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2017/06/26 12:01:39

oldsnapper

Could worn throttle shaft cause engine to race?

My old 8 hp L-Head B&S engine (Model 190707) on a Snapper Rear Engine Rider from the mid 80's has developed a tendency to race exceeedly fast when started, hunt for a speed, and then if I can't find a sweet spot with the throttle control where it will run, it may die out. My first thought was the govenor, and have tried adjusting it, but still races. It also will not idle or run at any speed other than really fast. I've tried adjusting the fast speed and the idle speed screws, and that's when I noticed how jittery the throttle shaft was. Could a worn throttle shaft cause my problem? Also, in case it is, can't find any YouTube videos showing how to replace. I have a left over large welch plug from when I overhauled the rest of the carb ... must the welch plate on the carb be removed for access to screw holding throttle plate or valve to shaft? If so, then how is it best removed and then replaced? And finally, is there a throttle shaft kit for this model, or do I just have to order individual pieces?
EDIT: Removed pictures of mower and added pictures of carb. First two show amount of side play in throttle shaft. Foam washer apparently wore out/fell out. Third and fourth pictures show the amount of rotation of shaft... very limited amount of movement, and very rough. Last picture shows amount of wear in hole of throttle stop for shaft. Similar amount of wear in top of throttle stop where pin is inserted. After taking off carb and examining more closely, I may have answered my own question ... no doubt the throttle shaft and throttle stop need to be replaced. Still would like any feedback / suggestions on replacement procedure, whether this is likely to be cause of racing / overrevving, welch plug removal and replacement, and any hints or tips would be appreciated.

5 comments Leave a comment

Roy

The throttle stop is part of the carb casting so not much you can do there. Pick up a copy of B&S part number 270962 manual, it will give you everything you need to know. I think there is a procedure for rebushing the bore if parts are still available.

2017/06/27 23:16:32

oldsnapper

Thanks for your reply Roy ... couldn't find a copy of the manual that you referenced except for sale, and don't want to invest that much in something I may never need again. The holes in the body for the throttle shaft don't seem too worn (see pics below), so don't think I'll try any kind of bushings. The wear on the throttle stop I was referring to was not part of the casting, but the enlarged holes on the part that fits on the end of the throttle shaft ... I called it a throttle stop ... parts list calls it Stop -Throttle.

So, I dove in, figuring I couldn’t do too much damage since it wasn’t working now, and the worst case scenario might be new engine or new carb (just a carb body no longer available, unless I could find one used).

Welch plate popped out with a screwdriver and light hammer blow accessed thru air intake opening.

Screws came out of choke plate with no problem; had a lock washer under the heads in lieu of staked threads at end as I was told there might be.

The pin that anchors one end of choke return spring and that extends into carb body to act as a stop to prevent choke plate from opening past 90 degrees was another story … couldn’t pull it out, so drove it in instead; once flush with body of carb had to drill into it and next to it until it broke off. Filed and sanded small broken projectile inside body until smooth enough. Will find new pin to fit my newly drilled hole (possibly a broken drill bit).

Once that pin was out, could remove venturi piece, and get to throttle valve. At this point, throttle plate fell out! There used to be a screw holding it, as evidenced by lighter color and marks on throttle shaft where screw had been, but it apparently worked loose, fell out, and somehow the engine survived after it went into the cylinder.

Finally got the throttle shaft out (another @#$%^& pin), and think I will try getting new parts.

After all reassembled will report back if the loose unattached throttle plate (which was no doubt the primary cause of binding) was also the root cause of the over revving.

2017/06/28 12:44:01

Mikel1

Can you manually slow the engine down controlling throttle at carb?
Usually throttle shaft leakage leans out fuel mixture due to extra air passing the shaft.
I must have been typing as you were.lol
I couldn't find a free service manual.

2017/06/28 12:50:32

AVB

That old flojet has multiple problems.

First yes air leaking around the shaft can cause surging and die out as the engine will lean out. Also with throttle floppin around the governor will not be able to fully control the carburetor. Here is the fun part. Even after you can install throttle shaft, you need to install throttle bushing. This not something provided by Briggs. Here I have a local machine shop to machine a custom bushing for the newly enlarge throttle shaft through hole. The enlarge hole must be in line with the opposite hole or there will be binding. And while that of hole size it looks both are egged shape so two bushings will be needed. In the case of the homeowner it is better to just buy a new carburetor.

As for the repair manual it depend if it is before 1981 or not but I will post the link for both of them that are on my Google Drive. The first digits of the date code will be the year.

Repairman's Handbook 1919-1981
https://drive.google.com/open?id=0B-g4TpKUJnh7VzlaODFvcUZNdG8

Single L-Head Engine repair manual
https://drive.google.com/open?id=0B-g4TpKUJnh7ZzF0MEpoakFzQlU

2017/06/28 15:23:11

oldsnapper

Am happy to report that got new parts installed, no parts left over, started and ran briefly, and seems to run fine.
Thanks AVB for links to manuals ... my engine is 1985 vintage, so only needed one, but went ahead and downloaded both for future reference if the need should arise. While mine might have benefited from machining and new bushings, the wear on the holes was apparently not enough to make it necessary at this time. The wear on the throttle shaft and throttle stop, plus missing the screw securing the throttle plate, appear to have been main problems. With the trottle plate not secured, throttle was getting stuck wide open, and then the govenor couldn't do its job properly. Pictures below show amount of wear on old parts as compared to new parts. Thanks again to everyone who replied and offered advice and suggestions.

2017/07/07 09:13:28

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Could worn throttle shaft cause engine to race?

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