Adjusting Dwell

[Bitsyncmaster]

It is very important if you want long rest pressure times to have your mixture screw adjusted when the fuel distributor is hot (after a long drive). So make your dwell adjustment then.

What happens is the length of the piston in the fuel distributor gets longer when heated and that little length change makes a big difference for rest pressure leaking down.

Also, make sure the outer surface of the roller bearing that the piston rides on is clean. You have to pull the FD off to get at that bearing. Use some Q-tips with solvent to clean it and make sure it spins freely. A drop of 3 in 1 oil may help it spin freely. I don't think it is a sealed bearing so the solvent alone made my bearing spin freely.

The adjustment screw is a very fine thread and the FD arm setups even drops the movement by 1/3 the amount that screw moves. The screw moves 0.019" each turn so one turn moves the piston 0.006". Normal adjustment is a 1/4 turn so about 0.0015" swing.

I discovered this requirement doing my rest pressure timing. Now that it is cold in my garage the rest pressure time exceeds 4 hours.

When summer time comes, I plan to read the dwell during normal driving conditions to see if it holds what I set it at idle. I would think setting it with idle RPM higher and engine under load would would help. Put the lights and AC on. My AC on would bump the idle up to 950 RPM anyway.

If your doing rest pressure timing, remember the fuel accumulator takes 40 seconds of fuel pump running to fully charge.

[Michael]

This may just raise a shitstorm with the tech gurus, but would it be prudent for those of us with wildly diverse climates (single digits in the winter to triple digits in the summer) to adjust the mixture twice a year? Say once coming into fall and once coming into spring/summer?

[Bitsyncmaster]

This may just raise a shitstorm with the tech gurus, but would it be prudent for those of us with wildly diverse climates (single digits in the winter to triple digits in the summer) to adjust the mixture twice a year? Say once coming into fall and once coming into spring/summer?

Good question. I don't think it makes much difference in starting the engine. So maybe your cold running would be leaner or richer but the lambda may have enough to adjust it after it goes closed loop.

I'm doing another test today. My FD has adjustable rates on each injector. I just backed out those 6 adjustments 1 turn each. It started and ran good but I had to turn the dwell clockwise about a 1/4 to 1/2 turn after it warmed up. So I'm thinking that reduced fuel flow and leaned the mixture. I guess the injector volume is above the screw. I was thinking I was making it richer. Now it did seem to make the dwell adjustment less sensitive which has been my problem forever.

[Bitsyncmaster]

I tried backing the adjusters in 1 turn past where I started at and the lambda never came active. So I went out 2 turn from where it started at. I had to race the engine to clear the O2 sensor to get it to go active. But I will have to do injector flow testing with the bottles. I may have messed it all up with the many adjustments. I did get the dwell adjusted but it goes higher 10 deg when I up the RPM. Not that would a problem since it would still hold 14.7 AFR. But that is the average of all 6 cylinders so one or more could be off.

[Bitsyncmaster]

I'm still not happy with my results. Still seems to sensitive to adjusting the dwell. It just seems like it needs to be adjusted right near the point of injectors leaking. After I made the FD provide more fuel, I now get 2.5 hours of rest pressure and still have sensitive dwell. Also dwell goes up 5 to 10 deg with higher RPM. It must be my non-stock FD is different. I'm thinking now maybe I have to reduce the counter weight on the air flap to get where it runs like a stock FD. Lucky I have extra mixture parts to play with. It runs fine so will have to let it bother me some more.

Also still have the shorter rest pressure time after an engine run to temp. Runs at 1 hour 50 minuets rest pressure time.

[Bitsyncmaster]

Just thought of quicker test for counter weight. Add a little weight to the air plate should do the same as removing some of the counter weight.

[Bitsyncmaster]

A 15 gram washer glued on the air plate with double stick servo tape did fix the dwell change with higher RPM. It also bought my old setting dwell down about 5 deg so I had to turn the mixture screw a little clock wise to center it at 40 deg.

So reducing the FD fuel amount I think made it less sensitive to dwell adjustment. I think it was so rich, setting the 40 deg dwell was just at the point of no adjustment.

I will keep that 15 gram washer and see how it all holds up when road testing.

It's funny the air plate does very little movement from idle to higher RPM. I guess it moves more under load.

[Michael]

A 15 gram washer glued on the air plate with double stick servo tape did fix the dwell change with higher RPM. It also bought my old setting dwell down about 5 deg so I had to turn the mixture screw a little clock wise to center it at 40 deg.

So reducing the FD fuel amount I think made it less sensitive to dwell adjustment. I think it was so rich, setting the 40 deg dwell was just at the point of no adjustment.

I will keep that 15 gram washer and see how it all holds up when road testing.

It's funny the air plate does very little movement from idle to higher RPM. I guess it moves more under load.

I wonder if a neodymium magnet would be more secure since you wouldn't have to worry about the adhesive failing, or is the plate made of aluminum?

[Bitsyncmaster]

I wonder if a neodymium magnet would be more secure since you wouldn't have to worry about the adhesive failing, or is the plate made of aluminum?

The large center washer is steel. The plate is aluminum. This double stick foam I use is really permanent. It's used to hold servos in remote control aircraft. Even if it were to fall off, it would just sit in the mixture unit. I could machine a new washer to replace the original. That one bolt holding the plate on did use locktite. I pulled it from my spare parts but just decided the servo "tape" would do fine.

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