MUTT Idle surging and loss fuel in line

[leny828]

I have a 1972 M151A2 with emissions control system. Recently I replaced the standard duty head with an NOS heavy duty head, new intake and exhaust manifold. I have checked the valve adjustment twice since installing the new head. At 600 rpm I get approximately 19-20" of manifold pressure. I set the timing at 6 degrees BTDC as the manual suggests. I used the super copper gasket sealer on the exhaust manifold, and the form a gasket on the intake manifold. I used grade "8" bolts instead of the stainless steel bolts. The bolts were all torked to the specification in the manual. I installed a new fuel pump, the metal fuel filter that threads into the carburetor, as well as a clear plastic filter upline from the metal fuel filter.

Since doing all this, at idle after warmup (no choke), the engine will idle OK, then surge, then stumble, then return to a normal idle, and keeps repeating this cycle. Can anyone help me with this issue. I am assuming with a intake manifold pressure of 19-20 when idling normal, is the problem in the carburetor? In reading some of the other strings, do I have an intake manifold leak?

The other problem I am having is, if the MUTT sits for a couple of weeks without being started, I notice all the fuel is gone from the clear plastic fuel filter. When attempting to start the MUTT, it won't pump fuel into the fuel filter. Once I use a vacuum pump to prime the fuel filter, the pump will supply fuel to keep the clear fuel filter full and there is a good flow of gas from the pump to the carburetor. What does anyone think is causing this problem?

Thanks in advance for your suggestions.

Len

[Rickf]

Len, That is a lot of vacuum at 600 rpm! Are you sure the timing is not too far advanced? The timing should be set right on the pointer. This sounds like a clogged in tank filter. You can pull the filter out or rig up a vacuum gauge on the fuel line between the pump and the tank. The reading should be almost 0. Any kind of readable vacuum would tell me the filter is clogged in the tank. The next thing I would check is the float level in the carburetor. You can try spraying carb cleaner around the intake manifold where it meets the head to check for vacuum leaks but this does not sound like that.



Rick

[leny828]

Hi Rick,

Thanks for your reply to my posting, I have been doing a lot of head scratching! The manual said the timing should be 6 degrees BTDC. I set it to the manual specs. My timing light allows me to set a BTDC setting, and that shows the pointer and mark on the pulley to be dead on. I measured the intake manifold pressure by removing the rear pipe plug on the intake manifold, per the manual. I thought the manual said, that you should see a steady reading of between 17-21 inches at 600 rpm's. The reading is steady when the idle is smooth, and of course it goes down when you push on the accelerator. On the emission M151A2's is there a fuel filter also in the tank? My Zenith carb. is a 13660A, do you know the benefits of the 13660B, and is there a difference in the rebuild kits for the "A" and "B" versions.

I still can't figure why the gas drains out of the plastic fuel filter that I added upstream from the metal fuel filter that screws into the carb. I can only assume there is some vacuum leak somewhere?

Thanks,

Len

[Horst]

Len,

The soft kit for the carbs are equal for all carbs. The full kits containing all the gaskets from the soft kit plus various jets are different as the jetting changed from the non-emission to the emission type carb. To my knowledge the full kits for a 13660A or B version should be equal.

see also http://g838.org/wiki/doku.php?id=zenith" onclick="window.open(this.href);return false;

On the in tank fuel filter question I am not sure, the non-emission A2 for sure has it. I have removed it as I have the clear filter similar to your set-up. On a non-emission tank you can simply grab in the tank and pull it off the pick-up tube. Depending on how strong your arms are, you might want to outsource this exercise to your girl friend or wife.

===removed, my idea of the reason was crap===

[muttguru]

I still can't figure why the gas drains out of the plastic fuel filter that I added upstream from the metal fuel filter that screws into the carb. I can only assume there is some vacuum leak somewhere?
Thanks, Len

Len,
there are two valves in the fuel pump. One allows fuel in from the tank, the other allows fuel out from the pump to the carb. The idea is that when the pump "strokes", fuel is drawn by the diaphragm into the pump. The inlet valve closes to prevent the pump pushing the fuel back to the tank. As the pump continues to"stroke", the fuel is pushed past the outlet valve up to the carb. The outlet valve then closes to prevent the pump sucking fuel back from the carb. If the valves in the pump are worn or clogged, or if the diaphragm has gone brittle and/or porous, then these can allow fuel to gravitate back to the tank, or worse still, into the sump oil. Additionally, if you have the wrong gas-cap fitted (such as a non-vented cap on a pre-emission control tank) then you can end up with all sorts of fuel supply problems. If there's no sign of fuel leakage around the connections at the carb and at either side of the fuel pump, and if you have the correct gas-cap fitted, then it could be a worn fuel pump.

Hope this helps....a bit....

[Rickf]

I know you said you replaced the pump but I wonder if the replacement was not bad.


Rick

[RonG]

Just a thought but when you say "My timing light allows me to set a BTDC setting, and that shows the pointer and mark on the pulley to be dead on." doesn't that make your timing either 0 or 12? I have an old Sun timing light that fires when the when it detects the ignition spark and I am not familiar with an adjustable timeing light.

Regarding the fuel pump, I've had the fuel pump valves stick open and it won't pump (as Ken has explained). For fuel to drain back to the tank, both valves would have to be leaking.

One commnet on the plastic fuel filter: I used one several years ago and liked that I could see the status of gasoline inthe line but I noticed a fair amount of flexing in the plastic. It occured to me that if it ever fatigued enough to break the plastic, there would be gasoline spraying all over a hot engine. I took it out.

[Rickf]

Ron is correct in what he says about the timing light. If you set your light for 6 degrees and it lines up with the mark then you are six degrees off since the mark and pointer are set at six degrees and will line up with a normal light. You would have to have your light set at zero to line up the pointer at 6 degrees. Ron, I noticed that flexing on the cheaper plastic filters with a flat back on them. I have some that have a metal back and I also use the type that is tapered at both ends.


Rick

[artificer]

Vacuum seems to be very normal for sea level indicating the engine is fine...
Fault finding starts with eliminating things which are know to be good then narrowing down to most likely.
With fuel lines disconnected check the fuel pump for inlet vacuum and outlet pressure using your Vac Gauge.
If both readings are good it means the pump is fine so check for issues...
Line integrity, filters, tightness of fittings etc between tank and pump as there appears to be something inhibiting fuel getting to the pump.
This could be a vacuum/air leak (most probable as you have the leak down/back issue) or blockage/restriction.
Importantly have you done anything to the tank?

[Cliffaliff]

I think the reason for the plastic filters draining after it has been sitting is gravity. The carb is the highest point, the gas is flowing back to the tank. I had to add a kink in rubber hose before the fuel pump. Seemed to help.

[Rickf]

That kink in the line before the pump will definitely cause you to lose fuel pressure at driving speed because it will severely restrict flow from the tank. The fuel pump will draw a decent vacuum but it is not really designed to do that all the time. The problem with the drain back is a bad or stuck open valve in the pump.