Loss of power and backfire: help!

[John L]

Guys:

I have totally delayed making this post as I have truly tried everything within, and beyond, my mechanical limits to fix my problem. Before I got started, the engine was running really bad. It would start and idle but when given any load whatsoever she would almost die, backfire out of either the carb or the tail pipe and may run a tad bit better afterwards, or not. It was not driveable. So, in the quest to upgrade my 40 year old baby (1975 710M) I decided to make some upgrades. Following my upgrades she now runs and is driveable; but when leaving a stop sign or stop light as I give her fuel she will lose power, backfire out of the carb, then seem to clear itself and run fine until the next shift and fuel dump. Once I have it up to highway speed she runs with more power and responsiveness than she ever has since I have owned her. But, when you stop for a light it starts all over again. It actually seems that as she warms up she gets more cranky. (I guess she and I have been around each other too long ) Anyway...

Aside from searching the forum every which way, including Sunday, here is what I have done to her over the last few months:
*New Zenith Carbs and gaskets
*new intake manifold gaskets (cleaned them as well and removed 40 years worth of crap off the top of the engine)
*new vacuum line between the intake manifold gaskets
*new vacuum line between the oil filler pipe and the engine
*Pinz SSI installed, including new plugs and wires
*new fuel lines from tank to carbs
*Compression test: seemed fine
*Leak down test: seemed fine
*Fuel pump cranking out 2.5 psi on outbound side (perfect according to the manual)

What am I missing???? Could it be as simple as I don't have the carbs dialed in properly? The engine starts great, idles great... it's only as I give her fuel that she fusses at me and belches by backfiring out the top of the carbs. I synched the carbs and they seemed fine but I am not a Jim nor an Andre. I currently have the idle a tad bit fast as I thought that may help the situation; but to no avail. I have read every post I could find that deals with this and can't seem to find the solution to my problem. Could my current problem be on the exhaust side? If so, I don't understand why a hole in a muffler, for example, would cause loss of power and a backfire out of the top of the carbs when I give her fuel.

Any thoughts or help would be greatly appreciated. Rick with the SSI system has been great and I have trouble shooted it to the best of my ability short of replacing the module. I have the grounds in place, the jumper grounds, checked the voltage to the system, timing, etc... I'll be bald before I finish this project

Thanks for taking a minute to try and assist with this problem Merry Christmas if we don't chat before!

John

[pinzinator]

Have you verified that the float level is correct? Is the vacuum plug in place where the rubber line to the distributor used to be connected? Is the accelerator pump plunger connected to the correct hole in the lever?

[4x4Pinz]

one issue that has not been covered is the 4500 rpm module. I would look into making sure it has been bypassed. If it has not been bypassed, that would be an easy task. They can cause the symptoms you are talking about.
http://www.pinzgauer.com/showdetails.ph ... 112850360R

[Jimm391730]

one issue that has not been covered is the 4500 rpm module

That should not be an issue, as the SSI doesn't use power from the module anymore. Without the SSI, a weak spark due to the 4500 rpm module will kill the performance above idle (and sometimes idle, too).

But it may be that the accelerator pumps are not set right, I agree with Pinzinator -- IIRC the Pinz needs the maximum amount of fuel pumped, with the pump rod in the outermost hole of the arm (to move the pump piston the most). I thought I heard that the "normal" carb setting was for less fuel from the accelerator pump, and the linkage needs to be changed for the Pinz. I could be wrong here, but since your problem appears to be ONLY during initial acceleration then this could possibly be the problem. I drove my EFI system for a year, babying the throttle off idle, until I increased the accelerator "pump" setting (gives a dash of extra fuel as you start to move the throttle) and wow, what a difference in driveability. You should also be able to see a significant stream of fuel into the venturis with the engine off when you open the throttle (like the amount of water you get from the average squirt gun).

[John L]

Interesting comments on the accelerator pump lever. I checked the repair manual. Two drawings show the lever connected to the hole closest to the pivot point at the top of the carb. This is where mine are connected and how the carbs came. But a photo in the repair manual notes the lever mounted at the lowest hole where the return springs mount. So there's a bit of a discrepancy. Do you suggest I move the lever so it's mounted at the lowest hole where the return spring mounts? I'll post photos tomorrow of what I'm referring to in the manual.

Thanks for the help.

John

[John L]

Here's the other diagram. My lever is connected like these diagrams.

[John L]

Here's the pic in the manual which appears to show it mounted at the spring return hole which is the lowest hole and/or the one farthest from the top pivot point on the carb.

[pinzinator]

I believe the rod has to be a certain length, too. This might affect the volume of gas pumped, since the piston would be higher or lower in the cylinder.

[Jimm391730]

I believe that the drawing is correct, with the linkage in the innermost hole -- this will give the largest piston movement and the most amount of fuel (if the lower lever had multiple holes then the linkage would need to be in the outermost hole there for the most movement. Pardon my confusion). I did not realize the rest of the accelerator pump valving like the pump upstroke valve, pressure valve, and plunger relief valve - any of these could be causing trouble, too. But the first question is: how much fuel do you see squirting into the carbs (with the bowls full of fuel, just after shutdown)? The cylinder bore and piston travel probably can squirt something close to a tablespoon worth of fuel going from idle to wide open throttle. You should see the same amount flowing in both carbs. IIRC it will come out as a stream of fuel when you look down the carb venturis, not a dribble or atomized spray. If you see a good stream then the accelerator pumps may not be your issue.

Likewise, if you look down the venturis with the engine idling you should NOT see any liquid fuel - no drops or dribbles. If so, the fuel level in the bowls may be TOO high and the engine may stumble from running too rich.

[John L]

Thanks guys. I will check the fuel coming into the carbs tonight when I get home. I figured since they were new they would come set up properly from the factory. I really thought this would turn out to be a grounding problem. But it should be ground perfectly now and I'm still having the issue. It has got to be something simple; but significant. It's driving me nuts!
s d

[hatzlibutzli]

John, have you synchronized the carbs?
Have you measured the CO @ idle?
What is the rev-speed @ idle?
Have you checked the cut down solenoids @ the carbs? (drop in revs, when one is unplugged @ idling ... click, if unplugged, when the engine is not running, but the ignition on)

Sounds a bit, if your carbs are running to rich ... turn in each of the four idle mix screws 1/2 wind in ... rev going up?

Happy Holydays ... Simon

[John L]

Simon

Happy Hollidays to you as well. You ask some good questions, that I neglected to answer. I did check the low idle fuel solenoids and they all seem to be operational. I get a click out of them when plugging them in. As for synching the carbs... yes I did do that. But, given what I've read, including your comments, I think what I will do this weekend is pull her out of the garage, disconnect the throttle linkages, re-synch the carbs then try and dial them in better. I have never dialed in carbs before. Maybe I either have them too rich or too lien. Before I disconnect anything, I"ll look into them and see what happens as the throttle is depressed while she's running and right after she's turned off. Hopefully this will tell me which direction to lean when adjusting the carbs again. Thanks for your input and I'll report back after I play with the carbs a bit.

John

[pinzinator]

Let me know the voltage of each battery, and what the output of the alternator is. The PinzSSI is voltage sensitive, it needs a minimum of 12 volts at the module and 24 at the relay wire (yellow). Check those!

[one2many]

Before you muck around too much with your carbs if you haven't already done so you need to check your valve clearances, overly tight clearances will definitely cause your symptoms and they need to be done routinely on air cooled engines IMO.

You did say that your compression and leak down tests "seemed fine" , I'm curious about what was the percentage of leak down across all cylinders?

It's true that good compression and acceptable leak down results will rule out valve clearances as the culprit but if you have any doubt then check your clearances, it's good measure in any case.

[Pinzcat]

Nobody speaks about ignition breaker: it could be a good reason for bad running and backfire, I had already the problem...