My New Bike is a Flamethrower!

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The Chicken Man

Active Member
Apr 2, 2021
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Alright so hi everyone!

If you're bored and wanna read about my bike I gave you all the back story. If you just wanna skip to my question scroll to the bottom.

Backstory of my bike:
I bought my Vmax (1989) about a week ago from a private party as a none runner, how I buy all my bikes. Story was when I bought it that it was running fine when parked and the only reason the previous owner stopped riding it was because they bought another bike. And so it had sat the corner of this guys garage gathering dust since sometime in 2015. Talking to the previous owner I got the impression that he was being honest as he seemed like a really cool guy. He did everything he could to be as honest as possible, including not moving the bike from where it had been sitting until I saw it first. If you're curious I paid $2,500 cash and I bought it in Massachusetts.

What I've done so far:
Now being an owner one of the legendary Vmax's I couldn't help but dive right into fixing her up. So the first thing I did was buy a brand new Interstate batter. With that I found out that all the electrics seemed to be working with the exception of the starter solenoid, sigh. So after a new starter and confirming the engine had oil in it I gave her a crank and found the engine was spinning freely. I then dove into the fuel system, draining about 1-1/2 gallons of old gas out of the tank before filling her back up about half way with fresh gas and a appropriate dosage of Sea Foam (1 ounce/gal). Next I found that the line from the fuel filter to the fuel pump was disconnected, maybe because the previous owner ran her dry for storage? Either way connected that back up then pulled of the carbs. I pulled of a bowl or two and everything surprisingly looked pretty clean, no big chunks of crap or anything. All the slide moves smoothly but I pulled of a couple diaphragm covers anyways and they looked good as well. At that point I let my excitement get the best of me so I just threw the carbs back together and with some brand new carb boots I put them back on the bike. After getting the fuel pump to suck up gas and fill the carbs I turned on the choke, gave her a crank, and lone behold she fired right up! In my opinion it even sounded pretty healthy too.

Still eager to unleash the furry of my new machine on the quiet back roads of my small town I pushed her out side to take her for a ride, but I quickly found that things weren't as good as they first seemed. Turns out even as it warmed up it would not idle without the choke unless I gave it a little throttle. Even so the idle quality was quite poor with a noticeable amount of sputtering and the occasional misfire. But because I know bikes sometimes need a moment to collect themselves after sitting for long stretches I still took her for a short ride. So my first ride ever on a Vmax, woah. And I mean WOAH! Even running like a pile of dog doo doo this thing was impressive (for reference my last bike was a 96 Honda Magna). Lots of low end torque, didn't seem like it wanted to stall at all, and the top end power was otherworldly. After my ride it seemed to be running a little bit better but overall the consensus was that it ran good at high rpms but really stumbled at the low-end. So y'all know what this means... piolet jets.

Following the routine for the shotgun cleaning that I read about on this website I gave the carbs a good cleaning. I also took the time to check that all plugs were sparking and preformed a compression test just to see if there was anything obviously wrong. The compression test was done cold, with the throttle wide open, and the bike fully assembled. The image below shows the compression test results and the position of the idle screws when I took them out. Note I did put them back in the same positions.


After doing all that I started her up once again noticed a significant improvement. Still used a good bit of choke to get her started but didn't need it turned all the way on this time. AND! After a couple minutes I was actually able to turn off the choke completely! But it was still idling a little low (maybe 700rpms) so a raised it up to 1000rpm before pushing her back outside to take her for another test flight. Once outside though just before I was about to start her back up however I noticed that I had accidently left one of the plug wires disconnected after doing the compression test. So even on three cylinders it was running that much better than before. So I plugged in that last wire, put the choke to same spot I did previously, cranked her up, and BOOM! She roared to life with the furry of one thousand and ten screaming banchees! Did I mentioned it has an aftermarket exhaust? So I quickly shutoff the choke and just like that she is idling good, on a cold day, with no help from the choke. (It was probably still a little warm from when I started it inside). I then had to turn the idle back down.

My Question:
So now, after all of that, my bike seems to be running decent. That afternoon (yesterday) it was idling fairly well with only the occasional sputter. So I took it for a ride in the evening to the local coffee shop and noticed that it still seemed to stumbling quite a bit. Now, that occasional sputter at idle seems to gradually get worse as you rev until about 3000rpm. Between 3000-5000ish the sputter becomes a consistent noticeable puttery popping sound and makes a notable negative impact on performance. Above those rpms the it seems to get all it's power back and run good, maybe because of the vboost? Once at the coffee shop, when it became dark outside, I noticed something interesting though. When idling every time it would do a little putter it would shoot a little flame out the exhaust. On my ride home, now being dark outside I noticed it was also shooting even bigger flames out the exhaust especially when I was in unsweet spot between 3000-5000rpm. Probably worth noting that this only seemed to be happening when on the throttle either holding a consistent speed or lightly acceleration. When doing hard pulls it seemed to run pretty good and it's quiet when decelerating.

So I'm just curious what y'all think? I've read a lot about how backfires or afterfires can be caused by both lean or rich conditions. I've felt similar stumbling on bikes that were running lean but in all of my experience it was with popping/backfires on deceleration and misfires while accelerating. I've never seen flames before either.

Next thing I'm gonna do, probably tomorrow, is pick up a new set of plugs just to rule out a fowled plug. But what do y'all think? Any thoughts are much appreciated and ask any questions you want.

Couple other things worth noting. My bike has about 29,000 miles on it. It also has an a 4 into 1 Monza exhaust on it. Beyond the fact that it's loud I don't know much about it, it's what came on the bike when I bought it. Another thing I noticed is a small light puff white-ish smoke out exhaust after you give her a quick high rev. But maybe that's just because of the cold weather lately?
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Just curious, have you put a new set of plugs in it by chance? You may want to check valve adjustment also. Lippy
I'd never-pay what you did for a bike non-running, unless it had $3-4K in extras on it. Why? These bikes are prone to 2nd gear issues. That's a 'split the cases, FIAT (for it, another thousand [$])' issue. For your sake, I hope it's OK. Anyway, as to your problem, did you remove and clean the pilot jets, and other passages? I saw you said you removed a float bowl and did a shotgun, but ethanol-laced gasoline, left sitting for a few months, without draining the float bowls, in my experience means a splitting of the carbs into L/R, removing the bowls and jet blocks, removing the jet block brass, and an ultrasonic bath. Yes to CV diaphragm removal. Blow-out things well.

The bike's been sitting since 2015, that's 6 years. It's probably got gummed-up piston rings, and you'll need some bring to op temp and then cool-down cycles to free-up the rings fully. I wouldn't expect things to be OK until you completely tear-down the carbs for a thorough cleaning, and ensure that the fuel tank's not corroded or gummed-up passing contaminated gas (yes I know you changed the gas, but if the tank's rusty or gummed-up, the 'clean gas' will just contaminate the carbs again). With the carbs apart, check the float levels, that is a critical issue on these bikes. In the pic below, the carbs are upside-down. You should check the float needles, the spring on the pin may be gummed-up and non-functional, a shot of carb cleaner administered to the float needle out of the float needle valve seat will free them-up, and confirmation of them being freed-up is easy to do. I suspect the white smoke will lessen as the piston rings become ungummed, and free-up.

Exhaust flames on overrun says to me, 'too-rich.' Misadjusted floats, stuck float valves/needles.
VMax FloatLevel bowl off.jpg
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It might still be a pilot jet clogged. Ive done shotguns before, and NOT been able to fully remove a clog from a pilot jet until I removed PAJ#2, for a full blast of air thru it. Usually flames out the exhaust is due to a lean condition, and the fuel doesnt ignite in the cylinder. It then enters the hot exhaust pipe, and then lights off. I would do another shotgun, with PAJ#2 out for as full blast of air, and then see what happens.
Thank you all for all the support! I really appreciate the very detailed responses, pictures, and all.

An update since my last post:

To determine whether it was running rich or lean I tried just giving the idle jets a full turn out, all around to see if it would run better, and woah it did! Instantly the back firing was all gone and it was starting, idling, and running a whole lot better. I know that's not the proper way to adjust pilot jets but I did for the sake of testing. Definitely was lean.

I then changed the spark plugs, for the sake of maintenance. Gapped to 0.035". Didn't really notice any change. (Image of old plugs shown below)

Was bored and a couple beers deep last Saturday so I hooked up my vacuum synchronizer just for shits and giggles. And uh wow, were they off. I wish I took a picture. The right size was sucking way more air than the left. So I synced them up and I definitely made a huge difference. Throttle response became smoother and the idle just sounded healthier overall.

Also noted that by this point the smoke on sharp revs has stopped. So the rings must be ungumming.

Responding to other points:


As stated I did changed the plugs but haven't checked the valves yet. It's on my to do list though.

Yeah I know the bike was definitely a gamble, but I'm a gambler. I do agree a purchase like this is not exactly a smart one though. Luckily the transmission seems to shift good though. You're probably right about cleaning the carbs more thoroughly, next time we get a stretch of rainy weather I'll pull them and clean all the areas you mentions thoroughly. The tank is actually very clean inside though, not any rust at all and I sucked all the junk at the bottom out when I drained it. I inspected it with a borescope before refilling. Can't thank you enough for all the details about setting the float level correctly though! When I pull the carbs for cleaning I'll make sure to check the float levels and update the thread with what I find.

I did remove all PAJ#2's, blew them out with carb clean and air, and inspected them to ensure they were clear. But you're probably still right that further cleaning would help.

P.S. Feel free to correct me on any forum etiquette mistakes, I'm still a bit new.
We all appreciate the feedback. Your input will likely help someone else who makes use of the search function if they have similar symptoms. Thank you for your input.
I was going to say the tank was probably full of rust but see you said about it isn’t. That thing definitely must have had ethinol free gas in it to have sat that long and the tank not be full of rust. You got lucky there. I sure hope that baby looks pristine for what you paid for it not running , but As long as everything turns out to be good you did fine i think. With all of that said a full complete carb clean will do the trick hopefully. Perhaps while it’s sitting during the carb cleaning you could soak the cylinders for a couple of days to help with any sticking rings.
I'd be checking 2 other things.
Vacuum leaks on the carb to intake rubbers AND airbox to carb rubbers as a possible.

Otherwise that sounds awfully like back pressure from an incompatible aftermarket exhaust or a compatible exhaust that has not had the other required modifications done (gen to carb jets etc).

There are a lot other threads on here about that sort of thing. Here is one the most recent. Check out the videos I linked for more info how exhausts work.
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