Bucking at prolonged highway speeds

aircav · Unread post by **aircav** » September 8th, 2009, 6:39 pm

This is a new one on me... m151 a1 drives great around town, but get it on a secondary road where I can do a prolonged run @ 50-55mph and after about 5min it starts to buck badly ... slow to 45 and it clears up... run back to 50-55 and in another few minutes it bucks again..... new fuel filter and a carburetor rebuild did not solve the problem... this a1 has a mechanical fuel pump ... I was under the impression that the OEM was an electric in the tank model... could this be an issue?

Any thoughts would be greatly appreciated....

Unread post by **rickf** » September 8th, 2009, 7:17 pm

Welcome to the board Aircav. This could be a lot of things but the first thing that comes to mind is fuel pump possibly going bad. Pull the vent line off of the pump that goes up to the air cleaner and see if there is any gas in it. Is there is then the pump is bad. It could be a sign of a coil starting to go bad but not as likely because it never really dies out. If it starts to die out and backfire out the exhaust that is most likely a coil.

Rick

aircav · Unread post by **aircav** » September 8th, 2009, 11:35 pm

Hi Rick,

I hadn't thought of that (fuel pump) in that way, I will check that out tomorrow ... I guess I was keying in on the fact that it had a mechanical pump as opposed to an electric ..thought a mechanical might not be able to maintain enough pressure at the offending rpm to feed the beast.... I will let you know what I find out... The ignition somewhere along the way was upgraded to an electronic pickup it runs to strong at lower rpm for me to suspect that... buttttt you just don't know when it comes to odd things... Thank you sir!!

Cheers

Scott

Cobra5 · Unread post by **Cobra5** » September 8th, 2009, 11:44 pm

Aircav welcome to the group.

Rick,

What about the float drop? According to a thread on the G, Muttguru talks about the float drop measurement in the manual not being correct. Sounds kind of similar as far as the engine starving at higher rpm. I only mention this because it sounds like he rebuilt the carb.

Tim

aircav · Unread post by **aircav** » September 8th, 2009, 11:58 pm

The float drop measurements in the book are way too small,,,, after setting it up accordingly there was less than 1/16 inch travel, I lengthened that out a bit to what would seem a more normal travel ... It was doing this before the rebuild ... (which prompted the rebuild) but I could check it again and see... does anyone have what the travel should be for the float?

Cheers
Scott
Scott

Cobra5 · Unread post by **Cobra5** » September 9th, 2009, 12:05 am

Here's a link to what I read on another form. http://www.g503.com/forums/viewtopic.php?f=14&t=148966

Unread post by **rickf** » September 9th, 2009, 7:35 am

Float drop should be a minimum of 3/8 inch.

Rick

raymond · Unread post by **raymond** » September 9th, 2009, 9:37 pm

Welcome to G838 aircav

BG6 · Unread post by **BG6** » September 10th, 2009, 12:02 am

aircav wrote:This is a new one on me... m151 a1 drives great around town, but get it on a secondary road where I can do a prolonged run @ 50-55mph and after about 5min it starts to buck badly ... slow to 45 and it clears up... run back to 50-55 and in another few minutes it bucks again...

When did you last change the points, condenser and coil?

I had a Suburban a few years ago which gave me the same problem. I added an electric fuel pump, thinking it was vapor issues, swapped out the carb, filters, etc, then swapped the ignition module (electronic) and it cleared right up.

aircav · Unread post by **aircav** » September 10th, 2009, 12:07 am

Since this is an electronic module I suppose that could be the culprit as well... There was a way when the modules first came out to meter them in action but I don’t remember how we did that....rats this getting old is for the birds!!!

aircav · Unread post by **aircav** » September 10th, 2009, 12:11 am

BTW, thank you for the many welcome a boards, I'll try to add what I can to the discussions but I am afraid my talents lay elsewhere as of late

aircav · Unread post by **aircav** » November 20th, 2009, 2:43 pm

I know it has been awhile, but I think I have found the problem, I filled up at a station that was 100% gas, and within a few miles it ran a whole lot better. It kind of wants to back if I am doing 55 for a longer period of time, say 10 min, but is now only hesitating a bit. I think when I get time I will drop the float some more and see if that also help. Is a bit difficult to tell for sure since for some reason this mutt seems to run better in the cooler weather than the hot.. Has anyone else found this out?

Unread post by **rickf** » November 20th, 2009, 4:50 pm

Cooler air is denser so you get more air in the cylinder. Why would you drop the float? Float level is critical in these things, 7/32" float to gasket. Any lower and you will have exactly what you are describing. Have you checked to see what main jet is in it? It could have a #28 main jet which will run very lean and also give those symptoms. I may have mentioned this before but I did not go back over the whole post.

Rick

aircav · Unread post by **aircav** » November 20th, 2009, 5:52 pm

Hi Rick , I don't think you had, the main jet number is it safe to assume it is stamped on the jet? which one should it be?

overall float movement is almost nonexistent, set to the specs on the book, which it currently is,... not familiar enough with the side draft zenith to know if this is about right, good, or bad. Originally I thought it might be fuel delivery, mine has a mechanical pump on the side of the engine, most of my reading and the manual I have reference an electric pump. so I don't know which is correct. I know in our A/C we had a mechanical as a backup to the electric but didn't think they did that on the mutts. Again I am pretty much on novice on these. 100% gas helped greatly but still there.. at the top end after a bit.... Also I checked the return line for the tank and there is no overflow of gas in it.
I will be putting in a new PVC valve, and I will be pulling the top of the tank off, I have a ceramic fuel filter to replace what is presumably in there, however the seal I ordered came in as the one for the mechanical pump not the tank.

I can't think of any other info to add at the moment, I greatly appreciate the thoughts and suggestions from every one, Boy have I got a lot to learn, fortunately I have a mechanics background from 30 years ago so at least I know what end of the hammer to hold

.....

raymond · Unread post by **raymond** » November 21st, 2009, 2:30 am

aircav wrote:
at least I know what end of the hammer to hold .....

Watch it saying the "H" word, it just rattles seabee's chain.

It's easy to make the gaskets for the round fittings on the top of the fuel tank. Get some gasket material and a compass and draw your circles on the gasket material. Cut it out with an Xacto knife. For the bolt holes I keep a bunch of old various caliber spent shell casings. Use them as a hole punch. You only get about 3 holes per spent brass shell casing though before the brass deforms. Good Luck.

Bucking at prolonged highway speeds

Bucking at prolonged highway speeds

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