.

[charbly]

So I found my heat riser valve was staying closed always, drove around town a few times until car was completely warm (thermostat popped). The riser seemed to have too much tension from the thermo-coil spring and was staying closed. There was no binding in the hinge and after removing the exhaust manifold I could verify it was all free to move inside/out, just too much tension, the counterweight had no effect on the spring. I held the riser flap in the open position and ran the throttle for a while, and monitored my afr's and there was no change in the rich Condition in the slow circuit. I'll try to adjust my riser issue so it works correctly, and look back up trail at making sure my vacuum advance is working correctly thru the throttle and not just at idle? And then start playing with my carb jetting? Does anyone know where to procure new jets for the hitachi Dch-340? Are there any compatible jets from other sources that are available to buy?  

[datzenmike]

Exhaust pressure pushes it open, normally closed when cold but the spring tension relaxes somewhat when hot.

[charbly]

I was surprised to find with a hot engine no effect on the coil.  Maybe someone had installed incorrectly when attempting to adress problem. Nevertheless, I imagine this doesn't account for the fat slow circuit I'm experiencing. 

[datzenmike]

Wire it open and drive for a tank full to confirm.

 

15 would be reasonable if short trips and mostly in town. Highway is the most efficient speed and brings the average up. On a cross Canada trip I was able to get 28 in perfect conditions.

 

You can increase your mileage well over 10% by just modifying how you drive.

 

Inflate tires to maximum

avoid short trips

empty 'junk from the trunk'

anticipate stops and slow downs ahead and begin slowing and try to roll through as the light turns green.

try to drive without using your brakes, leave more space between you and the car ahead

every time you have to use the brakes you are throwing away the gas used to get you up to speed.

avoid 'warm ups' totally unnecessary except on extremely cold days. Cars will warm up as you drive.