Briggs V Twin RPMs Fluctuating

BS V twin, 52" ZT, kept garaged, 500 hrs, reg. maint, syn oil, new plugs actually did it twice, checked fire on both cylinders with an inline neon lamp adapter designed for the purpose both at WOT and idle, re gapped magnetos to flywheel, checked tappet clearance (in spec). Air cleaner off or on. Blade engagement, even at idle does not lug engine, takes it in stride. Runs up in the 3500 area with blade engaged, no grass, a little slow on the up take, about as fast as a long stroke diesel 0 to max. Compression hits about 150 on the 4th stroke on one and about 135 on the other. Manual says within 25% ok. Rotated the valves (with a pair of pliers off the seat) on the weaker one. Didn't retest compression but miss is still present.

Runs right at 3600 (spec) at WOT, after snake oil treatments in fresh fuel and in crankcase, clean oil, hasn't run that good even when new (wasn't broken in)....usually 3300+. Idles at about 1650, (spec is 1750). For a long time it would increase idle as a function of temp. After mowing, idle would be up around 2200. Not doing that now.

Once you come off WOT (a small amount of the throttle reduction) it starts missing intermittently, but consistently intermittent. Once I get to idle it settles down but sounds like it's running on one lung even though I checked with the neon bulb and both firing evenly.....course that doesn't say that the cylinder is producing hp.

It seems to run smoother when first started which led me to think a coil or valve adjustment too tight but neither of those have proven to be a problem.

Can't say it's fuel supply or I'd have problems at WOT.

I thought I knew a few things about engines. This one has me stumped and I don't want to rip it apart just to find I'm chasing a ghost.

I originally thought the vacuum/pressure hose from the valve cover to the fuel pump was the problem due to numerous cracks, but with my new one, problem remains.

Help would be appreciated and thanks in advance.

Mark

Update: Fails the tissue paper test. I pulled up a newish BS V twin next to it with the same muffler system and got out a paper towel. The new one always blew the towel away from the exhaust source. The ailing one would do both.

Ok now we have an output port sucking when the piston is going down on the intake stroke. Exhaust valve didn't close in time or didn't completely close. Symptom is intermittent.

Burned exhaust valve on low comp. cylinder? One would think that temp wouldn't be affecting it. This engine runs worse as it heats up. Also problem would/should be a solid malfunction regardless of rpms and especially not pronounced at the mid rpms.

Sticking exhaust valve or weak exhaust spring....why would it show out worse at mid throttle, backfiring intermittently and missing when it's time for the power stroke. However this problem could be intermittent if it's sticking. Why is it sticking? Never had black oil in it's life. Always full syn, always changed at around 50 hrs per year.

New BS engine I pulled up along side is smooth with very little vibration as a V twin ought to be all the way through the rpm range. Got vibrations from the ailing one with the coughing and pfarting.

When I was checking the valves, I ran it for a minute or two with the cover off and the oil pump was doing it's job in keeping oil on the upper cylinder parts.

Will be listening for any suggestions and meanwhile putting time on the engine. Sea Foam said when you put it in an auto crankcase, do it 100-300 miles before you do an oil change. 100 @ 50 mph (for a minimum number) is 2 hours. I have at least that now. Will wait.

Wellllll, we can prove a point here. I have the two Husqvarna tractors with Briggs Vtech twin engines. They be 21 and 22hp. I have had problems trying to run regular gas. One of them HATES regular gas. Almost what you describe. Try hy-test gas and a little sea foam first. If they persist then the next step is Swap the carbs on the engines. Bet you the problems follow the carb. I changed out the old one on the 22hp engine with a new NIKI carb and it was like a pixie came and bonked the engine with her wand. Instant magic performance. They have come down in price from last year, so you may want to give a new carb a shot. Your engine is not running on one cylinder when it is acting up. It is running one and a half one one side and one half on the other cylinder. Carb goes out of balance internally. Here is how to make a simple test. Grab a skwert can of gas. Take off the air filter and start up the engine. Now you want to squirt a couple of shots all of the way to the right and then a couple of shots to the left. If the engine suddenly runs smooth for a second or two you have carb troubles. Just buy a new one. Beleve me, you don't want to bother with a rebuild. New carb is cheaper!!

That makes a lot of sense. I did do the squirts with aerosol Sea Foam and got varying results. I was looking for a change in performance and I was not careful where or how I shot the charge but I did move it around the inlet (filter removed area). I did not realize what you are saying when doing so. The engine hasn't been used a lot lately and even though I use 3 types of fuel preservative (testing which works) the fuel was at least 6 months and probably older.

It's hard to visualize instantaneous missing not being ignition but if the carb is dumping fuel over what's needed (flooding) one side, it makes a case for the fuel fumes I smell when the engine is running. I guess the misleading (to me) instantaneous explosions are due to build up of gas in the exhaust system and when reaching an explosive level, boom and a backfire....or what I thought was a backfire....never felt a blast out of the carb when that happened, course I never had my hand in front of the carb to check for it either. I have a manual ordered which will be here in a couple of days and I can get a number, but the compression tests, and I did 3 of them at different running temps, just didn't support a bad valve.....so spark tests were positive, compression tests positive......daaaaaaa. Fuel but didn't act like it till you put 2+2.

Well Jeff, I'm going to hop on your suggestion and since you've been there and done that, what can I say but thanks a bunch. You may have saved me an engine teardown.

I'm going to town today and will get some premium gas and run that for awhile with the SF. If that doesn't do it, I'll do a tear down of the carbs and intake system and look for clues. After that if nothing found, it's off to look for a carb. I use the other BS engines either daily or several times a week so I don't want to disturb them.

I really appreciate your input sir!!!!
Thanks,
Mark

Jeff, had to update. Went though what you listed and found 3 obvious problems not counting non-obvious problems.

The float needle valve had a ring around it and I know what kind of flooding trouble that causes.

The plastic intake manifold was loose on the bottom two screws; thought initially that was it but turned out not to be. After removing it to have a look at the carb, the seals were silicon square rings (like an oring but square), badly burned and appeared to be damaged. This was one of my smoking guns as when I sprayed carb cleaner around the outside of the carb prior to deciding to pull it off, I found a spot or two where the spray made the engine combust properly....aka intake manifold leak.

The venturi of the side not combusting properly was coated with black residue which I suppose is degraded ?" fuel line. Since the engine was purchased in 2005 time for that to be a problem; was in my 2002 2 stroke outboard.

Thanks to your picture, I found the exact model number, advertised to replace the numbers you listed on ebay new for $52 with free shipping; guy has 9 left now as I bought one. I guess there is a reason why carb kits aren't available and new, redesigned carbs are offered for sale. Will be here Wed. or Thurs.

Thanks again for your support.

Mark

Wow....sounds like you really dug into it. Never had a problem with those goofy square rings and that goes for the plastic manifold. BUT when you mount your new carb check for warping between the carb and manifold a tiny bit of sand paper on a steel table and just a couple of ruba dubs to get a good fit. Found a bit of warping on mine. The carb you will be getting is a Third Revision unit. You will be shocked how nicely it is going to work. You will have a mile wide grin. You will have instant throttle response and a sinfully smooth idle. Just a shame to not rebuild a carb but in this case...new is SO much better! Very glad I could help and report back to us.

PS .if the bottom of that manifold is damged then get a new one or a good used one off of a blown engine. That damage sounds as if you're carb has been running out of ballance for a long time and was burning stuff up. Those little Niki carbs are actually two seperate carbs in one body. Did you ever try to balance the carbs on a British sports car from the 60s ? There are enough guys out there who get the message. Keep us informed. By the way, do you have any regular lawn / garden tractors with headlights on them? I am going to post some thing in a couple of days about those machines.

PPSS...do yourself another big favor. Those engines use the NGK Copper core plugs. Mine take the BPR6ES plugs. Now just for giggles pickup a set of Irridium plugs made by NGK. They are a bit pricey but your engine will love you for them. Instant starts ! Be sure you put a dab of copper never seize on them. Life time plugs they are. Yoda says so.

PPPPSSS... when you mount up that new carb follow the instruction sheet. Make sure that deflector plate is installed correctly! !!!!

The plastic does not show signs of overheating to the point of distortion. I took a steel rule and ran it across the flange in several directions with a light behind it. Did not see distortion. I think the new flat gasket will help to cover minor irregularities. Sand paper is a good idea.

I did some research on plugs. I found that NGK heat range 5 is the same as Champion heat range 12. Also on the NGK site went through several issues of their plugs that are drop ins for the Champions. The Champion www said that the plug gap was .035 and the NGKs I bought (5 sets for my other BS engines too) are .035. Also bought their V tips. Didn't go with the $8 vs $2 as felt I didn't need them....I do have Irridium . OEM plugs in my Silverado spec'd at 100k mile change interval.

Had a respondent somewhere in my search that said he sets them to .030. I ran gaps on my engine from .025 out to .045 and didn't seem to matter all that much on the 30 and 35. Magneto gap is .010 which is in accordance with the BS 8-12 and this respondents 10. I swapped coils on the dead cylinder and checked spark and spark is sound. Really looking forward to the carb coming in which should be in the next 3-4 days.

Side note: Some guy on ebay was advertising a used carb for the 40H777 engine at $40. Had to laugh at that and feel sorry for the poor purchaser if any. Glad Nikie (carb builder for BS) is in their 3rd iteration of this thing. Must have gotten it right this time around.

Was pondering the design aspect of carbs, especially this one......how do you put in the little slots and holes and where and how big and on and on........

I did notice that....first time I saw such a thing in a carb course this thing looks like no carb I have ever seen.

Thanks Jeff again. Hats off to you man.

Mark