Diagnosing poorly running Mikuni setup. Not the usual causes

[zKars]

Hey gang, thought I'd share an interesting Mikuni troubleshooting session that occured this past weekend.

A good friend from Edmonton has a Datsun roadster 2000 with a pair of Mikuni 44's. After converting to them a few years ago, he's had nothing but trouble trying to get them run properly. Bought them fresh from Wolf Creek, so had no reason to think the carbs themselves were the problem. 

He's had several local "experts" try to get them working properly with no luck. In what can only be called a final desperate act, he asked me if I'd give this a shot. Being a sucker for punishment, I agreed. 

Now I did find the problem, but it was not anything that I'v encountered before, so I felt I had to pass on the details, as I think this type of problem may be more common than we might want to believe.

I won't bore you with the all the details, but I checked ALL the common issues such as fuel pressure, pump flow into a pail, float levels, jettings, balance, as well as "related" items like spark strength, timing and plug condition. Even checked idle vacuum. Decent 15 inHg, no big vacuum leaks at least.

To jump to the answer a bit, the problem was with "fuel delivery" so I'll concentrate on the specific steps we took to track down the actual problem.

First thing "Dave" said was that he has an electric pump but no regulator. "Aha" I'm thinking. So we stick a fuel pressure guage in line and turn on the pump, car not running. Solid 4psi. This should be no problem. MAYBE is the needle seats are ancient, they might have a problem sealing 4, but these are virtually new. We then proceeded to go through a carb balancing act, and turns out they were SO out of balance at idle it was laughable. front carb was OFF while back carb was so far open to get the thing to idle it was sad.

It did require that we remove the air horns to get a good seal on the throats to get a good flow reading, I couldn't get the same reading from both barrels on a single carb using the snyc plate on air horns. Maybe previous troubleshooters were fooled by this. 

After much idle screw adjusting and idle tweaking and balancing games it still ran like crap. No throttle response, massive misfire, no power. Lean lean lean.  Jetting?  140 main, 200 airs, 60 pilots, idle screws out 1.25 turns, visual confirmation all barrels can squirt accel pump jets, nothing crazy here.

 

 

[zKars]

Somewhere around this time, I glanced at the fuel pressure gauge .. WTF???? It's reading 0!. The engine is still running (if you call this running....).. Zero ???.  Turn the car off, turn the pump on. Gauge reads a touch off 0, but not much. Dang the pump has died. Or is the gauge fried. The car is running, and long enough to empty the bowls too, so the pump MUST be supplying enough gas. Pressure can't be zero.. 

We stop and take the hose off the gauge and try to pump fuel into a pail to test it. Turn on the pump, yup, nice steady stream into the pail. It's not the pump, it's just fine. Stupid pressure guage.... Put the fuel line back on.

Back to scratching my head about what the real problem might be.

We cleaned the plugs, tried a different coil, drank beer, nothing helped.

Okay enough torture. The fuel pump pressure being suspiciously low is bugging me. Never had a problem with this gauge before. Could it be the pump after all? Let's swap it out and see what happens. 

Changed the pump and like magic, the car runs like it's new. Smooth, powerful, no lag, torque out of the hole is amazing, this dang little Roadster is actually fun to drive! 

 

[zKars]

So let's review. 

The old pump appeared to work fine, but actually it does not work worth crap. It could provide the needed volume only under no load. When pouring open into a pail, it has tons of volume, but I did one important test that proved the point. I turned the pump on and put my finger over the end of the hose trying to stop the flow. It was easy to stop it dead with no squirt trying to get past my finger. No jam.

With the new pump, it was almost impossible to stop flow with my finger.  The wide open into a pail flow looked EXACTLY like the bad pump, but it has the power to provide volume while working against a closed needle valve. 

Now the carbs have proper fuel level in the bowls at all time and are not being starved with the resulting lean condition

So we learn the difference and inter-relationship between pressure and flow rate. 

One more diagnostic nugget. At the end of the job, I remembered to remove my little manifold with the fuel pressure gauge before it drove away back to Edmonton never to be seen again. Car had been off for about 10 minutes while we were celebrating. 

I pulled the hose off and got a nice residual pressure squirt of fuel all over my hands and sleeves and in my face (safety glasses on, no problem). THIS NEVER happened with the old pump. It was unable to maintain a check valve seal when off. 

[Patcon]

Good catch...

[grannyknot]

Thanks for posting this, pump with no jam, try and stop flow with thumb before ruling it out. Noted

[Captain Obvious]

That's really interesting. I guess it could come down to the pump's internal design.

So was the new pump the same brand and style as the old one?

Good sleuthing!

[zKars]

The old pump was different than the new one. Both are your basic parts store CheapO pumps.... The new one is a AirTex E8012S, the old one was a Carter 9217A.

[madkaw]

Makes me wonder about my pump and fuel pressure/supply. The dyno showed a lean condition under heavy load. Pressure barely hangs in at 3psi though it's rated 4 psi and 40 gph. Probably should have gone big with a regulator myself . Getting ready to go the $$ route and do the bypass regulator and bigger pump to eliminate any ?? about fuel supply.
This was a carter pump .


Sent from my iPhone using Classic Zcar Club

[zKars]

Don't go too big. You're asking for premature burnout or noisy pump. Pumps like to flow, not sit there buzzing trying to pump through their bypass or cavitating. Bad.

A nice Nismo low pressure fuel pump or a Facet FEP60SV (2.5-4psi) are my favorites.

[madkaw]

Trying to cover my carb and FI in one sweep. Going to install larger 3/8" fuel lines so bypass can flow freely. Think I will go areomotive 13222 with a FI pump . Not going to dwell too much longer with the Triples , but still like to see if I can get a little more out of the L24.

i might consider a low pressure pump and just sell off later. 4070 Carter maybe

[zKars]

47 minutes ago, madkaw said:

Trying to cover my carb and FI in one sweep. Going to install larger 3/8" fuel lines so bypass can flow freely. Think I will go areomotive 13222 with a FI pump . Not going to dwell too much longer with the Triples , but still like to see if I can get a little more out of the L24.

i might consider a low pressure pump and just sell off later. 4070 Carter maybe

I put an in-tank EFI pump conversion (fit real slick) from TanksInc in my tank a few years back when I switched to a 280 tank for my 73. I had these same thoughts about "when I go EFI" but so far, have only gone from triple 44's to SU's, which are pretty darn nice. Hope that in-tank pump marinades ok....

Certainly heard of lots of folks with a stock EFI pump with a reg to get it down to carb levels being really happy too. 

[madkaw]

That's exactly my plan. Talked to Tank inc today about there FI module . It is pretty slick.