Briggs Intek STILL Backfiring Through Carb

Joe2

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Howdy all. I have a model 31F777 17HP (busted rod) and 31F707 18HP (flat intake cam lobe) that I mixed and matched parts to make the 31F707 18HP run. I checked all part numbers on B&S website prior to starting this project and all numbers where the same. Basically this is what I did, I used the cam from the 17HP and installed it in the 18HP. The ACR was broken on the 17HP so I robbed it off the 18HP, along with the stator and flywheel cuz I needed that for the electric PTO. Installed 18HP in mower and fired er up, easy enough start up, ran great at high rpms, but backfired bad at low rpms. Rechecked my valve adjustment, still within specs, rechecked flywheel key way, not sheared, cleaned carb, not dirty, changed spark plug, no change, also sprayed carb cleaner all around intake with no changes. So then I brought piston up to TDC, held flywheel bolt with wrench, and with my rubber tipped air nozzle pumped approx 100 psi of air in cylinder. It yanked the wrench out of my hand pretty quick and jammed my finger at the same time, ouch :confused2: So I repeated this process again with a firmer grip and did not hear any air escaping, anywhere. Hmmm... So at this point I pulled the head and inspected valves thinking that one of the guides dropped, not the case and both valves have good seats and aren't sticking. Then I checked the head gasket, blown at weak point at push rod side, Hmmmm.. why didn't it leak when I pumped air in the cylinder? So I clean er all up, install new head gasket, adjust valves, etc, and fire er up again. Same results, backfiring through the carb. :frown: What I'm thinking now is that maybe the ACR is not functioning properly and holding the intake open. I did operate the ACR 1/2 dozen times prior to install and it seemed OK. If anyone knos where to look next, please let me know. Thanks again all :thumbsup:
 

motoman

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Sounds like you have background in these motors. Have you changed the basic timing with the e.g., cam gear? Try marking TDC and putting a dial gauge on the intake valve to see wha's happening. Then do a static spark timing test with a light .
 

Joe2

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Motoman, appreciate the quick response. I'm not an expert on Briggs but do alot of research before I post. I'm a retired heavy equipment mechanic and have rebuilt 100's of Cat, Komatsu, Cummins, Detroit Diesel (screaming demons), diesel engines, transmissions, final drives, hydraulic pumps, etc. Also small block chevys from my racing days. In regards to cam timing, I know I have the 2 dots lined up. I'll put my indicator on intake, if I can find it, and see what it does. So how does the static timing test go? With an automotive timing light? I think I still have one someplace. Thanks again :thumbsup:
 

motoman

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Please understand that many of this forum have long experience with specific setups. I only suggest based upon lifelong hobby experience. The suggestion on the timing is just to assure things are reasonably in synch, because you know how timing can affect things. Anyway , if you have a flywheel triggered electronic (magneto ) system a brighter spark depends upon minimum rotation speed like that of the starter driven rpm. Mr Bert says you can use a slow speed,drill driven socket on the flywheel nut. I have not tried that, but I once used an inductive auto timing light on my Intek in a darkened garage to see an idling engine. (The strobe was a barely visible red glow due to the low voltage) I have never tried to put an easily accessible timing mark on the cowl or somewhere for the strobe. Also you may not see any advance if you get that far (normal I think.)
 

Joe2

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Howdy all. Issue resolved :thumbsup: and shame on me :ashamed: Removed and tore the engine down again thinking that the cam was damaged or twisted or something. Verified that dots where lined up, bit the 80 some dollar bullet and bought new shaft. Installed, reassembled, valves. etc and fired her up again. Same, same , same, BIG UGH and almost decided to take my 20# sledge to it. As a last resort I swapped out carbs and guess what, success. :thumbsup: No backfire and runs great. Thanks again all.
 

Mikel1

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Ironically I had this exact problem a few days ago on a Briggs intek 31P667, air leak at throttle shaft on carb. Put new carb on and popping is gone. I found the leak with carb cleaner (rpms climbed), tried wd40 due to a member here but no change when used. Was this also a Nikki carb?
 

Joe2

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Yup it was a Nikki carb and I swapped it out with a Walbro. One of the first things that I did was spray carb and intake with cleaner and check the throttle shaft for excessive movement. None and none. Oh well, t'aint gonna cry over spilt milk :wink:
 
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