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I have an old Murray wit the 20HP Brigg opposed flat head engine. I have done a lot to this thing to keep it running, and have "fixed" this problem before but it keeps coming back.

The problem doesn't always go by the same sequence, but it sometimes does (clear as mud huh?) but often, it will run fine on startup and for first 5-10 minutes. I thought I had it fixed once today and mowed with it for about 190 minutes and it came back.

What it does, is say you are at 1/2-3/4 throttle, the engine will rev up to the throttle setting level, then the revs drop off, almost to an idle, then right back up again. It will keep repeating this process the whole time, from then on. It is not shutting off, it just sounds like you are mowing and playing with the throttle, rev it up, pull it back to idle, rev it up, pull it back to idle, just about that fast.

I have gone through the carb, several times, and replaced the fuel pump once and rebuilt it once. Each time I have done the work, mostly on the fuel pump, it corrects the problem, for a few hrs of run time, then back again. I have also replaced the fuel filter (it's one of those red screens actually) twice. This thing cannot be going through fuel pumps that fast.

This really does not run like it has ignition issues, I believe it HAS to be fuel related. Is there a possibility, something tiny is floating around in there driving me crazy? Should I replace the screen with a REAL fuel filter, and the lines while I am at it? Does that sound like I am headed in the right direction?

The fuel line comes directly from the bottom of the tank, up to the screen, and on to the fuel pump.

This is the three screw fuel pump, for those familiar with the old opposed twins. There is a 4 screw model also, but not mine.

Thanks

Well, I got no responses, so I went ahead and pulled the carb off of it. Went through the carb, and put it back together. I did NOT go through the fuel pump though. I got rid of about 90% of the surge, but now I think the other 10% might be in the fuel pump.

I did throw away the crappy little lawn mower filter and installed a paper element automotive type, so hopefully once I get this cleared up it will not return.

A few things I would try. First I would install a clear plastic fuel filter so I can monitor the amount of fuel going to the carb. It should be constant, but probably not full. Kohler makes a good small one. Second did you check for a cracked vacuum lineto the pump? You might not see it unless you look real close. If you have a small funnel and some fuel line you could make a gravity fuel system to rule out a pump problem. When you cleaned the carb, did you put in a rebuild kit. Just cleaning without replacing the needle valves may not solve the problem.

Rivets
Thanks. No I did not put in a kit, this time, but I did just a month or two ago, so that really ought not to be an issue. The cleaning was to rid any possible foreign objects. I did also just lately (after posting) installed a clear, paper element filter, got tired of the little useless screen. The tank outlet is about 8-10" below the carb/fuel pump and the filter is in line between the tank and carb. It does have gas in it all the time, about 70% full if I remember correctly.

I was wondering just a few minutes ago about the vacuum line. Is that the one (about a 3/8" or larger hose?) that connects to the bottom of the fuel pump? Seems like if I remember correctly, there MIGHT be a crack in the hose, where it connects to the pump. Will have to look that over a bit closer tomorrow.

IF that does not fix it, I will try the gravity feed and see if that makes it run better. I was also wondering about one of those small micro 12v fuel pumps. They seem to go for about $35 or so. I think they are outputting about 1-1.5 psi. Thinking about putting one in right near fuel tank outlet.

Would that be too much psi you think?

Well a few days ago I pulled the fuel pump off, went through it and re-installed and it still ran crappy.

Yesterday, I took the mower to my shop to work on it AGAIN, and getting there it was running fine, so I thought, "maybe I will see if it will keep running", and lo and behold it kept running fine. Went out and went over the little mowers work, the grass that standing AFTER it was mowed :thumbsup: and worked it for a good 30 min and not a hiccup. Sooo I will see how it goes from now on.

I cannot figure this thing out to save me.:frown:

Be careful with the electric pumps. Some carbs are not setup to handle the pressure. Also, try using ethynal free high octane fuel from now on.

handirifle said:

Rivets
IF that does not fix it, I will try the gravity feed and see if that makes it run better. I was also wondering about one of those small micro 12v fuel pumps. They seem to go for about $35 or so. I think they are outputting about 1-1.5 psi. Thinking about putting one in right near fuel tank outlet.

Would that be too much psi you think?

Click to expand...

That is the pressure that the vacuum fuel pump puts out

A vacuum pump normally has less than 2.5 psi. The difference is electric pumps and vacuum pumps is that I have found that some carb float needles and seats don't like the constant pressure of an electric pump, even though the needle' seat and float is designed to hold up to 9 psi. Don't ask me why, because I don't know. Just experience.

Rivets said:

Be careful with the electric pumps. Some carbs are not setup to handle the pressure. Also, try using ethynal free high octane fuel from now on.

Click to expand...

No can do, here in CA ALL you can get has ethenal in it.

OK IF it starts acting up again, what about going with another brand of external vacuum pump? I just thought, even if I used an electric pump, will it feed to the carb through the old one? The factory pump is part of the carb itself on the B&S engine.