Briggs 875 Intek issues

justjake450

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I have a Briggs 875 Intek #1213120114B8 on a craftsman lawn vac I bought that has a hard time running at speed over 3/4 throttle, starts and runs fine until loaded in the upper RPM range it spudders and loses engine speed, playing with choke will help but not much. I have cleaned the carb, new plug, valves within spec and air filter clean with fresh gas all that. compression is around 50PSI while pulling and leak down test shows about 20% leakage clearly going past rings as all air is coming from oil cap. with the fan for the vacuum removed engine runs strong and to full RPM without issue it only has an issue when engine is loaded. Compression seems to be low to me specially from the leak down test, wanting to have a 2ND opinion if that is causing my issue before I rebuild or replace engine as I have yet to even use this since buying well used. and engines with this tapered shaft are not quite as easy to come by it seems as straight 3/4 or 1" engines. Thanks for any help or input
 

Scrubcadet10

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50 psi is on the low end for that engine with the automatic compression relief on the camshaft. generally on a good/new 6hp or so pushmower engine i will see about 90 psi.... used ones down to about 60.
Is there any thing in the blower portion that could be wrong and could be causing excess load on the engine?
 

ILENGINE

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50 psi and 20% are not really out of the norm. Would like to see more compression but with compression release you can get some weird numbers sometimes. Just did a twin intek the other day that showed that exact figures on both cylinders. starts and runs fine after a plug change. Worked on a generator a couple years ago that showed 5% bypass on the leakdown test but compression was 5 psi.

Anyway enough rabbit hole stuff. Could you throw a spark tester on this thing and see what it is doing at the poor run range. I am wondering if this isn't an ignition module issue. Misfiring at higher rpm under load due to a fault in the module. Acting like a faulty rev limiter situation.
 

justjake450

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I have not tried running at full load with spark tester I will try that tonight. spark seems fine at idle but agree could change with load.
 

ILENGINE

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I have not tried running at full load with spark tester I will try that tonight. spark seems fine at idle but agree could change with load.
Just thinking along those lines, due to the number of ignition modules I have seen the last few years doing weird things. Like starting at idle, but would act like it is running out of fuel when rev'ed up. Or run for like 20 minutes and then act like out of fuel but actually the module is changing the timing so fires at wrong time, and never loose spark.

Have replaced probably 10 modules over the last 4-5 years where the only symptom was poor running, and nothing else fixed the problem. Replace module and back in business. So many of today's ignition system on saws, trimmer, and mowers contain electronics that change the spark timing for increased power, and easier starting, and none of those features are listed in the description for the part. But will cause erratic running conditions that will make you want to pull your hair out. Nothing like working on a piece of equipment with a faulty ignition module and never loose spark.
 

justjake450

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I will look into it for sure thanks for the input. Also is this MTD tapered shaft universal if i ever do end up needing to find a engine for this.
 
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