Starts loosing power, gas milage is bad

Cusser

rev10eight reinstall the plates

Confirm that the plates are PERFECTLY flat, or sand them flat on a sheet of glass. See the Tech Section.

rev10eight

Pulled and plug vacuum line from brake booster, auto trans and distributor. No change.

Pulled the carb and plates. Inspected intake manifold and also made sure bolts were tight- all good.

Found I had mard the bottom plate with the carb bolt on first install but that was not the problem . Done Both plates on a window I had. Started with 120 finished with 600. I marked all over the plates surface with a magic marker. Once all the marker was gone, I knew I had it flat and smooth. Inspected carb for cracks, all looked good. Reinstalled and guess what? Still have the same problem.

Going to check my fuel pressure tomorrow to make sure my new mechanical pump hasn't started crapping out on me. I'm still suspecting this carb.

kz6fittycent

rev10eight if your plugs are reading good, the question is, was that after a run down the road or was it from just idling? There's a difference because of the different circuits in the carb. A lean fuel mixture can cause backfiring through the carburetor. Adjusting the air/fuel mixture can help, but if the problem persists, it might indicate a deeper issue such as a flat camshaft lobe or a tight valve.

Consider this from a "process of elimination" perspective:

You've already changed basically the entire fuel delivery system
The timing is correct and appears to be advancing
The carb is clean and you've contacted the vendor - the vendor has indicated there's not a problem
You've checked for vacuum leaks and repaired the adapter plates
You stated it starts getting sluggish when the second bbl kicks in and the engine is under load
a. This is what indicates a lean condition IMO. You may need to buy a jet kit and go up a couple sizes and see if there's a change. I had to do this!

rev10eight

kz6fittycent
Thank you

"plugs are reading good, the question is, was that after a run down the road or was it from just idling?

I ran it down the road

There's a difference because of the different circuits in the carb. A lean fuel mixture can cause backfiring through the carburetor. Adjusting the air/fuel mixture can help, but if the problem persists, it might indicate a deeper issue such as a flat camshaft lobe or a tight valve."

I can check the valves but have not did that yet.

Consider this from a "process of elimination" perspective:

You've already changed basically the entire fuel delivery system
The timing is correct and appears to be advancing
The carb is clean and you've contacted the vendor - the vendor has indicated there's not a problem
You've checked for vacuum leaks and repaired the adapter plates
You stated it starts getting sluggish when the second bbl kicks in and the engine is under load
a. This is what indicates a lean condition IMO. You may need to buy a jet kit and go up a couple sizes and see if there's a change. I had to do this!

That's is what we did while testing with the factory tech. We changed main and secondary idle jets and main and secondary jets. Did the bowl jets first and checked, then the idel jets. After doing the jets, we rechecked the timing and best lean air mixture.

Valve adj might be too tight, I'll check that.

The tech said they may have changed the fuel in my area for winter. That's why we uped the jet sized to make up for the lean condition. Ethenol fuel required more fuel volume to compensate for fule burn.

I'm going to drain the last of the tank
And put non ethenol in it. That's what I'm going to run if I ever get it running right again.

When I put the new head on this, I didn't rebuild the lower. It has probably 60k on it. Could be something to consider.

kz6fittycent

rev10eight heat can definitely impact cylinder pressures which would induce lower power, but it's the backfiring that really indicates the main two things I was saying. So, if you know you're jetting is right (and hey, I run e10 here in TX - all I can get really), then that leaves the valve adjustment. The other issue, hopefully not, is that you have a bad cam or valve train components, leading to the backfire and power loss.

rev10eight

Thank you so much for the information. It gives me something to look into.

rev10eight

My mechanic dropped my other vehicle off today as I Had just finished working on the truck. I had just finished checking the fuel pressure and adjusting all the valves to 10 while cold. I had a couple that was a little looser than they should have been. He asked me what was going on and I told him what it was doing and what all I had done. He will come by Monday and check the compression of each cylinder for me. He said he had a newer vehicle doing this and it drove him nuts until he found it. one of the valves was not closing all the way and was causing blow back combustion. its possible. This new head has about 3500 miles on it but it did get hot once. I've checked all I can check. Going to let him take a stab at it.

rev10eight

I Finally had the compression checked. Took my mechanic till today to get here.
I had to go rent a compression test tool after he left. The one he was using, you would set the piston to top dead center where both intake and exhaust valves were closed and pressurize the cylinder to about 100psi. As soon as he shot the pressure to it, the cylinder would shoot down and rotate the engine. We tried it on every cylinder and it would do the same thing. I would have to hold the crank with a socket wrench to keep it from partially turning over/pushing the piston down. We couldn't get a good reading like this. When I did hold it in place, it seemed like it would leak and not hold pressure. I went and got a regular compression test tool and crank the engine on each cylinder (about 6 to 8 revolutions on each one) to get the top pressure of each one.

My results with the compression tool:
cyl1 145PSI
cyl2 145PSI
cyl3 142/143PSI
cyl4 142/143PSI

Its odd to me that #3 and #4 were exactly the same, little lower than #1 and #2.
What's the chances?

Axel-Breaker-Earl

The system that your mechanic was using was not a Compression Tester.......he was trying to do a Cylinder Leak Down Test. In a Cylinder Leak Down Test, you have two pressure gauges. One gauge reads the air pressure amount that you are feeding into a cylinder. The other gauge reads how much pressure is being lost out of the cylinder, or the percentage of air that is lost. Typically most mechanics will set the air pressure going into the cylinder at 100 psi. If your other gauge is reading 90 psi or above, that cylinder is sealing properly.......once you get down to 70 psi or so, that cylinder has an issue.

You can also check where the air is escaping to (crankcase, intake manifold, exhaust, etc) and therefore determine if your rings are bad, if you have a burnt (or not sealing) valve, and so on. A Compression Test lets you know if there is a issue with a certain cylinder, but a Leak Down Test can actually hint to you where the problem is within that cylinder.

rev10eight

Thank you. I am really depending on you guys too help get this going.

I was hitting a bit of language barrier with him but we did see they were leaking.

~~Question: should the 100psi pressure cause the piston to go down and partially turn the engine. I had to hold it on place while he did it? He wasn't expecting that.~~

To answer my own question, yes it will rotate the engine if it's not top dead center. You will have to hold it out keep rotating it untill you have achieved TDC.

rev10eight

We completed all the compression and leak down tests. Compression is 143 to 145 across all four.
All pressure leak test held great, minimal to no perceived leakage. Now Im just stumped. Ain't no need to tear it open to look.

Just filled it up with non ethanol. That helped but still doing it. Once I hit 2800/2900/3000 rpms its starts that stutter/pop
I'm staring at that carb again.

MazdaMitch

rev10eight I just reread the original post and it crossed my mind
.... is your vacuum advance hose connected to the base of the carburetor or did it come off?

Kind of makes me think the vacuum advance is not advancing the timing at higher RPMs.

rev10eight

Thank you Mitch for your assist. I'm nearly bald at this point.
I made sure that the vacuum advance line is connected and it is.
I've also thought that its possibly not advancing correctly. Only way to eliminate that possibility is a new dist.

I just said screw it, threw my hands up and ordered another Weber Carb.
I can send it back if its not the carb or keep it for my next truck.
I've been fighting this beast since mid Dec.

MazdaMitch

rev10eight where did you order your carb from... let's hope it's a genuine and not a clone....

rev10eight

Let me go double check its location.

Just checked. Its in the right place on the bottom of the carb . I blew through the line to check for obstructions and blew thorough the carb vac port and the dist vac port, all is clear.

rev10eight

Parts geek. Last one I got from them was real.

Axel-Breaker-Earl

I'm looking at your fuel pick-up in your gas tank......

Is that a small filter on the end of it? With a spring around it?

I believe that you removed the auto trans in-tank fuel pump and went with a mechanical fuel pump......is that correct?

rev10eight

That is correct, in tank pump is gone. And yes that is a small strainer on the end of the pickup fitting.

Axel-Breaker-Earl

I wonder if you are getting the fuel that you need from that revised pick-up. Can you change that out to the regular pick-up for a mechanical fuel pump?

MazdaMitch

On mine I run the regular pickup of course and no fuel sock on the filter.
All I have is the fuel filter itself and the small filter inside the carburetor

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